Why does reverse thrust work?

How did you do this without blasting out all the windows in the terminal?!

It was only certain gates. Also the MD-80 is a relatively light airplane, with the engines waaaay in the back. You use the minimum power required to breakaway, and then reverse idle the rest of the way, with maybe one more addition of power if you had to make a turn.

Are you saying that the initial explosion of the fuel doesn’t push the aircraft forward?it has to wait for the force to exit the pipe for the craft to move? It is the explosion or combustion in the engine that pushes on all points in the aircraft (engine) and takes the easiest routes out which because of the design of the engine is forward by the wheels, and-more easier- out back(more out back than front).. imagine a grenade thrown into a fortified engine, the force would be in all directions; the force tries to go up and down but can’t because it is contained and counteracted by the opposite sides, it pushes against the front also and moves the aircraft because the wheels allow the aircraft to overcome some friction, (if the brakes were on , all energy would go out the back,) it also goes out the back because of the opening, otherwise it would be just a contained controlled explosion/combustion counteracting itself.. the forward motion is a result of the energy acting on the front of the engine trying to go that way . what you feel out of the back is unused wasted energy going the path of least resistance. Imagine you are in the middle of two walls, one 100 lbs and one 10 lbs.. left hand on one wall and right hand on other, you push with equal force, the 10 lb wall will move faster that the 100 lb . that is how the aircraft moves.the 10 lb wall is the exhaust (path of least resistance) and the 100lb is the aircraft, much more resistance. We want to go the direction of the 100 lb wall.
hope that is correct and clear
vy VY

Uh, no. This is no more true than the idea that you can lift yourself into the air by pulling up on your own head. The fact that the airflow is redirected in the forward direction is critical.

The airflow is redirected to the front of the plane by metal ducts/buckets/whatever. As it is redirected, the air exerts a force on the metal components, partially in the backward direction, that slows the plane.

If the airflow were redirected 90 degrees to either side, the force exerted would not have any component in the backward direction, and no effect on the plane’s speed.

I can’t believe you don’t understand this. There are forces acting on an airplane in all sorts of directions all the time. It goes in the direction where the forces are greater than any counteracting forces. When you walk forward there is a backward force on you from the resistance of the air, but you still manage to move forward. That’s because you generate more force in the forward direction than the backward force on you. When a jet reverses thrust it changes the direction of the exhaust, in the simplest form so that the exhaust goes forward putting a backward force on the plane. That exceeds any forces pushing it forward that are produced anywhere in the engine.

Yeah, I am aware of all this…how is this different from what I said?

I’m not saying that the airflow has to be redirected totally 180 degrees - just that there must be some forward component to the redirected airflow in order for there to be a backward component to the force vector and decelerate the plane. I was responding to Chessic Sense’s post, in which he said that the forward redirection of the air was “beside the point” and that thrust reverser acted more as a parachute. This implies (erroneously) that it is the simple force of the engine thrust incident on the reverser itself that decelerates the plane, not the fact that the reverser redirects the airflow to the forward direction. I was trying to correct that misconception. His post implies that you could deploy a totally flat thrust reverser, that simply redirects the air 90 degrees to each side without any forward component, and still get deceleration, which is obviously incorrect.

I might be misreading his post, or may have miswritten mine…I think I might be getting the effing flu…but I’m not seeing what you’re objecting to here…

I have to apologize, I had intertwined my reading of your post and the one before it into something that didn’t make sense. I’m not sure what yours is saying, I have to go look back at Chessic’s to get the context, but it wasn’t what I thought I was responding to.

exactly! its the force pushing on/against the metal that causes the reaction. otherwise it would all just be hot air, pardon the pun
vy VY

you two are saying the same thing.. but the plane is going backward using some of the force that acts directly on the metal and the waste is going the other way… both correct
vyVy

The air being directed as you say is actually air being bounced off the bucket, when it hits the bucket some energy is used to stop the plane-trying to push it backwards- and the excess that can’t be used because of the weight or mass of the plane bounces forward, a bit less that then original force some of which was absorbed by the buckets, which slows the plane
vy VY

Of course the bucket has to have the correct curvature, a flat board would just cancel itself out.
vy VY

You are half right. If you contained the explosion in all directions then there is the same pressure in all directions and the aeroplane goes no where. If you put an opening at the back then there is high pressure on the sides and front but low pressure at the back so the engine and aeroplane move forwards. But if you redirect the rearward exhaust before it leaves the aeroplane/engine system then the reaction is no longer forwards. By redirection the thrust downward it is effectively the same as having the exhaust itself pointing downward. If this wasn’t the case then aircraft like the Harrier Jump Jet wouldn’t work and neither would thrust reversers. As I say, it doesn’t really matter what happens to the gas stream within the engine, it is the final exhaust vector as the gas stream leaves the aeroplane/engine system that provides the thrust vector in the opposite direction.

Have a look at this PW100 series turbo prop engine. The combustion chambers are actually facing forwards so the initial gas expansion is toward the front of the engine but the gas flow is then redirected rearward where it drives the power turbine. The exhaust for these engines is ultimately directed to the rear of the aircraft and 10% of total thrust from the engine is from the exhaust, even though the combustion chambers are facing backwards.

So once again, it doesn’t matter what the gas stream does within the engine, it is the direction and velocity that it leaves the aeroplane/engine system that counts and thrust reversers act to change that velocity vector.

