Wheel brake effectiveness standards

[Kenn Sebesta]

Does anyone have any data, preferably quantitative, about what sort of braking performance is required? On the one hand, it would seem that effective braking is primordial for safe landing in the event of an outlanding, but on the other hand many gliders seem to have inadequate brakes, to put it charitably. And these brakes oftentimes are not easily actuated, for instance in a B-4 or L-23 where squeezing the wheel brake handle requires releasing the air brake. So it's fair to conclude that brake performance is (or was) a very distant thought.

I've looked through CS-22, but there are no given standards for wheel brakes, only a loose admonition that "If the main landing gear consists only of one or more wheels, the sailplane must be equipped with mechanical braking devices, such as wheel brakes."

In particular, I'm trying to calculate how much energy the brakes need to absorb. An easy analysis is simply calculating the kinetic energy of the plane when landing 5kts faster than stall (since it's hard to glue the plane to the ground when going much faster). However, this grossly underestimates the amount of energy dissipated through rolling and air resistance. It also doesn't account for what might occur if brake forces were so high that the plane tips forward and skids on its nose.

Still, since the consequence of underspeccing the brakes is brake fade and glazing, and the consequence of overspeccing is additional weight and cost, it's worth trying to right-size the system.

Does anyone have any domain specific experience they could share?

[Rakel]

On Friday, October 16, 2020 at 10:55:16 PM UTC-4, Kenn Sebesta wrote:
>
> Does anyone have any domain specific experience they could share?


Any properly adjusted brake will work well enough to stop your glider.

I fly an LS 1-f with the brakes and spoilers on the same control. The spoilers will open fully and a little more pull on the spoiler control will actuate the brakes. I have the standard drum brakes and I can brake hard enough to cause the nose of my glider to pitch forward and rub on the ground. I do not do this very often.

My point is, there is enough brake power to stop your glider without over thinking this. Proper maintenance is the most important. You need to adjust the brakes to work as intended.

[krasw]

I'm pretty sure that in some cerfification standard there is this specification: "pilot should be able to move some lever is the cockpit marked as wheel brake, and preferably hear scratching sound while pulling it, so that he feels like he is applying wheel brake. No decelaration is needed, however."

At least this is how they were made for decades, before someone invented hydraulic disc brakes.

[AS]

On Friday, October 16, 2020 at 10:55:16 PM UTC-4, Kenn Sebesta wrote:
> Does anyone have any data, preferably quantitative, about what sort of braking performance is required? On the one hand, it would seem that effective braking is primordial for safe landing in the event of an outlanding, but on the other hand many gliders seem to have inadequate brakes, to put it charitably. And these brakes oftentimes are not easily actuated, for instance in a B-4 or L-23 where squeezing the wheel brake handle requires releasing the air brake. So it's fair to conclude that brake performance is (or was) a very distant thought.
>
> I've looked through CS-22, but there are no given standards for wheel brakes, only a loose admonition that "If the main landing gear consists only of one or more wheels, the sailplane must be equipped with mechanical braking devices, such as wheel brakes."
>
> In particular, I'm trying to calculate how much energy the brakes need to absorb. An easy analysis is simply calculating the kinetic energy of the plane when landing 5kts faster than stall (since it's hard to glue the plane to the ground when going much faster). However, this grossly underestimates the amount of energy dissipated through rolling and air resistance. It also doesn't account for what might occur if brake forces were so high that the plane tips forward and skids on its nose.
>
> Still, since the consequence of underspeccing the brakes is brake fade and glazing, and the consequence of overspeccing is additional weight and cost, it's worth trying to right-size the system.
>
> Does anyone have any domain specific experience they could share?

>> ... for instance in a B-4 or L-23 where squeezing the wheel brake handle requires releasing the air brake. <<
I flew my club's B4 a bit and as far as I remember, the wheel brake was actuated by a bike-brake type handle on the stick, so one didn't have to release the spoiler handle.
Anyhow, I owned a H301 with a low serial number. It had the hub of a German post-WW-II Zündapp moped wheel, which was somewhat borderline for stopping a mass of 300kg from 80km/h. However, if properly cared for and adjusted, I could lock up the wheel on grass or make the glider lift the tail on hard surfaces.
The old Schleicher gliders Ka6, Ka8, etc. had a 'steel band over the tire' brake, which worked ok in dry conditions but were useless on wet grass, yet they were certified gliders. IIRC, the POH called it an 'emergency' brake, i.e. only use them if you are about to go through a fence.
I would call 'locking up the wheel at touchdown-speed and MTOW' the upper spec limits of any requirement.
Unless your glider is equipped with an ABS system, locking up the wheel (or wheels in my case) doesn't make you stop any faster but only produces flat-spots on the tread.
The old drum brakes are still adequate as built but require some more attention than the modern hydraulic disc brakes and that may be the core of their bad reputation.

Uli
'AS'

[Brian[_1_]]

IMO the aero dynamic forces are mostly not relevant to the brake sizing.
Since you should have a theoretical Drag number and be able calculate about how much drag your airframe and drag devices (flaps and spoilers) produce you could probably calculate a stopping distance with no brake. Probably need to add a small factor for wheel and bearing friction. And then decide how much more energy you want the brake to absorb.

But more realistically you want the brake to probably stop you in less than 500-1000 feet depending on the landing speed and weight of the aircraft. The Aerodyamic braking is going drop off of so fast that you can just consider it a design safety factor for the wheel brake system.

That leaves you with just the Kinetic energy of the aircraft at touch down being converted to the amount of the heat the brake can absorb. Again the stopping happens so fast you can pretty much assume none of the heat is going to be dissipated during the braking action. So how much heat does the brake need to be able to absorb?

The last consideration is the braking force that the brake can generate. Can it generate enough torque to put the glider up on its nose? You cann make some assumptions about how much elevator forces and resist and or assist this.

I am not an engineer but have done some design work an brakes for a couple different aircraft. So this information may be worth less than you paid for it.

Brian

[Eric Greenwell[_4_]]

krasw wrote on 10/17/2020 6:50 AM:
> I'm pretty sure that in some cerfification standard there is this specification: "pilot should be able to move some lever is the cockpit marked as wheel brake, and preferably hear scratching sound while pulling it, so that he feels like he is applying wheel brake. No decelaration is needed, however."
>
> At least this is how they were made for decades, before someone invented hydraulic disc brakes.
>
You describe my experience with my Std Cirrus in the early '80s :^)

I am told owners now know much more about improving the braking ability and maintaining it. If
the OP opts for a Std Cirrus, he should contact other owners, and take their brake advice.

--
Eric Greenwell - Washington State, USA (change ".netto" to ".us" to email me)
- "A Guide to Self-Launching Sailplane Operation"
https://sites.google.com/site/motorgliders/publications/download-the-guide-1

[jfitch]

The Tost drum brake on my PIK20D, if you turned the drum, relined the shoes and had them arced perfectly to the drum, and carefully assembled everything clean and neat, would stop the glider (or put it on its nose). For the first flight. By the fourth or fifth flight, just adequate. By the tenth flight, you'd be better off opening the canopy and dragging your foot. In contrast, on my ASH26, the Cleveland disk brake will skid the wheel flight after flight, year after year. If I have the choice of a couple of extra pounds to overkill the brake, or dragging my foot to stop, give me the former.

On Saturday, October 17, 2020 at 6:50:35 AM UTC-7, krasw wrote:
> I'm pretty sure that in some cerfification standard there is this specification: "pilot should be able to move some lever is the cockpit marked as wheel brake, and preferably hear scratching sound while pulling it, so that he feels like he is applying wheel brake. No decelaration is needed, however."
>
> At least this is how they were made for decades, before someone invented hydraulic disc brakes.

[Kenn Sebesta]

On Saturday, October 17, 2020 at 10:29:49 AM UTC-4, Brian wrote:
> IMO the aero dynamic forces are mostly not relevant to the brake sizing.

Hmmm, how sure are you on this? I can land my AC-5M at 45kts and roll to a stop within 700' of touchdown. That's some pretty good drag. A lot is obviously coming from the grass, but I suspect with full airbrakesÂ*there's a lot of aerodynamic drag as well.Â*Â*

> Since you should have a theoretical Drag number and be able calculate about how much drag your airframe andÂ* drag devices (flaps and spoilers) produce you could probably calculate a stopping distance with no brake. Probably need to add a small factor for wheel and bearing friction. And then decide how much more energy you want the brake to absorb.

That's a great idea. Where would I get that drag number? It's probably not too different between airplanes so any airplane would probablyÂ* be typical enough for rough calculations.Â*

> But more realistically you want the brake to probably stop you in less than 500-1000 feet depending on the landing speed and weight of the aircraft. The Aerodyamic braking is going drop off of so fast that you can just consider it a design safety factor for the wheel brake system.

How much energy is lost to rolling and air resistance is the critical value to sizing the rotor. As you rightly point out, we can assume the brake rotorÂ*system is adiabatic and so all energy goes into heating up the steel. Twice as much steel means half as much temperature rise. So more steel helps keep temps lower but once you're below the critical glazing and warping temperatures extra thermal mass doesn't have any benefit.Â*

>That leaves you with just the Kinetic energy of the aircraft at touch down being converted to the amount of the heat the brake can absorb. Again the stopping happens so fast you can pretty much assume none of the heat is going to be dissipated during the braking action.Â* So how much heat does the brake need to be able to absorb?

We can calculate the energy as 1/2 * m* v^2 minus whatever energy is lost to rolling and air resistance. So if we assume 50% for standard landings then we can get away with a 50% smaller disc rotor. That's pretty significant!Â*

> The last consideration is the braking force that theÂ* brake can generate. Can it generate enough torque to put the glider up on its nose? You cann make some assumptions about how much elevator forces and resist and or assist this.

Is burying the nose skid more effective than pure wheel braking? I feel like the answer is yes, but it'd be great to have this confirmed.

[Kenn Sebesta]

On Saturday, October 17, 2020 at 10:29:03 AM UTC-4, AS wrote:
> I flew my club's B4 a bit and as far as I remember, the wheel brake was actuated by a bike-brake type handle on the stick, so one didn't have to release the spoiler handle.

If you think about the ergonomics of that handle, you need to release pressure on the airbrake handle in order to open your hand and warp your fingers around the wheel brake lever. It's extremely unergonomic to actuate the two at the same time with any degree of precision, which is what leads me to believe that they didn't really design the system to have great braking performance. It strikes me more like an afterthought, something they threw in so that we could control how far we roll once we're below 10km/h.

> The old Schleicher gliders Ka6, Ka8, etc. had a 'steel band over the tire' brake, which worked ok in dry conditions but were useless on wet grass, yet they were certified gliders. IIRC, the POH called it an 'emergency' brake, i.e. only use them if you are about to go through a fence.

This is a great data point!

> I would call 'locking up the wheel at touchdown-speed and MTOW' the upper spec limits of any requirement.

Immediately on touchdown there's very little force on the wheel because most of the plane's weight is still carried by lift. Do you mean that you should be able to lock it up at any point during touchdown?

