1.   Reference is made to Inter-Office Memorandum, P.W. Nosker:ffc: 51 dated 27 June, 1944 and Lt. D. B. Parker:PC:51 dated 10 July, 1944 from Chief, Aircraft Laboratory to Chief, Flight Section requesting that compressibility dive tests be conducted on the P-51D airplane.

            2.   A series of thirty-one dives was conducted by the Flight Research Branch between 3 August, 1944 and 16 September, 1944. These dives included high and low altitude tests and limited stability tests at high Mach numbers. A complete report of these tests is in the process of preparation at the present: however, the necessary information is forwarded so that it may be made available immediately for operating instructions.

            3.   The results indicate that the airplane should be restricted to a Mach number of 0.80 due to compressibility difficulties which become increasingly dangerous beyond that point. It is recommended that the airplane be placarded with the following limit diving speeds:-

Pressure Altitude (Ft.)	Pilot’s IAS (m.p.h.)
40,000	275
35,000	310
30,000	345
25,000	385
20,000	425
15,000	470
10,000	505
5,000	505

            4.   Porpoising.- The P-51D airplane, at high speeds, is subject to the longitudinal instability commonly referred to as porpoising. The results to date indicate that the condition may be induced at a Mach number of 0.70 and above, but may be encountered at somewhat lower Mach numbers at low altitude. It is known that the fabric bulge in the elevator surfaces is more critical at low altitudes and may be related to the airplane’s increased tendency to porpoise at lower Mach numbers in that range.

The/

‘B’ REPORT

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            The porpoising is not a severe condition and can be controlled. In the first place, the condition is usually induced by the pilot, since any ununiform elevator stick force will result in the porpoising at high Mach numbers. Any effort on the part of the pilot to counteract this effect will result in increasing amplitude. It may actually be stopped by holding the stick firmly in one position or, in fact, eliminated by trimming forward gradually to near zero stick forces as the dive is entered, thus reducing the amount of forward stick forces necessary to maintain the dive angle.

            5.   Rolling.- As a Mach number of 0.75 is approached , a slight amount of rolling may become apparent with a simultaneous reduction in aileron sensitivity. This rolling does not become severe, and may be easily controlled.

            6.   Vibration.- At a Mach number of 0.76 a true effect of compressibility becomes evident in the form of a complete vibration of the airplane. This vibration is caused by a combination of compressibility effects on the wing and the horizontal stabilizer. The condition becomes increasingly severe as the Mach number increases and could eventually cause a primary structural failure.

            7.   Maximum Limit of Combat.- The airplane has been dived to a maximum Mach number of 0.85 and on several occasions to 0.84. In each case the pilots reported that the vibration became extremely heavy beyond 0.80. In each dive to 0.84 or above the vibration became so severe that the airplane was damaged. The leading edge skin of the wing flap was buckled between rivets, a coolant radiator cracked and hydraulic line broken due to vibration on various dives to 0.84 and above. In extreme war emergency the airplane can be dived to a Mach number of 0.83 (400 m.p.h. Indicated Airspeed at 25,000 ft.), if a very gradual pull-out is made.

            A relatively low acceleration could result in a primary structural failure if applied during the vibration.

            8.   Entering a Dive.- Relative to starting a dive, no difficulties are encountered in nosing over or in entering from a diving turn, however, extreme care should be exercised in starting from a half roll since high Mach numbers may be reached within a few seconds. Entry of a dive (at rated power) from a half roll above 36,000 ft. would probably result in a primary structural failure.

            9.   Recovery Technique.- Recovery in any case must be gradual and executed with extreme caution since relatively light elevator stick forces or rapid application of trim may very easily result in the application of excessive load factors. As acceleration is applied at the beginning of the pull-out some increase in vibration may occur. This will gradually decrease as the recovery is completed. In no case is elevator trim necessary to aid recovery. The normal pull-out distance stated in Pilot’s Information File apply. The pull-out distances stated in Sec.II, Para.21, page 26 of Pilot’s Operating Instructions AN-01-60JE-1, are not correct.

            10.   Effect of Yaw and Change of Power.- In no case has the airplane tended to “tuck-under” when power was either increased or decreased. The rudders become stiff in the compressibility range, but the airplane exhibits no unusual tendencies when force is applied to either right or left.

            11.   At present the test airplane (P-51D, No.44-143134) is being equipped with a new horizontal stabilizer having a positive one degree angle of incidence and metal covered elevators. A standard P-51B pitot system, a long boom pitot system and additional camera equipment are also being installed for further dive tests.

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JM.	(Signed):-	Mark E. Bradley, Jr.
		Colonel, Air Corps
		Chief, Flight Section

Transcribed by Brent Erickson

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