Examination of stopping distances. A/S hurts.

EDIT: As this topic has just been bumped, I will stress that this was done with Beta 4. I beleive that the surface friction model was tweaked for the final release so the results below may now be redundant.

There have been many posts reguarding braking and stopping distances. Though many suggestions have been made all seem to be based on anecdotal evidence and texts. I have now tested over 50 landings and present the following empirical results. (Only the conclusion needs to be read, the rest is just detail).

Conclusion:

1) Anti-skid GREATLY increases stopping distance under all conditions tested.

2) Best stopping distance is A/S off, 3/4 wheelbrakes, Full airbrakes.

3) Anti-skid greatly aids handling especially at speeds < 50 kts, but 3/4 wheelbrakes with no A/S is just as good.

4) Stopping times were LONGER with less weight. (Same as an underweight car).

5) Pumping the brakes made stopping distances much longer.

6) suggested landing configuration A/S on for safety, 3/4 wheelbrakes, full airbrakes. Turn off A/S if runway overrun is looming (long landing).

THE FOLLOWING IS JUST DETAIL DON'T BOTHER READING UNLESS INTERESTED.

Method:

A track was created with overspeed landing at idle. Tests were conducted by taking control of the track at 130-140kts and applying brakes (slider assigned) at 120kts. Results were ALWAYS reproduceable within 1 second (OK sometimes 2), due to this, many measurements were performed only once.

nb. My SaitekX52 timer was 20% slow (though it keeps good time) so it was not used.

Results: (Beta 4)

Notes:

-Stopping time was measured not distance (too hard), but by eye it seemed a fairly accurate substitute for distance.

-Using 3/4 brakes was almost always better than full brakes and even more important when A/S was on.

-3/4 brakes (slider estimate by my eye) appears to be about 60% according to Ctrl-Enter Graphic.

-Hydrolics were never seriously affected and remained near full pressure.

-No tires were popped except under deliberate oversteer.

-% Effect of turning A/S off was greater with higher grossweight and airbrakes extended.

-With A/S off, airbrakes only had a minor effect! (by percentage)

-Airbrakes may spoil lift and increase down pressure on ground thereby reducing skidding.

-Wet weather may alter results and crosswinds may make A/S more important for control (not tested).

It appears that the A/S is tuned for stability and safety rather than stopping power. Skidding was more effective than maintaining traction under these conditions.

Good work :thumbup:

Brilliant post. Especially for a first post. :thumbup:

I think we can conclude that the A/S is the culprit, as suspected. Thank you!

4) Stopping times were LONGER with less weight. (Same as an underweight car).

Interesting.

-Airbrakes may spoil lift and increase down pressure on ground thereby reducing skidding.

Trailing-edge symmetrical airbrakes should have a relatively small effect on lift. They will disturb the airflow over the outer segment of the wing, but in ground attitude the wing is likely to be near its zero-lift angle of attack. There's probably a degree of washout in the wing, meaning the outer part will then be generating negative lift, if any. That's guesswork though, as I don't know the specifics of the design, but based on sound knowledge. Either way, the impact of the airbrakes on overall lift should be negligible.

It appears that the A/S is tuned for stability and safety rather than stopping power. Skidding was more effective than maintaining traction under these conditions.

Hoping for B5, 6 or 7 to correct this. :)

Cheers,

Fred

b5 6 or 7 how many is suppose to be

b5 6 or 7 how many is suppose to be

B5 is supposed to be the last beta before release iirc.

Ofc everything is subject to change and can do so at any time ;)

b5 6 or 7 how many is suppose to be

That's WIP... ;)

(I have no idea - as many as it takes, I'd assume, but luza may be right. Can't say I really care, I'm happier flying ED betas than flying the final releases of most other companies! :) )

b5 6 or 7 how many is suppose to be

as many as it takes.

I don't know that there is a 'culprit.' I have never had a problem stopping with ample space, even with A/S on. I hold off in the flare for as long as possible, land with air brakes out, and keep 5-10 degrees nose up as long as possible, get on the brakes as soon as the nose comes down.

I don't know that there is a 'culprit.' I have never had a problem stopping with ample space, even with A/S on. I hold off in the flare for as long as possible, land with air brakes out, and keep 5-10 degrees nose up as long as possible, get on the brakes as soon as the nose comes down.

And if one engine caught fire every time you turned left, you could fly on one engine. It still shouldn't happen, even if the consequences can be managed. Aircraft have powerful brakes, for good reason. The simulated A-10 doesn't. This seems to be due to the A/S operation. Fits my definition of culprit.

Allow me to quote the dash 1:

"If available runway does not require maximum braking, speed can be reduced by aerodynamic braking or by extending speed brakes."

If you want an aircraft to stop in a short distance, you plant it hard, get all tyres on the ground and hit the brakes. That's more or less universal across types. AD braking is a way of saving on the equipment. It is not what you do if you want short field performance.

...Aircraft have powerful brakes, for good reason. The simulated A-10 doesn't. This seems to be due to the A/S operation...

And this is based on what?

It's not good enough to state that the systems modelling is incomplete/erroneous. You have to be prepared to back it up with charts/documentation etc etc. Affecting change where necessary is not going to happen without substantiation ;)

Do not misunderstand me - I'd be the first to hammer down doors to ensure that relevant systems are modelled as close to the real thing as possible. In the present circumstances however I'd have to bet on the Devs vs unsubstantiated opinion :)

And this is based on what?

