Chronology Current Month Current Thread Current Date
[Year List] [Month List (current year)] [Date Index] [Thread Index] [Thread Prev] [Thread Next] [Date Prev] [Date Next]

Re: [Phys-l] Bad physics in National park

Donald Smith wrote:
Hi,

No, that explanation makes more sense than what they said at the national
park. What they said at the park was more like
"the air that goes over the top of the wing has to travel further than the air going under the bottom of the wing.
OK
The two flows must travel across the wing in the same amount of time.
Not OK
Therefore,
the top one must travel faster, and since it is travelling faster, the
pressure is lower above the wing than below. This difference in pressure results in an upward force we call "lift", which is what holds up the airplane." I'm paraphrasing from memory, but that is as close as I can remember to an exact quote.
I didn't like the word "therefore".
Never mind that this "explanation" would imply that planes cannot fly upside
down, which obviously they can.
An upside down asymmetric foil still sees a faster flow on the blue side than
the brown side. Airflow travels a path that doesn't slavishly follow the surface.
That's what bugged me. Not that they invoked Bernoulli at all, but that
they used this bogus and completely unfounded assertion that the flows take equal time to get around the wing to make an argument that had patently false
implications.

Yours,

Don

It is a plausible position, until one first sees a an airfoil with coincident smoke bars on the leading edge which do not rejoin at the trailing edge.

There is a wonderfully elegant stall warning fitted to old Cessna 150's.
It consists of a slot just under the leading edge of the wing opening into a little air box, which is piped to the wing root at the cabin, where a mouth organ reed is placed. The reed sounds if the tube is sucked, but is silent if the flow is into the cabin.
As the angle of attack approaches 15 degrees, the stagnation point of the airflow proceeds down the leading edge, until the flow reverses near that slot which senses the
relatively low pressure there (compared with the cabin ambient pressure.)

Verifying its operation, a standard operating procedure, was not always popular with people who objected to placing their lips on the wing slot and sucking. There was a possibility of inspiring flysquash at times.

Brian W