Neutral point change with AOA

[Mohammad Bahmani]

Hello to all

I was doing some analizes for my configuration and i found out that Neutral point varries greatly with AOA change. I know there is something wrong with my modelig but i don't quit understand what it is. I know that neutral point can have a slight change in different AOA's but in my case its not "Slight". I attached my model and .stab file

Any help would be appreciated. 

[Rob McDonald]

You are using the Carlson Pressure Coefficient stall model (on the advanced tab).  And you are at pretty high alpha (11-13 deg).

I did a sweep of alpha (no stab runs, just plain runs) from zero to high alpha (above what you were looking at).

This shows that the Carlson stall model predicts stall somewhere between 11 and 12 deg.

To look at static longitudinal stability, we can look at CM vs CL.  The slope of this line is the static margin.

The left-most point is zero alpha -- the mostly horizontal part of the line is the linear range of the CL vs. alpha curve.  The curved part 'under' the straight part is the post-stall behavior.

In the linear region, your CL vs. alpha is nearly horizontal -- that is neutrally stable.  You should move the CG to get that portion of the curve to slope down to the right.  The airplane will trim where the CM curve crosses zero (you can shift this curve by deflecting control surfaces of course).

It is useful to look at the load distribution (cl*c/cref).  I've shown 11 deg in red and 12 deg in yellow (post stall).

We see from these charts that the main element stalled first.

You can also look at how hard each lifting surface is being worked (cl).

The rule of thumb for a canard design is that you want to design the canard to stall first -- that way you will get a nose-down pitching moment at stall that prevents deepening of the stall.

I can look at the high alpha runs and I see that the canard first stalls at 18 alpha.  So, if I increase the incidence of the canard by about seven degrees (and turn twist reference to relative), we should expect it to stall first...

To see this behavior, you really have to look at the load distributions, or run a bunch of alphas in the near-stall region.  The point of having a canard stall first is that you'll never get to the really high alphas.  However, VSPAERO does not know that.  It can still run a simulation as if you reached 20 deg -- even though there is no way to maneuver and control the aircraft to reach that attitude.  So, when looking at the stall characteristics of a canard, you have to look at the CM vs. CL behavior in detail around stall.

Rob

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[Mohammad Bahmani]

Hello Rob. Sorry for my late response. My intention of using STAB mode is to get dynamic derivatives of the airplane but the "x_np" that the ".stab" file shows is not promising so I started doubting other outputs. I ran the model at low AOA's and the value of "x_np" from the ".stab" file changes rather unusual. from my little experience, the Neutral point should not vary this much with a change in AOA. My question is should I trust in the value of "x_np" or not?

[Rob McDonald]

Please try to take the time to understand my reply.

I expect you should get pretty consistent x_np for alpha from 4 to 10.  Above that, stall will make a big difference.  I'm not sure why the moment looks a little off for alpha=0,2, but it shouldn't be a huge deal.

Rob