To AussieEngineer. Check the graph on the site porvided by MustangFLyer for the difference in roll rate for the P38 without and with power boost on the ailerons.

There is one graph on the site from MustangFlyer that compared P38J with and without boost to P47D and P51B, but am unable to find it now. There is another that simply compares the P38J with and without boost. It indicates a roll rate of 90 deg. in just under 0.5 sec. with boost. The best roll rate indicated without boost was 90 deg. in about 1.8 sec. Note that roll rate more than tripled due to the aileron boost.

I've seen the one with the no boost vs boost. I find that one odd in that it does indicate a roll speed of ~200 deg/s at 400 mph, which is higher than the peak roll speed of Fw-190. The other thing is that it rolls so much faster than the no-boost P-38, you can see the verticle line on the no-boost curve that shows where it was no longer maximum aileron deflection. I would have expected no-boost and boost to be very similar up to that point.

I suspect different test regimes. The other one is the time to roll 90 degrees. The NACA one is contineous rolling rates at a given stick pressure and speed. I suspect expect most planes to be very quick to 90 degress then be slower, overall, for a complete roll. Just the AoA of the wings will cause differential forces, plus any trim , rudder, elevator effects. I know the Mustang was very quick to 90 degress, I just touched the stick and it was on its ear, seemed awfully fast to me (didn't time it, just went "Whoa"). We need to read the NACA study in more detail and determine how they did the study. I'd say they had the more scientifuc approach and were more reliable for overall comparisons.

To AussieEngineer: Perhaps the alterations included in installation of the boosted controls increased the available deflection of the ailerons??

To MustangFlyer. First time I got into that site I saw a roll rate chart for the P38 with and without boost, conpared to a P51B and a P47D. Haven't been able to find it when I went back. Could you direct me to it?? The NACA graph was discussed earlier. I suspect it has a lot to do with 'lightness' of the controls. At the indicated pressure, the Spitfire would certainly be at max. roll, but some of the others (for example the F6F which was notoriously 'heavy') would still have a good deal left. The Wright Field tests were specifically for maximum roll rates. I believe that the Wright Field facility did most (if not all) of the actual test flying for NACA at the time, but I could be wrong about that.

Will do, I'll chase it up later, they've been adding a lot of stuff recently and changing the format. The NACA tests (after reading the whole document) were a specific set of tests to endure a baseline for comparison. Level flight (as diving can affect roll rate as well as yaw .. re late Me109s). 5 rolls, constant stick pressure (50lb), no rudder or trim tab correction. Basically a controlled experiment. Interesting, I didn't know how many things could affect roll rate, so many factors are involved. It does confirm my earlier hypothesis, a contineous roll rate will be slower than just an instantaneous 90 degree turn, especially for a good pilot who can coordinate their stick and rudder well. As you roll through the clock all these other things start to come into effect (AoA, dihederal, design and hinge type of aileron, rudder size, etc, etc, etc). The different characteristics of rolling in a dive are very interesting, things like yaw become very important, to avoid over stressing the airframe. In the SpitfirePerformance website there are the some comments on this for the late model Me-109s, where aileron use is not advised in high speed dives because of yawing. Still amazing how taking a few feet off a wing can change things so much, especially since this was an in the field job, not something major. I think the 50lb force amount was chosen as it was about an average limit for most pilots, especially if you had to repeat the manouver over any period of time. Of course this was exceeded in combat, especially the good (and strong) pilots who could coordinate controls well and prevent yawing. Any yawing (as well as being potentailly dangerous) would slow the contineous rate of roll. A single flick roll by a good pilot would probabaly be faster than those figures for all the planes involved, though you would expect the differentials to remain reasonably similar (rudder effectiveness would probably become a bigger factor then). Interesting about wing twist, the p47 lost 36% of its roll rate though twist at high speed reducing aileron effectiveness, the Spitfire 65%, just shows you don't get something for nothing. At a particular point the wings will twist enough that you will get aileron ineffectiveness then actual aileron reversal! Happened to all planes of the time, some resisted it better (e.g. P47 & Mustang). The price was lower mach limit, greater weight, etc, etc. Complex thing this aerodynamics, no easy answers, just different compromises. Perhaps the big improvement in the clipped wing spit had something to do with reducing wing twist over and above the duction in the moment of inertia and rolling drag.

Larry here is the P38 90 degree roll rates. http://www.spitfireperformance.com/p-38/p-38j-roll.jpg I should add that the NACA report says that they measured rates at all sort of different stick forces and deflections. The chart only covers 50llb and full deflection.

Could well be. The fairly minor span reduction seems to have had a disproportionate effect. Note that the roll rate for the '109 on AEs chart is for 66 lbs pressure, so at least that much increase must have been available. Probably a case of NACA using data on hand for 50 lbs and got some German data for 30kg. The F6F was considered quite agile, but needed a LOT of muscle on the stick to get the performance. Most pilots flew combat with both hands on the stick, with an occassional grab at the throttle. I would think that the P38s lack of torque effect would be a positive factor in roll as it should have reduced the yaw effect of rotation.

I think that I'd mentioned before that there is an old engineering adage:"No such thing as a free lunch." Anything you do to improve performance of one sort is going to cost you somewhere else. If an a/c had one or two outstanding qualities, tactics could maximize for them, whatever they were.