Someone asked why the Sopwith Camel was so unstable?

I belong to a WWI modelers email list. These guys are primarily static modelers but to allow us RCers to be a part of the group. Some one asked why the Sopwith Camel was so unstable. It was reputed to turn very quickly to the left and not to the right. Also it had a tendency to pitch down with sudden changes in power. (I think this is the correct directions...it's late!) I think a lot can be explained by a rotary engine on an aircraft that is short coupled. I understand the gyroscopic principles of rigidity in plane and the effect of a torque being applied showing up 90 degrees in the direction of rotation.

Several folks tried to relate it to CG which I know isn't the answer. Having a CG that is too far forward or aft has a whole different set of problems! I just don't know quite how to explain what happens with short coupled aircraft. I know how to treat RC models with this problem. But not how to explain why these behaviors exist. Could you help me out?

From : Don Stackhouse

Actually, C/G is a BIG part of the answer, but there are a number of other factors as well. Like many WW I aircraft, the Camel was badly tailheavy, which resulted in a reduction in static pitch and yaw stability.

The effects of C/G on dynamic stability (the ability to damp out oscillations) are more complicated, but the Camel wasn't very good in that regard, either, mainly because the tail surfaces were too small. The additional moment of inertia about the pitch axis because of the biplane layout (having the weight of one wing down low and the other at quite a distance above it does the same thing to inertia about the pitch axis that having a lot of weight at the nose and tail, far from the C/G, would do) adds to the problem. In short, there's a lot of things that can keep an oscillation in pitch or yaw started, but not much tail authority to make the oscillations stop. The pathetically small fixed area on the fin/rudder made the problems in yaw even worse. A pilot generally doesn't hold the rudder pedals stiffly enough for the rudder to contribute very much to yaw stability. In full-scale design it comes up in the category of what we call "stick-fixed" vs. "stick-free" (or in this case, "rudder-free") stability. The Fokker Triplane was even worse in this regard, with no fixed vertical fin area at all and a huge aerodynamic counter-balance on the front of the rudder. The Triplane's almost total indifference to which way it was pointed in the yaw direction was legendary. The situation on R/C models is a bit better in this regard, since a servo with a good linkage can hold the rudder stiffly enough for the rudder to act like a fixed fin.

Then there's the gyroscopic precession you alluded to. When you try to make a gyroscope tilt in one plane that's parallel to its axis of rotation, it reacts by trying to tilt in another plane that's exactly 90 degrees further around its axis of rotation. In the case of the Camel, with approximately 400 pounds of gyroscope hanging on the nose, a turn to the left would cause a gyroscopic precession moment that would try to raise the nose. A turn to the right would shove the nose down. In a right turn this acts like down elevator. If you roll into a steep bank and shove in enough down, you will find yourself in knife-edge flight with no turn rate. OTOH, for a steep turn you normally need a pretty fair dose of "up", and the precession in a left turn would provide essentially the same thing. Pulling the nose up would cause a yaw to the right, and pushing the nose down would yaw the airplane to the left. If you stall, the rapid pitch-down at the stall break would cause a strong and rapid yaw to the left, just as if you had just stomped on the left rudder. And we all know what happens when you boot the rudder right at the stall break.

Thanks in large part to the studies of the Wright brothers, which were very meticulous but did not properly account for Reynolds number effects, the airfoils everyone back then (with the possible exception of A.Fokker and H.Junkers) believed were the best were thin and highly cambered. Besides condemning the airplanes of the time to biplanes with external bracing (the wings were too thin for cantilever spars), it also condemned them to poor stall characteristics. The airfoils that work best on models often don't work well for full-scale, just as full-scale airfoils usually don't work well on models. Add this to the gyroscopic effects and a spin was almost inevitable.

Then there's all the torque from that big, slow turning prop. Horsepower is a combination of RPM and torque. If the RPM is lower, then the torque must be higher in order to get the same horsepower. Those big, slow-turning props on those low RPM motors were relatively efficient (which is why replicas with modern high-RPM motors often don't perform nearly as well despite having greater installed horsepower), but the had gobs of torque. This torque tried to roll the Camel to the left, which made rolling into left turns easier and rolling into right turns more difficult. It also increased the necessary lift coefficient on the left wing, which set the stage for a magnificent left wing drop at the stall.

Perhaps the final nail in the beginning Camel-pilot's coffin was the carburation system on the Clerget engine. It had a little quirk that required the mixture to be re-adjusted at about 200 ft altitude after takeoff. Imagine that you're a first-time Camel pilot, with just a few hours in non-rotary, low power, worn-out Avros and Be-2's, up for your first flight in a Camel. You take off, start a normal climb (nose well up, airspeed low, power at max), and just as you start into that standard 90 degree left turn after takeoff, the engine begins to sputter, instantly drawing your attention away from the horizon and into the cockpit. Meanwhile, unbeknownst to the pilot (whose head is in the cockpit, and who has never experienced a plane that automatically tries to change its pitch attitude dramatically whenever it turns) the left turn plus the gyroscopic effects starts inexorably pulling the nose up. The airplane's nearly non-existent pitch stability (due to the aft C/G) doesn't put up much resistance to this. The more nose-up attitude adds to the already decaying airspeed (caused by the loss of power from the sputtering engine), and sure enough, the airflow lets go from those thin wings and the plane stalls violently. The pilot, suddenly attentive but completely disoriented, looks up (far too late to do anything about the situation) just in time to see the nose yaw violently to the left as it falls through in the stall break. In this era long before the relationship between stall, angle of attack and airspeed was properly understood, our neophite pilot reacts (naturally) by pulling the stick back the rest of the way, and probably adding full right aileron as well. With utterly no chance for a recovery at this point, the airplane does about a quarter turn before smiting itself against terra-firma. The rotation is slight enough that his buddies on the ground don't recognize that it was indeed a spin, perpetuating the mystery and the myth, and delaying any meaningful insight into solving the problem. Today, however, we can recognize from the photos that it was indeed a spin that added our hapless hero to the Camel's list of statistics. Clearly visible in the photo is that fact that one set of wings are wrapped around the nose, and the other set are wrapped around the tail, indicating that the plane was rotating when it hit.

If I remember correctly, the Brits lost about 1100 Camels in combat, and about 1700 in training accidents.

Don Stackhouse
DJ Aerotech