motorgliders as towplanes

K13s and Bocians (both types I fly regularly) also aerotow in a noticeably
nose down attitude. The Bocian is such a slow glider that it unlikely that
any normal tug could fly slowly enough to get close to its stalling speed,
without stalling itself, but the K13 definitely starts 'wallowing' below
about 52knots, whereas its normal free flight stalling speed is about
36knots.

Derek Copeland

At 12:15 16 March 2009, Bob Cook wrote:

I also noted, in the case of a 2-33 and a Blanik L-13 (the crap that I
usually have to fly) that BOTH of these gliders fly NOTICIBLY nose down
on
tow! (which menas the rope is imparting a nose up force vector, even in
normal, tow position. )

Anyone who has flow a 2-33 knows that it takes CONSIDERABLE forward
stick
force when on tow.

I have experienced the "too slow tow" phenomonon in the '33. (never
got a too-slow in the Blanik)

Again, my point being that I am still not convinced that anyone has
completly explained the phenomonon, that we all agree exists!

Cookie







forward and downward pull (relative) to the glider! =A0

I'd expect high tow to give a downwards pull and low tow to give an
upwards one.


Seems a lot to me as well. What are these people towing with - chain?

Ian

On 16 Mar, 10:15, Alan Garside wrote:
Can I add another question, why does the adverse yaw at 60 on tow appear to
be more than 60 in free flight, many students learing to aerotow have
difficulty with the rudder.

Do you think it's an aerodynamic problem? I'd always thought, based on
my own experience, that it came from having a much clearer indication
of yaw (the tow rop) than normal and overcorrecting some PIOs into the
system.

But I hadn't really considered the aerodynamics. Hmmm.

Ian

On Mar 14, 8:43*am, The Real Doctor wrote:
On 14 Mar, 13:15, Bob Cook wrote:

To beter understand how the lift gets less as the climb angle gets
greater, let's look at teh "extreme". Consider a glider attached by a
nose hook to a huge construction crane. *The crane operator *applies POWER
to the lifting cable and the glider is slowly lifted, vertically into the
air.

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

Ian

Never been towed behind an Ag-cat, have you?

Nothing horizontal about that evolution!

Kirk
66

Sorry Nyal - excessive vocab use. Dimer == related pair of.

In this case two vortices - one off each wingtip that interact to create
a roughly symmetrical "geared disk" shape behind the wing. With the
downward part of the vortex from each wingtip merging with the downward
flow from the other.

If you drive behind a (modern / streamlined not SUV) car in the rain or
snow you can see the dimer it creates. Formula one and Nascar rear wings
also create impressive examples...



Nyal Williams wrote:
Help; what is "dimer" ?


At 14:09 15 March 2009, Bruce wrote:
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

I know this is not alt.usage.english, but what is the etymology of this
word? Is it slang? Jargon? Engineer language? Is it Di-mer or Dim-er or
dimer... something or other? I'm just a poor musicologist trying to figure
stuff out.

At 14:27 16 March 2009, Bruce wrote:
Sorry Nyal - excessive vocab use. Dimer == related pair of.

In this case two vortices - one off each wingtip that interact to create

a roughly symmetrical "geared disk" shape behind the wing. With the
downward part of the vortex from each wingtip merging with the downward
flow from the other.

If you drive behind a (modern / streamlined not SUV) car in the rain or
snow you can see the dimer it creates. Formula one and Nascar rear wings

also create impressive examples...



Nyal Williams wrote:
Help; what is "dimer" ?


At 14:09 15 March 2009, Bruce wrote:
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

monomer
Dimer
Trimer
....


Nyal Williams wrote:
I know this is not alt.usage.english, but what is the etymology of this
word? Is it slang? Jargon? Engineer language? Is it Di-mer or Dim-er or
dimer... something or other? I'm just a poor musicologist trying to figure
stuff out.

At 14:27 16 March 2009, Bruce wrote:
Sorry Nyal - excessive vocab use. Dimer == related pair of.

