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'[EE]:: Lift - airfoils, flat plate.'
You probably have thought of this already but my understanding is
that, to "first order", a flat plate is just an "airflow deflector",
so that it gets maximum L/D at 45 deg AOA, and at that point, L=D.
Furthermore, a stalled airfoil approximates a flat plate.
The most satisfying explanation of lift I've found is the Coanda
effect (wiki has a decent article IIRC). The typical "air passes
faster over the curved surface than over the flat one" explanation is
wrong or at least reverses cause and effect as I understand it.
Basically, the Coanda effect says that airflow will tend to stick to
surfaces unless the surface geometry would require too abrupt a
momentum change, in which case it separates and causes turbulence. So,
an airfoil is a device for smoothly directing airflow. With an
asymmetrical airfoil or a symmetrical one with an AOA > 0, the net
airflow off the back of the wing is DOWN, producing an upward reaction
force on the aircraft.
On Tue, Apr 29, 2008 at 8:04 AM, Apptech <paradise.net.nz> wrote: apptech
> You probably have thought of this already but my
> understanding is
> that, to "first order", a flat plate is just an "airflow
> so that it gets maximum L/D at 45 deg AOA, and at that
> point, L=D.
> Furthermore, a stalled airfoil approximates a flat plate.
45 degrees AOA as max lift is as per sites quoted BUT
stalling usually occurs well below that. Maybe not for flat
plates. NASA sim I quoted allows you to model a flat plate
well. Doesn't allow my "bent plate" rotors and it's not
obvious how to handle them analytically and easily.
With the flat-bent plate rotor machines the plate goes
through 0-180 degrees of aoa and back again so analysis may
I'm contemplating just measuring torque for a locked rotor
at various aoa's and air speeds which should give me easy
results and then vary variables (blade to bent section
ratio and bend angle) and see where that takes me.
Is that rotor for an airplane/helicopter or a ventilator or something else?
If that is for airplane, do you need only the statical torque or also the
Did you try xfoil by the way? That's only for foil analysis, but I think
there are some extensions for that for 3D plane, blade etc.
On Tue, Apr 29, 2008 at 2:26 PM, Apptech <paradise.net.nz> wrote: apptech
|> Is that rotor for an airplane/helicopter or a ventilator
> or something else?
> If that is for airplane, do you need only the statical
> torque or also the
> dynamic one?
The static torque and dynamic torque are, in my present
ie if I have a rotor with air passing over it at a certain
rate and from a certain angle in a steady state then this
will (hopefully) closefully enough approximate this
condition when it transiently occurs during rotor rotation.
ie (again) I'm saying that I am just changing the frame of
reference. Moving air and moving rotor or moving air at
another velocity and locked rotor. For a mainly lift based
machine this is difficult as tip to wind speed is high
(typically 3:1 - 6:1 range and can be 12:1+) but for a drag
based machine tip speed i <= wind speed. I think the
flat-bent plate machines are mainly drag based but have a
lift element but by simply measuring tip speed in real world
unloaded use this will become evident.
> Did you try xfoil by the way? That's only for foil
> analysis, but I think
> there are some extensions for that for 3D plane, blade
Haven't done more than play as yet.
On Tue, Apr 29, 2008 at 9:26 AM, Apptech <paradise.net.nz> wrote: apptech
> 45 degrees AOA as max lift is as per sites quoted BUT
> stalling usually occurs well below that. Maybe not for flat
> plates. NASA sim I quoted allows you to model a flat plate
> well. Doesn't allow my "bent plate" rotors and it's not
> obvious how to handle them analytically and easily.
45 deg AOA for max lift is ONLY for flat plates. Typical light
aircraft airfoils have max lift at just below stall AOA, around 15 or
20 deg. Delta winged high performance aircraft often have a much
higher usable AOA (35 deg or so), and in fact will often also have
significant, decreasing lift at AOA higher than stall. (This can lead
to the paradoxical situation that pulling "up" on the stick results in
less lift, without the characteristic "break" at stall which would
warn you of what is going on)
Flat plates are always "stalled" in the sense that their behavior at
any AOA is like that of a normal airfoil NEAR its stall AOA.
