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AIRPLANE DRAG IN FLIGHT AND WIND TUNNEL



Although Ames engineers, as just noted, developed procedures for predicting the handling qualities of airplanes from wind-tunnel model tests, the prediction of performance factors, such as drag and speed, from such tests was quite a different problem. The test models were usually idealized with smoother, truer surfaces than the originals and lacked the gaps, excrescences, and rivet heads that were found in the actual airplanes. Moreover, there were other influences peculiar to the wind tunnel that were often difficult to evaluate. Such influences included the interference of the struts on which the models were mounted, the turbulence in the airstream, and the subtle effects of the surrounding walls. The rather indeterminate effects of these many factors placed in some question the accuracy with which the drag, and thus the speed, of an airplane could be determined from wind tunnel model tests.

To obtain information on the subject just mentioned, Ames engineers undertook to make a comparison of the drag of airplanes as determined first by model tests in a wind tunnel and then by measurements made on the airplanes in actual flight. The comparison was made for only two airplanes, the P-51 and the P-80, but the results were expected to have general significance. The first airplane selected for the test was North American's new P-51 "Mustang" fighter, an airplane on which Ed Schmued, Ed Horkey, and [82] others of the NAA design group had lavished much attention. The P-51 was a good selection for the test. It was the first of a new class of extra clean fighter airplanes and the first to use the new laminar-flow wing sections developed by NACA. There was still a question of just how much laminar flow one could expect to get in an actual application of these sections considering the effects of propeller slipstream and all the unavoidable surface roughness resulting from conventional, or even refined, manufacturing methods There was also the question of whether the 16-foot tunnel, in which the model was to be tested, was sufficiently free of turbulence to allow the wings of the P-51 model to develop their full laminar-flow potential. These questions, however, merely added spice to the experiment.

A one-third scale model of the P-51, without propeller, was carefully tested in the wind tunnel through a range of lift coefficients and speeds. This phase of the experiment was not unusually difficult. It was pretty much a conventional wind-tunnel test. The problem came in running the flight test-without a propeller. The propeller had been eliminated because there was no good way of measuring the thrust of either the propeller or the engine exhaust in flight, and these uncertain forces would totally obscure the drag of the airplane, the force which was to be measured. So the propeller was removed, the carburetor inlet blocked off, and the whole airplane polished and waxed to resemble the surface conditions of the model. The usual load of special NACA flight-research instrumentation had been installed in the airplane, and this included a sensitive accelerometer that would measure accelerations in the longitudinal direction with an accuracy of 0.01 g. The drag of the airplane would be measured by the deceleration it produced.

With no propeller, the P-51 would have to be towed to altitude and there released to descend along some prescribed path to a dead-stick landing. The NACA pilot, James M. (Jimmy) Nissen, recognized the hazards involved. True, he did not expect to get any special financial reward for undertaking work involving unusual danger-NACA pilots never did-but if he felt any concern over these dangers, it was buried in his great enthusiasm for the project. In any case, the flights were to be made from the Army base at Rogers-more commonly called Muroc-Dry Lake where the maximum opportunity for a safe landing would be provided.

The airplane chosen to do the tow job was the Northrop P-61 Black Widow. It would be connected to the P-51 by means of two long tow cables having at the P-51 end a special release mechanism which Jimmy could operate if he got into trouble.

The whole operation was a very tricky business. The towed takeoff, the climb to 28,000 feet, the release of the cables, and the descent to a dead-stick landing all added to the thrill of the experiment. But everything went off fine. The first flight was completed successfully and so was the second. On the third flight, however, difficulty arose. For some unexplained reason, the cable released from the Black Widow. It flew back and wrapped around the P-51 like spaghetti around a hot dog. Jimmy was in real trouble. Though trussed up like a Christmas turkey, he found he was still able to control the airplane. Gingerly he brought the P-51 down: in fact, he couldn't stop it from coming down as it had no propeller. He landed in a rather rough area, a quarry. The structure of the P-51 crumpled. When the dust had settled, there was Nissen crawling out of his wrecked plane, shaken but hopefully not seriously harmed. He was taken to the base hospital to be X-rayed for broken bones. Unfortunately, the X-ray machine was not working; Jimmy had clipped the powerline to the hospital on his way in for his ill-fated landing. The instrumentation fortunately survived the crash and provided the evidence that was sought. The flight data confirmed to an acceptable degree the results of the wind-tunnel tests.

A description of the whole project is given in TR 916 (ref. A-6) authored by James Nissen, Burnett Gadeberg, and William Hamilton. A foreword was added to the report by NACA Chairman Jerome Hunsaker in recognition of the special contribution made by Nissen.

The hazards to which NACA test pilots were subjected were considered acceptable only if they could not by any reasonable means be avoided. In this case, the whole project had been rushed and a question remained whether' with a little more deliberation, a little more care and checking, the failure of the cable attachment could have been avoided. The lesson learned was reasonably cheap, but it could have been otherwise.

http://history.nasa.gov/SP-4302/ch1.10.htm

On the third flight, however, difficulty arose. For some unexplained reason, the cable released from the Black Widow. It flew back and wrapped around the P-51 like spaghetti around a hot dog. Jimmy was in real trouble

The P-61A was painted standard Army olive drab .

So what was it called before it was the "Black Widow?"