poor lateral control on a slow tow?

On Jan 6, 5:22*am, Eric Greenwell wrote:
On 1/5/2011 10:52 AM, 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.

(big snip)

Andreas' posting was the clearest description for me of the wake effect.
I'd love to see "3-D" perspective view of the wake behind a towplane, as
I doubt I'm visualizing it well.

--
Eric Greenwell - Washington State, USA (change ".netto" to ".us" to
email me)- Hide quoted text -

- Show quoted text -

One possible explanation for the slow tow effect is that although the
glider is (or should be) above the main wake and prop wash from the
tug, it is flying through air that has been pushed down by the tugs
wing. Hence it has to fly at a higher angle of attack to maintain
position. As this would only be a transitory effect that does not
extend too far behind the tug, using a longer rope should reduce this
effect. Certainly it seems easier to aerotow on a long rope than a
short one.

Derek C

At 05:22 06 January 2011, Eric Greenwell wrote:
On 1/5/2011 10:52 AM, 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.

(big snip)

Andreas' posting was the clearest description for me of the wake effect.

I'd love to see "3-D" perspective view of the wake behind a towplane,
as
I doubt I'm visualizing it well.

--
Eric Greenwell - Washington State, USA (change ".netto" to ".us" to
email me)

someone else posted this link before - there's a good picture half way
down of the tip vortex behind a typical towplane

http://www.sciencebuddies.org/scienc...Zoo_p057.shtml

On Wed, 05 Jan 2011 21:22:53 -0800, Eric Greenwell
wrote:


I'd love to see "3-D" perspective view of the wake behind a towplane, as
I doubt I'm visualizing it well.

Have you seen this?
http://www.centennialofflight.gov/es...tex/TH15G5.htm



BTW: Have you already seen this? (starts at 0:55):
http://www.youtube.com/watch?v=__pyxPb6gMc

Note how long the air behind the plane continues to sink after the
plane has passed... and how the wing tip vortices and the downwash
behind the wing interact.



Andreas

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.

On Thu, 06 Jan 2011 09:09:39 +0000, Doug Greenwell
wrote:


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 (!!).

Coooooooooool.



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.

Definitely. I think that the slipstream and the turbulence of that
huge propellor might have an influence, too.


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.

Seems like some gliders actually stabilize themselves behind a tow
plane.

Here's an example of a free-flight test of a space shuttle model that
flew well in aerotow, but worse in free flight.


Ladies and gents, Great Britains only serious contribution to
spaceflight - the Reliant Shuttle:
http://www.youtube.com/watch?v=pJdrlWR-yFM



Andreas

On Jan 5, 1:52*pm, 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

Best explaination so far!

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