Wheel brake effectiveness standards

On Saturday, October 17, 2020 at 5:51:58 PM UTC-4, Martin Gregorie wrote:
On Sat, 17 Oct 2020 13:51:16 -0700, AS 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.

Most of the single seaters I've flown (Libelle, Discus 1, Pegase 90 use
that arrangement, but I flown a few fairly common types that don't:

- ASK-21: the wheel-brake is applied by pulling the air-brake handle back
past the (spring-loaded) fully air-brake stop. Both brakes work well.

- SZD Puchacz: the air-brake handle is too far back which makes it
awkward enough that some people can't get full air-brake, not that this
is a problem because the air-brakes and HUGE, fully speed-limiting and
tend to stay where you leave them. Just as well because the wheel brake
is a black knob on the left just in front of the air-brake handle's
forward position. Both brakes work well.

- the SZD Junior originally has a bicycle handbrake type wheel-brake but
it was on the air-brake handle rather than the stick, where its pivot
severely weakened the air-brake control assembly. There was an AD to fix
this by deleting the bicycle handbrake control and connecting the wheel
brake to the air-brake handle so that pulling against the stop with the
brakes fully out applies the wheel-brake.

- IIRC the Grop G.103 Acro also has the wheel-brake connected to the
air-brake lever but its been a long time since I flew a G.103 and our
club no longer has one so I can't check.

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

IIRC the Grop G.103 Acro also has the wheel-brake connected to the
air-brake lever but its been a long time since I flew a G.103 and our
club no longer has one so I can't check.
You are correct, Martin! That's how our G103 - III-Acro os set up.
To add to the list of strange brake systems: the Blechnik L13 has a lever on the floor-board next to the seat. The brake itself worked well; probably due to the great leverage one has by pulling up on a handle.

Uli
'AS'

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

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.

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'

IMO the aero dynamic forces are mostly not relevant to the brake sizing.
Since you should have a theoretical Drag number and be able calculate about how much drag your airframe and drag devices (flaps and spoilers) produce you could probably calculate a stopping distance with no brake. Probably need to add a small factor for wheel and bearing friction. And then decide how much more energy you want the brake to absorb.

But more realistically you want the brake to probably stop you in less than 500-1000 feet depending on the landing speed and weight of the aircraft. The Aerodyamic braking is going drop off of so fast that you can just consider it a design safety factor for the wheel brake system.

That leaves you with just the Kinetic energy of the aircraft at touch down being converted to the amount of the heat the brake can absorb. Again the stopping happens so fast you can pretty much assume none of the heat is going to be dissipated during the braking action. So how much heat does the brake need to be able to absorb?

The last consideration is the braking force that the brake can generate. Can it generate enough torque to put the glider up on its nose? You cann make some assumptions about how much elevator forces and resist and or assist this.

I am not an engineer but have done some design work an brakes for a couple different aircraft. So this information may be worth less than you paid for it.

Brian

On Saturday, October 17, 2020 at 10:29:49 AM UTC-4, Brian wrote:
IMO the aero dynamic forces are mostly not relevant to the brake sizing.

Hmmm, how sure are you on this? I can land my AC-5M at 45kts and roll to a stop within 700' of touchdown. That's some pretty good drag. A lot is obviously coming from the grass, but I suspect with full airbrakesÂ*there's a lot of aerodynamic drag as well.Â*Â*

Since you should have a theoretical Drag number and be able calculate about how much drag your airframe andÂ* drag devices (flaps and spoilers) produce you could probably calculate a stopping distance with no brake. Probably need to add a small factor for wheel and bearing friction. And then decide how much more energy you want the brake to absorb.

That's a great idea. Where would I get that drag number? It's probably not too different between airplanes so any airplane would probablyÂ* be typical enough for rough calculations.Â*

But more realistically you want the brake to probably stop you in less than 500-1000 feet depending on the landing speed and weight of the aircraft. The Aerodyamic braking is going drop off of so fast that you can just consider it a design safety factor for the wheel brake system.

How much energy is lost to rolling and air resistance is the critical value to sizing the rotor. As you rightly point out, we can assume the brake rotorÂ*system is adiabatic and so all energy goes into heating up the steel. Twice as much steel means half as much temperature rise. So more steel helps keep temps lower but once you're below the critical glazing and warping temperatures extra thermal mass doesn't have any benefit.Â*

That leaves you with just the Kinetic energy of the aircraft at touch down being converted to the amount of the heat the brake can absorb. Again the stopping happens so fast you can pretty much assume none of the heat is going to be dissipated during the braking action.Â* So how much heat does the brake need to be able to absorb?

We can calculate the energy as 1/2 * m* v^2 minus whatever energy is lost to rolling and air resistance. So if we assume 50% for standard landings then we can get away with a 50% smaller disc rotor. That's pretty significant!Â*

The last consideration is the braking force that theÂ* brake can generate. Can it generate enough torque to put the glider up on its nose? You cann make some assumptions about how much elevator forces and resist and or assist this.

Is burying the nose skid more effective than pure wheel braking? I feel like the answer is yes, but it'd be great to have this confirmed.

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

On Saturday, October 17, 2020 at 11:59:10 AM UTC-4, Kenn Sebesta wrote:
On Saturday, October 17, 2020 at 10:29:49 AM UTC-4, Brian wrote:
IMO the aero dynamic forces are mostly not relevant to the brake sizing..
Hmmm, how sure are you on this? I can land my AC-5M at 45kts and roll to a stop within 700' of touchdown. That's some pretty good drag. A lot is obviously coming from the grass, but I suspect with full airbrakes there's a lot of aerodynamic drag as well.
Since you should have a theoretical Drag number and be able calculate about how much drag your airframe and drag devices (flaps and spoilers) produce you could probably calculate a stopping distance with no brake. Probably need to add a small factor for wheel and bearing friction. And then decide how much more energy you want the brake to absorb.
That's a great idea. Where would I get that drag number? It's probably not too different between airplanes so any airplane would probably be typical enough for rough calculations.
But more realistically you want the brake to probably stop you in less than 500-1000 feet depending on the landing speed and weight of the aircraft. The Aerodyamic braking is going drop off of so fast that you can just consider it a design safety factor for the wheel brake system.
How much energy is lost to rolling and air resistance is the critical value to sizing the rotor. As you rightly point out, we can assume the brake rotor system is adiabatic and so all energy goes into heating up the steel. Twice as much steel means half as much temperature rise. So more steel helps keep temps lower but once you're below the critical glazing and warping temperatures extra thermal mass doesn't have any benefit.
That leaves you with just the Kinetic energy of the aircraft at touch down being converted to the amount of the heat the brake can absorb. Again the stopping happens so fast you can pretty much assume none of the heat is going to be dissipated during the braking action. So how much heat does the brake need to be able to absorb?
We can calculate the energy as 1/2 * m* v^2 minus whatever energy is lost to rolling and air resistance. So if we assume 50% for standard landings then we can get away with a 50% smaller disc rotor. That's pretty significant!
The last consideration is the braking force that the brake can generate.. Can it generate enough torque to put the glider up on its nose? You cann make some assumptions about how much elevator forces and resist and or assist this.
Is burying the nose skid more effective than pure wheel braking? I feel like the answer is yes, but it'd be great to have this confirmed.

On Saturday, October 17, 2020 at 1:37:06 PM UTC-4, Tango Eight wrote:
Let me provide another perspective.

Should have included this (love those clouds!):

https://www.youtube.com/watch?v=oBexl9GfKK0

T8

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.

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