New trainer from SZD Bielsko

At 19:24 29 June 2007, Brtlmj wrote:
structurally damage a glider during a winch launch
from overspeeding.
Providing the correct weak is used, that is not possible.
Old myths
die hard though.
Why, then, is the winch launch maximum speed not Vne?

If I understand it correctly, flying at maximum winch
speed or below
ensures that you won't lose your wings even if you
use a wrong weak
link or no weak link at all.

Bartek

VNE in free air is determined by the amount of lift
a wing can generate vs. it's load strength; ie the
wings can only generate as much lift as the spar/structure
can safely handle.--During a winch launch there is
significantly more stress for a given airspeed due
to the downward force being applied to it by the winch
pulling it, counteracting some the lift the wings are
generating. The wings generate the same amount of lift
at any given airspeed, whether on or off the cable,
but while it is attached to the cable a lot more of
the force is being directed into the airframe, since
the hook/cable is keeping it from being able to zoom
in it's normal path physics would otherwise dictate,
were it not attached at that speed; and naturally the
forces increase with increases in speed.
I have a novel idea--how about a new thread about winching,
so this one can remain on the subject: New Trainer
from SZD Bielsko. I personally ca not wait to fly one
of these beautiful new aircraft. I think it is about
time that an alternative to the K-21, 1000, Duo, and
Fox be available. It seems like it will make a great
trainer for XC, acro, or maybe even beginners that
show aptitude. All this and for much less money than
the other options (OK, priced similar to the Fox, but
the Fox is not really an XC ship). Not knocking any
of the other ships, they all have their ups and downs.
I still say hats off to SZD!!

Paul Hanson
"Do the usual, unusually well"--Len Niemi

Paul Hanson wrote:
VNE in free air is determined by the amount of lift
a wing can generate vs. it's load strength; ie the
wings can only generate as much lift as the spar/structure
can safely handle.

With respect, this is entirely wrong. In straight free flight the wings
generate exactly enough life to counter the weight of the airframe and
its contents. If the wings generate more lift than that the aircraft
will loop: if they generate less its called a "stall".

I suspect that Vne is more often determined by the torsional resistance
of the wing. That's certainly the case for an ASW-20.

--During a winch launch there is
significantly more stress for a given airspeed.

Correct. The minimum amount of lift needed must counter the weight of
the airframe and contents plus the weight of the cable plus the vertical
component of the tension in the cable.

The upper maximum amount of lift is the point at which the wings fail in
bending unless other factors such as elevator power or the wing's
accelerated stalling performance intervene to set a lower limit.


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

Vaughn Simon wrote:
"Martin Gregorie" wrote in message
...
Sally W wrote:
I see no reason to deviate from the standard CBSIFTCBE checklist,
so "Flaps: not fitted" accompanied by a glance to see that there
is indeed no flap handle is part of my checklist for a non-flapped
glider.

This has the benefit of keeping instructors happy on check rides
without straining my brain to remember what checklist is expected as compared
with what I might do or say when no instructor is present.

Perhaps it is just the way that my mind works, but I have to respectfully
disagree.

If you fly something with fixed gear for your first 1000 flights, each time
dutifully reciting something unnecessary like "UNDERCARRIAGE" and each time
DOING NOTHING but just skipping on to the next item, then when you finally get
in something with retractable gear, you are liable to do the same thing you have
always done and land gear up as a result. I believe that checklists should be
ideally posted in the cockpit and should be made specific to each aircraft so
that each step on the list has real meaning each and every time.

Isn't that a different situation?

I regard it as different because, unlike the pre-landing checks, its
done on the ground and without anything like the same time pressure to
complete it and without competing claims on your attention.

I was never taught a pre-landing checklist for just the reasons you
give. When I was flying an ASW-20 I taught myself to use WUF (Water,
u/c, flaps) as a pre-landing check. Now I have an early Std Libelle (not
B series, so no water) I've reverted to no mnemonic checklist because U
seems a bit silly. In any case the club flying orders now say that the
u/c should be lowered and a radio call made as soon as you decide to
join the circuit, which is much too early for the only other checklist
item (trimming for the approach). A second radio call is made at high
key and trim for landing toward the end of the downwind leg, so there's
really no point when a more formal checklist should be run.


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

At 19:06 30 June 2007, Martin Gregorie wrote:
Paul Hanson wrote:
VNE in free air is determined by the amount of lift
a wing can generate vs. it's load strength; ie the
wings can only generate as much lift as the spar/structure
can safely handle.

