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poor lateral control on a slow tow?



 
 
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  #11  
Old December 31st 10, 06:21 PM posted to rec.aviation.soaring
Andy[_1_]
external usenet poster
 
Posts: 1,565
Default poor lateral control on a slow tow?

On Dec 31, 9:35*am, 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.

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


Since the possible influence of hook position has been introduced,
I'll have to add that all my tows in the 28 have been on the CG hook
as were all my tows in the 19 I had before that.

One factor that has not been mentioned yet is the fact that a
ballasted glider has much higher roll inertia that a dry one. This
will require larger aileron inputs to get the same roll response and I
suppose could lead to an aileron stall.

I don't remember ever having loss of control authority on an
unballasted tow and maybe there is more to that than just the
difference in stall speeds.

Andy
  #12  
Old December 31st 10, 07:19 PM posted to rec.aviation.soaring
bildan
external usenet poster
 
Posts: 646
Default poor lateral control on a slow tow?

On Dec 31, 4:40*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


I suspect, but can't know unless I flew with you, that you are
unconsciously trying to "steer" the glider with ailerons. Overuse of
ailerons is very common and it makes aero tow 'wobbly'. If you
consciously use rudder to aim the nose at the tug's tail and just keep
the same bank angle as the tug with ailerons, it might work better.

Wake effects are generally favorable if you stay at the right height
relative to the tug. Using a slightly higher tow position can
sometimes help a lot.

The tip vortices rotate inward above the propwash which, if allowed to
do so, will drift the glider to the center position and help keep it
there. I haven't noticed any tendency for them to yaw a glider
towards a tugs wing tip.
  #13  
Old December 31st 10, 07:52 PM posted to rec.aviation.soaring
Doug Greenwell
external usenet poster
 
Posts: 67
Default poor lateral control on a slow tow?

At 18:19 31 December 2010, bildan wrote:
On Dec 31, 4:40=A0am, "Doug" wrote:
As an aerodynamicist/flight dynamicist recently re-soloed after 25

years
off, people keep asking me hard questions. =A0One that has come up

recent=
ly is
why a heavy glider on tow feels horrible, but thermalling in the same

gli=
der
at lower speeds is fine? (see also Mike Fox's article on aerotowing

in
th=
e
October issue of S&G).

I did some calculations, and I reckon it's probably due to the tug

wing
w=
ake
(tip vortices generating a downwash inboard, upwash outboard) changing

th=
e
lift distribution on the glider wing - with an increased angle of

attack
=
out
at the tips reducing aileron effectiveness. =A0There's possibly an

intere=
sting
academic research project here, but it's always best to get a reality

che=
ck
first ...

Is poor handling at low speed on tow a common experience? =A0I'd

apprecia=
te
any thoughts/comments/war stories ... particularly bad

tug/glider/speed
combinations, incidents of wing drop during a tow etc etc?

Doug Greenwell


I suspect, but can't know unless I flew with you, that you are
unconsciously trying to "steer" the glider with ailerons. Overuse of
ailerons is very common and it makes aero tow 'wobbly'. If you
consciously use rudder to aim the nose at the tug's tail and just keep
the same bank angle as the tug with ailerons, it might work better.

Wake effects are generally favorable if you stay at the right height
relative to the tug. Using a slightly higher tow position can
sometimes help a lot.

The tip vortices rotate inward above the propwash which, if allowed to
do so, will drift the glider to the center position and help keep it
there. I haven't noticed any tendency for them to yaw a glider
towards a tugs wing tip.


Certainly my early aerotows were a bit wobbly, although they've settled
down now - I (re-)soloed on an aerotow, which I think is relatively
unusual in the UK, but most of my launches since then have been on the
winch.

The problem is a bit more than wobbliness though - there does seem to be a
common theme of experienced pilots having real control difficulties when
heavy and a bit slow on tow (not a situation I've been in myself ...
yet).

Part of the difficulty in analysing this is splitting out the effects of
the aerodynamic wake/wing/fin interaction(s) and the mechanical cable
dynamics.

It just occured to me that it might be interesting to do this by flying a
motor glider in formation with a tug - no cable, so only the aerodynamic
effects to contend with. There has been a lot of work done on
aerodynamics of flrmation flight and towing, but everything published has
been on aircraft with either the same span or with the lead aircraft much
larger than the trailing aircraft.

  #14  
Old December 31st 10, 08:07 PM posted to rec.aviation.soaring
John Cochrane[_2_]
external usenet poster
 
Posts: 237
Default poor lateral control on a slow tow?


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


The D2 also has a very low angle of incidence and thus a high nose
attitude on tow. Could this be part of the issue -- it's just darn
uncomfortable to fly with the nose pointed above the towplane? That
would also account for why 15 meter seems easier. We fly with flaps;
they are interconnected to the ailerons so we're not getting great
roll rates on tow. But it does give a nose down attitude so we can see
the towplane.

John Cochrane
  #15  
Old December 31st 10, 09:06 PM posted to rec.aviation.soaring
Todd
external usenet poster
 
Posts: 73
Default poor lateral control on a slow tow?

