WIND TUNNELS OF NASA

 

Chapter 4 - Propellers to Jets: The Impetus of World War II

Tailspin: The Pilots' Terror

 

[41] The most dreaded type of tailspin is the " flat spin" in which the aircraft whirls to the ground out of control like a maple seedpod. Fear of tailspin was justified in the early days of flight. Many pilots died fighting their all but useless controls in a vain effort to recover. Even today, approximately 10 percent of all military aircraft accidents and 25 percent of the fatalities are attributed to stall and spin accidents. During the period 1965-1971 some 250 U.S. military planes were lost in such mishaps. Considering the cost of aircraft and the incalculable value of lives lost, it is far cheaper to thoroughly check out aircraft in wind tunnels before introducing them to military operations

The NACA began its spin research in the 1920s, employing three techniques:

1. Drop tests of aircraft models from high buildings.

2. A 5-foot vertical wind tunnel in which typical models were subjected to rotation tests.

3. Flight tests of various full-scale aircraft.

The objective was to provide aircraft designers with criteria with which they could get an early qualitative feel for whether planes on their drawing boards would have acceptable spin-recovery characteristics.

[42] Such rule of thumb guidelines were of course no substitute for the testing of the final models in wind tunnels or for flight testing of the actual aircraft.

NACA ultimately built a series of two freespinning vertical tunnels at Langley. A 15-foot diameter spin tunnel was placed in operation in 1935. Langley's present 20-foot Free Spinning Tunnel began its work in 1941. In all these tunnels, air is drawn upward through the test section by a fan at the top. After passing through the fan, the air circulates through turning vanes that direct it down into an annular return passage and up through the test section again. The current 20- foot free-spinning tunnel has a 1300-horsepower motor that provides 100 foot per second air in the test section. Spin tunnels are extremely simple. No provisions need be made for mounting a model or measuring aerodynamic forces. A rising column of air is all that is needed.

The researcher launches the aircraft model into the rising air by hand from a platform. A flick of the hand imparts a spin and, as the model spins downward, the operator increases the tunnel wind speed until the model's fall is just balanced by the uprushing air, like a circling hawk buoyed by rising thermals. Then the control surfaces of the model, which are...

 


Cross section of the Langley 20-foot spin tunnel

 Cross section of the Langley 20-foot spin tunnel. Air flows up through the center and down the annular space between the test section and the building walls. Models are launched into the ascending airstream by hand in Frisbee fashion.

 


image of time sequence

 Time sequence of an aircraft in a spin. A free spinning tunnel can study the regime of the developed spin and the initiation of recovery.

 

...driven by tiny electric servo-actuators, are activated electromagnetically to initiate recovery from the spin.

During World War II, every fighter, light bomber, attack plane, and trainer-over 300 designs-had to be tested in Langley's spin tunnels. Subsequently, over half of these aircraft were modified in some way to ensure that their controls would be able to pull them out of a spin.

After the war came the jets with their small swept wings and long heavy fuselages. The whole spin recovery picture changed with these bulletlike craft. A set of spin recovery rules had to be evolved in the spin tunnels. But a new problem had arisen. Because of their small sizes, the spin models often exhibited aerodynamic characteristics quite different from their full-scale prototypes. A full-scale spin tunnel to solve this problem was out of the question. But a small spin model could be modified locally (via wing leadingedge radius, fuselage strakes, vortex generators, etc.) to make it behave as if it were of larger scale. The Ames 12-foot pressure tunnel was uniquely suited for [43] this task, for it could span the Reynolds number test range from model to full-scale flight. This unlikely combination of facilities solved the problem of model scale and reinforced the validity of the free-spinning model technique.

The value of spin tests can be illustrated with two modern aircraft, the F-4 fighter and the variablesweep F- 111 fighter-bomber. In the former, spin tunnel tests demonstrated that two types of spins were possible: a steep, nose-down spin from which recovery was easy and, second, a deadly flat spin. The steep spin had occurred occasionally in peacetime service but posed no real problem. The flat spin was not encountered until the plane entered combat in Vietnam, when more severe maneuvers and inexperienced pilots combined to create conditions unanticipated by the plane designers. The Langley spin-tunnel team quickly found a new piloting technique that allowed pilots to recover from this kind of spin..

Focusing on the variable-sweep F-111, Langley spin tests found an uncontrollable type of spin in the designs first tested. This spin occurred only under flight conditions well outside those expected during actual service. Nevertheless, Langley aerodynamicists provided the manufacturer with data for an emergency spin recovery parachute and recommended that it be incorporated on test aircraft. The manufacturer (General Dynamics) did install the chute on the aircraft scheduled to perform stall tests. It was a fortunate decision because in flight tests the aircraft did enter this uncontrollable spin mode, and both plane and pilot were saved by the chute. A design fix later eliminated this type of spin.


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