Pilot DAR wrote:
So far in my flying career, I have come to accept that if you are gliding for the purpose of best glide ratio, you want the greatest L/D. If you extend flaps, you upset that ratio. Once upset, you can't just put it back as before, without a loss. The loss is the setting with the loss of lift, and the need to loose altitude to do a power off acceleration, to restore the aircraft to the flaps up best glide speed. I suspect that the net effect of these losses would exceed any benefit resulting from the reduced drag of the flaps being retracted.
This last sentence does not match the experience of many glider pilots.
One of the reasons why your chosen flap setting for the approach might turn out to be wrong is hitting heavy sink. 10kt down (1,000 fpm) is not unknown in the UK, and in hotter parts of the world even stronger sink might be encountered.
In these circumstances you need to (a) increase speed (to spend as little time as possible in the sink) and (b) improve the glide angle and reduce drag by, inter alia, reducing flap. The short-lived loss of lift on retracting flaps is more than compensated by the improved L/D, assuming you have sufficient height to make these adjustments. Of course, you need to know the effect of raising flaps on your aircraft - if, say, you lose 50 ft and are only 51ft agl then it's hopeless. If you're at 250ft, in most gliders you'll probably improve the situation if your speed is such that reducing flap doesn't put you on the back slope of the drag curve (or worse, cause you to stall, but you shouldn't be approaching that slowly in most instances)..
In a Cessna/Piper XXX the height at which you can benefit is clearly greater, so if you don't know the effect on the aircraft it might be best not to fiddle with the flaps. But if you
do know, ideally from practice, it would be foolish to stick with a losing flap setting which will only put you in the hedge.