on the subject, Dr. Von Kármán and Dr. Milliken at Cal Tech. I told them what I proposed to do in design, how we intended to compute performance, and our concern about stability and control.
“We don’t know anything different. Go ahead,” they agreed. Dr. Von Kármán had recently delivered a technical paper in Italy on the expectation of compressibility at higher speeds and predicted that shock waves would form on the wing, but he did not develop resultant effects on the airplane itself.
We encountered compressibility, but not immediately.
Our Air Corps project officer for the XP-38 was a pilot, a young first lieutenant, Benjamin S. Kelsey. He was excellent. In those days a project officer with that rank had more authority than many four-star generals do today. If we asked Ben for a decision, we got it—on the spot.
We had trouble before we got off the ground. We had trucked the airplane, under wraps for secrecy, to the Air Corps’ March Field near Riverside, Calif., and reassembled it for first flight. The brakes had been received just the day before, because they had to be qualified first back at Wright Field in Dayton, Ohio, before we could install them. We had loaded the rudder with a 500-pound pedal force and the usual type oflinkage to the brakes.
So, on a bright, sunny morning, Kelsey started up those wonderful-sounding Allison engines. He decided to make a high-speed taxi run. He got up to speed, then stepped on the brakes. No deceleration. He pushed and pushed—in fact he bent those pedals that we had tested to 500 pounds of pressure the night before. Fortunately, he was able to stop the airplane short of the end of the runway.
On disassembly, we found a small residue of grease from the oily rags in which the brake shoes had been packed. That didn’t help the braking power at all. Well, we had located our trouble and were ready to fly. No committee inspections, no review. In those days, when we were ready to fly, we flew. Next day, first flight.
One of the design features of this airplane was the high-lift maneuvering flaps—it was the first fighter to have them. They were a fundamental part of the design and had to work.
Kelsey used a few degrees of flap setting, and the takeoff appeared to be splendid. Then the flap links broke, and all the flaps were sticking above the wing. Kelsey describes the experience:
“We developed wing flutter on takeoff. Looking out, we could see the flaps coming up above the trailing edge, so we retracted the flaps; the flutter stopped, but we had to come in on that first landing without any flaps—which was a little unusual.”
We put a full-scale section of the wing with an actual flap and flap mechanism in our brand-new wind tunnel at Burbank, and that first series of tests proved the value of the tunnel. We found the solution quickly. We had a very fine streamlined aerodynamic gap between the flap and the wing, very sensitive to any small imperfections in air flow. So we just drilled holes in the fairing above the flap to let the basic structure control the air flow; and with air flow stabilized, the flap would not be buffeted. In the 10,000 airplanes that were produced, we never had flap flutter again.
When we finally ran into compressibility later in the airplane’sdevelopment, many people thought it was a case of tail flutter because of the unorthodox appearance of the airplane. Kelsey was our staunch supporter in insisting that the problem was, instead, compressibility. Because the airplane was the first to get high enough and go fast enough to reach Mach numbers approaching the speed of sound—Mach 1—it was difficult to convince anyone that we had encountered this phenomenon.
When the first squadron of P-38s was delivered to Selfridge Field in Michigan, it was Col. Signa Gilkey who first encountered it in accelerated service testing, going through combat maneuvers and other performance tests that would subject the airplane to higher stresses than in
“We don’t know anything different. Go ahead,” they agreed. Dr. Von Kármán had recently delivered a technical paper in Italy on the expectation of compressibility at higher speeds and predicted that shock waves would form on the wing, but he did not develop resultant effects on the airplane itself.
We encountered compressibility, but not immediately.
Our Air Corps project officer for the XP-38 was a pilot, a young first lieutenant, Benjamin S. Kelsey. He was excellent. In those days a project officer with that rank had more authority than many four-star generals do today. If we asked Ben for a decision, we got it—on the spot.
We had trouble before we got off the ground. We had trucked the airplane, under wraps for secrecy, to the Air Corps’ March Field near Riverside, Calif., and reassembled it for first flight. The brakes had been received just the day before, because they had to be qualified first back at Wright Field in Dayton, Ohio, before we could install them. We had loaded the rudder with a 500-pound pedal force and the usual type oflinkage to the brakes.
So, on a bright, sunny morning, Kelsey started up those wonderful-sounding Allison engines. He decided to make a high-speed taxi run. He got up to speed, then stepped on the brakes. No deceleration. He pushed and pushed—in fact he bent those pedals that we had tested to 500 pounds of pressure the night before. Fortunately, he was able to stop the airplane short of the end of the runway.
On disassembly, we found a small residue of grease from the oily rags in which the brake shoes had been packed. That didn’t help the braking power at all. Well, we had located our trouble and were ready to fly. No committee inspections, no review. In those days, when we were ready to fly, we flew. Next day, first flight.
One of the design features of this airplane was the high-lift maneuvering flaps—it was the first fighter to have them. They were a fundamental part of the design and had to work.
Kelsey used a few degrees of flap setting, and the takeoff appeared to be splendid. Then the flap links broke, and all the flaps were sticking above the wing. Kelsey describes the experience:
“We developed wing flutter on takeoff. Looking out, we could see the flaps coming up above the trailing edge, so we retracted the flaps; the flutter stopped, but we had to come in on that first landing without any flaps—which was a little unusual.”
We put a full-scale section of the wing with an actual flap and flap mechanism in our brand-new wind tunnel at Burbank, and that first series of tests proved the value of the tunnel. We found the solution quickly. We had a very fine streamlined aerodynamic gap between the flap and the wing, very sensitive to any small imperfections in air flow. So we just drilled holes in the fairing above the flap to let the basic structure control the air flow; and with air flow stabilized, the flap would not be buffeted. In the 10,000 airplanes that were produced, we never had flap flutter again.
When we finally ran into compressibility later in the airplane’sdevelopment, many people thought it was a case of tail flutter because of the unorthodox appearance of the airplane. Kelsey was our staunch supporter in insisting that the problem was, instead, compressibility. Because the airplane was the first to get high enough and go fast enough to reach Mach numbers approaching the speed of sound—Mach 1—it was difficult to convince anyone that we had encountered this phenomenon.
When the first squadron of P-38s was delivered to Selfridge Field in Michigan, it was Col. Signa Gilkey who first encountered it in accelerated service testing, going through combat maneuvers and other performance tests that would subject the airplane to higher stresses than in
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