I do have a bit of follow-up on your tip about maintaining airspeed on small sailplanes.

Are they designed to have to fly fast or is it just an inherent characteristic of small sailplanes? The reason I wonder is that I understand that low-level thermals tend to be very small, only blooming with altitude. Since an HLG spends a lot of time near the ground, is there a trick to staying in the small areas of lift while flying at a high-ish speed?

From : Don Stackhouse

It's the natural result of the laws of physics. When you decrease Reynolds number (either make the chord smaller, fly slower, or fly in thinner air), the drag goes up. It is possible to develop airfoils that do better at lower Reynolds numbers, but even with them, as you slow down, the drag goes up. I looked at the published data for one popular airfoil a while back, and found that between a Reynolds number of 120,000 (about equivalent to a 6.2" chord at 25 mph at sea level) and 80,000 (same airfoil at 17 mph) the drag coefficient doubled, and it doubled AGAIN between 80,000 and 60,000 (same airfoil at 12 mph).

In addition, when flying at high lift coefficients (tight turns and low speeds) the induced drag (the drag that's a natural by-product of making lift with a finite-span wing) goes up. The amount of induced drag depends on the "mass flow" of the wing, i.e.: how much air it's acting on to produce its lift. Think of it as the volume of a cylinder of air with a diameter equal to the wingspan and a length equal to the distance the airplane travels in one second. If you fly slower, the length of this imaginary cylinder decreases, the mass flow of the wing decreases, and therefore the induced drag must increase.

The combination of these two effects increases the total drag of the aircraft, which then decreases the L/D, which brutally murders the sink rate.

However, just as you point out, it's necessary to turn tight enough to stay in the core of the thermal, and having to fly faster doesn't help that requirement. We can help this situation by:

1. developing airfoils that perform better at low Reynolds numbers. 2. reducing weight to minimize induced drag. If we don't have to make as much lift, the induced drag goes down.

Reducing aspect ratio (making the wing wider) can help increase Reynolds numbers, but may also have other side effects.

All of these things are mainly factors in the design problem, not a matter of flying technique. Once the model is designed and built, your primary role as pilot is to learn to use the best speed and bank angle to get you the best climb rate. Some of the older designs used very high lift airfoils and had their lowest sink rate near stall. With those it was common to hang them on the edge of the stall and watch them go up. A modern HLG doesn't do so well if you hang it on the edge of a stall like that, but its sink rate at its (somewhat higher) best thermalling speed will be better than the best that the old-style hlg can do.

Probably the biggest influence you the builder have on that is weight. The first thing that deteriorates when a model is too heavy is turning radius. The lighter you can build it, the tighter it will turn. Because low Reynolds numbers reduce the max lift of the wing, a Mosquito class model like the Nymph generally cannot turn quite as tight as a 1.5 meter HLG. The Nymph seems to be comfortable at turn radii around 7-10 feet.

Mosquitos are extremely sensitive to weight, more so than any other class of sailplane. Induced drag is related to the square of the wingspan. Since the wingspan of a Mosquito class model is half that of a regular HLG, the weight compares to that of a normal HLG four times the weight. For example, a 3 ounce Mosquito is comparable to a 12 ounce 1.5 meter HLG, a 3.5 ounce Mosquito compares to a 14 ounce 1.5 meter HLG, a 4 ounce Mosquito compares to a 16 ounce 1.5 meter HLG, and a 5 ounce Mosquito compares to a 20 ounce 1.5 meter HLG. Some of the other Mosquito kits out there come out at 7 ounces or more, which compares to a 28 ounce 1.5 meter model. No wonder those heavier ones generally only do well on the slope, or in localities with extremely strong thermals! Be fanatical about saving weight when you build a Mosquito class model, and you will be rewarded!

The biggest factor in flying is the "P" word: PRACTICE. Go out in still air and see what sort of bank angles, fuselage attitudes and bank angles give you the longest flight times while turning for the whole flight. At amount of up elevator in the turn do you get noticeable sluggishness in the controls? At what point does the model start to seem reluctant to roll out of a turn? Reynolds number related drag rise will hurt the inboard wingtip the worst, causing a proverse yaw (yaw into the direction of the turn) that can fight the rudder when trying to roll back to level. Your model talks to you, but sometimes only in whispers. It's part of your job to learn to listen to those subtle cues.

Don Stackhouse
DJ Aerotech