In rec.aviation.student WingFlaps wrote:
On Apr 27, 6:04?am, wrote:
? ? ? ? Lots of people had the impression you were talking about the
dreaded downwind turn, with all the talk about the energy required to
accelerate to maintain airspeed. The energy required, as pointed out
in a much earlier post with several very good references, is so tiny
that it's not worth fooling with at all.

Perhaps you could put a number on that? Could you try a gliding turn
with stopwatch and altimeter and compare that to a straight glide?

In the optimal 45-degree-banked turn the load factor will be about 1.4.
Your best glide speed and min sink speed will increase by the square root
of that, or 20%. The glide angle remains the same if you increase your
airspeed appropriately, so your sink rate will also increase by 20%. So
instead of 650fpm you'll be coming down at 780fpm. At 78kts (65kts best
glide speed from previous post plus 20%) and a 45 degree bank you're
making a circle a bit over 500ft across which will take you 13 seconds to
complete half of. The extra sink rate from the turn will therefore cost
you 30 feet over what you would have experienced in a straight glide for
the same amount of time.

You'll also lose about 80 feet to accelerate from 65kts to 78kts. But
you'll gain this back at the end, so as long as the end of your turn ends
at a reasonable height it can be ignored.

The numbers will, of course, vary between aircraft but it would appear
that the extra energy loss due to the turn itself isn't all that
significant. If 30 feet is the difference between making it and not making
it you probably should not be turning around in the first place.

Did I miss anything?

--
Michael Ash
Rogue Amoeba Software