motorgliders as towplanes

On 15 Mar, 05:45, Bob Cook wrote:
For purposes of analysis, the angle at which the tow rope meets the glider
is the angle to consider that THRUST is acting on the glider. *A rope can
only be in tension. *It cannot impart any rotational "moments" to the
glider.

Unless the hook is on the centreline and the tow rope is pulling
stright ahead, it certainly can apply a moment. Simplest case: low
down belly hook, pull straight ahead, pitch up.

Ian

On 15 Mar, 01:15, Bob Cook wrote:
At 13:43 14 March 2009, The Real Doctor wrote:

Bad example, since tow planes pull - give or take a wee bit -
horizontally, regardless of climb angle.

Well, our tow plane certainly does not pull horizontally!

I figure it pulls at about a 10:1 angle or about 5.5 degrees.

The only thing that determines the angle of the pull is the angle of
the tow rope at the glider end. This will be slightly affected by the
caternary sag in the cable, but will by and large depend on the
relative positions of glider and tug.

In high tow, the glider is more or less level with the tug, so the tow
force is more or less horizontal. The tug does NOT pull the glider up:
the glider pulls itself up and the tug supplies the energy. It's just
the same as with a climbing powered aircraft: the prop does not pull
it up to any significant extent - it allows the wings to pull it up.

Ian

Good point!

Re-reading my own posts I came to the same conclusion this morning.

So it is possible to be in "good" tow position, and have the rope impart
a forward and downward pull (relative) to the glider!

20 degrees still seems like a lot to me. I guess it depends on the weight
of the rope, and the drag on the glider.

Cookie




At 10:58 15 March 2009, The Real Doctor wrote:
On 15 Mar, 05:30, Bob Cook wrote:

If the tow line formed a 20 degree =A0angle to the glider, (from tow
plan=
e
upward to glider) the glider would have to be 72 feet =A0ABOVE the tow
plane.

Only if the tow rope were straight. He has already said that it was in
a clearly visible catenary curve.

Ian

Let me clarify:

The rope itself is in pure tension.

Yes,this tension force applied to the glider, combined with other forces
acting on the glider can form a moment. (on the glider)

If the rope were magically "rigid" like an I beam, and welded to the
glider, the it could impart moments to the glider. But a rope is
flexible.

When forces applied to the glider act in a direction other than through
the CG, moments are formed in the glider.

Like your example, tow rope attached low, so the "thrust" acts on a line
below the CG. Drag acts in the opposite direction but on a line above
the CG. (like a high winged glider) A nose up moment is formed.

But there is still not moment transmitted directly form the rope to the
glider since the rope is flexible and the tow ring would not allow it
anyway.

Cookie


At 11:00 15 March 2009, The Real Doctor wrote:
On 15 Mar, 05:45, Bob Cook wrote:
For purposes of analysis, the angle at which the tow rope meets the
glide=
r
is the angle to consider that THRUST is acting on the glider. =A0A
rope
c=
an
only be in tension. =A0It cannot impart any rotational "moments" to
the
glider.

Unless the hook is on the centreline and the tow rope is pulling
stright ahead, it certainly can apply a moment. Simplest case: low
down belly hook, pull straight ahead, pitch up.

Ian

Ok, we do have to consider the angle with which the rope meets the glider,
not the angle of the glider relative to the tow plane.

So if the glider /towplane combination is climbing at say 5 degees, it is
reasonable to see how the rope may be pullling exactly horizontally, or
even a bit downward. (due to sag)

Cookie





The tow plane does At 11:06 15 March 2009, The Real Doctor wrote:
On 15 Mar, 01:15, Bob Cook wrote:
At 13:43 14 March 2009, The Real Doctor wrote:

Bad example, since tow planes pull - give or take a wee bit -
horizontally, regardless of climb angle.

Well, our tow plane certainly does not pull horizontally!

I figure it pulls at about a 10:1 angle or about 5.5 degrees.

The only thing that determines the angle of the pull is the angle of
the tow rope at the glider end. This will be slightly affected by the
caternary sag in the cable, but will by and large depend on the
relative positions of glider and tug.

In high tow, the glider is more or less level with the tug, so the tow
force is more or less horizontal. The tug does NOT pull the glider up:
the glider pulls itself up and the tug supplies the energy. It's just
the same as with a climbing powered aircraft: the prop does not pull
it up to any significant extent - it allows the wings to pull it up.