> With the flat-bent plate rotor machines the plate goes
> through 0-180 degrees of aoa and back again so analysis may
> be "difficult".
Do you have a drawing or photo of what you are trying to model?
Yes, I think it would be difficult. If you could assume that there is
enough time for the turbulent air to get out of the way before the AOA
changes significantly, then you could probably model it in a stepwise
fashion (two regions - simple geometrical flat plate (where lift is
just airflow times drag times sin(angle)) and airfoil-like, where L/D
is greater than one and dependent on AOA squared, like a normal wing)
> I'm contemplating just measuring torque for a locked rotor
> at various aoa's and air speeds which should give me easy
> results and then vary variables (blade to bent section
> ratio and bend angle) and see where that takes me.
Would probably be worth it even just to develop some more insight into
On Tue, Apr 29, 2008 at 11:08 AM, Sean Breheny <cornell.edu> wrote: shb7
> just airflow times drag times sin(angle)) and airfoil-like, where L/D
> is greater than one and dependent on AOA squared, like a normal wing)
I think this should have said "dependent on AOA linearly".
Russell, take a look at blade calculations for impulse turbines. A flat plate
has no lift as such (the paths are equally long around both sides), it works as
a deflector mostly. And its efficiency is low.
> I'm trying to understand flat-plate lift well enough to
> model "bent flat plate" vertical axis wind turbines. While I
> started off thinking that these were a trivial or simplified
> version of a Savonius Rotor it appears that they are a
> combination lift and drag machine.
It's not clear what machine exactly you are referring to. It is possible to
use flat plates in a pure drag design, like my idea explained at
Most of the literature I looked at automatically assumes that lift is
somehow inherently better than drag, but I don't think that is true and I
haven't seen any justification for it. I think lift was probably shown to
be better than drag for a particular application, and the conclusion has
stuck but the specific constraints forgotten. Lift designs can intercept
more wind area than drag for the same airfoil area, but that is just one of
many tradeoffs in wind turbine design.
One advantage with drag designs is that the parts move slower than the wind
and therefore don't pose a hazard to birds. The optimum power point for a
flat plate getting pushed straight downwind is 1/3 the wind speed.
A commercial group is trying to build a wind farm in Nantucket Sound off the
coast of Massachusetts, and the NIMBY crowd (even though it's miles from the
nearest back yard) are trying to squash it on the basis of birds getting
killed, so this is more a issue than it might appear at first.
Embed Inc, Littleton Massachusetts, http://www.embedinc.com/products
(978) 742-9014. Gold level PIC consultants since 2000.
> Russell, take a look at blade calculations for impulse
> turbines. A flat plate
> has no lift as such (the paths are equally long around
> both sides), it works as
> a deflector mostly. And its efficiency is low.
Surprisingly, and not overly intuitively until you've
wandered through the various descriptive pages a few times,
a flat plate does provide lift - as Sean says, for a flat
plate, maximum lift occurs at 45 degrees (!). It also has
substantial drag. Some specialist aircraft have wings
approaching flat plates. One example is very fast aircraft -
especially it seems older fast ones. Starfighter comes to
mind without going and looking how flat they are.
Lift occurs when you deflect an airstream such that there is
an "easy" and "hard" path (or lower and higher resistance,
or preferred and non preferred or ...). How you view what
happens after that depends on which method of analysis you
choose. There are various nice effects to complicate things.
(Messrs Coanda, Bernoulli, Reynolds and more stand in the,
'[EE]:: Lift - airfoils, flat plate.'
> Test, ignore
Apptech <apptech <at> paradise.net.nz> writes:
This is not funny, I wrote two messages yesterday and they went to the bit
bucket somehow. One of them contained the T-word. I guess they put them on a
server in Gitmo now, until they clarify what and why.
I wrote about aerofoils (symmetrical ones not flat ones - the short version: see
Darreius rotor aerofoils and Wells Turbine).
The other message suggested in not so many words that towing a vehicle without
police papers is a good way for T's to get a V of their choice for an E. For
which reason many places make towing arbitrarily parked vehicles without a
police agent being present (physically riding the truck in the case of city
parked cars) and suitable paperwork illegal.
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