Ted,
I think I disagree with your definition of angle of attack. I think that angle of attack has to do with the direction of the relative air as it meets the wing as a whole. It is my opinion that our stunt ships have variable angles of incidence rather than variable angles of attack when flaps are deployed.
Al
HI Al,
An interesting observation. One with which, at least in part, I totally agree.
First of all, under all conditions you are absolutely correct that deflecting flaps alters the angle of incidence, irrespective of the relative airflow. To make it clear to all the readers, "incidence" is the angle at which a lifting surface diverges from the "datum" line of a vehicle. The datum is generally the fore and aft centerline about which the airplane was designed and will usually be roughly in line with the thrust line ... not always, but it is a reasonable reference for conventional planforms.
The angle of incidence of, say, the wing (also true of the tail and or canard, if utilized) is the angle at which its chordline differs from the "datum" line. By definition, the chordline of an airfoil is a straight line from the extreme forward location of leading edge to the the extreme aft location of the trailing edge. Generally speaking, the chordline of most stunt ships is parallel to the datum and, with no flap deflection the angle of incidence is, thus, zero. If the airplane has no flaps the angle of incidence will always be zero.
On a flapped ship, any time the flaps are deflected (up or down) the angle of incidence is changed by virtue of the lowering or raising of the trailing edge of the wing (remember, the flap is merely a part of the wing which happens to be "adjustable" ... not a separate surface as many like to suggest). Thus, as Al properly states, the angle of incidence of a flapped airplane is infinitely variable as the degree of flap deflection is changed.
Our apparent difference of opinion isn't really a difference at all, however.
The angle of attack of any surface, flapped or not, is the angle at which the profile chordline of the surface strikes the ambient air through which it is moving. It is true, as Al is postulating, that a flapped surface need not act at a positive angle of attack to create lift. The act of deflecting the flap changes the airfoil from a symetrical configuration to a "cambered" one. A cambered surface is capable of producing positive lift even when in a slightly negative angle of attack. In other words, the zero lift angle can actually be "negative". This is "interesting" to us stunt flyers but not of significant interest because every time we deflect our flaps we also deflect the elevator and, assuming good design and trim the airplane pitches in the desired direction.
It is, nonetheless possible, as I'm sure Al will point out, for a lightly loaded, flapped stunt ship to actually have very little positive angle of attack while maneuvering. That's one of the reasons we talk about a flapped ship being more visually accurate in turning high G manevuers than an unflapped one, because the body angle (think "datum" from the above discussion) won't diverge a great deal from the direction of travel of the entire ship. The unflapped ship "must" have the "datum" at a positive angle to the pitching radius in order to achieve the necessary angle of attack to produce the lift necessary to support the G induced weight of the ship in the maneuver. Thus, the nose will always be at least slightly inside the radius of the track the airplane is following. Very light wing loadings of good unflapped stunt ships minimize this but, no matter how small the angle, it must be there to produce the required lift.
Notwithstanding all the above, however, the act of deflecting a flap on a three or four pound airplane traveling 55 or so MPH in a straight line will produce a positive angle or attack until such time as other aerodynamic forces intercede. For instance, if the pushrod to the elevator were to fail at the instant of the flap deflection, the cambering of the airfoil due to flap deflection will produce a negative pitching moment and the airplane will actually pitch forward. Don't ask me how I know this to be true ... on several occassions over the last fifty or so years.
If, for instance, you haven't done a good job of balancing the lift produced to the lift required for the radius of turn, you might well have an airplane that produces more lift than necessary by virtue of not only the increased incidence of the airfoil relative to our datum but also from a greater than necessary angle of attack relative to the ambient air mass. In the simplest case this could be a ship with a large area, large flaps and excessive flap deflection combined with a tail surface which is too "powerful" (Larger than necessary and with too much deflection). This situation will result in an airplane which "leaps" through corners rather than tracking around the desired radius as it would do if the lift were not in excess of the lift required. The "leap" is the result of the excess lift which is directed at right angles to the chordline and thus literally elevates the entire ship "into/inside of" the desired radius.
The opposite of that situation is a wing which can't produce the lift required despite all the flap deflection and angle of attack it can stand. When that happens the wing stalls. Stalls occur as the result of excessive angle of attack (exceeding the "critical" AOA as we say in the "biz".) All airfoils have a critical angle of attack whether flapped or not. Exceeding the critical angle of attack in your stunt patterns is not a good thing.
So, you can see that it isn't a simple one or the other question. Angles of incidence and angles of attack are related, but different. Almost by definition, a successful stunt ship needs to be capable of a reasonable amount of "excess" lift to insure that stalls are avoided. Thus, almost all will fly at some angle of attack during maneuvers unless they are in that class of ships that have "too much" lift and "accelerate" into corners as a result of that excessive lift. IMHO, ships that fly that way should have the flap travel backed off or (Sparky will shoot me for this) weight added to the CG so that the acceleration into corners is almost, but not quite, eliminated. The "not quite" in that sentence is your insurance against stalls.
The bottom line is that I'll agree with Al that stunt ships always maneuver with significant amounts of "incidence". Incidence is merely a geometric relationship of the datum and the chordline. Whether or not a little or a lot of angle of attack is also present is a more complex question, the answer to which will vary infinitely with wing loading, air speed, air density and the efficiency of the airfoil (its "coefficient of lift" for a given state of camber).
The length of this response probably tells you a lot about why I generalized in earlier responses which didn't seem to require this degree of specificity. Pretty much everyone that flys stunt ships is conversant with the concept of Angles of Attack and the distinction for basic understanding didn't seem worthy of the expanded commentary.
Al's comments, on the other hand, lend themselves well to the expansion. I'm sure he'll have some responses as well. I look forward to them.
By the way, the experiments with flaps on the Imitation design were very illuminating and my comments in the article relate almost directly to this conversation. If any of you have found this discussion interesting I think you'll enjoy reading the article, especially the part about varying the span of the flaps and the effect on the flight characteristics.
The article is available through the AMA/Model Aviation link, I believe.
Ted