good thermaling design characteristics?

[ciurpita]

i recently asked about how the L/D of a sailplane could be improved. L/D is important when moving between thermals to minimize the loss of altitude and maximize the chances of reaching the next thermal. high L/D is not needed to thermal. so what characteristics of a sailplane make it competitive while thermaling?

the june '04 rcsd contains a note by mark drela discussing planform design. i was hoping someone could help me better understand the things discussed in marks note. for example, i can understand that you want to avoid tip stalls, but why does a "smaller Cl" help this?

http://www.rcsoaringdigest.com/pdfs/RCSD-2004/RCSD-2004-06.pdf

[JetPlaneFlyer]

For thermal performance alone the single most important factor is wing loading.. so maximise wing area and minimise weight.
However L/D still plays it's part in thermal performance so induced drag reduction measures like high aspect ratio and near elliptical spanwise lift distribution cant be forgotten.

Regarding Mark's comment about reducing Cl toward the tip.. An airfoil stalls when it's Cl max is exceeded.. keep the Cl lower at the tip and you should prevent tip stall. Washout is a well known method of reducing Cl at the tip but a wing planform with little or no taper also reduced tip Cl.

[HerkS]

As you said, the best glide ratio occurs when the lift to drag ratio of a design is a maximum. Lift (for normal glide angles) can be assumed to equal weight. So, for a given design, reduce drag to get best L/D. (interesting note on this later)

Minimum sink occurs when the ratio of (Lift to the 1.5 power) divided by drag is a maximum. So as JPF says - reduction of drag works for both L/D and min sink.

Light weight - low wing loading is also important, but I once built a free-flight sailplane that was so light that it would not glide. It was quite large and extremely light. When hand tossed, it would just slow down and then fall like a feather to the ground. It would not fly until I added weight at the CG.

It's hard to picture the air as viscous, but it was the viscosity of air that caused this. This is also an effect on larger sailplanes. Some very light models actually thermal better when weight is added, because the wing performs better in terms of providing sufficient lift at lower drag. This is where Reynolds Number (the ratio of dynamic to viscous forces) comes in to the play.

[nmasters]

Minimum sink occurs when the ratio of (Lift to the 1.5 power) divided by drag is a maximum.

Herk--

Would that be equivalent to the span squared load (W/b²)? Full size motorgliders typically have a span squared load of a bit under 0.6 while good,un-powered, sailplanes are half that and less. IIRC the Carbon Dragon is around 0.18 and can stay up on squirrel farts

--Norm

[HerkS]

Hi Norm, Weight/span squared ---

What span loading says is that light is good, and high aspect ratio is good. Both very important. The problem with that, on models, is that it can push the design toward operating at very low RN. So the value of it as a criterion diminishes if the model is very light and the wing chord very small.

My sense is that if the RN at min-sink flying speed gets much below 100,000-150,000 performance will suffer regardless of other considerations. If your design needs to operate down there, airfoil selection gets very critical. And the airfoil chosen needs to be carefully matched to the design objectives - i.e. free flight glider as opposed to a DLG hand launch design.