Aircraft brake systems

[Dafydd Llewellyn]

Picking up from the discussion on brakes in the "another Jabiru engine" thread -

 

Firstly, Turbs is quite right that comparing aircraft brakes to motorcycle brakes is apples and oranges. If you have ever seen an undercarriage drop test, you will understand why brake disc diameters are so limited on aircraft whose undercarriages deflect outwards. And if you look at the Cleveland website, they have some pretty pertinent comments on the use of stainless steel brake discs.

 

The worst aircraft brakes are those that use a brake drum inside the wheel; and in my experience the choice of which is the absolute worst is difficult to choose between the Auster series and the Super Cub; they are both foul, but for entirely different reasons. The least-worst I have encountered are Cleveland, and there are quite a number of clones of that system; but the discs do get hot, and as a consequence are generally prone to corrosion - so you find fancy finishes like "Black Steel" (whatever that is) that aim to improve that situation.

 

The point I'd like to discuss, is the concept of using an anti-locking valve actuated by the weight (or rather, the lack of weight) on the tailwheel of a taildragger, to reduce the number of accidents where somebody drives the propeller into the ground during engine run-up. This is VERY expensive, because it invariably requires an engine bulk strip and possibly a crankshaft replacement. The valve must not altogether prevent braking, but could perhaps reduce the pressure at the calipers by half when the tailwheel is off the ground.

 

Comments?

 

 

[SDQDI]

How would that affect tail up operations Dafydd?

 

For example when I fly in and out of paddocks I prefer the tail wheel to be off the ground as it doesn't handle the bumps like the mains do so after touchdown I balance brakes and power to slow the aircraft but keep the tail up until I'm at the edge of the paddock. I like the idea of anti lock while the tail wheel is off the ground but would still like full control.

 

 

[Old Koreelah]

Good idea, Dafydd. The technology is already highly developed in motorcycle anti-dive systems, so adapting that to an aircraft should be easy. On the other hand, your system is adding a technological fix to a handling limitation. The purists might object and prefer to hone their knife-edge braking skills. Those with common sense would admit that many tail wheel pilots have slowing reflexes, so they should welcome the assistance of a little bypass valve.

 

My own solution is to have such a weak brake system that lockup is unlikely. Bicycle disks and cable-operated callipers off a pocket bike. After a decade of use the pads show little wear.

 

 

[facthunter]

I cannot recall in the "OLDEN" days of a t/w doing that. With the tail on the ground, the CofG is further back and with full back stick there should be a sufficient down force. I would suggest that you don't apply the hand brake but rely on the feet holding them on and never take your hand off the throttle, being ready to close it, but NOT if the tail has already raised, and is still lifting. Have room to roll forward too and never apply the brakes hard at low speed. Full Power should be treated with caution. An L-188 Electra Jumped the chocks at Essendon. IF you are doing full power runs tether it to something strong or heavy like a Four wheel drive... Nev

 

 

[SDQDI]

I just reread your post Dafydd, I haven't heard of a tw doing that what model had trouble with that? I've heard of plenty of prop strikes on landing and a few on takeoff but I've never really thought of it during run up.

 

I was always taught to never use the park brake during run up and I think that is a must. (Although the reason for that was the parking valve was switched after you put pressure on the pedals and it held that pressure but locked out the pedals if you needed more pressure because you started to move you first had to release the park brake before you could apply the brakes.)

 

Most of my run ups now are done while taxiing to avoid gravel and dirt pickup.

 

 

[Dafydd Llewellyn]

How would that affect tail up operations Dafydd?For example when I fly in and out of paddocks I prefer the tail wheel to be off the ground as it doesn't handle the bumps like the mains do so after touchdown I balance brakes and power to slow the aircraft but keep the tail up until I'm at the edge of the paddock. I like the idea of anti lock while the tail wheel is off the ground but would still like full control.

