HiStart with a Turnaround?

[AkiP]

Hello everybody!

Anyone tried using a high start with a turnaround? In other words, have the bungee extending away from you with the line going through a raised winch turnaround back toward you. The glider doesn't have to lift the bungee, and it would allow for a more compact setup... :cool:

I've done tons of searches here, at rcuniverse and on yahoo groups soaring, but only found this and the accompanying link in the thread....

http://www.rcgroups.com/forums/showthread.phps=&threadid=120915&highlight=start

What are the minuses that I am missing? Or is it the added complication of the turnaround that turns people off the idea, since a high start is otherwise so simple? :D

Aki

[tempest411]

The link didn't work, but I had the same idea some time ago, and asked here as well. The consensus seemed to be that the drag of the bungee on the ground is more than that created by the plane pulling it up into the air with it. I suppose you could find some way of suspending the bungee off the ground, but now your talking about something that's 10X more of a hassle than a winch. I'm sure Ollie could tell you a lot more, as it was he that had some experiences with this that he shared with me...

Rick

[Ollie]

I tried a turnaround with a highstart about 25 years ago. I was motivated by the same thinking: lifting the rubber is a disadvantage. However, it turns out that lifting the rubber is a great advantage.

Even on a slippery surface like wet grass, the drag of the rubber on the ground very greatly reduces the tension on the tow line available to launch the model. The frictional force on the rubber is proportional to the weight of the rubber for any given ground condition. The tension on a highstart is greatest at the time of launch. With a turnaround, the friction of the rubber laying on the ground leaves the rubber partly stetched after the plane releases. Not only is much of the energy stored in the stretched rubber dissipated in friction but some of it is left unused at the end of launch.

With a conventional highstart the tension lessens to near the weight of the rubber at the end of the launch. The friction is overcome initially when tension is high and, as the rubber is lifted, the friction is reduced. When the rubber is off the ground, the friction is zero. Zero friction in the later part of the launch is a big advantage because the shortened rubber is producing less tension and there is less tension to spare.

[AkiP]

Thanks for the replies! I was hoping Ollie would answer...

I found another link...

http://home.clara.net/barcs/barcs-nf/easimoore-launch.htm

Would this system, where the turnaround is lifted off the ground, help with the friction associated problems? Of course it depends how much of the rubber is actually off the ground...

The thing that interests me in the above link is that they talk about zoom launches off the high start! :D Though the bungee restretching is funny, unless there's a headwind...

Aki

[Mr B....]

It would be to much drag on the ground.


Fly Safe!

Merrill Brady
AMA 276833

LSF 7247
Level V in the works

Visit our website at http://www.mmglidertech.com

[Ollie]

It would be interesting to experiment with the BARKS Easimore system to see how much the turnaround pulley's height reduced ground friction. The rubber will hang in a catenary curve which deepens as the tension on the rubber lessens during launch. The point at which the line attaches to the rubber also sags closer to the ground as the launch progresses. Making the anchor of the end of the rubber above ground will also help. Here in Florida there are lots of small ponds. If the rubber were suspended over water, the friction would be greatly reduced. Also, if the anchor and turnaround were mounted on small hills ground friction would be further reduced. By using a stronger than normal high start or launching in wind, the model will launch quicker and with less line travel than otherwise. This will keep more tension on the rubber during launch which will in turn reduce sag and ground friction. There are many variables to work with and I think there is a possibility that a practical combination can be developed.

[AkiP]

Thanks again for the replies...

Only really one way to find out how it works: time to go out and build me a turnaround!

Will report back...

Aki

[uscra112]

I did this a couple of years ago. Drag of the bungee on the ground makes the system not work. Best arrangement used two pulleys.

Double up the bungee. Stake out your turnaround pulley. Secure one end of the towline to that same stake. Attach the second pulley to one end of the doubled bungee. Stake out the other end of the bungee. Route the free end of the towline thru that pulley and then thru the turnaround pulley. Still with me? Now when you pull on the towline to stretch the bungee, you pull the line two feet for every foot that the bungee stretches. When you let it go, the bungee moves half as fast thru the grass. Was better, but drag was still a problem, though. So I elevated it anyway. Easy for me, there's an old rusty combine in the field right where I fly, so I just tied it off to the topmost part of that.

Pulleys are a problem - the hardware-store varieties I tried had far too much friction - they got blazing hot from just one launch. I modded a couple by installing ball-bearings in them.

Pulleys need to have as little inertia as possible. They have to be spun up to speed, and the energy has to come from the bungee.

I eventually just said the heck with it and went electric.

[Miami Mike]

I've been experimenting with this arrangement for use with smaller, lightweight planes:

http://www.smacaw.com/ezone/alternate hi-start.gif

Note that it has a 2:1 mechanical advantage. I haven't gotten it to work too well yet, but I think it has potential.

[Aten W Arthog]

I Don't think it's pull strength you need so much as speed.... a minimum airspeed pull...

[T. Lyttle]

Restretch: either breeze or thermal will allow you to restretch the bungee. My Oly used to go up on 20 paces stretch by zigzagging. Once a thermal is found, the model can be forced to follow the lift, eventually restretching the bungee to at least twice its length and a zoom launch releases the model, really high! A breeze at ground level cna easily be triple that speed higher up, same procedure, force the model...