How Low to Spin??

Nyal Williams wrote:
At 18:24 01 September 2004, Chris Ocallaghan wrote:

I'd always assumed there were two factors in choosing
a pattern speed.
First, safety, thus the +5 for turbulence. The other
was to place the
glider at best speed to fly. That way if you have to
put the spoilers
away, you are guaranteed to cover the maximum distance.
If I recall,
the simple formula for best speed was best l/d speed
plus 1/2 the
headwind. Don't recall the second ever being explained
though. Just
seemed to fit.


It appears that if you draw a tangent to your glider's
polar beginning, not at zero, but at any given headwind
speed, the line will touch the polar at a point that
is best L/D plus half that headwind.

I was under the impression it was added to give you a margin for gusts
and turbulence, which are usually less than the average wind speed. The
"half" was likely chosen empirically, as something that was adequate
almost all the time.

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

Eric Greenwell
Washington State
USA

It appears that if you draw a tangent to your glider's
polar beginning, not at zero, but at any given headwind
speed, the line will touch the polar at a point that
is best L/D plus half that headwind.

Close to it but, yes - as you can see in the lines drawn on the L23
polar of the article
http://home.comcast.net/~verhulst/GB.../headwind.html

I was under the impression it was added to give you a margin for gusts
and turbulence, which are usually less than the average wind speed. The
"half" was likely chosen empirically, as something that was adequate
almost all the time.

I suspect that you may be confusing the "best speed to cover the most
ground in a headwind" with "the best speed to make a safe approach to
landing".

Tony V.

At 05:42 04 September 2004, Eric Greenwell wrote:
Nyal Williams wrote:
At 18:24 01 September 2004, Chris Ocallaghan wrote:

I'd always assumed there were two factors in choosing
a pattern speed.
First, safety, thus the +5 for turbulence. The other
was to place the
glider at best speed to fly. That way if you have to
put the spoilers
away, you are guaranteed to cover the maximum distance.
If I recall,
the simple formula for best speed was best l/d speed
plus 1/2 the
headwind. Don't recall the second ever being explained
though. Just
seemed to fit.


It appears that if you draw a tangent to your glider's
polar beginning, not at zero, but at any given headwind
speed, the line will touch the polar at a point that
is best L/D plus half that headwind.

I was under the impression it was added to give you
a margin for gusts
and turbulence, which are usually less than the average
wind speed. The
'half' was likely chosen empirically, as something
that was adequate
almost all the time.

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

Eric,

Chris already accounted for the safety factor (gusts
and turbulence) with his statement about plus 5 knots.
His second factor was best speed to fly if you have
to close spoilers and need the guaranteed best speed
to fly for maximum distance.

My answer was to that part of his statement. One might
add another 5 knots for the gusts, etc., if desired.

Nyal Williams wrote:


It appears that if you draw a tangent to your glider's
polar beginning, not at zero, but at any given headwind
speed, the line will touch the polar at a point that
is best L/D plus half that headwind.

I was under the impression it was added to give you
a margin for gusts
and turbulence, which are usually less than the average
wind speed. The
'half' was likely chosen empirically, as something
that was adequate
almost all the time.

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


Eric,

Chris already accounted for the safety factor (gusts
and turbulence) with his statement about plus 5 knots.

Unfortunately, +5 knots is not very good insurance against gusts and
turbulence, which typically increase with wind speed. Or was this
supposed to be added on top of the "1/2 the wind speed"? If so, I
suggest the +5 knots is redundant in general (specific sites [hill
sites, for example] may require much higher speeds, of course).

His second factor was best speed to fly if you have
to close spoilers and need the guaranteed best speed
to fly for maximum distance.

I doubt it was chosen this way, though the correspondence with the best
L/D in wind is appealing. Since we routinely fly final approach at well
above best L/D glide slope (typically, the moderately steep glide slope
that is achieved with half spoilers), having "best L/D speed" available
when the spoilers are closed doesn't seem like a good way to pick
approach speed.

I believe, but have no direct evidence for it, that it was chosen
empirically: over many years, people that used that value had it work
out well, so it became the recommendation. I suspect the origin is now
shrouded in the fog of history.


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

Eric Greenwell
Washington State
USA

Eric Greenwell wrote:

Unfortunately, +5 knots is not very good insurance against gusts and
turbulence, which typically increase with wind speed. Or was this
supposed to be added on top of the "1/2 the wind speed"? If so, I
suggest the +5 knots is redundant in general (specific sites [hill
sites, for example] may require much higher speeds, of course).

US flying handbooks seem to suggest 1.3 times stall speed for
approach. Perhaps the aircraft POH differ (so I won't address that,
as there are too many reasons for that and too many POHs to
speak clearly to). But at 1.3 times Vso, for the gliders I know
of that stall at 30kts+, this means 40kts, which is 10 kts over
stall speed, and seems pretty good. With a 20 knot headwind,
now one would be on final at (1.3 x 30) + (1/2 x 20) = 50kts.

That seems pretty good for a glider that stalls at 30 kts.

So yep, it's a SWAG (scientific wild arse guess), but it seems
ok. Now a thermal right at flare and touchdown is a much
more interesting problem, but hey, ya gotta land SOMETIME!

I believe, but have no direct evidence for it, that it was chosen
empirically: over many years, people that used that value had it work
out well, so it became the recommendation. I suspect the origin is now
shrouded in the fog of history.

