Examination of stopping distances. A/S hurts.

[effte]

That generates a rough calculation of the drag produced by the airbrakes.

 

The air brakes reduce ground roll distance by about 700 feet at 50,000 lbm gross weight, and by about 300 feet at 25,000 lbm gross.

 

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".

 

At 50,000 lbm and sea-level standard conditions, the A-10 has a brake energy limit speed of 140 knots with the speed brakes closed. In other words, it's really only a concern when hot and high in this aircraft. Not that you'll become too popular with your crew chief if you come to a stop with glowing brakes and 1000 meters of runway to spare with any regularity...

 

So assuming the wings are producing zero lift (a little unlikely though, given the 20deg of flap)

 

Confirmed for the simulated aircraft. At ground attitude with 20 degrees of flap, level flight can be achieved (178 KIAS @ 25,850 lbm gross). In other words, at 130 KIAS the wings would still be generating around 15,400 lbf of lift, without adjusting for ground effect. Once you get on the brakes, the nose lowers a bit though so much of that lift should be dumped.

 

It should not be forgotten that the flaps generate a fair bit of drag as well. In fact, the same amount as 40% speed brakes. The lift also diminishes rapidly as speed decays (9,100 lbf @ 100 KIAS).

 

While the theory is correct, in reality you are unlikely to be able to get much of a benefit from flap retraction on the roll-out, and you are likely to get yourself in trouble if you start messing with the flaps when you should be focusing on getting the aircraft going straight down the runway and getting on the brakes. Retracting the flaps on the landing roll isn't a standard procedure on many aircraft for good reasons. Slight benefit if done just right, large screw-up potential - leave'em and sort them out once you are off the runway, as per the book. Home-made procedures are generally not a good idea.

 

 

Yo-yo,

can't see where you contradict anything I was saying, but we seem to agree that the A/S needs to be checked, so it's all good. That's my only point, so as long as we reach the same conclusion there's no need to argue the details. :thumbup:

 

Once we have the A/S working, I suspect we'll see by-the book performance as you seem to have reasonable coefficients. We'll revisit the subject then!

 

Where do we find the timestamped TAS? In the track recordings?

 

 

Viper,

I'd have an issue with braking performance below the book numbers even on a 20,000 meter runway. The issue isn't running out of runway, it's performance. ;)

 

It also tends to become an issue in severe crosswinds, which have a nasty habit of consuming runway at an alarming rate.

 

 

Finally, I'd like to add that the book numbers are on the assumption of rolling without braking for three seconds after touchdown, in order to let the aircraft settle down for optimum braking. They also assume that you do not go to full speed brakes prior to three seconds after touch down. To compare our results to the book figures, we need to keep this in mind.

 

Checking my landing roll in TacView, I achieved a landing roll of 2670 feet. Due to TacView limitations, that's from derotation to full stop, so add a couple of hundred feet for the distance from touchdown. It's also without the prescribed three free-rolling seconds, and with the airbrakes at 100% before touching down. All in all, it's likely to end well above 3000 feet, while the book says 1600. More late nights in the office, Yo-Yo. Sorry! ;)

 

Finally, I have an additional gripe! This simulator is making performance checking a lot harder than it is in many other simulators. I find myself unable to focus on the task at hand, as I sit there staring at this beautiful aircraft moving around on my computer screen, with flight dynamics which really make it come alive! :P

[159th_Viper]

Checking my landing roll in TacView, I achieved a landing roll of 2670 feet. Due to TacView limitations, that's from derotation to full stop, so add a couple of hundred feet for the distance from touchdown. It's also without the prescribed three free-rolling seconds, and with the airbrakes at 100% before touching down. All in all, it's likely to end well above 3000 feet, while the book says 1600. More late nights in the office, Yo-Yo. Sorry! ;)

 

Feet or Metres?

 

Edit: Nm, got it - brain-hiccup :)

Edited January 24, 2011 by 159th_Viper

[BiPod]

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 never thought to test this as I just assumed the extra drag would be the dominant factor, but you are right (at least for beta 4).

 

Light weight (56%), Full brakes, FULL FLAPS, Airbrakes retracted: 46, 47, 47

Light weight (56%), Full brakes, NO FLAPS, Airbrakes retracted: 45,45

 

Heavy weight (87%), Full brakes, FULL FLAPS, Airbrakes retracted: 35, 35, 34, 35

Heavy weight (87%), Full brakes, NO FLAPS , Airbrakes retracted: 30 ,31

 

So retracting flaps does reduce stopping distance. Though as Effte points out, this may not be worth the extra pilot workload. Interestingly (Read: this goes against my hypothesis) the effect was only minimal at low weight. Perhaps a tipping point was crossed at high weight that was not crossed at low weight? Or perhaps I am just plain wrong.

 

Taken together, this just strengthens the consensus which I beleive Effte, Yo-Yo and myself are reaching. A/S and skidding factors may need tweaking.

 

@mvsgas

As per PheonixBvo's answer.

 

You raised a good point about mentioning the A/P. All tests were done at Gudauta with a quick test at Kobuleti to confirm the results. Different (beta) surface factors will totally change these results. I should have mentioned this.

[Yo-Yo]

Yo-yo,

 

Where do we find the timestamped TAS? In the track recordings?

 

 

 

Yes, you can export it via lua script. I think there are many guys here who could help ypu to use this option. As far as I can remember, you can even export Nx value directly.

 

If you do not want to use it you can use model timer and XZ position in the status bar in F2 view.

[Sickdog]

anyone notice how quickly the AI A-10's stop on the runway after landing? It appears to be half the distance as when I try to stop the A-10 with maximum braking ability.

