poor lateral control on a slow tow?

At 18:52 05 January 2011, Andreas Maurer wrote:
On Wed, 5 Jan 2011 09:23:29 -0800 (PST), Derek C
wrote:

Gliders appear to get near to the stall during slow aerotows at much
greater than their normal free flight stalling airspeeds. I would
suggest that aerotowing must increase the wing loading in some way.

I have to admit that I didn't bother to read all the 120+ postings
about this topic, so please forgive me if the things that I'm going to
post have already been mentioned in this thread.


The main factor for the seemingly odd flying characteristics behind
the tow plane is the downwash of the latter.


Let me explain:
The downwash has a significant angle (the air is deflected downwards
behind the tow plane's wing to up to four degrees!), but due to the
larger span of the glider it only affects the inner part of the
glider's wing.

Therefore, if the glider if lying laterally displaced, only one wing
is affected by the downwash of the tow plane - four degrees of AoA
difference between left and right wing need a lot of aileron to
correct.

Likeise, if the glider is flying straight behind the tow plane, the
downwash *decreases* the AoA of the affected inner part of the wing.
Getting the nose up by pulling back will restore the lift of the inner
part of the glider's wing, but now the outer parts of the wing have a
much higher AoA than they have in free flight.
Voila, meet the the conditions for poor alieron efficiency (high AoA!)
and tip stall.


The downwash is reduced by
- wingloading of the tow plane
- wing span of the tow plane

In other words: The more a tow plane looks like a motorglider (say, a
Dimona, or Katana Extreme), the less the flight characteristics of the
glider are affected.
Anyone who has ever been towed behind a motorglider or a microlight
will testify that problems like poor lateral control or running out
of elevator don't exist there, despite a far slower tow (55 kts
compared to a typical 70-75 kts behind a typical tow plane like
Reorqeur or Pawnee).


One interesting fact:
When Akaflieg Braunschweig flight-tested their SB-13 flying wing (with
a back-swept wing), they encountered a nose-down momentum after
lift-off that could not be recovered and usually lead to a crash
immediately after lift-off.

Explanation:
The downwash of the tow plane (Robin Remorqeur) hit the inner part of
the wing, decreasing its AoA (and lift) and therefore shifting the
center of lift backwards due to the sweepback.

Increasing the length of the tow rope helped.



Greetings from a snowy Germany
Andreas

Interesting experience with the SB-13.

There's a chapter in Eric Brown's book 'Wings of the Weird &
Wonderful' in which he describes flight tests of the GAL 56 flying wing
glider in 1946. This was a 28deg swept wing with an aspect ratio of 5.8
towed by a Spitfire IX* (!!!) to 20000ft (!!).

He describes the opposite effect, with a very strong (often
uncontrollable) nose-up pitch on take-off - this was thought to be due to
ground effect. In this case the tug span was similar (37ft) to the glider
span (45ft), so the wake/wing interaction would be different.

Interestingly he also reports that the GAL56 could be flown hands-free on
the tow - unless the tug slipstream was entered, in which case all lateral
and longitudinal control was lost. Robert Kronfield was later killed
spinning this aircraft.