I have finally come up with a bulletproof system for quickly knowing if a new model is tail heavy or just out of trim.
I do a lot of maiden flights of experimental designs. Part of the process of flight testing a new design involves figuring out the useful range of the center of gravity (CG). In other words, how nose heavy and how tail heavy can the model be flown and have it still be safely controllable?
Since I usually hand-launch my models, I get to watch them in the air for a few seconds to help me diagnose what is going on. A very puzzling situation was when I would launch the model and it would pitch up.
Is the center of gravity too far back, making the model uncontrollable, or is it just out of elevator trim? From just one launch, it was impossible to tell these two cases apart. I would usually adjust the pitch trim down and launch it again. Adjusting the trim is easier than adjusting the CG. But now I would find myself in a long cycle of small adjustments, basically using trial and error until the model became controllable. There had to be a better way.
The pitch or elevator trim of a model airplane is an adjustment that is made to make the model easier to fly. In theory, you do not need trim tabs on your transmitter. You could just hold the control sticks offset from neutral. If you did that, the model would not know the difference. It would definitely make the model harder to control, but it would not fundamentally change the intrinsic handling qualities of the model.
I have never seen a model airplane pilot do this, but it is common in full-size airplanes to continually adjust the elevator trim lever throughout a flight. Say you take-off and then want to climb 9,000 feet (3,000 m). Do you really want to hold the control stick back while the airplane makes that long climb? Of course not. What full-size airplane pilots do is adjust the pitch trim until the airplane holds the climbing attitude on its own. Then they can relax and focus on the other tasks required to safely fly an airplane.
On the other hand, pitch (or longitudinal) stability is an intrinsic handling quality of an airplane. You cannot make a model airplane more stable in pitch by programming your transmitter. You could do it by adding a gyro, but that is something else entirely.
A model airplane is called nose heavy if the center of gravity is further forward than is desirable. What is a nose heavy model to one pilot could be perfectly balanced to another. It all has to do with the handling qualities that you are looking for.
By definition, a tail heavy airplane has its center of gravity too far back for good pitch stability. If the center of gravity is so far back that it is actually behind the center of lift, then a model with neutral elevator will pitch up when launched. Knowing this is key for making my flight testing strategy work.
Breaking Down The Problem
In the past, I would always launch my experimental designs with a little bit of up elevator, just like I am supposed to do. The problem was that then I found myself optimizing two variables at the same time: pitch trim and center of gravity. It would usually take multiple launches to sort these two variables apart. Was the CG in the wrong place, or the elevator trim out of whack?
This was a tedious, time consuming, and frustrating process. Like everybody else, I hate crashing and breaking my model airplanes. I hate it even more on a maiden flight.
The it finally occurred to me to break down the problem. Since I could not take the CG location out of the equation, I was going to neutralize the elevator trim issue, literally.
Bulletproof CG Test
If you suspect that the model airplane design may be tail heavy, launch it with neutral elevator. Make sure the throw is straight and level. You will not learn anything from a bad throw.
If the model dives for the ground, then it has at least a small amount of pitch stability. Even if the pitch stability is marginal, I can usually fly the model around enough to decide if the center of gravity needs to be moved forward some more.
If the model goes straight or noses up and stalls, then it is definitely tail heavy. Guaranteed. Add some weight to the nose, and try again.
Making It Work
I am not telling you to stand idly by while your model dives to the ground or stalls. You are just looking for the first clear sign of what the model wants to do. The instant that you see this clear signal, jump on the controls. There is no need to be a spectator as your model crashes to the ground. Once you have jumped on the controls, then either bring it down for a safe landing or take it up for a spin around the flying field.
Only follow the above procedure with those models where you truly do not have good information on where the CG is supposed to go. If the plans or kit instructions tell you where to put the CG on a maiden flight, then by all means follow their advice.