# [fluids] airplane

1. Nov 29, 2006

### kbyws37

An airplane flies on a level path. There is a pressure difference of 540 Pa between the lower and upper surfaces of the wings. The area of each wing surface is about 150 m^2. The air moves below the wings at a speed of 81.0 m/s.
(a) Estimate the weight of the plane.
(b) Estimate the air speed above the wings.

I figured out (a) weight of plane.
(150 m^2)(540 Pa) = 81,000 N

I am having trouble figuring out the air speed.
I am guessing that I have to use Bernouli's equation b/c there are two different points (upper and lower surfaces).
When using Bernouli's equation, don't I need density ? How can I find density if no mass is given?

2. Nov 29, 2006

### OlderDan

You can look up the density of still air at standard temperature and pressure.

You might want to look at this link

http://www.grc.nasa.gov/WWW/K-12/airplane/right2.html

and hit the next button at the bottom until you get to the end. The Bernoulli approach is not wrong, but the assumption that the air over the top of the wing arrives at the same time as the air going under the wing is not correct. Since you have velocities given above and below (which in fact are not nearly constant over the top surface, so take it to be an average and use it) you can do a Bernoulli calculation.

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