[Tango Eight]

Let me provide another perspective.

I had around 300 landings in my ASW-20B. I don't immediately recall the number of field landings, but likely 10 ish. That glider, built in 1985, has a 5.00-5 Cleveland wheel / hydraulic disk. That glider weighs as much as a modern 18m glider (no engine). 380 kg dry, with me in it.

My normal habit following a field landing is to walk off the landing roll and self assess. Those landing rolls were never over 250', all but one or two were 200, right on the nose.

So the short answer is: that problem has been solved for 35 years. Copy what works, worry about more important things.

T8

On Saturday, October 17, 2020 at 11:59:10 AM UTC-4, Kenn Sebesta wrote:
> On Saturday, October 17, 2020 at 10:29:49 AM UTC-4, Brian wrote:
> > IMO the aero dynamic forces are mostly not relevant to the brake sizing..
> Hmmm, how sure are you on this? I can land my AC-5M at 45kts and roll to a stop within 700' of touchdown. That's some pretty good drag. A lot is obviously coming from the grass, but I suspect with full airbrakes there's a lot of aerodynamic drag as well.
> > Since you should have a theoretical Drag number and be able calculate about how much drag your airframe and drag devices (flaps and spoilers) produce you could probably calculate a stopping distance with no brake. Probably need to add a small factor for wheel and bearing friction. And then decide how much more energy you want the brake to absorb.
> That's a great idea. Where would I get that drag number? It's probably not too different between airplanes so any airplane would probably be typical enough for rough calculations.
> > But more realistically you want the brake to probably stop you in less than 500-1000 feet depending on the landing speed and weight of the aircraft. The Aerodyamic braking is going drop off of so fast that you can just consider it a design safety factor for the wheel brake system.
> How much energy is lost to rolling and air resistance is the critical value to sizing the rotor. As you rightly point out, we can assume the brake rotor system is adiabatic and so all energy goes into heating up the steel. Twice as much steel means half as much temperature rise. So more steel helps keep temps lower but once you're below the critical glazing and warping temperatures extra thermal mass doesn't have any benefit.
> >That leaves you with just the Kinetic energy of the aircraft at touch down being converted to the amount of the heat the brake can absorb. Again the stopping happens so fast you can pretty much assume none of the heat is going to be dissipated during the braking action. So how much heat does the brake need to be able to absorb?
> We can calculate the energy as 1/2 * m* v^2 minus whatever energy is lost to rolling and air resistance. So if we assume 50% for standard landings then we can get away with a 50% smaller disc rotor. That's pretty significant!
> > The last consideration is the braking force that the brake can generate.. Can it generate enough torque to put the glider up on its nose? You cann make some assumptions about how much elevator forces and resist and or assist this.
> Is burying the nose skid more effective than pure wheel braking? I feel like the answer is yes, but it'd be great to have this confirmed.

[Tango Eight]

On Saturday, October 17, 2020 at 1:37:06 PM UTC-4, Tango Eight wrote:
> Let me provide another perspective.

Should have included this (love those clouds!):

https://www.youtube.com/watch?v=oBexl9GfKK0

T8

[AS]

On Saturday, October 17, 2020 at 12:01:39 PM UTC-4, Kenn Sebesta wrote:
> On Saturday, October 17, 2020 at 10:29:03 AM UTC-4, AS wrote:
> > I flew my club's B4 a bit and as far as I remember, the wheel brake was actuated by a bike-brake type handle on the stick, so one didn't have to release the spoiler handle.
>
> If you think about the ergonomics of that handle, you need to release pressure on the airbrake handle in order to open your hand and warp your fingers around the wheel brake lever. It's extremely unergonomic to actuate the two at the same time with any degree of precision, which is what leads me to believe that they didn't really design the system to have great braking performance. It strikes me more like an afterthought, something they threw in so that we could control how far we roll once we're below 10km/h.
>
> > The old Schleicher gliders Ka6, Ka8, etc. had a 'steel band over the tire' brake, which worked ok in dry conditions but were useless on wet grass, yet they were certified gliders. IIRC, the POH called it an 'emergency' brake, i.e. only use them if you are about to go through a fence.
>
> This is a great data point!
>
> > I would call 'locking up the wheel at touchdown-speed and MTOW' the upper spec limits of any requirement.
>
> Immediately on touchdown there's very little force on the wheel because most of the plane's weight is still carried by lift. Do you mean that you should be able to lock it up at any point during touchdown?

>> ... you need to release pressure on the airbrake handle in order to open your hand and warp your fingers around the wheel brake lever. It's extremely unergonomic to actuate the two at the same time with any degree of precision, which is what leads me to believe that they didn't really design the system to have great braking performance. <<

Hi Kenn - not sure I understand! In the B4 and any other glider I am familiar with, the spoiler handle is on the left side and there is no brake actuation via the spoiler handle - not by pulling it back fully or by a brake lever on that handle. The right hand is on the stick and the brake handle is mounted on it to the front of it. It does not take a lot of dexterity of the hand to wrap two or three fingers around the brake handle and squeeze it while continuing to hold the stick back.

>> Do you mean that you should be able to lock it up at any point during touchdown? <<
No - only after the wheel(s) are firmly planted on terra firma can any meaningful brake action begin. That's why the big planes have 'squat switches', which look at the condition of the landing gear struts/dampers and only allow braking to start in earnest when they are activated, i.e. after the landing gear is 'loaded'.

Uli
'AS'

[Martin Gregorie[_6_]]

On Sat, 17 Oct 2020 13:51:16 -0700, AS wrote:

> Hi Kenn - not sure I understand! In the B4 and any other glider I am
> familiar with, the spoiler handle is on the left side and there is no
> brake actuation via the spoiler handle - not by pulling it back fully or
> by a brake lever on that handle. The right hand is on the stick and the
> brake handle is mounted on it to the front of it. It does not take a lot
> of dexterity of the hand to wrap two or three fingers around the brake
> handle and squeeze it while continuing to hold the stick back.
>
Most of the single seaters I've flown (Libelle, Discus 1, Pegase 90 use
that arrangement, but I flown a few fairly common types that don't:

- ASK-21: the wheel-brake is applied by pulling the air-brake handle back
past the (spring-loaded) fully air-brake stop. Both brakes work well.

- SZD Puchacz: the air-brake handle is too far back which makes it
awkward enough that some people can't get full air-brake, not that this
is a problem because the air-brakes and HUGE, fully speed-limiting and
tend to stay where you leave them. Just as well because the wheel brake
is a black knob on the left just in front of the air-brake handle's
forward position. Both brakes work well.

- the SZD Junior originally has a bicycle handbrake type wheel-brake but
it was on the air-brake handle rather than the stick, where its pivot
severely weakened the air-brake control assembly. There was an AD to fix
this by deleting the bicycle handbrake control and connecting the wheel
brake to the air-brake handle so that pulling against the stop with the
brakes fully out applies the wheel-brake.

- IIRC the Grop G.103 Acro also has the wheel-brake connected to the
air-brake lever but its been a long time since I flew a G.103 and our
club no longer has one so I can't check.

And lets not forget the much older gliders with nose skids (Slingsby
T.21, Schweitzer 2-33, unmodified ASK-13s*) which don't have a wheel-
brake: you just put the nose skid on the ground and maybe push on the
stick a bit to make them stop quicker.

* most of the K-13s I've flown were retro-fitted with a nose-wheel and
wheel-brake.


--
Martin | martin at
Gregorie | gregorie dot org

[Dan Daly[_2_]]

On Saturday, October 17, 2020 at 5:51:58 PM UTC-4, Martin Gregorie wrote:
> On Sat, 17 Oct 2020 13:51:16 -0700, AS wrote:
>
> > Hi Kenn - not sure I understand! In the B4 and any other glider I am
> > familiar with, the spoiler handle is on the left side and there is no
> > brake actuation via the spoiler handle - not by pulling it back fully or
> > by a brake lever on that handle. The right hand is on the stick and the
> > brake handle is mounted on it to the front of it. It does not take a lot
> > of dexterity of the hand to wrap two or three fingers around the brake
> > handle and squeeze it while continuing to hold the stick back.
> >
> Most of the single seaters I've flown (Libelle, Discus 1, Pegase 90 use
> that arrangement, but I flown a few fairly common types that don't:
>
> - ASK-21: the wheel-brake is applied by pulling the air-brake handle back
> past the (spring-loaded) fully air-brake stop. Both brakes work well.
>
> - SZD Puchacz: the air-brake handle is too far back which makes it
> awkward enough that some people can't get full air-brake, not that this
> is a problem because the air-brakes and HUGE, fully speed-limiting and
> tend to stay where you leave them. Just as well because the wheel brake
> is a black knob on the left just in front of the air-brake handle's
> forward position. Both brakes work well.
>
> - the SZD Junior originally has a bicycle handbrake type wheel-brake but
> it was on the air-brake handle rather than the stick, where its pivot
> severely weakened the air-brake control assembly. There was an AD to fix
> this by deleting the bicycle handbrake control and connecting the wheel
> brake to the air-brake handle so that pulling against the stop with the
> brakes fully out applies the wheel-brake.
>
> - IIRC the Grop G.103 Acro also has the wheel-brake connected to the
> air-brake lever but its been a long time since I flew a G.103 and our
> club no longer has one so I can't check.
>
> And lets not forget the much older gliders with nose skids (Slingsby
> T.21, Schweitzer 2-33, unmodified ASK-13s*) which don't have a wheel-
> brake: you just put the nose skid on the ground and maybe push on the
> stick a bit to make them stop quicker.
>
> * most of the K-13s I've flown were retro-fitted with a nose-wheel and
> wheel-brake.
>
>
> --
> Martin | martin at
> Gregorie | gregorie dot org

and SZD-55 wheel brake is at the rear of spoiler handle travel as well.

