Aerodynamics of aero towing

[Martin Gregorie[_5_]]

This is a quick observation report. Its really a follow up to a sub-topic
on the "Poor lateral control on a slow tow" thread.

Last Friday I was sitting outside the CGC clubhouse, which was at the
upwind end of the active runway, when our Pawnee went past at about 200
ft with a Ventus on the line. As it went away I had a clear rear quarter
view which made the attitudes if the two aircraft really easy to see. The
Ventus looked to be at the same height as the Pawnee, but was clearly
flying much more nose-up. I'd estimate that it was at around 4 degrees
nose-up relative to the Pawnee.


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

[Karen]

On Mar 7, 2:15*am, Martin Gregorie >
wrote:
> This is a quick observation report. Its really a follow up to a sub-topic
> on the "Poor lateral control on a slow tow" thread.
AIRSPEED! AIRSPEED!

It is way more disconcerting when you see the tops of the wings of a
Pawnee from the grounf just before the DG300 behind him stalls,
releases and and spins in from 100 ft.

> Last Friday I was sitting outside the CGC clubhouse, which was at the
> upwind end of the active runway, when our Pawnee went past at about 200
> ft with a Ventus on the line. As it went away I had a clear rear quarter
> view which made the attitudes if the two aircraft really easy to see. The
> Ventus looked to be at the same height as the Pawnee, but was clearly
> flying much more nose-up. I'd estimate that it was at around 4 degrees
> nose-up relative to the Pawnee.
>
> --
> martin@ * | Martin Gregorie
> gregorie. | Essex, UK
> org * * * |

[Mike the Strike]

On Mar 7, 7:43*am, Karen > wrote:
> On Mar 7, 2:15*am, Martin Gregorie >
> wrote:> This is a quick observation report. Its really a follow up to a sub-topic
> > on the "Poor lateral control on a slow tow" thread.
>
> AIRSPEED! AIRSPEED!
>
> It is way more disconcerting when you see the tops of the wings of a
> Pawnee from the grounf just before the DG300 behind him stalls,
> releases and and spins in from 100 ft.
>
>
>
>
>
>
>
> > Last Friday I was sitting outside the CGC clubhouse, which was at the
> > upwind end of the active runway, when our Pawnee went past at about 200
> > ft with a Ventus on the line. As it went away I had a clear rear quarter
> > view which made the attitudes if the two aircraft really easy to see. The
> > Ventus looked to be at the same height as the Pawnee, but was clearly
> > flying much more nose-up. I'd estimate that it was at around 4 degrees
> > nose-up relative to the Pawnee.
>
> > --
> > martin@ * | Martin Gregorie
> > gregorie. | Essex, UK
> > org * * * |

We have discussed this many times before. The nose-up attitude of the
glider results in extra forces that have to be overcome by the wings
and tail surfaces. In particular, modern standard-class ships with a
low angle of incidence of the wing have a pronounced nose-up attitude
that results in loss of control at speeds well above free-flight stall
speed.

I find my Discus 2 won't high-tow very well below 60 knots, even
though its stall speed is nearer 40 or 45. One strategy I use if the
tow plane won't or can't speed up is to descend into low tow, which
buys an extra 5 knots or so.

Extra speed is great, but not under control of the glider pilot!

Mike

[Eric Greenwell[_4_]]

On 3/7/2011 7:24 AM, Mike the Strike wrote:

>
> We have discussed this many times before. The nose-up attitude of the
> glider results in extra forces that have to be overcome by the wings
> and tail surfaces. In particular, modern standard-class ships with a
> low angle of incidence of the wing have a pronounced nose-up attitude
> that results in loss of control at speeds well above free-flight stall
> speed.

I'm having trouble picturing the influence of a low angle of incidence
of the wing (I assume that's compared to the fuselage) on loss of
control. Why would the wing be affected by the fuselage attitude being a
few degrees higher, relative to the air, than on an older glider?

