Hi guys
I’ve just read through the earlier pages of this thread and I notice a number of messages that appear to be discussing the merits of flap use, some saying that prolonged use will harm turning performance others that it won’t, and the striking thing about those posts is that there is some truth in both points of view. For the split flap configuration illustrated in the diagram below, taken from page 79 of Perkins & Hage, the benefit depends on how high the lift coefficient is. For example, at low values of lift coefficient use of flaps is not good, and you can see that the two points A and B in that diagram have exactly the same lift coefficient, but point B, has a higher corresponding drag coefficient. That explains why you shouldn’t try to use flaps during maximum rate climbs, or power off glides, the drag penalty makes it prohibitive. That situation continues up to a relatively high lift coefficient where the two polars cross each other. The point where the polars cross is quite high, and that means that most turns at high G conducted at speeds close to corner velocity will suffer higher drag and lower sustained turning ability with flaps extended, which also explains the advice given by Lockheed. However, there is more, if you look at the diagram again, you notice that the drag is exactly the same at point C and D, but that the lift coefficient is much higher at D, meaning that once the crossover point is exceeded, you can get more lift for the same drag, making flap usage advantageous. In practice the region of the envelope where pilots can take advantage of this, occurs generally at much lower speeds, where high coefficient of lift values can be achieved at tolerable G levels and can result in better sustained turns. The point is that both better and worse sustained turns are possible, it just depends on the particular conditions under which the turn is being executed, which basically means that so far everyone is right
The next question is, how does all of that apply to the P-38? Here is an EM diagram for the AH P-38 showing a clean configuration and what happens with 4 and 5 notches of flaps extended.
Notice that at higher speeds, the use of flaps is expensive in terms of loss of energy, reducing the sustained turn dramatically, but notice that at very low speeds the sustained turn rate is just as high as it was without flaps, the main difference being that the turn radius is now much smaller than before, and this is the advantage that good P-38 drivers are able to exploit. I believe this diagram also holds the secret behind the success of the cloverleaf maneuver, just look where the instantaneous turn rate and radius goes at the higher speeds within the placard limit. I think the good P-38 drivers in AH already use this fact appropriately.
The last question regarding the P-38 and its maximum coefficient of lift brings up a recurring topic on these boards and it is important to remember the following point. The maximum coefficient of lift for an airfoil is not constant. There isn’t a single catch all value that can be quoted. In simple terms it varies with speed (Mach number and Reynolds number) which means it has different values at different points in the envelope. Also, most quoted values are power off values. For example, in the NACA TN 1044 report quoted earlier in this thread the results came from both wind tunnel and flight test data that were obtained under throttled and feathered conditions. Under full power the effective coefficient of lift is significantly higher, and the reason for that is amplified due to the configuration of the P-38. Allow me to explain.
Propeller driven aircraft (pullers only) have a power on stall speed that is lower than their power off stall speed. The reason for that is due to the fact that the wings are in the slipstream of the propellers and the wash speeds up and energizes the air over them, thereby reducing the stall speed. Because of that, obvious safety concerns have resulted in power off stall speeds being more commonly quoted in flight manuals, while in some sources both are quoted. Of course that is equally true of single engine types, one wing receiving a slight upward flow the other downwards, and that doesn't change the fact that they still have a lower power on stall. That they still have a lower power on stall speed reflects the fact that even at relatively high angles of attack, the prop' wash is still being driven over the wing at a very low angle of attack because the prop is generally normal (90 degrees to the axis of the aircraft) to it. That will always have the effect of energizing the flow over the wings delaying separation. This is shown in the diagram below:
Of course this is also true for other aircraft, so what’s so special about the P-38?
Two other factors that are often neglected for the P-38 and serve to improve its turning performance, are due to its twin engine configuration, they are... Firstly, during low speed high AoA maneuvers the engine thrust has a component that contributes to the radial load factor, at high angle of attack this is almost double that for single engine fighters. Another benefit of this is that 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, provides a nose up pitching moment that reduces the downward tail force, thus enhancing the lift even further. Those factors along with the previously mentioned effect of the slipstream speeding up and energizing the air over wings, thereby increasing the lift, means that the P-38 was indeed better in practice than the average flight sim pilot (or for that matter your average aero graduate) would normally expect. The big question is not that this happened, but how much difference it made in practice?
Well, the effective increase in the coefficient of lift due to the slipstream depends on a number of factors, such as the area of wing influenced by the slipstream, the forward distance of the prop from the leading edge of the wing. the thrust coefficient, and the forward fuselage shape… all factors that were enhanced for twin engine fighters such as the P-38 and the Bf110. If you take a look at the diagram above for example, not only is the component of thrust contributing to the radial load factor double that for a single engine fighter, but you can see that the area of wing influenced by the slip stream is also significantly greater. For example, if you do the calculations for an aircraft with a Clmax of 1.4 at Mach and Reynolds numbers corresponding to its 1g stall speed of 103mph it shows that at full power the effective coefficient of lift is actually 1.66, reducing the stall speed to 94mph. However, keeping everything else the same, but repeating the calculations for a twin engine configuration the coefficient of lift is increased to 1.86 and the stall speed reduced to 86mph, and that helps to explain why aircraft like the P-38 and Bf110 are so good at low speeds.
Hope that helps…
Leon "Badboy" Smith
