How does jet propulsion work?

I can push off from a wall on a skateboard and will be propelled foward by the reacting force. But if I push against the air, no such force will occur. How does it happen that say a leaf blower or a strong fan will propel me forward while only pushing against air? Does not the air move away when the jet stream pushes against it, providing no reactive force? Does the displaced air have sufficient inertia to provide the reactive force that propels you forward?

 

Log in or Sign up to hide all adverts.

Because a jet engines pushes vast amounts of air at high speed.
And the reaction takes place inside the engine itself: most of the propulsive force comes from the combustion chamber and the reaction on the rear of the fan blades:

Please Register or Log in to view the hidden image!

 

Log in or Sign up to hide all adverts.

So for a jet engine, the forward propulsion is derived BEFORE the air leaves the engine? What is all that air inside the engine pushing against?

 

Log in or Sign up to hide all adverts.

If you stand on your skateboard and throw a ball, you will move in the opposite direction. This is nothing to do with pushing on the air, it is the reactive force from the ball acting on you. You push on the ball to accelerate it and it pushes back on you, accelerating you in the opposite direction. The jet engine accelerates air and hot gas rearwards and the reaction is to propel it forwards.

The French understand this: in French, a jet plane is called "un avion à réaction".

 

H

He's just told you: the blades of the fan.

 

So it has nothing to do with the force of that air reacting against an opposing mass? It's more like it's ejecting the mass of that air and thus creating less weight that translates into a forward propulsion?

 

I'm asking for clarification because the blade assembly is attached to the engine itself.

 

The fan is pushing the air backwards. There is an equal and opposite reaction pushing on the fan blades, and thus the shaft, the engine and the airplane.

It's just the pushing on the mass of air. The air pushes back.

 

Nothing to do with "less weight".
It's action/ reaction.
Simply ejecting the air produces the reaction.

If the blades weren't attached to the engine then they couldn't transfer that thrust to the engine (which is, in turn, attached to the aircraft).

 

So why must the air blow on fan blades? Why can't the air that comes out the engine simply blow out the rear, exerting a forward force? It seems the propulsion force would only be expended in pushing the blades around.

 

Something has to push on the air. That something is the blades. In a simpler case (say a SCUBA tank opened all the way) then the air is pushing against all parts of the container equally - except for the hole where the air is escaping. That unbalances the forces and produces a net forward force. (Same way rockets work.)

 

But a fan isn't creating more force is it? The output force of the fan must equal the input force, right? Or isn't it actually a little less due to the mass of the fan wheel?

 

You can't effectively "push" air with your hands. Only a small amount of air will be moved, and therefore the reaction is too small to overcome even the friction in the wheel bearings.

Because they are effective in moving a lot more air, and moving it quickly. The reactive force is proportional to the mass of air moved, and to the square (power of 2) of the velocity of the moving air.

Subject to the reaction I mentioned above. Air moving against air causes the pressure to rise, so there is a reaction -- a force -- in the opposite direction.

If it has sufficient mass and velocity per the rule I mentioned. But it (the effective sail) also needs to have adequate surface area.

 

Compressed air is entering the front of the combustion chamber. Slowing that air flow mass which relative to the air plane down a great deal (to the speed of combusting gases), is a "negative thrust." Some of the combustion pressure probably does help slow the entering flow down as to some extent the combustion gas pressure, not the gas, does "back up" to push on the compressor blades, but this is relatively unimportant. The forward force on the compressor blades come more as the reaction force of them throwing the now much denser air towards the rear.

Much more important than all this is the huge pressure of the combustion products gas expanding towards the rear, but it is slowed by energy extracted to spin the blades on the same shaft as the compressor blades are attached to. Few realize that the energy captured by these "aft blades" is several (perhaps five) times more than the energy left in the exhaust gas! I.e. the jet engine uses most of the fuel energy to compress the entering gas, but it is not 'lost" - just recirculating internally before leaving in the exhaust gas. An insignificant part of the thrust, especially at high altitude, is the exhaust gas "pushing on the air" as many believe. If a jet had its own oxidizer supply, it would work better - make more thrust - if there were no air to push against, but then we call it a "rocket."

These "aft blades" let the combustion gasses, now with less energy pass thru them and that is what most think the jet's thrust comes from, but as just discussed all of the net thus is reaction forces on various surfaces that do throw the exhaust out the back of the engine. Hope that goes a long way towards answering your question.

 

Yes it does. So the fan is actually lowering the forward thrust? That's interesting. A related question..When the plane lands, where does it get all that force to "brake"? Or is the braking all in the wheels? Tks for all the answers so far..

 

"More" force? It's creating as much force forward as it is spending by pushing the air back.

Yes, the forward/back forces equal (forward on the fan, backward on the air.) There is some additional rotational torque that is lost to drag. In airplanes this torque is countered by the rudder. (It is a large torque on propeller aircraft, small on jets.)

The "thing in the front of the engine" (fan + compressor) is generating forward thrust. The combustors are also generating forward thrust. The turbine (thing in the back) is using some of that force to drive the fan+compressor, so that lowers the forward thrust. It is, however, necessary for the engine to work.

Three places. One, additional drag from speedbrakes on the wings. Two, engine reverse thrust. Three, brakes on the wheels. Small aircraft often only have wheel brakes.

 

Thank Gawd for backup brakes.

How is the engine reverse thrusting? I thought it was just basically a hot compressor ejecting masses of air?

 

A few ways. One is a set of clamshell or bucket doors that close over the back of the engine and direct the thrust forward. A second way is called a "cold stream reverser" and are basically air vents around the edge of the engine that allow most of the air to escape forward. They are hideously inefficient, but since they are only used for a few seconds a flight (and the engines have power to spare) that doesn't matter too much.

 

So that's that unnerving roaring noise we hear on touch down. Ok..I think that about does it. Unless more comes up later. Tks!

 

Landing is via aerodynamic braking, thrust reversers, spoilers, and mechanical braking...depending on the airplane.

Edit: I see Billvon types faster than I do

Please Register or Log in to view the hidden image!

In a small airplane aerodynamic braking does much of the braking before the actual mechanical brakes are used...meaning holding the nose up for as long as possible.