# How is thrust generated by a propeller? [duplicate]

I get confused sometimes because I hear different theories of how propellers generate thrust.

• One is that the propeller pushes air back to provide thrust.
• Another is the blades generate lift creating a high pressure behind the blade pushing the aircraft forward.

Is it just one of these true or is it both? For you guys its the same way of generating thrust, but to me its two ways thats my confusion. And why does a propeller need to generate lift if it just accelerates air backwards to provide thrust. That question is my main confusion.

• I assume by "under the blade" you mean behind. In this case air is not washed forward, but backward, and Federico's answer is better understood. Move is created by pushing air opposite to the move direction, to move air (by deflection) you need the blade lift effect. – mins Sep 14 '15 at 12:34
• Read discussions of lift production on wings. The arguments are exactly the same if you replace the words "back" to "down" and "forward" to "up". – casey Sep 14 '15 at 13:30
• Why did you ask a new question when you already have a very good answer here - aviation.stackexchange.com/questions/19582/… – Simon Sep 14 '15 at 16:43
• Those two statements seem to say the exact same thing to me. – Octopus Sep 14 '15 at 20:22
• You already accepted an answer on your first question, with effectively the same title. Edit or comment your first question if there is something you don't understand about it. Generating lift is equivalent to accelerating air; Newton's laws of motion. – fooot Sep 14 '15 at 21:49

It's the same thing, it changes only the point of view, see Newton's third law: for each action there is an equal and opposite reaction, i.e. to create thrust, the engine has to push air backwards. Pushing the air backwards means that the air is pushing you forward and this, in the reference system of the propeller blade, is lift.

Both the methods you are describing are the same:

• The propeller pushes the air behind if you are viewing from the aircraft (or propeller).
• The propeller generates lift by creating a high pressure if you are in a frame of reference out of the aircraft.

Both are equivalent and can be used to describe what is going on.

• So both produce thrust. – Ethan Sep 14 '15 at 18:34
• @Ethan: Yes, in the sense that a car and an automobile are both vehicles. They're literally the same thing, just described in different words. – Ilmari Karonen Sep 14 '15 at 19:20
• So wait so I thought was generated by the higher pressure under the blade pushes the aircraft forward .I am confused. – Ethan Sep 14 '15 at 21:13
• @Ethan read Newtons 3rd law. – casey Sep 15 '15 at 4:37
• @ethan. Lift is generated by creating higher pressure under the blade and lower pressure above. You do that by pushing air behind the blade :-) they're the same thing but described in different ways (action and result, cause and effect) – Jon Story Sep 15 '15 at 15:47

If you imagine one of the propeller blades as a wing it can help you understand. It spins around, moving air over both sides of its blade. It is producing thrust the SAME way a wing produces lift. Airflow over the top surface, which is in front of the airplane, causes lower pressure. That negative pressure sucks the airplane forward the same way negative pressure over a wing has the effect of lifting the plane.

Thinking of it this way also helps with understanding P-factor.

• negative pressure over a wing has the effect of lifting the plane: True, but that's not the main source of the lift. – mins Sep 14 '15 at 20:44
• What do you mean by sucks the airplane. How I think it is the blade generates lift and the high pressure behind the blade pushes the aircraft forward. Am I correct because I am really confused. – Ethan Sep 14 '15 at 21:26
• Ethan, that is the same thing. If high pressure is below and low pressure is above, the plane moves from high to low going "up". Whether you prefer to say you are pushed up or pulled up is the same thing. – Charlie Sep 14 '15 at 21:33

When I think of fluids and pressure gradients, I go back to Newton's first and second laws. A fluid can only accelerate in the presence of a pressure gradient, and if there is a pressure gradient then there must be acceleration (ignoring gravity for the moment). So your propeller accelerating air past it only occurs because the propeller induces a pressure gradient in the air (high pressure behind the propeller, low pressure in front), or the pressure gradient occurs because the propeller accelerates the air. They are essentially one and the same fenomenon, either side of the equation F=MA.