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I have to do some calculations to get the drag from an experiment with a wake rake. In the equation I have to enter the total pressure coefficient $C_{pt}$, but in my calculations it seems to always be equal to +1. Is this correct?

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To get the drag from a wake survey, use the following formula from Fundamentals of Aerodynamics by John D. Anderson:

$D' = \int_{-h}^h\rho u\left(U_\infty - u\right)dy$

I'm assuming you know $U_\infty$ from a freestream Pitot tube. You can find $u$ from the wake rake using Bernoulli's equation. The equation above can be derived using a control volume of the entire test section.

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Thanks. I already found the solution. This was exactly what I did. –  Aaron de Windt May 9 '13 at 10:58
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