While flying my Chrysalis, I need to keep constant up elevator to stay level.

From : Don Stackhouse

That sounds noseheavy, assuming your incidences are not way off.

Doing a dive test, the plane goes almost vertical, which I believe means it is tail heavy (starts to tuck under).

I prefer something along similar lines in principle, but much more gentle. If you go significantly outside of the range of angles of attack you normally see in regular flight, then you're introducing things like aeroelastic deformations, and aerodynamic nonlinearities.

Instead, get it in a stabilized, normal, hands-off glide. Now, blip some up elevator, just enough to pull the nose up about 5 degrees or so, then center the stick and watch what happens. If the C/G is ahead of the neutral point, the plane will return to its original pitch attitude. Most airplanes will overshoot the original attitude, end up in a very shallow dive, then pull up again, gradually damping out the pitch oscillations so that it's pretty much damped out after about two or three cycles. This is what's called "positive, underdamped dynamic stability".

The Chrysalis has so much tail moment arm and tail area that it will probably just ooze slowly back to the original glide attitude. We call this "positive, overdamped dynamic stability".

The case where it comes back to the original attitude as quickly as possible but without overshooting into any oscillations is called "critically damped".

A model with not enough tail moment arm and/or tail area will oscillate, but each oscillation will get bigger than the last. We call this "positive static stability, negative dynamic stability", or simply "dynamically divergent". You should not see this problem with your Chrysalis!

Getting back to your stability tests, if your C/G is ahead of the neutral point, and you blip the nose down a few degrees and release the stick, the nose should come back up (indicating positive static stability. Whether it oscillates in pitch after that once again depends on the tail design and the resulting dynamic stability.

The further back you move the C/G, the less static pitch stability your model has, so it comes back to the original pitch attitude more slowly. If you move the C/G back to the exact location where the model no longer tries to return to the original attitude, but instead exactly holds whatever new pitch attitude you give it, the model now has "neutral static stability", and that C/G location is called the "neutral point". On the Chrysalis, that should be near the aft limit of the C/G range marked on the plans.

With C/G locations aft of the neutral point, when you pull the nose up, the nose will then want to pull up even further by itself, and if you push the nose down (even a small amount), the model will want to dive increasingly steeply.

Your symptoms are at first glance contradictory. The case of needing lots of up elevator in level flight suggests nose heaviness, but a tendency to tuck nose-down in a dive seems to indicate static divergence, which suggests tail heaviness.

As I stated before, things like aeroelastic deformations in a high speed dive could help explain this. However, another possibility is tail heaviness, but also an incidence problem.

The incidence of the wing that matters is the overall average of the individual incidences along the wing, not just the incidence at the root. For example, if you built the model with much more washout than the plans and instructions called for, that would reduce the average incidence of the entire wing, which would act like down elevator. You would have to continuously hold some extra "up" elevator to counteract this.

If you have a tail heavy condition, to the point that the C/G is in the "statically divergent" condition (which typically requires a C/G a little behind the aft limit shown on the plans), then the plane could diverge downwards during a dive test. However, if you had an incidence problem, such as a considerable amount of excess washout, then that same plane could require up elevator to sustain a normal glide.

Of course there are also other possibilities. For example, if your elevator pushrod is buckling under load in the dive test, the plane will lose a large amount of the "up" elevator input you are giving it, causing the plane to nose down further.

I think the solution is to go through your setup again and make sure everything is within limits. Recheck the wing's washout and make sure it is correct (1/8" on the inboard panels, the outer panels should be flat). Double check that the tail section of the fuselage is not warped (unlikely problem). Make sure the C/G is correct. I find that mine seems to fly best with the C/G just a little ahead of the aft limit. Double check that the tail linkages are stiff enough that the pushrods don't buckle if you push down on the elevators. One more thing that I've seen happen: check that the elevator servo is not floating around loose in the fuselage, but is firmly anchored to its mount.

After making sure the setup is correct, try those less abusive tests I described. Odds are that the model will be much better behaved. If it isn't, contact me and we'll dig into it further.

Don Stackhouse
DJ Aerotech