Brake Chute Bug

[Random]

If you deploy the chute, then throttle up you can move backwards, obviously not right! Reheat will detach it.

[zaelu]

You sure? I used to break off my chute by putting AB on it... till I found out you need to raise a safety of the disconnect switch.

[Random]

Im talking about the aircraft reversing with high thrust (but no reheat) if you dont detach the chute

[Buzzles]

I tested this earlier.

 

Around 80% throttle the Mig will creep forward very slowly.

 

At about 95% throttle, the Mig21 does indeed start to reverse when the chute is out.

 

Yes, going to 101%+ does force it to tear off.

[Corrigan]

It should reverse.

 

EDIT: I'll expand. Assuming an ideal system (chute doesn't break or leak, etc), and that all the hot gasses from the engine hit the chute, then applying throttle will result in a net backwards force. It's basically like a (crappy) thrust reverser.

Edited October 5, 2014 by Corrigan

[Flagrum]

It should reverse.

 

EDIT: I'll expand. Assuming an ideal system (chute doesn't break or leak, etc), and that all the hot gasses from the engine hit the chute, then applying throttle will result in a net backwards force. It's basically like a (crappy) thrust reverser.

I can't believe that. That sounds like the old joke question: if you put a huge ventilator onto a sailing boat so it blows directly into the sail, will the boat go forward, driven by it's own generated wind? (Answer: ofc not, but the ventilator acts as propellor and drags the boat backwards).

 

The chute can not catch all the hot gases the engine produces and therefore the energy pushing the aircraft forward will always be greater than the force generated by the chute. At maximum, if the chute catches all the hot gases, it would be able to stop the aircraft, but never reverse its direction, as the chute never generates more drag than the engine produces thrust (system in equillibrium - external factors like wind aside, ofc).

[Corrigan]

Not to argue from authority, but I do have a degree in physics, so at least you know I'm not talking 100% out of my ass.

 

The thing to realize is that the exhaust gas doesn't stop the moment it reaches the chute, but recoils from it. That's also the reason blowing a fan into a sail actually does work, only rather poorly.

I'd also ask you to consider a thrust reverser and ask yourself what the difference is (conceptually there are none).

 

Imagine this scenario: You're standing on a wagon, holding a ball. You throw the ball forwards, off the wagon, and momentum conservation in the system means you roll backwards.

 

Now, imagine there's a wall mounted on top of the wagon, in front of you. You throw the ball against the wall, the ball stops dead and drops to the floor of the wagon. You'll impart momentum on the ball, and the ball will put it back into the wagon, and you'll have no speed after the ball has stopped.

 

Ok, now the crux: You throw the ball against the wall, and it bounces off the wall and lands slightly behind the wall. Clearly, compared to the previous case, there will result a forward net translation.

 

It's obviously much less efficient as a mode of propulsion than just throwing the ball backwards to start with (analogous to mounting the jet engine backwards in our case), but it does work.

Edited October 5, 2014 by Corrigan

[Flagrum]

My thoughts in blue:

Not to argue from authority, but I do have a degree in physics, so at least you know I'm not talking 100% out of my ass.

Allright, I am just trying to apply common sense ... ;o)

The thing to realize is that the exhaust gas doesn't stop the moment it reaches the chute, but recoils from it. That's also the reason blowing a fan into a sail actually does work, only rather poorly.

I'd also ask you to consider a thrust reverser and ask yourself what the difference is (conceptually there are none).

See longer remark beneath the quoted text.

 

Imagine this scenario: You're standing on a wagon, holding a ball. You throw the ball forwards, off the wagon, and momentum conservation in the system means you roll backwards.

*nods*

 

Now, imagine there's a wall mounted on top of the wagon, in front of you. You throw the ball against the wall, the ball stops dead and drops to the floor of the wagon. You'll impart momentum on the ball, and the ball will put it back into the wagon, and you'll have no speed after the ball has stopped.

