### Airspeed vs GPS speed

After de-briefing my flight today with almost no wind,  I did some comparison between AirSpeed and GPS speed (I added airspeed to my logs).
The difference is massive and explains some of the behavior that I have seen.
It looks like I have to change the AIRSPEED_RATIO to 0.7-0.8 to have closer speeds, which does not make send (should be 2/volumetric density of air (1.2/1.3) = 1.5/1.6.
My pitot is parrellel to the ground but the plane (skywalker) may have a bit of tail down attitude. Would it be due to this and I need to change the position of my pitot tubes ?

#### Replies

• Developer
@Flick - I agree that a curve from measured data is the most accurate. It is not something we want to implement though, as we are trying to get the code to work well with as little user interaction as possible. However, if you want to insert that functionality in your own code I think it is the most accurate approach.
• Developer
Matthias - I missed your post on the 23rd.

The range on the two sensors is different, but the sensitivity is the same, and is what matters. We use the same constant for both sensors.

Past changes probably came from changes to the ADC. We are using 2 more bits here than we did on the previous hardware.
• Developer
@Mark
That is the reason that pilots fly by calibrated versus true airspeed. When you fly at higher altitudes where the air is less dense and you maintain the same calibrated airspeed you had at lower altitude the wing is flying at the same angle of attack.... Stall speed remains at the same calibrated airspeed.....

For ArduPilot, if we were to flying significantly higher than 400' then the place that changes in density would be most interesting is in our barometric altimeter calculation. We assume a certain pressure profile with altitude based on standard atmospheric temperature lapse rates. However it is rare that the temperature profile matches this ideal.
• The problem probably is not the dinalmyc pressure but the static, where is the atmospheric pressure taken? inside the aircraft just a simple opening can make the internal pressure raise and make a static port is really tricky at that size.
for this reason I use a curve in my pitot reading that takes in account all the error of ports postions.
• I don't see any measure of air temperature. I know from calculations I made for bike riders (bikepower.c) that temp makes a BIG difference on air density and the force it applies at speed.

From my program:

Force of air resistance
F_a = A2 * (V + H)^2 + A1 * (V + H)

From W&W p85ff, air resistance coefficient C_a
C_a = F_a / (dynamic_pressure_of_air * frontal_area)

Where F_a is force of air resistance. At bicycling speeds:
dynamic_pressure_of_air = air_density * (V + H)^2 / 2

where V + H is the relative air velocity.

So...
F_a = C_a * air_density * frontal_area * (V + H)^2 / 2

p90 suggests that the Reynolds number is in the ballpark of
200000, so there may be some advantage to wearing rough,
rather than smooth clothing.

air_density is 1.2047 kg/m^3 at 20^C = 68^F.
p91 suggests that roughly, C_a = 0.9 for a bicycle and rider.

F_a = (0.9 * 1.2047 / 2) * frontal_area * (V + H)^2
= 0.54 * frontal_area * (V + H)^2

This doesn't directly apply. But you can see how much temp changes things here. http://bikecalculator.com/veloUS.html
• Developer
Matthias,

I have tested with the value in the code and it worked VERY well for me in the range from 10 to 20 m/s. Much closer than your results with the modified value.

What sensor are you using?

Also know that we are using a simplified equation, which will only provide reasonable results over a relatively small range of airspeeds. I get good results between about 8 and 18 m/s. At lower speeds the equation will under-report the value and at higher speeds it will over-report the value. If your nominal speed is in a higher range then adjusting the value is a good way to compensate.

We use the simplified equation as the more correct equation requires significantly more computation time. You can find it and experiment with it if you like. Look for a term raised to the 2/7 (or maybe it was 7/2 power) inside the square root. That is the one.
• your tail down attitude will only affect your flight attitude , but not your pitot. You will experiment something like the plane will always go down, but nothing to worry about, if the case is that the tail is glued and the position cant be fixed, just adjust the elevator trim to correct it. Once you correct it on the air, turn on the plane on the ground and radio, measure the angle of correction applied with the trim on the radio, turn back your trim to zero and try to apply the angle given but by adjusting the elevator pushrod. Remember always fly your airplane with trims on zero... you never know when you will reaaly need them, so you wanna make shure you have full to both sides.
Other thing is that you should check the plane balance since this could be another reason of your tail down attitude...it could be tail heavy... Your model instructions should tell where to locate de CG on the wing!
Good Luck
• Developer
Matthias - are you using the latest code from the repository. R1266 corrected an issue that was causing the airspeed to be off by a factor of two. From your graph above it looks like you have an earlier version of the code. Update your code and you should be OK.
• I started to say wind, but I see there was almost none. Maybe wind higher up? What altitude were you flying?

Just a thought, I'm pretty new to this so I can't help you on the programming aspect.

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