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

[Martin Gregorie[_5_]]

On Mon, 07 Mar 2011 13:14:04 -0800, Eric Greenwell wrote:

> On 3/7/2011 7:24 AM, Mike the Strike wrote:
>
>
>> We have discussed this many times before. The nose-up attitude of the
>> glider results in extra forces that have to be overcome by the wings
>> and tail surfaces. In particular, modern standard-class ships with a
>> low angle of incidence of the wing have a pronounced nose-up attitude
>> that results in loss of control at speeds well above free-flight stall
>> speed.
>
> I'm having trouble picturing the influence of a low angle of incidence
> of the wing (I assume that's compared to the fuselage) on loss of
> control. Why would the wing be affected by the fuselage attitude being a
> few degrees higher, relative to the air, than on an older glider?

I should add that there was plenty of speed - the Pawnee had its tail
well up. From where I was sitting its fuselage looked to be pretty much
horizontal. The Ventus looked to be well under control - it was flying
very steadily. I ascribed most of the glider's nose-high attitude to the
fact that it was flying in the downwash field behind the tug.

Don't forget that the downwash behind a lift generating wing is roughly
1/3 of the wing's AOA, so this was actually a great visualisation of the
downwash.


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

[Mike the Strike]

On Mar 7, 2:14*pm, Eric Greenwell > wrote:
> On 3/7/2011 7:24 AM, Mike the Strike wrote:
>
>
>
> > We have discussed this many times before. *The nose-up attitude of the
> > glider results in extra forces that have to be overcome by the wings
> > and tail surfaces. *In particular, modern standard-class ships with a
> > low angle of incidence of the wing have a pronounced nose-up attitude
> > that results in loss of control at speeds well above free-flight stall
> > speed.
>
> I'm having trouble picturing the influence of a low angle of incidence
> of the wing (I assume that's compared to the fuselage) on loss of
> control. Why would the wing be affected by the fuselage attitude being a
> few degrees higher, relative to the air, than on an older glider?
>
> --
> Eric Greenwell - Washington State, USA (change ".netto" to ".us" to
> email me)

The angle of incidence of the wing (relative to the fuselage) is a lot
lower on modern standard class ships. The result is that they have a
pronounced nose-up attitude when flying slowly - you can see this both
on tow and when thermalling. On aero-tow, a nose-mounted tow hook
will result in a downward force on the nose that has to be compensated
by an upward force from the tailplane and extra lift from the wings.
The point at which you lose control under these dynamics is at a lot
higher speed than the free-flight stall speed. The effect doesn't
seem to be as strong with older gliders or flapped ships, but
definitely is a lot less in low tow.

Mike

[Eric Greenwell[_4_]]

On 3/7/2011 2:47 PM, Mike the Strike wrote:
> On Mar 7, 2:14 pm, Eric > wrote:
>> On 3/7/2011 7:24 AM, Mike the Strike wrote:
>>
>>
>>
>>> We have discussed this many times before. The nose-up attitude of the
>>> glider results in extra forces that have to be overcome by the wings
>>> and tail surfaces. In particular, modern standard-class ships with a
>>> low angle of incidence of the wing have a pronounced nose-up attitude
>>> that results in loss of control at speeds well above free-flight stall
>>> speed.
>>
>> I'm having trouble picturing the influence of a low angle of incidence
>> of the wing (I assume that's compared to the fuselage) on loss of
>> control. Why would the wing be affected by the fuselage attitude being a
>> few degrees higher, relative to the air, than on an older glider?
>>
>> --
>> Eric Greenwell - Washington State, USA (change ".netto" to ".us" to
>> email me)
>
> The angle of incidence of the wing (relative to the fuselage) is a lot
> lower on modern standard class ships. The result is that they have a
> pronounced nose-up attitude when flying slowly - you can see this both
> on tow and when thermalling. On aero-tow, a nose-mounted tow hook
> will result in a downward force on the nose that has to be compensated
> by an upward force from the tailplane and extra lift from the wings.
> The point at which you lose control under these dynamics is at a lot
> higher speed than the free-flight stall speed. The effect doesn't
> seem to be as strong with older gliders or flapped ships, but
> definitely is a lot less in low tow.

The incidence difference is less that 5 degrees, isn't it? That would
mean the nose hook was only about 6" higher than otherwise. I just can't
see how moving the rope up 6" is going to change the forces
significantly. There is a bigger difference between a CG hook and nose
hook vertical separation, and nobody comments on how they lose control
sooner with the nose hook. I sure didn't notice any difference on my
previous glider, an ASW 20 B, where I used neutral flap to tow.

