Being towed too slow?

In article , Ben
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.

Er, yes. I think you have got this a bit wrong.

We have to do an exercise in our annual check called "boxing the wake".

One of the objects is to show you where the turbulence behind the tug
is. You fly out to one side, go lower and lower, still out to the side,
then gradually fly to a position behind the tug but below the wake. It
never stops surprising me how far below the tug you have to be to be
clear of the turbulence.

This is a bit unpleasant for the tuggie, who has to use a bootfull of
rudder to keep straight.

In the normal, high tow position we use, the tug appears about a wing
span or a wingspan and a half above the horizon, and the air in that
position is usually pretty quiet.

So it looks to me as if the disturbed air is deflected downwards,
probably about 20 degrees. Which is what you'd expect from the way a
wing works. The plane in the picture was just taking off, not in free
air, so not really comparable.

As to gaining energy from the upgoing bit of the tugs wingtip vortices,
I'll bet you a pizza that the increased drag your rudder would produce
to fly in that position would more than compensate for any advantage
you'd get.

http://upload.wikimedia.org/wikipedi...ortex_edit.jpg

--
Mike Lindsay

Mike Lindsay wrote:
In article , Ben

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.


Er, yes. I think you have got this a bit wrong.


So it looks to me as if the disturbed air is deflected downwards,
probably about 20 degrees. Which is what you'd expect from the way a
wing works. The plane in the picture was just taking off, not in free
air, so not really comparable.

As to gaining energy from the upgoing bit of the tugs wingtip vortices,
I'll bet you a pizza that the increased drag your rudder would produce
to fly in that position would more than compensate for any advantage
you'd get.


http://upload.wikimedia.org/wikipedi...ortex_edit.jpg



It looks like in this wind tunnel picture that the airflow is deflected
downward even at quite a distance above the wing.

http://www.jefflewis.net/graphics/ai..._P0001845.html


It seems like even in
high tow you might still be in an airstream that is deflected downward
significantly compared to the airstream that the towplane is feeling.
I was speculating to myself that with a glider like a 1-26 or 2-33, you
don't really notice the problem, since your stall speed is so low
relative to the towplane, but when a heavily ballasted glider with a
stall speed in the mid to high 40kt range gets towed by a towplane with
a stall speed in the low 50kt range, you're starting to get where
perhaps that downward component of the airstream that the glider is
flying in compared to what the towplane is flying in becomes
significant, and the glider may come close to flying in a near stalled
condition even though the towplane is still happy as a clam. Has
there been any actual scientific wind tunnel data published on the
towplane - glider configuration to actually analyze this in a systematic
way as opposed to the valuable, yet still anecdotal, opinions expressed
so far on the thread? I was wondering if any of the Akafliegs had
studied this but couldn't find anything on a Google search. I did see
one study by NASA on a 747 towplane - towing a proposed freighter
glider but you had to order it, so i didn't read it.

On the subject of trying to gain energy from the towplane's wingtip
vortices, I saw a video of a goose flying formation with an ultralight
trike where the goose was flying just in front of the trike's wing and
not flapping at all, getting a free ride from the upward
deflected air in front of the wing. Could this be used by advanced
students of leeching technology? The other example might be Dr.
MacCready's little hand tossed glider you could push along with the air
deflected by your hands. Perhaps when getting towed, we have the
opposite problem, since we are usually flying in the downwash from both
wingtip vortices of the towplane, we lose energy rather than gain it. I
was thinking maybe if you hold some bank relative to the towplane,
rather than using rudder to get out to the side, and perhaps with a CG
hook rather than a nose hook, it might not create as much drag to fly
out to the side and get in the upward part of the wingtip vortex and
perhaps get a boost. Somebody should do some real scientific tests on
this. Is Dick Johnson listening?

I was wondering if any of the Akafliegs had
studied this but couldn't find anything on a Google search. I did see
one study by NASA on a 747 towplane - towing a proposed freighter
glider but you had to order it, so i didn't read it.

Apologies for the above, I found some pretty relavent links by searching
on Formation Flight Aerodyamics, so I'm sure it's all been worked out.
But maybe not as well analysed yet for the particular problem of a heavy
glider on tow.

Ben wrote:

It looks like in this wind tunnel picture that the airflow is deflected
downward even at quite a distance above the wing.

http://www.jefflewis.net/graphics/ai..._P0001845.html

It seems like even in
high tow you might still be in an airstream that is deflected downward
significantly compared to the airstream that the towplane is feeling.

The other thing it shows is that, to keep a normal AOA on tow, you'll be
pitched up further than in free gliding flight.

If you hit PREV a few times in that sequence there's a nice head-on view
of smoke tracing the tip vortex. The top of the vortex slopes slightly
down as it travels rearward, so away from the ground there is no
significant upward flow behind the top plane's tip as somebody else
suggested.


--
martin@ | Martin Gregorie
gregorie. | Essex, UK
org |