From: Brian [mailto:[EMAIL PROTECTED]]
>
> Hi Guys, (oops I forget I'm Aussie) so G'Day Cobbers as well.
G'Day Mate!
First off, I assume you're talking about FAI type winch launching,
not the US TD stuff.
> I have just come from our latest club meeting and a point of discussion
> was turn-around safety. Lately we have had a few incidents where the
> turn-around has been propelled back towards the winches.
I've seen a few of these also, and would group them into two
categories:
First are those who are still using the same turnaround (and winch)
securing techniques they've always used, despite the fact they're
now launching 3+ meter ships with full span camber and carbon spars
instead of built-up floaters.
Second are those who simply don't understand the loads involved
and the direction they're applied in. This especially applies to
people running elevated turnarounds where there's a leverage effect,
often in a similar direction to that in which they've hammered their
pegs in...
> As the club membership grows and model technology advances and launches
> get harder we are trying to think of the best way to secure turn-around
> stakes.
>
> Fixed points set in concrete are of consideration for a home field, but
> what about when flying at a nearby park or other club's field that isn't
> so equipped?
>
> Safety cables securing the stake in case it departs company with the
> ground will probably be the answer but I'm interested in what other
> clubs are doing around the world.
Cables are nice because they only carry loads in tension, so it's
real easy to know which direction the forces act and hence secure
them. A lot of us here in NZ now use a simple pulley attached to
a short chain yoke, with the ends of the chain pegged down. Joe Wurts
introduced us to this method a few years back, and it works really
well with the added advantage that the lines are right on the ground
until the model is actually launched, which minimises the chances of
line cuts when others don't wind down fast enough.
> So here are the questions, has any one measured or worked out the forces
> involved during a hard launch at the turn-around and come up with
> solutions regarding:
If the line tension is T and the angle formed between the ground
and top lines is A, then ignoring losses the force on the turnaround
pulley is: F = 2T x cos(A/2)
This has a max value of twice the line tension, so around 150kg
for a reasonable F3B launch. It's acting in the direction that
bisects the angle formed by the lines, i.e. initially straight
back toward the winch, then as the model climbs the force starts
acting in a more upward direction. If the model is directly over
the turnaround the force will be acting at a 45 degree angle. At
this point the magnitude of the force is reduced to about 1.4
times the line tension due to the angle between the lines
(cos(45) = 0.707), however a good launch will be building tension
which will offset this effect.
The chain yoke system mentioned above transfers this force directly
(in both direction and magnitude) to the pegs which is really easy
to secure against by angling them away from the winch and hammering
them right in so the attactment point is close to the ground.
Elevated turnarounds will act as levers and apply the force in an
upward direction to the rear pegs, unless they use cable or chain
stays as rear supports.
> a. Different types.....star picket, rod, bar, plate and peg etc?
>
> b. Different material.......stainless, bright steel etc?
>
> c. Different diameters?
A lot of these factors will be dependant on the ground. However
if it's real easy to drive the stake in, chances are it's also
easy to pull it out... Most people over here use a couple of
round steel pegs about 250-300mm and about 8mm diameter. I've
seen these pull out and my team use two 400mm long, 10mm diameter
pegs. We launch as hard as anyone over here (harder than most)
and have never had these pull out (or even loosen slightly).
> d. Best angle to drive in the stake? (What about when flying past the
> turn-around to ping off back towards yourself?)
Angled away from the winch and each other about 30 degrees seems
to work well. Flying around the turnaround and zooming back
toward yourself puts less upward loading on the turnaround than
you might think, as the angle between the lines is now very large
and the cos(A/2) term in the force equation gets small fast. The
90 degree weave launch is probably as bad or worse.
> e. Stake length?
See above. I think most people don't use long enough stakes.
> f. Safety cable type, position and setup?
For the chain yoke type of turnaround I don't think safety cables
will help unless the stakes used are bigger than those on the
original chain. In this case why not just use the bigger stakes
in the first case? For other types of turnaround they may well
be useful.
> g. Best way to secure for varying soil types, moisture levels?
>
> h. Turn-around pulley above ground height? (Above ground would appear
> better if using a safety cable)
And probably the most likely to need one also!
> i. Ease of portability?
>
> j. Ease of removal?
Ciao - Chris
******
Chris Kaiser
Auckland, NEW ZEALAND
http://www.rcmodels.com/clubs/asfcnz/
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