ORIGINAL: northwest
Hi all. Think its my first post on RCU. Only two of us here in Jamaica ever fly pure sailplanes so that's not enough to buy a winch, so we use a hi-start (pinnacle large). I have launched the Spirit, the Daydream (both 2m) and also the Polecat Thermal Dancer (3M) and the Ava with it. Hi-starts are great and they work, but so far nobody has discussed any disadvantages/downsides. When launching the big boys it can get real exciting and is very much a physical work-out! Hi-start launching is where the model is at the most risk. These are things to look out for...especially when launching large models w/o assistance.
-Once the pull gets really strong if you have sweaty/slippery hands you will have a problem holding the plane and positioning for the launch. Two hands are needed for the pull-back, but transitioning to the one-handed launch moment can be tricky. A wood fuse can actually be crushed from the pressure at this point, so for larger planes I actually prefer a strong composite body.
-Now the acceleration is really fast with this amount of pull...so keep your head clear of the stab. You have like 3 seconds to "hold with one hand, level the wings, duck & throw". If the stab clips your head the plane could be destroyed.
When using a hi-start you can adjust how much pull by the distance you walk back. It is not necessary to pull back 3X the rubber length for every plane.
My hi-start is a pinnical XL, which is stronger than yours. When launching my Supra or the AVA, I might pull back 250 feet and fell that is plenty. That will give me about 24 pounds of pull. Considering the AVA weighs about 2.5 pounds, that is 9X the plane's weight. More than enough. It would launch every well at 12 pounds of pull.
The Supra wrigths 4 pounds, so that gives me 6X the plane's weight. More than enough. On a lighter hi-start, like the Pinnical Large I would imagine you get about 16-20 punds of pull at 300 foot pull. That can be a lot for some people to handle alone, but you don't have to pull back that far, especially if you are launching into a breeze. And, as I recall, there is almost always a breeze in Jamica.
When I launch my Spirit, I only pull back about 150 feet on the Pinnical XL, which gives me about 14 pounds of pull, or about 5.5X the plane's weight. Way more than enough. It would launch very nicely at 10 pounds using a lighter hi-start.
-Some planes seem to be specifically designed for a winch. My Ava is a good example. Because the belly curves upward in the tow hook area the hi-start ring has a difficult time sliding off at the top of the launch. We don't get pop-offs with this plane but you must zoom pretty good (winch-style) to come off the line. This plane is big and draggy for the hi-start so acceleration is a bit slow (and in any case the plane will not go up faster than the parachute drag will allow) so the technique is to push over for the zoom before reaching the top. In the zoom with the Ava you must pull to almost vertical to slip the ring off the hook. Without adequate speed the plane simply stalls at the top and remains connected. Imagine your glider being pulled back downwards by the hi-start-not good! My ava has gone completely inverted, me looking at the top. When I managed to disconnect it recovered at about 10 ft flying straight downwind back towards me. The hook is tough and I can't bend it open, so I filed off the inside edge. Things improved a bit.
On another occasion I dived so hard for a good zoom that the plane actually accelerated off the hook and overtook the parachute. The chute snagged the stab and the plane went in (but no damage, the nose stuck into soft mud).
That's what hi-starting with large planes is all about. Be careful and make sure your radio is on!
While the AVA and the Supra are designed to withstand the higher stress of a winch, there is nothing about them that would make them difficult for hi-start launching. I have never seen any of the tendancies you mention when launching off a hi-start.
I think what you are seeing is caused by the fact that these planes have large wing areas. If there is a breeze, they can actually pull back on the hi-start as they climb preventing it from fully contracting. This is VERY good in that it will give you higher launches. But if you just want to float off the line, you have to take the tension out of the line.
This has nothing to do with the design of the plane and everything do do with how the pilot flies the hi-start. If you are getting too much pull from the plane, move the hook forward. But, of course you will give up some altitude this way.
Personally when I launch my Supra or AVA on a breezy day, I add 100 feet of line and get awesome launches.