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poor lateral control on a slow tow?



 
 
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  #1  
Old December 31st 10, 12:40 PM posted to rec.aviation.soaring
Doug[_5_]
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Posts: 2
Default poor lateral control on a slow tow?

As an aerodynamicist/flight dynamicist recently re-soloed after 25 years
off, people keep asking me hard questions. One that has come up recently is
why a heavy glider on tow feels horrible, but thermalling in the same glider
at lower speeds is fine? (see also Mike Fox's article on aerotowing in the
October issue of S&G).

I did some calculations, and I reckon it's probably due to the tug wing wake
(tip vortices generating a downwash inboard, upwash outboard) changing the
lift distribution on the glider wing - with an increased angle of attack out
at the tips reducing aileron effectiveness. There's possibly an interesting
academic research project here, but it's always best to get a reality check
first ...

Is poor handling at low speed on tow a common experience? I'd appreciate
any thoughts/comments/war stories ... particularly bad tug/glider/speed
combinations, incidents of wing drop during a tow etc etc?

Doug Greenwell


  #2  
Old December 31st 10, 03:03 PM posted to rec.aviation.soaring
wladkummer76
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Posts: 15
Default poor lateral control on a slow tow?

On Dec 31, 9:40*am, "Doug" wrote:
As an aerodynamicist/flight dynamicist recently re-soloed after 25 years
off, people keep asking me hard questions. *One that has come up recently is
why a heavy glider on tow feels horrible, but thermalling in the same glider
at lower speeds is fine? (see also Mike Fox's article on aerotowing in the
October issue of S&G).

I did some calculations, and I reckon it's probably due to the tug wing wake
(tip vortices generating a downwash inboard, upwash outboard) changing the
lift distribution on the glider wing - with an increased angle of attack out
at the tips reducing aileron effectiveness. *There's possibly an interesting
academic research project here, but it's always best to get a reality check
first ...

Is poor handling at low speed on tow a common experience? *I'd appreciate
any thoughts/comments/war stories ... particularly bad tug/glider/speed
combinations, incidents of wing drop during a tow etc etc?

Doug Greenwell


Sure, even more with flaps equipped ships! But yaw control seems to be
more affected than roll. Can we explain that?
Just a little faster please is what doctor ordered here...usually 5
mph more is enough to a more pleasant tow.
  #3  
Old December 31st 10, 03:44 PM posted to rec.aviation.soaring
Andy[_1_]
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Posts: 1,565
Default poor lateral control on a slow tow?

On Dec 31, 4:40*am, "Doug" wrote:

Is poor handling at low speed on tow a common experience?


Well not for me, but only because I don't often get towed slow. For
my ballasted ASW-28, 65kts indicated in the glider is as slow as I'd
like to be. On the few tows that the speed has dropped to 60kts it's
barely possible to stay in control in any sort of turbulence. Both
pitch and roll control are greatly reduced, I never noticed a problem
with yaw control.

Slow tows only seem to be a problem for me when I go to an site that
uses tow pilots that have no high performance glider experience but
they still know better than the glider pilot what speeds are needed.
Fortunately there are not too many of those.

The worse possible case is the tow pilot that refuses to follow the
simple instruction to stay in ground effect until a safe tow speed is
reached.

Some glider pilots don't realize how hopelessly inaccurate the tug
airspeed indicator may be on tow and they ask for the speed they'd
like to see in the glider. Bad idea. Far better to ask for a speed x
kts or mph higher than they use for the gliders they tow everyday.

Andy

  #4  
Old December 31st 10, 03:59 PM posted to rec.aviation.soaring
Doug[_5_]
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Posts: 2
Default poor lateral control on a slow tow?


"wladkummer76" wrote in message
...
On Dec 31, 9:40 am, "Doug" wrote:
As an aerodynamicist/flight dynamicist recently re-soloed after 25 years
off, people keep asking me hard questions. One that has come up recently
is
why a heavy glider on tow feels horrible, but thermalling in the same
glider
at lower speeds is fine? (see also Mike Fox's article on aerotowing in the
October issue of S&G).

I did some calculations, and I reckon it's probably due to the tug wing
wake
(tip vortices generating a downwash inboard, upwash outboard) changing the
lift distribution on the glider wing - with an increased angle of attack
out
at the tips reducing aileron effectiveness. There's possibly an
interesting
academic research project here, but it's always best to get a reality
check
first ...