Maybe the difference between older and newer gliders is wing loading?

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

[Bruce Hoult]

On Mar 8, 4:24*am, Mike the Strike > wrote:
> I find my Discus 2 won't high-tow very well below 60 knots, even
> though its stall speed is nearer 40 or 45. *One strategy I use if the
> tow plane won't or can't speed up is to descend into low tow, which
> buys an extra 5 knots or so.

I suspect you're flying the tow too high behind a powerful tug that is
climbing at a steep angle e.g. flying with the tug on the horizon
rather than just above its slipstream.

The rope can't possibly make enough of a difference to the lift needed
from the wings to make a difference of 5 knots, but it can make you
run out of elevator power.

If low tow is ok then a high tow just above the slipstream will be
too.

[[email protected]]

On Mar 7, 5:47*pm, Mike the Strike > wrote:
> On Mar 7, 2:14*pm, Eric Greenwell > wrote:
>
>
>
>
>
> > On 3/7/2011 7:24 AM, Mike the Strike wrote:
>
> > > We have discussed this many times before. *The nose-up attitude of the
> > > glider results in extra forces that have to be overcome by the wings
> > > and tail surfaces. *In particular, modern standard-class ships with a
> > > low angle of incidence of the wing have a pronounced nose-up attitude
> > > that results in loss of control at speeds well above free-flight stall
> > > speed.
>
> > I'm having trouble picturing the influence of a low angle of incidence
> > of the wing (I assume that's compared to the fuselage) on loss of
> > control. Why would the wing be affected by the fuselage attitude being a
> > few degrees higher, relative to the air, than on an older glider?
>
> > --
> > Eric Greenwell - Washington State, USA (change ".netto" to ".us" to
> > email me)
>
> The angle of incidence of the wing (relative to the fuselage) is a lot
> lower on modern standard class ships. *The result is that they have a
> pronounced nose-up attitude when flying slowly - you can see this both
> on tow and when thermalling. *On aero-tow, a nose-mounted tow hook
> will result in a downward force on the nose that has to be compensated
> by an upward force from the tailplane and extra lift from the wings.
> The point at which you lose control under these dynamics is at a lot
> higher speed than the free-flight stall speed. *The effect doesn't
> seem to be as strong with older gliders or flapped ships, but
> definitely is a lot less in low tow.
>
> Mike- Hide quoted text -
>
> - Show quoted text -

I suspect you intended to say the tail was generating more down force
to over come the down vector on the nose.
Simple math based on a guess at tow rope tension of, say 50 lb or so
( 1000 lb glider/40 to 1 and double for sake of discussion), would
lead me to guess the down force on the nose to be not more than a few
lb.(maybe 10?). Assume a tail moment arm of 3 times the nose arm and
the extra lift the wing needs to generate is maybe 15 lb. at the most.
Your observation that the glider flies better in low tow makes sense
in that you are in cleaner air.
FWIW on a rainy winter night.
UH

[CindyB[_2_]]

On Mar 7, 2:47*pm, Mike the Strike > wrote:
*On aero-tow, a nose-mounted tow hook
> will result in a downward force on the nose that has to be compensated
> by an upward force from the tailplane and extra lift from the wings.
> The point at which you lose control under these dynamics is at a lot
> higher speed than the free-flight stall speed. *The effect doesn't
> seem to be as strong with older gliders or flapped ships, but
> definitely is a lot less in low tow.


Wow.

and all these years I was thinking that the center of gravity was
ahead of the center of lift; that there was a downward force
'forward' of the wing by the limits on CG location; and that the
horizontal tail
had to make a downward force to counteract the nose dropping tendency.
I think they called this Trim Drag, but I could be confused. I am
blond.

I better watch out for those nose-hook mounted gliders and
their scary tendencies. Maybe that's why the folks at EASU changed
the design requirements from aerotowing on CG towhooks to
requiring them to be aerotowed on 'forward' hooks. And that forward
hook placement was under the knees ......(even lower on the
ariframe than those dastardly true nosehooks. How does that fit into
the discussion?)