[AS]

On Saturday, October 17, 2020 at 5:51:58 PM UTC-4, Martin Gregorie wrote:
> On Sat, 17 Oct 2020 13:51:16 -0700, AS wrote:
>
> > Hi Kenn - not sure I understand! In the B4 and any other glider I am
> > familiar with, the spoiler handle is on the left side and there is no
> > brake actuation via the spoiler handle - not by pulling it back fully or
> > by a brake lever on that handle. The right hand is on the stick and the
> > brake handle is mounted on it to the front of it. It does not take a lot
> > of dexterity of the hand to wrap two or three fingers around the brake
> > handle and squeeze it while continuing to hold the stick back.
> >
> Most of the single seaters I've flown (Libelle, Discus 1, Pegase 90 use
> that arrangement, but I flown a few fairly common types that don't:
>
> - ASK-21: the wheel-brake is applied by pulling the air-brake handle back
> past the (spring-loaded) fully air-brake stop. Both brakes work well.
>
> - SZD Puchacz: the air-brake handle is too far back which makes it
> awkward enough that some people can't get full air-brake, not that this
> is a problem because the air-brakes and HUGE, fully speed-limiting and
> tend to stay where you leave them. Just as well because the wheel brake
> is a black knob on the left just in front of the air-brake handle's
> forward position. Both brakes work well.
>
> - the SZD Junior originally has a bicycle handbrake type wheel-brake but
> it was on the air-brake handle rather than the stick, where its pivot
> severely weakened the air-brake control assembly. There was an AD to fix
> this by deleting the bicycle handbrake control and connecting the wheel
> brake to the air-brake handle so that pulling against the stop with the
> brakes fully out applies the wheel-brake.
>
> - IIRC the Grop G.103 Acro also has the wheel-brake connected to the
> air-brake lever but its been a long time since I flew a G.103 and our
> club no longer has one so I can't check.
>
> And lets not forget the much older gliders with nose skids (Slingsby
> T.21, Schweitzer 2-33, unmodified ASK-13s*) which don't have a wheel-
> brake: you just put the nose skid on the ground and maybe push on the
> stick a bit to make them stop quicker.
>
> * most of the K-13s I've flown were retro-fitted with a nose-wheel and
> wheel-brake.
>
>
> --
> Martin | martin at
> Gregorie | gregorie dot org

>> IIRC the Grop G.103 Acro also has the wheel-brake connected to the
air-brake lever but its been a long time since I flew a G.103 and our
club no longer has one so I can't check. <<
You are correct, Martin! That's how our G103 - III-Acro os set up.
To add to the list of strange brake systems: the Blechnik L13 has a lever on the floor-board next to the seat. The brake itself worked well; probably due to the great leverage one has by pulling up on a handle.

Uli
'AS'

[jmurtari]

Folks,
I'm former USAF and have really enjoyed flying gliders. I love this topic as it has mystified me and I've heard some amazing things..."if you fly a glider properly you don't need brakes." I also saw a 1-26 brake that was a flexible piece of metal that rubbed against the tread. Personally, I expect a fully applied brake to stop wheel rotation and cause a skid. Clearly the FAA does not feel that is essential in a glider....

Best regards!
John Murtari

[Brian[_1_]]

@On Saturday, October 17, 2020 at 7:35:10 PM UTC-6, wrote:
> Folks,
> I'm former USAF and have really enjoyed flying gliders. I love this topic as it has mystified me and I've heard some amazing things..."if you fly a glider properly you don't need brakes." I also saw a 1-26 brake that was a flexible piece of metal that rubbed against the tread. Personally, I expect a fully applied brake to stop wheel rotation and cause a skid. Clearly the FAA does not feel that is essential in a glider....
>
> Best regards!
> John Murtari

When I did my glider transition training the Blanik L-13 i was using was waiting for brake parts from the factory and as a result did not have a working wheel brake. I did all my dual flights, most of my solo flights and my check ride in this glider with no working wheel brake.
Definitely possible to fly without a wheel brake in many, maybe even most situations, but a good wheel brake is a great safety device.

Brian

[Kenn Sebesta]

> Hi Kenn - not sure I understand! In the B4 and any other glider I am familiar with, the spoiler handle is on the left side and there is no brake actuation via the spoiler handle - not by pulling it back fully or by a brake lever on that handle. The right hand is on the stick and the brake handle is mounted on it to the front of it. It does not take a lot of dexterity of the hand to wrap two or three fingers around the brake handle and squeeze it while continuing to hold the stick back.

Oh, that's interesting. I've only ever flown the one B-4, and its wheel brake is a separate lever on the airbrake handle. This means that you can't squeeze it at the same time as you're pulling back against the airbrake springs.

The B-4 in this video has the same setup: https://www.youtube.com/watch?v=Zws1Fy_yNGE.

[Kenn Sebesta]

I think the summary so far is that there's an amazing range of brake effectiveness. One takeaway is that we like having brakes, but so far there are no stories about why having highly effective brakes saved the day, or alternatively why having no brakes led to an unpleasant outcome. This is a not altogether surprising result, considering that my experience mirrors the accounts here.

I'm unsure what conclusion to draw here. It certainly seems that, arguably, effective wheel brakes are seen as a nice-to-have and great wheel brakes are an unneeded luxury. As unsettling as that is to me, if after all these years there's no data to support their need, and even CS-22 barely pays them lip service, then it doesn't seem wholly unjustifiable.

I'd love some hard numbers, if they're out there.

[AS]

On Saturday, October 17, 2020 at 10:19:53 PM UTC-4, Kenn Sebesta wrote:
> > Hi Kenn - not sure I understand! In the B4 and any other glider I am familiar with, the spoiler handle is on the left side and there is no brake actuation via the spoiler handle - not by pulling it back fully or by a brake lever on that handle. The right hand is on the stick and the brake handle is mounted on it to the front of it. It does not take a lot of dexterity of the hand to wrap two or three fingers around the brake handle and squeeze it while continuing to hold the stick back.
>
> Oh, that's interesting. I've only ever flown the one B-4, and its wheel brake is a separate lever on the airbrake handle. This means that you can't squeeze it at the same time as you're pulling back against the airbrake springs.
>
> The B-4 in this video has the same setup: https://www.youtube.com/watch?v=Zws1Fy_yNGE.

I see what you mean now! This is not the way I remember the set-up in our B4. Does your B4 have the fixed or retractable gear?

Uli
'AS'

[Delta8]

On Saturday, October 17, 2020 at 10:19:53 PM UTC-4, Kenn Sebesta wrote:
Hi Kenn - not sure I understand! In the B4 and any other glider I am familiar with, the spoiler handle is on the left side and there is no brake actuation via the spoiler handle - not by pulling it back fully or by a brake lever on that handle. The right hand is on the stick and the brake handle is mounted on it to the front of it. It does not take a lot of dexterity of the hand to wrap two or three fingers around the brake handle and squeeze it while continuing to hold the stick back.

Oh, that's interesting. I've only ever flown the one B-4, and its wheel brake is a separate lever on the airbrake handle. This means that you can't squeeze it at the same time as you're pulling back against the airbrake springs.

The B-4 in this video has the same setup: https://www.youtube.com/watch?v=Zws1Fy_yNGE.

I see what you mean now! This is not the way I remember the set-up in our B4. Does your B4 have the fixed or retractable gear?

Uli
'AS'

Something not in the conversation so far .I'm wondering how much braking a tail skid adds ? I fly off a grass strip and have a tail skid vs wheel ,I've noticed some up elevator after touchdown ( below stall speed) adds some resistance .Granted not much but I have an old ASW-20a the braking is rather poor but I land flap setting 6 almost every landing, so far I've only need to use the brakes once or twice .

[Delta8]

Folks,
I'm former USAF and have really enjoyed flying gliders. I love this topic as it has mystified me and I've heard some amazing things..."if you fly a glider properly you don't need brakes." I also saw a 1-26 brake that was a flexible piece of metal that rubbed against the tread. Personally, I expect a fully applied brake to stop wheel rotation and cause a skid. Clearly the FAA does not feel that is essential in a glider....

Best regards!
John Murtari

I have a lot of time in 1-26's , they have the effective brake system you mentioned .
But I had the most longest and terrifying off field landing roll in a 1-26.
I had a failed ridge transition and picked a very long field ,it had some rolls but pretty much level. After touchdown I kept going ,and going ,and going.
I pulled and puled on the brake but Nothing! Coming to a stop 50' from the trees after what seemed like a quarter mile I exited the canopy and got down on my knees to check the brake and found the main wheel off the ground by 4"!!
Apparently the crop was not soy beans like I thought it was Alfalfa . Very dense and when crushed produces water and a slimy substance .
I guess 1-26s are not very ground loop prone in high crops LOL .

[jfitch]

I consider that a very peculiar conclusion. No you don't "need" a brake. You don't "need" 50:1 glide either, though it's nice to have. You don't need a parachute, except every once in a great while. You really don't "need" a glider at all.

There are plenty fo gliderports where it is expected that you will roll clear of the active runway, without hitting parked gliders, cars, spectators - for that you need a brake. If a motorglider that does any taxiing, you need a brake. If there is ever a chance of landing off field, you need a brake..

I would not own a glider without a brake. And I am quite happy to own one with a very effective brake. And most especially if designing a glider or a brake system for one, I would consider it abject incompetence in this day and age to design an ineffective one. Would you skimp on the brake to save a pound, so you can add a pound of water ballast? What exactly is the point?

On Saturday, October 17, 2020 at 7:54:28 PM UTC-7, Kenn Sebesta wrote:
> I think the summary so far is that there's an amazing range of brake effectiveness. One takeaway is that we like having brakes, but so far there are no stories about why having highly effective brakes saved the day, or alternatively why having no brakes led to an unpleasant outcome. This is a not altogether surprising result, considering that my experience mirrors the accounts here.
>
> I'm unsure what conclusion to draw here. It certainly seems that, arguably, effective wheel brakes are seen as a nice-to-have and great wheel brakes are an unneeded luxury. As unsettling as that is to me, if after all these years there's no data to support their need, and even CS-22 barely pays them lip service, then it doesn't seem wholly unjustifiable.
>
> I'd love some hard numbers, if they're out there.

[[email protected]]

On Saturday, October 17, 2020 at 1:37:06 PM UTC-4, Tango Eight wrote:
> Let me provide another perspective.
>
> I had around 300 landings in my ASW-20B. I don't immediately recall the number of field landings, but likely 10 ish. That glider, built in 1985, has a 5.00-5 Cleveland wheel / hydraulic disk. That glider weighs as much as a modern 18m glider (no engine). 380 kg dry, with me in it.
>
> My normal habit following a field landing is to walk off the landing roll and self assess. Those landing rolls were never over 250', all but one or two were 200, right on the nose.
>
> So the short answer is: that problem has been solved for 35 years. Copy what works, worry about more important things.
>
> T8


Tango: you weren't clear whether you used the brake to get that short roll, or were they that short without braking?

In my off-airport landings the ground was soft enough that the roll was much less than 200 feet without any brake use. Good thing, since the brake on my glider is rather ineffective. I'll try not to land on the Lake Tahoe golf course :-)

I use the brake some times at my home airport, on grass, to try and stop reasonably close to my trailer, but have to plan the maneuver so that I have a clear space to keep on rolling into if necessary.

