final glide estimates

In article cCUlhtvFIYkV-pn2-g3gnB5bUeG9L@localhost,
"Ian Johnston" wrote:

On Sun, 12 Mar 2006 00:24:17 UTC, Bruce Hoult wrote:

: Rough air places lower loads on a structure than do extreme control
: deflections

Surely that depends entirely on just how rough the air is?

True.

The point of rough air speed is that below that speed the structure will
be protected from breaking under the load from vertical gusts by the
wing stalling, which dramatically decreases the constant factor of the
aerodynamic forces.

There is still of course the V^2 factor, so the total force will rise
again to structural breaking point if the gusts are sufficiently strong
-- 100 m/s, say.

But at that point the speed you are flying at is irrelevant, so this
situation has no bearing on the setting of rough air speed.

--
Bruce | 41.1670S | \ spoken | -+-
Hoult | 174.8263E | /\ here. | ----------O----------

John Galloway wrote:
Yes 119 knots for both rough air and VNE for the Standard
Cirrus with a manouevring speed of only 81knots.

I notice the Hornet has it reversed: Rough air = 75 knots; maneuvering =
81. It's hard to believe the Hornet might be in trouble at 76 knots and
the Cirrus could blast on past at 119 knots!

A quick scout through the BGA datasheets seems to show
a trend that for modern single seaters the rough air
and manouevring speeds are the same but for some older
singles and current deep spar two seaters the rough
air is higher. I don't know how the speeds are determined.

http://www.gliding.co.uk/bgainfo/tec...datasheets.htm




--
Change "netto" to "net" to email me directly

Eric Greenwell - Washington State, USA

www.motorglider.org - Download "A Guide to Self-launching Sailplane
Operation"

Maneuvering speed and rough air speed are different
animals. For the curious, refer to JAR-22.341, which
begins with 'in the absence of a more rational analysis..'
and continues with an equation. The equation involves
a number of parameters, including the slope of the
wing lift curve and the wing chord - so it will vary
with sailplane model. There is no obvious relationship
between the rough air and maneuvering speeds, but JAR-22
also requires that the rough air speed must not be
less than the maneuvering speed. Of course, not all
sailplanes are subject to JAR-22.

http://www.tux.org/~milgram/temp/jar22.pdf

Ian

At 01:06 14 March 2006, Bob Faris wrote:
There is a common misconception of maneuvering speed
among pilots
that
somehow has been interpreted to be the maximum speed
that will not
cause airframe damage with full and abrupt control
deflections. THIS
IS WRONG! The November 2001 crash of the AA Airbus
in New York
was
determined to be from abrupt rudder deflections below
maneuvering
speed
resulting in the loss of the vertical fin.
http://www.flyingmag.com/article.asp...article_id=527

I might be wrong here, but the AA Airbus break-up was
due to a repair
on the vertical fin spar. The Airbus flight controler
positions the controles
according to a program designed to limit the amount
of deflection so that
over stress does not occur. Everything runs through
the box.
Chuck

bagmaker wrote:
Newbie help required!
Lets suppose I am on a shallow final glide, 60k out in my 40:1 ship,
cruising at 60 knots. Recent thermals have been at least 5 knots and I
am coming into some big lift. For the discussion rough air Vne is 100
knots.
What strength thermal should I take to increase finishing speed to Vne
and how much (if any) time will this save me?
How do I estimate this at the time? What is the latest point on track
to take such a thermal?

Once you HAVE final glide, you should not stop for lift unless you fall
below the glide path. If you have a 5kt thermal day, you will be on
final glide at a speed somewhat faster than 60kts!

As a rule of thumb, if you are in a gaggle climbing to achieve final
glide, the first one to leave will get home first (provided they have
enough height). I have tried staying in a 6 kt thermal and flying a 6kt
final glide rather than leaving at the 4kt final glide mark (when other
gliders left) and I did not catch them up, despite my higher speed.

On final glide, if I do hit lift, I may slow slightly to use the good
air but generally I hold my speed and keep flying. On a good day, you
can leave below final glide and pick up enough height to get home this way.