With respect, this is entirely wrong. In straight free
flight the wings
generate exactly enough life to counter the weight
of the airframe and
its contents. If the wings generate more lift than
that the aircraft
will loop: if they generate less its called a 'stall'.

I suspect that Vne is more often determined by the
torsional resistance
of the wing. That's certainly the case for an ASW-20.


You suspect incorrectly. The faster you fly, the more
lift the wing is generating, until it can no longer
safely bear the bending (compression) loads, or in
some cases I suppose can no longer be countered by
the other flight control surfaces, but certainly not
as a function of parasite drag (especially on a 20).
Perhaps some gliders wings twist (torsional load) a
bit more than others, but most bend upwards more than
they twist. I'm sure this varies with washout as well,
but just watch a high speed finish or any high speed
flying, particularly on a long wing. You can see them
bending upwards (not backwards or twisting) due to
the excessive (excessive in this case meaning more
than is needed to simply offset the glider against
gravity) lift being generated at higher speeds, and
it most certainly increases as a function of speed.

By the very same phenomenon, the outboard wing generates
more lift in turning flight, since the outboard wing
is moving faster through the air--hence the over banking
tendency. This is why when once established in a bank
, it usually requires somewhere between neutral stick
and top aileron to maintain the same bank angle without
increasing (not on all gliders though). Of course while
turning other forces are at play too, like increased
drag creating adverse yaw, diving tendency etc, but
that is a different subject.
I stand by my statement. The wing can only take so
much stress from EXCESS LIFT generated at higher speeds,
and that usually determines a glider's VNE. Other
factors (besides the center of lift usually closely
coinciding with the center of gravity) keeping the
glider form 'looping' at higher speeds are being applied
by other flight control surfaces, like the elevator
for instance. There may be some specific cases where
VNE is determined by the speed at which the other flight
controls are no longer effective enough to counter
the lift the wings generate, but no examples I can
site off hand. The generation of lift is in direct
mathematical relation to the speed of the relative
wind, period.
BTW, a stall only in the simplest sense is from the
wing generating 'not enough lift'. It is from exceeding
the critical angle of attack for any given loading
condition, and can happen at any airspeed, any gross
weight. It happens when the airflow over the wing becomes
too turbulent to provide the needed aerodynamic reaction
to offset it's current load requirement, any angle,
any speed.

Paul Hanson
"Do the usual, unusually well"--Len Niemi

On 29 Jun 2007 22:56:02 GMT, Paul Hanson
wrote:


VNE in free air is determined by the amount of lift
a wing can generate vs. it's load strength; ie the
wings can only generate as much lift as the spar/structure
can safely handle.

Close, but no cigar...

The above is basically the definition of Va (maneuver speed).


Bye
Andreas

I suspect that Vne is more often determined by the
torsional resistance
of the wing. That's certainly the case for an ASW-20.
You suspect incorrectly. The faster you fly, the more
lift the wing is generating,

...._at_a_constant_angle_of_attack_. Fortunately, we have this nice
device called "elevator" and can change angle of attack.

Vne is limited by flutter speed. I am not entirely sure, but I think
there is strict dependence between Vne and the theoretical speed when
flutter should occur.

Bartek

At 01:24 01 July 2007, Brtlmj wrote:
I suspect that Vne is more often determined by the
torsional resistance
of the wing. That's certainly the case for an ASW-20.
You suspect incorrectly. The faster you fly, the more
lift the wing is generating,

...._at_a_constant_angle_of_attack_. Fortunately, we
have this nice
device called 'elevator' and can change angle of attack.

Ahh yes, the elevator. so thats what thats for :-)
You CAN stall an aircraft at any angle of attack though,
it is a matter of exceeding the CRITICAL AOA, which
can happen at any speed, AOA, or load condition. Like
an accelerated stall for entry of snap maneuvers for
instance. This occurs at much higher than normal stall
speed, by intentionally exceeding the critical AOA,
meaning the AOA in which creates too much turbulence
on the top of the wing to keep it from stalling. Add
full rudder next and whala, you just did a snap roll,
great fun BTW.

Vne is limited by flutter speed. I am not entirely
sure, but I think
there is strict dependence between Vne and the theoretical
speed when
flutter should occur.

Bartek

With this statement you are correct for some gliders,
but not all. The same goes for the aerodynamic loading
I have been hammering on, for too long now--some gliders
but not all. VNE is usually based on aerodynamic loading,
and if well designed, flutter should not occur until
much higher speeds than that. Not always the case though.

