Towrope tension??

Looking for information covering tension applied to a towrope during tow and any other maneuvers while on tow. Twins vs. singles.
Scientifically applied or Kentucky windage guesses also welcome to be posted.
Thanks,
R

On Mon, 15 Oct 2018 04:34:27 -0700, Retting wrote:

Looking for information covering tension applied to a towrope during tow
and any other maneuvers while on tow.

Here's the result of a spreadsheet calculation I did some time back for
my Libelle being flown by myself, so the weights are accurate. Of course
I still have the spreadsheet.

Libelle on tow
==============
Glider weight 216 kg
Pilot+parachute 73 kg
Flying weight 289 kg

Glide ratio 36
Drag at best glide 8.03 kg
Best glide 53 kts
Tow speed 60 kts
Drag at tow speed 10.29 kg

Climb angle 600 fpm (5.67 degrees)
Load due to climb 28.54 kg

Total tow load 38.83 kg

Explanation
===========
I calculated drag at best glide as all-up weight / glide ratio and then
scaled it for tow speed as (towspeed^2 / bestglidespeed^2) since drag
varies as the square of flying speed.

Load due to climb: I calculated the angle as asin(climbrate/towspeed)
where both speeds are in kts. The tow load due to climbing is
flying weight*(climbrate/towspeed)

So the load in the tow rope is drag + climbing load.


--
Martin | martin at
Gregorie | gregorie dot org

What would you think the increase (%) would be at start of roll thru aceleration?
A heavy twin at 700 kg?

R

Well start with you can work out the average acceleration...and Newton’s second law...

What are you actually trying to get done here? This will vary from such trivial stuff (like the above) to very complex modeling.

On Mon, 15 Oct 2018 05:28:59 -0700, Retting wrote:

What would you think the increase (%) would be at start of roll thru
aceleration?
A heavy twin at 700 kg?

That would depend on the total of:

a) rolling resistance, i.e. the type of surface, how hard it is,
how long the grass, etc. is and tyre pressure.

b) the acceleration, which depends on the type of towplane, engine size
and the throttle-bending tendencies of the tuggie.

c) the slope, if any, of the runway. That, at least, is easy once you
know the slope because its the same calculation as the climbing
contribution to tow load in the air.

I have little idea of the values of parameters a and b except that the
acceleration load would be the all-up glider weight multiplied by the
acceleration expressed in G, so a 0.1G acceleration would have a tow load
of 50kg for an all-up weight of 500kg. However, the initial acceleration
is almost certainly less than 0.1G

As for rolling resistance, I suggest you fit a decent spring balance to a
short length of towline and rings. Then two or three of you can easily
measure low speed rolling resistance for a glider or two on a variety of
surfaces. It would be interesting to know how much difference there is
between rolling resistance on a tarmac surface and a dry, recently mown
grass strip. Last but not least, and it would be something to do at the
field on a non-flying day.


--
Martin | martin at
Gregorie | gregorie dot org

A typical 235Hp Pawnee generates about 400kg of static thurst, that would also be the maximum tension on the rope that the towplane can generate.
On the other hand can the glider generate a towrop tension high enough to break the tope or the weak link if used.

R
Uli, has some actual data, from a load cell with telemetry, from our winch launches. Which should be much greater than anything you would see on tow. when winch launching the two areas of maximum rope stress are during the initial ground acceleration, and at the top when the winch can pull the the glider down while the glider is trying to go up. Only the first part of that is applicable to aerotow, and the initial winch pull is much greater than the relatively gentle acceleration of a tow plane.

If the rope does not break with the glider on or just off the ground, it shouldn't break during tow, unless the glider pilot does some unusual maneuvers behind the tow plane. The last time a student broke the rope on tow, I was relieved, because I could just land, and make it stop.

If you want the data e-mail me, and I'll hook you up with Uli.

SF

On Monday, October 15, 2018 at 9:22:43 AM UTC-4, Per Carlin wrote:
A typical 235Hp Pawnee generates about 400kg of static thurst, that would also be the maximum tension on the rope that the towplane can generate.
On the other hand can the glider generate a towrop tension high enough to break the tope or the weak link if used.

Instead of calculating and modeling these conditions, why not measuring it very accurately? We wanted to know what the force distribution throughout a winch launch is - a far more complex thing than an aero-tow - and built a slick little recorder. It was housed in a 2l soda bottle attached to the apex of drogue chute and recorded the forces and accelerations of multiple launches. The data was compressed and stored on a micro SD card and was read at the end of the day.

Uli
'AS'

100 Uganda Shillings to the first to provide the best response to the original thread.
R

Starting new thread.....