SAFETY ALERT

On Mon, 22 Aug 2011 22:02:17 -0700, Eric Greenwell wrote:

Now I'm curious - what glider and what initial airspeed did you use?

That may well apply to anything up to and including early glass and to
selected later gliders too: Juniors and ASK-23 both come to mind.

I checked my Std Libelle's zoom capability yesterday - at 3000 ft and
2800 ft, pulling up at a stabilised 100 kts both times. Each time I
pushed over as the speed came down toward 50 kts. Both went no lower that
42 kts and both gained precisely 300 ft. The rate of speed drop-off below
50 kts is dramatic.

Bottom line: As I'd guessed, I don't think a low pass and pull up is a
safe option in a Libelle.

Re elevator flutter stories: surely you'll only get control surface
flutter if you're over Vne or flying a badly maintained glider? Doing
either is unsafe at any altitude.


--
martin@ | Martin Gregorie
gregorie. | Essex, UK
org |

On 8/23/2011 2:30 PM, Martin Gregorie wrote:
Snip...

Re elevator flutter stories: surely you'll only get control surface
flutter if you're over Vne or flying a badly maintained glider? Doing
either is unsafe at any altitude.

Being a strong believer in Murphy, and knowing enough about engineering and
airplane design to be dangerous, and allowing for uncertainties difficult to
convey in short snippets of writing, I - myself - would have used a different
word than 'surely' in the posed question. In any event, you're theoretically
correct...but the pilot in me isn't comfortable pushing that part of the
envelope in close proximity to the ground. There are reasons many (most?)
professional test pilots tend to rank flutter testing toward the top of their
least favorite tests.

And I suppose it's quite possible each case of zoomie-related, contest finish
flutter about which I've read involved flight exceeding Vne and/or 'lousy
maintenance.' If so what might that suggest about some subset of contest
pilots...flagrant disregard of flight limitations? Dubious ability to maintain
precise speed control at high speeds, in thermic turbulence, near to the
ground? Slapdash maintainers of their ships? Hidden pre-existing damage? Etc.
The simple fact of flutter existence in this particular flight regime raises
seriously perturbing questions in my mind.

Without intending to kick a wounded horse (while noting no one so far has
bothered to address most of the *non*-rhetorical questions posed elsewhere
about zoomies), my larger point in posing the questions is to encourage
readers of the thread to examine themselves, their motivations, and their
comfort levels in performing this particular task. Whether individuals decide
to perform zoomies is up to them, and I'm philosophically OK with that.

As I noted elsewhe BTDT; stopped doing them ~1980; have seen (and enjoyed
watching) many since (while simultaneously mentally cringing and
hoping/praying nothing bad happens); wouldn't consider my future significantly
poorer if I never see another one; sincerely hope I don't personally know
(even via RAS) anyone who may be a part of a zoomie gone bad in the future.

And to paraphrase Forrest Gump, that's all I have to say about zoomies in this
thread.

Regards,
Bob W.

On Aug 24, 8:30*am, Martin Gregorie
wrote:
On Mon, 22 Aug 2011 22:02:17 -0700, Eric Greenwell wrote:
Now I'm curious - what glider and what initial airspeed did you use?

That may well apply to anything up to and including early glass and to
selected later gliders too: Juniors and ASK-23 both come to mind.

I checked my Std Libelle's zoom capability yesterday - at 3000 ft and
2800 ft, pulling up at a stabilised 100 kts both times. Each time I
pushed over as the speed came down toward 50 kts. Both went no lower that
42 kts and both gained precisely 300 ft. The rate of speed drop-off below
50 kts is dramatic.

That is exactly what I'd have predicted.

A rough mental calculation of how much height a given airspeed can be
converted to (at zero final airspeed e.g. top of a tail slide) is
speed in knots divided by five, squared.

So 100 knots can be converted to (100/5)^2 = 20^2 = 400 ft

If you still want to have 50 knots at the top then you need to
subtract the height that 50 knots is "worth": (50/5)^2 = 10^2 = 100
ft.

Giving 300 ft net.

(the theoretical frictionless physics says to divide by 4.748 not 5,
but 5 is both easier to work with in your head and closer to what
you'll actually get)

Bottom line: As I'd guessed, I don't think a low pass and pull up is a
safe option in a Libelle.

100 knots is certainly on the slow side. 120 is much better. That
gives you an expected (120/5)^2 - 100 = 476 ft to play with.


Wikipedia says the Std Libelle has a 250 km/h (135 knot) Vne. Is that
incorrect?

http://en.wikipedia.org/wiki/Glasflügel_H-201

On Tue, 23 Aug 2011 21:07:57 -0700, Bruce Hoult wrote:

100 knots is certainly on the slow side. 120 is much better. That gives
you an expected (120/5)^2 - 100 = 476 ft to play with.

Agreed, but mine is not a B series, so that's just over Vne, so I can't
see a good reason for intentionally going there outside an emergency in a
42 year old glider. 110 kts maybe, but thats 384 ft with 50kt pushover -
still pretty marginal.

Wikipedia says the Std Libelle has a 250 km/h (135 knot) Vne. Is that
incorrect?

That's a B series.

Mine is earlier (balsa wing skins, top & bottom airbrakes, s/n 82) the BGA
data sheet quotes 119kts for Vne. All other limiting speeds apart from
Vne are the same for both original and B series.


--
martin@ | Martin Gregorie
gregorie. | Essex, UK
org |

On Aug 24, 8:47*pm, Martin Gregorie
wrote:
On Tue, 23 Aug 2011 21:07:57 -0700, Bruce Hoult wrote:
100 knots is certainly on the slow side. 120 is much better. That gives
you an expected (120/5)^2 - 100 = 476 ft to play with.

Agreed, but mine is not a B series, so that's just over Vne, so I can't
see a good reason for intentionally going there outside an emergency in a
42 year old glider. 110 kts maybe, but thats 384 ft with 50kt pushover -
still pretty marginal.

Wikipedia says the Std Libelle has a 250 km/h (135 knot) Vne. Is that
incorrect?

That's a B series.

Mine is earlier (balsa wing skins, top & bottom airbrakes, s/n 82) the BGA
data sheet quotes 119kts for Vne. *All other limiting speeds apart from
Vne are the same for both original and B series.

--
martin@ * | Martin Gregorie
gregorie. | Essex, UK
org * * * |

I have pics of 201 Libelles doing low finishes... with the wing tips
drooping noticeably. I don't know if the wing is twisting or just has
a bunch of washout to begin with, but it doesn't look happy going that
fast. And that was brand new.

-Evan Ludeman / T8

At 04:07 24 August 2011, Bruce Hoult wrote:
Vne for the Std Libelle is 118kts.

Vne for Std Cirrus is about the same.

135kts is probably quite exciting in either

PF

Wikipedia says the Std Libelle has a 250 km/h (135 knot) Vne. Is that
incorrect?

http://en.wikipedia.org/wiki/Glasfl=FCgel_H-201

On Thu, 25 Aug 2011 09:03:54 +0000, Peter F wrote:

At 04:07 24 August 2011, Bruce Hoult wrote:
Vne for the Std Libelle is 118kts.

Vne for Std Cirrus is about the same.

135kts is probably quite exciting in either

I thought I'd seen the H.201B Vne given as 123 kts. In fact the BGA Data
Sheets quote 124kts but the B series Operators Manual, as issued by
Glasfaser says 135 kts, 250 km/h

I'm with you: 135kts sounds rather too exciting.


--
martin@ | Martin Gregorie
gregorie. | Essex, UK
org |