Is poor handling at low speed on tow a common experience? I'd appreciate
any thoughts/comments/war stories ... particularly bad tug/glider/speed
combinations, incidents of wing drop during a tow etc etc?

Doug Greenwell


Sure, even more with flaps equipped ships! But yaw control seems to be
more affected than roll. Can we explain that?
Just a little faster please is what doctor ordered here...usually 5
mph more is enough to a more pleasant tow.

Good question ... the only thing I can think of at the moment is that the
tug tip vortices would also generate an inwards 'sidewash' flow above the
wake, which would be stronger the heavier and slower the tug was. If you
are then for some reason off to one side of the tug the sidewash at the fin
from the nearer vortex would tend to yaw you 'nose-out', in the wrong
direction.

Doug


  #5  
Old December 31st 10, 04:05 PM posted to rec.aviation.soaring
John Cochrane[_2_]
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Posts: 237
Default poor lateral control on a slow tow?

Is poor handling at low speed on tow a common experience?

Well not for me, but only because I don't often get towed slow. *For
my ballasted ASW-28, 65kts indicated in the glider is as slow as I'd
like to be. *On the few tows that the speed has dropped to 60kts it's
barely possible to stay in control in any sort of turbulence. *Both
pitch and roll control are greatly reduced, I never noticed a problem
with yaw control.


This is precisely the aerodynamic puzzle. The same standard class
glider stalls in the low 40s and is perfectly happy thermaling at
45-50. So why does 60 feel so awful on tow?

This does seem a lot worse in standard class than 15 meter, another
hint for the puzzle.

If it were downwash, one would think that just flying higher would
solve it. But that's the best theory I've heard so far.

I have seen quite a few contests with tow pilots who had little
experience with fully ballasted gliders. It's really important to get
the word out to them 1) yes, we really want to tow that fast. 2) get
up to speed in ground effect, then start climbing.

I distinctly remember that helpless feeling sitting in a tanked up
discus, crossing the end of the runway, as the towplane departed what
looked like straight up at about 50 knots.

On another occasion, every single tow by one pilot was accompanied by
a chorus of demands for more speed on the radio. Eventually he piped
up "what do y'all want to fly so fast for anyway?'' I guess the 2-33
never wanted to go 70 on tow.

John Cochrane
  #6  
Old December 31st 10, 04:25 PM posted to rec.aviation.soaring
jcarlyle
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Posts: 522
Default poor lateral control on a slow tow?

The worst towing experiences I've had has been during aero retrieves.
Once the tugs (I've seen this behind an L-19 and a Pawnee) reached
cruising altitude, it became extremely difficult to stay behind them.
I asked them to slow down (went from 85 to 65 kt), got into low tow,
slipped and even pulled on some spoiler, but my LS8 (unballasted) was
a handful in roll and pitch. Once on a turbulent blue day I
deliberately released 25 miles before I had final glide, preferring
the excitement of finding a means of staying up rather than continuing
on tow.

-John
  #7  
Old December 31st 10, 04:31 PM posted to rec.aviation.soaring
Doug Greenwell
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Posts: 67
Default poor lateral control on a slow tow?

At 15:05 31 December 2010, John Cochrane wrote:
Is poor handling at low speed on tow a common experience?


Well not for me, but only because I don't often get towed slow.

=A0For
my ballasted ASW-28, 65kts indicated in the glider is as slow as I'd
like to be. =A0On the few tows that the speed has dropped to 60kts

it's
barely possible to stay in control in any sort of turbulence. =A0Both
pitch and roll control are greatly reduced, I never noticed a problem
with yaw control.


This is precisely the aerodynamic puzzle. The same standard class
glider stalls in the low 40s and is perfectly happy thermaling at
45-50. So why does 60 feel so awful on tow?

This does seem a lot worse in standard class than 15 meter, another
hint for the puzzle.

If it were downwash, one would think that just flying higher would
solve it. But that's the best theory I've heard so far.

I have seen quite a few contests with tow pilots who had little
experience with fully ballasted gliders. It's really important to get
the word out to them 1) yes, we really want to tow that fast. 2) get
up to speed in ground effect, then start climbing.

I distinctly remember that helpless feeling sitting in a tanked up
discus, crossing the end of the runway, as the towplane departed what
looked like straight up at about 50 knots.

