Weight Reduction on the Monarch HLG Sailplane

When Joe Hahn and I began work on the 'Monarch', the typical competition hand-launched sailplane weighed about 13 to 15 ounces, a little lighter if it used a built-up wing and tail, and a little more if fully sheeted and fiberglassed. Here in Southwest Ohio the winds tend to be moderate but not insignificant, also the glide ratio of a HLG limits the amount of sky the model can search. What we needed was a model with the low speed capability of the built-up ships and the glide ratio of the glass airplanes.

We decided to optimize the aerodynamics for high speed, and reduce weight to obtain the low speed performance. To some extent these goals were actually complementary; for example, the minimum whetted-area fuselage required for good high speed performance also helped minimize its weight. The tail boom could be very small if we used the strength of the fiberglass effectively, reducing not only whetted area and tail weight, but additional nose weight as well. There are few things more discouraging than shaving every gram possible all through construction, then having to add an ounce of lead to the nose to get the C/G correct. We also developed a lighter alternative to the heavy plastic tubing typically used for pushrod guides. Because of the lightweight tail, most of the Monarchs have actually come out a little nose heavy, even using a 150 mah battery pack.

Speaking of batteries, our tests show that a fully charged 110 mAh, 4-cell battery pack will power 2 micro-servos and a micro 4-channel FM receiver for over 45 minutes, with the transmitter sticks in motion continuously! Since the typical contest round in hand-launch is 8 to 10 minutes, this battery life is more than sufficient. For all-day contests it is probably a good idea to swap batteries every 2 or 3 rounds.

The other big weight reduction came in the skinning method for the foam wings. Our first model used the traditional spray contact adhesive to attach the balsa skins to the foam cores. On the next we used aliphatic resin (such as Franklin's, "Titebond" or Elmer's Carpenter's Glue) thinned about 3 or 4 parts glue to one part water. We used a sponge-rubber paint roller to apply the glue to one side of the core, put the wing skin in the foam block, and put the core on top of it. Next we rolled glue onto the other side, folded the other wing skin over on top of it, put the other foam block on top and weighted the whole stack down on a flat surface for 2 or 3 days. It's slow, but it's cheap, and it gives you several minutes to adjust the position of the skins on the core before the glue begins to set. It forms a rigid bond between the skin and the core to minimize warps, and best of all, saves about half an ounce per wing compared to the spray glue. In later wings we used a thin epoxy to apply the 1" fiberglass tape to the inside of the upper and lower skins at the trailing edge, since the aliphatic resin showed some warpage problems in this area. The thinned aliphatic resin does seem to work well for applying tape reinforcements at the dihedral joints. We don't have a good comparison yet with the epoxy and vacuum bag method, but we would expect an even bigger weight difference in favor of the aliphatic resin method.

Wood selection is important. We weigh each piece of wood in our kits and try to average at about 6 lb./cu.ft. on the sheeting. Heavier wood could add more than an ounce per wing. One area frequently overlooked is tip blocks. These can have a significant percentage of the total volume of wood in the wing or tail, and besides the extra weight, the inertia of all that extra mass at the tips can seriously hurt roll response. We also tried obeechi skins and found them to be about equal to 10 lb. balsa in finished weight. It is probably good for 2-meter ships, but the extra strength is probably overkill for a hand-launch unless you like to fly during hurricanes.

Tired of ironing bubbles out of your iron-on covering? We found that a painted finish directly on the wood saves about 1/2 to 3/4 ounce, depending on product and technique. We use water-based polyurethanes applied with a sponge paint brush or cloth.

The finished, ready to fly Monarch typically weighs about 10 ounces. It is possible to build one at a little more than 9 ounces by not painting the wing and tail. Thermalling ability will be breathtaking: however, penetration starts to suffer. If you tend to have very light air with little or no wind, this is a good option. Unless you have very high winds, there is no need to go over 10-1/2 ounces; at that weight a Monarch penetrates about as well as a typical 14 or 15 ounce model. With the right aerodynamics, it isn't necessary to be heavy to go fast!