Calculating Air Speed for Low Flying RC Airplanes

[woodturner550]

I need a formula to calculate the speed of air moving over an RC airplane. Because the plane will not be flying above 2000 ft and this plane flies at less than 15 miles per hour max, the formula can be very basic I think.
Example of sensors:
I have two very sensitive barometer (temperature compensated), one is for the altitude and the other is for speed of the aircraft in air( similar to a pitot tube). The plane has a GPS for the ground speed of the plane. The need for the air speed of the aircraft is to be used to guide the control computer as to the amount of input into the controls to accomplish tasks (turn, climb rate etc). Because the airplane flies so slowly power must be applied to get enough air flow across the rudder to cause a turn. Many times the plane is standing still in a slight breeze or even appears to fly backward.

 

[boneh3ad]

So what are you actually looking for? You said you essentially have a Pitot probe, so you already have airspeed. What other speed do you need?

 

[woodturner550]

I only have the pressure reading from the sensor in the pitot tube. I need to relate that reading to airspeed of the aircraft. I also have the barometric pressure without the pitot tube.

Thanks,

 

[rcgldr]

Wiki has some formulas:

http://en.wikipedia.org/wiki/Airspeed

 

[woodturner550]

Thanks, I was aware of the wikipedia.org/wiki/Airspeed. The big problem is that takes a lot of calculation. I was looking for a simplier way to figure air speed...this is for a radio controlled airplane. Less that 15 miles per hour and flight time in the minutes. I was just hopeing that there was a simpler formula.

I guess I will have to fly it with a data logger and go from there. Total difference in the starting pressure and the max pressure logged devided by 15(mph) will give an approximation good enought for this project.

Thanks again.

 

[cjl]

That'll be hideously inaccurate (to the point of uselessness). The correct function is based on the square of velocity, so modeling it as linear defeats the purpose of having a sensor in the first place.

Probably the easiest accurate formula I can think of is to use V = sqrt(2*(P_total - P_static)/rho). If you're near sea level, using 1.225 kg/m³ for rho is close enough, otherwise you can correct for your starting altitude.

 

[woodturner550]

That is what I am looking for. Thanks.

I will be using the starting pressure due to the fact that the airplane will be flown from several places, one in the mountains.