My IEEE Spectrum Letter - RF Cafe Forums

 Post subject: My IEEE Spectrum Letter

Posted: Mon Oct 01, 2007 4:38 pm 

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?
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_________________
- Kirt Blattenberger
RF Cafe Progenitor & Webmaster

 Post subject:

Posted: Fri Oct 05, 2007 7:04 am 

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

 Post subject:

Posted: Fri Oct 05, 2007 8:06 am 

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

 Post subject:

Posted: Fri Oct 05, 2007 10:23 am 

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