"Jmarc99" wrote in message
.. .

"Chris Reed" wrote in message
...
Jmarc99 wrote:
You wrote that something like 90 degrees= nil wind. So if someone
do a slip in the wind direction in order to sayt alligned with the
runway
during the final leg, the pitch angle of the sailplane is somewhere
greater
than supposed! In this case, with 90 degree wind, the sailplaine have
to travel a longer way through the air mass.

Though, the sailplane fly more faster than supposed, and the pilot
doesn't use much airbrake! Is that what effectively happen in that
wind condition?

Jmarc


I'm not suer I understand the question properly.

My crosswind landing technique is to crab into wind, and then kick off
the drift after the roundout. This works fine for, say, a 10kt crosswind
at 90 degrees, and at that wind speed there's no real danger of wind
shear.

I also prefer crabing than slipping!
I also agree that a little wind, coming from any direction, can't produce
dangerous wind shear. No wind means no real effective change in the
wind speed or wind direction.


However, if the wind is gusting 20kts or more at 90 degrees, that's
outside the demonstrated crosswind limits of my glider. Ideally, I choose
a better landing direction. If I had no choice (e.g. poor or late field
selection), then this would be one of those "interesting" landings which
test one's improvisational and piloting skills. I'd be using a
combination of crabbing and slipping, to give me less drift to kick
off,and I'd certainly add 10 kts or more to allow for gusts and wind
shear unless that seemed more dangerous than approaching at a slower
speed.

I guess the point I was really trying to make is that if on every landing
you add extra speed for "safety", the result is that you never learn to
land with the minimum safe energy.

No need for extra speed when there is no wind at all! I agree again
that is important to learn to land with the minimum safe energy..!

This is a skill that I think every XC pilot needs. However, in gusty or
high winds, you have to allow for the possibility of needing extra
control authority or suddenly losing 15kts airspeed through wind shear.
So minimum safe energy landings are for benign conditions.

The real point to talk in this thread is "how much" speed to add to
the 1.3 Vstall speed, depending on the wind speed? Especially if
the wind come from a side or the back!

All pilots seams to say that only the heading portion of the wind
should be took in care, when choosing the landing (circuit) speed.

Is someone might agree that a 15-20 knots wind, at low altitude
(like the final leg) may produce windshear that may lower the
the relative airspeed, instead of increasing it, even when the
wind come frome the back?

This is the real question. I think that very experienced glider
pilots, lurkink in this newsgroup, might have someting very
interesting to say about that!

Sure, the good thing is to land upwind, anyway! But, is that
right to reduce the speed to only 1.3 Vstall when you realize,
when doing the base leg, you're going to land tailwind?

Thanks Chris, for your interest to this question!

Jmarc..

One answer is based on understanding what the "reported wind" really means.
15kt gusting to 25 kt means that the average speed over time (I have
forgotten the exact time interval but about 30 to 60 sec) is 15kt. If peak
gusts are 25kt, then that means it can fall as low as 5kt on the average,
and for some moments can be 0kt. The gusts come from turbulence, and can
therefore come from *any* direction. The danger is not so much the 25kt gust
but the 5kt lull. Hence the advice to add the gust differential to the
approach speed. Some advise "half the wind plus half the gust
differential"--more or less the same result. What is needed is to remain
above stall speed until touchdown! Thankfully, the wind is much less very
close to the ground---3-4 ft (1 meter). My glider stalls at 38kt --1.3 Vs
is therefore ~ 50kt. So, with a "negative" (opposite direction) gust, I
would be very close to stall without the extra velocity.

As to the tailwind question---the wing only knows airspeed. But the landing
field does not move with the wind, so the ground speed is eventually
important. The pilot will be tempted to slow at this point as the ground
speed seems high. Then there is the loss of control issue then the glider is
rolling on the ground, but has little aerodynamic control due to low
airspeed. Finally there is a wind shear issue at about 200 ft. If you are
landing downwind, you will descend into a decreasing tailwind, and inertial
forces will give you an increase in airspeed. It is best to be on final
approach before descending into this shear, which BTW, always exists. It is
just the magnitude that varies. This is so speed adjustments can be made
without the complications of turning--i.e. raised stall speed, increased
workload, etc.

Sooner or later, you will have to slow down in order to land and stop. So
fly the "no wind" approach speed and add the gust differential, until it is
time to roundout and land. Use the brakes if the tailwind is leading to a
imminent loss of control.

Happy Landings
Hartley Falbaum
ASW27B KF USA