Over many beers and margaritas on our trip to OSH, a question came up about the horizontal stab/elevator under different configurations and exactly what is going on. To summarize: standard introductory textbooks show that in most aircraft, the CG is forward of the main wing CL, thus causing a "pitch down" moment which is counteracted with the HS/Elevator's downward-directed "lift" vector.
We all know that loading an aircraft near the aft CG limit reduces the amount of elevator down force required, thus reducing induced drag and (theoretically) allowing for increased velocity.
The question is: on an RV, does the elevator ever generate *upward* lift (assuming one keeps the CG in the envelope)? Or is only a reduction in the downward "lift"?
My understanding was that it was always downward, due partly to the CG-CL relationship and partly to the downwash on the tailplane, but the counterargument was that because the angle of incidence of the RV HS is 0 degrees, any amount of downward elevator beyond neutral means that the HS is now generating UP-ward lift. I submitted that this can't be true unless the CG >= CL (which should be a longitudinally unstable situation).
I'm a systems engineer, not an aero, so I'm sure there are lots of factors involved here...but hoping someone who is can help us settle an Oshkosh "bar bet"
We all know that loading an aircraft near the aft CG limit reduces the amount of elevator down force required, thus reducing induced drag and (theoretically) allowing for increased velocity.
The question is: on an RV, does the elevator ever generate *upward* lift (assuming one keeps the CG in the envelope)? Or is only a reduction in the downward "lift"?
My understanding was that it was always downward, due partly to the CG-CL relationship and partly to the downwash on the tailplane, but the counterargument was that because the angle of incidence of the RV HS is 0 degrees, any amount of downward elevator beyond neutral means that the HS is now generating UP-ward lift. I submitted that this can't be true unless the CG >= CL (which should be a longitudinally unstable situation).
I'm a systems engineer, not an aero, so I'm sure there are lots of factors involved here...but hoping someone who is can help us settle an Oshkosh "bar bet"