In the late 1930s, in the period just before WW-II, designers learned how to dramatically increase the performance of airplanes. Fighters led the way. Cantilever monoplanes replaced biplanes. Greater knowledge of drag reduction plus the advent of retractable landing gear led to the production of aerodynamically clean, low-drag fighter airframes.
At the same time, engines became much more powerful, and turbocharging made it possible for fighters to operate at ever-higher altitudes. All of these advancements dramatically improved the performance of fighter aircraft.
In the early 1930s, the Boeing P-26 Peashooter and the British Gloster Gladiator—both front-line fighters—had top speeds between 200 and 250 mph and ceilings on the order of 20,000 feet. In 1939, the Lockheed P-38 Lightning made its first flight. It had a top speed of almost 450 mph and a ceiling above 30,000 feet. This increase in performance over the Peashooter and Gladiator was achieved over a period of just seven years.
In the process of greatly improving the performance of their creations, designers inadvertently outran their knowledge of high-speed aerodynamics. The combination of higher true airspeed and higher altitude meant that this new generation of fighters operated at a high enough Mach number that the effects of “compressibility” of the air and the formation of shock waves on flying surfaces became very significant.
Some issues were only discovered after the aircraft got into flight test. Several types—notably the P-38—exhibited serious Mach number effects (called “compressibility” effects at the time). While the airplane behaved well in level flight, as airspeed increased in a dive from high altitude, the nose would drop uncontrollably, steepening the dive. The elevators did not have enough control power to raise the nose, and the plane would be locked into a high-speed dive until it got to a lower altitude where the speed of sound is higher. This higher speed of sound caused the Mach number-induced effects established at high altitude to dissipate. It was then possible to recover. The pull-out was still very risky because it was easy to break the airplane by pulling too hard at very high airspeed and overloading the structure.
This “Mach tuck” or “compressibility dive” was dangerous and also hurt combat capability. If the pilot dived too aggressively trying to attack or escape, he could find himself unable to raise the nose and diving helplessly out of the fight. When the Mach tuck problem first appeared in service on the P-38, Lockheed and the Army Air Corps had a major problem. The aerodynamic phenomena causing the uncontrolled dive were poorly understood, and there was no way to change the fundamental design of the airplane to fix it quickly. They needed a solution that would eliminate the uncontrollable “compressibility dive” by giving the pilot some way of maintaining or regaining control if they got into the overspeed condition. That solution had to be something that could be retrofitted to existing airplanes or installed during production without changing the overall design of the airplane.
Dive-Recovery Flaps
The interim solution was an aerodynamic device called a dive-recovery flap. These are split flaps on the lower surface of the wing, with their hinge line at about 30% of the chord of the wing. They look like spoilers but are on the bottom of the wing rather than on top (see Figure 1).
When they open, the air pressure ahead of the flap increases, while the pressure behind it decreases. This change in the pressure distribution produces a nose-up pitching moment. Deploying the dive-recovery flap also produces a small increase in lift and an increase in drag. The combination of nose-up pitching moment and positive lift increment help the airplane recover from a high-speed dive, while the drag increment helped keep the airspeed increase in check.
For a detailed description of the aerodynamics of the dive-recovery flap and a collection of aerodynamic test data, see: NACA RM No. A7F09 “A Summary And Analysis of Data on Dive-Recovery Flaps.”
Dive-recovery flaps were first used on the P-38. Initially, kits were produced that could be retrofitted to existing airplanes. Later, the flaps were installed when the airplanes were built. The designers of the Republic P-47 Thunderbolt learned from the Lockheed discoveries on the P-38 and incorporated dive-recovery flaps into their design.
Dive-recovery flaps were an interim fix for the first airplanes that flew at high subsonic Mach numbers. They disappeared later as designers understood high-speed aerodynamics better and were able to design airframes that had acceptable flying qualities over their entire Mach number envelope. Devices similar to dive-recovery flaps are still used as aerodynamic speedbrakes on some aircraft, often being deployed in combination with upper-surface spoilers.
Interesting and informative as always!
As a kid in the fifties, I remember reading about “mach tuck”, “dive flaps”, and other exotic aerodynamic aspects of late-WW2/Korean War fighters. But I never realized that dive flaps were doubly hinged. What is the purpose of the aft panel? Does it keep the airflow from becoming turbulent? Lower activation forces at high speeds?