Naked Science Forum

Non Life Sciences => Technology => Topic started by: Peter A. Blacksberg on 04/04/2011 18:30:03

Title: Do propellers work the same way as wings?
Post by: Peter A. Blacksberg on 04/04/2011 18:30:03

Peter A. Blacksberg  asked the Naked Scientists:
   
Hello again,

So airplanes depend on downward thrust?  Okayyy that differs from the "bernoulli lower pressure on top of the wing" I had learned.  I buy it because it stands to reason that even if large aircraft wing had zero pressure on the upper surface one would presume the lower surface would only have the ambient pressure for lift.   

And thusly are we to understand the same explanation should be used for propellers? Aircraft and marine?

Thanks.
Peter a Blacksberg

What do you think?

Title: Do propellers work the same way as wings?
Post by: JMLCarter on 06/04/2011 23:33:30

Airplanes depend on downward deflection for low speed and high lift situations. During take off some of the thrust (from engine not wing) is also directed downward.
Also note that downward deflection can be understood as creating a high pressure area under the wing. But during flat and level flight the wing can rely solely on it's shape for a pressure differential and it can give enough lift.

Atmospheric pressure is about 30KPa at 30000ft, giving a lift of up to about 3000Kg/m² (and the relevant area is the area the wing moves each second, not just the area of the wing). This is key to cruising at altitude. The downward deflection approach creates a lot of drag making it very inefficient so it needs to be used minimally.

There are many demonstrations of the power of air pressure at ground level, most famously http://en.wikipedia.org/wiki/Magdeburg_hemispheres

Anyway back to the main question...

Helicopter rotor blades are a kind of half way house between wings and propellers, and there is a massive down-draft from them, I don't think there's much doubt that they make some use of the downward deflection method. However whether it is the main contributor to lift is another question. At those speeds the whole area swept through by the wing may be considered available to provide a pressure differential for lift at, say, v. roughly, the 3000Kg/m² mentioned earlier.

Propellers:
"Propellers are similar in aerofoil section to a low drag wing and as such are poor in operation when at other than their optimum angle of attack." - see http://en.wikipedia.org/wiki/Propeller_(aircraft)#Forces_acting_on_a_propeller

Title: Do propellers work the same way as wings?
Post by: Geezer on 07/04/2011 00:30:00

Quote from: JMLCarter on 06/04/2011 23:33:30

 and there is a massive down-draft from them, I don't think there's much doubt that they make some use of the downward deflection method.

There is much doubt in my mind that they do. The thrust is produced in the direction of the axis of rotation. In horizontal flight, that axis is horizontal too, so any downthrust produced is going to be a function of the attitude of the aircraft (unless we are talking about something like an Osprey of course).