motorgliders as towplanes

Ah, chemistry; I never did that either. Shoulda tried Wikipedia first; was
sure it was some sort of shortened up slang. Thanks.


At 15:13 16 March 2009, Bruce wrote:
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

This theory certainly explains the poor aileron control or wallowing at
slow towing speeds, even though it seems the wing, (on average) is no
where near the stall angle of attack.

I have seen guys try to fly model planes with wash in instead of wash out
in the wings. Doesn't work too well!


Cookie





At 15:33 16 March 2009, The Real Doctor wrote:
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

Agree with Ian,

I thought we agreed that the force of the tow rope acts at whatever angle
the rope meets the glider. This means it is a factor the RELATIVE
position of the glider to the towplane, NOT the direction of flight.

That being said, I once was at an airshow where Oscar Bosch was towed by
the famous plane called "Sampson". Sampson held all kinds of "time to
climb" records.

Old Oscar never really caught up with Sampson, and was in the lowest low
tow I think I have ever seen. There was plenty of up thrust on that tow
rope!


Cookie








At 15:29 16 March 2009, The Real Doctor wrote:
On 16 Mar, 13:28, " wrote:
On Mar 14, 8:43=A0am, 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 Mar 16, 9:33*am, The Real Doctor wrote:
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

I fly a 20 meter glider that sometimes is loaded with 60 gallons of
water and I have to say that I've not noticed this effect when towing
behind either a Pawnee or a Cub. Aileron control on tow is no worse
than in free flight. The slower Cub is easier to follow if anything.
Over the years, I've been towed by everything from a 85HP 7AC champ to
a 400HP Pawnee Brave. It's all pretty much the same - just follow the
tug.

What I have noticed is that a glider in high tow will tend to drift
laterally toward the center which is generally a good thing. I
haven't noticed any of the pitch related stuff people are talking
about.

On one occasion a pilot was complaining that his glider needed more
than 80 knots on tow to feel "right". I flew the glider and radioed
the tow pilot to reduce towing speed until he started complaining
about control authority but I saw no problems with this glider. The
only thing I could point to was the pilots tendency to over control
his glider and to use positive flap "to see the tow plane better". I
suspect the excess flap was limiting aileron authority. He would have
been better off with some cushions.

On 16 Mar, 16:41, danlj wrote:

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 was once approaching my then club - Borders GC - to land when I saw
a Bocian lining up for takeoff behind the club's Pawnee 150. "Oh, this
should be interesting" I thought and I found a bit of lift to park in
and watch the fun.

The fun turned out to be the combination making three full circuits of
the airfield, never getting above a couple of hundred feet, then the
tug waving off and the two landing together (no on-tow landings in the
UK, unlike France).

Nice wee tug for a single seater, and whisper quiet with a four blade
prop and silencer, but it couldn't pull the skin off a two-seater rice
pudding. It later had the engine upped to 160hp (as far as the CAA
would go without an STC), a 2-blade prop and lost the silencer.

Ian

On 16 Mar, 17:08, sisu1a wrote:

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...)

Thanks! I'm finding this interesting too. I might have to tuft a wing
and go towing later this year.

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.

Good point. It's a lot easier with a belly hook and I think AN-2s can
tow off the interplane struts, which would give a bit of offset.
Unfortunately I don't know anyone with an AN-2 ...

Ian

I ran a quick calculation of the expected catenary sag in a towrope.
With a 200-foot nylon/poly towrope and 75 pounds of tension, the sag
amounts to less than a foot, ignoring aerodynamic forces. Under
steady towing conditions, it can probably be ignored.

Mike

Mike,

I agree that the sag of the rope is small and can probably be ignored.

My calculations do not show drag values as high as 75 lbs however, so the
sag may be (a little) more that your calculations.

I figure a modern glider will have something in the neighborhood of 30 lbs
drag, while an old "boat" of a glider will be in the 50 lb range. (Am I
missing something here?)