Whilst the tail is up (depending how far up, of course), you normally have quite a bit of lift on the wings - so even at half-pressure, brakes that can skid the wheel under full weight, will give about as much control as you can handle without flat-spotting the tyres, with the tail up. I doubt you'd notice the difference.

 

 

[Dafydd Llewellyn]

I just reread your post Dafydd, I haven't heard of a tw doing that what model had trouble with that? I've heard of plenty of prop strikes on landing and a few on takeoff but I've never really thought of it during run up.I was always taught to never use the park brake during run up and I think that is a must. (Although the reason for that was the parking valve was switched after you put pressure on the pedals and it held that pressure but locked out the pedals if you needed more pressure because you started to move you first had to release the park brake before you could apply the brakes.)

Most of my run ups now are done while taxiing to avoid gravel and dirt pickup.

I've seen several taildraggers stood on their nose on run-up; especially with toe brakes; If the tail starts to lift, people instinctively push themselves backwards with their toes. I'm thinking about the insurance costs, here.

 

 

[turboplanner]

I cannot recall in the "OLDEN" days of a t/w doing that. With the tail on the ground, the CofG is further back and with full back stick there should be a sufficient down force. I would suggest that you don't apply the hand brake but rely on the feet holding them on and never take your hand off the throttle, being ready to close it, but NOT if the tail has already raised, and is still lifting. Have room to roll forward too and never apply the brakes hard at low speed. Full Power should be treated with caution. An L-188 Electra Jumped the chocks at Essendon. IF you are doing full power runs tether it to something strong or heavy like a Four wheel drive... Nev

https://www.youtube.com/watch?v=gMKw-5LBiAM

 

 

[turboplanner]

This is what I posted on the other thread:

 

The biggest factor in improving brake performance is the disc diameter, and that can't realistically be less than a deflated tyre - around rim size, and with small wheels that may not be enough. Just 25 mm increase in diameter (with the pads correspondingly 25 mm further out) makes a big difference.

 

The second biggest factor is the leverage from the centre of the hand point to the master cylinder.

 

When either of these is less than optimum it becomes very difficult getting a workable result, and in such a light airctraft this is likely to be the big challenge.

 

There are a number of reasons why good brakes are needed to prevent aircraft damage, for example:

 

• A shortfield forced landing into a confined area less than the aircraft's landing roll where survival is your prime objective
   
   
  
• A landing on a very short runway where you have floated longer than expected
   
   
  
• To avoid a runway incursion, or another aircraft's runway incursion
   
   
  

 

 

 

Sure you can fly an aircraft without brakes, or with just enough brakes to do your runups - older aircraft like Tiger Moths had no brakes, but they were designed to operate from all-over fields.

 

While some people have referred to motorcycle brakes, the discs are usually a lot bigger than can be fitted to a recreational aircraft with it's small wheels. There's nothing wrong with adapting motor cycle calipers, lines, master cylinder etc. but if you can't get the fundamental of the diameter optimised, the rest doesn't help that much. Motor cycle brakes are designed to operate from 300 km/hr to zero repeatedly, whereas with an aircraft it's usually one spike brake only, so heat dissipation is not so much of an issue. If you can get an aircraft brake disc red hot, you can pretty much assume the pad/caliper/pressure design is optimum.

 

 

[turboplanner]

"I watched the owner of a Storch have a lot of problems trying to get it to stop. Took a while but once convinced to try a softer brake pad braking power increased no end."

 

Brake pad compounds are usually designated by two letters which in turn refer to performance characteristics.

 

On another thread someone was talking about changing Jab overheating characteristics by fitting a bigger jet (without specifying what size - which could lead to other engine issues).

 

and the same applies with unspecified brake linings.

 

As a rule softer linings increase brake performance, but at the expense of pad wear and heat, but this is not something to test on your aircraft unless you are happy to bear the cost of aircraft repairs if the wheels lock up due to the aggressive performance. So some very sound technical advice from a brake expert is required.

 

Another byproduct of different brake compounds is harmonic squealing. Aggressive performance and loud squeal are not a problem on race cars, but I've seen hundreds of thousands of dollars spent on trying to build out squeal with special pad compounds.

