Propellers And Thrust

[Mr Garrison]

Hi All,

Just reading through some theory notes and have come across a statement that I'm not sure about regarding propellers.

Contrary to popular belief, the thrust that moves an aeroplane forward is NOT produced by the blast of air going backwards from the propeller but rather the lift force produced by the propeller blade.

So is it true that all propeller driven aircraft are pulled through the air by the propeller or pushed through the air via the propeller.

Interested to hear people's thoughts


Thanks.......


Mr.G

[Ex FSO GRIFFO]

Props & Thrust / Lift?

G'day Mr "G",

I see by your profile you list CPL(H) as one of your accomplishments.

Tell me, how does your helo rotor get you off the ground?

Same thing, only in a different direction, perhaps?

Another Mr 'G'.

p.s. There is a VERY good book on the Principles Of Flight by A.C.Kermode, I highly recommend it.

[FFP]

If I remember rightly, think the answer is (c) . .. .

[smith]

The normal propeller "sucks" you through the air unless of course its a pusher prop located at the rear of the aircraft.

[High Wing Drifter]

I'm sure the ATPL's grossly oversimplify this problem.

Surely a wing creates lift through the combined effects of Bernoulli's transit time principle and Newtons 3rd law. Otherwise asymetrical wings would not fly upside down, lift would not be possible at low or even negative angles of attack and you wouldn't feel backdraft from a static prop.

Therefore, the prop both pulls and pushes its way through the air.

For the ATPL exams, I agree with FFP, definately answer C, either that or the longest.

[cadaha]

from what I gather the propeller (simplified) uses the same principles as the wing. As it rotates it is presented to the airflow at a certain angle of attack and the airflow over the top (front of the prop) is faster creating a lower static pressure therefore creating lift (thrust) so in escence sucking the aircraft forward. As long as the force is not less than drag then you will move forward. If the "lift" (thrust) created by the propeller is greater than the drag you will accelerate until the lift (thrust) is equal to the drag and then you will maintain speed.

Am I on the right Lines??

[Send Clowns]

The distinction is false. They are differnet parts of the same physical process. You cannot poduce lift in a prop without directing a stream of airflow backwards, and cannot produce the airflow without producing lift.

[Dick Whittingham]

I left this thread to log in, and when I got back I found that Send Clowns had already given the answer. However, here's my tuppence worth.

There can be no lift on an airfoil, or no thrust on a prop blade, unless there has been downward (backward) momentum added to the airflow. So no downwash - no lift.

This downwash can be induced by a straight change of direction of the airflow, as in a flat plate with positive angle of attack. It can also be induced by pressure and speed changes over cambered wings. As Clowns says, two parts of the same effect.

Dick W

[cadaha]

OK getting confused now.

Doesn't the prop generate its own airspeed (as such) by spinning?

If so then isn't it creating an airflow across the surface and therefore create its own horizontal lift called thrust because the static pressure in front of the prop is less than behind it gets (for want of a better word) sucked along the direction of travel. ???

[smith]

Although I stated the prop "sucks" you forward, there is an element of push as well, just as in a normal aerofoil (ie aircraft wing). The air above the wing is accelerated at a greater rate than the air below. Hence the majority of the lift comes from the upper surface.

As with the prop therefore the majority of the thrust comes from the front of the blade.

[High Wing Drifter]

As with the prop therefore the majority of the thrust comes from the front of the blade.

May I refer you to my answer above. If that is the case, how can an asymetrical wing generate lift when upside down? I suspect the majority of the lift comes from Newtons 3rd law, a reaction to the wings surface pushing the air down due to its angle of attack.

However. That still is unsatisfactory. Newton or Bernoulli, at the end of the day you have a lower pressure on top and a higher pressure underneath, the same result for two entirely different and yet combined reasons. I would venture to state that you cannot say pull/suck or push. You can only say that the aerofoil will want to move away from the high pressure.