*nods*

 

Ok, now the crux: You throw the ball against the wall, and it bounces off the wall and lands slightly behind the wall. Clearly, compared to the previous case, there will result a forward net translation.

See further below.

 

It's obviously much less efficient as a mode of propulsion than just throwing the ball backwards to start with (analogous to mounting the jet engine backwards in our case), but it does work.

 

I think, a thrust reverser works a bit differently than a drag chute. The point is whether or not the hot gases transfer their energy to the chute/thrust reverser (what is that called? the impulse?).

 

If you throw a lump of clay at the wall on your waggon, all energy is transfered back to the waggon and thus stopping it. But you can not make that lump of clay bounce off the wall - as you did in your second example. For that to happen, the design of the system must allow this, i.e. by using a ball that can bounce - but that will not transfer all its energy back to the wagon. Therefore in this case, the wagon would keep rolling backwards.

 

This is comparable to the drag chute situation: not all energy of the hot gases are transfered to the chute/aircraft and the net energy pushes the a/c still forward.

 

A thrust reverser on the other hand would have - so my imagination - a special design to reduce the amount of directly transfered energy. It does not try to stop the hot gases, but to deflect the flow of the gases so that only (relatively) minor friction happens:

Edited October 5, 2014 by Flagrum

[Tango]

Assuming the chute catches 100% of the thrust output of the engine, don't forget that the thrust coming out of the engine is trying to push the aircraft forwards (result is zero net thrust - forces cancel). As the air spilled from the chute is not all going in the opposite direction, the net thrust is in the forward direction.

 

We know the chute is not sealed, so instantly there are inefficiencies in the redirection of airflow, too.

 

Thrust reversers work because they force the airflow forwards, with practically zero leakage.

 

Best regards,

Tango.

[Corrigan]

Brake chutes and reversers are designed differently, yes, but that doesn't mean that a chute can't act as a crappy reverser.

 

When your clay sticks to the wall, that's what's called a totally inelastic collision. You spend some of the kinetic energy the clay had to reshape it, etc. Molecules, when they hit the chute, however, basically collide elastically. That's why this situation is like case 3 in my wagon example and not case 2.

 

Also, in your drawn chute, you can't just have an eternal density buildup inside that box. If gas escapes forwards (the way your nose is pointing) (we need to assume that the chute doesn't leak or break), you have that momentum we are looking for.

[cichlidfan]

I think the air turbulence created inside and in front of the chute complete negates any significant thrust reversal. Thrust reversers cleanly ( or fairly cleanly) redirect nearly all of the airflow directly back around the engine not back into the original path of airflow.

[Corrigan]

I think the air turbulence created inside and in front of the chute complete negates any significant thrust reversal. Thrust reversers cleanly ( or fairly cleanly) redirect nearly all of the airflow directly back around the engine not back into the original path of airflow.

 

That's quite possible too, I guess. The only real way of knowing is to do an experiment. All I'm really talking about is an idealized scenario (which is what a simulator needs to do as well).

 

It's interesting to note you can easily make a boat-mounted fan blowing into a sail propel the boat (I think mythbusters did this once, but I haven't seen it). I guess we want to know what happens if you replace the fan with a jet engine. Intuitively, I'd guess "the same thing".

[Fishbreath]

 

I'm with Corrigan—I feel like it's basically the same case. It's an unintuitive and seemingly impossible result, but physics is occasionally unintuitive and seemingly impossible. :P

[Flagrum]

That's quite possible too, I guess. The only real way of knowing is to do an experiment. All I'm really talking about is an idealized scenario (which is what a simulator needs to do as well).

 

It's interesting to note you can easily make a boat-mounted fan blowing into a sail propel the boat (I think mythbusters did this once, but I haven't seen it). I guess we want to know what happens if you replace the fan with a jet engine. Intuitively, I'd guess "the same thing".

The point I was trying to make is, in the case of the chute, the hot gases (that actually hit the chute) transfer all of its impulse to the chute. The air molecules do not bounce back, they have spent all their energy when colliding with the chute.

