An obviously frustrated Doug Holverson writes, with regard to his rocket-launched boost glider experiments:

Second flight of the Golden Hawk shredded on a C6-3. My main frustration with these 'Hawk gliders is that they don't upscale well. I'm guessing that the tip rudders add a lot of bad drag and inertia loading and possibly tip flutter out there. Anybody else have any insights?

From : Don Stackhouse

Flutter and structural divergence are both possibilities.

A big factor in flutter is the torsional stiffness of the wing. You might try a trick we use on our "Roadkill Series" models to improve the stiffness of your wing. Instead of making each wing panel from a single sheet of wood, split it down the 50% chord location (in other words, a front half and a back half of about equal chords). Make the grain of the front half parallel to the leading edge, and the grain of the rear half parallel to the trailing edge. This creates a sort of "A" frame effect that significantly improves the torsional stiffness.

Other big factors in flutter are the chordwise locations of the panel's C/G and "shear center". Both are important, although the panel's C/G location is probably the most important.

Imagine holding the root of the panel rigid, then applying a pure twisting (no up, down, forwards or backwards bending) to the tip of the panel. The leading edge will flex one way, the trailing edge will flex in the opposite direction, and some point in the middle will be stationary, the "axis" about which the rest of the panel is twisting. This axis is the shear center. On a flat, uniform, rectangular cross-section slab of wood the shear center will be at the 50% chord location.

To combat flutter, you need to get both the panel's C/G and its shear center to coincide with its aerodynamic center, in other words at 25% of the chord aft of the leading edge. One way to do this is to make the forward 20-25% of the panel out of something stiffer than the rest of the panel. With the older style wooden blades for model helicopter rotors, they typically make the forward portion out of maple and the aft portion out of pine, or even balsa. On both model and full-scale rotor blades it's also common to add weights to the leading edge to get the blade's C/G forward, in addition to the added inertia to help store energy for the landing flare during autorotations. Lead is commonly used on model helicopter blades, and on full scale I've even seen cases of a tungsten bar built into the leading edge of the blade. A Bell Jet Ranger has 25 pounds of lead in the tip of each blade, and I can assure you that it's located as close to the leading edge as possible.

Control surface flutter on airplanes is a similar problem, and similar solutions apply. Mass balancing is frequently the solution there as well. If the hinge line is aft of the leading edge (such as the rudder on a DC-3), the mass balance can be inside the leading edge of the control surface itself. If not, then the mass balance may be inside of an aerodynamic counterbalance or strut that sticks forward from the tip (very common on light aircraft elevators and ailerons), or even a simple weight mounted on a forward-angled strut, such as above and below the elevator on a P-38.

It's possible to add a strut sticking forwards from the tip of your wing, with a lead weight on the end. However, safety aspects of the possibility of that snagging on everything in sight (including people) is a potential problem.

One alternative would be to get some of the lead foil tape they use as a sensor strip for windows in burglar alarm systems. It's thin, adhesive-backed lead foil typically about 1/2" wide. They use a strip of it around the perimeters of windows, so that if a burglar breaks the glass, the lead foil tape is broken, electrically triggering the alarm. Radio Shack used to sell it for their do-it-yourself home alarm systems, although with all the paranoia about lead these days I don't know if they still carry it. Carve your wing panel to shape, making the leading edge cross-section shape a little thinner than the final airfoil shape. Next, apply one or more (as required) strips of this tape along the leading edge, half above and half below, sort of like a de-ice boot, until the panel balances at the 25% chord location. If you're worried about the possible lead exposure, paint over the tape with a tough paint, or cover it with a slightly wider strip of thin plastic tape.

Don Stackhouse
DJ Aerotech