[John Sinclair[_5_]]

I had an emergency breaking situation where really good breaking saved the day! I was flying our ASH-25 with a new partner out of Williams, Ca. I had several flights with the new guy, but he’d never landed the bird, so I asked if he’d like to make the landing? He said NO, rather firmly and in retrospect, I should have listened, but we had plenty of runway and low winds, so I kinda forced the issue.........bad idea, if someone doesn’t want to do something, that means he isn’t comfortable with it! Well he turned final way too soon.....kinda like where he’d have turned when flying his ASW-20, but here we were, on final a good 300 feet to high! I thought about doing a 360, but thought that might be more dangerous, so we proceeded down final with full spoilers and landing flaps on. Next thing I knew, we were still inches in the air as the hangar went by.........and the south end of the hangar is about 100 feet from the fence! Finally touched down with both of us on the brakes as hard as we could pull aft on the spoiled handle! I could smell burning rubber as smoke rolled out of the wheel well accompanied by loud squealing sound!
We rolled right up the the fence and stopped ...........I kid you not, 1 foot from the fence!
JJ

[Tango Eight]

On Sunday, October 18, 2020 at 12:10:31 PM UTC-4, wrote:
> On Saturday, October 17, 2020 at 1:37:06 PM UTC-4, Tango Eight wrote:
> > Let me provide another perspective.
> >
> > I had around 300 landings in my ASW-20B. I don't immediately recall the number of field landings, but likely 10 ish. That glider, built in 1985, has a 5.00-5 Cleveland wheel / hydraulic disk. That glider weighs as much as a modern 18m glider (no engine). 380 kg dry, with me in it.
> >
> > My normal habit following a field landing is to walk off the landing roll and self assess. Those landing rolls were never over 250', all but one or two were 200, right on the nose.
> >
> > So the short answer is: that problem has been solved for 35 years. Copy what works, worry about more important things.
> >
> > T8
> Tango: you weren't clear whether you used the brake to get that short roll, or were they that short without braking?
>
> In my off-airport landings the ground was soft enough that the roll was much less than 200 feet without any brake use. Good thing, since the brake on my glider is rather ineffective. I'll try not to land on the Lake Tahoe golf course :-)
>
> I use the brake some times at my home airport, on grass, to try and stop reasonably close to my trailer, but have to plan the maneuver so that I have a clear space to keep on rolling into if necessary.

(In the video) Landing flaps + full spoilers on final, 48 KIAS, then tail first landing, spoilers still full open, maximum braking without rubbing the nose in the dirt. Even with landing flaps, minimum touch down speed is nearly 50 mph with spoilers open (that's the price of 7.6# wing loading and a 13% thick wing section). Getting stopped in 200' requires a powerful, easy to modulate brake. It's a great system, but it would be seriously less great with a crummy brake. More recent competition oriented 15 & 18m gliders land faster still and the brake is proportionately even more important.

Your Russia with you in it is 5.x # wing loading, thicker airfoil section, not really an apples and apples comparison.

T8

[Chris Behm]

On Saturday, October 17, 2020 at 2:51:58 PM UTC-7, Martin Gregorie wrote:
> On Sat, 17 Oct 2020 13:51:16 -0700, AS wrote:
>
>
> And lets not forget the much older gliders with nose skids (Slingsby
> T.21, Schweitzer 2-33, unmodified ASK-13s*) which don't have a wheel-
> brake: you just put the nose skid on the ground and maybe push on the
> stick a bit to make them stop quicker.
>
> * most of the K-13s I've flown were retro-fitted with a nose-wheel and
> wheel-brake.
>
>
> --
> Martin | martin at
> Gregorie | gregorie dot org


The 2-33 at our club has a wheel brake that is actuated at the end of the airbrake handle travel. However, shortly after that you will be rubbing the nose skid to the ground. But it DOES have a wheel brake.
R,
Target

[Guy Acheson[_2_]]

Seems pretty simple to me.
If actuating the brake will put the glider on its nose...the brakes are working just fine.

[2G]

On Friday, October 16, 2020 at 7:55:16 PM UTC-7, Kenn Sebesta wrote:
> Does anyone have any data, preferably quantitative, about what sort of braking performance is required? On the one hand, it would seem that effective braking is primordial for safe landing in the event of an outlanding, but on the other hand many gliders seem to have inadequate brakes, to put it charitably. And these brakes oftentimes are not easily actuated, for instance in a B-4 or L-23 where squeezing the wheel brake handle requires releasing the air brake. So it's fair to conclude that brake performance is (or was) a very distant thought.
>
> I've looked through CS-22, but there are no given standards for wheel brakes, only a loose admonition that "If the main landing gear consists only of one or more wheels, the sailplane must be equipped with mechanical braking devices, such as wheel brakes."
>
> In particular, I'm trying to calculate how much energy the brakes need to absorb. An easy analysis is simply calculating the kinetic energy of the plane when landing 5kts faster than stall (since it's hard to glue the plane to the ground when going much faster). However, this grossly underestimates the amount of energy dissipated through rolling and air resistance. It also doesn't account for what might occur if brake forces were so high that the plane tips forward and skids on its nose.
>
> Still, since the consequence of underspeccing the brakes is brake fade and glazing, and the consequence of overspeccing is additional weight and cost, it's worth trying to right-size the system.
>
> Does anyone have any domain specific experience they could share?

Brakes on gliders were almost an afterthought until the advent of motorgliders, which are heavier and require more braking authority. My DG400 had a Tost drum brake that was marginal. Schleicher introduced disk brakes which are much more effective. But one point that hasn't been mentioned is how much tail weight does the glider has. Braking will be limited to the moment arm of the tail; a light glider can't apply as much braking force as a glider with a heavier tail. And the Schleicher MGs have very heavy tails.

As you already found out, there are no standards for a glider's braking ability. But more is better, especially at congested glider operations like Williams.

Tom

[Kenn Sebesta]

> Brakes on gliders were almost an afterthought until the advent of motorgliders, which are heavier and require more braking authority. My DG400 had a Tost drum brake that was marginal. Schleicher introduced disk brakes which are much more effective.

This is an excellent data point.

> But one point that hasn't been mentioned is how much tail weight does the glider has. Braking will be limited to the moment arm of the tail; a light glider can't apply as much braking force as a glider with a heavier tail. And the Schleicher MGs have very heavy tails.

I was initially under this assumption as well, but then I gave it a quick analysis and now I'm convinced the tail weight has very little to do with stopping distance.

Just working off the moment required to tip a modern glass glider forward on its main-- as quantified by hard numbers for a few select aircraft and more generally guesstimated by the effort required to lift the tail to get a dolly under it-- we're looking at around 100Nm per 100kg of plane MTOM.Â*

What this means is that for a 30cm-ish tire diameter, each revolution burns 600J per 100kg MTOM per meter rolled. Nicely, when comparing to kinetic energy the mass cancels out and we can roughly determine that the stopping distance for this maximally effective brake is d=v^2/3.Â*

So for a light plane touching down at 30kts, we're looking at <20m stopping distance without tipping forward on the nose. For a heavier plane touching down at 40kts we're at <35m. Interestingly, those are basically good car stopping distances.

I think all agree that these distances are far shorter than anything we're seeing or can even reasonably expect. We can, therefore, conclude that the tail moment is not the limiting factor.

So why does the tail weight seem important at first glance? Because at anything over a few knots of airspeed you can use the elevator to unload the tailwheel. So it's not the tailwheel weight distribution that's allowing the plane to tip forward when braking hard, it's the (lack of) elevator control..

--------------------------------

It's interesting to consider, in light of this thread, which factors are predominant-- right now I'm hewing toward saying surface quality (no alfalfa!), winds, and airspeed and altitude control are the biggest driver of distance between the start of where a plane could feasibly land and where it ultimately stops. If design choices result in weaker brakes but landing 1kt slower and 500fpm steeperÂ*we might find that the actual stopping distance is improved. Very surprising!

[Dan Marotta]

A very nice analysis except for your question:

So why does the tail weight seem important at first glance?

Tail weight is important for center of gravity considerations.Â* But I'll
bet you knew that.

On 10/20/2020 8:29 AM, Kenn Sebesta wrote:
>> Brakes on gliders were almost an afterthought until the advent of motorgliders, which are heavier and require more braking authority. My DG400 had a Tost drum brake that was marginal. Schleicher introduced disk brakes which are much more effective.
> This is an excellent data point.
>
>> But one point that hasn't been mentioned is how much tail weight does the glider has. Braking will be limited to the moment arm of the tail; a light glider can't apply as much braking force as a glider with a heavier tail. And the Schleicher MGs have very heavy tails.
> I was initially under this assumption as well, but then I gave it a quick analysis and now I'm convinced the tail weight has very little to do with stopping distance.
>
> Just working off the moment required to tip a modern glass glider forward on its main-- as quantified by hard numbers for a few select aircraft and more generally guesstimated by the effort required to lift the tail to get a dolly under it-- we're looking at around 100Nm per 100kg of plane MTOM.
>
> What this means is that for a 30cm-ish tire diameter, each revolution burns 600J per 100kg MTOM per meter rolled. Nicely, when comparing to kinetic energy the mass cancels out and we can roughly determine that the stopping distance for this maximally effective brake is d=v^2/3.
>
> So for a light plane touching down at 30kts, we're looking at <20m stopping distance without tipping forward on the nose. For a heavier plane touching down at 40kts we're at <35m. Interestingly, those are basically good car stopping distances.
>
> I think all agree that these distances are far shorter than anything we're seeing or can even reasonably expect. We can, therefore, conclude that the tail moment is not the limiting factor.
>
> So why does the tail weight seem important at first glance? Because at anything over a few knots of airspeed you can use the elevator to unload the tailwheel. So it's not the tailwheel weight distribution that's allowing the plane to tip forward when braking hard, it's the (lack of) elevator control.
>
> --------------------------------
>
> It's interesting to consider, in light of this thread, which factors are predominant-- right now I'm hewing toward saying surface quality (no alfalfa!), winds, and airspeed and altitude control are the biggest driver of distance between the start of where a plane could feasibly land and where it ultimately stops. If design choices result in weaker brakes but landing 1kt slower and 500fpm steeperÂ*we might find that the actual stopping distance is improved. Very surprising!

--
Dan, 5J

[Tango Whisky]

You've got your units pretty much messed up, and when you correct for that, your calculation doesn't make any sense.

Le mardi 20 octobre 2020 Ã* 16:29:41 UTC+2, Kenn Sebesta a écritÂ*:

> I was initially under this assumption as well, but then I gave it a quick analysis and now I'm convinced the tail weight has very little to do with stopping distance.
>
> Just working off the moment required to tip a modern glass glider forward on its main-- as quantified by hard numbers for a few select aircraft and more generally guesstimated by the effort required to lift the tail to get a dolly under it-- we're looking at around 100Nm per 100kg of plane MTOM.
>
> What this means is that for a 30cm-ish tire diameter, each revolution burns 600J per 100kg MTOM per meter rolled. Nicely, when comparing to kinetic energy the mass cancels out and we can roughly determine that the stopping distance for this maximally effective brake is d=v^2/3.
>
> So for a light plane touching down at 30kts, we're looking at <20m stopping distance without tipping forward on the nose. For a heavier plane touching down at 40kts we're at <35m. Interestingly, those are basically good car stopping distances.
>
> I think all agree that these distances are far shorter than anything we're seeing or can even reasonably expect. We can, therefore, conclude that the tail moment is not the limiting factor.
>
> So why does the tail weight seem important at first glance? Because at anything over a few knots of airspeed you can use the elevator to unload the tailwheel. So it's not the tailwheel weight distribution that's allowing the plane to tip forward when braking hard, it's the (lack of) elevator control.
>
> --------------------------------
>
> It's interesting to consider, in light of this thread, which factors are predominant-- right now I'm hewing toward saying surface quality (no alfalfa!), winds, and airspeed and altitude control are the biggest driver of distance between the start of where a plane could feasibly land and where it ultimately stops. If design choices result in weaker brakes but landing 1kt slower and 500fpm steeper we might find that the actual stopping distance is improved. Very surprising!

