motorgliders as towplanes

Hi Mike, what are you towing.. A Horsa Glider?
75 pounds x (L/D) of 40 = 3000 Lbs
I would think the tow rope 'Thrust' would be nearer 20-25 lbs.
Even so, your sag only increases to 3 ft.

Pilot Pete

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

On 17 Mar, 10:00, Peter Higgs wrote:
Hi Mike, what are you towing.. *A Horsa Glider?
75 pounds x (L/D) of 40 * = * 3000 Lbs
I would think the tow rope 'Thrust' would be nearer 20-25 lbs.

We covered this a few posts back. During the two the glider's flight
path is - obviously - inclined upwards, so the lift vector is inclined
backwards, giving a horizontal component of lift for the tug to
overcome. If the glider is doing 60kt, climbing at 4kt and weighs
1000lb then this backwards lift is close to 1000 * sin (arctan (4/60))
which is 67lb.

Ian

Ian,

Of course I was missing something! (I think better in the morning than
evening.)

When you figure in the DIRECTION OF FLIGHT (upward and forward) the
Thrust vector (tension on the rope) has to be greater than drag alone.

Our tow plane can tow a 1000# glider at about 1:10, so the pull on the
rope would be even a little more than you calculated, (1:15).

I do the vector calcualtions with the following "standards":

Gravity acts downward, lifts acts perpendicular to the direction of
flight, drag acts parallel to the direction of flight. Although it is
nice to think of thrust as acting parallel to the direction of flight, we
have shown that this is not necesarily true and thrust can act slightly
upward or downward compared to the direction of flight.

I think that we can conclude the following: On tow, lift is almost
unchanged, even at different climb angles. Even if the thrust from the
rope is not parallel to the direction of flight, lift is hardly changed.

The tension on the rope, however varies with the direction of flight, is
greater than drag alone, and is signifigant.

Cookie





At 10:15 17 March 2009, The Real Doctor wrote:
On 17 Mar, 10:00, Peter Higgs wrote:
Hi Mike, what are you towing.. =A0A Horsa Glider?
75 pounds x (L/D) of 40 =A0 =3D =A0 3000 Lbs
I would think the tow rope 'Thrust' would be nearer 20-25 lbs.

We covered this a few posts back. During the two the glider's flight
path is - obviously - inclined upwards, so the lift vector is inclined
backwards, giving a horizontal component of lift for the tug to
overcome. If the glider is doing 60kt, climbing at 4kt and weighs
1000lb then this backwards lift is close to 1000 * sin (arctan (4/60))
which is 67lb.

Ian

Ian,

I do think of this (Climbing flight) as the "lift vector being tilted
rearward."

But we should add (vector addition) the force of gravity (downward) the to
force of lift (upward and rearward).

We find that these two forces nearly cancel each other except for a
realtively small resultant force, parallel to the line of flight.

This resultant force could be added to drag, therefore thrust would have
to be the sum of this force plus drag, but in the opposite direction.


Cookie






At 10:15 17 March 2009, The Real Doctor wrote:

We covered this a few posts back. During the two the glider's flight
path is - obviously - inclined upwards, so the lift vector is inclined
backwards, giving a horizontal component of lift for the tug to
overcome. If the glider is doing 60kt, climbing at 4kt and weighs
1000lb then this backwards lift is close to 1000 * sin (arctan (4/60))
which is 67lb.

Ian

OK, all this talk about the aerodynamics of towing brings me to something
about the aerodynamics of gliding!

The following question came up at a recent club meeting. I believe the
question was originally on the Bronze Badge Exam.

Q) What force provides the forward motion necessary to move a glider
through the air?

a) lift
b) centripetal force
c) gravity


I contend that it is a flawed question, and that none of the choices for
answers are (completly) correct!

Any thoughts?


Cookie

On Sun, 15 Mar 2009 10:36:59 +0200, Ian wrote:

Does anybody know what the certification situation is for towing with
ultralight catorgory aircraft (450kg AUW) is in Europe these days? How
many tow planes are certified for tugging and what mass of glider are
they permitted to tow?