[159th_Viper]

anyone notice how quickly the AI A-10's stop on the runway after landing? It appears to be half the distance as when I try to stop the A-10 with maximum braking ability.

 

AI planes are not modelled to the same fidelity as the player-controlled craft.

[Sickdog]

AI planes are not modelled to the same fidelity as the player-controlled craft.

 

Ahh, makes sense... Thanks! Just making sure that my longer landing distance is normal, and I'm not doing something wrong.

[Jack McCoy]

Static and kinetic friction coefficients

 

Sorry for the necro-post, but I feel I'm getting poor braking performance with the latest patch.

 

ED Team, do you use the static or kinetic friction coefficient? I know it should be the STATIC one for this particular situation. The static coefficient is always higher than the kinetic one, sometime by a factor of 2. With anti-skid, the maximum brake torque should be [a little less than the static coeff] x [mass on wheel] x [tire radius], or else it would be commanding to make the wheel skid. If the wheel starts skidding, the A/S needs to drop braking to below the kinetic coefficient so the wheel stops skidding.

 

Raising the flaps is not recommended on the runway under normal operations, but not because of braking performance. I was told that the pilot risks moving the wrong ancilary control (maybe landing gear) and blocking the runway. Of course, strong winds and other factors might call for raising flaps early.

 

As a side note, in the 172 POH, Cessna recommends to retract the flaps when applying emergency braking after an engine failure and landing on the (short) remainder of the runway.

Edited September 29, 2011 by Jack McCoy
Spelling

[Garfieldo]

Excuse the OT post, but shouldn't the brakes be capable of holding the aircraft in place when you power up for takeoff? When I go to max power, the aircraft starts rolling slowly, even with full brakes.

 

Cars are designed so the brakes are stronger than the motor for obvious reasons, I would have thought the same goes for aircraft (maybe excluding the SR-71:D).

[GripenNG]

Excuse the OT post, but shouldn't the brakes be capable of holding the aircraft in place when you power up for takeoff? When I go to max power, the aircraft starts rolling slowly, even with full brakes.

 

Cars are designed so the brakes are stronger than the motor for obvious reasons, I would have thought the same goes for aircraft (maybe excluding the SR-71:D).

 

Well there are few brakes that are stronger than a max-powered jet engine, instead you should apply 75% FAN speed, and when releasing the brakes, go full power! :)

[Jack McCoy]

Excuse the OT post, but shouldn't the brakes be capable of holding the aircraft in place when you power up for takeoff? When I go to max power, the aircraft starts rolling slowly, even with full brakes.

 

No. With Anti-Skid, the brakes will unlock by themselves. Without A/S, when the engines supply more thrust than the static friction force, the wheels would start skidding and screeching...

 

Cars are designed so the brakes are stronger than the motor...

 

Not true. The brakes are designed according to the car's weight and performance expectations.

[gear_monkey]

No. With Anti-Skid, the brakes will unlock by themselves. Without A/S, when the engines supply more thrust than the static friction force, the wheels would start skidding and screeching...

 

 

 

Not true. The brakes are designed according to the car's weight and performance expectations.

 

anti-skid systems on aircraft are typically deactivated below about 15-20 kts depending on aircraft. So to say that the system would allow wheel spin below this speed is not true.

 

I am a pilot in the civilian commercial world but I have F-16 friends who tell me that above 80% power, with the brakes set, the aircraft will move forward due to the rubber tires turning around the locked rims.

[Jack McCoy]

anti-skid systems on aircraft are typically deactivated below about 15-20 kts depending on aircraft. So to say that the system would allow wheel spin below this speed is not true.

 

I just said that given an anti-skid system that's doing its job (not disabled under the typical behavior you mention), it would release braking to prevent skidding. It was to put in contrast that without the anti-skid, brakes locked, and near-full power, things would happen as you described below. In the end, this being a Warthog forum, my statement could be misleading. But who's to say the Sim implements the typical behavior you describe? Is this in the Flight Manual? I only read 500 of the 671 pages! :megalol:

 

I am a pilot in the civilian commercial world but I have F-16 friends who tell me that above 80% power, with the brakes set, the aircraft will move forward due to the rubber tires turning around the locked rims.

 

I agree and believe that. Thanks for the input.

[BigBlue]

Sorry for the necro-post, but I feel I'm getting poor braking performance with the latest patch.

 

ED Team, do you use the static or kinetic friction coefficient? I know it should be the STATIC one for this particular situation. The static coefficient is always higher than the kinetic one, sometime by a factor of 2. With anti-skid, the maximum brake torque should be [a little less than the static coeff] x [mass on wheel] x [tire radius], or else it would be commanding to make the wheel skid. If the wheel starts skidding, the A/S needs to drop braking to below the kinetic coefficient so the wheel stops skidding.

 

Raising the flaps is not recommended on the runway under normal operations, but not because of braking performance. I was told that the pilot risks moving the wrong ancilary control (maybe landing gear) and blocking the runway. Of course, strong winds and other factors might call for raising flaps early.

 

As a side note, in the 172 POH, Cessna recommends to retract the flaps when applying emergency braking after an engine failure and landing on the (short) remainder of the runway.

 

 

That's what i understand as well for the light Piper AC. High performance landing procedure for those, including raising flaps on the roll-out to place more weight on the mains. Perhaps because the Piper's big clumsy flap handle is hard to miss, they figure that method can be included in the "normal Procedures" section.

Edited September 30, 2011 by BigBlue
another thought........

[doright]

Many aircraft anti-skid system aren't just lock then dump systems, they are optimized to detect impending lock-up and then regulate pressure to hold as much continuous braking force as possible. Of course that is for modern aircraft, the 40 year old A10's system I can't speak to.