Paul Hanson
"Do the usual, unusually well"--Len Niemi

At 23:42 30 June 2007, Andreas Maurer wrote:
On 29 Jun 2007 22:56:02 GMT, Paul Hanson
wrote:


VNE in free air is determined by the amount of lift
a wing can generate vs. it's load strength; ie the
wings can only generate as much lift as the spar/structure
can safely handle.

Close, but no cigar...

The above is basically the definition of Va (maneuver
speed).


Bye
Andreas

Not at all, my rough explanation of deriving VNE is
correct (but possibly not complete) for straight and
level flight, at sea level, in free air. BTW, by 'Safely
handle' I mean with a safety factor of 1.5 of course.
Va on the other hand, is the maximum speed at which
you can fly and still make abrupt and full control
deflections without overloading any part of the airframe
and is primarily associated with imposing G-loads as
opposed to imposing aerodynamic loads, again at sea
level with a safety margin of 1.5. This means the wings
are still generating little enough lift at Va that
you can make a full abrupt elevator pull back and still
not overload them with the additional G's you will
pull. If flying at VNE on the other hand, you are
right at at 1.5 times below the actual load limit,
but this being aerodynamically imposed as a function
of lift generation. At these speeds a strong gust has
the potential provide the overload (I think it has
to be stronger than 15 m/s for JAR 22, but I am not
positive), and a full abrupt pull back on the stick
would most certainly snap any ordinary wings due to
adding the G-loads to the inherent aerodynamic loads
already being imposed on the wings, from increased
lift being generated at those speeds.
There is more to this equation though, although is
not generally often considered to apply towards sailplanes
(but has definitely come into play in the past) due
to the average altitude bands we normally operate in.
It comes with increasing in altitude. The higher you
go, the lower your VNE (IAS) gets, although not due
to increased aerodynamic loads ( again meaning increased
lift generation), but rather due to G-load considerations
due to increased TAS. As you get higher in altitude
the air thins and your TAS vs IAS is increasing, and
TAS is responsible for imposing G-loads due to control
inputs, as it represents your true velocity. VNE (IAS)
and Va (TAS) eventually cross each other in whic case
you should stick with the lower figure to fly safely,
and if you go high enough, VNE (TAS) and stall speed
(always a function of IAS) cross each other. That's
called the 'Coffin Corner'.

Back to the point though, and from a different angle,
if lift does not increase as a function of speed, why
then do you need to fly faster when flying wet, or
turning, or with passengers? Increasing the load increases
the minimum lift requirements, meaning stall speed
increases, no? The extra lift needed is generated by
flying faster, no? This increase in lift generation
continues on up throughout the speed range, until you
achieve catastrophic failure, usually from generating
more lift than the spar/structure can endure, since
the wing roots are attached at the fuse and can not
continue to rise as the tips can while the speed increases.
Again, this is why the wings bend up higher, the faster
you fly. This is really pronounced on open class ships
with flexible wings. You can only bend the wings so
far...

You can still keep the cigar though ;-), although I
do miss them a bit since I quit smoking.

Paul Hanson

"Do the usual, unusually well"--Len Niemi

Ahh yes, the elevator. so thats what thats for :-)
You CAN stall an aircraft at any angle of attack though,

A given airfoil stalls at constant AOA, regardless of airspeed. You
can't stall at smaller AOA. Or did you mean to say "at any speed"?

I have been hammering on, for too long now--some gliders
but not all. VNE is usually based on aerodynamic loading,

Do you really think that when you fly at 100 knots, straight and
level, your wings produce four times more lift than when You fly at 50
knots, straight and level?

Bartek

Paul Hanson wrote:
At 19:06 30 June 2007, Martin Gregorie wrote:

Paul Hanson wrote:

VNE in free air is determined by the amount of lift
a wing can generate vs. it's load strength; ie the
wings can only generate as much lift as the spar/structure
can safely handle.


With respect, this is entirely wrong. In straight free
flight the wings
generate exactly enough life to counter the weight
of the airframe and
its contents. If the wings generate more lift than
that the aircraft
will loop: if they generate less its called a 'stall'.

I suspect that Vne is more often determined by the
torsional resistance
of the wing. That's certainly the case for an ASW-20.



You suspect incorrectly. The faster you fly, the more
lift the wing is generating,
stuff snipped...

"Not quite."

Writing as a non-practicing aerospace engineer (that's what they called
aeronautical engineering in the 1960's in the U.S.), a dormant teacher
gene compels me to comment. I'd have written "Agreed," IF the words
"capable of" were inserted between "is" and "generating."

