Wheel brake effectiveness standards

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.

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.

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.

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

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.

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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.

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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.

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.

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.

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 heÂ*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.

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

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.