Mini-Winch for FES

On Friday, June 5, 2020 at 8:59:21 AM UTC-6, wrote:

https://youtu.be/wwsa4PVYMJY

Excellent.

For an FES ground assist, you need enough energy for a rope break like return to the field when the FES does not work.

I there a safe and economical way to make a winch that can just get you to 300 feet?

Might be some minor issues with the design example in the video.

Good and cheap are mutually exclusive. A good, safe machine won't be cheap.

Good and cheap are mutually exclusive. A good, safe machine won't be cheap.

Right, but I thought the idea of the original question was to see if sacrificing some good (higher than 300 foot tow?) might make cheap and safe possible.

Seems like a small corner case until there are lots more FES, but still fun to think about.

On Friday, June 5, 2020 at 12:26:21 PM UTC-5, wrote:
Good and cheap are mutually exclusive. A good, safe machine won't be cheap.

Right, but I thought the idea of the original question was to see if sacrificing some good (higher than 300 foot tow?) might make cheap and safe possible.

Seems like a small corner case until there are lots more FES, but still fun to think about.

A light duty winch to get you to 300 feet would still require all the same components as a heavy duty winch designed to get a Duo to 2000'. Assuming you will climb at a relatively shallow angle, you could get away with somewhat less power than a typical glider winch because you won't need to climb steeply as is done in a regular winch launch. Also, because line tension will be less in the shallow climb, your winch drum can be lighter. Brakes are critical on a winch to manage inertia of the drum/line and to put some tension on the in when reeling it out. A lighter system could have correspondingly lighter brakes. Your winch line will be under less tension, and with a low launch rate, wear would not be a big issue. You could forego expensive Dyneema line and just use polypropylene (might be a false economy eventually). These would all be small increments of "lighter, smaller, cheaper". Probably not enough to justify building it. My guess is it would be better to buy/build a regular winch or go with auto launch. Hard to get cheaper and simpler than an old car, a bucket of rope, and a tow release on a trailer hitch.

On Friday, June 5, 2020 at 1:50:59 PM UTC-4, WB wrote:
On Friday, June 5, 2020 at 12:26:21 PM UTC-5, wrote:
Good and cheap are mutually exclusive. A good, safe machine won't be cheap.

Right, but I thought the idea of the original question was to see if sacrificing some good (higher than 300 foot tow?) might make cheap and safe possible.

Seems like a small corner case until there are lots more FES, but still fun to think about.

A light duty winch to get you to 300 feet would still require all the same components as a heavy duty winch designed to get a Duo to 2000'. Assuming you will climb at a relatively shallow angle, you could get away with somewhat less power than a typical glider winch because you won't need to climb steeply as is done in a regular winch launch. Also, because line tension will be less in the shallow climb, your winch drum can be lighter. Brakes are critical on a winch to manage inertia of the drum/line and to put some tension on the in when reeling it out. A lighter system could have correspondingly lighter brakes. Your winch line will be under less tension, and with a low launch rate, wear would not be a big issue. You could forego expensive Dyneema line and just use polypropylene (might be a false economy eventually). These would all be small increments of "lighter, smaller, cheaper". Probably not enough to justify building it. My guess is it would be better to buy/build a regular winch or go with auto launch. Hard to get cheaper and simpler than an old car, a bucket of rope, and a tow release on a trailer hitch.

You could forego expensive Dyneema line and just use polypropylene (might be a false economy eventually).

I would advise against using polypropylene line since it has a much higher elongation compared to UHMWPE lines. It would most likely lead to a very unsteady launch with a lot of surging - like hanging on a bungee cord. Spectra or Dyneema have an elongation very close to what a steel cable has but at about 1/10 of the weight. That also plays into the overall equation for accelerating the line and then stopping everything with your drum brakes.
Rather than building something on the 'Hold my beer - watch this' principle, spend the money and get the right stuff.

Uli
'AS'

On Friday, June 5, 2020 at 1:26:21 PM UTC-4, wrote:
Good and cheap are mutually exclusive. A good, safe machine won't be cheap.

Right, but I thought the idea of the original question was to see if sacrificing some good (higher than 300 foot tow?) might make cheap and safe possible.

Seems like a small corner case until there are lots more FES, but still fun to think about.

Agreed! It's a fun thought experiment for a growing sector of the sport.

For me, the issue is that if I have a setup which can get me to 300', it can get me a lot higher for minimal extra effort. 300' would mean about 600' of linear space, which is unlikely to fit in the area before a runway. So once I'm using any part of the runway, I might was well use the entire length.

From a mechanical perspective, the only substantial difference between 300' and 3000' is the length of rope and the total energy required. The angles, forces, and power stays the same.

Some crucial distinctions of a 100' long catapult-style launch are that there's no hardware on the runway so it can plausibly be completely autonomous; the angles are purely horizontal so the spooling system is simplified and the hook cannot drift down onto the runway; a 3 second launch means that motor cooling is not an issue due to the thermal mass of the motor; and a launch failure is very graceful, with a worst case result of rolling down the runway at a leisurely 30-40kts.

On Fri, 05 Jun 2020 11:00:11 -0700, sebesta wrote:

On Friday, June 5, 2020 at 1:26:21 PM UTC-4, wrote:
Good and cheap are mutually exclusive. A good, safe machine won't be
cheap.

Right, but I thought the idea of the original question was to see if
sacrificing some good (higher than 300 foot tow?) might make cheap and
safe possible.

Seems like a small corner case until there are lots more FES, but still
fun to think about.

Agreed! It's a fun thought experiment for a growing sector of the sport.

For me, the issue is that if I have a setup which can get me to 300', it
can get me a lot higher for minimal extra effort. 300' would mean about
600' of linear space, which is unlikely to fit in the area before a
runway. So once I'm using any part of the runway, I might was well use
the entire length.

The rule of thumb over here is that you get to 1/3 of the cable length,
not 1/2, but it depends on wind too. Our Skylaunch (around 400 hp from V8
on LPG and 1000m (3270 ft) of cable gets me to 1200ft on a light breeze
and 1400 ft plus in more normal summer weather with a reasonable wind
gradient (Std Libelle, not pulling hard, so around 1/2.5 of line length
on average.

A low power winch won't do nearly as well.



--
Martin | martin at
Gregorie | gregorie dot org