motorgliders as towplanes

At 21:55 13 March 2009, The Real Doctor wrote:
How much extra lift do you think is required to climb?

Without going into the math (euphemism for I'm not sure how to!).

If you are in a free 60 knot glide sinking at 1.5 knots and you want to
climb at 4 knots you have to increase the angle of attack ie pitch up to a
more nose high position to achieve that.

That of course causes a corresponding increase in drag and the glider
starts to slow down immediately.

On tow the tug produces the necessary force to keep the glider flying at
the same speed while climbing but for a given IAS (ignoring any position
error) surely the angle of attack and thus the stall speed must be higher
than in free flight.

On Mar 13, 8:45*pm, Z Goudie wrote:
At 21:55 13 March 2009, The Real Doctor wrote:

How much extra lift do you think is required to climb?

Without going into the math (euphemism for I'm not sure how to!).

If you are not sure how to do the math, how can you be sure that you
are correct ?

In fact, I am able to do the math ( practicing aero engineer) and you
are not correct. The difference between lift ( and thus angle of
attack ) in a STEADY descent and a STEADY climb is practically non
existent.

To do the math, you should draw out a diagram of the drag, lift,
weight and towline. The aircraft is climbing on a line that is an
angle "gamma". Draw the towline force on this line pulling the
aircraft forward and up. Draw the drag in the opposite direction.
Draw lift at a right angle to this line and finally draw weight
pulling straight down.

Total up these 4 forces and making them balance out in the up-down and
forward-back directions gives you the relationship that lift = cosine
( gamma ) * weight. The steeper you climb, then less the lift !!!

If you make different assumption of the direction of the towplane
force, then you would get a different result.


Todd Smith
3S

At 01:03 14 March 2009, wrote:
If you are not sure how to do the math, how can you be sure that you
are correct ?

In fact, I am able to do the math ( practicing aero engineer) and you
are not correct. The difference between lift ( and thus angle of
attack ) in a STEADY descent and a STEADY climb is practically non
existent.

I should have known better than get into this discussion!

Yes you are of course correct in that the VERTICAL component of lift equal
to the gliders weight is exactly the same whether it is freely descending
at 60kts (at 1.5kts sink, 1:40 slope down) or being towed behind a tug at
60kts (at 10kts climb, 1:6 slope up).

There is however an increase in the angle between that Vertical component
required and the actual wing lift which is near enough at right angles to
the airflow.

This will require a higher wing loading (admittedly only a couple of
percent perhaps) to produce the vertical vector with a consequent related
change in stalling speed.

To small to be noticeable at reasonable towing speeds but if the tug gets
a bit slow and erratic then it will get interesting.

On Mar 14, 10:57*am, The Real Doctor
wrote:
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?

I have no idea. That would I imagine be a question for our chief
instructor. Personally I would not use it even if it was enabled, as
the effectiveness of the remaining controls is perfectly adequate and
exceeds that of more recent designs e.g. the Duo Discus.

As you all know I know nothing about aerodynamics but... surely you only
need to produce more lift to produce an acceleration. If you climb at a
steady 400ft / min at a constant speed you are not accelerating so you do
not need more lift to go up.

Surely the point of the tug is to overcome the drag that would otherwise
burn off potential energy and add energy into the system which the glider
converts into height.

I do not see why the wing has to produce more lift than in level flight at
the same speed. As the vertical component of the flight is now up rather
than down the aircraft must assume a more nose up attitude in order to
provide the same angle of attack at the leading edge giving the impression
that the angle of attack is increased leading those that know less than I
about aerodynamics (Delboy) the idea that the aircraft is producing more
lift.

Any aerodynamicists out there?

Jim



At 00:45 14 March 2009, Z Goudie wrote:
At 21:55 13 March 2009, The Real Doctor wrote:
How much extra lift do you think is required to climb?

Without going into the math (euphemism for I'm not sure how to!).

If you are in a free 60 knot glide sinking at 1.5 knots and you want to
climb at 4 knots you have to increase the angle of attack ie pitch up to
a
more nose high position to achieve that.

That of course causes a corresponding increase in drag and the glider
starts to slow down immediately.

On tow the tug produces the necessary force to keep the glider flying at
the same speed while climbing but for a given IAS (ignoring any position
error) surely the angle of attack and thus the stall speed must be
higher
than in free flight.

On Mar 14, 5:45*pm, Z Goudie wrote:
At 01:03 14 March 2009, wrote:

If you are not sure how to do the math, how can you be sure that you
are correct ?

In fact, I am able to do the math ( practicing aero engineer) and you
are not correct. *The difference between lift ( and thus angle of
attack ) in a STEADY descent and a STEADY climb is practically non
existent.

I should have known better than get into this discussion!

I can do the math also (note that I gave the same formula in an
earlier post as Todd just gave you).


Yes you are of course correct in that the VERTICAL component of lift equal
to the gliders weight is exactly the same whether it is freely descending
at 60kts (at 1.5kts sink, 1:40 slope down) or being towed behind a tug at
60kts (at 10kts climb, 1:6 slope up).