[Kenn Sebesta]

On Tuesday, October 20, 2020 at 10:51:44 AM UTC-4, Tango Whisky wrote:
> You've got your units pretty much messed up, and when you correct for that, your calculation doesn't make any sense.

Before investigating my numbers, I'm going to wait until you provide any evidence of this. Otherwise, I think in 2020 we've learned that faceless internet commenters who dispute but don't provide evidence are to be approached with a certain degree of skepticism.

[Kenn Sebesta]

On Tuesday, October 20, 2020 at 10:51:44 AM UTC-4, Tango Whisky wrote:
> You've got your units pretty much messed up, and when you correct for that, your calculation doesn't make any sense.

I'm not immune from errors, but before again recalculating I'm going to wait until you provide any evidence of this. Otherwise, I think in 2020 we've learned that faceless internet commenters who dispute but don't provide evidence are to be approached with a certain degree of skepticism.

[John Galloway[_2_]]

On Tuesday, 20 October 2020 at 16:01:04 UTC+1, Kenn Sebesta wrote:
> On Tuesday, October 20, 2020 at 10:51:44 AM UTC-4, Tango Whisky wrote:
> > You've got your units pretty much messed up, and when you correct for that, your calculation doesn't make any sense.
> I'm not immune from errors, but before again recalculating I'm going to wait until you provide any evidence of this. Otherwise, I think in 2020 we've learned that faceless internet commenters who dispute but don't provide evidence are to be approached with a certain degree of skepticism.

[John Galloway[_2_]]

On Tuesday, 20 October 2020 at 16:01:04 UTC+1, Kenn Sebesta wrote:
> On Tuesday, October 20, 2020 at 10:51:44 AM UTC-4, Tango Whisky wrote:
> > You've got your units pretty much messed up, and when you correct for that, your calculation doesn't make any sense.
> I'm not immune from errors, but before again recalculating I'm going to wait until you provide any evidence of this. Otherwise, I think in 2020 we've learned that faceless internet commenters who dispute but don't provide evidence are to be approached with a certain degree of skepticism.

Whether or not you can raise the tail enough to nose over also depends on how much downwards lift is exerted by holding the stick back on the ground run. That is more effective, obviously, just after touch down so it is better to put the brake on immediately (if it is needed at all). Irrespective of any calculation my V3M, with a very heavy tail and an effective disc brake, can certainly nose over later in the landing ground run.

[MNLou]

I've been working all year on limiting my ground roll.

I have a LAK17AT that, supposedly, has weak drum brakes. I dispelled that rumor years ago by having full stick back and dropping the nose twice due to hard braking.

I added a 7# brass tailwheel last year which helped both the CG and helped keep the tail on the ground when braking. (Also, helped tail bouncing on early takeoff ground roll.)

I did run into one situation that caused me some angst a few weeks ago. Landed well, stick back, lots of brake. Turned off the runway and hit the brake again. Nope, no joy - big brake fade. I squeezed as hard as I could and stopped with my nose just into the cropland.

Any good ideas on how to limit brake fade? Other than, of course, limit high speed brake use?

Lou

[jfitch]

Have you ever actually flown a glider? It is an innocent question, prompted by the seeming naiveté of your posts. Almost never do you touch down with maximum braking, you brake when you need to, often late in the rollout when the elevator has lost any effect. The reason Tost drum brakes were acceptable in light '80s gliders is any more would put the glider on its nose.. An ASH motorglider on the other hand can skid the tire to a stop, because the tailwheel load is well over 100 lbs. If you had flown a variety of gliders you would have experienced this. Nose wheel trainers can have very effective brakes because they cannot nose over. You need to step away from the calculator and fly more.

On Tuesday, October 20, 2020 at 7:29:41 AM UTC-7, Kenn Sebesta wrote:
> > Brakes on gliders were almost an afterthought until the advent of motorgliders, which are heavier and require more braking authority. My DG400 had a Tost drum brake that was marginal. Schleicher introduced disk brakes which are much more effective.
> This is an excellent data point.
> > But one point that hasn't been mentioned is how much tail weight does the glider has. Braking will be limited to the moment arm of the tail; a light glider can't apply as much braking force as a glider with a heavier tail.. And the Schleicher MGs have very heavy tails.
> I was initially under this assumption as well, but then I gave it a quick analysis and now I'm convinced the tail weight has very little to do with stopping distance.
>
> Just working off the moment required to tip a modern glass glider forward on its main-- as quantified by hard numbers for a few select aircraft and more generally guesstimated by the effort required to lift the tail to get a dolly under it-- we're looking at around 100Nm per 100kg of plane MTOM.
>
> What this means is that for a 30cm-ish tire diameter, each revolution burns 600J per 100kg MTOM per meter rolled. Nicely, when comparing to kinetic energy the mass cancels out and we can roughly determine that the stopping distance for this maximally effective brake is d=v^2/3.
>
> So for a light plane touching down at 30kts, we're looking at <20m stopping distance without tipping forward on the nose. For a heavier plane touching down at 40kts we're at <35m. Interestingly, those are basically good car stopping distances.
>
> I think all agree that these distances are far shorter than anything we're seeing or can even reasonably expect. We can, therefore, conclude that the tail moment is not the limiting factor.
>
> So why does the tail weight seem important at first glance? Because at anything over a few knots of airspeed you can use the elevator to unload the tailwheel. So it's not the tailwheel weight distribution that's allowing the plane to tip forward when braking hard, it's the (lack of) elevator control.
>
> --------------------------------
>
> It's interesting to consider, in light of this thread, which factors are predominant-- right now I'm hewing toward saying surface quality (no alfalfa!), winds, and airspeed and altitude control are the biggest driver of distance between the start of where a plane could feasibly land and where it ultimately stops. If design choices result in weaker brakes but landing 1kt slower and 500fpm steeper we might find that the actual stopping distance is improved. Very surprising!

[Tango Whisky]

40 kts corresponds to 20.58 m/s. (20.58 m/s) ^2/3 doesn't make any sense unit-wise, and the numerical result would be 7.36.
My Ventus cM touches down at 40 kts and has a hydraulic disc brake which works pretty well. Stopping distance without hitting the nose on the ground (on grass) is 170 m.

Bert
D-KHTW "TW"

Le mardi 20 octobre 2020 Ã* 17:01:04 UTC+2, Kenn Sebesta a écritÂ*:
> On Tuesday, October 20, 2020 at 10:51:44 AM UTC-4, Tango Whisky wrote:
> > You've got your units pretty much messed up, and when you correct for that, your calculation doesn't make any sense.
> I'm not immune from errors, but before again recalculating I'm going to wait until you provide any evidence of this. Otherwise, I think in 2020 we've learned that faceless internet commenters who dispute but don't provide evidence are to be approached with a certain degree of skepticism.

[John Sinclair[_5_]]

On Tuesday, October 20, 2020 at 8:50:32 AM UTC-7, jfitch wrote:
> Have you ever actually flown a glider? It is an innocent question, prompted by the seeming naiveté of your posts. Almost never do you touch down with maximum braking, you brake when you need to, often late in the rollout when the elevator has lost any effect. The reason Tost drum brakes were acceptable in light '80s gliders is any more would put the glider on its nose. An ASH motorglider on the other hand can skid the tire to a stop, because the tailwheel load is well over 100 lbs. If you had flown a variety of gliders you would have experienced this. Nose wheel trainers can have very effective brakes because they cannot nose over. You need to step away from the calculator and fly more.
> On Tuesday, October 20, 2020 at 7:29:41 AM UTC-7, Kenn Sebesta wrote:
> > > Brakes on gliders were almost an afterthought until the advent of motorgliders, which are heavier and require more braking authority. My DG400 had a Tost drum brake that was marginal. Schleicher introduced disk brakes which are much more effective.
> > This is an excellent data point.
> > > But one point that hasn't been mentioned is how much tail weight does the glider has. Braking will be limited to the moment arm of the tail; a light glider can't apply as much braking force as a glider with a heavier tail. And the Schleicher MGs have very heavy tails.
> > I was initially under this assumption as well, but then I gave it a quick analysis and now I'm convinced the tail weight has very little to do with stopping distance.
> >
> > Just working off the moment required to tip a modern glass glider forward on its main-- as quantified by hard numbers for a few select aircraft and more generally guesstimated by the effort required to lift the tail to get a dolly under it-- we're looking at around 100Nm per 100kg of plane MTOM.
> >
> > What this means is that for a 30cm-ish tire diameter, each revolution burns 600J per 100kg MTOM per meter rolled. Nicely, when comparing to kinetic energy the mass cancels out and we can roughly determine that the stopping distance for this maximally effective brake is d=v^2/3.
> >
> > So for a light plane touching down at 30kts, we're looking at <20m stopping distance without tipping forward on the nose. For a heavier plane touching down at 40kts we're at <35m. Interestingly, those are basically good car stopping distances.
> >
> > I think all agree that these distances are far shorter than anything we're seeing or can even reasonably expect. We can, therefore, conclude that the tail moment is not the limiting factor.
> >
> > So why does the tail weight seem important at first glance? Because at anything over a few knots of airspeed you can use the elevator to unload the tailwheel. So it's not the tailwheel weight distribution that's allowing the plane to tip forward when braking hard, it's the (lack of) elevator control.
> >
> > --------------------------------
> >
> > It's interesting to consider, in light of this thread, which factors are predominant-- right now I'm hewing toward saying surface quality (no alfalfa!), winds, and airspeed and altitude control are the biggest driver of distance between the start of where a plane could feasibly land and where it ultimately stops. If design choices result in weaker brakes but landing 1kt slower and 500fpm steeper we might find that the actual stopping distance is improved. Very surprising!


A couple of quick notes............
+++Nosing over while breaking hard is related more to where the main gear is located in respect to the inflight CG. The ASH-25 has the main gear well forward, but the 301 Libelle’s gear is just about over the CG.

+++If you have a hard breaking incident, it’s a good idea to replace the brake pads because the pads are going to be glazed over and thereafter very ineffective. New Cleveland Pads are only $15 bucks each and can be changed without disconnecting the hydraulic lines.
JJ

[Kenn Sebesta]

On Tuesday, October 20, 2020 at 11:41:08 AM UTC-4, MNLou wrote:
> I've been working all year on limiting my ground roll.
>
> I have a LAK17AT that, supposedly, has weak drum brakes. I dispelled that rumor years ago by having full stick back and dropping the nose twice due to hard braking.
>
> I added a 7# brass tailwheel last year which helped both the CG and helped keep the tail on the ground when braking. (Also, helped tail bouncing on early takeoff ground roll.)
>
> I did run into one situation that caused me some angst a few weeks ago. Landed well, stick back, lots of brake. Turned off the runway and hit the brake again. Nope, no joy - big brake fade. I squeezed as hard as I could and stopped with my nose just into the cropland.
>
> Any good ideas on how to limit brake fade? Other than, of course, limit high speed brake use?
>
> Lou
Brake fade is what happens when component temperature passes some critical value. Thus, there are two approaches we can take: (1) change the critical value and (2) keep temperatures lower.

