Downwash....does it really explain how airplanes fly?

Question

When I learned about how airplanes fly I understood that the difference in pressures is what creates lift....greater pressure under the wing vs. less pressure over, thus creating lift. It's a simple way of understanding flight until, of course, I learned of a new theory. I'm reading a book called Understanding Flight and in the book it relates newton's laws with lift. It states, essentially, that it's the downwash that creates the lift. The air "pushes" down and since newton states "for every action there is an equal and opposite reaction", there must be lift. What do you guys think. I can sort of see it in one sense but in another I think the 1st theory I have understood makes more sense in explaining lift.

However, the theory I understand and like doesn't explain symmetrical wings, which leads me to believe that there is a better explanation.

thanks

Answer 1

I'm a fan of the Level 3 explanation on Principles of flight on this site. Top menu bar.

See if it makes it easier to understand.

Answer 2

In aviation, 'downwash' usually refers to microbursts from thunderstorms pushing the plane down, which can be quite hazardous at low altitudes. I have never heard of it in that sense before...

Answer 3

Downwash doesn't lift an airplane any more than a wake propels a boat.  Both are just the result of applied forces.  An airplane must have lift to fly and lift happens AT the wing....not AFTER the wing.  You understood it right the first time.  Either you missed something in the book's explanation or the author was unclear in his explanation.  

With symmetrical wings, angle of attack effectively adds to the camber of the top of the wing.  When you look at it that way, the theory you understand does work.

Answer 4

"Downwash" is not a widely accepted explanation for fixed-wing airplanes.  Helicopters produce massive downwash.

The differential pressure/Bernoulli Effect explanation is the generally accepted explanation for the generation of lift by fixed-wing airplanes.

Answer 5

If you want to start a really heated discussion at a flight school, ask the bernoulli vs newton question.

Best answer is that they are both essential parts of efficient flight.

As most B52 pilots will explain, you can fly anything if you put enough engines on it - so I believe the "efficiency" part of the equation is important.

Answer 6

The answer is actually a combination of both explanations you have read. Newton's Law and the Bernoulli Principle are both working to make a wing fly.  As to the degree to which each contribute something to lift, the Bernoulli Principle of differential pressure is by far the more important of the two, but down-wash plays a considerable part in drag and pitching moment of the wing as well as imparting a measurable "equal but opposite reaction" vector in the direction of lift.

By the way, the symmetrical airfoil creates lift because it still has to meet the air at some positive angle of attack, just like an asymmetrical airfoil. In other words, the wing is not neutral to the airflow, it must be inclined so the air going over the top has to travel a greater distance than the air flowing underneath. The difference in velocity creates the difference in pressure (Bernoulli) and the positive angle of attack also creates a downwash (Newton), just like a non-symmetrical wing. One advantage of the symmetrical airfoil is that it is equally efficient in inverted flight as it is in normal flight, so it is often used for military or aerobatic aircraft. Asymmetrical airfoils generally produce somewhat more lift per square foot though.

Answer 7

well think aboutit this way...

the downwash /aka vectoring the passing air fownwards obviously appears to certain degree with both airplanes and helicopters.

Now here you have the reaction and you have to find the action to it.

if only the first part was true, than the air behind the wing would travel parallel /relative to the wing/ or would just stay still relative to the earth/which it does not./

Symetric wings do not produce lift unless they are provided with non-zero angle of attack - in which case they obviously stop being aerodynamically symmetrical

So basically think of the pressure differential as initiating force to vectoring the passing air downwards /or pushing it simply downwards

aerostar> the wake behind the boat are two mutually eliminating waves of drag or whatever - unlike the downwash which is oriented downwards and because of the orientation MUST generate reaction - lift at its originator - the wing. so basically the downwash is similar to the movement of a skimship than a general boat. or did you confuse that with wingtip vortices?