Ian

To clarify some more - we tow behind a Pawnee using a 120-foot rope
and my Discus 2 has a nose hook. On tow, the Discus has a notable
nose-up attitude, so the 20 degrees I report is the apparent angle
from my seat. I am positioned immediately behind the towplane just
above the wake. I did have one tow last year in which I was unable to
maintain low tow and had to release.

The pull of the towrope can exert a moment if the force vector is not
aligned with the center of mass of the glider.

The catenary depends on the weight of the rope, but is very notable.

Mike

Paul

There is a large scale vortex dimer operating behind any aircraft, and
particularly behind high wing loading, heavy short winged things like
Pawnees.

The wake we fly above in high tow is the turbulent propeller wake, but
we would have to be impossibly high and/or far back to avoid the
downward moving centre section of the dimer.

I saw a picture using smoke trails that demonstrates the scale and power
of this some years back -
http://www.nasa.gov/audience/forstud...ry/Vortex.html

There is a more impressive video at
http://www.youtube.com/watch?v=uy0hgG2pkUs&NR=1

So - given that you are flying in a field of air that has a significant
downward component, maybe you do have a higher angle of attack on the
wings.


Bottom line is that even in the smooth air above the propwash you are
still in air affected by the tug.

Bruce


sisu1a wrote:
Agreed. My money is on the towplane wake.


I put my monies on the elevator authority/AoA ratio. We fly above
the wing wake (USA...) in most cases, in relatively clean air, but
sometimes in the clean air below it. Box the wake, it will tell you
where it is and where it isn't...

But typically glider's noses, on tow, are unnaturally high (and thus
AoA is higher...) for a given airspeed, in addition to being more
forcefully held there, both effects of course due to the rope's
pull. The elevator is the same size whether on tow or free flight
though, so the authority it can exert against the countering forces is
proportionately lower than in free flight...

The fix is the same regardless of why though- more speed... please!
(wings rocking in vain...)

-Paul

On Mar 15, 8:30*am, Bob Cook wrote:
Let me clarify:

The rope itself is in pure tension.

Yes,this tension force applied to the glider, combined with other forces
acting on the glider can form a moment. (on the glider)

If the rope were magically "rigid" like an I beam, and welded to the
glider, the it could impart moments to the glider. *But a rope is
flexible.

When forces applied to the glider act in a direction other than through
the CG, moments are formed in the glider.

Like your example, tow rope attached low, so the "thrust" acts on a line
below the CG. * Drag acts in the opposite direction but on *a line above
the CG. * (like a high winged glider) A nose up moment is formed.

But there is still not moment transmitted directly form the rope to the
glider since the rope is flexible and the tow ring would not allow it
anyway.

Cookie

At 11:00 15 March 2009, The Real Doctor wrote:



On 15 Mar, 05:45, Bob Cook *wrote:
For purposes of analysis, the angle at which the tow rope meets the
glide=
r
is the angle to consider that THRUST is acting on the glider. =A0A
rope
c=
an
only be in tension. =A0It cannot impart any rotational "moments" to
the
glider.

Unless the hook is on the centreline and the tow rope is pulling
stright ahead, it certainly can apply a moment. Simplest case: low
down belly hook, pull straight ahead, pitch up.

Ian- Hide quoted text -

- Show quoted text -

Maybe the moment caused by the towhook location relative to the center
of drag helps explain the elevator authority issue. At slower speeds,
elevator is less effective allowing this moment to overtake it. The
aileron issue may be similar to what we experience on take-off. High-
alpha (sitting up on gear with tail on ground) combined with
relatively low-speed airflow over the wings and the effect feels very
much like initial take-off roll. As someone mentioned, the pull of the
rope keeps the glider in this state, yet prevents the stall break.

Just a complete guess on my part. Interesting discussion. I had a slow
tow last year and experienced the same phenomenon.

Mike the Strike wrote:
To clarify some more - we tow behind a Pawnee using a 120-foot rope
and my Discus 2 has a nose hook. On tow, the Discus has a notable
nose-up attitude, so the 20 degrees I report is the apparent angle
from my seat. I am positioned immediately behind the towplane just
above the wake. I did have one tow last year in which I was unable to
maintain low tow and had to release.

The pull of the towrope can exert a moment if the force vector is not
aligned with the center of mass of the glider.

The catenary depends on the weight of the rope, but is very notable.

