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My IEEE Spectrum Letter - RF Cafe Forums
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Kirt Blattenberger
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Post subject: My IEEE Spectrum Letter
Posted: Mon Oct 01, 2007 4:38 pm
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Site Admin |
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Joined: Sun Aug 03, 2003 2:02 pm Posts: 878
Location: Erie, PA |
Greetings: As you might have inferred from some of the material I have posted
over the years, I have been involved in model and, to a lesser extent, full-size aviation
for many decades. There has been a long-standing discussion over how lift is produced
by an airfoil. Most of us learned that the total lift is a combination of the downward
force created by the air moving past and deflecting off the bottom of the wing (equal-and-opposite
reaction), and the Bernoulli effect that creates an area of relatively low pressure
on the top surface of the airfoil by virtue of the longer airflow path on the curved
top surface. NASA, the ESA, and probably all other authoritative sources employ the
multiple factor view when calculating lift and other aerodynamic forces. I am
always amazed when someone is willing to put his ignorance - or maybe it is pure arrogance
- on display by proclaiming a viewpoint (usually an uneducated opinion) and then implying
that anyone who does not sign on is inferior. Then, even when confronted with the truth,
those people are often unrelenting. In the August 2007 edition of the IEEE Spectrum
magazine, a reader wrote in to criticize the author of an earlier article for invoking
the Bernoulli mechanism of lift generation when, in the reader’s opinion, lift is generated
primarily by the equal-and-opposite effect of deflected air. He uses an inverted airfoil
as an example of how Bernoulli cannot work. A search of NASA’s website quickly turns
up an explanation of how, in fact, it does predict lift for an inverted airfoil due
to Bernoulli, provided that the angle of attack is great enough to offset the more curved
bottom surface. A fully symmetrical airfoil needs only a small positive angle of attack
(AoA) to generate a lot of lift (assuming the wing loading is not excessive). To assign
the difference between zero lift at zero degrees AoA and that generated with a couple
degrees of positive AoA (great enough to lift a 1,500 lb plane + pilot) primarily to
an equal-and-opposite force reaction is nonsensical. You can read my polite
response to the reader as printed in the October 2007 edition. The editors cut out about
half of what I wrote, but it gets the point across. I do wish that they had left the
last sentence in its original form where I said that the NASA website is replete with
examples of Bernoulli, including downloadable software and instruction kits for teachers
at all educational levels. See “Foiled Again” (title given by Spectrum)
http://www.spectrum.ieee.org/oct07/5564
Here is the reader’s letter (“Newton, Not Bernoulli”)
http://www.spectrum.ieee.org/aug07/5396
Unlike the reader’s claim, I attribute the authority of my position to NASA,
not myself. Comments? : :
_________________ - Kirt Blattenberger
RF Cafe Progenitor & Webmaster
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nubbage
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Post subject:
Posted: Fri Oct 05, 2007 7:04 am
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General |
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Joined: Fri Feb 17, 2006 12:07 pm Posts: 365
Location: London UK |
Hi Kirt Whether by coincidence, or because you have raised this matter previously
in these august pages, I know not, but I recently wished to demonstrate my prowess as
a model constructor to my adoring, hero-worshipping grandson. I had a large sheet of
quarter inch thick polystyrene and some very long kebab sticks. So I cut the wings and
talplane surfaces from the polystyrene and made the fuselage from the kebab sticks.
It looked convincing, anyways to the 6 year-old. I thought: perhaps an aerofoil surface
is irrelevant, and the only important vector forces were the imbalance between low forward
drag and high vertical drag. The acid test: did it fly? Well, a shuttle pilot summed
up the shuttle's aerodynamic performance, and it applies as well to my non-aerofoil
model: it flew like a brick. Absolutely no lift whatsoever, irrespective of the AoA
employed to launch it. It did not even stall spectacularly, it just ran out of forward
momentum, and sank slowly to the groud. Rather like my credibility did with my grandson.
Nubbage
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Kirt Blattenberger |
Post subject:
Posted: Fri Oct 05, 2007 8:06 am
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Site Admin |
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Joined: Sun Aug 03, 2003 2:02 pm Posts: 878
Location: Erie, PA |
Greetings nubbage: Sounds like a familiar result if the aeroplane is tail heavy.
If you still have the craft and want to try again, then her's a good rule of thumb for
balance. Assuming the plane "looks" proportionally correct, then lift it by two fingers
from under the wings at a point about 1/3 of the way back between the leading and trailing
edges. If it the tail falls, it needs weight in the nose until it hangs level.
Do it while your grandson is not around, and then do the hero thing when he returns
- after you get it trimmed to fly.
Take care.
_________________ - Kirt Blattenberger
RF Cafe Progenitor & Webmaster
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nubbage
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Post subject:
Posted: Fri Oct 05, 2007 10:23 am
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General |
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Joined: Fri Feb 17, 2006 12:07 pm Posts: 365
Location: London UK |
Hi Kirt Well, I built a lot of flying models that really flew during my illustrious
career. The balance point was just where you recommend, just aft of the leading
edge of the wing. My next attempt, in private as you recommended, will be to glue
a strip on the upper wing surface down the center line, and two strips on leading and
trailing edges on the underside of the wings, thus creating a crude aerofoil. My 50
bucks says it will then "fly" instead of "flop". nubbage
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Posted 11/12/2012
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