---------------
Addressing (1) can only be done through material changes. Different material brake pads have different heat characteristics. Some can handle twice the temperature before overheating. However, there seems to be an inverse relationship between fade temperature and braking effectiveness. You also want to make sure increased temperatures don't lead to nasty unintended-consequences, such as melting rubber sidewalls.

---------------
Compensating for (2) is a matter of thermal mass. There are many ways to add thermal mass. You can very easily calculate an upper limit on how much thermal dissipation you need by taking your worst case touchdown speed and your MTOM, and then calculating your kinetic energy (1/2 m *v^2). This heat needs to be absorbed by the braking system. There are two easy ways to reduce temperatures while absorbing this fixed quantity of kinetic energy-- heat transfer to the air and temperature increase. There's also an interesting way which we won't go into here except to say that it'd be amazingly effective if you could figure out the engineering and that's phase change materials (e.g. boiling water or meltingÂ*paraffin wax).

Dissipating energy to the environment is hard, there just isn't enough time or ventilation. However, for a limit case-- which it sounds like you have-- you could maybe just nudge things over the line by using a fan which blows on the drum. Depending on a number of factors, this could increase heat transfer effectiveness by an order of magnitude, which could be enough to buy you a few more seconds of braking. Easy enough to test, get a car to tow you down the runway, release, and then brake hard.

However, in a more general sense-- and ignoring the prior comment about phase change materials-- the only option we've got is increased thermal mass. Steel is easy because the brake drum is already made of it, but it has pretty poor specific heat capacity at around 0.5 kJ/kgC. Aluminum is almost twice better at 0.9kJ/kgC. You've just got to figure out how to thermally couple the aluminum to your steel drum. It's not hard, but it's not as easy as wrapping some old aluminum foil around the drum and calling it a day.Â*

One outside possibility is that If your wheel is aluminum, you *might* even be able to use a thermally conductive pad between the wheel and the drum in order to more quickly transfer heat to the wheel. This has the effect of reducing thermal resistance to a very nice mass of aluminum. With a low enough thermal resistance the wheel can serve as an effective thermal sink.Â*

So it really depends on your appetite for experimentation and budget. Easiest might be different brake pads, if you can find such things.

P.S. One thing we haven't talked about is brake fluid boiling. I don't know if your brake is cable driven or hydraulic, but if it is hydraulic then there is a possibility you experiencedÂ*this instead of pad fade.

[[email protected]]

I don't think brakes in sail planes were not thought out, but technology has improved quite a bit since the original status quo. Both disk and drum can absorb the energy, but the disk seems able to keep doing it over and over without causing maintenance issues.

My last glider had a drum. First with a cracked drum and glazed linings and then with an whole new system freshly tuned from Tost. The old system would eventually stop the glider, but wasn't that great. The fresh system was good enough to put the nose on the ground in soft dirt. Not sure how long it would have kept doing that, but a lot of drum problems may well be maintenance issues.

The current glider is heavier and has disk with hydraulic assist. These definitely work better when glazed than the drums work fresh. Fortunately, I've not yet needed to see how much better. I didn't really plan to have disk, but I can see how if one were concerned about really short landings, then these are worth considering. If I had know this when I redid the drum system in the last glider, I would have switched to disk.

In terms of stopping power It's hard to beat a tire running sideways in a low speed, 180 degree ground loop. YMMV depending on the tail boom structure and control inputs to full stop. Still, it beats a collision even if you to have to inspect the wheel and tail structures and pilot's shorts after the manouver.

[Martin Gregorie[_6_]]

On Tue, 20 Oct 2020 07:29:37 -0700, Kenn Sebesta wrote:

> the stopping distance for this maximally effective brake is d=v^2/3.
>

Is that d = (v*v)/3 or d = v^0.67



--
Martin | martin at
Gregorie | gregorie dot org

[Kenn Sebesta]

On Tuesday, October 20, 2020 at 12:02:17 PM UTC-4, Tango Whisky wrote:
> 40 kts corresponds to 20.58 m/s. (20.58 m/s) ^2/3 doesn't make any sense unit-wise, and the numerical result would be 7.36.
> My Ventus cM touches down at 40 kts and has a hydraulic disc brake which works pretty well. Stopping distance without hitting the nose on the ground (on grass) is 170 m.

Ah, I see the problems. You've made a mistake in the order of operations AND I've made a typo. The exponential resolves before the division so it's not v^(2/3). However, even worse is the typo: the equation is (v^2)/12.Â*

Derivation is here:Â*https://gist.github.com/kubark42/61a14d547bc6beb8dee0f6e7abefe643

Despite the typo, the calculation was correct for your plane 20^2/12 = 33.33m. Please do note that this calculation is meaningless beyond giving how much the tailwheel moment limits you to before you tip forward onto your nose.

However, your real-world 170m distance supports my theory that for modern glass planes the limiting factor is not the weight distribution between the main and tail wheels.

[Kenn Sebesta]

On Tuesday, October 20, 2020 at 11:50:32 AM UTC-4, jfitch wrote:
> Have you ever actually flown a glider? It is an innocent question, prompted by the seeming naiveté of your posts. Almost never do you touch down with maximum braking, you brake when you need to, often late in the rollout when the elevator has lost any effect. The reason Tost drum brakes were acceptable in light '80s gliders is any more would put the glider on its nose. An ASH motorglider on the other hand can skid the tire to a stop, because the tailwheel load is well over 100 lbs. If you had flown a variety of gliders you would have experienced this. Nose wheel trainers can have very effective brakes because they cannot nose over. You need to step away from the calculator and fly more.

Since your question was innocent, I'll accept your implicit apology. I don't think anyone here would mind if you made it explicit, though.

On my last aerotow I had a full brake failure predicated by an old habit of always touching the brakes to stop the wheel. In this case, my theory is that the lack of inertia led the system to have a high jerk, snap, crackle, and pop (yes, those are real terms of the art), leading to a cascading failure. My landing was by necessity brakeless and-- no surprise-- it went perfectly fine with a reasonably short roll-out.

Since I'm going to have to repair a fair amount of my landing gear, and seeing as I've got a lifetime of experience working on planes, I want to take a first principles approach to understanding it. I'm not telling anyone anything new by pointing out that in aviation there's always a tradeoff between weight in one area and lower performance in all others. There's always a way to make a plane stop faster (drag chutes, wider and taller tires, bigger airbrakes, etc...), and the challenge in good design is to find the maximal ratio between cost and benefit.

This has been a very educational thread, thanks to all who have participated so far. Despite the few times when we have sometimes lost sight of our civility I have a very clear idea and theoretical foundation for further experimentation.

[India November[_2_]]

On Tuesday, October 20, 2020 at 1:11:20 PM UTC-4, Kenn Sebesta wrote:
> On Tuesday, October 20, 2020 at 11:50:32 AM UTC-4, jfitch wrote:
> > Have you ever actually flown a glider? It is an innocent question, prompted by the seeming naiveté of your posts. Almost never do you touch down with maximum braking, you brake when you need to, often late in the rollout when the elevator has lost any effect. The reason Tost drum brakes were acceptable in light '80s gliders is any more would put the glider on its nose. An ASH motorglider on the other hand can skid the tire to a stop, because the tailwheel load is well over 100 lbs. If you had flown a variety of gliders you would have experienced this. Nose wheel trainers can have very effective brakes because they cannot nose over. You need to step away from the calculator and fly more.
> Since your question was innocent, I'll accept your implicit apology. I don't think anyone here would mind if you made it explicit, though.
>
> On my last aerotow I had a full brake failure predicated by an old habit of always touching the brakes to stop the wheel. In this case, my theory is that the lack of inertia led the system to have a high jerk, snap, crackle, and pop (yes, those are real terms of the art), leading to a cascading failure. My landing was by necessity brakeless and-- no surprise-- it went perfectly fine with a reasonably short roll-out.
>
> Since I'm going to have to repair a fair amount of my landing gear, and seeing as I've got a lifetime of experience working on planes, I want to take a first principles approach to understanding it. I'm not telling anyone anything new by pointing out that in aviation there's always a tradeoff between weight in one area and lower performance in all others. There's always a way to make a plane stop faster (drag chutes, wider and taller tires, bigger airbrakes, etc...), and the challenge in good design is to find the maximal ratio between cost and benefit.
>
> This has been a very educational thread, thanks to all who have participated so far. Despite the few times when we have sometimes lost sight of our civility I have a very clear idea and theoretical foundation for further experimentation.


FYI Tost lists design brake momentum in Nm for different sizes of wheel with drum and shoe brakes. A 5-inch wheel with drum brake is rated at 200 Nm whereas the 5-inch disk brake is rated at 370 Nm. Might be a starting point.. https://www.tost.de/PDF/Catalog_English_2019_web.pdf

Ian IN

[Martin Gregorie[_6_]]

On Tue, 20 Oct 2020 09:53:58 -0700, Kenn Sebesta wrote:

> On Tuesday, October 20, 2020 at 12:02:17 PM UTC-4, Tango Whisky wrote:
>> 40 kts corresponds to 20.58 m/s. (20.58 m/s) ^2/3 doesn't make any
>> sense unit-wise, and the numerical result would be 7.36.
>> My Ventus cM touches down at 40 kts and has a hydraulic disc brake
>> which works pretty well. Stopping distance without hitting the nose on
>> the ground (on grass) is 170 m.
>
> Ah, I see the problems. You've made a mistake in the order of operations
> AND I've made a typo. The exponential resolves before the division so
> it's not v^(2/3). However, even worse is the typo: the equation is
> (v^2)/12.
>
That still seems a bit long: that revised calculation gives 307m to stop
after a 33kt touchdown: this number assumes I flew finals at 55kt on a
calm day before rounding out for a fully held-off landing in my 201
Libelle, which stalls a little below 35 kts, so 33kts seems about right
for the speed at which the main wheel hits the floor.

However, I know that if I fly a 55 kt approach into a light breeze with
my roundout aim point 15m past the theshhold of our mown grass airfield
I'll be down and stopped 300-325m from the threshhold. Since Libelles
have famously weak airbrakes, I'll have covered at least another 100m
after roundout before my wheels hit the ground.

By comparison an SZD Junior stops sooner thanks to better airbrakes and a
draggier airframe. Both gliders have drum wheelbrakes and a tailwheel, so
not real powerful braking once on the ground.

So I wonder: is your calculation intended to apply to a hard (tarmac/
concrete) runway with the glider being put down above stall speed on just
the mainwheel and with airbrakes being dumped shortly after touchdown?

If so, that would explain the difference very nicely.


--
Martin | martin at
Gregorie | gregorie dot org

[Kenn Sebesta]

> That still seems a bit long: that revised calculation gives 307m to stop
> after a 33kt touchdown: this number assumes I flew finals at 55kt on a
> calm day before rounding out for a fully held-off landing in my 201
> Libelle, which stalls a little below 35 kts, so 33kts seems about right
> for the speed at which the main wheel hits the floor.