Mike

I would bet you are in the top of the wake, not just above it. Matches
the symptoms. I have done the same thing since I like to fly very close
to the top of the wake. If you fly to put the towplane the same place on
the canopy (not uncommon in mountainous areas with no well defined
horizon), a slower tow speed will have you riding low and in the wake.
Guess how I know. Flaps really help here.

By all the reports I've read on the D2, seeing the towplane from normal
high tow position below about 65 kts is a problem. So going to a little
higher tow position may not be a sane option.

Your shortish tow rope may exacerbate this since you will need to be
higher to be above the wake, the wake will be stronger and the angles
will all be exaggerated compared to a longer rope you may well be used to.

The best option may indeed be more speed on tow. Or get some stinking
flaps

-Dave

There is a large scale vortex dimer operating behind any aircraft, and
particularly behind high wing loading, heavy short winged things like
Pawnees.

Hmm, I'm of the understanding that we use Pawnees because they are so
lightly wingloaded (relative to other tugs) and have such good power/
weight ratios when not full of bug juice and spray gear. I thought
this is also what allows them to happily fly too slow for our tastes
as well. While a Pawnee is perfectly content tugging at 55mph, I'm
not.


The wake we fly above in high tow is the turbulent propeller wake, but
we would have to be impossibly high and/or far back to avoid the
downward moving centre section of the dimer.

I disagree, I think we are flying well clear of this phenomenon, at
least in high tow. To me, the video you link illustrates just how far
below and behind the a/c this is taking place. The C-5 flies over, and
well past, and they even skip some time in the footage before the
vorticies reach down where the smokers are, which really shows the
downward trailing shape of this effect. It does not compute that you
would be subject to this effect if you were level with it and not
overly close behind (Isn't there a ratio of minimum rope length to
wingspan that is used as a rule of thumb?)

I saw a picture using smoke trails that demonstrates the scale and power
of this some years back -http://www.nasa.gov/audience/forstudents/k-4/dictionary/Vortex.html

Neat pic, (very neat actually...) but tells little in the way of what
is happening to a glider on normal tow. A side shot, and one well
clear of the ground would show us what we are actually looking for for
this discussion, but this plane is in ground effect and the shot is
from behind. All bets are off when the disturbed air can't escape
below the flightpath where it wants to go... (and we are thus stuck
flying in this disturbance, which I don't recall as being very
disturbing either) Pretty pic, but somewhat useless as evidence for
this debate, or at least for what I am describing, which is not the
mechanics of towing while in ground effect but rather why gliders feel
like they are on the verge of a stall while on tow despite being well
above normal stall speeds.

There is a more impressive video athttp://www.youtube.com/watch?v=uy0hgG2pkUs&NR=1

So - given that you are flying in a field of air that has a significant
downward component, maybe you do have a higher angle of attack on the
wings.

I think you are misinterpreting the photo/video and drawing incorrect
conclusions form them. Intuition (dangerous in aerodynamics, I
know..) tells me that if your vortex dimer was indeed striking the
glider as you suggest, the downward moving air would actually serve to
effectively decrease AoA since it would be striking the top of the
wing and not the bottom. I still think we fly pretty clean air, above
or below the really affected air, and are only suffering the
butterffly effect of this phenomenon when not actually inside or right
on the edge of the wake.

Bottom line is that even in the smooth air above the propwash you are
still in air affected by the tug.

Affected? Sure. But my money is STILL on the pitching up of the nose
due to the rope's pull as being the primary cause of the sensation of
being on the verge of a stall while on tow, since the pull of the rope
is causing the glider to be drug through the air at an unnaturally
high AoA for any given airspeed, while at the same time drastically
reducing elevator effectiveness from reduced airflow and the fact the
nose is tethered. This increased AoA also greatly affects aileron
performance as well, since they too are operating at higher AoA's for
any given speed.

As mentioned elsewhere here, flaps most certainly help this effect
too, by pitching your nose back down some and thus reducing your AoA
for whatever given speed the tug is pulling at.

Unflapped,
-Paul

PS. the propwash to wingwash ratio should be pretty easy to figure
out. The main wing has to support the entire a/c (couple thousand
pounds) while the little spinning wing only needs to provide thrust.
(couple hundred pounds?) Which do YOU think is dominating the scene?
Unless there are tugs out there approaching 1:1 (thrust:weight) the
main wing is the main show, and hence the main contributor of
disturbed air. Ian, please do your power off tug test and please
post to youtube! Be sure to box the wake too though, because it
IS most certainly there, prop or not.