motorgliders as towplanes

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 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

"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

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.

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 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.