From the old MOSP 50. Section 3 on Form 2s
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3-2-2
(3) Wing Frequency Check
All new sailplanes delivered now have stated in their airworthiness
documentation the natural vibrational frequency of at least the wings.
We all know that a drinking glass will “ring” when tapped, but if
it is cracked it will be “dead”. Similarly our sailplane structure
will respond to internal damage, loos wing root fittings etc by
changing the frequency at which it wants to vibrate.
Clearly it will be important to KNOW the frequency of the structure
when new and keep track of any natural lowering of this value with age
such that a sudden change can be detected and become meaningful.
WHEN:
At each Form 2 and after any air load or ground load incident
particularly heavy landings and ground loops.
HOW:
With the sailplane sitting on its undercarriage, tyres at correct
pressure, on a paved surface, empty, wings level, gently grasp one
wing tip and shake it up and down. The wing will flex at a steady
rate. It will find its own frequency and it is next to impossible for
you to change it. Using a suitable watch, time the natural frequency,
a complete cycle being UP and DOWN. This value should then be compared
to what was found last time or what is specified for that sailplane.
As a guide only, some typical values:-
F.R.P …… 135 cycles per minute
Wood …… 180 cycles per minute
Metal …… 192 cycles per minute
Remember - excessive heaving and shoving at the wing tip can be
quite damaging.
============================================
Regards
SWK
----- Original Message -----
From: "Discussion of issues relating to Soaring in Australia."
To:"Discussion of issues relating to Soaring in Australia."
Cc:
Sent:Tue, 1 Dec 2015 15:15:48 +1100
Subject:Re: [Aus-soaring] Wing frequency Procedure
>>a document on how the GFA would like it performed
Doesn't this vary from manufacturer to manufacturer? Some allow wing
frequency checks while on the wheel while others require the fuselage
to be in a cradle or similar. My guess is that the GFA would say
'follow the manufacturer's recommendations'.
Here's one they prepared earlier:
Wing oscillation frequency:
approx. 160/min 15 m span with winglets
approx. 130/min: 18 m span without parting
approx. 124/min: 18 m span with parting and winglets
Aircraft should rest on both wheels during frequency measurements.
I think the last part is critical for each manufacturer.
D
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