Bi-Fli (oz823)

About this Plan

Bi-Fli. Early RC stunt model. 47in RC Stunt .15+, 3-chan escapement bipe, Phil Kraft, Oct 1959 MAN.

Update 24/11/2019: Added article, thanks to MikeFoster.

Quote: "Full-scale biplanes are no longer produced except for special purpose types such as Grumman's new crop duster. Their relatively low efficiency clue to the higher induced drag of two wings has relegated this oldest aircraft type to rapid extinction. However, for our purposes, the lower lift-drag ratio is not a disadvantage and, from the theoretical standpoint, the biplane seems to be the most logical approach to RC stunt. There is little question that the biplane is the most maneuverable type of aircraft. Virtually all of the full scale airplanes used in stunt competition were biplanes.

The reason for the biplane's inherently superior maneuverability lies primarily in its shorter moments and concentration of mass and area which reduce the effects of inertia. Thus, the biplane can respond more quickly and smoothly to control movements. Besides maneuverability, the biplane offers greatly increased stall resistance. This is because the rear wing is generally set at a slightly lower angle of attack than the front. In a stall, the rear wing retains its lift longer than the front and since it lies aft of the center of gravity the nose of the airpla'ne will drop to regain flying speed without the abrupt loss of lift and sharp dive associated with monoplanes. Ground handling, take-off, and landing characteristics are also excellent. From the model standpoint, the reduced overall dimensions of the biplane provide a stronger, lighter aircraft which is easier to transport.

Theory and practice do not always coincide, but the apparent advantages of the biplane layout prompted the design of the Bi-Fli. A great deal of thought went into the design in an attempt to produce a maneuverable yet stable, smooth-flying model suitable for any type of control from rudder-only to multi-channel equipment. For flight smoothness, a moderate wing aspect ratio (6-1) and fairly long tail moment with plenty of stabilizer area has been used. Logically, the two wings should be the same size, but for appearance sake the lower wing was designed 10% smaller. While free-flight experience has taught that an unduly scientific approach to airfoil selection is rather senseless, the NACA 23015 section was selected because its characteristics seemed ideal as to center of pressure travel, etc. More important, the section looked 'right.' Negative stagger was seriously considered, but the more conventional positive stagger was decided upon to simplify landing gear installation.

The first Bi-Fli weighed only 37 ounces and was powered with the K&B .09. The ship was a beautiful flier, living up to all expectations, but it was a bit slow due to the very light wing loading..."

Supplementary file notes

Article.

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