Whassa Dutch Roll ???

From : Don Stackhouse

It's a maneuver that combines yawing and rolling motion, sort of like a falling leaf. The model wags its tail side-to side, and rolls in concert (but out of phase) with the tail-wagging. It's sort of like a falling leaf's motion, but regular, with a specific frequency. It's the result of a number of parameters, the main ones being the wing dihedral, the effective fin area, and the inertia about the yaw axis. In general, too much dihedral and/or not enough fin area will result in a dutch-rolling tendency.

Spiral instability is the tendency of the model to wind itself up into a "graveyard spiral" if held in a sustained turn (such as a thermal turn). It's generally the result of not enough dihedral, or too much fin. Also, it's not unusual for a model to have positive spiral stability (i.e.: tends to roll out of a turn, back to wings-level) for some bank angles, and slightly negative spiral stability for others (typically for steeper banks)

Note, the fix for one problem is the opposite of the fix for the other. Depending on how much inertia the model has about the yaw and roll axes, the ranges of dihedrals and fin areas that result in one problem or the other may:

1. Overlap. No matter what fin area and dihedral you use, it will either dutch roll, have spiral instability, or both. This is usually the result of having too much inertia. That's one more reason for trying to keep the weight out of the model's extremities!

2. Touch at one spot in the middle. There will be ONE, and only one, combination of wing dihedral and fin area that has neither dutch roll nor spiral instability problems.

3. Have a range of dihedrals and fin areas that are free of both dutch roll and spiral instability. Once again, keeping the inertia to a minimum is usually the key to the size of this "sweet spot". Because they tend to have slightly higher inertia, the size of the sweet spot for a conventional tail tends to be a little bit narrower than for a V-tail.

Because the dutch roll/spiral stability balance is so sensitive, it becomes a good measuring tool for how equivalent two different types of tail are. If you fine-tune a V-tail design till it's in the middle of that "sweet spot", then switch to a conventional tail, it should still be in the middle of that sweet spot if the two tails are truly equivalent.

We have a number of designs that offer both conventional tail and V-tail options, and we go to great pains to make sure the two versions have as identical handling as possible. The dutch roll/spiral stability balance is one very useful measure of that equivalency.

Don Stackhouse
DJ Aerotech