Martin--
I'm not quite sure where we're diverging. Here's my math: 33kts *.511m/s / kts = 16.9m/s. 16.9^2/12 = 23.8m.

However, and this is a big however, this is not an analysis of a plane's best possible touchdown. It's a rough estimate for modern glass ships to show how short the stop would be if the ONLY consideration were not nosing over AND your elevator beingÂ*nonexistant. This calculation relies on several implausible factors, several of which you rightly point out. You'd have to have no wing lift (in order to place weight on the tire); no elevator lift (theÂ*premise of the question); you'd have to have great tire/surface friction (in order not to lock up the tire); there should be no other drag (or else we'd stop even faster!); your braking system would have to be up to the task (hah!); etc...

It's only useful so that we know if we should look at the main/tail wheel mass distribution as a limiting factor in braking distance. The stark difference between the optimal and the real-world numbers (10x!) let us conclude that it is not.

[[email protected]]

On Tuesday, October 20, 2020 at 10:49:33 AM UTC-4, Dan Marotta wrote:
> A very nice analysis except for your question:
>
> So why does the tail weight seem important at first glance?
>
> Tail weight is important for center of gravity considerations.Â* But I'll
> bet you knew that.
>
> On 10/20/2020 8:29 AM, Kenn Sebesta wrote:
> >> Brakes on gliders were almost an afterthought until the advent of motorgliders, which are heavier and require more braking authority. My DG400 had a Tost drum brake that was marginal. Schleicher introduced disk brakes which are much more effective.
> > This is an excellent data point.
> >
> >> But one point that hasn't been mentioned is how much tail weight does the glider has. Braking will be limited to the moment arm of the tail; a light glider can't apply as much braking force as a glider with a heavier tail. And the Schleicher MGs have very heavy tails.
> > I was initially under this assumption as well, but then I gave it a quick analysis and now I'm convinced the tail weight has very little to do with stopping distance.
> >
> > Just working off the moment required to tip a modern glass glider forward on its main-- as quantified by hard numbers for a few select aircraft and more generally guesstimated by the effort required to lift the tail to get a dolly under it-- we're looking at around 100Nm per 100kg of plane MTOM.
> >
> > What this means is that for a 30cm-ish tire diameter, each revolution burns 600J per 100kg MTOM per meter rolled. Nicely, when comparing to kinetic energy the mass cancels out and we can roughly determine that the stopping distance for this maximally effective brake is d=v^2/3.
> >
> > So for a light plane touching down at 30kts, we're looking at <20m stopping distance without tipping forward on the nose. For a heavier plane touching down at 40kts we're at <35m. Interestingly, those are basically good car stopping distances.
> >
> > I think all agree that these distances are far shorter than anything we're seeing or can even reasonably expect. We can, therefore, conclude that the tail moment is not the limiting factor.
> >
> > So why does the tail weight seem important at first glance? Because at anything over a few knots of airspeed you can use the elevator to unload the tailwheel. So it's not the tailwheel weight distribution that's allowing the plane to tip forward when braking hard, it's the (lack of) elevator control.
> >
> > --------------------------------
> >
> > It's interesting to consider, in light of this thread, which factors are predominant-- right now I'm hewing toward saying surface quality (no alfalfa!), winds, and airspeed and altitude control are the biggest driver of distance between the start of where a plane could feasibly land and where it ultimately stops. If design choices result in weaker brakes but landing 1kt slower and 500fpm steeperÂ*we might find that the actual stopping distance is improved. Very surprising!
>
> --
> Dan, 5J

The "tail weight" being talked about here is how much weight does one need to hoist if lifting the tail while the glider is on the ground (with the pilot seated). For the same tail design and CG and aerodynamics, this "weight" can be changed by moving the wheel forward or backward. The wheel location of course has no effect on the aerodynamics, as long as the CG is still in the same location (relative to the wing and tail).

[Dan Marotta]

Why not simply hit glazed pads with sand paper rather than replacing them?

On 10/20/2020 10:15 AM, John Sinclair wrote:
> On Tuesday, October 20, 2020 at 8:50:32 AM UTC-7, jfitch wrote:
>> Have you ever actually flown a glider? It is an innocent question, prompted by the seeming naiveté of your posts. Almost never do you touch down with maximum braking, you brake when you need to, often late in the rollout when the elevator has lost any effect. The reason Tost drum brakes were acceptable in light '80s gliders is any more would put the glider on its nose. An ASH motorglider on the other hand can skid the tire to a stop, because the tailwheel load is well over 100 lbs. If you had flown a variety of gliders you would have experienced this. Nose wheel trainers can have very effective brakes because they cannot nose over. You need to step away from the calculator and fly more.
>> On Tuesday, October 20, 2020 at 7:29:41 AM UTC-7, Kenn Sebesta wrote:
>>>> Brakes on gliders were almost an afterthought until the advent of motorgliders, which are heavier and require more braking authority. My DG400 had a Tost drum brake that was marginal. Schleicher introduced disk brakes which are much more effective.
>>> This is an excellent data point.
>>>> But one point that hasn't been mentioned is how much tail weight does the glider has. Braking will be limited to the moment arm of the tail; a light glider can't apply as much braking force as a glider with a heavier tail. And the Schleicher MGs have very heavy tails.
>>> I was initially under this assumption as well, but then I gave it a quick analysis and now I'm convinced the tail weight has very little to do with stopping distance.
>>>
>>> Just working off the moment required to tip a modern glass glider forward on its main-- as quantified by hard numbers for a few select aircraft and more generally guesstimated by the effort required to lift the tail to get a dolly under it-- we're looking at around 100Nm per 100kg of plane MTOM.
>>>
>>> What this means is that for a 30cm-ish tire diameter, each revolution burns 600J per 100kg MTOM per meter rolled. Nicely, when comparing to kinetic energy the mass cancels out and we can roughly determine that the stopping distance for this maximally effective brake is d=v^2/3.
>>>
>>> So for a light plane touching down at 30kts, we're looking at <20m stopping distance without tipping forward on the nose. For a heavier plane touching down at 40kts we're at <35m. Interestingly, those are basically good car stopping distances.
>>>
>>> I think all agree that these distances are far shorter than anything we're seeing or can even reasonably expect. We can, therefore, conclude that the tail moment is not the limiting factor.
>>>
>>> So why does the tail weight seem important at first glance? Because at anything over a few knots of airspeed you can use the elevator to unload the tailwheel. So it's not the tailwheel weight distribution that's allowing the plane to tip forward when braking hard, it's the (lack of) elevator control.
>>>
>>> --------------------------------
>>>
>>> It's interesting to consider, in light of this thread, which factors are predominant-- right now I'm hewing toward saying surface quality (no alfalfa!), winds, and airspeed and altitude control are the biggest driver of distance between the start of where a plane could feasibly land and where it ultimately stops. If design choices result in weaker brakes but landing 1kt slower and 500fpm steeper we might find that the actual stopping distance is improved. Very surprising!
>
> A couple of quick notes............
> +++Nosing over while breaking hard is related more to where the main gear is located in respect to the inflight CG. The ASH-25 has the main gear well forward, but the 301 Libelle’s gear is just about over the CG.
>
> +++If you have a hard breaking incident, it’s a good idea to replace the brake pads because the pads are going to be glazed over and thereafter very ineffective. New Cleveland Pads are only $15 bucks each and can be changed without disconnecting the hydraulic lines.
> JJ

--
Dan, 5J

[John Sinclair[_5_]]

On Tuesday, October 20, 2020 at 4:56:04 PM UTC-7, Dan Marotta wrote:
> Why not simply hit glazed pads with sand paper rather than replacing them?
> On 10/20/2020 10:15 AM, John Sinclair wrote:
> > On Tuesday, October 20, 2020 at 8:50:32 AM UTC-7, jfitch wrote:
> >> Have you ever actually flown a glider? It is an innocent question, prompted by the seeming naiveté of your posts. Almost never do you touch down with maximum braking, you brake when you need to, often late in the rollout when the elevator has lost any effect. The reason Tost drum brakes were acceptable in light '80s gliders is any more would put the glider on its nose. An ASH motorglider on the other hand can skid the tire to a stop, because the tailwheel load is well over 100 lbs. If you had flown a variety of gliders you would have experienced this. Nose wheel trainers can have very effective brakes because they cannot nose over. You need to step away from the calculator and fly more.
> >> On Tuesday, October 20, 2020 at 7:29:41 AM UTC-7, Kenn Sebesta wrote:
> >>>> Brakes on gliders were almost an afterthought until the advent of motorgliders, which are heavier and require more braking authority. My DG400 had a Tost drum brake that was marginal. Schleicher introduced disk brakes which are much more effective.
> >>> This is an excellent data point.
> >>>> But one point that hasn't been mentioned is how much tail weight does the glider has. Braking will be limited to the moment arm of the tail; a light glider can't apply as much braking force as a glider with a heavier tail. And the Schleicher MGs have very heavy tails.
> >>> I was initially under this assumption as well, but then I gave it a quick analysis and now I'm convinced the tail weight has very little to do with stopping distance.
> >>>
> >>> Just working off the moment required to tip a modern glass glider forward on its main-- as quantified by hard numbers for a few select aircraft and more generally guesstimated by the effort required to lift the tail to get a dolly under it-- we're looking at around 100Nm per 100kg of plane MTOM.
> >>>
> >>> What this means is that for a 30cm-ish tire diameter, each revolution burns 600J per 100kg MTOM per meter rolled. Nicely, when comparing to kinetic energy the mass cancels out and we can roughly determine that the stopping distance for this maximally effective brake is d=v^2/3.
> >>>
> >>> So for a light plane touching down at 30kts, we're looking at <20m stopping distance without tipping forward on the nose. For a heavier plane touching down at 40kts we're at <35m. Interestingly, those are basically good car stopping distances.
> >>>
> >>> I think all agree that these distances are far shorter than anything we're seeing or can even reasonably expect. We can, therefore, conclude that the tail moment is not the limiting factor.
> >>>
> >>> So why does the tail weight seem important at first glance? Because at anything over a few knots of airspeed you can use the elevator to unload the tailwheel. So it's not the tailwheel weight distribution that's allowing the plane to tip forward when braking hard, it's the (lack of) elevator control.
> >>>
> >>> --------------------------------
> >>>
> >>> It's interesting to consider, in light of this thread, which factors are predominant-- right now I'm hewing toward saying surface quality (no alfalfa!), winds, and airspeed and altitude control are the biggest driver of distance between the start of where a plane could feasibly land and where it ultimately stops. If design choices result in weaker brakes but landing 1kt slower and 500fpm steeper we might find that the actual stopping distance is improved. Very surprising!
> >
> > A couple of quick notes............
> > +++Nosing over while breaking hard is related more to where the main gear is located in respect to the inflight CG. The ASH-25 has the main gear well forward, but the 301 Libelle’s gear is just about over the CG.
> >
> > +++If you have a hard breaking incident, it’s a good idea to replace the brake pads because the pads are going to be glazed over and thereafter very ineffective. New Cleveland Pads are only $15 bucks each and can be changed without disconnecting the hydraulic lines.
> > JJ
> --
> Dan, 5J
You can sand off the glaze, but after I’ve gone to the trouble of removing the caliper, might as well put on new brake pads, unless they are nearly new in thickness. Aircraft Spruce sells a neat little tool that removes the rivets and curls over the new rivets. $15 bucks for the tool and $15 bucks per pad, not bad!
JJ

