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From: paul@ubvax.UUCP (Paul Fries)
Newsgroups: net.rec.skydive
Subject: Re: wierd canopies
Message-ID: <434@ubvax.UUCP>
Date: Mon, 10-Feb-86 14:04:47 EST
Article-I.D.: ubvax.434
Posted: Mon Feb 10 14:04:47 1986
Date-Received: Wed, 12-Feb-86 21:32:02 EST
References: <650@wjvax.UUCP> <41800060@hpcvlo.UUCP>
Reply-To: paul@ubvax.UUCP (Paul Fries)
Organization: Ungermann-Bass, Inc., Santa Clara, Ca.
Lines: 59

In article <41800060@hpcvlo.UUCP> kas@hp-pcd.UUCP (kas) writes:
>
>     I think the remarks made in the "Conclusions" section of that report said
>it best.  There wasn't enough data presented to ascertain the cause of that
>accident.  As I said before, it is physically impossible for the canopy to
>"weird out" totally on its own.  It MUST be acted on by some external force
>for it to do what it did.  Downdrafts, wind gusts from behind, too deep in the
>brakes, turbulence, following too close behind another canopy, etc., etc., all 
>could be factors in a situation like that, whether or not the canopy was in
>a turn at the time.  My point is, it is imperative to understand and recognize
>all the things that COULD go wrong, and then make sure you don't get yourself
>into a situation that invites disaster.

This sounds fine to me.  Recognition of the dangerous possibilities and
dealing with them BEFORE they become disasters is the key.  However, I
must take (small) exception to the following:

>The best hedge against such problems is AIRSPEED.  The faster the canopy is
>going relative to the air, the less likely it is to be affected by one of
>those gremlins.  That's why it is so important to make your final approach
>at nearly full airspeed, and flare only when you are 3 or 4 seconds from
>touchdown.  (Canopies vary, so the time may vary.

The flight manuals for my canopies (CRUISLITE, RAVEN IV, RAVEN III) all
call for reduced airspeed (not to exceed 1/2 brakes) through turbulence.
This is so that the canopy is NOT subjected to so radically different
forces over too short period of time.  Turbulence, after all, is simply
air that is moving in a different direction, which in turn alters the
relative pressures found within the canopy.  When the canopy is subjected
to varying forces too quickly, the airfoil will be disturbed.  Slowing
the canopy allows it to adjust and hence, stay more stable.  There are
some things we simply have to accept involved here.  As the canopy has
no power of its own, running into a downdraft is going to increase the
rate-of-descent.  There is nothing we can do about that.  The best we
can hope for is to keep the canopy fully inflated to get the most we can
from it.

Ideally, we SHOULD approach the ground with the highest possible airspeed.
The "flare" that we use during landing requires airspeed to happen.  What
we are actually doing is increasing the angle of attack of the airfoil to
trade airspeed for lift.  As the brakes are applied, you pendulum forward
under the canopy.  The canopy pitches up, which is actually a rotation
about the center of lift.  The airspeed drops rapidly, but we get that
momentary dramatic increase in lift, hence, the reduced rate-of-descent.
Again, as the canopy has no power of its own, we can only get away with
this once when landing (the only way to get airspeed back is to let it
sink for a while), so the timing is critical (as we well know).

So overall, the "correct" airspeed is one that will keep the canopy fully
pressurized, while still having enough speed for the flare.  Of course,
there will be times when keeping the canopy inflated will mean that you
will NOT be able to get a full flare, so we should be ready (and willing)
to do a good PLF.

Anyway, thanks for all the good comments!  I am sure that the novice will
be better off for them.

	Paul Fries
	C-17147