 

You could be lucky, but just tread carefully, particularly if you have a taildragger. I've done wonderful things with aggressive brake linings on trucks - for a little while.

 

 

[Oscar]

Brake performance requires a mixture of characteristics. Let's confine our discussion to disc-type brakes, for the sake of both simplicity and relevance to the aircraft most of us fly.

 

As Turbs has unchallengeably identified, for disc brakes, disc diameter is paramount - a simple appreciation of the principle of leverage. But we are constrained by the small size of wheels we use and the problems of u/c leg flex.

 

Yet: racing motorcycle dry clutch packs averaging around 125mm diameter and about 40 mm depth including the spring packs, transmit 200+ horsepower and sustain gear changes at full power often several times in a very few seconds, for an hour or more of repeated use. The technology is there, with selection of the friction materials, to produce vastly better braking capability in a small footprint than most aircraft brakes. Selection of friction materials - both for disc and pad - is important; any old motorcycle rider will remember the moment of sheer terror when first applying the brakes on a Honda with the 'new' stainless discs in the wet.

 

Given that we have with aircraft wheels a small area for the brake, there are a number of other significant parameters.

 

Probably the most important of these parameters is clamping pressure of the pads to the disc. Factors that influence this are: Hydraulic advantage, brake line swelling, and caliper stiffness.

 

Hydraulic advantage is the matching of the master cylinder area and stroke to the caliper piston area and stroke. It's no use limiting the master cylinder stroke if the required stroke exceeds the lever travel - and that is partially a consequence of line swelling and caliper stiffness.

 

Lever travel needs to be greater than the movement required to overcome any pad knock-off effects, line swelling, and caliper stiffness.

 

Pad knock-off is mitigated by floating discs; I suspect very few U/L aircraft brakes feature that.

 

Line swelling is mitigated by the use of braided stainless brake lines: quite standard practice, but is it adopted by the U/L industry?

 

Caliper stiffness.. Anybody who has looked at early Jabiru calipers has, surely, said: 'you HAVE to be joking'. There are VASTLY better alternatives out there: for Kart racing, for one. Top-line motorcycle racing uses 'monoblock' calipers (that cost about the same as the GDP of a medium-size third-world country), but even half-decent calipers are available for small cost.

 

You can NOT have too much braking efficiency. You certainly CAN have too much braking APPLICATION for the circumstances, which lot of people apply in panic situations (hence the development of ABS), but it's a matter of training and ability to reduce the brake application vs. achieving the system limits and it won't do what's needed - which no amount of skill can rectify.

 

 

[kaz3g]

Picking up from the discussion on brakes in the "another Jabiru engine" thread - <snip>The point I'd like to discuss, is the concept of using an anti-locking valve actuated by the weight (or rather, the lack of weight) on the tailwheel of a taildragger, to reduce the number of accidents where somebody drives the propeller into the ground during engine run-up. This is VERY expensive, because it invariably requires an engine bulk strip and possibly a crankshaft replacement. The valve must not altogether prevent braking, but could perhaps reduce the pressure at the calipers by half when the tailwheel is off the ground.

 

Comments?

Probably more cost effective to keep the stick back in your guts when performing run-ups?

 

Kaz

 

 

[Oscar]

Pressure-limiting valves were commonplace in brake systems from around the mid-70's for the rear brakes on cars (my Alfa GTV had one, actuated by a lever from the rear axle) - but I think they only worked to limit incoming pressure, not relax existing pressure that would happen when an engine run-up lifts the tail on a taildragger.

 

 

[aro]

The point I'd like to discuss, is the concept of using an anti-locking valve actuated by the weight (or rather, the lack of weight) on the tailwheel of a taildragger, to reduce the number of accidents where somebody drives the propeller into the ground during engine run-up. This is VERY expensive, because it invariably requires an engine bulk strip and possibly a crankshaft replacement. The valve must not altogether prevent braking, but could perhaps reduce the pressure at the calipers by half when the tailwheel is off the ground.

