On Tue, 20 Oct 2020 09:53:58 -0700, Kenn Sebesta wrote:

On Tuesday, October 20, 2020 at 12:02:17 PM UTC-4, Tango Whisky wrote:
40 kts corresponds to 20.58 m/s. (20.58 m/s) ^2/3 doesn't make any
sense unit-wise, and the numerical result would be 7.36.
My Ventus cM touches down at 40 kts and has a hydraulic disc brake
which works pretty well. Stopping distance without hitting the nose on
the ground (on grass) is 170 m.

Ah, I see the problems. You've made a mistake in the order of operations
AND I've made a typo. The exponential resolves before the division so
it's not v^(2/3). However, even worse is the typo: the equation is
(v^2)/12.

That still seems a bit long: that revised calculation gives 307m to stop
after a 33kt touchdown: this number assumes I flew finals at 55kt on a
calm day before rounding out for a fully held-off landing in my 201
Libelle, which stalls a little below 35 kts, so 33kts seems about right
for the speed at which the main wheel hits the floor.

However, I know that if I fly a 55 kt approach into a light breeze with
my roundout aim point 15m past the theshhold of our mown grass airfield
I'll be down and stopped 300-325m from the threshhold. Since Libelles
have famously weak airbrakes, I'll have covered at least another 100m
after roundout before my wheels hit the ground.

By comparison an SZD Junior stops sooner thanks to better airbrakes and a
draggier airframe. Both gliders have drum wheelbrakes and a tailwheel, so
not real powerful braking once on the ground.

So I wonder: is your calculation intended to apply to a hard (tarmac/
concrete) runway with the glider being put down above stall speed on just
the mainwheel and with airbrakes being dumped shortly after touchdown?

If so, that would explain the difference very nicely.


--
Martin | martin at
Gregorie | gregorie dot org