Damping Spirits and Chasing Tail

After going through the dynamic properties of the preliminary Carbon Eagle design, it is evident that a near-plank type layout provides very little pitch damping... and this leads to some interesting design compromises.

First off, a pitch damping primer:
Pitch damping is different than pitch stability. Imagine that the suspension in your car consisted only of coil springs... no shock absorbers. You would not have any trouble on a perfectly smooth road, but roads are never perfectly smooth. Once you hit a bump, this springs-only car will continue to rock back and forth for a long time.. It has stability, but little damping, so it overshoots the neutral point and causes a lot of uncomfortable oscillations.. If the bumps are big enough and spaced properly, the overshoot from the last bump is added to the next bump, then the next... at the right bump amplitude and frequency, the car will actually flip! NOT GOOD! So we add shock absorbers to add damping, greatly reducing, if not eliminating overshoot.. Now the car will not oscillate or flip, and driving over that same series of bumps turns into a comfortable, safe experience.

The same principle applies to an aircraft. Flying in smooth air is OK with low pitch damping, but add a few bumps --that is, turbulence, thermals and wind gusts-- and the pilot will begin to loose control... perhaps even flipping over in some conditions. Again.. NOT GOOD!

To make matters worse, slow-flying aircraft are more affected by these bumps than faster aircraft. For example: A pitch stable, under-damped glider is cruising along safely at 25 mph, when it suddenly encounters a 5 mph vertical updraft. Its angle of attack, or pitch angle, increases by 11 degrees, which approximately doubles its lift force (+2Gs). Since it's a pitch-stable wing, it will pitch back down towards the neutral pitch angle, but it's under-damped, so it keeps pitching down well past past the nuetral point, just like the car with no shocks.. Now it encounters a 5 mph downdraft .. the result: sudden strong negative Gs.. A slow, under-damped glider can very quickly become uncontrollable in strong turbulence, perhaps even resulting in structural failure in extreme conditions. A faster under-damped glider cruising at 50 mph would experience only half that angle-of-attack change as the 25 mph glider in the same flying conditions, so its induced pitch oscillation is reduced, and it therefore has far fewer risks imposed by low pitch damping in turbulent flying conditions.

So, even if a higher speed plank-type sailplane design (i.e. Marske Pioneer) may be successful, you cannot conclude that the same design concept will be successful when implemented as a much slower flying hang glider.

As a result, I've decided the Carbon Eagle is going to have a tail! This is a major change to the design trade space. Now that some of the pitch stability can be shifted to a tailplane, positive pitching moment airfoil sections are no longer needed. We can now use neutral, even slightly negative pitching moment airfoil sections, which are generally higher performing, allowing a smaller wing area, so that's good news. However, since this is a foot-launched glider, static balance is very important, so the tail plane should be kept as light and small as possible. Therefore, the highly negative section pitching moments are still undesirable, as they drive the need for a larger horizontal stabilizer area and/or tail boom length.

By the way, my first hang glider in 1975, the Quicksilver B, had a large tail and could not be launched without a tail-runner. Its single surface airfoil had a highly negative pitching moment, and the somewhat crude tail structure was fairly heavy aluminum.. not optimally engineered for minimum weight. Because we will be utilizing lightweight composite materials, a lower pitching moment wing, and more engineering effort, the Carbon Eagle will have a much smaller and lighter tail in order to meet the requirement of being foot-launchable without assistance... so after 35+ years, I'm coming around full circle .. back to a tailed glider!