pchapman(at)ionsys.com
Guest
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 Posted: Sat Jan 26, 2008 6:19 pm Post subject: origin of lift [was Re: Cabin airfoil] |
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At 09:33 26-01-08, you wrote:
| Quote: | --> Zenith-List message posted by: Jaybannist(at)cs.com
A lot of our list posters seem to have bought into the premise that an airplane flies because of the lift provided by the Bernoulli effect of the upper surface of the wing. In fact, most of the lift is provided by Newton, pushing on the bottom surface of the wing (With enough power, you can fly a barn door). This speaks very loudly about the "expertise" of some of our most prolific posters, right? |
This is a topic that could go on forever and divert attention from actually building Zeniths... but I can't help sticking my nose in.
I say there's more pulling up than pushing up.
There is no single way to describe lift. The same forces can be described in different terms. Lift does depend on suction pressures on top of the wing and positive pressures underneath (that is, the pressures all around the wing!), and there's a net downward deflection of air through this process - upwards lift from pushing air down. So the Bernoulli stuff explains in detail how the Newton stuff happens overall. If one prefers to talk of circulation, well, a net circulation is also a result of the whole process.
Yes one can fly a barn door with enough power. Max lift will be poor (say, halved) and drag will be way way up, and angle of attack will be very high to create the lift. So on the one hand, no one doesn't need a nice rounded airfoil to fly, on the other hand, it sure helps a lot.
As for whether there's more pulling on top or pushing on bottom, I'm still in the camp that says the lift on top is the larger contributor in normal circumstances.
The inline attachment (if it transmits OK) shows an aerodynamic code's results for a typical light plane airfoil at a typical higher speed flight and lower speed flight -- A NACA 2412 at 4 degrees (smooth line) and 8 degrees (line with boxes) angle of attack relative to the chord line.
Pressure distributions are shown.
Without getting into too much detail, for each line, the area above the horizontal axis (suction upwards from the top of the wing) is a lot more than the area below the axis (upwards push from the bottom of the wing). So for typical airplane applications, there is a lot more pulling up than pushing up!
(Now in the extreme case of a barn door at high angles of attack, there's not much lift off the entirely stalled top surface, and a lot of pressure on the bottom. That's one case where the pushing might become more important than the pulling. Although even then there may be some very useful vortex lift from the edges. But most of us prefer to use rounded airfoils at low angles of attack, that takes advantage of their incredible ability to create a lot of lift off their top surface, without creating a ton of drag at the same time. I personally only go the barn door route when flying a skydiving wingsuit at a L/D ratio of under 2:1. And I still need to deploy a real airfoil wing to survive the landing. )
[img]cid:.0[/img]
Peter Chapman
Toronto, ON
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