Hacker News Reader

Comment by exabrial | Hacker News Reader

Sort of off topic, but reverse thrust sort of boggles my mind. If you're sucking air in from the front of the aircraft and ejecting it out the front, don't those forces counterbalance each other?

View on HN

To elaborate on seiferteric: If the engine were just an unducted fan, the input and output volumes and velocities would be the same, and if they're pointed in opposite directions, there would be zero net force.

In a jet engine though, you have a nozzle on the back, so the input area is greater than the output area. Heating the air also causes it to take up much more volume. These put together, mean that the output velocity is much greater than the input velocity, so there is a momentum transfer from the atmosphere to the plane.

"Sucking" doesn't actually generate any thrust - In highschool, my physics teacher did a demo with a T-shaped pipe with a fan in the middle, suspended from some string.

Without the T, the pipe moved as you'd expect. With the T directing the airflow in equal proportions perpendicular to the axis of the pipe, the pipe stayed still as if the fan wasn't on at all.

physics says no. There was something wrong with that demo if this was the conclusion.

Feynman says yes, except at very low speeds. https://en.wikipedia.org/wiki/Feynman_sprinkler

that's very interesting! but it's about angular momentum and I'm not confident that I can take the lesson over to linear momentum.

Here's my caveman f=ma thought experiment:

1. make it 2-d.

2. replace the fan with a person sitting on a chair on a frictionless surface.

3. instead of air it's an endless field cinder blocks ahead of him.

the person reaches out, and pulls in a cinder block. f=ma says they each move toward the other while the center of mass of the combination of them does not move.

Now, if he throws the cinderblock behind him, he moves further forward - this would be analogous to an airplane propeller. or a fan in an open pipe.

If he, um, splits the cinderblock in two and places each half directly off to his sides there is no net force exerted on him by this. This is the fan in a T-shaped pipe.

the fan+pipe grabs air from ahead, moves this mass backward and then sets it aside. it's not a jet-engine, but it is moving the air mass toward itself and must be moved equally and oppositely.

I don't think it's essential to worry about how the air/blocks rearrange themselves after this - but if the blocks surround and jostle, that's just another effect layered in super-position over this one, and if we don't agree so far then it will only make things more confusing

You have to look for the equal and opposite reaction. A normal jet creates a stream of fast air behind it. There's no such thing for a reverse jet that sucks air in.

With your concrete block example, as I start pulling the block towards me I experience an impulse forward. But when it approaches my body I slow it down to zero speed, creating an opposite impulse. So although I might have moved forward a few inches during the motion, my momentum is zero at the end. When you scale this up to large numbers of air molecules, the result is the same.

thanks for following up. I think this is one of those conversations that works better in front of a white board - it's surprisingly tricky to express in words.

if the demo was mainly to show that it's the jet of air expelled out the back that's providing thrust, then, fine it does that and its a valuable lesson. I guess I'm hung up on the technicality that there is actually a real movement of air mass even without that rearward jet and that has to be felt by the apparatus - I guess it's just unnoticeably small in the real world demo.

anyhow, my confidence in physics intuition has been shaken. thanks bunches.

Nah the demo was right. The impulse from the sucking (ie air molecules bouncing off the inside of the fan blade) is countered by the impulse from the air molecules bouncing off the inside off the back T-pipe. Different at a low Reynolds number with a reversible flow.

Reynolds number, compressibility, reversibility don't really matter for this - the principle under discussion is a simple momentum-balance. Find the force on the pipe by analyzing what is happening at the boundaries, you can ignore what happens inside. (in engineering this is called control-volume analysis).

The exiting flows out the sides neatly cancel. So what's happening from front-to-back? There is air flowing in at some velocity x cross-sectional area x air density. this momentum has to be balanced completely for the pipe to stay still - but there is no source of momentum in the other direction so the pipe will feel this force and move.

if the pipe was open at the back, there would be momentum exiting the pipe balancing the incoming momentum - or even over-balancing (as in a jet engine)

here's a video going through the math on a similar problem which is a little more complex in detail, but the same in principle: https://www.youtube.com/watch?v=hXApWf1r0Eo

Reverse thrust doesn't come directly out the front of the engine - it's vented out the side (or, in very old engines, redirected from the back exhaust):

https://upload.wikimedia.org/wikipedia/commons/d/df/F-GTAR_A...

That makes it even more surprisingly counterintuitive.

I think the important thing to consider is the relative velocities of the intake and exhaust air (the latter being much larger).

that is a misleading way to put it - the reverse-thrust air does indeed move toward the front of the plane. it also moves outward, and is often taken from the sides of the engine, but a component of the (reversed) thrust is definitely going opposite the usual direction.

here is a thermal image of a modern jet with thrust reverses on : http://www.rusaviainsider.com/russias-mc-21-tested-for-rever...

Your forgetting that the cool dense intake air is expanded (greatly) in the engine via combustion with fuel, so the volume of air being exhausted is much greater than went in and at much higher pressure, this is where the thrust comes from in the first place. It's not just a big fan :)

Modern turbofans are basically ducted fans spun by a smaller turbojet engine, so they are basically huge fans!

But is it the hot air out of the engine being reversed or the air from the big fan? I thought it was the latter.

It's mostly bypass air.

Think of the net momentum balance. Some mass of air is entering the intake at low speed, the engine does work on the air, then the air exits the front at very high speed. The intake speed is approximately the airplane's speed (150 mph), while the exhaust speed is probably 500 mph.

If the suck force and the blow force were equal, you wouldn't need to burn fuel inside the engine during normal operation. Combustion produces a lot higher volume of gasses at higher energy than the air that goes in the front.

I initially thought this as well, but it turns out that the reverse thrust is actually made of bypass air in most engines

No - because it's going faster when it exits the engine than when it enters.

The net effect is moving a mass of air one way, and generating an opposite force on the aircraft.

Just draw arrows for how the air is moving the whole time.

The force on the plane is in the opposite direction.

Reply on news.ycombinator.com