Ed - The need to apply right rudder in 3 control airplanes when operating at higher angles of attack is primarily due to P factor.
P factor is the effect of the fact that at higher angles of attack, the descending blade of the prop has more effective pitch than does the ascending blade, creating thereby more thrust from the right side of the prop disc (as viewed from the cockpit) than the left side of the prop disc produces. In European engines, most of which rotate backwards to American engines, the pilot applies left rudder when in higher angles of attack. Jerry E. -----Original Message----- From: [email protected] [mailto:[email protected]]on Behalf Of Ed Burkhead Sent: Monday, March 29, 2010 7:07 AM To: 'Hartmut Beil'; ety Subject: RE: [ercoupe-tech] Funny Tales About Ercoupes I wrote: > Putting the vertical stabilizers outside the slipstream keeps > the slipstream from causing trim changes when going from > idle to full power. Hartmut wrote: > The Ercoupe had no rudder trim. So that could not have been the reason. Sure it could have been the reason. Planes are normally designed to fly straight in cruise. In some planes, the vertical stabilizer has a tiny cant, as if there were a tiny bit of right rudder. This counters the spiral slipstream hitting the left side of the vertical stabilizer and rudder and it's tweaked to be just right in cruise. At low airspeeds and high power, there's more effect of the spiral slipstream and this pushers the tail to the left, hence the pilot applies right rudder to compensate. Thus, these planes have a "trim" change in yaw from idle to full power and need pilot controllable rudders to counter this. In designing the Coupe, Fred Weick wanted to eliminate this misbehavior. He did this by putting the vertical surfaces at the ends of the horizontal stabilizer, outside of the slipstream. This is effective for Fred's purpose of a well behaved airplane. I suspect it was nearly essential for development of a well behaved two-control airplane. Hartmut wrote: > most of my information is from the book "The Ercoupe" by > Stanley Thomas. Do you have the book? If not, I recommend > reading it. Awesome details. > > Thomas also speaks about the small "stall strips" at the wing root, > added not for stalling purposes, but for creating an extra air burble > in a slow flight configuration. That burbled air hits the elevator > and is needed to keep elevator authority at slow speeds with a 13 > degree restriction. The tested prototypes had a lesser restriction > on the elevator. [emphasis added - Ed] Please recheck the bottom of page 47 of "The Ercoupe." You'll find this: "Third, the loss of lift in the center of the wing reduced the downwash on the tail, thus reducing the effectiveness of the elevator in forcing the tail down and increasing the angle of attack." [emphasis added - Ed] Hartmut wrote: > That indicates that the split elevator is the ultimate design in > Ercoupe elevators and should be added where ever possible. Agreed. However, I don't think it's so important that I'd put it ahead of other safety improvements. I can't remember his exact phrasing, but when I asked Fred what he thought of the split elevator, he said it was effective in doing for the elevator what the Coupe tail did for the vertical stabilizers and rudder - get it out of the slipstream and reduce the effect of the slipstream on the behavior of the plane as the power changes between idle and full power. Fred said he didn't think of it himself but he wished he had and he was complimentary towards Sanders for thinking of it. Ed
