Pitot tube vs. GPS speed

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Pitot tube vs. GPS speed

Postby Tom » Mon Mar 29, 2010 8:23 am

A lot of attitude estimation algorithms use a pitot tube to determine the speed. This is usefull for variations in speed / accelerations, but I can not understand how it can be used to calculate eg centripetal forces. When flying in strong winds, the plane can sometimes barely move (ground GPS speed ~ 0), but the pitot tube will measure a high speed!

Any thoughts on that?
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Re: Pitot tube vs. GPS speed

Postby Tom » Sun Apr 11, 2010 4:59 pm

I suppose the biggest importance of the pitot tube is gain scheduling (relative motion of the wind over your plane).
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Re: Pitot tube vs. GPS speed

Postby magellan » Wed Sep 22, 2010 4:37 pm

Hi Tom,

Airspeed is better for our application than GPS.

What matters to the airframe is its speed relative to the airmass it evolves in. The ground speed is of no relevance. If the plane is flying at 40 Kmh against a 10 Kmh wind, its speed relative to the ground is 30 Kmh but the speed at which it is flying through the air is 40 Kmh, that is why it would be better to use airspeed to compute the centripetal factor. My 2 cents worth.
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Re: Pitot tube vs. GPS speed

Postby Tom » Wed Sep 22, 2010 4:44 pm

Hi Oliver,

Indeed, the airspeed is what counts in all control-related tasks. However when you need to calculate the centripetal acceleration, you can only count on the GPS speed. Imagine 40km/h speed and 40km/h winds. The plane is still as it would be in your hands. When the plane turns, it will change heading without centripetal acceleration.

There is an interesting paper on how to compute the wind speed based on the GPS data and the plane's attitude (yaw gyro mostly). Maybe something to work on during the winter :-)
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Re: Pitot tube vs. GPS speed

Postby magellan » Thu Sep 23, 2010 4:48 pm

Hi Tom,

This has always been a confusing issue, always fun to debate. It's true, for control, we need airspeed, for the centripetal reference, we need GPS as it computes the speed in relation to the earth, the earth being the reference body responsible for all the forces we are dealing with here. Thanks for clarifying the matter!

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Re: Pitot tube vs. GPS speed

Postby Mitch » Mon Sep 27, 2010 5:16 pm

Another input which is used in almost all high performance airplanes is angle of attack (AOA). It would be very simple to rig a vane using a potentiometer and voltage divider. Then the voltage would be proportional to the AOA. Seems to be neglected in the model world but would be a simple way to obtain more valuable inflight data.
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Re: Pitot tube vs. GPS speed

Postby magellan » Wed Sep 29, 2010 4:34 pm

Mitch, that's a great idea.

We could use it to prevent unwanted stalls, it can also tell if the plane is upside down.
It's a great backup too.

If you build one, let us know. I am adding it to my to do list too!

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Re: Pitot tube vs. GPS speed

Postby Chris » Wed Sep 29, 2010 8:50 pm

Yea, I agree the AoA vane is a great idea.

I know someone who built one by shining an LED through 2 polarized films, one attached to the axle of a vane and the other film was stationary. There was a light detector on the other side of the films from the LED and the light intensity varied as the angles between the polarized films changed from perpendicular to parallel. I don't remember if its a linear relationship between angle and light intensity, but it was a cool system with only friction form the bearing. Since you want the vane to be as small and light as possible, the forces can be small and the friction from the potentiometer becomes important.

Another good "non contact" measuring system is the AS5043 magnetostrictive sensor (they give free samples of chips and magnets too!).
http://www.austriamicrosystems.at/Products/Magnetic-Encoders/Rotary-Encoders/AS5043

Depending on the size and accuracy you want, balancing the vane statically and aerodynamically can also be challenging, but I bet you could something functional with a little tinkering.
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Re: Pitot tube vs. GPS speed

Postby Tom » Wed Sep 29, 2010 9:08 pm

Can anyone provide some kind of picture? I don't really understand how it works....
What would the advantage be?
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Re: Pitot tube vs. GPS speed

Postby Chris » Wed Sep 29, 2010 10:52 pm

Image
Image

So when you're flying along, you are usually at some "angle of attack" to your direction of travel through the air and since your pitot-static tube needs to be directly into the wind to be accurate, this angle is important. You can kind of back calculate what your air speed is based on the pitot static reading and the AoA, to an extent. Also, In aerodynamics, its important because it is related to how much lift and drag is on the vehicle, which can be important for your control law. I don't think gluonpilot currently takes aerodynamic effects into account, but that is something you'll probably need for high performance maneuvers.

The vane is free to swivel and will try to stay in line with direction of travel, and so indicate your angle of attack (also called AoA or alpha)
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