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Wheel brake effectiveness standards



 
 
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  #21  
Old October 18th 20, 01:17 PM
Delta8 Delta8 is offline
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Originally Posted by AS View Post
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 .
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  #22  
Old October 18th 20, 01:54 PM
Delta8 Delta8 is offline
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Originally Posted by jmurtari View Post
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 .
  #23  
Old October 18th 20, 04:28 PM posted to rec.aviation.soaring
jfitch
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Default Wheel brake effectiveness standards

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.

  #24  
Old October 18th 20, 05:10 PM posted to rec.aviation.soaring
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Default Wheel brake effectiveness standards

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.
  #25  
Old October 18th 20, 06:25 PM posted to rec.aviation.soaring
John Sinclair[_5_]
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Default Wheel brake effectiveness standards


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
  #26  
Old October 18th 20, 07:21 PM posted to rec.aviation.soaring
Tango Eight
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Default Wheel brake effectiveness standards

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
  #27  
Old October 18th 20, 11:53 PM posted to rec.aviation.soaring
Chris Behm
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Default Wheel brake effectiveness standards

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
  #28  
Old October 19th 20, 12:08 AM posted to rec.aviation.soaring
Guy Acheson[_2_]
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Default Wheel brake effectiveness standards

Seems pretty simple to me.
If actuating the brake will put the glider on its nose...the brakes are working just fine.
  #29  
Old October 20th 20, 05:42 AM posted to rec.aviation.soaring
2G
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Default Wheel brake effectiveness standards

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
  #30  
Old October 20th 20, 03:29 PM posted to rec.aviation.soaring
Kenn Sebesta
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Default Wheel brake effectiveness standards

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!
 




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