Nose up problem

[OBIO]

I am tinkering around with the freeware Alphasim RA-5C Vigilante. Panel set up, new paints, yada yada. The stock flight dynamics yields a plane that stalls very very easily. I swapped in the flight dynamics from Bob Cichilo's (I hope I got his last name right) 3rd update of the Vigilante. The stalling is gone, but the plane goes into a very hard nose up attitude once it gets some decent air speed built up. Even with power rolled back to 80% (as depicted on the RMP gauges), the plane still goes nose up. It takes nearly full down trim to get the plane to fly level.

I moved the fuel tanks forward to shift more weight toward the nose. They were set at the center of gravity. I combined the three center tanks into one larger 1090 gallon tank and moved it forward 7 feet from the COG. The wing tanks I moved forward 3 or 4 feet of COG. And still the plane noses up.

Anyone have any tips for getting the Vigilante to fly level without having to use the AP or max down trim?

OBIO

 

[sparks]

This is not simple to explain or solve, but the root cause of the problem is the lift curve.

In order for an aircraft to fly level at high speed, the lift coefficient has to be large enough at zero AoA (angle of attack) to provide the lift needed to support the weight of the aircraft. If the lift coefficient is too low, the plane will need to fly nose up to get the higher lift coefficient needed.

The specific solution to the problem is:

CL = W / (1/2 ρ V2 S)

Plug in the values of W (weight), V (velocity), S (wing area) and ρ (rho, i.e. air density) and solve for CL (lift coefficient). Once you've done that, set up your lift curve so you have the value you calculated at zero AoA. Now all you have to do is figure out what the maximum value of CL should be and what the critical angle of attack should be for a Vigilante. A good way to estimate maximum CL is to solve the above equation using the stall speed, and anywhere around 15 degrees usually gives satisfactory results for an FS flight model.