Design primer - stability (part one)
 

Aerodynamic Stability
1) Rigging for Positive Stability.

First, a definition. Rigging, when speaking of an airplane, includes the incidence angles of the wing(s) and horizontal stabilizer, the incidence difference between multiple wings of a biplane or triplane, also correctly referred to as decalage, and to a lesser extent the horizontal displacement of the leading edge of the vertical fin, and the engine thrust line. Collectively referred to as the rigging.

This discussion will not include the rigging, just the aerodynamics of a plane properly rigged for positive stability.

Wolfgang Langeweische, in his book “Stick and Rudder,” said that power controls altitude, elevator controls speed. In an airplane rigged for positive stability, and trimmed for level flight at a given power setting this can be used as an effectively true statement. Increase the power with no other changes, the plane climbs, decrease the power and it descends. Even with no knowledge of the aerodynamics that sounds logical, and it’s easy to accept. The other side of the coin is a lot harder. Push a little down elevator, and while there will be a slight dive, the speed will come up and the plane will come back to level flight at a higher rate of speed. Conversely, pull a little up, and again, there will be a slight climb, but level flight again will be attained, this time at a lower speed. That’s the hard one to accept.

It’s all caused by the way the airplane is rigged. And for we who play with R/C, this is mainly the trainer type aircraft. Aerobatic planes will be a later discussion.

The wing, at a given angle of attack, (AOA) will support the plane in level flight at one particular airspeed. Any slower and it descends, any faster and it climbs. So when we increase power without changing the angle of attack we climb, decrease power, down she goes. To go faster in level flight, the AOA has to decrease, to go slower, the AOA has to increase. And this is where it gets tricky

The horizontal stabilizer is not a lifting surface. It actually pushes the tail down, working against the wing.

Why, you will ask, would we want to waste some of our lift that way? The early designers would have asked the same question, because they did have lifting surfaces, and flying the early planes wasn’t much easier than balancing a needle on a pin point. If you are ever offered the opportunity to fly a Curtiss “June Bug,” a “Wright flyer,” or similar airplane decline with thanks.

But back to the horizontal stabilizer. We have seen that the main wing needs a set airspeed to maintain level flight at a given angle of attack. By having the center of lift behind the center of gravity, the plane has a natural tendency to drop the nose. When it does the air speed picks up, the increased air flow over the stabilizer generates more down thrust , the tail drops, and the plane assumes level flight with the nose weight and the down force of the stab balanced. So we cut the power, the plane slows down, we lose some of the stab’s down force, the plane noses down until the airspeed comes back to get more down force, and we are descending at the same airspeed we had in level flight. Now add power. The airspeed increases, the down force increases, the tail goes down and we are climbing. The rate of climb will increase until the airspeed reaches the point where, once again, the down force equals the nose weight, and we have a stable climb. All at the same airspeed. So, by changing the power level we have not made a lasting change in air speed, we have merely changed from level flight, to a descent, and back to a climb. All at the same airspeed.

N.B.There are exceptions to the stab with downforce, notably free flight models that do use a lifting stab, but the exceptions are outside the purview of this discussion, we ‘re going to ignore them.

So you now ask, how do we change air speed?

Once you understand the relation of power to rate of climb this part is a LOT easier. If you don’t understand that part you can continue, maybe this part will give you the insight you need, or you can go back and study the last part some more.

Remember the wing? At a given angle of attack (AOA) it will generate enough lift for level flight at one particular air speed. How about changing the AOA? If we decrease the AOA it will need a higher airspeed to support the plane. Conversely, a higher AOA will support the plane at a lower air speed. So, if we want to go faster, how do we decrease the AOA? Right. Push in a little down elevator. We lose some down force, the nose drops, the airspeed picks up until the down force stabilizes things again, level flight at a higher airspeed. To fly slower use a little up elevator. Yes, we get more down force, the nose goes up, and when things stabilize again we’re in level flight at a lower airspeed.

So now you see it. Power controls altitude, elevator controls speed.

Yes, I know that’s an over simplification, but let’s all get this down before we get into complexities. Besides, we still need to look at roll and yaw stability, as well as thrust line matters first. This is enough for one lesson.

I welcome any and all comments pointing out my errors in this, amplifying my statements, or adding something I have forgotten.

Bill.