Let's take a look at some basic aerodynamics and some reasons your plane flies the way it does. First, everyone has an opinion as to what a "good flying airplane" is. I don't plan to argue opinions on that subject because I probably have different ideas of a "good' flying airplane-although they may claim to - it is what YOU like that counts. What I'm going to talk about is the "why" to RC flight. I'll also throw in a few ideas on design, since I like to design and fly my own designs.
Did you ever wonder why you see so many symmetrical airfoils in RC? Full scale planes, except for the Laser type competition planes rarely use them. I think there are two reasons. One is that we fly inverted more than any full scale pilot. The experienced RC pilot doesn't think anything about flying upside down or about doing outside loops. Symmetrical airfoils work either way, upright or inverted. For the next reason, think about where your throttle is most of a flight. A large percentage of us stay at full throttle for most of a flight. Have you ever heard of a full scale pilot using full throttle for anything except take off and maybe climb? Of course not. The airfoils on a full scale plane were designed for the airspeed and attitude it was intended to fly. At cruise power, it was meant to be at its most efficient attitude and fly level with level trim. So what do we modelers do? We take this scale model and overpower it and fly inverted. Now we complain when it climbs at full throttle and when we have to hold a lot of down when inverted. It's no wonder that a lot of designers and manufacturers use non-scale airfoils which are either symmetrical or a very close semi-symmetrical. These just work better for the way the majority of us fly models.
Take the Sig V4 scale Cub, for example. One of the guys in my club, French Brown, has one. His is powered by an OS 90 2-stroke, more than enough power. The Cub has a typical old flat bottom, thick airfoil and a flat stab, most probably set at zero incidence. The plane easily takes off at half power, or less. After take off, you throttle back or climb like mad. I you want to go fast, you have to use a lot of down trim. This is OK, but you must remember to retrim before landing or you'll have to hold a lot of back stick on final approach.
Examine the set up of the Cub. It has a flat bottom airfoil which produces more and more lift as speed increases. The flat stab does nothing to compensate. And don't forget the location of the engine. It is below the wing, trying to swing the nose upward like it was on the end of pendulum. Naturally, it heads for the sky when you open the throttle. Keep the power down and fly "scale like," - have you heard that before - and it cruises along with neutral trim, just like the real one.
If you look at the real Cub and at some of the 1/3 scale models, you'll find that the front of the stab, the aluminum tube, is adjustable up and down, for a small amount. This allows you to adjust the stab incidence (angle relative to a reference line down the fuselage) to compensate for more or less powerful engines. If you are going to use a lot of power, you raise the front of the stab a slight amount. Now, when you add power and gain speed, the stab lifts the tail and points the nose downward, compensating for the increased lift. Pretty clever, those designers, huh?
You may think that raising the leading edge of the stab
and trimming the elevator down both accomplish the same thing. It
appears that this should be true, but it isn't. At least in some cases.
They are definitely not the same when you are using other
controls, especially the rudder. On pattern planes in knife edge flight,
having the elevator trimmed down can cause a fairly good pitching
movement, due to a partial blanking of the stab. Having the stab
set correctly for level flight, will not. Let me put it this way. If you
can adjust both the stab and the elevator, you can usually (nothing
is always) get rid of the pitching moment in knife edge. With
elevator alone, you usually can't. More on basic aerodynamics
next time.