Heres why this is a dumb idea
(this answer is meant to be facetious,
not condescending! Im the devils advocate you were asking for!)
I had read through the discussion in the comments about four times to
be sure I had your setup right.
From what I can tell, what you basically have is a helicopter without
the helicopter. Basically something that looks like a frisbee or an
upside down saucer with a ducted fan built in the top that blows air
all around the airfoil-shaped surface.
When you get this device airborne are you going to get an upward
force? Yes. Are you going to get a downward force? Yes, and it will be
much larger than the upward force. But lets concentrate on the
For the incompressible case that you are interested in you can figure
out the pressure of the air over the top by using bernoullis
equation. The pressure on the bottom is simply the ambient air. The
pressure on the top goes as the speed across the top surface, and the
pressure on the bottom is simply ambient. Whala... Lift!
What I dont think you are accounting for is the momentum of the air
coming down on the disk. Momentum can be calculated as 1/2 emm vee
squared. Or if you are talking about fluids, its easier to think in
forces. 1/2 rho volume/second vee^squared (momentum transfer per
second... a force). And its all coming down- on your wing! If you
calculate this for your particular configuration you will likely see
that the momentum transfer on the top of the wing plus the pressure
over the top is likely higher than that on the bottom. What you end
up with is essentially negative lift, not lift. The suction force on
top of the ducted fan (producing an upward force) is likely to be very
small because the air going into the fan is travelling at very low
Heres a Demon... dont think up, think sideways. Your configuration
is basically the front of a fuselage without the rest of the fuselage.
Air coming straight on the disk, no fuse or wings behind it. LOTS of
drag. There is no simple way to calculate this, either stick it in a
wind tunnel or use computational fluid dynamics (that takes turbulence
into account). I doubt there is a way to get very highspeed airflow
over the top surface of your device without getting very turbulent,
One last demon... When the airflow coming off of the disk meets the
edge it will likely try to curl around it. This is bad news because
when this happens the air will separate (become spoiled) and reduce
the high pressure on the bottom. In other words, the air on the
bottom is actually moving... reducing the ambient pressure quite a
I think that about kills it. Its probably worth the time to take the
numbers that you have for your proposed configuration and run them
through this model just to be sure. If I've glossed over a detail you
need, just request it and I'll be happy to provide it.
If Ive screwed something up in the above explanation, please ask for
a clarification... Also, if my conception of your device is flawed, I
can take a second try... this was a very neat question!
FYI... You keep referring to camber. A wing (or airfoil) has camber
for two reasons. 1) to help keep flow attached, 2) to get the maximum
pressure differential between the top and bottom of the wing
(somewhat, but not completely related to 1). In this device's case,
neither one nor two will apply because your device does not satisfy
the Kutta condition that your average wing does. The Kutta condition
states that the flow meets somewhere near the tail end of an airfoil.
Hopefully (for drag's sake) at the trailing edge of the airfoil. For
this device, in a real world configuration, the flow will meet
somewhere on the -bottom- of the disk, because the flow is bending
around the edge. It will also be unstable, because the flow will be
Request for Answer Clarification by
22 Dec 2002 18:20 PST
krobert - Thank you for being the Devil. I asked for a $50.00 answer
and have gotten much more than $50 worth of info and enjoyment. But,
you're point about the momentum of the air pushing on the airfoil is
incorrect. The second stage of the counter rotating ducted fan
re-accelerates and re-directs the airflow (about 60 degrees) parallel
to the lifting surface.
It's going to be just like blowing air over the top of a sheet of
paper, at 200 kts.
The Kutta thing might be the Achilles heal, and the vortices, and God
knows what else. So, I've got an appointment with the wind tunnel
guys and UW, we're going to try to jury rig some of his gear so the
airflow is directed parallel to the lifing surface. I did that with a
leaf blower and lifted the shell off the ground.
Thank you for your time, and pls respond to the momentum thing. I'll
consider that an answer to we can take this thing off the Google
I'd be happy to report the wind tunnel results, if I know to whom to
Thank you for your time
Request for Answer Clarification by
28 Dec 2002 19:45 PST
Krobert: Assuming your question was not rhetoric, "no" the shell
did not "hop," it "flew" up into the diffuser and stayed there untill
I turned off the airflow.
If your "downward force" argument held water helicopters could not
hover. We already have PACV's (Air cushion vehicles) that "fly" on
the force of a single stage ducted fan. The first stage of my ducted
fan will "pull" air straight down into the second (counter-rotating)
stage which will re-accelerate and re-direct the airflow
(approximately) 60 degrees, parallel to the lifing surface. The "lift"
from the second stage will be marginal because the lift vector will be
deflected 60 degrees from vertical. I'm discounting the lift from the
fans altogether, hoping to get significant lift from the difference
between the low pressure, high speed, turbulent air over the top of
the lifting surface and the zero airspeed, ambient, 14.7 psi air
I've obviously explained this thing badly and apologise for taking up
I'm going to leave this discussion now. Am happy to pay the $50.00
because of the the thoughtful, evocative, responses. Still, my
original question: "how wouold I figure the lift on a large cambered
wing with approx. 200 kts over the top and zero airspeed underneath?"
was never addressed.
On to the wind tunnel...