Inputs needed for modeling practices

[Eric Wang]

Hello,

I am working on a special configuration that the cruciform of the engine nacelle degen has no contacts to the pylon surface as shown in the first figure. The pylons are not perpendicular nor parallel to the cruciform.  The tip of the pylon is  just long enough to touch surface of the nacelle under the normal mode (the second figure). I wonder if it is a proper way to model the aircraft for VLM analysis? 

regards,

Eric 

[Rob McDonald]

You could try it as-is, you could also drop the pylons entirely, you could try it in panel mode...

How much difference is there in the results?  You could also extend the winglet to the center of the nacelle -- that might help close the gap in thin-surface mode.

Flow goes through the nacelles, so the thin-surface representation may actually be better than the solid-body thick-surface blob.  You could also build up a flow-through duct to consider (if this is a turbofan).

The nacelles will provide somewhat of a winglet effect for the pylons.

Pylons are usually designed to be very short (perhaps this is an UDF, so they must be long) -- but they are generally not designed to be a lifting or trimming surface.  If I was designing one, I would probably try to design it to carry no aerodynamic load in cruise.

Rob

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[Eric Wang]

Hello Rob,

I was trying to debug the model, since the pitching moment output from OpenVSP has quite a bit different from the result obtained by other CFD tool.
That is why the pylon modeling method came into my attention. This long pylon has some effects on the aircraft longitudinal stability. 

BTW, it is a propeller driven aircraft. The long pylon is to provide clearance for the props. 
I extend the pylon to the center of the nacelle and the aircraft pitching moment increases 50%. The original position of the pylon tip is marked by the red arrow. 
I am surprised that this little difference makes so much change in pitching moment. 

regards,

Eric 

Rob McDonald 在 2022年1月6日 星期四下午9:50:16 [UTC-8] 的信中寫道：

[Rob McDonald]

I would investigate both models for hot-spots -- is there anything blowing up in either case that might lead you to believe that the answers were being thrown off.

Your horizontal tail looks very small -- the pylon is nearly as large as the tail.  So, in this case, you will likely see more contribution to moment than you might expect.  I agree that the extension should probably not cause a large change in result.

Rob

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[Eric Wang]

Hello Rob,

Beside connecting the tip to the center of the cruciform degen, I also tried to connect the tip to the nacelle horizontal surface of the cruciform plane. It gave me the same effect that increases pitching moment quite a bit. I wonder maybe it has something to do with how VLM interprets fuselage components. Could you point me to some references how OpenVSP calculate fuselage aero coefficients?

regards,

Eric  

Rob McDonald 在 2022年1月7日 星期五上午8:36:30 [UTC-8] 的信中寫道：

[Rob McDonald]

Panel force contributions are calculated the same no matter what kind of component they occur on.  The force on each vortex element is summed.

Objects without a wake will not be able to support a force -- the trailing separation line will move to a location that produces no force.  However, those components can support moments.

Objects with a wake are capable of supporting a force (and moments).

Although non-lifting bodies can't support a force, they can change the force on other bodies.

You should be able to see what changes significantly on the Cp plots of the two cases.

Rob

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