[AS]

On Tuesday, October 20, 2020 at 8:54:20 PM UTC-4, John Sinclair wrote:
> On Tuesday, October 20, 2020 at 4:56:04 PM UTC-7, Dan Marotta wrote:
> > Why not simply hit glazed pads with sand paper rather than replacing them?
> > On 10/20/2020 10:15 AM, John Sinclair wrote:
> > > On Tuesday, October 20, 2020 at 8:50:32 AM UTC-7, jfitch wrote:
> > >> Have you ever actually flown a glider? It is an innocent question, prompted by the seeming naiveté of your posts. Almost never do you touch down with maximum braking, you brake when you need to, often late in the rollout when the elevator has lost any effect. The reason Tost drum brakes were acceptable in light '80s gliders is any more would put the glider on its nose. An ASH motorglider on the other hand can skid the tire to a stop, because the tailwheel load is well over 100 lbs. If you had flown a variety of gliders you would have experienced this. Nose wheel trainers can have very effective brakes because they cannot nose over. You need to step away from the calculator and fly more.
> > >> On Tuesday, October 20, 2020 at 7:29:41 AM UTC-7, Kenn Sebesta wrote:
> > >>>> Brakes on gliders were almost an afterthought until the advent of motorgliders, which are heavier and require more braking authority. My DG400 had a Tost drum brake that was marginal. Schleicher introduced disk brakes which are much more effective.
> > >>> This is an excellent data point.
> > >>>> But one point that hasn't been mentioned is how much tail weight does the glider has. Braking will be limited to the moment arm of the tail; a light glider can't apply as much braking force as a glider with a heavier tail. And the Schleicher MGs have very heavy tails.
> > >>> I was initially under this assumption as well, but then I gave it a quick analysis and now I'm convinced the tail weight has very little to do with stopping distance.
> > >>>
> > >>> Just working off the moment required to tip a modern glass glider forward on its main-- as quantified by hard numbers for a few select aircraft and more generally guesstimated by the effort required to lift the tail to get a dolly under it-- we're looking at around 100Nm per 100kg of plane MTOM.
> > >>>
> > >>> What this means is that for a 30cm-ish tire diameter, each revolution burns 600J per 100kg MTOM per meter rolled. Nicely, when comparing to kinetic energy the mass cancels out and we can roughly determine that the stopping distance for this maximally effective brake is d=v^2/3.
> > >>>
> > >>> So for a light plane touching down at 30kts, we're looking at <20m stopping distance without tipping forward on the nose. For a heavier plane touching down at 40kts we're at <35m. Interestingly, those are basically good car stopping distances.
> > >>>
> > >>> I think all agree that these distances are far shorter than anything we're seeing or can even reasonably expect. We can, therefore, conclude that the tail moment is not the limiting factor.
> > >>>
> > >>> So why does the tail weight seem important at first glance? Because at anything over a few knots of airspeed you can use the elevator to unload the tailwheel. So it's not the tailwheel weight distribution that's allowing the plane to tip forward when braking hard, it's the (lack of) elevator control.
> > >>>
> > >>> --------------------------------
> > >>>
> > >>> It's interesting to consider, in light of this thread, which factors are predominant-- right now I'm hewing toward saying surface quality (no alfalfa!), winds, and airspeed and altitude control are the biggest driver of distance between the start of where a plane could feasibly land and where it ultimately stops. If design choices result in weaker brakes but landing 1kt slower and 500fpm steeper we might find that the actual stopping distance is improved. Very surprising!
> > >
> > > A couple of quick notes............
> > > +++Nosing over while breaking hard is related more to where the main gear is located in respect to the inflight CG. The ASH-25 has the main gear well forward, but the 301 Libelle’s gear is just about over the CG..
> > >
> > > +++If you have a hard breaking incident, it’s a good idea to replace the brake pads because the pads are going to be glazed over and thereafter very ineffective. New Cleveland Pads are only $15 bucks each and can be changed without disconnecting the hydraulic lines.
> > > JJ
> > --
> > Dan, 5J
> You can sand off the glaze, but after I’ve gone to the trouble of removing the caliper, might as well put on new brake pads, unless they are nearly new in thickness. Aircraft Spruce sells a neat little tool that removes the rivets and curls over the new rivets. $15 bucks for the tool and $15 bucks per pad, not bad!
> JJ

If de-glazing does not work or if you are dissatisfied with the performance of you drum brake, you can find a brake shop for vintage cars, that can remove the original lining and replace it with one that is a bit more 'grabby'. They can also adjust the curvature of said new linings to make sure they contact the I.D. of the drum with the maximum surface. That was done many years ago by a H301 owner, which had a weak brake to begin with. He reported a remarkable improvement.

Uli
'AS'

[Delta8]

The perfect brake 2:35

https://www.youtube.com/watch?v=X9frBKmg30Q&ab_channel=EmpireStateStudios

[Tango Whisky]

Velocity squared is NOT a distance. It's just nonsense.

Le mardi 20 octobre 2020 Ã* 18:54:01 UTC+2, Kenn Sebesta a écritÂ*:
> On Tuesday, October 20, 2020 at 12:02:17 PM UTC-4, Tango Whisky wrote:
> > 40 kts corresponds to 20.58 m/s. (20.58 m/s) ^2/3 doesn't make any sense unit-wise, and the numerical result would be 7.36.
> > My Ventus cM touches down at 40 kts and has a hydraulic disc brake which works pretty well. Stopping distance without hitting the nose on the ground (on grass) is 170 m.
> Ah, I see the problems. You've made a mistake in the order of operations AND I've made a typo. The exponential resolves before the division so it's not v^(2/3). However, even worse is the typo: the equation is (v^2)/12.
>
> Derivation is here: https://gist.github.com/kubark42/61a14d547bc6beb8dee0f6e7abefe643
>
> Despite the typo, the calculation was correct for your plane 20^2/12 = 33.33m. Please do note that this calculation is meaningless beyond giving how much the tailwheel moment limits you to before you tip forward onto your nose.
>
> However, your real-world 170m distance supports my theory that for modern glass planes the limiting factor is not the weight distribution between the main and tail wheels.

[Dan Marotta]

Yes, I've relined brakes but being a cheap glider pilot, I like to save
$15 when I can. :-D

On 10/20/2020 6:54 PM, John Sinclair wrote:
> On Tuesday, October 20, 2020 at 4:56:04 PM UTC-7, Dan Marotta wrote:
>> Why not simply hit glazed pads with sand paper rather than replacing them?
>> On 10/20/2020 10:15 AM, John Sinclair wrote:
>>> On Tuesday, October 20, 2020 at 8:50:32 AM UTC-7, jfitch wrote:
>>>> Have you ever actually flown a glider? It is an innocent question, prompted by the seeming naiveté of your posts. Almost never do you touch down with maximum braking, you brake when you need to, often late in the rollout when the elevator has lost any effect. The reason Tost drum brakes were acceptable in light '80s gliders is any more would put the glider on its nose. An ASH motorglider on the other hand can skid the tire to a stop, because the tailwheel load is well over 100 lbs. If you had flown a variety of gliders you would have experienced this. Nose wheel trainers can have very effective brakes because they cannot nose over. You need to step away from the calculator and fly more.
>>>> On Tuesday, October 20, 2020 at 7:29:41 AM UTC-7, Kenn Sebesta wrote:
>>>>>> Brakes on gliders were almost an afterthought until the advent of motorgliders, which are heavier and require more braking authority. My DG400 had a Tost drum brake that was marginal. Schleicher introduced disk brakes which are much more effective.
>>>>> This is an excellent data point.
>>>>>> But one point that hasn't been mentioned is how much tail weight does the glider has. Braking will be limited to the moment arm of the tail; a light glider can't apply as much braking force as a glider with a heavier tail. And the Schleicher MGs have very heavy tails.
>>>>> I was initially under this assumption as well, but then I gave it a quick analysis and now I'm convinced the tail weight has very little to do with stopping distance.
>>>>>
>>>>> Just working off the moment required to tip a modern glass glider forward on its main-- as quantified by hard numbers for a few select aircraft and more generally guesstimated by the effort required to lift the tail to get a dolly under it-- we're looking at around 100Nm per 100kg of plane MTOM.
>>>>>
>>>>> What this means is that for a 30cm-ish tire diameter, each revolution burns 600J per 100kg MTOM per meter rolled. Nicely, when comparing to kinetic energy the mass cancels out and we can roughly determine that the stopping distance for this maximally effective brake is d=v^2/3.
>>>>>
>>>>> So for a light plane touching down at 30kts, we're looking at <20m stopping distance without tipping forward on the nose. For a heavier plane touching down at 40kts we're at <35m. Interestingly, those are basically good car stopping distances.
>>>>>
>>>>> I think all agree that these distances are far shorter than anything we're seeing or can even reasonably expect. We can, therefore, conclude that the tail moment is not the limiting factor.
>>>>>
>>>>> So why does the tail weight seem important at first glance? Because at anything over a few knots of airspeed you can use the elevator to unload the tailwheel. So it's not the tailwheel weight distribution that's allowing the plane to tip forward when braking hard, it's the (lack of) elevator control.
>>>>>
>>>>> --------------------------------
>>>>>
>>>>> It's interesting to consider, in light of this thread, which factors are predominant-- right now I'm hewing toward saying surface quality (no alfalfa!), winds, and airspeed and altitude control are the biggest driver of distance between the start of where a plane could feasibly land and where it ultimately stops. If design choices result in weaker brakes but landing 1kt slower and 500fpm steeper we might find that the actual stopping distance is improved. Very surprising!
>>> A couple of quick notes............
>>> +++Nosing over while breaking hard is related more to where the main gear is located in respect to the inflight CG. The ASH-25 has the main gear well forward, but the 301 Libelle’s gear is just about over the CG.
>>>
>>> +++If you have a hard breaking incident, it’s a good idea to replace the brake pads because the pads are going to be glazed over and thereafter very ineffective. New Cleveland Pads are only $15 bucks each and can be changed without disconnecting the hydraulic lines.
>>> JJ
>> --
>> Dan, 5J
> You can sand off the glaze, but after I’ve gone to the trouble of removing the caliper, might as well put on new brake pads, unless they are nearly new in thickness. Aircraft Spruce sells a neat little tool that removes the rivets and curls over the new rivets. $15 bucks for the tool and $15 bucks per pad, not bad!
> JJ

--
Dan, 5J

[James Metcalfe]

At 06:35 21 October 2020, Tango Whisky wrote:
>Velocity squared is NOT a distance. It's just nonsense.

Actually, v^2 is proportional to energy per unit mass.
As is braking distance under constant braking force (unless your mass is
changing, e.g. still dumping ballast).
J.