Wheel brake effectiveness standards

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

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'

The perfect brake 2:35

https://www.youtube.com/watch?v=X9fr...reStateStudios

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 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.

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

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.