Wind Tunnel Explorations
In 1956 Robert Kirby explored the effectiveness of wings with large-chord flaps in deflecting propeller slipstream downward through the large angles needed for vertical takeoff.
The tests were carried out at the NACA Langley in Virginia on a model situated in the free-flight wind tunnel facility. The span of the model's wings was approximately equal to the theoretical diameter of the propeller slipstream, i.e. 70% of the propeller diameter (24 inches ). Robert Kirby summarized his research on deflected slipstream in NACA Technical Note 3800:
“The investigation showed that it was possible to turn the propeller slipstream 90° so that the resultant-force vector of the wing-propeller combination was normal to the propeller shaft and was 80 percent of the magnitude of the propeller thrust. When the model was near the ground, the slipstream was turned only about 75°, but the resultant force increased to about 88 percent of the thrust. The resultant force was reduced about 10 percent when a fuselage was added to the wing system.” He added in his conclusion, “End plates with an approximately semicircular shape on each wing (defined by the upper surface of the wing with the flaps deflected and a tie between the leading and trailing edge) were essential for obtaining high turning angles and efficiency. Larger end plates showed no improvement in the turning effectiveness of the wing system”.
Other wind tunnel studies of deflected slipstream VTOL mechanics were conducted in 1955 and 1956 by Richard Kuhn and John Draper at the NACA Langley Research Center. They published a number of Technical Notes on the subject for NACA. In Technical Note 3360, Kuhn and Draper discussed their goal:
An investigation of the effectiveness of monoplane wings and flaps in deflecting propeller slipstreams downward is being conducted at the Langley Aeronautical Laboratory. A part of this investigation is reported in references 1 and 2. The results of reference 1 indicate that a monoplane wing equipped with plain flaps and auxiliary vanes can deflect the slipstream through the large angles approaching the angles required for vertical take-off.
Kuhn and Draper concluded from their research:
On the basis of tests with flat plates of various chords, the best turning angle was obtained with a ratio of wing chord to propeller diameter equal to 1.00, which was the largest ratio investigated; however, increasing the ratio of wing chord to propeller diameter from 0.75 to 1.00 led to only a small improvement in turning effectiveness but caused a large increase in the diving moment.
This reference to a "diving moment", meaning pitching forward of the model as it approached the ground while hovering indicated one of the challenges facing the construction of full scale prototypes that used the deflected slipstream principle.
Read more about this topic: Deflected Slipstream
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