But hey, all that trim drag from different hook locations, gee, must
change the wing loading and subsequent stall speed by what......
a knot? Geesh.
Spend a few bucks on a few extra Spring tows,
and get past your winter nerves. Or break open a chapter of
Langewische. Or travel to a site that's still flying all winter
long and don't let the rust accumulate.

Think : First flight with a CFI-G for 2011. Promote annual
refresher training.... and less accidents.

Chuckling,

Cindy B
www.caracole-soaring.com

[Mike the Strike]

On Mar 7, 8:17*pm, CindyB > wrote:
> On Mar 7, 2:47*pm, Mike the Strike > wrote:
> **On aero-tow, a nose-mounted tow hook
>
> > will result in a downward force on the nose that has to be compensated
> > by an upward force from the tailplane and extra lift from the wings.
> > The point at which you lose control under these dynamics is at a lot
> > higher speed than the free-flight stall speed. *The effect doesn't
> > seem to be as strong with older gliders or flapped ships, but
> > definitely is a lot less in low tow.
>
> Wow.
>
> and all these years I was thinking that the center of gravity was
> ahead of the center of lift; that there was a downward force
> 'forward' of the wing by the limits on CG location; and that the
> horizontal tail
> had to make a downward force to counteract the nose dropping tendency.
> I think they called this Trim Drag, but I could be confused. I am
> blond.
>
> I better watch out for those nose-hook mounted gliders and
> their scary tendencies. *Maybe that's why the folks at EASU changed
> the design requirements from aerotowing on CG towhooks to
> requiring them to be aerotowed on 'forward' hooks. *And that forward
> hook placement was under the knees ......(even lower on the
> ariframe than those dastardly true nosehooks. How does that fit into
> the discussion?)
>
> But hey, all that trim drag from different hook locations, gee, must
> change the wing loading and subsequent stall speed by what......
> a knot? * * * *Geesh.
> Spend a few bucks on a few extra Spring tows,
> and get past your winter nerves. *Or break open a chapter of
> Langewische. *Or travel to a site that's still flying all winter
> long and don't let the rust accumulate.
>
> Think : * *First flight with a CFI-G for 2011. *Promote annual
> refresher training.... and less accidents.
>
> Chuckling,
>
> Cindy Bwww.caracole-soaring.com

Obviously a few don't like my explanation and without measured data I
can't substantiate it. I've done the math too and the numbers would
suggest that the effect should be small. I can, however, insist that
the effect is real and not just the result of a clumsy pilot. I won't
claim that my position on aerotow is always correct, but after 42
years of practice, I am usually in about the right place!

I also fly all sorts of ships, have owned a few, and have only ever
noted this on the Discus 2, although colleagues flying the LS-8 report
a similar, if less pronounced effect. I have tried my ship in low tow
too (that was standard procedure when I flew in South Africa) and it
suffers from the same effect, just at a slower speed. In one contest,
flying with lots of water ballast, I ran out of elevator authority
when the towplane slowed in a thermal and my glider sank from high tow
to low tow with full up elevator, after which I had to release.
Aileron control was only just satisfactory.

To lay the subject to rest, I will do a test with a tail-mounted video
camera and one of our helpful club tow pilots.

Any other skeptics can do the same - just borrow a Discus 2 and try to
aerotow at 45 to 50 knots and report back.

Mike

[Eric Greenwell[_4_]]

On 3/7/2011 9:26 PM, Mike the Strike wrote:

>
> To lay the subject to rest, I will do a test with a tail-mounted video
> camera and one of our helpful club tow pilots.
>
> Any other skeptics can do the same - just borrow a Discus 2 and try to
> aerotow at 45 to 50 knots and report back.

Sounds like a good experiment - actual data; however, I don't think
anyone has disputed your experience, just the explanation for it.

Maybe you can locate a glider with a nose hook and a CG hook to test. I
never noticed any difference in control between the two locations with
my ASW 20B (actually, a forward hook), but I did notice it went much
straighter during the initial ground roll.

--
Eric Greenwell - Washington State, USA (change ".netto" to ".us" to
email me)
- "A Guide to Self-launching Sailplane Operation Mar/2004" Much of what
you need to know tinyurl.com/yfs7tnz