Yeah, trying to figure out the polar during landing was probably just
too complicated. The GFH tries to simplify things to 7th grade math.
Yep, so a 14 year old can do it
--

------------+
Mark Boyd
Avenal, California, USA

The current BGA instuctors manual does not recommend
the use of 'downwind checks' or the use of a mnemonic
for pre landing checks. As soon as a pilot decides
to land the glider should be configured for landing.
With the exception of the RAFGSA, who you would think
would know better, (RAF pilots are restricted in the
number of touch and goes they can do because of the
danger of automatic response to checks) this is what
should be taught.
The big problem with 'challenge and response' checklists
where the challenge is chanted automatically is that
the response tends to become automatic too. Given that
most two seat traning gliders in the uk have a fixed
undercarriage, do not carry water and have no flaps
large numbers of students happily chant Water - not
carried, Undercarriage - fixed, Loose articles - stowed
Flaps - not fitted. This is fine until they fly a glider
which carries water, is fitted with a retractable undercarriage
and has flaps. In moments of stress, when it is all
going wrong they can revert to their early days when
they did not have these extras. Personally I would
rather rely on good airmanship to configure the glider
correctly for the phase of flight intended, and teach
that than rely on a chant which can become automatically
wrong.

WULF checks as they were are out. Some bright wag once
told me that they were W Wheel down and locked, Undercariage
- down, Landing gear - extended and Firestones - out
of the bay. :-)

At 16:36 04 September 2004, F.L. Whiteley wrote:
Is this a troll?

Personally I can't believe a UK basic instructor is
saying this publicly.
Are you still giving ab-initio lessons? Perhaps you
should chant this, or
something like WULFSTALL, in the circuit and think
about what each item is
and what the implications are if you don't do each
one. You seem to already
know about one, but the others are so embarassing they
can kill you.
Doubtless you do this appropriately during your annual
club checks, but lack
of clarity of what each item reflects or the need is
disturbing. Please
re-read your post, print a copy and hand carry it to
your CFI at Rattlesden.

Frank Whiteley
Colorado

'Chris Reed' wrote in message
...
I quite like the 'lookout' element of USTALL (though
I don't actually
chant
the checklist to myself on circuit). What I use it
for is a reminder that,
in addition to my normal lookout, I also need to pay
attention to the
other
side of the circuit, look for aircraft on long, straight
in approaches,
and
look at what's happening on the ground. This is a
different mode of
lookout
to XC or local soaring, and I usually find myself
muttering 'lookout' at
some point to remind me of the change of mode.

But I take the point - if the pilot doesn't lookout
except in response to
a
checklist, I'd like to be in a different part of the
sky.

S (straps or speed?) is pretty useless on downwind,
T (trim) ditto, and A
(airbrakes I think) is wierd - if you can't find them
you're in trouble,
though if I flew a flapped glider or had the UC lever
on the same side as
the airbrake (LS4s excepted) I'd add a mental note
to check which lever I
intended to use for approach control.

U is quite clear in my mind, having landed wheels
up once already, and
hoping not to do it again.

'Ian Johnston' wrote in message
news:cCUlhtvFIYkV-pn2-zikdWvOGpoiF@localhost...
You should see some of the downwind checklists/mnemonics
in use in the
UK. They include things like 'trim' - for people
who wouldn't normally
think of using the trimmer, I presume - and, most
bizarrely of all,
'lookout'. I'm not sure that I want to share the
sky with people who
need a mnemonic to remind them to look out...

On Sun, 5 Sep 2004 23:06:53 UTC, Don Johnstone
wrote:

: The big problem with 'challenge and response' checklists
: where the challenge is chanted automatically is that
: the response tends to become automatic too.

I agree completely.

I think there is also a problem in confusion between lists of checks
and lists of actions. In other words, don't put the undercarriage down
at "U", check that it is down. Is "S" the time to increase speed to
the approach speed, or to remember what approach speed is? If the
latter, how does that help if, because of the conditions, approach
speed should have been established previously? If the former, the
converse?

Ian
--

At 15:42 07 September 2004, Tony Verhulst wrote:

It appears that if you draw a tangent to your glider's
polar beginning, not at zero, but at any given headwind
speed, the line will touch the polar at a point that
is best L/D plus half that headwind.

Close to it but, yes - as you can see in the lines
drawn on the L23
polar of the article
http://home.comcast.net/~verhulst/GB.../headwind.html

I was under the impression it was added to give you
a margin for gusts
and turbulence, which are usually less than the average
wind speed. The
'half' was likely chosen empirically, as something
that was adequate
almost all the time.

I suspect that you may be confusing the 'best speed
to cover the most
ground in a headwind' with 'the best speed to make
a safe approach to
landing'.

Tony,

As I read his question it, he was asking for 'best
speed to cover the most ground in a headwind. Actually,
he seemed to confuse the two in the question.

In article ,
Nyal Williams wrote:

I suspect that you may be confusing the 'best speed
to cover the most
ground in a headwind' with 'the best speed to make
a safe approach to
landing'.

Tony,

As I read his question it, he was asking for 'best
speed to cover the most ground in a headwind. Actually,
he seemed to confuse the two in the question.

And why earth would you want to know that when you were in the circuit?
You are surely not going to go *that* far downwind that you need best
L/D into wind in order to get back.

The extra speed with wind is to provide extra guard against a tail gust
stalling you (though the +10 knots or *1.3 does a lot of that), but
mostly I think so that and likely wind gradient still leaves you with
flying speed.

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

Bruce Hoult wrote:

And why earth would you want to know that when you were in the circuit?
You are surely not going to go *that* far downwind that you need best
L/D into wind in order to get back.

Apparently this is a significant cause of crashes (undershot
landing from overshot downwind).

On another subject, can someone describe the
45/V type approach that I've heard is used in
some countries? Is it like this?

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--

------------+
Mark J. Boyd