On Tue, 19 May 2015 08:33:29 -0700, Andy Blackburn wrote:

I have to admit to having a bit of trouble with the idea that having two
steps in the polyhedral of 3 degrees each followed by a 84 degree angle
for the winglet has much impact on interference drag.

They're not at all common, and don't seem to help a lot in F1ABC class
models and are not commonly used. There's have been a few F1Cs using t5hem
but that's about it: I can't thing of an A or B that tried them, but they
probably don't add much in RN number range (40,000 - 100,000). What does
seem to work is the Hoerner tip in conjunction with a swept LE on the
outermost wing panels. as well as the effects Schueman described, fairing
the upper surface so it meets the lower surface at an acute angle, raking
the tip at least 30 degrees and making it meet the TE at a sharp point
all helps to reduce the tip vortex, maybe moves it outboard a bit and
certainly stabilises its location, all of which seem to help thermalling
performance and certainly don't hinder the model's ability to self centre
in thermals.

I notice that the F5B and F5F boys are using similar wing planforms to
current FF models, i.e usinh Hoerner tips rather than winglets and are
using them at a very much higher RN, combined with interesting low drag
aerofoils that differ radically from anything I've seen on either F1ABC
models or our bigger toys. They obviously glide and thermal well and do
so at surprisingly high airspeeds.


Gliders with span
limits (for class or structural weight considerations) still generally
have a vertical winglet at the tip rather than the Boeing-style flat
raked tip (though a winglet and a span extension have similar effects on
wingtip vortex and induced drag reduction for slightly different
tradeoffs in bending moment).

That's interesting. Maybe, if the class rules impose a span limit its
better to accept the additional surface drag of the vertical tiplet
because of the extra area you get from the blunter tip?



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martin@ | Martin Gregorie
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