otrcman

Whatever floats your boat. One of the things I love about building model airplanes is that we all get to do it just the way we want to.

But I put a premium on not only realistic static appearance, but also on realistic dynamic motions in flight. Even as a kid, I hated those phoney looking rocket ships in the old Flash Gordon serials.

As an aero engineer, I'm aware that simply scaling down an airplane means more than reducing all the linear dimensions by the same amount and painting it the same color. In a still photograph, a well scaled model can look very realistic. But in a video the motions don't look at all realistic. People get used to seeing models flit about. A trip to the local airport will show just how smoothly full scale airplanes do fly.

People comment on how much more realistic the big third scale models look in the air. That's true, and it's because the degree of scaling is much less than, say, 1/8 scale. What you see when watching a model in flight is that the inertias and damping of the model do not scale in proportion to the rest of the model. This is inescapible; it has to do with the density of the model and the distances of the various components from the center of gravity. Unfortunately, the only size that accurately reproduces the inertia and damping is 1 : 1. No other size, larger or smaller, reproduces these dynamic components.

A simple example of lack of inertia scaling can be seen in takeoff and landing distances in models. On takeoff, they accelerate like dragsters, getting airborne in a very short distance. On landing, they roll to a stop in just a few lengths of the fuselage. We can fix these visual defects by slowly increasing power on takeoff, and by carrying just a bit of power through the touchdown and then slowly reducing the power to idle so that the plane rolls further.

But what to do about the rapid rotary motions ? I'm talking about the sudden swerves on takeoff or landing, and about the twitches and dips in response to wind gusts. The answer ? A rate gyro. The gyro effectively increases the mass and damping of the model. It gives back what we have unavoidably removed from our scale model in a way that no amount of flying skill can accomplish.

Do you put washout in the wings of your scale model to reduce tip stalling ? Do you dial in some aileron differential to make the plane roll better ? How about a bit of expo or an adustment to elevator travel or moving the CG forward ? If you are really commited to exact scale, you won't do any of those things. If I remember correctly, the CG of a real Sopwith Pup is around 39%. Care to try that on your model ?

My point is that we have accepted many subtle modifications to make our scale models fly more safely and more realistically. These are time honored methods and most of us don't think twice about using them because they're necessary to make up for physical laws of aerodynamics. But along comes the modern rate gyro and we look at it with a wary eye. In fact, the gyro is just another useful accomodation that a scale modeler can use to make up for some of the inescapable side effects of making a plane smaller.

Dick