Steven - I tend to go with wing flex also. I have a mini QuickFly called a FleaFly + 10. Strange name but it is 10percent bigger than the FleaFli which was a Phil Kraft desing. It is a small airplane of conventional structure. It called for around a .19 to .25 in the original form (I think I am remembering this correctly). I am running an older model of )S .40 in it. It is a nice strong running motor but it is not as fast as the new OS 40 that I have.

Anyway the overall planform vs mass relationship is not that far removed from the P-51. Most airplanes that fly nicely will have the general layout the same. Scale them all to the same wingspan and it is interesting.

Anyway my FleaFly (never can remember if it's FleaFli or FleaFly) flies great. The big motor just makes it go faster. I would expect the same from the P-51 IF the wings are not flexing. A bigger motor on a strong enough platform just goes faster, it doesn't roll reverse.

rogwabbit - The mass considered at the CG and it's moment arm is small compared to the aerodynamic forces the airplane is experiencing at high speeds. A good example of low aero forces happens in a stall turn. When the airplane stops at the top of the straight line up, and a lot of rudder given to start the turn it will flop over and do a yaw wiggle until the speed builds up again.

The P-51 in a high speed turn pulling a lot of g's has a bank angle between 60 and 80 degrees. The moment due to weight could be around 25 inch pounds (5 pound airplane with a 5 inch moment arm from the wing location). That amount of moment is small compared to what the ailerons can produce. For an aileron moment arm of 24 inches both ailerons would have to be responsible for 1 pound of force or 1/2 pound per aileron. Pretty easy to do going at a high velocity.

Steven again - a limp and flexible wing will twist the opposite way of the input and causes a reversal in roll from what was commanded. It is a problem in full scale and model scale both. Most of our structures are so strongly built that we never have it happen. To stop it you just need to remove the covering and put in some diagonal wing ribs to give some more torsional rigidity.