That is not correct, and that is not what Todd said.

In a constant speed and constant angle climb or descent, the vertical
component of lift DECREASES the more the flight path departs from
horizontal.

The total force required to support the aircraft is what remains
constant. The difference is made up by the sum of the thrust (zero in
gliding flight of course) and the drag.


There is however an increase in the angle between that Vertical component
required and the actual wing lift which is near enough at right angles to
the airflow.

Right.

Except it is not "near enough". The wing lift is BY DEFINITION
exactly and always at right angles to the airflow.


This will require a higher wing loading (admittedly only a couple of
percent perhaps) to produce the vertical vector with a consequent related
change in stalling speed.

You are drawing the wrong triangle, with the right angle in the wrong
place. You would be correct if the glider was climbing while being
pulled by a horizontal rope. It isn't.

Hi Jim,

We cross swords on yet another forum! I am only trying to explain and
understand WHY gliders seem to require more speed to fly in a steady and
controlled manner on aerotow than is necessary in free flight. There seems
to be a general agreement that this is a real effect, by everyone who has
ever had a slow aerotow! I don't believe that it is purely a control
issue, as it still seem to be the case even in smooth air. I have at least
an average gliding instructor's knowledge of aerodynamics and theory of
flight BTW, and I am aware that angle of attack only relates to the
relative airflow.

In steady and unaccelerated flight, the theory states that lift acting
upwards at 90 degrees to the wings chord line equals weight acting
vertically downwards due to gravity, and that thrust (in this case
provided by the tug via the rope) equals drag. There is a vector effect
when climbing or descending, which is overcome in powered aircraft by
increasing or reducing the power setting, i.e. thrust.

If anyone can come up with a convincing theory as to why this 'aerotow
effect' exists, I would like to know it as much as anybody.

Derek Copeland


At 08:15 14 March 2009, Jim White wrote:
As you all know I know nothing about aerodynamics but... surely you only
need to produce more lift to produce an acceleration. If you climb at a
steady 400ft / min at a constant speed you are not accelerating so you
do
not need more lift to go up.

Surely the point of the tug is to overcome the drag that would otherwise
burn off potential energy and add energy into the system which the
glider
converts into height.

I do not see why the wing has to produce more lift than in level flight
at
the same speed. As the vertical component of the flight is now up rather
than down the aircraft must assume a more nose up attitude in order to
provide the same angle of attack at the leading edge giving the
impression
that the angle of attack is increased leading those that know less than
I
about aerodynamics (Delboy) the idea that the aircraft is producing more
lift.

Any aerodynamicists out there?

Jim



At 00:45 14 March 2009, Z Goudie wrote:
At 21:55 13 March 2009, The Real Doctor wrote:
How much extra lift do you think is required to climb?

Without going into the math (euphemism for I'm not sure how to!).

If you are in a free 60 knot glide sinking at 1.5 knots and you want to
climb at 4 knots you have to increase the angle of attack ie pitch up
to
a
more nose high position to achieve that.

That of course causes a corresponding increase in drag and the glider
starts to slow down immediately.

On tow the tug produces the necessary force to keep the glider flying
at
the same speed while climbing but for a given IAS (ignoring any
position
error) surely the angle of attack and thus the stall speed must be
higher
than in free flight.

On 14 Mar, 00:45, Z Goudie wrote:
At 21:55 13 March 2009, The Real Doctor wrote:

How much extra lift do you think is required to climb?

Without going into the math (euphemism for I'm not sure how to!).

If you are in a free 60 knot glide sinking at 1.5 knots and you want to
climb at 4 knots you have to increase the angle of attack ie pitch up to a
more nose high position to achieve that.

Not quite. Your flight path has move upwards, of course, but the
effect on angle of attck needed is more subtle.

That of course causes a corresponding increase in drag and the glider
starts to slow down immediately.

On tow the tug produces the necessary force to keep the glider flying at
the same speed while climbing but for a given IAS (ignoring any position
error) surely the angle of attack and thus the stall speed must be higher
than in free flight.

Only a little.

In free flight the vertical components of lift and drag both balance
the weight, and the horizontal component of lift balances the
horizontal component of drag. Since the drag is, for aerodynamic
reasons, much less than the left, a little horizontal lift component
balances a lot of horizontal drag component - in other words, the
glide path is quite shallow.

As a result, there is very little vertical component of drag. Roughly
speaking (and using the theta = sin theta = tan theta approximation,
if that means anything), if the glide angle is 40:1, only 1/40 of the
drag acts vertically, and since the drag is 1/40 of the lift in the
first place, the lift does about 1599/1600 of the holding up, with the
drag contributing the remaining 1/1600.

For all practical purposes, it's the lift which holds the glider up,
and "lift = weight" is a perfectly reasonable approximation for all
normal flight paths. The difference is actually 1 - cos(arctan(D/L))
which for 40:1 is 0.9997.

Including climbing.

