I learnt to glide and instruct in the late 1960s and 70s at a club that
used autotow and would suggest that if you are startng up and your site can
manage the rope issues with ground based cable launching then start with
straight autotow. 1500' to 1800' of cable and a pick up truck. It's the
simplest system. The turn round time will be slower but to start with that
won't be an issue.
We tried reverse pulley and couldn't make it work reliably because it was
very easy to launch too far and what happened then was that the cable
broke, thus making the whole thing a mess.
If you do then you will find that your pulley assembly needs to move about
two axes, both horizontally and vertically. The vertical axis handles the
situation where the glider is to one side of the runway.
But if the airfield is being used by other air traffic I'm not sure if any
wire launch system will work because of the risk to things such as lights
and the risks of mixing aircraft and cables.
Chris
At 09:39 02 January 2019, Frank Whiteley wrote:
On Wednesday, January 2, 2019 at 12:59:47 AM UTC-7, John Foster wrote:
On Tuesday, January 1, 2019 at 8:13:46 AM UTC-7, Martin Gregorie wrote:
On Mon, 31 Dec 2018 19:21:45 -0800, WB wrote:
=20
There is nothing new under the sun when it comes to ground
launching,
auto tow, winch, whatever. It's pretty much all been tried. Derek
Pig=
got
wrote a book that is a great resource for learning about ground
launc=
h
in it's many forms. Unfortunately, it appears to be out of print.
ITYM "Gliding - A handbook on Soaring Flight"
=20
Edition 7, which I have (blue cover showing a DG-800B with its
motor=20
out), has two chapters on ground launching and is currently on offer
fr=
om=20
Amazon for $3.60. His books may be out of print, but you can easily
fin=
d=20
copies. Amazon even has a Derek Piggott author page.=20
=20
=20
--=20
Martin | martin at
Gregorie | gregorie dot org
=20
That books looks promising. Thanks for the link.
=20
With regard to the pulley: what diameter would it need to be? I've
seen
=
somewhere that a higher grade of UHMWPE--New England STS-12 (formerly
Endur=
a 12), recommends a sheave:rope diameter ratio of 8:1. That would mean
for=
a 1/8" rope, one could get away with a sheave of only 1" in a pulley.
Som=
ehow that just doesn't seem like a good idea in this application though,
bu=
t I don't know. So I'm asking.
=20
If this were the case then, one could simply order these items off
Amazon=
and be done with it:
https://www.amazon.com/Smittybilt-27...k/dp/B003CG9P=
IC/ref=3Dsr_1_3?ie=3DUTF8&qid=3D1546415068&sr=3D8-3&keywords=3Dsnatch+block
https://www.amazon.com/dp/B01I25Z5F4...&colid=3D1CG4=
T3YDCVWF8&psc=3D0&ref_=3Dlv_ov_lig_dp_it
8 inches is okay, but there are other reverse pulley considerations. If
yo=
u want the wheel to track the glider climb and drift, as in the old
Cotswol=
d system, a larger diameter is probably better. It was an effective 50"
ra=
dius using using small rollers to reduce any stored energy in the system,
w=
hich was an issue with the Essex GC system. With UHMWPE rope, you want
to
=
eliminate both stored energy and the ability for slack rope to tuck into
an=
y gaps or result in loops that might impact on the ground or equipment.
La=
rger rope types might help with some of this, but there are always
compromi=
ses of performance and price. UV resistence, abrasive resistence,
fatigue
=
(as a result of loading cycles), and ease of handling (including
splicing)
=
are all considerations.
This might be a suitable substitute for UHMWPE ropes for reverse pulley,
ht=
tp://www.novabraid.com/rope/oletec-12/ though they make no claims on
abrasi=
on resistance. 3/8" might be so heavy as to cause abrasive issues. This
w=
as true on the Cotswold system using solid wire. 10 gauge was heavy
enough=
to cause excessive wear on the "memory loops" on the wire, so 13 gauge
was=
found to give the best performance. Use of solid wire is not
recommended
=
at public use airports wherever you are because a large clear zone is
neede=
d in case of wire breaks. Many of these new ropes have negligible
stretch,=
thus no recoil issues. Solid wire and steel wire ropes have significant
r=
ecoil hazards.
Frank Whiteley