On another occasion, every single tow by one pilot was accompanied by
a chorus of demands for more speed on the radio. Eventually he piped
up "what do y'all want to fly so fast for anyway?'' I guess the 2-33
never wanted to go 70 on tow.

John Cochrane


In the normal tow position I don't think you can get high enough to get
away from the wing wake without tipping the tug on its nose. On the other
hand, my simple modelling suggests that a typical low tow position should
be a lot better.

15m vs standard class ... Forgive my ignorance (I've never flown a
flapped glider), but presumably a 15m glider is towed with the flaps
deflected, which would tend to unload the tips and hence maintain aileron
authority at high incidence. Would the ailerons typically be drooped at
this flap setting?

Doug

  #8  
Old December 31st 10, 05:07 PM posted to rec.aviation.soaring
Andy[_1_]
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Posts: 1,565
Default poor lateral control on a slow tow?

On Dec 31, 8:05*am, John Cochrane
wrote:


The same standard class glider stalls in the low 40s and is perfectly happy
thermaling at 45-50.


Not mine, at least not when ballasted. Most of my flying is done
where the thermals are small cored and often rowdy. 50kts would only
work in a very smooth thermal that was big enough to use a low bank
angle.

However, the D2 which does seem to climb well at low speeds seems to
be really sensitive to being towed slow, at least that I understand
from one of the local pilots. Maybe D2 owners could comment on that?

Andy
  #9  
Old December 31st 10, 05:35 PM posted to rec.aviation.soaring
sisu1a
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Posts: 569
Default poor lateral control on a slow tow?


Sure, even more with flaps equipped ships! But yaw control seems to be
more affected than roll. Can we explain that?
Just a little faster please is what doctor ordered here...usually 5
mph more is enough to a more pleasant tow.


I'm going to assume your flying with a nosehook, in which case the big
tether is resisting motion in the yaw axis... whereas for roll the
towrope runs directly down the longitudinal axis, leaving airspeed/AoA
as the dominant external forces affecting roll.

The rope also holds the glider at unnatural attitudes for given
airspeeds, which I think contributes a lot to the lack of perceived
(less than usual for that particular airspeed) aileron authority,
contributing greatly to making the glider feel horrible on tow,
despite being well within it's stall speed.

Not as an endorsement for or against this arrangement but from an
aerodynamic prospective rather, a CG hook leaves the yaw axis pretty
free to swivel accordingly and likely reduces the rope induced AoA
affecting aileron authority as well *once in a steady climb. (*not to
be confused with the up pitching tendency these hooks are famous for
during the initial acceleration of a launch...)

-Paul
  #10  
Old December 31st 10, 06:19 PM posted to rec.aviation.soaring
Doug Greenwell
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Posts: 67
Default poor lateral control on a slow tow?

At 16:35 31 December 2010, sisu1a wrote:

Sure, even more with flaps equipped ships! But yaw control seems to be
more affected than roll. Can we explain that?
Just a little faster please is what doctor ordered here...usually 5
mph more is enough to a more pleasant tow.


I'm going to assume your flying with a nosehook, in which case the big
tether is resisting motion in the yaw axis... whereas for roll the
towrope runs directly down the longitudinal axis, leaving airspeed/AoA
as the dominant external forces affecting roll.


good point - a nose hook would tend to improve yaw and pitch stability on
tow, but have little effect on roll, so it's difficult to split out the
effects of aerodynamics and tow cable dynamics. I did some work years ago
on towed bodies (aerial targets and sonar fish) and the coupling effects
between cable and body motion get really complicated.

The rope also holds the glider at unnatural attitudes for given
airspeeds, which I think contributes a lot to the lack of perceived
(less than usual for that particular airspeed) aileron authority,
contributing greatly to making the glider feel horrible on tow,
despite being well within it's stall speed.


yes - I'm sure there must be a pyschological element as well


Not as an endorsement for or against this arrangement but from an
aerodynamic prospective rather, a CG hook leaves the yaw axis pretty
free to swivel accordingly and likely reduces the rope induced AoA
affecting aileron authority as well *once in a steady climb. (*not to
be confused with the up pitching tendency these hooks are famous for
during the initial acceleration of a launch...)

-Paul


I think the rope effects on stability with a nose hook are generally
favourable - its the aerodynamic effects of the tug wake that cause the
problems


 




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