At 11:22 07 November 2017, Peter Whitehead wrote:
On Monday, 6 November 2017 12:48:16 UTC, Dan Schmitz wrote:
I'm wondering what the consequences of overspeeding on winch launches
are=
? I understand that it places stress on the airframe, but to what extent?
=20
I find it near impossible to control the airspeed on the top third of a
l=
aunch and have found myself over speeding the placarded max. speed by 5 -
1=
0 knots a few times despite signalling to the winch driver.
=20
Should I be taking the approach of releasing at 2/3 of typical launch
hei=
ght (e.g. 700 - 800 foot) if my airspeed isn't under control at this
point?
=20
Thanks in advance
Dan, my response is "yes".
The physics suggest that there is substantially greater risk to the
airfram=
e (and you) over 1/2 way up the launch, and even more as it progresses
(wit=
h little risk below half-launch situation) because of increasing resolved
f=
orces acting 'downwards' (relative to the glider). Yes the weak link
should=
provide protection - BUT are you sure the weak link is the correct one?
Is=
your airframe perfect (in manufacture, later storage, repair,
infestation,=
rot, corrosion, etc )?=20
Fortunately we have not seen many situations where the structure has
failed=
through over-speeding. We have seen more damage and death with too low
air=
speeds or failed launches. A little off this particular thread but please
m=
ay I reiterate the advice that we should all take a (repeated) look at
the
=
BGA advice, arrived at through a proper scientific analyisis to the 'BGA
sa=
fe winch launching initiative' to reduce accidents (proven to help). The
ba=
sic advice is at=20
https://members.gliding.co.uk/wp-con...04/1430311979=
_HowToWinchSafely_Part1.mp4
The whole programme is at=20
https://members.gliding.co.uk/bga-sa...safe-winching/
Pete.
There is another aspect to this, which unfortunately cannot be evaluated.
My understanding from a BGA Technical Officer a long time ago, was that the
designer does not calculate the maximum speed that the specified gust load
can be withstood safely, but rather decides on a speed, and then does the
calculation to show that that speed is safe. We don't know, but it may well
be safe at a much higher speed. Some years ago the BGA National Coach tried
to get the Puchacz max winch speed increased above 59 kts. This gives a
very narrow window when you don't really want to be below 55 kts in the
full climb.
Dave