 

In case of a thrust reverser (ideally) no energy is transfered to the device, but instead just the direction of the air molecules is changed and they keep their energy. (ofc. friction happens, etc., but lets ignore that for the sake of the example).

 

The chute makes a so bad makeshift thrust reverser that I think it is safe to say, they have about nothing in common. ;o)

[Corrigan]

The point I was trying to make is, in the case of the chute, the hot gases (that actually hit the chute) transfer all of its impulse to the chute. The air molecules do not bounce back, they have spent all their energy when colliding with the chute.

 

On what basis do you say this? And why does blowing into your own sail work if this paragraph is true?

[Flagrum]

On what basis do you say this? And why does blowing into your own sail work if this paragraph is true?

Blowing into your own sail does not work. It is the same principle there: the thrust generated by the ventilator/propeller exceeds the braking force of the sail/chute as only a part of the air that is accellerated by the propeller actually hits the sail. If all air would be catched, then it would stop the propeller induced motion, but not reverse it. (imagine a closed system, a box with only an opening for the propeller, but otherwise no means for the air to escape the box. It would not move.)

 

And the gas molecules transfer all their energy because of the design of the chute. It is soft and rather tranforms exceeding energy into thermal energy than to give it back to the molecule, reversing the impulse. Similar to my clay example.

 

 

edit:

Illustration of the "closed system" example:

 

Which tube would move and which would not?

Edited October 5, 2014 by Flagrum

[zaelu]

Don't forget that this is not reality but merely o model of it... a model that just reached its limits here.

 

I couldn't move forward when I tested but maybe I blew the chute with too fast throttle or maybe catch the afterburner a bit.

 

However if the plane really moves backward is because the numbers get a bit off. You know... like in a simulation of a suspension for a car when the car bounces continuously... heck... I think this exists or existed even in DCS.

 

Regarding the hypothetical talks... if you have an engine that produces high thrust in the center of its nozzle... and say 70% gets fully reversed by an obstacle like a stubborn chute and than the leaked 30% might not hold it back enough.

 

And please... stop giving examples with Mithbusters guys... those are such fails. :D

Edited October 5, 2014 by zaelu

[Corrigan]

 

Which tube would move and which would not?

 

A obviously moves.

In B, pressure builds up inside the tube until the fan can't compress the air anymore, after which nothing happens. But this has no bearing on the scenarios we're discussing. In both the fan-in-sail scenario and the chute (basically the same thing), air can escape. A region of high pressure builds in the chute/sail, and air is displaced out of it.

[Fishbreath]

Blowing into your own sail does work—see the Mythbusters video, which is pretty solid practical evidence. (It's much less efficient than just using the blowing object as a jet, which is why it takes 95% throttle to move the MiG backward.) They even end up having directional control.

 

And the gas molecules transfer all their energy because of the design of the chute. It is soft and rather tranforms exceeding energy into thermal energy than to give it back to the molecule, reversing the impulse. Similar to my clay example.

 

This is wrong. Pick up a Kleenex or a piece of paper, then blow at it. If you adjust the angle slightly, so that you're blowing on the paper from slightly below its surface normal, you can feel the airflow hitting you in the eyes. Same effect with a parachute.

[Flagrum]

B) is an example of an "idealized brake chute", or rather one aspect of it: the part concerning the exhaust gases of the engine. The aspect of braking the whole system by also converting the energy of the relative airflow of the surrounding air into thermal energy is left out here. It illustrates that even if all hot gases are catched by the chute, no reversal of the movement can occur - at best it can only negate the thrust completely.

[Fishbreath]

B isn't a braking chute at all—it wouldn't provide any slowdown. Braking chutes aren't similar to thrust reversers, they are simple thrust reversers. Consider that the MiG, which needs more braking than the Su-25, has only one braking chute, placed directly behind the engine, while the Su-25 has two chutes—again, behind each engine.

