Fixes to improve lateral stability and flaring

[Aventador]

1. Lateral stability
I had some problems with a custom helicopter with a larger bottom rotor ascending in a conical spiral, and I was thinking it may be due to the lateral stability of the top rotor. I was told to put a sheet of wood in the empty section of the top rotor to achieve this. Also, can you do this on the bottom rotor? What would it do?

2. Flaring
On a stock FFM, we experienced an issue with the bottom rotor flaring too much. Previously, I had modified my helicopter to use the thicker carbon fiber (0.32" I believe) all throughout the entire helicopter, top and bottom TE and LE, to try to fix this problem. I also added a rib in the center of both bottom rotor blades, but my helicopter still went sideways during ascent. It stabilizes once it reaches the ceiling, however. Would a better change be to use original FFM instructed carbon fiber thicknesses but only modify by putting a rib in the center?

[coachchuckaahs]

For your first question, since it is a custom heli, we would need far more information to help diagnose the problem. Including type of rotor (X-rotor or conventional blade), tip pitch, pitch progression, CG, etc. However, if it is going in a conical spiral but making it to the ceiling, I would not worry too much about it.

Climb stability may be affected by a number of different things, such as unequal rotor pitch (blade to blade on each rotor), design of the rotor, axial alignment of the two rotors under load (MS bending), etc. If a conventional blade, then often a freely-rotating vane is added above the top rotor. If an x-rotor, filling in the center uncovered area, perhaps simply with scotch tape, has been reported to help. Some have suggested that moving the CG may help with stability. The FFM heli has much lower pitch (more power) on the bottom rotor, while the AreoMartin heli takes the opposite approach with lower pitch on the top rotor. Unfortunately, this probably means some experimentation is needed.

In general, several have reported that too low pitch on the FFM bottom rotor can cause climb instability at higher torques. If the instability goes away at lower torques, try slightly increasing the pitch of the bottom rotor.

On your second issue, the FFM lower rotor is designed to flare. This means that the leading edge is more flexible than the trailing edge, so that the rotor goes to a higher effective pitch at high torque. If this is what you are reporting is excessive, then what are the symptoms that you are seeing? Is it going to a high enough pitch that the heli cannot climb at all? Again to look at your specific issue, which appears to be a stability in climb, be sure your tip pitch of the bottom rotor is to spec, or at least 8 degrees if I recall. A lower pitch can lead to one blade going to reverse flare and severe instability.

Coach Chuck