A couple of popular misnomers here
a) That placarded Vne is at sea level and decreases with altitude in
relation to TAS
Since gliders spend little time at sea level this would be a rather useless
number to placard as Vne; most gliders the placarded Vne is IAS at 6000' or
10,000'.
(What does JAR22 say?)
In the Ventus flight manual, you do not need to make allowance for altitude
(TAS) until 19,680', at which altitude Vne reduces from 135 to 134 knots,
113 knots at 29,520' and 92 knots at 39,360'.
So the relationship doesn't even relate to TAS; about halfway between IAS
and TAS above 20,000' or so.
* Check the flight manual for your specific aircraft *
The hazard of this misconception is that for pilots flying in wave, where
winds speeds at altitude of 50-70 knots are possible (if not higher),
limiting their IAS to some unnecessarily low number may not give them
sufficient ground speed to (dare I say it) penetrate into wind.
The limit is purely for flutter reasons not stressing; The structure is good
for +4/-2.65G at 135Knots IAS no matter altitude or TAS.
b) That "Rough Air" equals "Not Smooth Air". Again, at Vne the structure is
good for +4/2.65G; as long as you are not subjecting the structure to loads
in excess of these numbers you are operating the aircraft inside its design
limits.
It will be a rough ride and you may put your head through the canopy,
however.
The Ventus flight manual also explicitly states that the max rough air limit
is applicable to mountain rotor or Cb, and that in normal weather conditions
the sailplane can be flown up to Vne. It doesn't say exactly what normal
weather is, but your typical summers days thermals should not be a problem.
It is likely the pilot will have a much lower Max Rough Air than the
aircraft i.e. based on comfort not structural reasons.
* Check the flight manual for your specific aircraft *
As far as test flights are concerned, unless you are prepared to load the
airframe up to +4/-2.65G at Vne and verify the structure (which most of us
aren't), what you are looking for is signs of flutter.
And you won't find out about that with a still air dive to Vne with the
controls held rigid; usually flutter is initiated by turbulence or control
input deflecting the control surfaces which then causes deformation of the
structure etc. etc.
Since flutter will occur at a particular critical speed, if you start off
below Vne and work your way up in increments of 5 knots or so with
deliberate excitation of the structure (by forcibly moving the ailerons for
example), if there is a serious problem with flutter at Vne it should show
itself as a damped oscillation (i.e. not destructive flutter) at some lower
airspeed.
If you haven't seen the GFA videos on "Aero-elastic Phenomenon" and the
German flutter films these are well worth having a look at.
I've put a brief extract from the German flutter film at
http://www.augc.aus-soaring.on.net/flutter.wmv - it's only 60k
my two bits worth
David
-----Original Message-----
From: [EMAIL PROTECTED]
[mailto:[EMAIL PROTECTED]]On Behalf Of
[EMAIL PROTECTED]
Sent: Monday, 6 August 2001 3:23 PM
To: [EMAIL PROTECTED]
Subject: [aus-soaring] Rough air
I was reading a glider flight manual recently, and noted that a max rough
airspeed was placarded. Further into the manual the definition of rough air
was rotor and turbulance near Cb's. No mention of strong thermal activity
(not related to the Cb mentioned).
For test flight purposes after a form 2, I have always avoided conducting
the flight when there has been significant thermal activity. So its
performed either early morning or late in the afternoon.To avoid turbulance
and allow a dive to Vne.
Also at a WA state comps in Jan on a strong day I was about 80km out at
10000' in a JantarIII on final glide into a 30kph headwind. I flew no faster
than 109kts, (compensated initially for the Vne reduction with altitude). I
was passing through some pretty strong thermals whilst on final, head
smacking on canopy stuff.
Anyone prepared to define rough air for Vne purposes,ie max rough airspeed.
regrds luke dodd
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