[Flagrum]

Blowing into your own sail does work—see the Mythbusters video, which is pretty solid practical evidence. (It's much less efficient than just using the blowing object as a jet, which is why it takes 95% throttle to move the MiG backward.) They even end up having directional control.

Ah, that was meant with the Mythbusters quote earlier. Hrm, haven't seen that one ... will have to check it out.

 

 

This is wrong. Pick up a Kleenex or a piece of paper, then blow at it. If you adjust the angle slightly, so that you're blowing on the paper from slightly below its surface normal, you can feel the airflow hitting you in the eyes. Same effect with a parachute.

Hrm, yes, you are right. But the difference compared to a thrust reverser that is designed for that job, a drag chute probably uses other effects as well. For example turbulences - like cichlidfan already said. Turbulences behind and around the cute and also the "reflected" molecules (which probably ineract again with the incoming molecules as well). All in all, I am still convinced that the "thrust reversal" capabilities of a drag chute are about nil in real life.

 

B isn't a braking chute at all—it wouldn't provide any slowdown. Braking chutes aren't similar to thrust reversers, they are simple thrust reversers. Consider that the MiG, which needs more braking than the Su-25, has only one braking chute, placed directly behind the engine, while the Su-25 has two chutes—again, behind each engine.

Imho B) illustrates one part of the effects - as I said earlier.

 

Drag chutes that operate in the airflow of the engine ... are more effective than if they would operate outside the reach of the engine's exhaust. Hrm. How much difference in terms of effectivity would that probably make (genuine question)?

 

But we do agree that drag chutes are not designed and thus not capable to overcompensate the thrust generated the thrust created by the engine? I.e. reversing the direction of motion?

Edited October 5, 2014 by Flagrum

[Flagrum]

Concering "Blowing your own sail":

 

Yes, seems that here indeed some sort of "thrust reversal" happens. But that is "just" the difference between science and engineering, theory and practice ... ;o) This example is not "pure".

 

But ok, for the real life application, this effect might play a role when it comes to effectiveness of drag chutes - but the question in my previous posting remains ...

[effte]

A jet engine at idle provides a significant amount of residual thrust. That's why you want to employ reversers if you have them, even if you only idle the engines. You want to kill that residual thrust or it will hurt your runway performance.

 

Depending on the design, how much of the exhaust you reverse with the chute (yes, it will reverse part of the exhaust), the intake vs exhaust velocity, the effective velocity of the turned flow vs the velocity of the peripheral flow which is not caught by the chute... you may, or may not, be able to achieve negative net thrust.

 

 

Replace the buckets with a theoretical perfect chute with the same performance in the same location and it will still be possible. Now, if you gradually convert that theoretical chute to a real chute, replacing the metal with mesh and moving it backwards, the efficiency will gradually reduce. At one point, you will no longer be able to achieve negative net thrust. But where that point lies is far from obvious. If you did the same exercise on a MiG-21, would that point be before you had degraded the bucket-analogue chute to the actual chute or not?

[Corrigan]

Flagrum, you seem to have an issue (and that's fine, I'm not insulting you) with the exhaust molecules rebounding or not off the chute. Let's say they stay very close to the chute: after a while, you'll have a region of high pressure, and a pressure gradient. Then, even if the chute is "sticky" like you think, you will have to have a flow of fluid out of the chute.

 

I don't know very much about materials science (a few courses in solid state physics, but I'm a high-energy theorist), but I'm sure the gas molecules will not stick just because the chute is fabric/soft/bendy/whatever. That's a special scenario, which requires special circumstances. The general case is that the gas molecule will rebound in some direction with some fraction of its kinetic energy. I see no reason why that should be 0, and the fan-into-sail experiment (done by laymen, sure) seems to support that.

 

Also, more on topic, I have no idea if this is modelled in DCS or if this behaviour actually is unintended (I suspect the latter since turbulence isn't modelled); all I'm saying is that it isn't as unphysical as the first few posters in this thread thought.