Idle Wheel Train Slipping Rate (anti skid)?

[mirkoni]

Just going through some jaa questions about landing gear and this one caught my attention...
It says that modern anti skid uses computer whose input data are:

- idle wheel speed
- braked wheel speed
- desired idle wheel train slipping rate.

Could somebody clarify what are these terms. I can understand idle wheel speed as the speed of the wheel that has no brakes applied, but what is meant by desired idle wheel train slipping rate?

[Dick W]

Have you a reference for this question?

Without any specialist knowledge I would guess as follows:

When the idle wheel has the same speed as the braked wheel there is no braking. As you apply the brakes the braked wheel begins to slip on the runway surface and slows down. Brake more, and the slip - the difference between the braked and idle wheels increases. Brake too hard and the slip goes off the clock as the braked wheel locks.

Figure out just how much slip gives you max braking before lockup and play the brake pressure to keep that value. On a slippery runway you will get the required slip at low brake pressure

It's the slip that wears the tyres out. If there was no slip in normal braking the tyres would last for ever

Any help?

Dick

[mirkoni]

Hello Dick,

Yeah it's question No 4495 in JAA TEST PREP from Aviationexam.com, issue year 2006.

No textbook defines "wheel slipping rate", not to mention "wheel TRAIN slipping rate".

Here is how I understand the process after your answer. Let's say we analyze airplane during the landing run:

1) If the wheels are not braked at all, they all follow kinematics, that is, the speed of the airplane (and wheel radius of course) directly determine their angular speed and deceleration. If we would use only spoilers and thrust reversers we would have pure kinematic deceleration of the wheels and hence no slipping (of course we presume that runway surface conditions are ideal). In this case, all wheels are on their "idle" speed all the time, in the sense that they are not being braked externally by the brakes, they just "naturally" decelerate together with the airplane. Idling all the time.

2) But, if the brakes ARE applied during landing roll, that means that at first moment NONE of the wheels is IDLE any more! Now they are not just decelerating with the airplane, they are having an additional deceleration from brakes being applied on them. That means that there is some small physical slipping at the contact area of the tire and the runway surface. That's causing the tyre wear you mention, right?

3)Now, DASU monitors the situation and has those braked wheels speed input. Now let's say for simplicity that one of the wheel for some reason gains such a high angular deceleration which would soon lead to blockage (zero angular speed) of that wheel. DASU prevents that by commanding servo on that wheel to release the brakes on that particular wheel. From that moment on, that particular wheel is IDLING, right? It is not slipping any more, it is again naturally "following" the airplane.

The term "desired idle wheel train slipping rate" is a bit contradictory then, because the idle wheel can not slip? Maybe it can "train slip", whatever that means. Or am I missing something?

Do you agree?

[Dick W]

Yes, except that I think when too much angular deceleration is detected the system will reduce brake pressure rather than fully release it, to keep things smooth

Dick