In this case two vortices - one off each wingtip that interact to create

a roughly symmetrical "geared disk" shape behind the wing. With the
downward part of the vortex from each wingtip merging with the downward
flow from the other.

If you drive behind a (modern / streamlined not SUV) car in the rain or
snow you can see the dimer it creates. Formula one and Nascar rear wings

also create impressive examples...



Nyal Williams wrote:
Help; what is "dimer" ?


At 14:09 15 March 2009, Bruce wrote:
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 16 Mar, 13:28, " wrote:
On Mar 14, 8:43*am, The Real Doctor wrote:

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

Never been towed behind an Ag-cat, have you?

265 horse Pawnee count?

Nothing horizontal about that evolution!

Tug wheels on the horizon, glider just above the prop wash, just like
everything else.

Ian

On 16 Mar, 13:00, Derek Copeland wrote:

... but the K13 definitely starts 'wallowing' below
about 52knots, whereas its normal free flight stalling speed is about
36knots.

OK, here's my latest theory. Gliders have bigger wingspans than tugs.
Therefore the outer bit of each glider wing is in the upwards moving
bit of the tug's tip vortices, and the centre bit is in the downwards
going bit. Effective result: much higher angle of attack at the tips,
particularly since the nose has to come up to maintain AoA at the
centre. Hence wash-in, tips near stall, downgoing aileron actually
stalling, reduced control effectiveness, wallowing.

Questions: does it happen as much out to one side hen boxing the wake?
Does it happen when the tug - a motorglider - has the same span as the
tug?

Ian

On Mar 8, 11:03*am, Brad wrote:
I know this has come up before in RAS. But thought I would bring up
the subject again.

For a club looking at long term projections, which at some point will
include either sticking a "new" engine on a Pawnee or getting rid of
it, does it make sense to start evaluating getting a 2 place
motorglider to serve as a tug and also as a touring/training tool?

Can a MG tug pull a loaded 2 place Blanik on a standard day at SL,
from a 1800' grass strip? Or perhaps such a tug could be used to tow
the members single place ships, and the heavier/ 2-place ships stow
behind the clubs remaining pawnee?

We are blessed with 3 towplanes in our club, there are upcoming
factors that will/are causing us to look at several different
scenarios and am wondering if tossing a MG into the mix might be one
such solution.

Brad

Ten - twelve years ago we had a little 4-man club that towed a fully-
loaded Blanik L-13 off a *paved* runway with a Piper Pacer. I think it
had a 135hp engine. I think the ground roll on asphalt for the Pacer
was about 2000'. We had grass available, and really didn't want to
find out how long the ground roll would be, partly because the
obstacle-clearance distance was very poor due to the anemic climb
rate. On a hot day, it might be 1 kt, and really needed thermals to
get up. There was no climb in sink. Our field has nearby landable farm
terrain, and believe me, I always made sure I knew exactly which field
I'd be landing in if the Pacer had an engine problem, and had a hand
on the yellow handle all the way to 500+ ft agl.

It's the "fully loaded Blanik" phrase that give me pause more than the
"motoglider as tug" phrase.

As other responders have implied, the proof of the pudding is very
much in the eating. Testing is always a good idea.

OK, here's my latest theory. Gliders have bigger wingspans than
tugs.
Therefore the outer bit of each glider wing is in the upwards moving
bit of the tug's tip vortices, and the centre bit is in the downwards
going bit. Effective result: much higher angle of attack at the tips,
particularly since the nose has to come up to maintain AoA at the
centre. Hence wash-in, tips near stall, downgoing aileron actually
stalling, reduced control effectiveness, wallowing.

Questions: does it happen as much out to one side hen boxing the wake?
Does it happen when the tug - a motorglider - has the same span as the
tug?

Ian

This thread is interesting and I thought it deserved a new title, even
though I have nothing to really add to it at this point (except that
rudder effectiveness is also reduced with a tethered nose...)

-Paul

PS. the rope's tugging of the nose is quite severe at the outside
positions while boxing the wake, and greatly affects control authority
(especially in draggy fat ships with ineffective controls like a
2-33...), so the experiment you propose might not yield as much
insight as we'd like.