The question is whether it is better to nose over, or release the brakes and tear off across the airfield under high power? At least nosing over is a stationary accident. Sure, you should close the throttle, but how many people will do that before they hit something vs. just standing harder on the brakes? Nosing over is not common, so presumably having the brakes release would come as a surprise - and remember the common question before an accident, "Why is it doing that?"

 

I could also imagine a situation where it makes it worse... the tailwheel lifts, the brakes release, you start moving forward and the tail comes back down, the brakes reapply, and now you have forward momentum + the rebound from the tail spring + braking all working towards a nose over.

 

If you had something like ABS that modulated an essentially constant braking force it might be OK - but you can do essentially the same thing by matching the braking power to the airframe. Are there taildraggers that can use more braking power when landing than it takes to nose over during runup? Runups with flaps down I guess is one situation...

 

As to how it might be implemented, the linked brakes on Honda motorcycles might provide some ideas. (Beware of patents, probably.) The torque from front calipers partially applies rear calipers and vice-versa.

 

Maybe a secondary master cylinder in the tail operating a second set of brake pistons. It could be operated by pressure from the primary master cylinder, but opposed by a spring that was linked to the weight on the tail. So as weight comes off, the spring works against the pressure on the secondary master cylinder. As weight is applied, it removes the spring pressure that was preventing pressure on the secondary master cylinder...

 

 

[facthunter]

ABS is good for stopping tyres blowing It doesn't modulate braking force to the extent you wouldn't nose over. A skidding wheel retards less than one that has anti skid, and it cycles and slows you up BETTER than a locked wheel.

 

That c-180 nose over was poor flying. I think people get half an idea from somewhere and try it out. Yes you will get more weight on the wheels by pushing the stick forward and you have the brakes more effective as a result. This retarding force is well under the Cof G location in the vertical sense. A consideration few people think of if they don't fly floats with high engine thrust lines, so you have a strong couple applied to rotate the aircraft over it's nose. Once you slow up and with the throttle closed, there is no airflow over the tail so it does nothing for pitch control.

 

Going over on the nose is a plane write off mostly and it could easily catch fire. Don't do it. Letting the brakes off is no big deal. IF you are taxiing with the tail up on a T/W, First of all you are a cowboy. If you don't realise that using any more brake than VERY light, you will come unstuck, expensively. Nev

 

 

[SDQDI]

The only time abs is detrimental in a car is stoping in loosish gravel but even then you will have more control. I can understand the need for better brakes overall but if you are putting another system (or part) in line it is another thing to go wrong, and if you are tipping up a tw on your run up then there are other issues there I would think although I'm lucky in that sense as the hornet won't do that sitting still.

 

Maybe the answer is for everyone to run tundras as that reduces the effectiveness of your brakes somewhat:wink:

 

 

[David Isaac]

I don't recall from any of my flying of the Citabria, Auster, C180 or C185 that you could of lift the tail while stationary at full power unless you were facing into a stiff head wind.

 

If you do what has been suggested is a problem, then surely you are either an idiot or not trained correctly, or am I being too harsh.

 

Are we attempting to create a solution for a problem that we don't have?

 

 

[Dafydd Llewellyn]

Probably more cost effective to keep the stick back in your guts when performing run-ups?Kaz

It's always worked for me, especially if you turn into wind before running up; however, I saw a Indonesian air-force Colonel stand the Seabird Seeker on its nose because he had never previously operated a tail-wheel aircraft; and I've seen a very experienced tailwheel pilot put a Bellanca Scout right over on its back, because a dust-devil formed just behind the aircraft during run-up, which negated the effect of holding back stick. It doesn't only occur at run-up, either; the Cessna 170 or whatever that flipped in the video may well not have done so had it had a device of this nature. I'm looking at something that can be added, to reduce the insurance hull rate for tail wheel aircraft. Simply teaching people to use correct run-up technique is of course essential; but it won't alter the actuarial facts that the bean-counters rely on. It needs something new to do that.

