motorgliders as towplanes

"Jim Beckman" wrote in message
...
At 09:25 11 March 2009, Bruce Hoult wrote:

Right. Try towing with a Huey :-)

It's been done. Well, maybe not with a Huey, but some big military
glider. Richard Schreder used to tell hilarious stories about flying in
some sort of contest in South America, and being retrieved from an
outlanding by a military helicopter. He went to great lengths trying to
communicate to the chopper crew that he wanted only horizontal flight,
*no* vertical maneuvers. I guess the rope was long enough and the
retrieve worked out OK.

Schreder was a fantastically entertaining speaker at any kind of glider
convention or seminar. Plenty good enough to make up for the silly rug he
wore on his head.

Jim Beckman


Here is the Bryan Times story about Schreder's trip to Chile.
http://www.soaridaho.com/Schreder/Sc..._in_Chile.html

The helicopter retrieve story can be found in the book "10,000 Feet and
Climbing."

Wayne
HP-14 "6F"
http://www.soaridaho.com/

Some gliders definitely will fly slower in free flight than on aero-
tow. The Discus 2 is a good example of this. I have run out of
elevator authority several times on tows slower than I'd like and
ended up slipping into low tow position or losing control altogether
and releasing.

I believe this is related to the angle of incidence of the wing, which
requires the Discus to adopt a high nose-up attitude at slow speeds.
Under aero-tow, the rope pulls the nose down, which requires extra
lift from the tailplane. At slow tow speeds, the tailplane cannot
develop enough lift to maintain the proper angle of attack of the wing
because of this pull from the rope.

When dry, my Discus 2 stalls below 40 knots and will thermal happily
between 40 and 50. I cannot safely aero-tow at 50 knots and usually
call for at least 65.

This effect is most notable with high performance standard class ships
whose angle of incidence is more optimized for high-speed flight.
Flapped ships by and large do not suffer from this problem, nor do
most training ships or lower performance single-seaters.

We are not imagining this!

Mike

On Mar 11, 11:07*am, Mike the Strike wrote:
Some gliders definitely will fly slower in free flight than on aero-
tow. *The Discus 2 is a good example of this. *I have run out of
elevator authority several times on tows slower than I'd like and
ended up slipping into low tow position or losing control altogether
and releasing.

I believe this is related to the angle of incidence of the wing, which
requires the Discus to adopt a high nose-up attitude at slow speeds.
Under aero-tow, the rope pulls the nose down, which requires extra
lift from the tailplane. *At slow tow speeds, the tailplane cannot
develop enough lift to maintain the proper angle of attack of the wing
because of this pull from the rope.

When dry, my Discus 2 stalls below 40 knots and will thermal happily
between 40 and 50. *I cannot safely aero-tow at 50 knots and usually
call for at least 65.

This effect is most notable with high performance standard class ships
whose angle of incidence is more optimized for high-speed flight.
Flapped ships by and large do not suffer from this problem, nor do
most training ships or lower performance single-seaters.

We are not imagining this!

Mike

If this is the case (and I think it is, combined with downwash
effects), then might low tow alleviate some of the low speed problems,
by changing the angle of the towrope at the glider end?

I agree that my LS6 sure feels "iffy" when the tow gets slow, even
though well above stall speed. I'll have to try to see if this effect
is as noticable at low tow next time I get a tow.

As far as vertical tows, I seem to remember they did a glider acro
routine at the NZ WGC that involved hanging a glider from it's nose
hook under a Hughes 500 in hover, then releasing for a tailslide to
start the routine. I recall the transition from normal tow to the
hover described as "interesting" in the glider (!!).

Kirk
66

K13s, which by no stretch of the imagination can be called 'modern',
behave in exactly the same manner during a slow and low powered aerotow
behind a motorglider! They are not comfortable to fly much below 55 knots
on tow. With a conventional tug (Robin DR400/Piper Pawnee) we get about 60
to 65 knots, which gives you much better control in turbulent conditions.

When I fly flapped gliders I tend to use some amount of positive flap,
mostly to get a better view of the tug, but also because it feels more
controllable.

BTW we have been specifically warned against aerotowing in low tow as our
motogliders can run out of forward elevator if you do.

Derek Copeland


At 16:07 11 March 2009, Mike the Strike wrote:
Some gliders definitely will fly slower in free flight than on aero-
tow. The Discus 2 is a good example of this. I have run out of
elevator authority several times on tows slower than I'd like and
ended up slipping into low tow position or losing control altogether
and releasing.

