26 February 1942
Burbank, California, USA
Being Lockheed's best test pilot for powered dives, Ralph Virden was once again tasked with putting the completely modified 009 through its paces. In the pre-flight planning he had gone over the airplane with the engineers in detail, reviewing and examining all of the changes. The last time he flew it, three weeks earlier, it had only recently been giving the new sliding canopy but was otherwise much the same as it had been in his earlier flights. Now, they had completed the final round of modifications to it which made it essentially identical to what would soon be manufactured as the P-38F.
The biggest change was in the engines. Allison had finally gotten their supply of new F5 engines figured out and Lockheed had completed fitting them to 009 over the past few weeks. The new engines promised to be more powerful with a Military Power rating of at least 1325 horsepower compared to the F2 engines 1150 horsepower. The increased power was largely thanks to higher boost settings permitting up to 47 in/Hg in Military Power. The engine was still “Normal Rated” at 1000 hp and 38 in/Hg manifold pressure, same as the old engines.
The exact upper power limit and the amount of boost was an unknown as the other big change with the airplane should enable even more than the 47” of boost that Allison and the Army estimated. That change was the new AiResearch core-type inter-coolers mounted under the engines. These should improve the charge air cooling and allow higher boost with less fear of detonation from high Carburetor Air Temperature.
The full power tests and boost ratings would be performed on behalf of the Army in April after the first few production P-38F's rolled off the factory floor. For now, Ralph was taking the airplane to do what what he did best: diving.
Ever since the NACA recommendation were developed they had not yet reached their true limit. First was the partially modified YP-38 which tested the wing adequately but didn't have the gondola and canopy modifications. Then was came the flight when the left turbo exploded which prevented Ralph from completing the dives. Since then they had been more focused on general testing and trying out individual systems. This would be the first full dive test designed to discover the absolute Mach limit of the airplane.
Due to the previous problems, the flight test engineers and Ralph agreed that all of the dives would begin at 30,000 feet and proceed until the airplane ran into compressibility with an altitude floor of 15,000 feet. The thought was that by starting in the thinner air above 26,000 feet the airplane should be able accelerate to its maximum more quickly and in the event of a control lock Ralph would still have enough altitude to recover.
Ralph took 009 through the first two dives at a shallow angle and with the throttle only at 42 inches and 2600 RPM just to get a feel for the new engines. The B-2 turbos could only maintain 42” up to about 25,000 feet after which the manifold pressures began to drop with altitude so that at 30,000 feet the plane was only pulling 36.5 inches, or about 22% more than sea-level pressure. This suited Ralph just fine as it avoided over-taxing both the engines and the turbos and the steady increase in the rated power as the dive descended ensured a smooth acceleration throughout the maneuver.
The next series of dives were performed under full power at 3000 RPM. Again, due to the limits of the turbochargers he kept the throttle in the 42” position (partway between Normal and Military power settings) until he was below 25,000 feet at which time he pressed the throttle controls fully forward. The airplane underwent a dramatic acceleration as the boost pressure increased until about 22,500 feet when the pressures held steady at 47”, 57% more than the normal air pressure at sea level.
With each dive he reached progressively greater speeds. The on the sixth dive, the airplane's acceleration dropped at 21,000 feet and Indicating just over 415 mph. Carrying the dive lower as the airspeed crawled up, Ralph felt the first signs of shudder a few hundred feet lower at nearly 421 mph. Pulling back on the throttles, Ralph slowly brought the plane out of the dive, the indicated airspeed continued to rise as he dropped into the thicker air but the shudder went away and the plane pulled level smoothly and without drama.
He decided to push harder in at a deeper angle to try to reach the limit sooner with the next several dives. During a slow climb back up to altitude under cruising power he ran a few quick estimates on his notepad. If he could reach an indicated speed above 400 at 22000 feet he would have the airplane at around the same True Airspeed. That should give him enough altitude to push the Mach limits all the way.
Dive seven started as the others except he was nosed over to negative 45 degrees. He watched the Airspeed Indicator closely as it spiraled up. Again he could feel a momentary pause in the acceleration followed by the start of a shudder in the elevators. The airspeed read 407 mph and the altimeter was at 21,800 feet and dropping. He rode it out.
At 19,600 feet and IAS of 432 mph the shudder started to become more violent and the nose began to tuck, just like it did with the old airplane. Not wanting to risk it becoming unrecoverable, he retarded the throttles back to 30 inches and dropped the RPM to 2280. Using his elevator trim he slowly brought the nose up a few degrees as the airplane slowed until the shudder stopped and he pulled the yoke back to level the airplane at 15,700 feet.
His eighth and final dive of the day repeated the seventh but he reached the start of the nose over at 20,200 feet while indicating 427 mph. Recovery was the same and Ralph felt confident that he was finally able to hit the dive limits of 009 after the last three dives.
* * *
Later, Ralph wrote up his report and took the time to compile the data from all eight dives, typing them up in a table. He was sure the Army, and Kelly Johnson, would be please with his results (although he suspected it might be time to request a new typewriter from the boss).
