Steve Leonard wrote:
Seems to be a common thing at contests or any other
time you are flying something different than what the
pilot is use to pulling. How about the next time it
happens to you, have your radio call be 'Towplane pulling
Glider (Insert your callsign here), please speed up
5 MPH' or some such if you aren't clearly able to identify
the towplane. Most of those guys know what glider
they are pulling at the time. They keep track so they
have a record of how many tows they made.
Or better yet, spend a little time talking with the
towpilots and getting to know them. When it is your
turn, watch and see who comes by to pull you into the
sky. If you are being pulled too slow or too fast,
call the towpilot by name with your request. But do
it very nicely, please. As John pointed out, these
guys work hard so we can go play. And sometimes, they
even get to go play, too.
Thanks again to all that have towed me!
Steve Leonard
Wichita, KS
I've been thinking about this and here is my simple minded non-expert,
but merely intuitive
explanation. Experts please tell me if I'm all wet.
Even though you are tied to the towplane and
get pulled along at what looks like from the ground to be the
same speed, the glider is actually experiencing a reduced indicated
airspeed and a relative wind at a different and unfavorable angle
compared to the towplane due to the disturbance to
the air from the towplane that the glider is flying through.
Check out this picture of what is happening to the air behind
an airplane not much bigger than many of our towplanes.
http://upload.wikimedia.org/wikipedi...ortex_edit.jpg
Anybody who has been in a sailboat race and in the wind shadow of
another sailboat and been "blanketed" will recognize the loss of energy
and sluggish
performance of his boat relative to the one ahead in the "clear air".
On tow, you're flying your glider between two large contrarotating
tornadoes right in the area of maximum downwash. Even in high tow, you
are still greatly affected by this downward component of the airflow
behind the
towplane, because the circulation extends well above the towplane's
wing. Relative to the angle of attack he is experiencing, at
the same tow speed, your effective angle of attack is not the same as
his, as
some of the air you are flying in behind him is actually being dragged
along in
the same direction as he is, so the indicated airspeed your glider feels
is less than what he feels. It's a bit like a continuous wind gradient.
The friction from the passage of the towplane causes the air to be
pulled along behind him and that's what your flying in.
Also there may also be some effects of being towed on longitudinal
and roll stability some of which were studied by NASA when towing an
QF-106 behind a C-141 Starlifter. Incidently, the Starlifter towplane
took off first at about 132 mph, while the QF-106 was still on the
ground. The QF-106 didn't lift off until they accelerated to 189 mph.
Maybe this same technique of towplane taking off first could be used at
some of the contests
to pull those really heavy gliders (only with good pre-flight
briefing between tow pilot and glider pilot, of course).
http://www.nasa.gov/centers/dryden/a...4-02-DFRC.html
Another interesting study involved some F-18's cruising in formation
in the upward part of the leader's vortexes and gaining as much as 100
mile of range by using that energy. Maybe if your glider is real heavy,
you should move way out to the side of the towplane, and try to get into
the upward going part of the vortex. You might be able to get some of
the opposite effects seen right behind the towplane where the downwash
is maximal. Has anyone ever experienced
a boost in glider tow behavior out to the side like that? Perhaps on
a double tow or something like that? Has anything like that been
formally studied and written up somewhere?