Where should I put the pressure taps on my aerofoil?

[liam1992]

For a uni project I have to build an aerofoil to measure the lift with a fowler flap. I was going to do it using pressure taps connected to a manometer but I don't know how many holes to do and where to place them. Is there a specific rule or can they just be anywhere as long as they are aligned? Also I am unsure as to how to connect the pipes to the taps on the flap because it will have to go through the main aerofoil body to go through the cantilever.

 

[RandomGuy88]

There are some general rules of thumb for placing pressure taps, but each airfoil is different. The maximum number of taps is limited by a couple of things including cost, space (do you have room for all of the tubing), the number of available pressure transducers...

As for placing the taps it is very helpful to have some idea as to what the pressure distribution will look like beforehand so you should try to do a calculation in xfoil or cfd if you can.

You will want to concentrate your taps in areas where the pressure is changing rapidly. On most subsonic airfoils this is on the upper surface near the leading edge. If your airfoil is heavily aft loaded it is possible that you will have a significant pressure gradient near the trailing edge, and this is why it helps to do some preliminary calculations. having these calculations can also be helpful for making sure your pressure transducers can handle the pressures you want to measure.

You will likely want a lot more taps on the uppers surface then the lower surface because the pressure gradients are larger on the upper surface.

Finally, you actually want to stagger your pressure taps in the spanwise direction, by which I mean you don't want them all in a straight line. This is because a pressure tap can prematurely trip the boundary layer and then your downstream taps would be in a turbulent boundary layer which would not have occurred if it weren't for the upstream taps. The actual amount by which you stagger the taps is not arbitrary either. You will want to take in account the rate at which the turbulent region from the upstream taps grow. But you must also consider wall effects. You don't want your pressure taps to get close to the wall, especially once airfoil has stalled because 3D effects can become significant.