[Physics] Given airplane mass, velocity of air under wing, and a wing area, find velocity of air over wing

fluid dynamicshomework-and-exerciseslift

I attempted to solve this problem as a tutor for a student and struggled, but want to be convince the professor didn't provide enough information.

The problem is essentially:
We wish to maintain a plane in flight. The plane has a mass of 1.9E6 kg, the wings have a surface area of 1500 m^2, and the velocity of the air underneath the wing is 97 m/s.

I setup:

P1 + 1/2*density*velocity1^2 + density*gravity*y1 = P2 + 1/2*density*velocity2^2 + density*gravity*y2

where the 1 sub terms are beneath the wing, and the 2 sub terms are above

we are essentially looking for velocity2

I realized that without a thickness of the wing, the professor is probably wanting us to recognize that (y2-y1) ~ 0, thus

P1 + 1/2*density*velocity1^2 = P2 + 1/2*density*velocity2^2

Recognizing that the upward and downward forces must be equal to the pressure exerted downward by the force of gravity on the mass of the plane only, and thus

     1.9E6 kg * g
P2 = ------------
         Area

P1 is different, however the force is the same, thus

     1.96E6 kg * g
P1 = -------------  since area is given, and I can only assume is the bottom area
       1500 m^2

We now have everything we need except the top area of the wing, which given the equation

A1*v1 = A2*v2

allows us to equate A of the top to area of the bottom, this results in

     A1*v1
A2 = -----
       v2

Putting all of this together results in a quadratic equation that results in essentially the same velocity over the top as was given for the bottom (our result was 97.02 m/s).

This of course was not the answer expected which is why I am asking for help? What did I do incorrectly here, or is there truly not enough information given?

Best Answer

If the plane is just flying at constant altitude then a vertical force balance requires that lift from the wings be equal to the plane's weight. The lift force, $L$ comes from a pressure difference above and below the wing so that $$ L = (p_1-p_2)A = mg $$

You can use the Bernoulli equation assuming a negligible difference in height to express the pressure difference as $$ p_1-p_2 = \rho/2 (v_2^2-v_1^2)$$

You should then be able to rearrange for $v_2$ and solve.

As @CarlWitthroft pointed out, this ain't how planes actually fly, but it does seem to answer your question.

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