Sound engineering practise. You severely limit the usability of an aircraft by degrading the brakes, without gaining much in either weight or complexity. Poor brakes really hurt your required landing distances and final usability of the aircraft. Check the dash 1 landing ground roll charts to see what happens as braking ability diminishes due to poor runway friction coefficients, and you'll see why you'll not want to cut down on the brakes. :)

Getting on the brakes, as per the dash 1 normal procedures and with anti-skid enabled, should give the shortest landing ground roll. Right now, as BiPod has documented, this is not true. Something is amiss.

Once we do get the shortest landing distances by utilizing the A/S to the max, we can start checking the landing distances versus real data and find out whether the problem is insufficient brake efficiency or exaggerated drag coefficients on the ground.

GW 37,000 lbm, flaps 20, speedbrakes 100%, SLS conditions. On the AoA indexer, TD speed just shy of 130 KIAS.

That gave a landing ground roll of approximately 29 seconds. TD speed of 130 knots equals 67 m/s. If we assume constant deceleration, we get 67/29 m/s^2, or 2.3 m/s^2. That's 0.2G.

The reason they want you to stay strapped in until the aircraft comes to a halt when flying commercially is that they don't want you to end up breaking any bones should they hit the brakes for some reason. Sounds like a bit more than 0.2G, doesn't it? You will go flying inside the cabin if it happens - trust me. That's in an airliner, not a tactical aircraft. Hard braking will have you hanging from the shoulder straps in most aircraft. It could be that they designed the A-10 to different criteria, as they figured it would never have to operate off short runways... but somehow, I doubt it.

Let us move on though. Again, with the assumption of constant deceleration, we can calculate the landing ground roll distance. 67*29/2 meters gives us 971 meters, or just short of 3,200 feet.

Now, let's hit the performance supplement. The charts (A-10A -1, figure A8-3) give an expected ground roll of around 1,600 feet under those same conditions. Again assuming constant deceleration, you'd get 4.32 m/s, or .44G. That's still less than I'd have expected, but then again, there's just one tyre to each side. While brakes give you good value for each added pound of mass, adding tyres isn't attractive to a designer so that may be the limitation of the design.

This is all a bit rough, and please double check my calculations. They are back-of-an-envelope - quite literally. :)

Cheers,

/Fred

To clarify, I have not stated that the modelling is wrong but do note that many posters including myself have felt that braking is sometimes lacking. As my pilots log indicates, I have had vey little time in the real C model (zero hours:D , lots of zeros in that log!). The tests were done to find the best stopping practices for accidental long landings.

The results indicate that braking power is not the limiting factor, but traction seems to be the problem. Hence the empty aircraft took longer to stop, 3/4 was better than full brakes, and slamming the brakes makes matters worse (extremely bad if you are using a push button for the brake). Cycling the brakes made things worse also.

I agree with effte, it seems very odd that A/S exacerbates rather than reduces problems with skidding, but I'm not qualified to state that it is wrong.

Cheers,

BiPod.

What the heck happens when an F18 hits the 3 wire at 130+ knots and comes to a stop in less than 3 seconds then?

What the heck happens when an F18 hits the 3 wire at 130+ knots and comes to a stop in less than 3 seconds then?

Under the same assumption, around 2.3G longitudinally is what happens. It's a crash which doesn't break the aircraft... ;)

BTW, I have around 370 times BiPod's hours in the C model. A whopping 0! I saw one once... :pilotfly:

Cheers,

Fred

Great post, nothing worse than coming home from a successfull mission and running off the end of the runway (done it a few times lol).

Braking force can not exceed mgF, where F is the coefficient for tire-surface friction.

Thats ignoring aerodynamic effects. Which at 130knts will be quite large. Fly straight and level at 130knts, apply full brake and then nose down to maintain speed. Then fly 130knots on idle throttle - what nose down angle do you need? Compare.. That generates a rough calculation of the drag produced by the airbrakes.

Even really crappy brakes can lock a wheel - and the A-10 has anti-skid, so, pretty much by definition, the A-10 brakes can lock a wheel.

Good brakes can absorb the kinetic energy of the aircraft and convert that into heat, and can do that for a long time. Crappy brakes can only do that for a short amount of time - so you "run out of brakes".

In general, for a given mass sitting on rubber on tarmac = ~1g static friction (again, ignoring aerodynamic effects including lift).

So assuming the wings are producing zero lift (a little unlikely though, given the 20deg of flap) you can do 1g.... however, as stated, some lift = less downforce on tyres = less stopping force. Dynamic friction (a locked tyre sliding over tarmac) results in about 0.7g.

From this, assuming the wings are producing some lift, at the beginning of your landing roll you'll have not much downforce on the wheels. Therefor you'll produce proportionally less wheel braking (and it will require *much* less braking force in the cockpit to lock wheels).

At you roll out (and slow), more weight comes onto the wheels, and more wheel braking can be generated, and it requires more braking force in the cockpit to lock a wheel.

Antiskid would help hide the variable braking force required to lock wheels during the landing roll.

So the question for the devs - does A-10 model the force of the wheels on the tarmac, including lift generated by the wings, and does it use that in braking calculations for the aircraft?

damn now my head is hurting :)

In order to increase weight on wheels quickly, raise your flaps as soon as you're rolling. Particularly at higher speed, this should make a big difference in the max achievable braking force.

I assume you guys are having problems stopping the aircraft ( I'm master of the obvious). Are you all using pedals? I normally land or cross the runway threshold at or about 120 knots indicated. I rarely use the brakes on landing roll. How much fuel/ammo/weapons do you guys have on landing? What runway are you guys testing on? Does anyone have a track we can watch?

The purpose is primarily for long landings, i.e. when you (for whatever reason) used up half the runway before touchdown. I agree that under normal conditions the problem is mostly non-existent.

The only operational runways that there might conceivably be a problem is Novo or Gelendzhik. That said, even with those two you should have no issues (1800m).