As has been pointed out, in steady state flight, pure speed has
esentially zero to do with the amount of lift a wing generates. It
generates an amount essentially equal to the glider's weight *if in
steady state flight*! Why no excess? That pesky elevator, which allows
the whole flying system to reduce the main wing's angle of attack (AOA),
in conjunction with an increasingly descending flight path. Not unless
a gust, or elevator use changes AOA will momentarily excess lift appear
(or, disappear).
- - - - - -

but just watch a high speed finish or any high speed
flying, particularly on a long wing. You can see them
bending upwards (not backwards or twisting) due to
the excessive (excessive in this case meaning more
than is needed to simply offset the glider against
gravity) lift being generated at higher speeds, and
it most certainly increases as a function of speed.

"It" (i.e. lift) does not directly increase as a function of speed.
(Just the *capability* of momentarily creating it does.) Considering
steady state high speed finishes of long wing birds (for the sake of
discussion...note that these principles hold true for any wing, with or
without flaps or spoilers), given the likelihood (either aerodynamic or
geometrical) washout does exist, the lift distribution DOES change for a
fixed trailing edge configuration with reduced/changing AOA (imagine
reducing it to the point of inverted flight). Decambering the trailing
edge with negative flap will of course further affect lift distribution.
The presence of wing bending may be due merely to the normal
(washout-affected) lift distribution of high speed flight, or it could
be increased by spoiler use or the presence of aft stick, but it is
incorrect to conclude it is entirely due to "excess lift due to speed."
So long as Joe Pilot does not create (or encounter a gust that
creates) excess lift, in what used to be steady state flight, note that:
Speed alone will NOT overstress the wing in bending.
- - - - - -

more snips
I stand by my statement. The wing can only take so
much stress from EXCESS LIFT generated at higher speeds,
Agreed, as stated. But see below...

and that usually determines a glider's VNE.

Um...unless I was the designer, I'd be loath to be so dogmatic.
Especially when enthusiastic elevator use above maneuvering speed
definitionally implies capability to generate lift generating G
exceeding design factors (which may or may not be the spar, incidentally).
- - - - - -

Other
factors (besides the center of lift usually closely
coinciding with the center of gravity) keeping the
glider form 'looping' at higher speeds are being applied
by other flight control surfaces, like the elevator
for instance. There may be some specific cases where
VNE is determined by the speed at which the other flight
controls are no longer effective enough to counter
the lift the wings generate, but no examples I can
site off hand. The generation of lift is in direct
mathematical relation to the speed of the relative
wind, period.
Um...With respect to the last sentence, I could have sworn AOA enters
the picture somewhere. An equation for a symmetrical airfoil comes to
mind...

CL = Lift/(0.5*air density*free stream velocity[squared]* wing area

For a wing SECTION (beloved of mathematical types), replace wing area
with wing chord, and (as way too many college teachers told me) "It can
be shown that" the section lift coefficient of a thin, low-speed,
symmetrical airfoil solves to 2*pi*AOA. Camber (which gliders obviously
have), changes the *location* of a lift curve when plotted vs. AOA
graph, but not the linear relationship with AOA.

So, "Agreed," speed has a BIG impact on (potential) lift for any given
airframe/glider. That pesky velocity squared term.

But it isn't speed that directly affects lift, rather it is AOA. This
(obviously!) isn't obvious, but further research and thought should
clarify things for you. The way I think of it is speed *depends* on
AOA, as does the potential for "excess lift." But (always assuming
steady state flight for ease of our thought experiment) speed by itself
is NOT the driver of things, it's merely along for the AOA ride.
_ _ _ _ _ _


BTW, a stall only in the simplest sense is from the
wing generating 'not enough lift'. It is from exceeding
the critical angle of attack for any given loading
condition,
Agreed.
and can happen at any airspeed, any gross
weight.
Agreed, despite what Tom Knauff semantically preaches (for
understandable if arguable reasons).
It happens when the airflow over the wing becomes
too turbulent to provide the needed aerodynamic reaction
to offset it's current load requirement, any angle,
any speed.

Discussion of the 3rd sentence of this paragraph is probably better left
for a real conversation. I'd need to hear more to decide if I agreed or
not. I've never heard, or thought, of the degree of turbulence being a
factor in where definitional stall separation occurs. (Let's ignore
laminar airfoils during our thought experiments.)

Paul Hanson
"Do the usual, unusually well"--Len Niemi

Len Niemi (whom I never met) was one of my heroes.

Regards,
Bob - pedantically apologetic - W.