I too agree with the real or perceived tow handling characteristics.

Looking at things from and aerodynamics standpoint (and I am about as
far from and aerodynamicist as you can get) it should seem that part
of the empirical data would suggest an experiment where you fly a
glider equipped with and Angel of Attack meter at your typical tow
speeds and record the AoA at various speeds. Then fly that glider on
tow at those same speeds and record the results.

I would suspect that we will find that the AoA is higher (e.g. closer
to the critical stall AoA ) and that this would explain the handling
characteristics. So the million dollar question is why? My guess is
that in soaring flight we are essentially coasting down hill at say
60Kt. In towing flight at 60Kt, we are being "dragged" up the hill
(plus, maybe, some effect from tow rope attachment point leverage) so
something, aerodynamically. has to be different and that has (???) to
be the AoA. As we all know from out stall training, as we approach
the critical angle of attack, we loose out roll (aileron)
effectiveness.

....or should I go back to learning to flip burgers at McD's?

  #16  
Old December 31st 10, 09:09 PM posted to rec.aviation.soaring
Derek C
external usenet poster
 
Posts: 114
Default poor lateral control on a slow tow?

On Dec 31, 6:19*pm, bildan wrote:
On Dec 31, 4:40*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


I suspect, but can't know unless I flew with you, that you are
unconsciously trying to "steer" the glider with ailerons. *Overuse of
ailerons is very common and it makes aero tow 'wobbly'. *If you
consciously use rudder to aim the nose at the tug's tail and just keep
the same bank angle as the tug with ailerons, it might work better.

Wake effects are generally favorable if you stay at the right height
relative to the tug. *Using a slightly higher tow position can
sometimes help a lot.

The tip vortices rotate inward above the propwash which, if allowed to
do so, will drift the glider to the center position and help keep it
there. *I haven't noticed any tendency for them to yaw a glider
towards a tugs wing tip.- Hide quoted text -

- Show quoted text -


There was a debate on our club forum about why gliders feel
uncomfortable on slow tows that are still well above their normal
stalling speed. We think the answer is that the glider is being asked
to climb with the tug providing the thrust via the rope. The glider is
still effectively in free flight and therefore has to fly at a greater
angle of attack for a given airspeed to produce the extra lift for
climbing. Hence its stalling speed is somewhat increased.

Derek C

  #17  
Old December 31st 10, 09:23 PM posted to rec.aviation.soaring
bildan
external usenet poster
 
Posts: 646
Default poor lateral control on a slow tow?

On Dec 31, 1:06*pm, Todd wrote:
I too agree with the real or perceived tow handling characteristics.

Looking at things *from and aerodynamics standpoint (and I am about as
far from and aerodynamicist as you can get) it should seem that part
of the empirical data would suggest an experiment where you fly a
glider equipped with and Angel of Attack meter at your typical tow
speeds and record the AoA at various speeds. *Then fly that glider on
tow at those same speeds and record the results.


Done that - and as nearly as I can see, there's no difference in AoA.

I've flown some pretty heavy high performance gliders behind some
pretty bad tow pilots - one of them stalled the tug with me on tow.
If I'm careful not to over-control the ailerons, there's no problem at
all.

Heavily ballasted gliders respond sluggishly in roll just due to the
extra roll inertia. A pilot trying to hold a precise position behind
a tug needs and expects crisp aileron response. When he doesn't get
it, he increases the amount and frequency of aileron with a
corresponding increase in adverse yaw. If he's less than equally
crisp with rudder to oppose the adverse yaw, it gets wobbly.
  #18  
Old December 31st 10, 10:47 PM posted to rec.aviation.soaring
Martin Gregorie[_5_]
external usenet poster
 
Posts: 1,224
Default poor lateral control on a slow tow?

On Fri, 31 Dec 2010 12:09:08 -0800, Derek C wrote:

On Dec 31, 6:19Â*pm, bildan wrote:
On Dec 31, 4:40Â*am, "Doug" wrote:





As an aerodynamicist/flight dynamicist recently re-soloed after 25
years off, people keep asking me hard questions. Â*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. Â*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? Â*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


I suspect, but can't know unless I flew with you, that you are
unconsciously trying to "steer" the glider with ailerons. Â*Overuse of
ailerons is very common and it makes aero tow 'wobbly'. Â*If you
consciously use rudder to aim the nose at the tug's tail and just keep
the same bank angle as the tug with ailerons, it might work better.

Wake effects are generally favorable if you stay at the right height
relative to the tug. Â*Using a slightly higher tow position can
sometimes help a lot.

The tip vortices rotate inward above the propwash which, if allowed to
do so, will drift the glider to the center position and help keep it
there. Â*I haven't noticed any tendency for them to yaw a glider towards
a tugs wing tip.- Hide quoted text -

- Show quoted text -


There was a debate on our club forum about why gliders feel
uncomfortable on slow tows that are still well above their normal
stalling speed. We think the answer is that the glider is being asked to
climb with the tug providing the thrust via the rope. The glider is
still effectively in free flight and therefore has to fly at a greater
angle of attack for a given airspeed to produce the extra lift for
climbing. Hence its stalling speed is somewhat increased.