So now we wanted to climb at 4kts, which is about 15:1. That's still a
very shallow angle, so the theta = sin theta = tan theta approximation
is still good. However, this time the horizontal component of lift now
acts backwards, and can't balance the horizontal component of drag. So
we need to pull the thing along. Hello tug. The simplest way of
looking at the tow force is as a combination of horizontal and
vertical forces. The precise ratio depends on lots of different
things: rope size, material and length, tow position and so on. It's
simplest just to think of it as a horizontal force and leave any
vertical component to modify the effective weight of the glider
slightly.

OK, so now we have a tow force. It has to oppose the horizontal
componet of the drag (which is a large proportion of a small force)
and the horizontal component of lift (which is a small component of a
large force). Overall, the two work out about the same, with the lift
component rather bigger in most cases.

Vertically the weight is now augmented by a downwards component of
drag, but this is still very small: if the wing is still working at
40:1 and the glide path is 15:1, the vertical component of lift has to
contribute 599/600 of the weight. The lift is slightly more tilted
than in free flight, but it makes hardly any difference - around 0.2%.

So in a 40:1 gliding descent, lift is 1599/1600 of weight, and in a
15:1 climb, lift is 599/600 of weight. The difference is 0.1% of
weight. If we want to climb at the same speed as we descended, that
certainly requires a change in angle of attack, but an absolutely
minuscule one. It won't make any difference to the L/D ratio, so my
assumption that it was 40:1 in the climb as well as the glide is
reasonable.

Summary: in a steady climb or glide, the lift needed from a glider's
wing is as equal to the weight as makes no difference and the angle of
attack needed is dependent on speed. Any differences in control
forces, feel and so on come from other factors: angle of the tow rope,
position of the tow hook, tug wing downwash and so on.

Ian

On 14 Mar, 08:06, Bruce Hoult wrote:
On Mar 14, 10:57*am, The Real Doctor
wrote:

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

I have no idea. *That would I imagine be a question for our chief
instructor. *Personally I would not use it even if it was enabled, as
the effectiveness of the remaining controls is perfectly adequate and
exceeds that of more recent designs e.g. the Duo Discus.

I don't doubt it - the tail chute was never, as far as I know, fitted
to my club's Cirrus. But it would be an interesting problem in case of
an overshoot accident, wouldn't it?

Ian

Todd,


Nice! You must be the only guy out there who understands this stuff, the
forces acting on a glider in flight. (Other than me!)

Yes, analysis shows that in CLIMBING flight, lift must just be LESS that
it is in level flight. Same for descending (gliding flight) Lift is less
than it would be in level flight.

The difference between descending flight, level flight and climbing
flights is POWER.

In the case of a glider on towplane, the power (energy) comes from the
fuel powering the engine which in turn produces thrust at the propeller
which in turn produces a pulling force throught the rope to the glider.
We could call this "Thrust"

Excess power makes an aircraft climb!

If Thrust is greater than drag, the aircraft will climb. If Thrust is
equal to drag the aircraft will fly level. If thrust is less than drag,
(or nonexistant as in a glider in free flight), the aircraft will
descend.

As you said, lift does vary, but very little if climb or descent angles
are kept reasonable.

Drag and Thrust are the important variables. Gravity MUST remain
constant, and lift hardly varies worth considering.

Note that power, we should say energy, can be imparted to a glider in
several ways that will result in climbing flight. Of course by a tow
plane as mentioned above, but it could be energy from a THERMAL, RIDGE,
WAVE etc. These will all make a glider climb!

To beter understand how the lift gets less as the climb angle gets
greater, let's look at teh "extreme". Consider a glider attached by a
nose hook to a huge construction crane. The crane operator applies POWER
to the lifting cable and the glider is slowly lifted, vertically into the
air.

The glider has only two forces acting on it now, THRUST from the lifting
cable, and gravity. Thrust acting vertically upward, and gravity acting
vertically downward. In fact, these forces woud be equal, but oppposite
to each other. LIFT would necessarily be ZERO!

Cookie (From blairstown)









At 01:03 14 March 2009, wrote:
On Mar 13, 8:45=A0pm, Z Goudie wrote:
At 21:55 13 March 2009, The Real Doctor wrote:

How much extra lift do you think is required to climb?

Without going into the math (euphemism for I'm not sure how to!).

If you are not sure how to do the math, how can you be sure that you
are correct ?

In fact, I am able to do the math ( practicing aero engineer) and you
are not correct. The difference between lift ( and thus angle of
attack ) in a STEADY descent and a STEADY climb is practically non
existent.

To do the math, you should draw out a diagram of the drag, lift,
weight and towline. The aircraft is climbing on a line that is an
angle "gamma". Draw the towline force on this line pulling the
aircraft forward and up. Draw the drag in the opposite direction.
Draw lift at a right angle to this line and finally draw weight
pulling straight down.

Total up these 4 forces and making them balance out in the up-down and
forward-back directions gives you the relationship that lift =3D cosine
( gamma ) * weight. The steeper you climb, then less the lift !!!

If you make different assumption of the direction of the towplane
force, then you would get a different result.


Todd Smith
3S