I think you're the guy to ask... I am struggling with the Wattage Extra 330L ARF. In case you are not familliar with it, it is a small electric plane with 200 square inches of wing and a target weight of 24 oz.

From : Don Stackhouse

OUCH! I've been designing indoor models too much lately! It's tough for me to conceive of trying to support a pound and a half on only a little more wing area, and probably not much more span, than what we use to support only a small fraction of a pound on one of our RK models!

However, that weight is unattainable with any sort of pack capable of flying the plane. Wattage, for example, recommends an 800 mAH NiMh pack, but it draws upwards of 15A static and I think that will toast a NiMh pack of that size in short order. Mine came in at 28 oz. with an 8 cell CP-1300 pack.

Again, OUCH!

In flight, any sudden up elevator can trigger a porpoising that can make it difficult or impossible to control. I have moved the CG forward from the factory recommended location, and it lessens the effect but it is still there. I think it is simply too heavy at 28 oz.... with a 200 square inch wing area, that is 21.5 oz/square foot.

My question is.... can CG changes really cure the porpoising, or is this a phenomenum related to the stall characteristics of the wing and angle of attack.

And is there anything else to try... like a slight aileron droop? Remember, this is an ARF so things like wing/stab incidence are factory set and have not been changed. There is visible down and right thrust. Flown fast, it is fine. But sudden changes to angle of attack can get.... interesting.

Dynamic stability depends on a number of things. It's the result of the balance between the inertia forces that try to keep the motion going during an oscillation (the mass of the plane, and especially how far from the C/G that mass is distributed), vs. the forces trying to stop the oscillation, mainly the aerodynamic forces on the tail.

Another factor is the amount of static stability. This determines how strongly the airplane tries to return to the trimmed angle of attack after a disturbance, and therefore how fast the mass of the airplane is moving about the C/G as it approaches that setpoint angle of attack. This is why moving the C/G aft can help. By reducing the airplane's desire to return to the trimmed pitch attitude, it doesn't build up as much kinetic energy for the tail to have to damp out as it passes throught the setpoint in the middle of each oscillation.

If you move the C/G all the way back to the neutral point, the oscillations will stop altogether, but the airplane will be very unpleasant to fly. Every time you try to put the pitch attitude at a new position, it will overshoot badly, and you will probably get into PIO's (Pilot Induced Oscillations) trying to compensate. This is not a good solution.

The real problem here is too much mass distributed away from the C/G, and/or too little tail area and tail moment. The mass is building up more inertia than the aerodynamic forces on the tail can damp out.

First, look at the structure and the big masses in the airplane, particularly the battery. Is there a lot of weight in the tail structure? Is the battery spread out for a considerable distance along the length of the fuselage? If you can take some weight out of the tail, re-configure the battery into a short brick right on the C/G instead of a long stick with its weight spread out, etc., you can reduce the overall mass and also concentrate the remaining mass closer to the C/G. This reduces the plane's inertia about the pitch axis, which makes the tail's job of damping out those inertia forces easier.

The other side of this issue is the tail size. Dynamic stability is linearly proportional to tail area, and proportional to the SQUARE of the tail moment arm. In other words, a 10% increase in horizontal tail area increases dynamic pitch stability by 10%, but a 10% increase in tail moment arm increases dynamic stability by 21% (i.e.: 1.10^2 = 1.21). This is because the increase in tail moment arm not only increases the tail's leverage, it also increases the tail's vertical velocities during the oscillations, which increase the tail's lift forces that are trying to damp those oscillations.

When you moved the C/G forward, you increased the tail moment arm, which improved the dynamic stability. However, in doing so you also increased the plane's static pitch stability, which means that it tries to come back to the trimmed attitude more aggressively, which increases the amount of energy the existing pitch damping has to damp out. These two effects are offsetting. You saw an improvement, indicating that the beneficial effects of the longer tail moment arm on damping were the bigger of the two, but much of the improvement from that was offset by the effects of the increased static stability.

In your case, you probably don't have a lot of options with this particular airplane. First, try to reduce the inertia as much as possible. The thing that would make the most improvement, lengthening the tail moment arm, is probably the most difficult thing to do from a structural standpoint. If you can't fix the problem by putting the plane on a diet, your next best move is probably an increase in tail area.

One other thing to double check is your control linkages and hinges. If they're not completely stiff and slop-free, then the elevator can move in trail a bit while these oscillations are occuring. This reduces the elevator's contributions to dynamic stability, which has the same effect as reducing the effective tail area.

In full-scale aircraft, they talk about "stick-fixed" stability vs. "stick-free" stability. In stick-free stability, the control surfaces are allowed to float during the oscillations, so that essentially all the damping forces come from the fixed portions of the tail.

If the controls are held rigid (i.e.: "stick-fixed"), then the aerodynamic forces of the control surfaces is added to the damping forces from the fixed portions of the tail, so the stability is greater than in the stick-free" mode. In an R/C model with good, stiff linkages, the servos continually hold the controls in what amounts to a "stick-fixed" mode.

Good luck, you'll probably need it! Don Stackhouse
DJ Aerotech