motorgliders as towplanes

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

On 10 Mar, 14:15, Derek Copeland wrote:

Remember that a glider has to produce more lift when climbing.

Why?

Ian

At 16:40 10 March 2009, The Real Doctor wrote:
On 10 Mar, 14:15, Derek Copeland wrote:

Remember that a glider has to produce more lift when climbing.

Why?

Ian


The glider does not just get pulled up by the tow rope! The tug acts as a
remote power source so that the glider can climb through the air without
losing speed. The wings have to produce more lift to support the weight of
the glider, plus the vector of the weight and the climb angle.

Gliders that have stalling speeds below 40 knots in free flight often
start buffeting and feeling slightly out of control if they are flown much
below 50 knots on aerotow.

Derek Copeland

On Mar 11, 3:15*am, 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.

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.

wrote:
On Mar 8, 1:03 pm, Brad wrote:
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?
Brad

http://www.youtube.com/watch?v=5UYPyNgf4Q4

Lambada motorglider towing
UFM 13 Rotax 912 100hp

Personally I would far rather be behind the 100hp Samba or Lambada with
my Std Cirrus - hot and high off grass the accelleration is slower, but
the propwash is limited and climb is better than the asthmatic 180hp
(must be really little ponies over there) super cub.

The wingloading on the Lambada is very similar and the tow combination
is nicely balanced. Behind a 235hp Rallye it is, by comparison
wonderful. Same time to altitude without all the - extreme speed just
off the deck waiting for the tug to leap skyward, then get to be like a
good martini and get all shaken up by the wake turbulence - then scream
around on the outside of all the thermals.

Give me a motorglider any time...

Bruce

Bruce wrote:
wrote:
On Mar 8, 1:03 pm, Brad wrote:
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?
Brad

http://www.youtube.com/watch?v=5UYPyNgf4Q4

Lambada motorglider towing
UFM 13 Rotax 912 100hp

Personally I would far rather be behind the 100hp Samba or Lambada with
my Std Cirrus - hot and high off grass the accelleration is slower, but
the propwash is limited and climb is better than the asthmatic 180hp
(must be really little ponies over there) super cub.

The wingloading on the Lambada is very similar and the tow combination
is nicely balanced. Behind a 235hp Rallye it is, by comparison
wonderful. Same time to altitude without all the - extreme speed just
off the deck waiting for the tug to leap skyward, then get to be like a
good martini and get all shaken up by the wake turbulence - then scream
around on the outside of all the thermals.

Give me a motorglider any time...

Bruce
In case anyone thought that the MG/UL has too little grunt.
Follow the link below.

The story behind it is - experienced pilot in getting current again on
Janus forgot the thing has a drogue chute.
On a check flight the Janus is towed up from an airfield (Orient) at
5100" MSL at around 20 centigrade.
As you can see the Samba accelerates slowly - then the drogue chute
deploys. (one bump too many )
Tuggie happens to be enormously experienced CFI - uses his head and
abuses the Rotax a little. Combination manages to make a circuit and the
Janus gets dropped over the threshold.
Samba engine did not over heat or suffer any apparent damage, although
it was kept in the time limited maximum power range for the entire circuit.
For information , low down there are very few options straight out on 36
that would not include reducing the Janus to kit form if he had been
dropped before getting back to the runway.

http://www.youtube.com/watch?v=4_w3n...eature=related

With a light single seater it beats the climb performance of the super cub.

On Mar 12, 1:09*pm, Bruce wrote:
Bruce wrote:
wrote:
On Mar 8, 1:03 pm, Brad wrote:
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?
Brad

http://www.youtube.com/watch?v=5UYPyNgf4Q4

Lambada motorglider towing
UFM 13 Rotax 912 100hp

Personally I would far rather be behind the 100hp Samba or Lambada with
my Std Cirrus - hot and high off grass the accelleration is slower, but
the propwash is limited and climb is better than the asthmatic 180hp
(must be really little ponies over there) super cub.

The wingloading on the Lambada is very similar and the tow combination
is nicely balanced. Behind a 235hp Rallye it is, by comparison
wonderful. Same time to altitude without all the - extreme speed just
off the deck waiting for the tug to leap skyward, then get to be like a
good martini and get all shaken up by the wake turbulence - then scream
around on the outside of all the thermals.

Give me a motorglider any time...

Bruce

In case anyone thought that the MG/UL has too little grunt.
Follow the link below.

The story behind it is - experienced pilot in getting current again on
Janus forgot the thing has a drogue chute.
On a check flight the Janus is towed up from an airfield (Orient) at
5100" MSL at around 20 centigrade.
As you can see the Samba accelerates slowly - then the drogue chute
deploys. (one bump too many )
Tuggie happens to be enormously experienced CFI - uses his head and
abuses the Rotax a little. Combination manages to make a circuit and the
Janus gets dropped over the threshold.
Samba engine did not over heat or suffer any apparent damage, although
it was kept in the time limited maximum power range for the entire circuit.

On Mar 13, 9:09*am, Bruce wrote:
The story behind it is - experienced pilot in getting current again on
Janus forgot the thing has a drogue chute.
On a check flight the Janus is towed up from an airfield (Orient) at
5100" MSL at around 20 centigrade.
As you can see the Samba accelerates slowly - then the drogue chute
deploys. (one bump too many )
Tuggie happens to be enormously experienced CFI - uses his head and
abuses the Rotax a little. Combination manages to make a circuit and the
Janus gets dropped over the threshold.

Nice.

We disable the parachute on our Janus. I'm not even sure why it has
one. The spoilers are about the same as a Duo, PLUS it has a landing
flap setting that is effective enough that you need a fairly steep
approach if you want to use half spoilers as well, PLUS it has one
heck of an effective slip which (unlike the parachute) can be
modulated or used multiple times as desired.

On 12 Mar, 22:29, Bruce Hoult wrote:

We disable the parachute on our Janus. *I'm not even sure why it has
one.

I believe the answer is "as a fudge to meet the old JAR speed-limiting-
in-a-45-degree-dive requirement.Most drag chutes were never seriously
intended to be used for approach, or indeed to be used at all.

Do your insurers know, by the way, that you have disabled a flight
control?

Ian