7 October 1941
Burbank, California, USA
A light knock on the side of the open door roused Kelly Johnson from his reverie. He looked up to see Hall Hibbard waiting for him. Putting aside the latest report from the manufacturing engineers—they were still having issues getting the dies to properly align on the P-38 line—he acknowledged the boss and could only hope he was not bringing more bad news.
“NACA finished their report on the P-38.” Hall came into the room and dropped a manila envelope on Kelly’s desk. “It’s not tail flutter causing the issue.”
“I told the Air Corps repeatedly that the Atalanta’s tail is too stiff for flutter!” It had been an on-going argument with the Air Corps and their pilots who seemed to be convinced that the diving problems with his beloved P-38 were from the tail flexing around. After their Lieutenant Putnam had been killed in the crash of number 699 in June, the Air Corps brass was convinced the twin-tail design was too unstable. Kelly had tried explaining to them that between the solid aluminum skin of the airplane and the internal structures of the booms the tail was too rigid to suffer flutter. They had even tried to force him to add external weights to the elevator even though the elevators were already internally balanced. Indeed, once of his engineers was working that exact thing just to prove to the Air Corps it makes no difference. Now, hopefully, they will not have to have to waste any more time on it. He was pleased to hear from Hall that NACA had vindicated him. “So, were they able to figure it out?”
“Take a look.”
Kelly opened the envelope and began scanning the document. The first few pages were standard introduction and description of the study and its methods—nothing too critical for detailed review at that moment. When he reached the third page, however, he stopped, re-reading it several times.
“…the local speed of sound will be reached in the wing-fuselage fillet at speed of 404 miles per hour…the entire region between the booms will…be subject to the accompanying compressibility separation.
“When the critical speed of a wing is reached and shock occurs on the upper wing surface, the flow separation which results is accompanied by a decrease in wing lift, a sharp increase in wing drag, and a strong diving tendency…The diving tendency is contributed to both by the increase in negative moment of the wing and decrease in lift…”
“Compressibility?” Kelly wondered. He had attended Theodore von Kármán’s--of the California Institute of Technology--lecture at the Aerodynamics session the previous January about aerodynamic compressibility at trans-sonic speeds but it was theoretical and thought to apply mostly to propeller limits. The idea of the whole airplane flying fast enough to run into compressibility was entirely new.
He continued reading on into the fourth page.
“The joint occurrence of tail buffeting and the diving tendency on the P-38 airplane appears to be satisfactorily explained…as an effect produced by attainment of critical speed over the entire section of the airplane between the booms…”
He repeated the last part aloud, “’…attainment of critical speed over the entire section of the airplane between the booms.’”
Hall smiled, “It looks like your Atalanta is faster than we thought and NACA has a way to get more than the fifteen knots the Air Corps wants.”
Kelly continued scanning the NACA report. “From their wind-tunnel testing it looks like they are estimating an additional sixty-four miles per hour with this point-two cee leading edge extension.”
“I’ll leave you to it. Review it a few times and grab your design team tomorrow to run the numbers. I want your recommendations by Friday. This is high priority but don’t let it stop the assembly tooling work. That’s still number one.”
4 November 1941
Burbank, California, USA
Johnson’s design team had been hard at work re-engineering the center section of YP-38 #689 to incorporate some of the NACA recommendations. After the initial review of the NACA report they elected to stage the improvements in various phases so as to limit the production line impact until the line was fully operational.
First they had implemented the wing filleting along the gondola and the nacelles. Early test flights, with strict profiles and hard limits on the dive speeds, seemed to bare out the NACA estimation on reduced vibration. With the help of the manufacturing engineering team they were able to get these added into the current production line a week earlier so the new P-38E’s that would soon start rolling off the factory floor would already have this improvement and the others would have them added in the field.
The next hurdle was the big one: the wing re-design and moving the radiators. Working with fabricators on the ground and a representative from Allison they were able to cobble together a hasty modification useful for proof-of-concept. Through Allison, they were able to acquire several low profile coolant radiators intended for P-39 production as well as oil radiators from the same. These were then mounted on hand made braces extending forward from the main spar of the inner-wing section of 689. Once the radiators were installed, the engine mechanics had the work to re-route all of the coolant and oil lines appropriately while the fabricators cut off the old radiator housings on the booms and hastily riveted some sheet aluminum over the resulting gaps and adding some faring over the old oil cooler inlets in the forward nacelles. Meanwhile, the designers were figuring out the best form for the leading edge extension and producing the drawing needed for the bracing and skin forms as well as updated filleting to accommodate the new wing.
