Aerodynamics of aero towing

On Mar 7, 8:17*pm, CindyB wrote:
On Mar 7, 2:47*pm, Mike the Strike wrote:
**On aero-tow, a nose-mounted tow hook

will result in a downward force on the nose that has to be compensated
by an upward force from the tailplane and extra lift from the wings.
The point at which you lose control under these dynamics is at a lot
higher speed than the free-flight stall speed. *The effect doesn't
seem to be as strong with older gliders or flapped ships, but
definitely is a lot less in low tow.

Wow.

and all these years I was thinking that the center of gravity was
ahead of the center of lift; that there was a downward force
'forward' of the wing by the limits on CG location; and that the
horizontal tail
had to make a downward force to counteract the nose dropping tendency.
I think they called this Trim Drag, but I could be confused. I am
blond.

I better watch out for those nose-hook mounted gliders and
their scary tendencies. *Maybe that's why the folks at EASU changed
the design requirements from aerotowing on CG towhooks to
requiring them to be aerotowed on 'forward' hooks. *And that forward
hook placement was under the knees ......(even lower on the
ariframe than those dastardly true nosehooks. How does that fit into
the discussion?)

But hey, all that trim drag from different hook locations, gee, must
change the wing loading and subsequent stall speed by what......
a knot? * * * *Geesh.
Spend a few bucks on a few extra Spring tows,
and get past your winter nerves. *Or break open a chapter of
Langewische. *Or travel to a site that's still flying all winter
long and don't let the rust accumulate.

Think : * *First flight with a CFI-G for 2011. *Promote annual
refresher training.... and less accidents.

Chuckling,

Cindy Bwww.caracole-soaring.com

Obviously a few don't like my explanation and without measured data I
can't substantiate it. I've done the math too and the numbers would
suggest that the effect should be small. I can, however, insist that
the effect is real and not just the result of a clumsy pilot. I won't
claim that my position on aerotow is always correct, but after 42
years of practice, I am usually in about the right place!

I also fly all sorts of ships, have owned a few, and have only ever
noted this on the Discus 2, although colleagues flying the LS-8 report
a similar, if less pronounced effect. I have tried my ship in low tow
too (that was standard procedure when I flew in South Africa) and it
suffers from the same effect, just at a slower speed. In one contest,
flying with lots of water ballast, I ran out of elevator authority
when the towplane slowed in a thermal and my glider sank from high tow
to low tow with full up elevator, after which I had to release.
Aileron control was only just satisfactory.

To lay the subject to rest, I will do a test with a tail-mounted video
camera and one of our helpful club tow pilots.

Any other skeptics can do the same - just borrow a Discus 2 and try to
aerotow at 45 to 50 knots and report back.

Mike

On 3/7/2011 9:26 PM, Mike the Strike wrote:


To lay the subject to rest, I will do a test with a tail-mounted video
camera and one of our helpful club tow pilots.

Any other skeptics can do the same - just borrow a Discus 2 and try to
aerotow at 45 to 50 knots and report back.

Sounds like a good experiment - actual data; however, I don't think
anyone has disputed your experience, just the explanation for it.

Maybe you can locate a glider with a nose hook and a CG hook to test. I
never noticed any difference in control between the two locations with
my ASW 20B (actually, a forward hook), but I did notice it went much
straighter during the initial ground roll.

--
Eric Greenwell - Washington State, USA (change ".netto" to ".us" to
email me)
- "A Guide to Self-launching Sailplane Operation Mar/2004" Much of what
you need to know tinyurl.com/yfs7tnz