Since modern airplanes have been attaining very high speeds at altitude, compressibility effects on performance have necessitated new design considerations.
Important changes in flight characteristics during a dive in which compressibility effects are encountered are
By designing compressibility dive flaps for the fast P-47 Thunderbolt, Republic Aviation engineers have provided means to counteract these undesirable changes and afford safe pullouts at critical speed and high altitude.
With the advent of heavy wing loading and good aerodynamic cleanliness, fighter planes have attained dive speeds approaching that of sound. It is to be expected that all airplanes will run into difficulty that is, exhibit unusual flying characteristics at such speeds. The ratio of airplane speed to sonic speed is known as the Mach number, and the same true airspeed corresponds to different Mach numbers at different altitudes since speed of sound decreases from 763 mph at sea level to 666 mph at approximately 35,000 ft, above which the speed remains constant.
Air flowing over a curved body experiences an increase in speed over that of the undisturbed air. Thus it may happen that although the speed of the body is not that of sound, the speed at some local point on the surface has attained this speed. Accordingly, the steadiness of the air adjacent to the airfoil breaks down, and there develops what is known to the aerodynamicist as a compression shock. The airplane speed at which this occurs is different for various bodies or airfoils, and th ratio of this speed to sonic speed is known as the critical Mach number, and is of great importance in all compressibility effects.
With the establishment of this shock wave, the flow over the wing, body, or tail surface changes in such way as to cause rearward movement of resultant loading over the surface, an increase in drag of the surface or body, and a material change in the width and degree of turbulence of the wake.
Because of increased diving moment which results from rearward movement of the center of pressure on the wing when compressibility shock occurs and the rearward movement of the center of pressure on the elevators, a greater force is required to pull out of a dive. In a steady dive at high Mach number it is likely that the horizontal tail surfaces will be in the thicker wake of the wing and therefore be subjected to buffeting. For the ailerons or control surfaces in which there is any tendency of the balance portion to protrude into the free stream, the resulting change in loading over this portion may be such that rapid reversal of stick force occurs.
As a result of high speed and high wing loading, the following phenomena which are direct effects of compressibility have occurred in high-speed high-altitude dives with the P-47:
These are indications that compressibility is occurring over the airplane's ailerons and that there is a possibility of the same existing at the tail.
It is important to note that speed is an extremely important factor in pulling out of high-speed dives. Flight test records on the P-47 show that whereas the pilot may run into difficulty at altitudes between 23,000 and 37,000 ft, in every instance known, the airplane is controllable at about 12,000 ft if the following diving recommendations are observed:
As the airplane loses altitude it recedes from the compressibility regime and the trim tabs become very effective for a very short period of time 2 or 3 seconds.
Had the pilot, during the maneuver and while the airplane was subjected to compressibility shock, trimmed the plane more tail heavy in an unsuccessful attempt to pull out, he would be confronted by a dangerous tail heavy condition when the craft reached lower altitude and compressibility shock cleared. The pilot would then have insufficient time to correct this over-trimmed condition and the pullout would be made at excessively high load factors, probably resulting in structural damage to the plane and physical injury to the pilot.
Power should never suddenly be reduced as this would appreciably change the stability of the plane and might put it into a steeper dive. However, it will be necessary for the pilot to adjust his throttle gradually so as to maintain, within reasonable limits, the condition with which he entered the dive. This is necessary because power tends to increase with and increase in speed due to the increase in RPMs.
All pullouts should be made at an altitude above 10,000 ft, since the altitude lost in coming out of the dive is very great on fast, heavily loaded planes.
Although recovery may be safely attained in dives at critical speeds if the procedure previously described is followed, nevertheless inherent tactical disadvantages result in the maneuver. An example: When diving sharply on an enemy plane which is flying at an altitude between 25,000 - 37,000 ft, the attacking pilot may pass the enemy plane without inflicting damage and should want to resume an advantageous position above it, and as soon as possible.
But if the dive were made at critical speeds, with recovery not possible until 12,000 ft were reached, valuable time and altitude would have been lost if the pilot desired to resume the attack.
Also, since altitude is lost so rapidly during the dive the human element suffers, bringing with it uncertainty in the proper correlation of controls required for recovery.
These conditions are solved by the dive flap installation, which permits a controlled pullout at a critical Mach number while still at a high altitude. The increased diving moment, which would ordinarily result from rearward movement of the center of pressure on the wing when compressibility shock occurs in the dive, is more than offset by the dive-flap action, which allows immediate recovery at a high altitude, holding G's to a safe limit in pullout.
One dive flap is located under each P-47 wing just outboard of the wheel well. Constructed of .188" 24ST sheet, 40" × 6", each flap is hinged (at forward edge) to the wing, and is limited in travel to an angle of 21° ± 1°.
The flap installation consists of:
When the switch in the cockpit is moved to either UP or Down, as the case may be, current is sent through the Up or Down limit switches in the wings to the recovery flap relay in the switch panel box. This, in turn, sends a higher current tot he actuating motors.
The magnetic brake and clutch in the motors are released electrically, and the motors operate to move the flaps up or down. When the flaps reach the fully up or down position, limit switches are actuated, breaking the circuit to the relay, which in turn breaks the circuit to the motor. As the current ceases to flow through the motor assembly, the brake is applied to prevent overtravel and slippage.
The cockpit switch has no Off position and must be left in either Up or Down position.
This "For Better Design" article was originally published in the February, 1945, issue of Aviation magazine, vol 44, no 2, pp 223-225.
The original feature includes 4 detail drawings and an electrical schematic.
Drawings are not credited.