With the news that the productions issues were nearly all cured, Johnson’s team now had more resources—and more pressure—to get the NACA modifications completed and tested. With the new wings and radiators installed on 689 now, Lockheed had ordered flight tests as soon as possible.
* * *
Lockheed test pilot Ralph Virden walked around the airplane on the tarmac at the Lockheed Air Terminal. The bright aluminum of the streamlined fighter was broken only by the red-white-and-blue stripes on the tail surfaces, the black anti-glare of the upper nose and inner-nacelles, and the blue and red roundels with their large white stars of the U.S. Army Air Corps adorning the wings.
He had already flown the airplane a dozen times but only in its original configuration. Now, looking at it without the radiator bulges on the booms and with the narrow radiator slots in the new leading edge he could not help but marvel at how sleek the airplane was as it sat hunched in front of him, nose pointing up as though it were anxious to be in the sky.
After his pre-flight walk-around checking the control surfaces and pitot tube, Ralph ducked under the left boom and came up to the small ladder extending from beneath the rear of the gondola. He always felt this was a poor way to get into an airplane. He needed to clamber up the couple narrow rungs, reach over the rear of the fuselage to the handle rising above it, then contort himself to get his right leg up over the trailing edge of the central wing. Using just the one leg, with his knee nearly in his chest, and the little handle he then hoisted himself up onto the wing—this time scraping his left knee as he raised it. Cursing from the pain, he lifted the handle and turned it down to raise the ladder then ensured it was fully stowed and locked in place—he made the mistake once of not doing so and had the ladder drop during his roll-out.
Once he stepped down into the cockpit and sat himself in the high backed seat the beauty of the arrangement became apparent. The seat was high in the cockpit with the wings and fuselage well below shoulder height the visibility was phenomenal, broken only by the canopy frames. He noticed that the extended leading edges blocked more than a few degrees of downward visibility compared to the previous wing, hoping it would not prove to be a problem during taxi.
The instruments and cockpit controls were unchanged from his previous rides which made his checks easier. After adjusting his seat and the pedals he started on the checklist the engineers had provided, methodically double checking each setting. He did not live to be a forty-three year old test pilot of high-performance military aircraft by cutting corners.
The airplane was connected to a remote battery cart for start-up so there was no risk of draining the internal battery while sitting on the apron so he ensured the battery was OFF. Then it was checking the fuel selectors, testing the fuel booster pumps and the low fuel warning lamps, and checking the pressure in the oxygen system. Then he had to set all the engine and propeller controls as indicated, turn the generator ON, and set fuel to use RESERVE. After getting all the miscellaneous items set appropriately it was time to get the engines started.
Ralph closed the cockpit hood and cranked up the right-side window. He called the ground crew out the open left window, “Turn them over.”
Two of the men assisting on the ground manhandled the Curtis Electric propellers through a few rotations each. While they were doing this, Ralph took the opportunity to double check the booster pumps and fuel pressure in the tanks. When the men backed away he selected the left engine on the hand primer and gave it a few pumps.
His eyes met those of the man to the left and he raised one finger, “Ready number one.”
The two ground crew stood by, one with a fire extinguisher at the ready and the other further forward. Once he indicated to Ralph that it was clear, Ralph set the ignition master switch ON and moved the left ignition switch to BOTH. With his left hand reaching past the yoke he pushed the starter toggle to LH with his middle finger and hovered over the engage toggle with his index finger. His right hand was on the engine primer pump ready to work.
The engine gave an electrical whine as the inertia starter began turning it over. The pitch of the sound climbed higher and higher as the starter revolutions built until it was a steady hum. Then, Ralph mashed his index finger forward to engage the left engine.
The electrical whine dropped as the starter engaged the crank, meeting sudden resistance in the mass of the 1,710 cubic inch V-12. The propeller started to spin and the cylinders sparked. Giving a few slow pumps to the primer the fuel flowed into the carburetor and the engine exploded into life with a muffled roar. Ralph quickly released the starter switches and pushed the fuel mixture lever all the way forward to AUTO RICH. A cloud of brown-black smoke spewed up from the exhaust outlet on top of the boom as the oil that had been sitting in the cylinders quickly burned away before turning into the familiar blue-grey of a clean gasoline burn.
Feeling the engine smooth out, he glanced at the oil pressure gauge and watched the needle turn until it was steady just above 60 pounds per square inch and made sure his voltmeter was registering current from the generator before priming the right engine. He held two fingers up to the ground crew and they hurried over to the other side of the airplane. Once they indicated they were set, Ralph turned the ignition on the right engine and started the engine.