No, the thrust that you feel is what is bounced off the metal when it hits the metal inside the engine or duct whatever that is the reaction. it couldn’t push the plane all the way because of the weight of the plane. so i.e ten percent was used to push the plane 10 feet and the rest got the hell outta there, so to speak doing no work. so if 100 lbs of thrust was produced 10 lbs went to move the aircraft and 90% is waste product. that is why a plane accelerates. as it goes faster up to speed you will find less pounds coming out the back and more going forward which is of course used to push the plane..technically , if the expansion of the combustion was 100 mph and the plane was moving forward at 100 mph you would feel nothing out the back, the plane would be 100% efficient. which because of mass and friction resistance would not happen. same for the harrier even though the vector of the thrust seems to be down, actually it is up, the air hits the skyward part of the can and tries to force it up so the plane goes up, what is not used is bounced away and out through the nozzle, as waste. Ifyou had another turbine not attached to the aircraft you could use that wasted thrust to power it..
vy VY

So initially the aircraft is being forced backwards which is then countered almost immediately by the force hitting the opposite side, as if it is one chamber. force on equal side counteracting each other. when a side doesn’t have an opposite side counteracting it, this side is free to move, so the open side which is called exhaust is there so there is no opposite reaction to the force being applied to a certain side of the chamber.. they use/harness some of this wasted force trying to follow the path of least resistance to turn a turbine and hence the propeller providing thrust, but the action on the can before it hits the turbines is still a force vector.. let’s use the dam turbine system for an example, you are basically saying that the water rushing out after the turbines is the thrust? if the water hit the turbines at 100mph pounds thrust, it would not come out of the back at the same speed or thrust. something will give, either speed; pounds thrust; or both. The energy has already been used, what is out the left is force that it couldn’t translate to energy. some diminishing returns law?
vyVY

You are not disagreeing with me as far as I can tell. Exhaust rearward means forward thrust. If you use something to change the exhaust vector such as thrust reverses or thrust vectoring then the exhaust vector is changed and so is the thrust vector. It doesn’t matter what twists and turns the gasses make within the engine/aeroplane, the thrust vector is ultimately defined by the exhaust vector, ie it is the opposite of the direction of gasses leaving the aeroplane.

What you feel coming out the engine is not thrust, that is what was not absorbed by the forward movement of the plane. if you put your hand through the exhaust and rest it on the metal.. you will feel your hand being crushed, that is the thrust, what goes the other way is unused energy, that couldn’t crush you hand anymore which results in the aircraft going forward
vyVY

In some ways, from the various videos that show reverse thrusters in action, it looks sort of like someone is sticking their legs out and driving their heels into the ground to keep the plane from going too fast. Except that their feet are made out of the exhaust that is being directed forward - onto the tarmac and into the air in front of the engines. It looks as if someone is being dragged forward and is digging in his heels to keep from moving too fast. That energy comes from the reverse thrusters, I guess. Isn’t that the point?

You still seem to be operating under some misconception regarding “wasted thrust”, thrust “absorbed by the forward motion of the plane”, “excess that can’t be used because of the weight or mass of the plane”, etc., but I am having a hard time discerning exactly what that misconception is.

There is no such thing as “excess that can’t be used because of the weight or mass of the plane”. There is inefficiency in this process, but the inefficiency is the result of frictional and turbulence losses in the airstream, as well as the fact that the redirected thrust vector does not point exactly forwards.

Pretend the molecules of air are little ball bearings.

A ball bearing enters the engine traveling at the speed of the aircraft, say 200 mph. It gets whacked by one of the spinning fan blades. Now the ball bearing is going faster (say 500 mph). But the fan blade has experienced the same force in the opposite direction. This force is passed from the blade to the axle to the engine housing to the rest of the plane to accelerate the plane forward.

Now say the ball bearing flies out of the back of the engine and ricochets off a thrust reverser. Its velocity goes from 500 mph in one direction to traveling say 100 mph in the *opposite *direction. Notice that the total change in velocity is actually greater. Going from 200 to 500 is a change of 300mph. But going from 500 to -100 is a change of 600 mph. A bigger change in velocity means a bigger force. The force involved in the collision with the thrust reverser is greater than the force involved in the collision with the spinning fan blade.

The thrust reverser experiences a force just like the fan did. That force is passed through its hinges to the rest of the plane and accelerates the plane backwards. But its bigger than the force from the spinning fan.

So for every ball bearing that passes through the system the plane experiences two opposing forces. The force from the spinning fan hitting the ball bearing pushes the plane forward, while the force from the ball bearing hitting the thrust reverser pushes the plane backwards. However, the force from the thrust reverser is larger, so the net effect is to slow the plane down.

If changing the visual appearance of the engine, would help clear things up, you could put one of these on the back of the engine, and it would reverse the thrust quite nicely.

If you’re concerned about the exhaust being ingested by the engine’s intake, you could add more ducting to the engine’s outlet so that the hot, high-speed exhaust is piped sideways a good 30 feet before turning to exit a forward-facing nozzle. You’ll achieve the same thrust-reversing effect, although the line of action of the reversed thrust will be wherever the exhaust stream is ultimately relocated to (in this example, 30 feet sideways from where the original engine is).

The Harrier aircraft was mentioned upthread. It’s able to take off like a conventional aircraft by directing its exhaust (and therefore its thrust) rearward, but it can also take off vertically by directing its exhaust (and therefore its thrust) downward. All of this is irrespective of the fact that the engine is oriented horizintally and sucks in air from the front, along a horizontal axis. this video demonstrates a Harrier vertical takeoff, and clearly shows the high-speed soot-laden exhaust stream being directed downward. Imagine a system that directs this exhaust stream forward instead of down, and that’s what a thrust reverser does.

the only important thing to understand is what direction is the high-speed exhaust stream traveling after its final interaction with the plane and anything attached to it (like thrust-reverser louvers/ducts/nozzles). When you know what direction that is, then the thrust experienced by the aircraft is in the opposite direction.