If the tug's downwash field extends back far enough to include the
glider, its AOA will be relative to the downwash streamlines. Add the
downwash angle to the climb angle of the tug-glider combination will make
the glider look quite nose-high to its pilot.

I know that the downwash angle is roughly 1/3 of the wing AOA at 4-5
chords behind the wing, i.e. about where the tailplane is, but not what
its angle might be at the end of a tow rope.


--
martin@ | Martin Gregorie
gregorie. | Essex, UK
org |
  #19  
Old January 1st 11, 12:25 AM posted to rec.aviation.soaring
Andy[_10_]
external usenet poster
 
Posts: 261
Default poor lateral control on a slow tow?

On Dec 31, 1:47*pm, Martin Gregorie
wrote:
On Fri, 31 Dec 2010 12:09:08 -0800, Derek C wrote:
On Dec 31, 6:19*pm, bildan wrote:
On Dec 31, 4:40*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


I suspect, but can't know unless I flew with you, that you are
unconsciously trying to "steer" the glider with ailerons. *Overuse of
ailerons is very common and it makes aero tow 'wobbly'. *If you
consciously use rudder to aim the nose at the tug's tail and just keep
the same bank angle as the tug with ailerons, it might work better.


Wake effects are generally favorable if you stay at the right height
relative to the tug. *Using a slightly higher tow position can
sometimes help a lot.


The tip vortices rotate inward above the propwash which, if allowed to
do so, will drift the glider to the center position and help keep it
there. *I haven't noticed any tendency for them to yaw a glider towards
a tugs wing tip.- Hide quoted text -


- Show quoted text -


There was a debate on our club forum about why gliders feel
uncomfortable on slow tows that are still well above their normal
stalling speed. We think the answer is that the glider is being asked to
climb with the tug providing the thrust via the rope. The glider is
still effectively in free flight and therefore has to fly at a greater
angle of attack for a given airspeed to produce the extra lift for
climbing. Hence its stalling speed is somewhat increased.


If the tug's downwash field extends back far enough to include the
glider, its AOA will be relative to the downwash streamlines. Add the
downwash angle to the climb angle of the tug-glider combination will make
the glider look quite nose-high to its pilot. *

I know that the downwash angle is roughly 1/3 of the wing AOA at 4-5
chords behind the wing, i.e. about where the tailplane is, but not what
its angle might be at the end of a tow rope.

--
martin@ * | Martin Gregorie
gregorie. | Essex, UK
org * * * |


I'd be surprised if the flow field from the towplane wake is
significant for gliders in normal high tow position. I do wonder if
the "sluggish controls" effect is to some extent psychological because
flying formation requires much more precision than normal slow flight
off tow. I'm most uncomfortable when I find myself slow and below the
towplane and need to climb up.

Unless the glider is accelerating vertically, I'm pretty sure that
steady climb requires the same amount of lift as steady glide. Steady
climb is not the same as accelerating climb. (F=MxA so if the lifting
force exceeds the glider's weight by definition it accelerates
vertically).

The towplane provides thrust to overcome the frictional and lift-
related drag losses, but unless you are well below the towplane the
force on the rope is, for all practical purposes, horizontal. If you
have a cg hook you will get a modest nose-up pitching moment from the
rope, but this is a trim issue more than an AOA issue I believe. The
tension on the rope could also provide some counter-force to rudder
and elevator inputs, but I don't think you'd feel much for small
angular displacements.

9B
  #20  
Old January 1st 11, 12:35 AM posted to rec.aviation.soaring
Eric Greenwell[_4_]
external usenet poster
 
Posts: 1,939
Default poor lateral control on a slow tow?

On 12/31/2010 3:40 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 ...


The wake behind a climbing towplane should be well below a glider in the
high tow position. How can it lift it's weight and the glider, if the
wake isn't descending? Recall one of the exercises a student does while
learning to tow is to start in the high tow position, then move straight
down until the wake turbulence is felt. With the usual 150' to 200'
rope, it's way below the high tow position. So, I don't think it's wake
turbulence, IF the glider is in the high tow position.

I think a big part of the answer is the pilot's perception of
"attitude": the glider has an additional attitude (relative to the
horizon) over it's normal angle of attack because the glider and tug are
ascending at about a 6 degree angle. T His is perceived by the pilot as
a very nose high attitude, and makes him feel uncomfortable; that, along
with reduced control response, makes him feel it's worse than it really
is. Usually, this happens close to the ground, making the perception
even worse.

The unusually nose-high attitude can keep the glider pilot from using
enough up elevator, with the consequence that he does sink down into the
wake. That will make the situation actually worse, not just perceptually
worse. But, it's because he is far from the high tow position, not just
because the speed is slower than normal.

This analysis obviously assumes a high tow as the normal situation, and
may not apply to the situation where low tow is the norm.

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




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