I calculated the sag effect in the rope a different way. I figure a
typical 200' poly rope weighs about 4 lbs. The tow plane supports one
end, while the glider supports the other. I figured in a 2 lb down force
on the glider. Since, during tow, lift is essentially the same as weight,
you have to get an extra 2 lbs of lift to compensate for the weight of the
rope. If you figure the weight of the glider in the 700 to 900 lb range,
it is a tiny percentage of extra lift needed due to the rope. (like 2/10
of one percent.)

Same is true if the rope pulls (slightly) upward or downward, the change
in lift is tiny.

Now consider free flight in a 45 degree bank. Lift is increased by 40%.
The glider still will fly pretty well at relatively slow speeds in a 45
degree bank. Now look at 60 degree bank where lift is doubled......

Just a couple more thoughts about the forces on that tow rope:

When the tug is flying without a glider, the rope does not hang straight
down. The "drag" on the rope causes the rope to fly at quite a flat
angle. We could say that some of the weight of the rope is being
supported by the air drag on the rope. But then when a glider is on tow,
and the rope is essentially level, with a sag, I guess the front half of
hte rope is being lifted by air drag, while the back half of the rope is
being pushed down by air drag. So I think that just adding the weight of
the rope as a downward component will work in calcualtions.

Long way to get to the point however. I don't see how the tow rope can
be the culprit in this "too slow tow" situation.

Cookie









At 23:30 16 March 2009, Mike the Strike wrote:
I ran a quick calculation of the expected catenary sag in a towrope.
With a 200-foot nylon/poly towrope and 75 pounds of tension, the sag
amounts to less than a foot, ignoring aerodynamic forces. Under
steady towing conditions, it can probably be ignored.

Mike

I calculated the rope tension as 25 pounds of aerodynamic drag (40:1
for a 1,000 pound glider) and 50 pounds to provide the force necessary
for the glider to climb. That's in the right ball park. The tension
obviously increases with the rate of climb since you have to impart
energy to the glider for it to ascend.

Mike

I understand that Bill's glider is a Nimbus 2, but don't know which
version.

I used to have a share in an early Nimbus 2 with an all flying tailplane
and just a belly hook. It was a bit twitchy in pitch, but otherwise very
easy to aerotow. I think the reasons for this are that this type has a
very large wing area (with a corresponding low stalling speed), and the
long wings give plenty of lateral damping. When retrieving from fields,
with negative flap selected, the wings would often level themselves in the
propwash when the tuggie opened up the throttle, even before the glider
started to roll. The little Standard Cirrus I own now is actually much
more of a handful on aerotow, particularly if you have to start without a
wingtip runner. It normally stalls at just below 40knots in free flight,
but doesn't feel comfortable below 60knots on tow.

I have noticed that in general, big span gliders seem to be easier to
aerotow than little gliders, despite the reduced aileron effectiveness.
Maybe this is because the outer part of the wings (where the ailerons are
located) are outside most of the slipstream of the usually much smaller
span tugs. This might also explain why tows behind motor gliders with
bigger wingspans seem to be more difficult, if the tug's slipstream is
the main problem.

Derek Copeland

At 20:45 16 March 2009, bildan wrote:

I fly a 20 meter glider that sometimes is loaded with 60 gallons of
water and I have to say that I've not noticed this effect when towing
behind either a Pawnee or a Cub. Aileron control on tow is no worse
than in free flight. The slower Cub is easier to follow if anything.
Over the years, I've been towed by everything from a 85HP 7AC champ to
a 400HP Pawnee Brave. It's all pretty much the same - just follow the
tug.

What I have noticed is that a glider in high tow will tend to drift
laterally toward the center which is generally a good thing. I
haven't noticed any of the pitch related stuff people are talking
about.

On one occasion a pilot was complaining that his glider needed more
than 80 knots on tow to feel "right". I flew the glider and radioed
the tow pilot to reduce towing speed until he started complaining
about control authority but I saw no problems with this glider. The
only thing I could point to was the pilots tendency to over control
his glider and to use positive flap "to see the tow plane better". I
suspect the excess flap was limiting aileron authority. He would have
been better off with some cushions.