Originally posted by F4UDOA:
BadBoy,
I have always thought the actual performance of the A/C would exceed the flight tunnel test because of addition lift from the body, tail surfaces, and propwash over the wings. However in the case of the Max cl for P-51 I still don't think it is much over 1.5 in a clean config. If you solve for the Max CL based on a 106MPH stall (That comes from the flight manual not AH) the Max Cl is 1.44
Which is generous because 9700LBS is heavy in a P-51D.
F4UDOA
Your calculations look fine, but they do neglect some important factors, also I disagree with your assessment of the weight. The P-51A had a maximum take off weight of 10,600lbs and the P-51D had a maximum take off weight of 12,100lbs. If you rework the calculations for the P-51D with a 106mph stall speed just after take off, your calculations require a Cl = 1.8.
Also, you can't consider any aditional lift from the "tail surfaces" because in a stable configuration the tail will be detracting from the radial g. But the P-51D wasn't always stable, as I'll explain later. However, one significant factor that was also neglected in the research paper you referenced earlier is the contribution of prop thrust to the radial g. As an example, consider a P-51D flying close to the 1g stall speed, at an AoA close to 18 degrees, the prop thrust will be contributing to the radial g enough to increase the apparent coefficient of lift from 1.44 to 1.6. During a flight test in which an aircraft apparently achieved a lift coefficent of 1.89 (USAAF data) the actual lift being generated aerodynamically would have been closer to a Cl of 1.7 with the component of prop thrust subtracted. That would still include the contribution from the fuselage and the effect of the prop wash energising the boundary layer, which when ignored (during wind tunnel tests on the airfoil for example) yields results much closer to the values that you are comfortable with.
As for the tail force mentioned earlier, during low speed high AoA, the engine thrust contributes to the radial g. At high AoA this can enhance the lift significantly and another benefit of this is that because normally the centre of lift and centre of gravity are relatively close together, with positive stability that requires a downward force on the tail, however, the component of prop thrust, with its large lever arm provides a strong nose up pitching moment that reduces the downward tail force, thus enhancing the lift even further. However, the Merlin powered versions, including the D we have in Aces High, were not so easy to analyse. During a turning engagement these aircraft could become unstable which could mean a stick force reversal, requiring the pilot to push forward on the stick in order to prevent the turn from tightening. This was only a problem with high fuel levels in the fuselage tank because that shifted the centre of gravity backwards creating a very small stability margin. That alone was not so much of a problem on its own, but the centre of pressure also moves forward considerably as the AoA increases until the aircraft became longitudinally unstable. In that situation the pilot needed to apply a push force to the stick in order to prevent further increase in AoA and the eventual stall. That reduction, (or in extreme cases, reversal) of the tail force would enhance the lift even further. My own calculations consider the contribution of all the factors previously mentioned, including prop thrust and longitudinal stability as described above.
As a point of interest, the USAAF data for the P-38L is for a maximum lift coefficient with out flaps of 2.17 and that can be explained because of its twin engine configuration. The P-38 generates even more lift than expected, due to the twin engines for two reasons. Firstly, during low speed high AoA, the engine thrust contributes to the radial load factor twice as much as the single engine fighters. Another benefit of this is the reduction in downward tail force, thus enhancing the lift even further. Secondly, and more importantly, the wings are in the slipstream of the propellers and the wash speeds up and energises the air over them, which significantly increases the lift and reduces the stall speed. Unlike the single engine fighters, this does not result in asymmetrical lift because of the counter rotation, so that also reduces the tendency to spin. All of these factors result in an actual coefficient of lift much greater than the average flight sim pilot, or for that matter, your average aero graduate would normally expect. Add to that, the fact that the unusually high aspect ratio wing gave the P-38 enhanced climb rate and sustained turning ability, all of which is often ignored in flight sim'modelling. For the real P-38, that means that in a clean configuration with flaps deployed, it could turn at VERY low speed, shuddering on the edge of stall, but would not snap into a spin (unless mishandled) because unlike the single engined fighters its counter rotating engines had no net torque and no asymmetrical lift. That meant that if a good pilot stalled in a tight turn, the P-38 would just mush outwards, and that could be recovered with forward stick and no loss of altitude. A good pilot could rack the P-38 around in and out of stall to safely effect a very tight turn, and get the best advantage from that high maximum coefficient of lift <g>.
Originally posted by F4UDOA:
Also it is my understanding that P-47 clean stall is 115Mph, FW190A5 is almost 110Mph and the F4U is at 100Mph. Is this the same data your working with? What do your tables show for these A/C relative to each other in turning ability? How do you feel they are represented in the Simm?
Andy only pointed me to AH a few days ago. I'm investigating those very questions and I promise to keep you up to date on my thinking.
Badboy
