WingFlaps

On Apr 27, 8:22 am, Michael Ash <m...@mikeash.com> wrote:
> In rec.aviation.student WingFlaps <Morefl...@> wrote:
>
> > On Apr 27, 6:04?am, Dan_Thomas_nos...m 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.
>

I make the turn diameter bigger than that using the formula
rad=(knots^2)/(11.26 x tan(bank)) (assuming it's right) or about
1080'? So, what would you consider the minimum height taking decision
time into account and a 225 degree turn followed by a 45 to line up
back on the runway?

Cheers