motorgliders as towplanes

On Mar 10, 9:56*am, 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.

The Lambada and similar modern motorgliders are very light (300kg) -
nearly half the weight of the older designs, such as the Grob 109 -
and perform very well.

I have flown a Grob 109 and it had enough trouble getting itself into
the air. *The Lambada also handles a lot better than the older heavy
beasts and is a joy to fly.

Mike

What the Lambada's towing performance at less than 1320 pounds gross
suggests to me is that the US Sport Light Aircraft regulations offer
the possibility of a formidable tug.

The Lambada's aerodynamics are great but 80 - 100 HP is inadequate for
heavy 2-seaters and ballasted singles. 135Hp or so combined with a
propeller optimized for max thrust below 60 knots would make a huge
difference. Aerodynamics doesn't count for much below takeoff speed -
there it's all weight, engine and propeller.

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

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

Why?

Ian

The G109B Turbo with constant speed prop. is a different beast altogether
with much more power and climbing capability.


I have flown a Grob 109 and it had enough trouble getting itself into
the air. =A0The Lambada also handles a lot better than the older heavy
beasts and is a joy to fly.

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

Also see what the adverse yaw is like on tow due to the increased lift
being generated


At 18:15 10 March 2009, Derek Copeland wrote:
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.

On 10 Mar, 18:15, Derek Copeland wrote:
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?

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...

How big do you think that effect is?

Ian

On Mar 11, 6:30*pm, Derek Copeland wrote:
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.

I agree that this effect exists, and I have noticed it, even behind
fast powerful tugs. For example I usually fly a Janus, which is
recommended to tow with +6 flaps. You'd normally switch to zero flap
anywhere above about 50 knots and be thinking about -4 at normal
towing speed. But if you go to zero flap while towing it feels very
mushy.

I'm not convinced that the stall speed is *actually* increased. I've
never been game to find out.

The only thing I can think of is that even though you're not flying
through the prop wash, you're still in the air that the wings of the
towplane have imparted a downward velocity to. The downwardly flowing
air exists maybe 5m or so both above and below the path that the wing
took. I don't know how to calculate the actual downward velocity, or
whether it is significant, but it may amount to the glider effectively
flying through significant sink.


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.

Right. Try towing with a Huey :-)

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