# Stagnation Pressure on Airfoil

#### recon223

First, let me say I am not a engineering student but just a average person trying to get his pilots license and enjoy working with aerospace problems.

My problem is:

“An aircraft is flying at 150 mph. The atomspheric pressure is 14.1 lbs./sq in and the temperature is 50 degrees F. Find the total or stagnation pressure at the point on the airfoil. If the pressure at a point on the upper surface of the airfoil is measured and found to be 13.9 lbs/sq in., what is the local air speed at this point?”

I am trying to use the Bernoulli principle to solve this problem

P1 + ½ρV^2 = P2 + ½ρV^2

I have converted a few things to help work with the equation:

150 mph x 1.4667 = 220 ft/sec
50 F + 459.6 = 509.6 degrees Rankine.
14.1 x 144 = 2030 lbs/sq. ft.

I understand that as the velocity increases the pressure decreases in the venturi tube and that as velocity decreases the pressure will increase but I am still having trouble solving the entire equation and I guess understanding how to calculate everything in the problem.

Here is what I have though and please correct me if I am wrong or have mathematical errors or failed reasoning 101.

Solving for P1 (dynamic pressure)

2030 + ½ (.0023769 slugs)(220)^2
=2030 + (.00118845)(48400)
=2030 +57.52
=2087 lbs/sq ft.

How do I go about solving the the second part of the word problem if the pressure is measured at 13.9 lbs/sq in., what is the local air speed? Do I just plug in 2001 lbs/sq. ft (13.9 x 144) for P2 and solve for V^2?

Also, let me apologize for the ole school units. My book came from a old bookstore and it by a professor at University of Kansas (Vincent Muirhead). I would like to have the Intro to Flight by Anderson but I refuse to give 180.00 for a book.

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Let me ask you a question first. If you take a pressure reading on the upper surface of the aerofoil, which pressure do you think you're measuring?

#### recon223

I would assume it would be P2 the stagnation pressure.

#### bigfooted

if the measurement device is a pitot tube (still the standard I guess) than yes, you measure stagnation pressure, also known as total pressure. It's the static pressure plus the dynamic (velocity) pressure.
The stagnation pressure is only equal to the static pressure P1 or P2 in the stagnation point.
and by the way: you can get the book of Anderson for a fraction of the pric you mention if you are satisfied with an older edition (5th edition, 2004, on abebooks, if I am allowed to mention that)

#### recon223

Thanks for the site for the book.

So, does anyone have any recommendations on my question. I think we all agree on how to calculate the pressure but are my calculations correct for P2? And how do i calculate the velocity if the pressure was 13.9 lbs /sq in?

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