General-Aviation Configurations

As part of its General Aviation Spin Research program in the 1970s, Langley included the development of a testing technique using powered radio-controlled models to study spin resistance, spin entry, and spin recovery during the incipient phase of the spin.[521] Equally important was a focus on developing a reliable, low-cost model testing technique that could be used by the industry for spin predictions in early design stages. The dynamically scaled models, which were about 1/5-scale (wingspan of about 4-5 feet), were powered and flown with hobby equipment.

Although resembling conventional radio-control models flown by hobbyists, the scaling process discussed earlier resulted in models that were much heavier (about 15-20 pounds) than conventional hobby models (about 6-8 pounds).

The radio-controlled model activities in the Langley program con­sisted of three distinct phases. Initially, model testing and analysis was directed at producing timely data for correlation with spin tunnel and full-scale flight results to establish the accuracy of the model results in predicting spin and recovery characteristics, and to gain experience with the testing technique. The second phase of the radio-controlled model program involved assessments of the effectiveness of NASA-developed wing leading-edge modifications to enhance the spin resistance of sev­eral general-aviation configurations. The focus of this research was a concept consisting of a drooped leading edge on the outboard wing panel with a sharp discontinuity at the inboard edge of the droop. The third phase of radio-controlled model testing involved cooperative stud­ies of specific general-aviation designs with industry. In this segment of the program, studies centered on industry’s assessment of the radio – controlled model technique.

Direct correlation of results for radio-controlled model tests and full – scale airplane results for a low wing NASA configuration was very good, especially with regard to susceptibility of the design to enter a fast, flat spin with poor or no recovery.[522] In addition, the effects of various con­trol input strategies agreed very well. For example, with normal pro-spin controls and any use of ailerons, the radio-controlled model and the air­plane were both reluctant to enter the flat spin mode that had been pre­dicted by spin tunnel tests; they only exhibited steeper spins from which recovery could still be accomplished. Subsequently, the test pilot and flight – test engineers of the full-scale airplane developed a unique control scheme during spin tests that would aggravate the steeper spin and propel the airplane into a flat spin requiring the emergency parachute for recovery. When a similar control technique was used on the radio-controlled model, it also would enter the flat spin, also requiring its parachute for recovery.

Some of the more impressive results of the radio-controlled model program for the low wing configuration related to the ability of the model to demonstrate effects of the discontinuous leading-edge droop concept that had been developed by Langley for improved spin resis­tance.[523] Several wing-leading-edge droop configurations had been derived in wind tunnel tests with the objective to delay wing autorotation and spin entry to high angles of attack. Tests with the radio-controlled model when modified with a full-span droop indicated better stall character­istics than the basic configuration did, but the resistance of the model to enter the unrecoverable flat spin was significantly degraded. The flat spin could be obtained on virtually every flight if pro-spin controls were maintained beyond about three turns after stall.

In contrast to this result, when the discontinuous droop was applied to the outer wing, the model would enter a very steep spin from which recovery could be obtained by simply neutralizing controls. When the discontinuity on the inboard edge of the droop was faired over, the model reverted to the same characteristics that had been displayed with the full-span droop and could easily be flown into the flat spin. Correlation between the radio-controlled model and aircraft results in this phase of the project was outstanding. The agreement was particularly noteworthy

in view of the large differences between the model and full-scale flight Reynolds numbers. All of the important stall/spin characteristics dis­played by the low wing, radio-controlled model with the full-span droop configuration and the outboard droop configuration (with and without the fairing on the discontinuous juncture) were nearly identical to those exhibited by the full-scale aircraft, including stall characteristics, spin modes, spin resistance, and recovery characteristics.[524]

While researchers were conducting the technical objectives of the radio-controlled model program, an effort was directed at developing test techniques that might be used by industry for relatively low-cost testing. Innovative instrumentation techniques were developed that used relatively inexpensive hobby-type onboard sensors to measure control positions, angle of attack, airspeed, angular rates, and other variables. Data output from the sensors was transmitted to a low-cost ground-based data acquisition station by modifying a conventional seven- channel radio-control model transmitter. The ground station consisted of separate receivers for monitoring angle of attack, angle of sideslip, and control commands. The receivers operated servos to drive potentiome­ters, whose signals were recorded on an oscillograph recorder. Tracking equipment and cameras were also developed. Other facets of the test technique development included the design and operational deployment of emergency spin recovery parachutes for the models.

One particularly innovative testing technique demonstrated by NASA in the radio-controlled model flight programs was the use of miniature auxiliary rockets mounted on the wingtips of models to artificially pro­mote flat spins. This approach was particularly useful in determining the potential existence of dangerous flat spins that were difficult to enter from conventional flight. In this application, the pilot remotely ignited one of the rockets during a spin entry, resulting in extremely high spin rates and a transition to very high angles of attack and flat-spin attitudes. After the "spin up” maneuver was complete, the rocket thrust subsided, and the model either remained in a stable flat spin or pitched down to a steeper spin mode. Beech Aircraft used this technique in its subse­quent applications to radio-controlled models.

General-aviation manufacturers maintained a close liaison with Langley researchers during the NASA stall/spin program, absorbing data produced by the coordinated testing of models and full-scale aircraft. The radio-controlled testing technique was of great interest, and following frequent interactions with Langley’s test team, industry conducted its own evaluations of radio-controlled mod­els for spin testing. In the mid-1970s, Beech Aircraft conducted radio-controlled testing of its T-34 trainer aircraft, the Model 77 Skipper trainer, and the twin-engine Model 76 Duchess.[525] Piper Aircraft also conducted radio-controlled model testing to explore the spin entry, developed spin, and recovery techniques of a light twin-engine config­uration.[526] Later in the 1980s, a joint program was conducted with the DeVore Aviation Corporation to evaluate the spin resistance of a model of a high wing trainer design that incorporated the NASA-developed leading-edge droop concept.[527]

As a result of these cooperative ventures, industry obtained valuable experience in model construction techniques, spin recovery parachute system technology, methods of measuring moments of inertia and scal­ing engine thrust, the cost and time required to conduct such programs, and correlation with full-scale flight-test results.