How does the temperature affect flying and trimming?

A friend and I were flying boost gliders (with and without tails) in 100 degree heat yesterday. It was fun! But anyway, the tailless boost gliders were being a little extra hard to trim and get to fly without stalling.

The higher temperature reduced the density of the air, which lowered your model's Reynolds numbers ("Re"). Your models have painfully low Re's to begin with, and any reduction at all is significant. The lowering of the Re's causes an increase in drag (which hurts L/D and also causes the model to decellerate faster during a stall entry), and also reduces the max lift coefficient.

Operation at low Re prompts some changes from normal practices in larger aircraft. For example, some classes of R/C sailplanes thermal at Re's of 60K to as low as 20K. Minimum sink in these models does not occur at a high lift coefficient, because the higher Cl causes a reduction in airspeed, which reduces Re, which dramatically increases Cd, which absolutely destroys the L/D and takes the sink rate with it. Instead, we use a much higher airspeed and a lower Cl in order to keep the Re's up as much as possible.

My friend (an ex-Estes employee) commented that it seemed that these gliders were really speed sensitive, which also seems to be said of tailless aircraft in general. Is this true of nurflugels? Is there any half-way easy fix?

A "tailless" aircraft usually is, in effect, extremely short-coupled (i.e.: has an extremely short tail moment arm). Short coupled aircraft in general tend to have a very narrow C/G range and tend to be very sensitive in pitch. If you built a conventional tailed model with a tail moment arm shorter than the mean aerodynamic chord of the wing, it would be very touchy too! Add to this the high drag (due to the low Re I mentioned above) and the low mass of the model, and you get an aircraft that stalls at the slightest provocation, with very little warning. It's to some extent the nature of the beast. Proper design and good flying techniques can at least partially compensate for this, but you have to be prepared to deal with this issue from the very beginning of the development process. Be prepared to design, build and fly to a higher standard of precision and accuracy than what you can get away with on a long-tailed model.

Don Stackhouse @ DJ Aerotech
djarotec@bright.net
http://www.bright.net/~djwerks/