Wheel brake effectiveness standards

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.

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!

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

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

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.

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

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