New trainer from SZD Bielsko

On 1 Jul, 02:56, Paul Hanson
wrote:

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

I find that very hard to believe.

it is a matter of exceeding the CRITICAL AOA, which
can happen at any speed, AOA, or load condition.

Care to expand on that a bit? Given that most wings stall at an AOA of
about 18 degrees, how would you go about getting one to stall at, say,
5 degrees?

Ian

At 05:12 01 July 2007, Bob Whelan wrote:
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,


'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.
- - - - - -


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.

Thanks Bob, it is nice to be corrected with correct
information. It all makes sense now (for the moment...),
and I will now use this new and corrected view on the
subject to get back to a point from earlier. VNE is
more often determined by the speed at which the aerodynamic
loads are too great for the spar/structure to handle
(most gliders), but not due to 'excess lift'. Rather
is it due to the bending loads being imposed on the
spars/structure (or elsewhere in the airframe of course)
by the downward force from the elevator conflicting
with the lift the wings are generating, and THAT load
increases as a function of speed due to the additional
downward elevator forces required to bring this mode
about, while the lift the wings generate remains basically
constant (steady state of course). Additionally, increasing
the angle of attack at high speeds WILL most certainly
affect the generation of lift, which also comes into
play for VNE considerations due to increased loading
from lift potential being converted into lift kinetic,
during these transitions.
Being a Sisu driver, I too am am a big Niemi fan, but
also was not fortunate enough to meet him before he
left us. Bummer he wasn't inducted into the Hall of
Fame sooner...

PS. Love your books, will you be at Albuquerque to
scribe in them?

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

Paul Hanson wrote:

A bunch of intervening stuff snipped...
Thanks Bob, it is nice to be corrected with correct
information. It all makes sense now (for the moment...),
I consider that an encouraging qualification there! This stuff is worth
thinking about if it interests a person, both because it's
fun/satisfying to learn stuff, and, because the knowledge may keep a
person alive longer when they're flying near the margins of a plane's
envelope (be those margins structural or aerodynamic [e.g. flutter]).


and I will now use this new and corrected view on the
subject to get back to a point from earlier. VNE is
more often determined by the speed at which the aerodynamic
loads are too great for the spar/structure to handle
(most gliders), but not due to 'excess lift'. Rather
is it due to the bending loads being imposed on the
spars/structure (or elsewhere in the airframe of course)
by the downward force from the elevator conflicting
with the lift the wings are generating, and THAT load
increases as a function of speed due to the additional
downward elevator forces required to bring this mode
about, while the lift the wings generate remains basically
constant (steady state of course). Additionally, increasing
the angle of attack at high speeds WILL most certainly
affect the generation of lift, which also comes into
play for VNE considerations due to increased loading
from lift potential being converted into lift kinetic,
during these transitions.
I think what you write here is exactly correct for most airframes
available to the GA pilot. Where uncertainty enters the picture in my
mind is in knowing the weak link on any given ship (impossible to know
without access to the designer's calculations, or, accident data...more
than a few earlier-production-run V-tailed Bonanza tails were ripped off
due to excessive downloads at higher speeds after 'continued VFR flight
into IMC conditions').

Another category of ship where what you write likely isn't true is the
first generation of 'pure fiberglass' gliders (as distinct from those
with carbon spar caps/skins, etc.). VNE on some (all?) of those ships
is more likely set by aerodynamic/flutter, or perhaps, control-run
mounting considerations than primary structure limitations. As a
general rule of thumb, glider spars absent carbon reinforcement are
'overstrong' in comparison to (say) metal glider spars, or (probably)
carbon-fiber-reinforced spars of a ship of equal span. As I recall,
ASW-12 spars were tested to at least 12 (20?) G without failure,
possibly the Open Cirrus, as well. Understandably, German bureaucracy
was cautious certifying fiberglass technology when it upset the
sailplane applecart in the 50's and 60's; they were almost undoubtedly
reinforced in their conservatism by the sad loss of Bjorn Stender in his
prototype BS-1 (which E. Hanle/Glasflugel re-engineered prior to
ultimately producing).

Perhaps someone (Bert Willing?) could shed more light on more current
sailplane spar structural testing now required by the LBA.


Being a Sisu driver, I too am am a big Niemi fan, but
also was not fortunate enough to meet him before he
left us. Bummer he wasn't inducted into the Hall of
Fame sooner...

"Roger that!"



PS. Love your books,
All feedback appreciated...some more than others! I'm gratified your
reading time wasn't wasted.


will you be at Albuquerque to
scribe in them?
I'd like to waste the rest of my youth perpetually bumming around the
glider world, but reality too often intrudes. It's too early to tell
about Albuquerque, but I'd *like* to attend...


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



Regards,
Bob W.

