motorgliders as towplanes

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