Hi. The engine pushes and the fan pulls through the air then you reverse the flow,wouldn’t that be an equaling effect? or less power for the reverse due to the turning and not full directional control? I know it works somehow else it wouldn’t be used, but it is not the same as reversing the blades on a propeller.
virtually yours,
Virtually Yours.
Reverse thrust does not reverse the engine, it reverses the direction of the exhaust as it exits the back of the engine.
A jet engine doesn’t work by pulling air in through the fan and pushing it out through the back. The jet engine pulls in air then combines the oxygen in that air with fuel to burn it, producing a high pressure gas which is then expelled through the back of the engine.
Thrust reversers simply redirect this expelled gas so that is pushed out in the direction of the front of the engine. So the engine will still be sucking in air, but the pressure created from the burning fuel is much greater than the pressure of the air being sucked in and the overall result is a large thrust in the reverse direction.
Um, yeah, it mostly does. Most of the thrust from a modern high-bypass turbofan engine does come from the fan, not the core. An airliner typically doesn’t have a reverser for the core anyway; it isn’t worth the cost/weight/trouble. On older planes, with low bypass (narrower) engines, yes, the entire exhaust is turned.
The reversed thrust isn’t turned through a full 180 degrees, btw. That, combined with other efficiency losses, limit max reverse thrust to something less than half of max forward thrust.
An even more puzzling related case is the pop-pop boat.
Here is the analogy that works for me: Suppose I am sitting on a frictionless disk on the middle of a hockey rink. It is right after the puck-toss promotion, so there are hockey pucks scattered all over the rink such that I can always reach one.
If I grab a puck and set it on my lap, I will move slightly in that direction so that the center of mass of where I was, and where the puck was, and my new position remain constant…If I keep doing this, I will move only slowly, and accumulate pucks on my lap. To keep moving, I have to keep grabbing pucks, even though there is no friction.
Since accumulating pucks is messy, I instead throw each puck as fast as I can in the direction opposite where I want to go, and then instead of just one step in position, I now acquire a velocity such that myself and the puck retain net-zero momentum. This is jet propulsion using hockey pucks instead of air. I keep moving even if I stop hurling pucks.
Now let it happen that I can’t reach a hockey puck from in front of me. So instead I grab one from behind me. This causes a slight loss of speed…but I can get that back, plus a lot more when I throw the puck.
So it turns out, it doesn’t matter much where I grab the pucks from, as long as I produce a stream of them, I will accelerate in the direction opposite the puck stream.
If you replace the pucks with packets of air or water, you can see how this applies to the airplane or pop-pop boat: Still air or water is grabbed by the vehicle and used to create a directed stream.
(constant or pulsating, it doesn’t matter)
Richard Feynman liked to use the idea of a lawn sprinkler submerged in a swimming pool to illustrate these principals. The action is very different depending on whether water flows to of from the lawn sprinkler.
You folks are missing my point. the thrust happens immediately after the blade and the burner cans. not an inch later or foot .. the bullet has left the casing..the thrust doesn’t happen two or three feet down the line. right where that blades bite the air and and the combustion it’s on the way forward.. when those fans bite the air the thrust you feel at the back is byproduct of that, the blade is on the way forward dragging the plane with it and the combustion is pushing the plane forward so to take that after the fact and then turn it around and send it forward should be self defeating but it works somehow.. hope I am making it clear..
virtually yours,
Virtually Yours
Thanks for the response but i fail to see where i said the engine itself is reversed.. the jet engine wouldn’t work at all
virtually yours,
Virtually Yours
The jet takes the air and sends it backwards, generating thrust. If you just had a “thrust stopper”, that stopped the jet exhaust, it would produce the same thrust, but in the opposite direction, cancelling the thrust from the jet, and it would be “self defeating”.
But a thrust reverser doesn’t just stop the engine exhaust, it redirects it forward, so it actually generates twice the thrust of the jet engine, more than canceling it out.
The physical metal that redirects the air flow is attached to the engine and the plane. The fan blade and the ignition cause low-pressure air to go to high pressure air. This is shot out of the back to make the plane go forward.
But when the reversers are turned on, the high pressure stream of air essentially hits a part of the plane itself. The air goes further to the front of the plane, sure, but as you point out, that’s beside the point. The part that slows down the plane is the fact that the high-pressure stream is striking the plane, slowing it down like a parachute.
The pop-pop works like a valveless pulse jet, which also has intake and exhaust from the same end of the jet. In all these cases it amounts to the same thing, there is greater force exterted on the object from the exhaust than the intake over time. The pulse jet and pop-pop just do it with a series of alternating intake and exhaust pulses.
To every action there is always an equal and opposite reaction. Issac Newton.
Also the reversed exhaust flows is such a way as to disturb the aerodynamics of the aircraft.
I’m not an ‘expert’; however I worked on DC-9s for a few years with Pratt & Whitney JT8D engines that had thrust reversers.
The parachute analogy is crucial. It makes them less ‘thrust reversers’ but more ‘drag inducers’. If an airliner were on a runway at a standstill and it engaged its thrust reversers and applied full power, it would start to move backwards but it wouldn’t be able to gain too much speed because of the huge inefficiency & waste of energy of the thrust’s redirection.