Bob Whelan wrote:
large snip

Another category of ship where what you write likely isn't true is the
first generation of 'pure fiberglass' gliders (as distinct from those
with carbon spar caps/skins, etc.). VNE on some (all?) of those ships
is more likely set by aerodynamic/flutter, or perhaps, control-run
mounting considerations than primary structure limitations. As a
general rule of thumb, glider spars absent carbon reinforcement are
'overstrong' in comparison to (say) metal glider spars, or (probably)
carbon-fiber-reinforced spars of a ship of equal span. As I recall,
ASW-12 spars were tested to at least 12 (20?) G without failure,
possibly the Open Cirrus, as well. Understandably, German bureaucracy
was cautious certifying fiberglass technology when it upset the
sailplane applecart in the 50's and 60's; they were almost undoubtedly
reinforced in their conservatism by the sad loss of Bjorn Stender in his
prototype BS-1 (which E. Hanle/Glasflugel re-engineered prior to
ultimately producing).

I've read that the Open Cirrus spar was tested to 15g (after which they
gave up, having failed to break it). When I rig mine I can believe this,
as the spars are as solid as a K21's.

Vne for the Open Cirrus was set on the basis of flutter, as you write
above. A damper (from a VW Variant/Squareback) was added to the rudder
circuit, following high speed flutter in some early competitions. I
believe the pilots were exceeding Vne and the damper may be unnecessary,
but I'd hate to find out the contrary - thus my damper was replaced
recently.

On 1 Jul, 08:35, Paul Hanson
wrote:

Rather
is it due to the bending loads being imposed on the
spars/structure (or elsewhere in the airframe of course)
by the downward force from the elevator conflicting
with the lift the wings are generating, and THAT load
increases as a function of speed due to the additional
downward elevator forces required to bring this mode
about, while the lift the wings generate remains basically
constant (steady state of course).

If the downforce on the tail has increased, and you're in a steady
state, then the upforce on the wing must also have increased to
balance it.

Ian

On Jul 2, 3:41 am, Chris Reed wrote:
Bob Whelan wrote:

large snip

Another category of ship where what you write likely isn't true is the
first generation of 'pure fiberglass' gliders (as distinct from those
with carbon spar caps/skins, etc.). VNE on some (all?) of those ships
is more likely set by aerodynamic/flutter, or perhaps, control-run
mounting considerations than primary structure limitations. As a
general rule of thumb, glider spars absent carbon reinforcement are
'overstrong' in comparison to (say) metal glider spars, or (probably)
carbon-fiber-reinforced spars of a ship of equal span. As I recall,
ASW-12 spars were tested to at least 12 (20?) G without failure,
possibly the Open Cirrus, as well. Understandably, German bureaucracy
was cautious certifying fiberglass technology when it upset the
sailplane applecart in the 50's and 60's; they were almost undoubtedly
reinforced in their conservatism by the sad loss of Bjorn Stender in his
prototype BS-1 (which E. Hanle/Glasflugel re-engineered prior to
ultimately producing).

I've read that the Open Cirrus spar was tested to 15g (after which they
gave up, having failed to break it). When I rig mine I can believe this,
as the spars are as solid as a K21's.

VNe for the Open Cirrus was set on the basis of flutter, as you write
above. A damper (from a VW Variant/Squareback) was added to the rudder
circuit, following high speed flutter in some early competitions. I
believe the pilots were exceeding Vne and the damper may be unnecessary,
but I'd hate to find out the contrary - thus my damper was replaced
recently.

My Open Cirrus didn't have the damper under after I sold it.
Inspectors never picked up on it until some major work was done.
Never a hint of flutter. I only got it 'really' fast once as it was
unnecessary in the UK.

IIRC, VNe was something like 15% under the flutter speed for that
generation of gliders. I've read it was common for competition pilots
to go through the gate at VNe+15%. I believe later glider design uses
6000m for an optimized altitude. Spar placement to accommodate
ballast and allow wider load ranges resulted in the true airspeed
tables for reducing VNe at altitude to prevent the onset of the
elastic mode of flutter as the center of pressure changed. Stiffening
against this would add weight and expense for the few that want to fly
faster a high altitudes. There was an OSTIV presentation on this
several years ago published in Technical Soaring. Several RAS threads
http://tinyurl.com/26nbu2

Frank Whiteley

Ian wrote:
On 1 Jul, 08:35, Paul Hanson
wrote:

Rather
is it due to the bending loads being imposed on the
spars/structure (or elsewhere in the airframe of course)
by the downward force from the elevator conflicting
with the lift the wings are generating, and THAT load
increases as a function of speed due to the additional
downward elevator forces required to bring this mode
about, while the lift the wings generate remains basically
constant (steady state of course).

If the downforce on the tail has increased, and you're in a steady
state, then the upforce on the wing must also have increased to
balance it.

If you're in a steady state, then the forces on all the surfaces will be
the same.

Your proposed balance doesn't work out because the wing and the elevator
aren't in the same horizontal position. An increased downforce on the tail
and upforce on the wing creates a twisting motion, which will raise the
nose.

