Replies to This Discussion

Permalink Reply by Steve Finley on February 28, 2009 at 12:01pm

Magnus,

By "power" do you mean thrust, or propeller rpm relative to an indexed servo position? Or are you talking about electrical power (I don't know anything about the electric engines) related to rpm, and calculated thrust from propeller size/pitch? What you would like to do for maintaining altitude, of course, is to keep lift constant, and that is obtained by a combination of thrust and angle of attack (pitch angle) in coordinated flight (what I think you're suggesting). It sounds like what you are going for is: if the state engine senses a departure from the target GPS altitude, how does lift need to be modified to get there? Some other questions you need to ask yourself are:

- How fast do I need the aircraft to converge to the target altitude? If you merely use full control, you'll induce a pitch oscillation - so you'll want some sort of dampening function.
- What amount of error is acceptable? Else you will continue to chase your target altitude.

I think you are on the right track to ignore SOG.

Steve

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Permalink Reply by Dean on March 1, 2009 at 12:08am

When your plane climbs, if you know the mass of the plane and climb rate, then it is easy to calculate the energy rate (Watts) that are going into raising your plane against gravity. So you take the level flight power, and add some more power based on the climb rate and mass of the plane, and a bit more based on motor efficiency (let's say 80% for a brushless outrunner), then you have a solution. Your equation above of added power being "pitch_angle * power_constant" does not take into account the Trig.

Climb Rate = (0.2777 m/s per km/h)*(Ground speed in km/h) * Tan(Pitch) I picked ground speed because that is what GPS give you.
Extra power for the climb in Watts = ((plane mass in kg)*(9.81m/s^2)*(climb rate in m/s)) / (0.8 for efficiency)

So let's say your 1 kg plane is cruising in level flight using 100 Watts at at some unknown airspeed but you know you want to maintain it in a climb, then you enter a 10 degree climb and want to maintain the same airspeed, you can easily estimate the extra power required to add to maintain the climb via additional power, and have the entire original 100 Watts there to maintain that drag for the airspeed. The added power would be calculated thus:

Climb Rate = (0.2777 m/s per km/h)*(60 km/h) * Tan(10 degrees) = +2.94 m/s
Climb power = ((1 kg)*(9.81m/s^2)*(climb rate in m/s)) / (0.8 for efficiency) = 36 Watts (accounting that motor is 80% efficinet)

So in this 10 degree climb, you would need to add 36 additional Watts to the original 100 to maintain airspeed, whatever that was. If instead you descended at 10 degrees, you could shave off some power.

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Permalink Reply by bGatti on March 1, 2009 at 9:51am

Dean,
This is quite thoughtful.
Are you using power sense to maintain Airspeed, or separately sensing airspeed?
Is the purpose of monitoring power to protect the batteries, or to regulate flight?
Thx.

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Permalink Reply by Dean on March 1, 2009 at 11:22am

bGatti - I was eloaborating on an idea I once had to maintain airspeed in absence of a Pitot tube. This idea requires inclusion of a voltage/current sensor, like the one I have selling on Sparkfun from AttoPilot. The core idea is in level flight a given motor power results in plane achieving an equilibrium speed where drag equals the thrust. When in a climb or descent, just add or take away power required for the gravity energy change rate. To be more accurate take into account motor efficiency.

In my AttoPilot, I use airspeed from a Pitot in a closed PI loop with throttle. Power is only measured for logging and telemetry purposes. Current is integrated to accumulate total mAh and that can be used for user-defined abort limit to return home.

So - in AttoPilot right now I am not monitoring power to regulate flight other than forced mission abort if mAh upper limit is exceeded.

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Permalink Reply by bGatti on March 1, 2009 at 2:01pm

Dean,
Interesting. Current sense for speed regulation is interesting. but while you appear to compare power to altitude as a means of deducing airspeed, I had thought to compare power to rmp as means of determining impedance, and from impedance reach airspeed.

if (rpm-power) = impedance
and airspeed = throttle / impedance

then perhaps airspeed = throttle / (throttle-current) within a moderate range of throttle.

(a pitot is so much easier)

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