OK Don, ... let me ask you a question to finish up something that was on my mind since you last commented on the Sky Crawler (Model Aviation - May 2001), when during your discussion (and unbeknownst to me at the time) someone mentioned that the elevator a two degree neg. incidence relative to the wing and the whole discussion jackknifed (I got >confused).

Would there be a difference if you take that two degree and applied it to the wing as opposed to the elevator? Relative to each other, there would still be a two degree incidence difference.

From : Don Stackhouse

In terms of the resulting airspeed, and assuming that the fuselage is slender enough that we can ignore its effects on flight trim, yes, those two situations would be nearly identical. When you say "Relative to each other, there would still be a two degree incidence difference", what you're referring to is what we call "decalage", the difference in incidence between the wing and the tail.

Decalage, in combination with the C/G location, elevator deflection, flap deflection, prop effects, aerodynamic pitching moments of the wing, tail and fuselage, vertical locations of various components (this has some influence on aerodynamic interactions, but the main effect is on the vertical locations of the drag and/or thrust of the various components, and the net effect that has on pitch trim), and probably a few other factors I've left out, determines the angle of attack at which the wing flies.

In terms of the airplane's flight attitude, there WOULD be a difference. Incidence alone does not determine the wing's angle of attack. The stuff I listed above does that. Note that incidence is conspicuously absent from that list?

So what does incidence of the wing alone determine? The wing and the tail (plus all that other stuff above) determine where the wing flies, and then the incidence between the wing and the fuselage gives us the angle of attack of the FUSELAGE.

In your example, changing the incidence of the tail only will result in the fuselage AND the wing ending up at a higher angle of attack. Changing the incidence of the wing (not the tail) will cause almost the same change in the wing's angle of attack, but the resulting angle of attack of the fuselage would be two degrees lower than in the other case.

Where this gets tricky is that the prop and thrust line move with the fuselage. Even if the fuselage's angle of attack itself has no measurable effect on overall pitch trim, the effects of the difference of two degrees of down thrust could easily be significant. The spinning prop acts like a combined vertical fin and horizontal stabilizer. When you change the angle of attack of the prop disk, you get sideways forces in the plane of the disk that act just like a tail surface in that location would. In addition, the component of the thrust that acts perpendicular to the flight path influences the total lift that the wing has to generate. All of these effects will show up in the final angle of attack of the wing, and the resulting airspeed after the change.

So , I guess the answer to your question is "Well, ALMOST the same."

So, if I understand it right, with the elevator centered and you give the wing a two degree positive incidence, everything being equal, would the plane should rise?

It will fly at a higher angle of attack. Whether it rises or not depends on power and on the overall aerodynamic characteristics of the airplane. In most typical powered flight cases it probably will. If you're already flying at max power and just barely above stall speed, no, it's probably not going to rise.

Again, everything being equal and giving the elevator a two degree negative incidence, it would rise too, right?

Same as above, although the exact amount of rise or descent might be a little different, as discussed in the previous part.

Am I wrong, sorta right or, should I forget airplanes and start collecting Barbies?

No, you're doing fine. Whether or not you should start collecting Barbies is a purely personal decision, and I'll leave the answer to that completely up to you! In my case, I am interested in full-scale flying and soaring, full-scale and R/C sailing, bike riding, horses, history and archaeology, watch repair, acoustic guitars, and playing and building violins/fiddles, among other things. However, I'm careful not to let any of that interfere with my R/C flying! ;-)

Don Stackhouse
DJ Aerotech