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Problem with static margin and neutral point

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Scheny

Well-Known Member
Joined
Feb 26, 2019
Messages
314
Location
Vienna, Austria
Hi!

I was already starting building my scale model (fortunately the wings where the design is already frozen), when I noticed I had one small error in my CoG calculation (CoG of fuselage estimated too much in the front, as front half has almost twice the carbon as the rear half). Now I did not want to take any risk and tried to verify the project three times more:
  1. I calculated the weight again with formulas from Raymer and guess what: 158,6kg (Raymer) vs. 160kg (my guess)
  2. I made a detailed analysis using VLM2 algorithm with a detailed fuselage panel model --> everything to my expectation
  3. I calculated everything with CFD and compared to VLM* data --> perfect match except for drag** (VLM)
Now I try to get the CoG right, but there is something which bothers me. According to literature:
  • the CoG should be located between 18-30% MAC
  • be in any case in front of the NP
  • static margin should be around 10% MAC
According to my simulations, the NP for the complete aircraft is located approximately at 84% MAC (26% for wing alone or 93% for wing+tail excl. fuselage).
This would mean that my static margin for CoG = 25% would be 59%!!! (SM=NP-XCg).

I checked this against other aircraft, but they look similar... CoG in the right spot at 25%, but the NP is way rear. I am now a little insecure about the static margin and hoped for some of your insights.
BR, Andreas

*VLM = vortex lattice method
**the viscous drag is not calculated by VLM, so it only adds a factor for it which is tuned for model aircraft
 

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