All bets are off if it’s on a treadmill… 
The statement above is wrong. Forward thrust is produced when the compressor and burner accelerate the air that is burned, and when the fan accelerates the bypass air. Reverse thrust happens when the hot air is used to spin the turbine, but this is less that the forward thrust. Finally, a large reverse thrust is generated when the buckets behind the engine take the jet stream from the engine and turn it to the forward direction.
But a far simpler way to look at is this:
Q: What was the air doing before the engine acted on it? That is to say before the airplane got there.
A: The air was just sitting there.
Q:What is the air doing after the engine acts on it?
A:A High velocity stream of it is blasting along in the same direction that the airplane is moving. It might be moving faster or slower than the airplane, it matters not.
Creating a high velocity stream of air moving in the same direction requires that there was a reaction force in the opposite direction. That force slows the airplane.
It is interesting that almost nobody has a problem seeing how reversing the pitch of a propeller (beta range) slows an airplane, but use buckets to turn around the stream coming out of a jet and their brains fall out. In both cases a wash of air moving the same direction as the airplane is created, and conservation of momentum requires that this slows the airplane down.
Some see it sooner if they picture an engine with a U-shaped tailpipe instead of the reversing buckets. The mass flowing into the intake and the mass flowing out the tailpipe are nearly equal (mass flow of fuel is also going out the exhaust). The forward thrust is going to be proportional to MVin. The reverse thrust is going to be proportional to MVout. Since the engine heats the air, Vout is going to be much higher than Vin, so you get a net reverse thrust. Because the thrust due to the intake is acting against the exhaust, it is less efficient than a straight engine, but is still works.
Unlike a propeller, compressor/fan blades do not accelerate air, but increase its pressure instead. The thrust “happens” at the nozzle when the pressure of the jet stream drops down to the atmospheric pressure.
the blades of the bypass act as a propeller and provide 80% of the thrust.. I am saying that the air you feel at the back is the result of the thrust. If the aircraft could move forward as fast as the air was being displaced you would feel nothing at the back, the aircraft would use all the energy produced and rocket forward. if you had a mile long pipe at the back of the engine, are you saying the aircraft would only begin to move when you start to feel the air out of the back? once again think of the bullet in the gun, the acceleration happens at the time of combustion not when the bullet exits the barrel. suppose you had a mile long barrel, are you saying you wouldnt feel the kick on the gun until the bullet exits the end??
vy
VY
Not quite as a propeller, since they are ducted and there is a nozzle behind them, but close enough…
And the engine would be producing zero thrust. From pilot’s POV, air enters the engine at one speed and exists at a higher speed. The difference in speeds multiplied by the mass flow produces the thrust. From a stationary POV, air is just sitting there before an aircraft passes through. Afterwards, air is moving in the opposite direction as the aircraft (as a result of the engine’s thrust).
Technically yes. Jet engine is a reaction engine. Thrust is produced when you change speed of some mass. As air enters a jet engine, it slows down a little, producing drag. As air exits through a nozzle, it accelerates and produces thrust. The rest of the engine produces a little of both, but the net effect is quite small.
OK, I’ll bite. Gunpowder in a cartridge burns and turns into high pressure gas. The gas acts on the base of the cartridge, producing part of the recoil. At the same time, the gas acts on the bullet, accelerating it down the barrel. A mile later, bullet exits the barrel and you are left with high pressure gas that is suddenly exposed to atmospheric pressure at the muzzle. The gas accelerates out of the muzzle until pressures equalize, producing the second part of recoil.
there is no nozzle behind the bypass..i am talking about a turbofan. unless I am not getting what you are saying
I think I figured it out, the initial thrust is counteracted when the air hits the reverse bucket i.e. if the acceleration was forward at 5 mph per sec per sec the deceleration would be basically -5 mph per sec per sec. hence the forward motion would be cancelled by the backward motion of the air hitting the buckets.. then after hitting the buckets it is directed forward as best it can hence producing thrust alone in the opposite direction of the aircraft movement..
vy VY
On a basic level, it doesn’t matter what happens to the gas stream inside the engine, all that matters is the direction and velocity of the airstream leaving the engine/aeroplane system. Example: Thrust vectoring on the Harrier Jump Jet.
I have backed up a jet airplane weighing 300,000 pounds using only reverse thrust. The old-style bucket reversers.
I have also backed up a turbo-prop airplane weighing 70,000 pounds by only reversing the props and increasing power.
Anyone fly on American Airlines years ago? At DFW we used to back the MD-80s out of the gates using reverse thrust.
I’m not going to get into the semantics of this discussion, but I will just say that I have over 20 years of experience of reverse thrust working, in all of its forms.
1962, Grey Army airfield, Ft. Lewis WA.
C-124 backed a little too far, we saw smoke from the right inboard. #3?
Some very unhappy brass came around. he he
Some poor mechanic they brought in was carrying 56 long reach, platinum tipped , mica insulated, spark plugs out to the plane, tripped on a tie down ring and dumped them all. They had to be replaced. More gnashing of teeth.
Sometimes big airplanes suck. he he he