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Bernoulli versus Coanda

2FlyAgain

By 2FlyAgain
in Landing and Flying

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2FlyAgain

2FlyAgain

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Greetings,

 

The recent discussion of angle of attack in another thread contained a mention of Bernoulli's principle. I suspect that practically all of us were taught Bernoulli's principle as the explanation of lift for airplane wings. About a decade ago, I ran across an argument that Bernoulli's principle was an inadequate description and that the Coanda effect was a better explanation. I have attached the 16 page article for those that are interested.

 

Briefly, an everyday observation of the Coanda effect can be seen observing water running from the kitchen faucet. Without an obstruction to the flow, the water runs straight down. When a curved object (e.g. a round glass held horizontally) is placed just barely into the water flow, the water curves around the object and direction of the flow changes from straight down.

 

The authors argue that for airplane wings the air flow coming off the trailing edge of the wing is not parallel to the bottom of the wing (Bernoulli thinking) but rather deflected downward at an angle roughly equal to the angle of attack. The force of the downward component (vector) of the airflow off the wing creates an equal an opposite reaction that lifts the wing upward according to Newton's third law. This explanation makes a sense according to my first year college physics.

 

The authors go on to elaborate that this explanation is consistent with wing efficiency, wing loading, power changes, drag effects, wing tip vortices and ground effect.

 

Since many readers of this forum seem to value both the true explanation for a phenomena and a good debate, I thought that I would offer this as food for thought...

 

My thanks to all of you for the many stimulating discussions on this forum!

 

Enjoy (...or not)

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Ed Cesnalis

Ed Cesnalis

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I'm the one that brought up Bernoulli's principal but I did qualify it by saying "if it has any merit." Personally I don't think you can use Bernoulli to explain lift because you can demonstrate in a wind tunnel that the air separated by the leading edge does not arrive at the trailing edge at the same time. Perhaps you can use Bernoulli to calculate what the air is doing, but it doesn't explain the mechanism responsible.

 

Its hard for me not to be a Coanda fan, didn't he build, fly and crash the first jet airplane about 30 years prior to the Germans?

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FastEddieB

FastEddieB

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I think there's a combination of things going on.

 

In a typical airfoil, I still think air accelerates over the upper cambered surface. Air is being slightly constricted as if in a venturi, with one side being the wing and the other the undisturbed air above the wing. That constricted air accelerates and per Bernoulli causes lower pressure on top and hence, lift.

 

Then again a brick or Snoopy's dog house will fly if given enough power and angle of attack. In that case, I think the downward deflection of air eclipses Bernoulli to keep the item aloft.

 

I had never heard of Coanda, but I suspect the mystery of flight is perhaps explained by some combination of the two descriptions.

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2FlyAgain

2FlyAgain

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I'm the one that brought up Bernoulli's principal but I did qualify it by saying "if it has any merit." Personally I don't think you can use Bernoulli to explain lift because you can demonstrate in a wind tunnel that the air separated by the leading edge does not arrive at the trailing edge at the same time. Perhaps you can use Bernoulli to calculate what the air is doing, but it doesn't explain the mechanism responsible.

 

Its hard for me not to be a Coanda fan, didn't he build, fly and crash the first jet airplane about 30 years prior to the Germans?

 

CT,

 

You make exactly the same point as the authors of article that I referred to. Namely, that wind tunnel evidence shows that air travels over the wing at a different rate than under the wing. Without "equal transit" times, the pressure differential described by Bernoulli is no longer readily understood.

 

I don't know if Coanda built a jet plane.

 

--Mark

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2FlyAgain

2FlyAgain

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This may be the discussion that explains lift. Caution - may take a while to load.

Disclaimer - I know Dr. Gurnett well. He is an expert glider pilot, owns a T6 and is a past U.S. RC National champion. He is in the Van Allen chair at the University of Iowa.

 

http://www-pw.physic..._Dec12-2003.pdf

 

Thanks Jim,

 

If I have read this correctly, Dr. Gurnett states on page 9 that he thinks that the Bernoulli approach is incorrect. Sadly, my knowledge of vector calculus is too old for me to determine how closely the Coanda effect resembles a simplified version of the Kutta-Joukowski Condition.

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Ed Cesnalis

Ed Cesnalis

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In Jet Age Airlanes of 1956, Coandă himself published an article entitled "The First Jet Flight". He submitted the same text that Caidin had written for Flying in May:

 

170px-IICCR_G240_Ceausescu_Coanda_crop.jpg

 

magnify-clip.png

Henri Coandă in 1967

 

 

"In December, we brought the airplane out of its hangar at Issy-les-Moulineaux and, after a bit of coaxing, started the motor. I must admit that I was never a very outstanding pilot. I always seemed unable to shake off a vague apprehension and, that morning, in addition to my usual uneasiness, I was rather excited. I climbed into the cockpit, accelerated the motor, and felt the power from the jet thrust straining the plane forward. I gave the signal to remove the wheel blocks, and the plane started moving slowly ahead. I had anticipated that I would not attempt to fly today, but would make only ground tests on the small field at Issy-les-Moulineaux. The controls seemed too loose to me, so I injected fuel into the turbine. Too much! In a moment I was surrounded by flames! I had to cut back and reduce my power quickly. I worked the throttle and the flames subsided. Only then did I have opportunity to lift my head. I saw that the plane had gained speed, and that the walls of the ancient fortifications bordering the field were lunging toward me. I pulled back on the stick, only much too hard. In a moment the plane was airborne, lunging upward at a steep angle. I was flying—I felt the plane tipping—then slipping down on one wing. Instinctively, I cut the gas with my left hand and the jet fuel with my right. The next thing I knew, I found myself thrown free of the plane, which slowly came down, and burst into flames. It was impossible to determine from the wreckage whether the celluloid or the fuel was the cause of the fire. But the test was over. I had flown the first jet airplane.
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Ian

Ian

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.........

 

Note, flaps cannot be used at speeds above landing, except for negative flaps, which can be used to reduce drag on the wing at higher speeds as the -6 flap setting FD has designed.

 

or -12 if you are in Europe - must be a difference in the air :blink:

 

But to say "flaps cannot be used at speeds above landing" is wrong - flaps can be deployed at any speed below the flap deployment limiting speed - indeed you might want to put some flap down for a "slow, safe cruise" for a whole variety of reasons.

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Doug G.

Doug G.

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CTLSi, where can I find that bit about not flying above landing speed - or did you just make that up?

If it is true, I would like to know because every time I take off I use flaps as the AOI tells me to and I fly faster than landing speed.

What you are saying has no basis.

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