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Using VSPAero to build a full FDM

Good sims require good FDMs (the "thing" that makes an aircraft behave like an aircraft).

Re: Using VSPAero to build a full FDM

Postby wlbragg » Wed Nov 30, 2016 1:35 am

A lot better as in wider range of flight envelope in the pitch and a bit more control.
If I had to describe by feel what the problem is I would say the engine doesn't have enough power and/or the elevators and rudder don't have enough drag (more the elevator).
Once you get close to a stall it is really hard to recover, instead of the nose dropping it simply stops flying forward and falls from the sky. Nose too far down too fast and the same effect in reverse, the elevator can't overcome or combat the pitch.
Even though flaps appear to provide lift they exaggerate the lack of power/drag because of the slower airspeed. It is very difficult to land with full flaps and no power. The aircraft becomes unstable in an un-powered glide with flaps.
But I still really like the feel of the FDM within the flight envelope.
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Re: Using VSPAero to build a full FDM

Postby Necolatis » Wed Nov 30, 2016 7:31 am

About using 25% MAC for AeroRP; Isn't neutral point that what you would normally use for AeroRP, since 25% MAC sounds like the approx aerodynamic center for the main wings, which will lie forward of the neutral point. At least that's how I understand of how to place AeroRP since its where the forces are applied, and if AeroRP is at aerodynamic center then for example the contribution of lift from the tail will also be generated at the aerodynamic center instead of where the net sum of lift is. And one needs the neutral point anyway, if want calculate cg position from static margin (I guess SMC stands for static margin in chord distance? don't remember seeing that abbreviation before).
"Airplane travel is nature's way of making you look like your passport photo."
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Re: Using VSPAero to build a full FDM

Postby Richard » Sun Dec 04, 2016 11:02 pm

@wlbragg - is the problem when you have flaps deployed. I'm not convinced that the flaps are even close to being correct.

@Niko - 25%MAC is indeed an approximation. I think maybe the point of MAC is that 25% is going to be about the same for many different wing sweeps. SMC is another (older?) way - Standard mean chord (SMC) is defined as wing area divided by wing span.
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Re: Using VSPAero to build a full FDM

Postby wlbragg » Mon Dec 05, 2016 12:54 am

@wlbragg - is the problem when you have flaps deployed.

Deploying flaps and reducing airspeed I think just enhances the bad effect of feeling like there is not enough friction or air flow over the elevators. So, yes, if the flaps are not correct I could see how it might aggravate the problem.
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Re: Using VSPAero to build a full FDM

Postby joshuawojo » Tue Dec 04, 2018 3:49 pm

I have gotten vspaero to show the vspaero viewer aero pressures and trailing waves in the newest openvsp. Is the .adb file a csv file now? I want to run my model in flightgear. jsbsim for now I cant run flight gear.
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