I believe this is related to the angle of incidence of the wing, which
requires the Discus to adopt a high nose-up attitude at slow speeds.
Under aero-tow, the rope pulls the nose down, which requires extra
lift from the tailplane. At slow tow speeds, the tailplane cannot
develop enough lift to maintain the proper angle of attack of the wing
because of this pull from the rope.

When dry, my Discus 2 stalls below 40 knots and will thermal happily
between 40 and 50. I cannot safely aero-tow at 50 knots and usually
call for at least 65.

This effect is most notable with high performance standard class ships
whose angle of incidence is more optimized for high-speed flight.
Flapped ships by and large do not suffer from this problem, nor do
most training ships or lower performance single-seaters.

We are not imagining this!

Mike

On Tue, 10 Mar 2009 08:56:47 -0700, Mike the Strike wrote:

I observed a 100HP Lambada equipped with 13m wings being used as a
towplane in the 2000 Worlds in South Africa. It seemed to perform very
well despite the moderate density altitude. They were launching from an
asphalt runway and the ground acceleration was slower than most other
towplanes, but once airborne no difference was notable.

I think you mean the Samba, sibling of the Lambada. This has a a shorter
wing + extensions to bring it to 12m. These have been used by a number of
clubs in South Afria and a Samba was also used for a number of years at
Gariep Dam (towing everything up to ASH 25's).

Some of the feedback I heard from the Gariep operation:

- The short wings result in a bit more drag, which is a problem with
marginal tows. But their are no aileron extensions on the wing extensions
so in long wing configuration it lacks aileron authority for good control
in strong weather.

- That Samba had a manually adjusted variable pitch prop. The pilot spent
a lot of effort adjusting the prop during the take off run and the tow to
get the most out of it. They also tried an electric auto variable pitch
prop but the electric motor burnt out very quickly.

The Lambada and similar modern motorgliders are very light (300kg) -
nearly half the weight of the older designs

This can also be a problem if the glider gets out of position on tow. A/T
training might get quite uncomfortable.

I did some research into M/G tugging a few years ago and put together
some notes. You can read them at

http://www.zsd.co.za/ian/gliding/cgc...gs/mgtugs.html

and some feedback from tow tests that we did at our club.

http://www.zsd.co.za/ian/gliding/cgc.../towtests.html

We never bought the Samba mentioned in the 2nd article. In hindsight I
think we made the right choice as the airframe of the Samba is just too
light and fragile to survive getting "clubbed".

But later we bought a 2nd hand 80 HP Rotax Falke. We had it equipped with
a tow hook, and did a few tows with it. We have a long hard runway near
sea level. The 80HP was fine with single seaters but not safe with two
seaters so we stopped using it for towing. (But we do use it for
training.)

Now we have just up-graded it 100HP and fitted a tugging fixed pitch
prop. We are optimistic this will make it a useful tug. In a year's time
I might be able to give some more feedback.

The Falke is much heavier, more robust and easier to fly then the Samba.
It has already stood up well to a few years of club abuse. But the tow
performance is going to be less than that of the Samba.

(In the meantime we have no plans to sell our 180 HP Super Cub tow plane
and there is still lots of training work for the Motor Glider to do.)

Turbochargers and variable pitch props help make up for the lack of
displacement of the Rotax 4 stroke. But they both add complexity and
costs which might not work out well in a club environment. Perhaps one
day someone will persuade Jabiru to water cool their 120HP motor. That
should make the basis of a decent M/G tug.


Ian

You're right - it was the Samba, the Lambada's cousin. I witnessed
several launches there, including some ballasted ships and none of
them looked scary, although ground run was long.

I'm a great believer in towplane mass and power - my all time favorite
is the 600 HP AgCat.

Mike

On Mar 12, 5:07*am, Mike the Strike wrote:
I believe this is related to the angle of incidence of the wing, which
requires the Discus to adopt a high nose-up attitude at slow speeds.
Under aero-tow, the rope pulls the nose down, which requires extra
lift from the tailplane. *At slow tow speeds, the tailplane cannot
develop enough lift to maintain the proper angle of attack of the wing
because of this pull from the rope.

Running out of elevator in such a situation does not imply that the
wing is anywhere near stalling.

I had a similar situation when I first flew a PW-5 behind a powerful
Pawnee tug. I was keeping the wheels of the tug on the horizon, and
it was horrid. I had almost no viz of the tug and the stick needed to
be waaay back.