 

 

[Keenaviator]

I thought you get maximum braking in a tail dragger in 3 point - proper braking puts more load on the mains increasing traction resulting in better retardation. If the tail comes up from 3 point you are overdoing it.

 

Laurie

 

 

[Dafydd Llewellyn]

I don't recall from any of my flying of the Citabria, Auster, C180 or C185 that you could of lift the tail while stationary at full power unless you were facing into a stiff head wind.If you do what has been suggested is a problem, then surely you are either an idiot or not trained correctly, or am I being too harsh.

 

Are we attempting to create a solution for a problem that we don't have?

In general, you are right; however I could quite easily lift the tail on my Auster if I used the brakes while they are cold, when taxiing slowly - and I had to be careful using power to taxi over a bump at the edge of the tarmac, too. As soon as the brakes warmed up, they lost the power to do that. American aircraft generally have their main wheels set a little further forward than English aircraft of that era; so much so, in the case of the Pawnee, that it would "hobby-horse" if you were not careful - which tended to break the shock-absorber, if it happened too often. However, that means that on the rare occasion when they do start to lift the tail, things happen very fast.

 

 

[Dafydd Llewellyn]

I thought you get maximum braking in a tail dragger in 3 point - proper braking puts more load on the mains increasing traction resulting in better retardation. If the tail comes up from 3 point you are overdoing it.Laurie

Yes, I agree fully.

 

 

[SDQDI]

That is all good Dafydd but you are adding another mechanical system which if it failed could cause more of a problem than is already there. I do appreciate the thought of reducing tail wheel insurance rates though but on a practical side it would have to be in service and proved for quiet a few years before the insurance companies would think about lowering your rates and if it did have a malfunction in that time well it could all be in vain.

 

 

[aro]

ABS is good for stopping tyres blowing It doesn't modulate braking force to the extent you wouldn't nose over.

ABS is just sensors, a servo and a computer. With the right sensors and computer program it can do whatever you like (case in point: stability control where the computer can apply brakes individually).

 

Actually there's a point, there's enough sensors and probably enough processing power in a Dynon to not only reduce the braking if the tail started rising, but it could even apply differential brake to stop a ground loop - with the right servos and the right programming (not necessarily as easy as it sounds).

 

Are we attempting to create a solution for a problem that we don't have?

Possibly. I'm happy in my taildragger as is but I can see the value of something to address these issues from the insurance perspective. Although the insurance companies would probably say we already have a fix for that problem - the nosewheel :-). Interesting discussion though.

 

 

[Dafydd Llewellyn]

Thanks to you all for the input. All the points you raise are valid; however passive rear wheel anti-locking systems have been around in the automotive scene for at least forty years now, so I think the reliability aspect isn't really a problem. First cost isn't likely to be much of an issue either, if one can use an off-the-shelf automotive part, and there are a large variety to choose from. You all seem to accept that it could be done without any significant adverse effect on the ground handling - and that's the point that concerned me. I've no current commercial interest in the idea - feel free to use it, so far as I am concerned. I'd put it on my own tail dragger.

 

 

[David Isaac]

There are a couple of rather obvious problems with ground handling with an ABS type braking system on a tail wheel aircraft:

 

1. When you want differential braking, for example when you want to ground loop the aircraft by stomping on say the left brake to turn the aircraft, the right wheel will travel faster and the ABS will attempt to release the left wheel brake to counter your intentions. That could also be a real problem taxiing in strong crosswinds as well as landing in real strong crosswinds where at the last moment on a full stall touch down you might want to ground loop the aircraft to maintain control.

 

2. It will not stop the roll over as we saw in that Cessna 180 unless the brakes locked and from what I saw he over braked and let the control yoke move forward ( might have had the seat move forward under heavy braking). ABS can't stop that kind of cock up unless it has a G positioning sensor that releases the brakes when the CofG moves too far forward as a result of the tail rising.