KR-netters,
There have been two excellent posts recently concerning full power stalls in the KR. Mark Langford's personal experience of flipping upside down after a hard roll out of a full power stall is not unique to the KR, but the math behind the snap roll says the high-powered short KR should respond this way. While most of us will not see an intentional full power stall in the KR, a more likely scenario is an accelerated stall during a hard turn or maneuver. If you are tail chasing a Glasair or RV around the patch this could be a problem if you are not paying attention to the rumble in the fuselage as it approaches stall. The feeling/sounds are the same as the stall series we all did in Phase I and are distinct from the sound of just having "G" on the plane. For my bird, Goliath, the rumble begins about 10-12 MPH before the calculated stall point and builds in intensity to a heavy rumble as approach to stall deepens. The School of Aviation Safety in their course, Fundamentals of Fixed-wing Aerodynamics covers critical conditions affecting directional stability. Two that are of interest to us in the KR world: "When an aircraft is flying at a high AOA the flow field over the vertical tail is blocked and disturbed. This causes a lost of lift which in turn results in a lower restoring moment. A technique to counter this effect is to add ventral fins on the bottom of the aircraft." I can tell you from personal experience this is why the F-16 has the huge ventral fins. Flying over the top at 40 KIAS in full blower would be impossible without them. "At high angles of side slip an aircraft can lose its directional stability because a large enough restoring force cannot be created. This would cause the aircraft to quickly diverge and depart controlled flight. To counter-act this less than desirable occurrence a dorsal fin is sometimes added to the vertical tail. This device reduces the effective aspect ratio of the tail and increases the angle at which stall occurs." This loss of directional stability is exacerbated by the P factor of the motor and helical prop wash on the aircraft. At higher AOA as the tail becomes more and more blanked eventually the motor is going to win and the plane is going to snap roll. Mark, I am glad you did not prove this in the ultimate manner. In the KR, the trend has been to add a dorsal fin to increase lateral stability. This certainly increases stability in normal flight for most who have tried it. Reducing the aspect ratio on the top of the fuselage would not help recovery from a deep stall or spin as it would be effectively blanked by the fuselage. I have not heard of anyone doing this, but has anyone put a ventral fin on a KR? This would have the effect of increasing stability in normal flight like the dorsal extension and should provide some increased measure of spin resistance. I am considering this addition to my plane this winter as I do the tail chase thing and a good deal of formation flying. IHS, Dave "Zipper" Goodman Vertical Avionics, Inc. www.verticalavionics.com 360 678 1602 (W)
