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Wheel brake effectiveness standards



 
 
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  #41  
Old October 20th 20, 05:23 PM posted to rec.aviation.soaring
Kenn Sebesta
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Default Wheel brake effectiveness standards

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 melting¬*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.¬*

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.¬*

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 experienced¬*this instead of pad fade.
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  #42  
Old October 20th 20, 05:24 PM posted to rec.aviation.soaring
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Default Wheel brake effectiveness standards

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.
  #43  
Old October 20th 20, 05:25 PM posted to rec.aviation.soaring
Martin Gregorie[_6_]
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Default Wheel brake effectiveness standards

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



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Gregorie | gregorie dot org

  #44  
Old October 20th 20, 05:53 PM posted to rec.aviation.soaring
Kenn Sebesta
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Default Wheel brake effectiveness standards

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 he¬*https://gist.github.com/kubark42/61a...e0f6e7abefe643

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.
  #45  
Old October 20th 20, 06:11 PM posted to rec.aviation.soaring
Kenn Sebesta
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Default Wheel brake effectiveness standards

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.

  #46  
Old October 20th 20, 06:35 PM posted to rec.aviation.soaring
India November[_2_]
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Default Wheel brake effectiveness standards

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

  #47  
Old October 20th 20, 09:02 PM posted to rec.aviation.soaring
Martin Gregorie[_6_]
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Default Wheel brake effectiveness standards

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.


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Gregorie | gregorie dot org

  #48  
Old October 20th 20, 09:16 PM posted to rec.aviation.soaring
Kenn Sebesta
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Posts: 48
Default Wheel brake effectiveness standards

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 being¬*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 (the¬*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.
  #49  
Old October 20th 20, 10:01 PM posted to rec.aviation.soaring
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Default Wheel brake effectiveness standards

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.¬* 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 steeper¬*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).
  #50  
Old October 21st 20, 12:55 AM posted to rec.aviation.soaring
Dan Marotta
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Default Wheel brake effectiveness standards

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
 




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