Most ultralights in Germany are certified to tow gliders, most of
them are limited to 650 kg AUW of the glider, some of them to 750 kg.

Bye
Andreas

On 17 Mar, 12:30, Bob Cook wrote:
OK, all this talk about the aerodynamics of towing brings me to something
about the aerodynamics of gliding!

The following question came up at a recent club meeting. *I believe the
question was originally on the Bronze Badge Exam.

Q) What force provides the forward motion necessary to move a glider
through the air?

a) lift
b) centripetal force
c) gravity

I contend that it is a flawed question, and that none of the choices for
answers are (completly) correct!

It's a flawed question because, as Newton pointed out, NO force is
required for steady motion.

Mind you, it's amazing how many people think the power source for
soaring flight is gravity.

Ian

On 17 Mar, 12:30, Bob Cook wrote:

But we should add (vector addition) the force of gravity (downward) the to
force of lift (upward and rearward).

We find that these two forces nearly cancel each other except for a
realtively small resultant force, parallel to the line of flight.

This resultant force could be added to drag, therefore thrust would have
to be the sum of this force plus drag, but in the opposite direction.

Agreed.

Ian

On Mar 17, 8:53*am, The Real Doctor wrote:
On 17 Mar, 12:30, Bob Cook wrote:

OK, all this talk about the aerodynamics of towing brings me to something
about the aerodynamics of gliding!

The following question came up at a recent club meeting. *I believe the
question was originally on the Bronze Badge Exam.

Q) What force provides the forward motion necessary to move a glider
through the air?

a) lift
b) centripetal force
c) gravity

I contend that it is a flawed question, and that none of the choices for
answers are (completly) correct!

It's a flawed question because, as Newton pointed out, NO force is
required for steady motion.

Mind you, it's amazing how many people think the power source for
soaring flight is gravity.

Ian

No force? Newton's first law tells you that for constant velocity the
forward component of lift must match the drag (or the glider would
accelerate or decelerate, and if so then the second law tells you how
much).

There is drag force so a forward force is required to counter that or
the glider would decelerate, and something has to overcome that force.
And that is the forward component of lift. Answer (a) is clearly the
best answer and this question as is I think is a suitable test of a
basic understanding of what is going on.

Gravitational potential energy gained by soaring (climbing in lift) or
by winching/towing/motorglider engine is the power source for gliding
flight. Thankfully we have several ways to charge that power source.

Darryl

Ian,

You are of course correct that once an object is in motion, there is no
force required to keep it in motion.

This is why the SUM of all forces acting on a glider in unaccelerated
flight is zero.



Accelerated flight: forces are not in balance, hence change, or
acceleration (speed and / or direction).

Now it did take some (unbalanced) force(s) to get the glider moving in the
first place.

But why does the glider take on the flight path that it does?

If we could say "drop" a glider from a helium balloon. The glider would
at first have no motion at all.

Eventually the glider would achieve a nearly horizontal flight path,
(40:1) with the correct control inputs. Initially there would be
unbalanced forces, resulting in increasing velocity, and changing
direction.

Once the forces became "in balance" (their sum = zero) the glider would
achieve steady flight.


My point is that Gravity alone will not have the above effect.

I don't like your use of the word power below however. Force, energy and
power are not the same. People often confuse them.

Cookie




At 15:53 17 March 2009, The Real Doctor wrote:
On 17 Mar, 12:30, Bob Cook wrote:
OK, all this talk about the aerodynamics of towing brings me to
something
about the aerodynamics of gliding!

The following question came up at a recent club meeting. =A0I believe
the
question was originally on the Bronze Badge Exam.

Q) What force provides the forward motion necessary to move a glider
through the air?

a) lift
b) centripetal force
c) gravity

I contend that it is a flawed question, and that none of the choices
for
answers are (completly) correct!

It's a flawed question because, as Newton pointed out, NO force is
required for steady motion.

Mind you, it's amazing how many people think the power source for
soaring flight is gravity.

Ian