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Elevon Design Problem

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Elevon Design Problem

Postby mrostami » Tue Jun 05, 2018 9:27 pm

I am trying to design an elevon configuration for my aircraft. The aircraft has 4 control surfaces altogether which the external surfaces acting as elevons and internal ones acting as elevator. How I can define a limit for the maximum deflection of the elevon? The thing is that once I am using the maximum aileron and then trying to use the elevator, the deflection goes further than what I have defined for each surface separately. I don't know where I should define the maximum that can be considered for both.
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Re: Elevon Design Problem

Postby Alant » Tue Jun 05, 2018 10:55 pm

This limit is usual for this design. It is usually only a problem at low speed, as at high speed large deflections are not used.
Here is one way:
Treat each surface independantly.
Sum the aileron and elevator demands to get the desired movement of the surface on one wing. Subtract aileron from elevator to get the deflection on the other wing. Beware of aileron sign convention as it is different in the older British system.
Now apply the physical or mechanical limits to each surface. These may vary according to airspeed.
Finally get the actual elevator by averaging all surface deflections, and the actual aileron by the difference between the left and right surfaces.
Do all of these calculations using degrees or radians, converting to and from flightgear´s normalised (i.e. range between +1 and -1) before and after doing these sums.
Don´t forget that the elevator command will be the sum of trim, control column, autostabilser and possibly autopilot. Same for aileron.
Look at the Flightgear FDMs for delta wing and most supersonic military aircraft for examples.
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