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Wheel brake effectiveness standards



 
 
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  #1  
Old October 17th 20, 03:55 AM posted to rec.aviation.soaring
Kenn Sebesta
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Default Wheel brake effectiveness standards

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?
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  #2  
Old October 17th 20, 12:34 PM posted to rec.aviation.soaring
Rakel
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Default Wheel brake effectiveness standards

On Friday, October 16, 2020 at 10:55:16 PM UTC-4, Kenn Sebesta wrote:

Does anyone have any domain specific experience they could share?



Any properly adjusted brake will work well enough to stop your glider.

I fly an LS 1-f with the brakes and spoilers on the same control. The spoilers will open fully and a little more pull on the spoiler control will actuate the brakes. I have the standard drum brakes and I can brake hard enough to cause the nose of my glider to pitch forward and rub on the ground. I do not do this very often.

My point is, there is enough brake power to stop your glider without over thinking this. Proper maintenance is the most important. You need to adjust the brakes to work as intended.

  #3  
Old October 17th 20, 02:50 PM posted to rec.aviation.soaring
krasw
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Default Wheel brake effectiveness standards

I'm pretty sure that in some cerfification standard there is this specification: "pilot should be able to move some lever is the cockpit marked as wheel brake, and preferably hear scratching sound while pulling it, so that he feels like he is applying wheel brake. No decelaration is needed, however."

At least this is how they were made for decades, before someone invented hydraulic disc brakes.
  #4  
Old October 17th 20, 03:29 PM posted to rec.aviation.soaring
AS
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Posts: 558
Default Wheel brake effectiveness standards

On Friday, October 16, 2020 at 10:55:16 PM UTC-4, 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?


... for instance in a B-4 or L-23 where squeezing the wheel brake handle requires releasing the air brake.

I flew my club's B4 a bit and as far as I remember, the wheel brake was actuated by a bike-brake type handle on the stick, so one didn't have to release the spoiler handle.
Anyhow, I owned a H301 with a low serial number. It had the hub of a German post-WW-II Z√ľndapp moped wheel, which was somewhat borderline for stopping a mass of 300kg from 80km/h. However, if properly cared for and adjusted, I could lock up the wheel on grass or make the glider lift the tail on hard surfaces.
The old Schleicher gliders Ka6, Ka8, etc. had a 'steel band over the tire' brake, which worked ok in dry conditions but were useless on wet grass, yet they were certified gliders. IIRC, the POH called it an 'emergency' brake, i.e. only use them if you are about to go through a fence.
I would call 'locking up the wheel at touchdown-speed and MTOW' the upper spec limits of any requirement.
Unless your glider is equipped with an ABS system, locking up the wheel (or wheels in my case) doesn't make you stop any faster but only produces flat-spots on the tread.
The old drum brakes are still adequate as built but require some more attention than the modern hydraulic disc brakes and that may be the core of their bad reputation.

Uli
'AS'
  #5  
Old October 17th 20, 03:29 PM posted to rec.aviation.soaring
Brian[_1_]
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Default Wheel brake effectiveness standards

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
  #6  
Old October 17th 20, 04:19 PM posted to rec.aviation.soaring
Eric Greenwell[_4_]
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Posts: 1,622
Default Wheel brake effectiveness standards

krasw wrote on 10/17/2020 6:50 AM:
I'm pretty sure that in some cerfification standard there is this specification: "pilot should be able to move some lever is the cockpit marked as wheel brake, and preferably hear scratching sound while pulling it, so that he feels like he is applying wheel brake. No decelaration is needed, however."

At least this is how they were made for decades, before someone invented hydraulic disc brakes.

You describe my experience with my Std Cirrus in the early '80s :^)

I am told owners now know much more about improving the braking ability and maintaining it. If
the OP opts for a Std Cirrus, he should contact other owners, and take their brake advice.

--
Eric Greenwell - Washington State, USA (change ".netto" to ".us" to email me)
- "A Guide to Self-Launching Sailplane Operation"
https://sites.google.com/site/motorg...ad-the-guide-1
  #7  
Old October 17th 20, 04:40 PM posted to rec.aviation.soaring
jfitch
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Default Wheel brake effectiveness standards

The Tost drum brake on my PIK20D, if you turned the drum, relined the shoes and had them arced perfectly to the drum, and carefully assembled everything clean and neat, would stop the glider (or put it on its nose). For the first flight. By the fourth or fifth flight, just adequate. By the tenth flight, you'd be better off opening the canopy and dragging your foot. In contrast, on my ASH26, the Cleveland disk brake will skid the wheel flight after flight, year after year. If I have the choice of a couple of extra pounds to overkill the brake, or dragging my foot to stop, give me the former.

On Saturday, October 17, 2020 at 6:50:35 AM UTC-7, krasw wrote:
I'm pretty sure that in some cerfification standard there is this specification: "pilot should be able to move some lever is the cockpit marked as wheel brake, and preferably hear scratching sound while pulling it, so that he feels like he is applying wheel brake. No decelaration is needed, however."

At least this is how they were made for decades, before someone invented hydraulic disc brakes.

  #8  
Old October 17th 20, 04:59 PM posted to rec.aviation.soaring
Kenn Sebesta
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Posts: 22
Default Wheel brake effectiveness standards

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.
  #9  
Old October 17th 20, 05:01 PM posted to rec.aviation.soaring
Kenn Sebesta
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Posts: 22
Default Wheel brake effectiveness standards

On Saturday, October 17, 2020 at 10:29:03 AM UTC-4, AS wrote:
I flew my club's B4 a bit and as far as I remember, the wheel brake was actuated by a bike-brake type handle on the stick, so one didn't have to release the spoiler handle.


If you think about the ergonomics of that handle, you need to release pressure on the airbrake handle in order to open your hand and warp your fingers around the wheel brake lever. It's extremely unergonomic to actuate the two at the same time with any degree of precision, which is what leads me to believe that they didn't really design the system to have great braking performance. It strikes me more like an afterthought, something they threw in so that we could control how far we roll once we're below 10km/h.

The old Schleicher gliders Ka6, Ka8, etc. had a 'steel band over the tire' brake, which worked ok in dry conditions but were useless on wet grass, yet they were certified gliders. IIRC, the POH called it an 'emergency' brake, i.e. only use them if you are about to go through a fence.


This is a great data point!

I would call 'locking up the wheel at touchdown-speed and MTOW' the upper spec limits of any requirement.


Immediately on touchdown there's very little force on the wheel because most of the plane's weight is still carried by lift. Do you mean that you should be able to lock it up at any point during touchdown?
  #10  
Old October 17th 20, 06:37 PM posted to rec.aviation.soaring
Tango Eight
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Default Wheel brake effectiveness standards

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.

 




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