After a while I realized that we were climbing so steeply that I
should be much lower. I was looking at the top of the fuselage! I
dropped down to where the tailplane was lined up with the wing and
things got much much better -- and the tug was *way* above the
horizon.

On Mar 12, 6:07*am, "
wrote:
As far as vertical tows, I seem to remember they did a glider acro
routine at the NZ WGC that involved hanging a glider from it's nose
hook under a Hughes 500 in hover, then releasing for a tailslide to
start the routine. *I recall the transition from normal tow to the
hover described as "interesting" in the glider (!!).

Yes, except it was a belly hook, with the glider hanging largely
upside down under the helicopter once it stalled (and stopped
oscillating).

You probably didn't see the practice run a day or two earlier. The
helicopter wasn't able to maintain height with the glider dangling
underneath it and called the glider to ask if it was planning to
release while there was still altitude. Bruce Drake in the glider
replied that he was going to have to do his seatbelts up tighter next
time because he was having trouble reaching the release! He did
eventually manage it.

I can't imagine why the stall speed would change on tow. The controls may
feel different because the tow rope is pulling on the nose, so any attempt
to turn or change the angle of attack will face an increased counter force,
but that's different than a change in the stall speed of the glider.

Mike Schumann

"Derek Copeland" wrote in message
...
Bruce,

So can you explain why the stalling speed definitely seems to increase
during an aerotow? Either the wing must be flying at a greater angle of
attack, i.e. producing more lift for a given airspeed, or the wing loading
must increase in some way.

As I said before, gliders that will quite happy fly at 40 knots in free
flight seem to need at least 50 knots on aerotow, even in smooth air. If
you aerotow behind a slow, low powered tug such as a motorglider, it often
seems to be quite difficult to keep up with its rate of climb, even though
it is very low. If you pull back the stick enough to do this, the glider
will start to buffet and the controls become rather ineffective. Both
symptoms of being close to the stall I believe! This is staying high
enough to avoid the tug's slipstream BTW, which could also produce
similar effects. If you do drop into the slipstream, it is often very
difficult to climb out of it again.

I agree that the accepted theory of flight says that in steady flight, the
vector of lift plus thrust must equal weight plus drag. I suppose that if
you had a tug powerful enough to produce enough thrust to more than equal
it's own weight plus the weight of the glider, then you could go
vertically up without the wings producing any lift.

Discuss!

Derek C


At 02:34 11 March 2009, Bruce Hoult wrote:
On Mar 11, 3:15=A0am, Derek Copeland wrote:
Remember that a glider has to produce more lift when climbing.


Er .. say what?

Any constant rate of climb (including flying level or constant
descent) requires exactly the same amount of upward force -- identical
to the weight of the aircraft.

In a powered aircraft flying level the weight and lift balance, and
the thrust and drag balance.

In a glider gliding, the lift from the wings is slightly less than the
weight (it is multiplied by the cosine of the glide angle), and the
balance of the upward force comes from drag (multiplied by the sine of
the glide angle).

In a glider being towed upwards, the lift from the wings is also less
than the weight (by the cosine of the climb angle), with the balance
of the upward force coming from the difference of the tow rope force
and the drag (multiplied by the sine of the climb angle).

If you're climbing at only a few hundred feet per minute while being
towed at 60 or 70 knots (6000 or 7000 fpm) then these angles are tiny
and the lift is essentially equal to the weight, but if a powerful
towplane could climb at, say, 45 degrees, then (far from having to
generate more lift than usual) your wings would only have to generate
lift equal to 70% of your weight.

On Mar 11, 6:33*pm, "Mike Schumann" mike-nos...@traditions-
nospam.com wrote:
I can't imagine why the stall speed would change on tow. *The controls may
feel different because the tow rope is pulling on the nose, so any attempt
to turn or change the angle of attack will face an increased counter force,
but that's different than a change in the stall speed of the glider.

Read my earlier post!

The tow rope in some gliders (especially those standard class racing
gliders with a shallow angle of incidence) acts to pull the nose down,
reducing the angle of attack of the wing and tailplane. The stall
speed depends not only on speed, but angle of attack - if you reduce
it by pulling down on the nose, lift will be reduced. As I mentioned
earlier, the Discus 2 runs out of elevator authority somewhere below
60 knots and descends into low tow, even though its free-flight stall
speed is less than 40 knots. It's not just a difference of feel - the
glider wallows and almost becomes uncontrollable.

Mike