After both engines were running smooth and they were up to operating temperatures he switched the battery ON and gave the hand signal to disconnect the battery cart. Once the airplane was clear, he completed all of the pre-flight checks and ran the engines up. With the engines back at idle he motioned out the window for the ground crew to remove the chocks before fully sealing the cockpit.
Taxiing was pretty similar to his previous experience, the reduced visibility from the new wing having little impact on his ability to follow the apron and taxi-way to the end of the runway. He wrote a brief line on the notepad strapped to thigh, “TAXI – GOOD.”
With the all clear from ground control and aligned straight on the runway he rolled his feet forward on the brakes, pushing them as far forward as he could. He moved the propeller controls to 3000 RRM and pushed his throttles to MILITARY. He watched the manifold pressures climb on each engine as they revved up. 35 inches…37 inches…39 inches... At 40 inches of mercury he released the brakes.
Old 689 leaped forward, slamming Ralph deep into his seat padded only by his parachute. The airplane sped down the runway, its airspeed quickly increasing. It only took about half of the 3600 foot runway before Ralph eased back the yoke, rotated the plane up, and left the ground.
With the gear up and altitude and speed increasing, Ralph slowly turned the airplane back over the airport, circling as he climbed. After five minutes he switched the tanks from RESERVE to MAIN and began preparing the tests the engineers back in Burbank had planned for him.
* * *
The first tests were pretty standard stability and flight dynamic evaluations at various altitudes and engine settings. Those were mostly just to confirm the 689, with its new wing, still behaved like the rest of the P-38’s in normal flight. The engine tests were a little more delicate as Ralph discovered the engine coolant was running a little hot with the new radiators. His note read “RAD. INSUF. ENG. 240+” Radiators insufficient, engines hotter than 240 degrees Fahrenheit. He was able to lower it a little by switching back from AUTO LEAN to AUTO RICH and opening the manual radiator flaps all the way but it was still making him a little a nervous.
Once the coolant temperatures came down, though, at 25,000 feet he was ready for the test that everyone was waiting for: the dive.
Increasing to max RPM and full MILITARY power, he said a quick prayer and nosed the airplane over.
At a sixty degree down angle his altimeter spun rapidly in the direction pilots never like to see as his Indicated Air Speed turned the other way. Ralph Virden was not an ordinary pilot though.
He stayed with the dive as his IAS climbed. He was indicating 260 at 25,000 feet when he started the dive which is right around 400 miles per hour. At 22,000 feet his instruments indicated his speed as 325—480 miles per hour. This was it, the moment they were all anticipating. Here was where they had always locked up before; but, 689 kept carrying Ralph faster and lower. As the altimeter passed 20,000 feet and the IAS climbed past 350, Ralph tested the controls and was relieved to find his dive shallowing up a few degrees as applied back pressure on the yoke. He slowly pulled out of the diving, leveling off at just below 17,000 feet and indicating 380 and dropping.
Once comfortable with the stability, he opened the radiator flaps again to cool off the engines and started a slow circling climb back up to 25,000 feet as he scribbled on his notepad: “D 1: A:250-200, IAS:260-350. GOOD”
Then, he dove again.
Over the next fifteen minutes he made a series of four dives. On the last dive he finally started to feel the nose drop and the controls lock up at 13,000 feet and an indicated speed of nearly 435 miles per hour. As soon as he felt the nose tuck he throttled back. His air speed continued to slowly climb and the nose dipped a few degrees farther while his windscreen started to fog over.
Ralph then saw the airplane as though he were watching from the airport below. He saw the streak of silver shudder and roll, locking into the dive. He could picture the tail arch before giving way completely, the remainder of the airplane spiraling out of control to become nothing more than fiery wreck as it slammed into the ground.
He shook the vision from his mind and took a breath remembering Major Gilkey’s recovery last spring. The altimeter dropped to 10,000 feet and he started turning the elevator trim tab. The thicker air was working and the airplane started to slow and the nose started to come up. Finally, after a breathless eternity he was able to regain full elevator control at 5,000 feet. His biceps strained as he began pulling back on the yoke with all his strength and 689 started to level off. At 3,500 feet he decided to call it a day—one close call was enough.
Back safely on the ground 10 minutes later he did a quick calculation: he peaked at nearly 550 miles per hour before the airplane tucked under him. Kelly Johnson would be thrilled. His wife, on the other hand, would not need to know about this!
