Lee Atwood on the P-51, pt 2
Regarding the Mustang, I have always referred to the work of F. W. Meredith of the RAE, whose report (RAE No. 1683) of August 1935, greatly influenced me as chief engineer for North American Aviation to offer the British Purchasing Commission the ducted radiator design configuration in 1940. That report showed how the momentum loss in the cooling radiator could be largely restored when excess cooling air was being forced through the radiator at high speed. As noted before, this involved closing the air exit enough to get a substantial back pressure behind the radiator which largely restored the momentum loss--which was quite large as described above. This was possible, in Meredith's words, because the outlet was "adjusted to suit the speed,o and back pressure was available accordingly.
Here again, while Meredith's analysis was coherent and mathematically instructive, he failed to convey the practical aspects through an example or two, although he did offer a chart showing drag reduction for various discharge area ratios and conditions. The point I am making was that his work was generally in unfamiliar mathematical terms and was poorly understood. In fact, in two cases I know about, it was described in terms of mild ridicule. In any case, some if not most of the designs of wartime aircraft, including the Spitfire, failed to get the full advantage of this available air pump.
It should be pointed out here that the controversy and misunderstanding of the Meredith Effect on the performance of the Mustang developed largely because it was essentially impossible to get a reasonable measure of the effect from wind tunnel models at the time. The mass flow and momentum could not be accurately measured on a scale model, and no large tunnels were fast enough--200 to 400 miles per hour--to get meaningful results.
It has been reported that Messerschmitt made extensive efforts to determine the reason for the low drag of the Mustang, but his wind tunnel measurements did not disclose the restoration of momentum to the radiator cooling air, and most probably could not have done so with the wind tunnel equipment available at the time.
At this point I would like to interpolate what is , to me, a most fascinating element in Meredith's 1935 report. As you may have noted, I have made no reference to the thermal element in the momentum recovery of the radiator cooling air and at the temperatures involved, the air expansion was relatively small and could be neglected. Real jet propulsion, however, involves fuel burning, and the velocity of the gases and heated air is greatly augmented by this high temperature.
In his report, undoubtedly independent of Whittle's jet engine work, Meredith suggests piping the engine exhaust heat and gases to discharge behind the radiator to heat the discharged air just as burning fuel would do. This would have increased the volume and velocity of the discharged air at the same back pressure and increased the favorable thrust force.
Of course, the thrust of the short stack exhausts had been recognized by Sir Stanley Hooker of Rolls-Royce in his book, NOT MUCH OF AN ENGINEER, and others, but Meredith's suggestion might have produced a much more powerful effect, but it involved complications and practical difficulties. As far as I can determine, it was never tried on any airplane.
This brings me to the Spitfire comparison, although that is probably a poor choice of words. That airplane was in a class by itself and at the top level of defense against the Luftwaffe in 1940, and was undoubtedly the most important defensive weapon in history. It was some 1,000 pounds lighter than the Mustang and was at the peak of interceptor efficiency and was essentially in classic conformity with the objectives of the RAF fighter command. It overmatched its opposition and was there when most needed.
In the cold illumination of hindsight, however, and probably for reasons I have outlined above, it missed the opportunity to restore much of the air flow momentum to the radiator cooling air and, with it, a possible speed increment of more than 20 miles per hour. The late Jeffrey Quill, Supermarine test pilot, describes the incorporation of the Meredith Effect in the Spitfire in his book, SPITFIRE, A TEST PILOT'S STORY, and that the radiators were enclosed in ducts under the wings. Here I would like to quote from an article "The Mustang Margin" I wrote for the AIR POWER HISTORY JOURNAL which involves some background and detail on the subject. It will, of course, be glad to try to answer any questions you may have at the end of my presentation.
"The most notable and probably the first application of the Meredith Effect was incorporated in the Supermarine Spitfire, one of the world's most successful airplanes. Over 20,000 were built in various models, but the Mark IX, with the Merlin -61 engine, was typical of the later wartime production, and a sketch of this model with detail of the radiator installation is shown. Two aspects of this design are significant. First, the radiator outlet has two positions--that is, fully open and partly closed--and cannot be progressively 'adjusted to suit the speed.' Second the inlet upper wall is a continuation of the lower surface of the wing and expands the duct cross section by rapidly curving upward.
"The first, the non-adjustable exit, of course, is a deviation from Meredith's dictum and precludes the progressive build-up of pressure behind the radiator with increasing speed. However, the second can only be judged in hindsight, from an airplane design point of view. The inlet seemed to be configured properly to recover the ram air pressure, and the first Mustang design had a similar entry opening. It was later apparent that the thin boundary layer of air flowing along the lower surface of the wing was progressively thickening ahead of the duct opening, and that the flow would break away at a point on the upward curve of the duct wall. While the resulting turbulent unsteady flow apparently did not create a serious vibration, it certainly reduced the efficiency of the radiator and prevented a more complete closure of the exit opening, which is necessary to develop the jet thrust. Very interestingly, the R.A.E. Subcommittee on Aerodynamics in 1936--in commenting on the Meredith and Capon reports--rather accurately predicted this problem: 'Experiments upon air-cooled engines in the 24-foot tunnel have shown that it is necessary to pay particular attention to the design of the entrance to cowlings and the cooling ducts in order to avoid loss of energy by the formation of eddies.' (Somewhat easier said than done at that time.)
"In the case of the Mustang, the duct volume was larger and flow instability more violent, creating an unacceptable vibration and rumble. Resourceful engineers at North American, working with wind tunnel models, overcame the problem by lowering the intake upper lip below the wing surface boundary layer, thus beginning a new upper duct surface. In this design, the flow expanded gradually as the duct velocity decreased, and the pressure at the radiator face was reasonably uniform. This permitted the appropriate closure of the exit with a temperature-controlled power actuator, and a minimum pressure drop across the radiator consistent with efficient radiator function and cooling demand.
"As a result, the cooling drag was estimated at only 3 percent of the total and used only something like 40 horsepower for cooling purposes. While the comparable power used for cooling by the Spitfire is not available to me, the measurements made by Rolls-Royce show a total power required for the same speed (400 mph) as 200 horsepower more for the Spitfire than for the Mustang.
"Records show the P-51D's speed was 437 mph and the Spitfire Mk IX speed was 405 mph. While the Spitfire had exposed tail wheel and other small differences from the Mustang, most of the speed difference was in the cooling drag. The Mark VIII with retracted tail wheel is rated at 414 mph at a somewhat higher altitude. Advanced models of both airplanes with higher performance were produced late in the war, but were not available in significant numbers before V-E Day, May 8, 1945.
"It seems that most other contemporary airplanes attempting to take advantage of the Meredith Effect failed for one reason or another to combine an efficient duct system with a properly designed and regulated exit-closing mechanism and did not develop the energy recovery inherent in the Meredith method. They generally used 10 percent or more of their power available at high speed to overcome cooling drag. A notable exception was the DeHavilland Mosquito multi-purpose plane with the same Rolls-Royce engines and which used a wing leading edge radiator mounting with a short and direct inlet duct. The controllable exit opening had a minimum area little more than half that of the Spitfire, and while it was a larger two-engine airplane, it had a speed of 425 mph.
"Since jet engines do not require cooling systems of the type described here, the subject has become moot and of little current importance. There was a time, however, when this rather insignificant subject made a critical difference."
