Winch Launching - Computing different cable weights

Changing from a 1/8' to a 3/16' 4,000' winch cable.

We know this is heavier cable, but none of the reference materials factor in cable weight and drag for the weak link and max payload of the aircraft.

We have had no trouble lifting 4,000 feet of 1/8' with an SGS 2-33 in a moderate wind at near max gross. Have not winched the heavier Blanik yet. (reason for our getting a bigger cable)

Is this a significant enough change and should this be a consideration combined with the gross weight of the glider, pilots and cable?

Thanks for any of your experience or references.

Michael

J. Michael Henderson NORTHWEST SOARING, INC.

Tel: (509) 674 0621, (206) 729 5358 Internet: www.northwestsoaring.com

Use the same (and correct) weak link.

With a nose hook, the 2-33 will run out of elevator and porpoise some as it reaches 600 ft or so. You can reduce this somewhat by backing off gradually on the power. If you don't, you will overspeed the glider as it reaches about 1000ft agl and pilots will release under high tension and provide some advanced winch driver training.

Whether we used 4000ft or 5000ft of 3/16' 7/7 wire rope, the net launch height was the same. The extra 1000ft added virtually nothing to the launch height. Adding a 1000' Spectra length to the end resulted in 200-300ft higher launches. The Schweizer C/G hook is only about 18' further aft and not a true C/G hook. Unless you're already fitted with one, I'd recommend keeping the $1000+ toward another glider to replace the 2-33. However, if a 100% Spectra winch ever proves practical, a 2-33 should get quite good launches without porpoising.

As long as you stay within placard limits, you should be fine.

Is your Blanik fitted with a CG hook, or with the yoke harness? Although Vitek will disagree with me, I'd highly recommend spending that $1000+ on refitting with a CG hook if you don't have one.

Frank Whiteley

We regularly launch Bergfalke 11 and 111 trainers similar to the 2-33 and a Blanik L13 on a wire only winch.

We get up to 2300' launches solo - with a headwind and fair reliability.

The set up is: Ford 302 cu inch V8 (Noah would have recognised it) with auto transmission. 4.65 mm annealed steel cable - we use 1600-1800m of 2000 available for a total length of upwards of 5000'. This wire is lighter than the 1/8' you are suggesting so it does not impede progress in the climb. Our aircraft all have CG hooks so the Bergies climb without loss of control authority.

The Blanik has the yoke cables. Some points, 1] These are a pain to continually swap becaue of the different leader. 2] The connector hooks develop fatigue and snap every now and again. 3] When the connectors wear the yoke releases under tension - resulting in advanced training on the winch... 4] The Blanik, launching with flaps deployed requires full throttle on this winch. When he rotates into the climb, the winch cannot maintain full RPM. If a cable breaks under this stress (most likely time) you get more interesting times on the winch.

We have not had any significant problems with this set up. You have to consider that the long cable can cause you a major headache if you have a cable break, especially early in the run. 4000' of 'knitting' can take a while to sort out.

Conversely, we regard anything under 1200' launch in still air, dual in a low performance trainer as a poor launch. We replace the cable once we reach around 15 repairs(knots), which lasts us around a year. Since the cable tends to fail at existing repairs, we can assume a 50% higher number of breaks. With an average of slightly over 30 launches a day and 50 launch days a year , that's about a 1.5% chance of a cable break.

The Blanik is responsible for a disproportionate number of these because of the higher stress it puts on the winch.

The porpoising you describe is caused by the tailplane stalling and then unstalling ad nauseum (don't forget it's working the other way) Reducing the amount of back pressure on the stick (reducing the angle of attack) will solve the problem as will fitting a CofG hook as has already been mentioned. Heavier cable on a non CofG hook will make the porpoising more likely.

At 05:06 10 June 2002, F.L. Whiteley wrote:

Our club (Memphis Soaring)has bitten the bullet! I ordered 4500'of 3/16' PLASMA (5500LBS breaking strengh, rope weight 1.5LBS/100' It should arrive in 3-4 weeks for us to install it on our new 2 drum winch. We are going to put it on one drum, and leave the 3/16' 7x7 steel on the other so very accurate results can be computed. Ofcourse the findings will be posted here on ras, if any of you have some ideas on spreadsheet data for the test please contact me. If you want to be part of the tests, them come fly with us (Forrest City, Arkansas 'KFCY' in about 4-6 weeks. Mark Busse

Way to go! Good luck!

Just get everyone to report their release height via radio and log all launches. Side by side comparisons with wire rope will be very interesting.

How many launches are on the existing wire? Longevity of the Plasma will be important, obviously.

Have you made any modifications to the winch for the Plasma drum?

Frank Whiteley Colorado

The weight of the cable doesn't directly affect the launch of the aircraft, provided that the winch has sufficient power. If you view the system in terms of the force at the hook, it doesn't matter whether that force comes from the weight of the cable or from the winch pulling at the other end.

It affects it indirectly, though, because the extra weight, and to a lesser extent the extra drag, causes the cable to bow more, which changes the angle of the force at the hook. We calculated about four degrees difference, going from 4mm cable to 4.5mm cable, on a 1300m launch run.

This means that there is a slightly higher vertical component of the force, which does affect the aircraft. It mainly means that the top of the launch happens earlier, and you lose four degrees worth of the main part of the climb - if you have 1000m of cable out in the middle of the climb, that's 65m off the launch height (65m subtends 4 degrees at 1000m, ish).

Well said, Geoff. However, I think the adverse effect of a heavy cable may be somewhat larger. While the vertical component of the pull angle limits the release height by loading the glider wing, there also seems to be some potential release height left on the table if a heavy cable sags enough to cause a premature automatic release as the cable angle reaches 70 degrees or so relative to the glider's longitudinal axis. (assuming a Tost release)

It doesn't matter how powerful the winch is near the top of the launch since the driver will have significantly reduced the power anyway. As the ratio of winch power to cable weight shifts toward cable weight, the pull angle at the glider will approach vertical - and the auto-release angle.

With 3/16' (4.5mm) cable, I almost always find that the glider releases automatically while the glider is still climbing. With light cable, I find that I must release the cable manually after the winch driver cuts the power.

It's going to be very interesting to see what the Memphis boys report.

Bill Daniels

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Just buffing out any burrs on the durm, and polishing the rollers...

'> We are going to re-spool one drum with the best steel from the two drums...Mark About 500 launches on each cable as is.

Funny to see that you're trying to re-invent the wheel. Go to this link and all your winch problems will be solved. http://www.proximedia.com/local/netherlands/m/machine- el/various.htm