As you go faster, the angle of attack needed to generate the required
amount of lift decreases, so the relative wind comes from a higher and
higher angle. This gives you a lower AoA on the wing, and also on the
elevator, which requires you to push the stick forward to compensate. But
the end result is the same vertical forces on the wing and elevator at all
speeds.

--
Michael Ash
Rogue Amoeba Software

On 2 Jul, 15:44, Michael Ash wrote:
Ian wrote:

If the downforce on the tail has increased, and you're in a steady
state, then the upforce on the wing must also have increased to
balance it.

If you're in a steady state, then the forces on all the surfaces will be
the same.

Almost. Both the forces and the moments will sum to zero, so there are
six equilibrium conditions to satisfy. The force on the wing isn't
equal to teh force on the tail, since they have (jointly) to balance
gravity as well.

Your proposed balance doesn't work out because the wing and the elevator
aren't in the same horizontal position. An increased downforce on the tail
and upforce on the wing creates a twisting motion, which will raise the
nose.

Ah. Do you know why a downforce on the tail is needed for a dive?

But
the end result is the same vertical forces on the wing and elevator at all
speeds.

Have you ever compared the size of wing and tailplane fittings?

Ian

Ian wrote:
On 2 Jul, 15:44, Michael Ash wrote:
Ian wrote:

If the downforce on the tail has increased, and you're in a steady
state, then the upforce on the wing must also have increased to
balance it.

If you're in a steady state, then the forces on all the surfaces will be
the same.

Almost. Both the forces and the moments will sum to zero, so there are
six equilibrium conditions to satisfy. The force on the wing isn't
equal to teh force on the tail, since they have (jointly) to balance
gravity as well.

My apologies, my wording was ambiguous. What I meant was that the net
(vertical) force on the wing at 50kts is the same as at 100kts, and the
net (vertical) force on the elevator at 50kts is the same as at 100kts. I
didn't mean to imply that the force on the elevator was the same as the
force on the wing, but I can see how it would read that way.

Your proposed balance doesn't work out because the wing and the elevator
aren't in the same horizontal position. An increased downforce on the tail
and upforce on the wing creates a twisting motion, which will raise the
nose.

Ah. Do you know why a downforce on the tail is needed for a dive?

I'm not sure which downforce you're referring to here. If you mean the
need to keep the stick forward, that's to compensate for the changed angle
of attack on the elevator.

But
the end result is the same vertical forces on the wing and elevator at all
speeds.

Have you ever compared the size of wing and tailplane fittings?

Presumably the same misunderstanding as above.

I should note that my background in all of this is just a couple of
semesters of college physics combined with not a whole lot of flying
experience and some inquisitiveness.

--
Michael Ash
Rogue Amoeba Software

On 2 Jul, 19:20, Michael Ash wrote:
Ian wrote:

Almost. Both the forces and the moments will sum to zero, so there are
six equilibrium conditions to satisfy. The force on the wing isn't
equal to teh force on the tail, since they have (jointly) to balance
gravity as well.

My apologies, my wording was ambiguous. What I meant was that the net
(vertical) force on the wing at 50kts is the same as at 100kts, and the
net (vertical) force on the elevator at 50kts is the same as at 100kts. I
didn't mean to imply that the force on the elevator was the same as the
force on the wing, but I can see how it would read that way.

OK. However, you are still mistaken, I fear. In general, the downwards
force on the tail increases with speed, so the upwards force on the
wing increases to keep the weight balanced.

Ah. Do you know why a downforce on the tail is needed for a dive?

I'm not sure which downforce you're referring to here. If you mean the
need to keep the stick forward, that's to compensate for the changed angle
of attack on the elevator.

Just because the glider is diving doesn't mean there's a changed angle
of attack.

What you're forgetting - or perhaps what nobody told you - is that
it's the motion of the centre of pressure which really matters.
Basically, as the AOA increases the net lift force on the wing appears
to move forwards, and as the AOA decreases it moves back.

This is unstable, since - all other things being equal: nose up -
increased AOA - cop moves forwards - nose up moment - nose up. So
what you do is stick a tail on the back. Now nose up - positive AOA
on tail - lift at tail and, if you get the sums and moment arms
right, this balances the pitching moment caused by the cop moving
forwards.

Similarly, diving involves a downwards force on the tail. Many people
find this counterintuitive - it seems far more likely that moving the
tail up should mean an upwards force, but it doesn't. Effectively you
are just using the tail to exploit the instability of the wing.
Incidentally, since the elevator goes down for a dive, when the tail
needs to produce a downforce, the tail is always working with camber
in the wrong direction. This is horribly inefficient and explains why
all-flying tailplanes work so well.

I should note that my background in all of this is just a couple of
semesters of college physics combined with not a whole lot of flying
experience and some inquisitiveness.

As you'll have guessed, this is an enthusiasm of mine. I think a lot
of people would fly better, and find it easier, if they understood the
physics better. I include instructors, I'm afraid - hardly any of them
know why you need back stick in a turn, for example.

Ian

PS Lots of simplification in the above!