Shortly before his death, Lee Atwood (head of the P-51 design team) spoke to an audience at the Yorkshire Air Museum. This is a portion of his address.
PART ONE:
"In 1940 we had a young, energetic, and first-class engineering department with competence in aerodynamics, structures, materials, and thermal technologies as developed up to that time, and the factory had a nucleus of expert machine shop, tooling, and production personnel. We gave the Mustang design credit to Edgar Schmued who led the design room effort and brought the components together under the direction of Raymond Rice, who succeeded me as a chief engineer, and the technical specialists.
All these and many others contributed significantly to the project, including Colonel, now General (Retired), Mark E. Bradley who directed the installation of the 85-gallon fuselage tank. He then demonstrated that the longitudinal instability created by this weight behind the pilot could be managed by the combat pilots, and the effective endurance could be increased by some two hours.
In considering the speed performance of the Mustang, which is really its primary advantage and distinction, it is necessary to adjust one's thinking and point of reference to a rather early period in the science of aerodynamics. In the 1930s, there was no jet propulsion, and by any measure of comparison, the technical resources, personnel employed, test equipment and financial expenditures were really insignificant when compared to the aerospace establishment of today. Of course, the basics were there--which involved derivations of Newton's laws and Bernoulli's hydraulic principles--and aero sciences had been basically defined by Prandl, von Karman, and many other mathematical and scientific authorities, but applications to actual aircraft were relatively crude and empirical. Wind tunnel models were the primary proving element in design, and there were still many elements of such testing that had to be estimated or extrapolated with opinion and hope.
In these circumstances it is not very surprising that, among these early practitioners of aeronautical engineering, there were discontinuities of information and differences of opinion on various fine points in the application of general aerodynamic science, as then known, to actual airplane design. This was most apparent in one of the critical aspects of airplane design during the period of reciprocating engines and propeller-driven airplanes. The liquid-cooled designs favored in England and Germany--and also used in the United States and other countries--were generally considered of lower drag because of their in-line cylinder configuration. Air-cooled engines were generally of radial design, with all cylinders facing the cooling air stream, and the diameter was considerably larger.
The well-known radiator became the automobile standard early on, and everyone in the pre-war era had various experiences with these installations and their belt-driven fans. The common experience usually involved adequate cooling at cruising speeds, with frequent over-heating on mountain grades or slow traffic, and the fans were not always adequate to control the temperature. Generally, most people had an occasional bad experience with an overheated engine.
Airplane radiators had a lot of the same troubles, and while separate cooling fans were not seriously considered, ground cooling from propeller circulation alone was frequently inadequate. Basically, the radiators were designed to cool the engines at full power in a climb--which was usually something like half the maximum possible level flight speed with the same power--so at high speed, the cooling capacity was much more than needed.
Now it is clear that we were then quite sure that, as in an automobile, there was no reasonable dynamic use for the warm air discharged from a radiator, and a low and medium speeds, up to say 200 miles per hour, that was quite true. The temperature rise was small, and the expansion was correspondingly modest, and heat energy recovery was insignificant.
However, as engine power increased and better aerodynamic shapes were developed in monoplane designs, we were all slow to realize that, with a normally ducted radiator at high speed, we had at our disposal a really remarkable air pump.
This air pump, like all pumps, had three elements--a compressor stage, a metering or valving stage (radiator core), and a discharge function through an air outlet. This began to be a considerable pumping action as speeds approached 300 miles per hour--and at 400 miles per hour, it had a large potential and could be a considerable fraction of the airplane's total power equation, since the pumping pressure increases as the square of the speed. To make this automatic pump effective, only one thing was required, and that was to choke the outlet enough to keep the pressurized airflow through the radiator just adequate for cooling and to discharge this compressed air at the highest speed possible.
This intuitively easy to follow and was also logical from a general streamlined design point of view--which all designers tried to follow as a matter of course. The potential magnitude of this effect was more difficult to appreciate, however, and since little or no data were available, these possibilities were overlooked in most cases.
In the case of the Mustang, the air duct pumping system at full speed at 25,000 feet was processing some 500 cubic feet of air per second, and discharge speed of the outlet was between 500 and 600 feet per second relative to the airplane. This air jet counteracted much of the radiator drag and had the effect of offsetting most of the total cooling drag. To offer some approximate numbers, the full power propeller thrust was about 1,000 pounds and the radiator drag (gross) was about 400 pounds, but the momentum recovery was some 350 pounds of compensating thrust--for a net cooling drag of only some 3% of the thrust of the propeller.
This air discharge had what can actually be called a regenerative effect. Maximum aircraft speed is the point where the line of power available, created in the engine and delivered by the propeller, crosses the line of power required to propel the plane through the air. Since the propelling force of the pressurized air from the radiator discharge increases as the square of the speed, we have the favorable situation where the faster you fly the more help you are getting from this regenerative air pumping system.
Since this high speed phenomenon could not be effectively measured by regular wind tunnel model test, it was viewed as ephemeral or even imaginary by many in the engineering practice. Actually, it is quite real and has a close relationship with jet propulsion.
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.
My regards,
Widewing
