poor lateral control on a slow tow?

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

On Jan 5, 6:52*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

The two most scary aerotows I have ever had we

1) 2 up in a K13 behind a Rotax engined Falke at about 50 knots
indicated airspeed

2) 2 up in a K13 behind a 150hp Piper Cub when we visited another
site. This tug wasn't very powerful anyway and its pilot seemed to be
trying to demonstrate how slowly he could fly. Indicated airspeed
slightly under 50 knots.

In both cases the glider wallowed about and it seemed very difficult
to keep above the wake turbulence/prop wash.

I have not been towed by a Dimona or Katana, but they seem to be a bit
faster than the above, so may not give the same problems. I think the
problem is more lack of airspeed than the type of the tug aircraft.

Greetings from (now) snow free England,

Derek C

I'm still not convinced by those who propose that the wings of the
glider generate no extra lift (or even generate less lift) when
climbing on tow.

We know that on a winch launch the glider climbs because the wings
generate more lift than in level/descending flight. This must be true
because there is nothing pulling it up.

However, we are told that on aerotow the wings generate the same (or
less) lift as in level/descending flight and the tug just pulls the
glider up the slope.

Does this mean that the tug climbs in the same way, i.e. wings
generate only enough lift to carry the weight of the tug, and the prop
drags the tug up the slope? This doesn't match what I've read about
how aircraft work. L=W only in level flight. I think the tug's wings
generate more lift than its weight, and thus it climbs.

If this is true, the same must be true for the glider behind it.

Bring on an aerodynamicist to show me I'm wrong.

Ok - so that would help in reducing stall speed slightly, but would not
help with the spanwise lift distribution.

Is the aileron/flap interconnect a standard arrangement, or are there
flapped gliders without it?

It's optional on a 1-35. (some have it, some don't) but I'm not sure
about the others. Those who have flown both don't report much
difference.

On Wed, 5 Jan 2011 15:51:05 -0800 (PST), ProfChrisReed
wrote:

L=W only in level flight.

Wrong.

The correct term is

L=W in STRAIGHT flight.

If both forces are not equal, the resulting flightpath is going to be
a circular arc.

It's simple vector addition.


Regards
Andreas

On Wed, 5 Jan 2011 15:09:39 -0800 (PST), Derek C
wrote:

Hi Derek,


The two most scary aerotows I have ever had we

1) 2 up in a K13 behind a Rotax engined Falke at about 50 knots
indicated airspeed

Well... 50 kts is pretty slow...

2) 2 up in a K13 behind a 150hp Piper Cub when we visited another
site. This tug wasn't very powerful anyway and its pilot seemed to be
trying to demonstrate how slowly he could fly. Indicated airspeed
slightly under 50 knots.

Clear case: Low aspect ratio, wing loading twice of the ASK-13. How
much above the stall speed of the Cub? 10 kts at maximum? Scary...

I guess you had a word with the tow pilot afterwards.

In both cases the glider wallowed about and it seemed very difficult
to keep above the wake turbulence/prop wash.

Yes, the typical situation for a very slow aerotow.


I have not been towed by a Dimona or Katana, but they seem to be a bit
faster than the above, so may not give the same problems. I think the
problem is more lack of airspeed than the type of the tug aircraft.

Well, I guess we both agree that this problem only manifests itself at
the low-speed area of the envelope, don't we?

The general consensus here in Germany (as well as my own experience)
is that an aerotow behind a motorglider is *much* easier to control
despite the fact that it takes place at 110-115 kph (60-63 kts)
instead of the 130-140 kph (70-75 kts) that are typical for Morane MS
893 and Robin Remorqeur.
Of course similar wing loadings result in similar reactions to gusts,
which helps to follow the tow plane.


Regards
Andreas

On Jan 6, 2:20*am, Andreas Maurer wrote:
On Wed, 5 Jan 2011 15:09:39 -0800 (PST), Derek C

wrote:

Hi Derek,

The two most scary aerotows I have ever had we

1) 2 up in a K13 behind a Rotax engined Falke at about 50 knots
indicated airspeed

Well... 50 kts is pretty slow...

2) 2 up in a K13 behind a 150hp Piper Cub when we visited another
site. This tug wasn't very powerful anyway and its pilot seemed to be
trying to demonstrate how slowly he could fly. Indicated airspeed
slightly under 50 knots.

Clear case: Low aspect ratio, wing loading twice of the ASK-13. How
much above the stall speed of the Cub? 10 kts at maximum? Scary...

I guess you had a word with the tow pilot afterwards.

In both cases the glider wallowed about and it seemed very difficult
to keep above the wake turbulence/prop wash.

Yes, the typical situation for a very slow aerotow.

I have not been towed by a Dimona or Katana, but they seem to be a bit
faster than the above, so may not give the same problems. I think the
problem is more lack of airspeed than the type of the tug aircraft.

Well, I guess we both agree that this problem only manifests itself at
the low-speed area of the envelope, don't we?

The general consensus here in Germany (as well as my own experience)
is that an aerotow behind a motorglider is *much* easier to control
despite the fact that it takes place at 110-115 kph (60-63 kts)
instead of the 130-140 kph (70-75 kts) *that are typical for Morane MS
893 and Robin Remorqeur.
Of course similar wing loadings result in similar reactions to gusts,
which helps to follow the tow plane.

Regards
Andreas

Yes, but 50 knots (93kph) should be OK for a K13 when its stalling
speed in free flight is only about 36 knots (67kph). But it clearly
isn't! Therefore either the wing loading or the angle of attack (or
both) must be greater during an aerotow than in free flight.

Cheers,
Derek C

On 1/5/2011 12:53 AM, Doug Greenwell wrote:
At 02:13 05 January 2011, AGL wrote:

Has anyone tried some flaps in an integrated flap machine (which
reduces stall speed) to see if the wallowing goes away?

Unfortunately, my trailer is in a snowbank.

That's a useful comment: the aerodynamic modelling I did suggests that
the lateral control problems on tow should be different (worse) than those
in a typical stall, because the wing is stalling at the tips rather than
the root.

No-one has yet admitted to actually stalling or dropping a wing on tow -
so the effect seems to be annoying rather than dangerous.

Flaps should (theoretically) improve matters by (a) reducing stall speed
and (b) shifting the spanwise lift distribution inboard and unloading the
tips. However, if the flaps are integrated with the ailerons then the
associated aileron droop would counteract (b).

My ASH 26E has the flaps and ailerons moving in unison in the 1,2, and 3
"cruise" positions; flap 4 (thermal) moves the flaps down a bit more
than the ailerons, so there is some washout in the wing. That might make
it handle better during a slow tow than it otherwise would. I've never
had a slow tow, so I can't say. If I ever take another tow, I'll compare
flap 3 and 4 with the tow plane going "slow".

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

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)

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