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How Airplanes Fly

Old 06-30-2009, 06:15 PM
  #26  
JetPlaneFlyer
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Originally Posted by Huffy01 View Post
I've got this other "thingy","question" about angle of attack.
I recently bought a DVD "Performance tuning for glider's". There is a big emphasis on wing incidence, tail decalge and fuselage datum.
All the angle's are at zero degrees to each other for best performance.
Is the 4 degree's angle of attack just for full size ,passenger carrying aircraft and trainer models.
That's kinda hard to answer without going into some depth, and it's probably getting rather off the original topic anyway..

Very briefly..
I'm not sure where you get 4 degrees from? Each airfoil has it's own optimum angle of attack and it also depends greatly on what you want it to be 'optimum for' .. speed, duration, load carrying etc.. optimum angle will be different for each task.

The angle that the wing is set at relative to the fuselage datum will rarely be the actual angle of attack that the wing flies at anyway.

Also measuring incidence is tricky because it depends on what airfoil datum you measure from. For flat bottom airfoils many measure from the bottom surface but this is technically incorrect. The 'correct' way to measure an airfoil's incidence is from it 'chord line' which is a line joining the extreme front point of the leading edge to the trailing edge.. For all but symmetrical airfoils neither of these datums represent the angle at which the airfoil produces no lift.. so even if the wing and tail are geometrically at 'zero' to the fuselage then aerodynamically the angles are probably different.

Maybe this stuff warrants a new thread if anyone is interested?
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Old 07-01-2009, 04:16 PM
  #27  
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"Angle of Attack" is mentioned in the text at the start of the thread. May be the "Wiki" stub needs expanding!
I got that 4 degree's from several text, I believe it's used in "broad" term's just like 15 degree's is the maxium angle before a stall occurs?
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Old 07-01-2009, 04:19 PM
  #28  
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Sorry ,angle of attack is in wiki-threads!
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Old 04-02-2010, 05:43 AM
  #29  
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Default Why it flys

Whew, lots going on here. Lucky for Wiki I guess.

What we utilise as lift is a combination of phenomona described in Bernoulli's principles, Coanda's effects, and Newton's laws.

Bernoulli talks about venturi effect, and our well designed assymetrical aerofoil results in half a venturi, so we therefore must see a decrease in static pressure above and a coincidental increase in static pressure below the wing section. Newton could ask Bernoulli if he thought there was an upward thrust force from pressure on the lower surface of the wing, and Bernoulli might ask Newton if he thought there was a suction force on the top skin of the aerofoil, as both disturbed air masses attempt to equalise themselves to the ambient static values.

Ambient static air pressure is altered as a result of an aerofoil passing through it - even if we do instead prefer to discuss 'relative air flow' over a stationary foil section. No movement, no pressure differential. The relative movement definitely causes the pressure differential. It is not a chicken or the egg scenario.

That nice aerofoil section also gives us laminar flow due to our utilisation of the coanda effect to separate air as cleanly as possible at the trailing edge of the section, thus generating less of what we call drag - so we require less motive mower to achieve a given airspeed or we get a better airspeed for the power we have available.

(I suspect there is more going on in laminar flow and the coanda effect relative to resultant lift. Entrainment of surrounding air into the flow around an aerofoil etc, but it seems to be an inexact science so far - can anyone fill me in on that? I'd be glad of some pointers...)

Formation of vorticies is a product of any disturbance of air. The energy lost to the formation of clean and controlled vorticies with desired separation characteristics from our laminar section foil is exactly the same as that energy lost in the formation of a dirty turbulent mess of eddy currents such as we might see from the airflow around a brick - if the two are of identical surface area, velocity and rho.

Newton would probably say that a good part of the upward force on an aerofoil moving through the air (ignoring compression as we do at subsonic velocities) is due to the impact or kinetic reaction of that air on the lower surface of the wing at a given angle of attack however - the downward acceleration of air disturbed by an aerofoil is secondary not causative to the production of lift. That downward accelerated airmass is the vortex - or by-product of our lift, which is of course drag.

Gliders produce lift and fly in exactly the same manner as above. The motive production of relative air flow over the glider's wing is the vertical force of gravity which is balanced by an engine in powered flight - or compounded by it during a failure.... Mr Newton would probably agree that the better it's 'Coandic' and 'Bernoullic' properties are the longer it will take to come down.

Hey does anyone have any really good stuff on the circular or 'ring' wing - or has anyone tried to power one up?

Regards Mic

Last edited by Mic; 04-02-2010 at 07:26 AM.
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Old 04-02-2010, 07:03 AM
  #30  
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I think it's a mistake to infer that Newton and Bernoulli are 'competing theories' or that Bernoulli applies only to the top of a wing and Newton only to the bottom. It's also incorrect to think of the top of the wing as a 'half venturi': http://www.grc.nasa.gov/WWW/K-12/airplane/wrong3.html
Newton and Bernoulli are both correct and both apply to any moving fluid, and to both top and bottom of a wing.

Steve
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Old 04-02-2010, 07:39 AM
  #31  
Mic
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Default Hi Steve

I agree - my point was that there are a series of complimentary relationships in play. It has always bothered me that CASA, EASA and other aviation and ED bodies like to teach Bernoulli (L = C Lift.1/2.rho.VV.S) without discussing the contributions made by others.

Have you got any good links for Coanda's work?

Cheers Mic
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Old 04-02-2010, 07:52 AM
  #32  
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Originally Posted by JetPlaneFlyer View Post
I think it's a mistake to infer that Newton and Bernoulli are 'competing theories' or that Bernoulli applies only to the top of a wing and Newton only to the bottom. It's also incorrect to think of the top of the wing as a 'half venturi': http://www.grc.nasa.gov/WWW/K-12/airplane/wrong3.html
Newton and Bernoulli are both correct and both apply to any moving fluid, and to both top and bottom of a wing.

Steve
I love that link Steve!

Thanks for passing that along
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Old 04-02-2010, 08:06 AM
  #33  
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Originally Posted by Mic View Post
It has always bothered me that CASA, EASA and other aviation and ED bodies like to teach Bernoulli (L = C Lift.1/2.rho.VV.S) without discussing the contributions made by others.

Have you got any good links for Coanda's work?

Cheers Mic
This is my favourite page on how wings really work: http://www.regenpress.com/

It does cover surface attachment (aka 'Coanda effect')

Steve

Last edited by JetPlaneFlyer; 04-02-2010 at 04:00 PM.
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Old 04-05-2010, 07:12 AM
  #34  
Mic
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Default Good stuff Steve, thanks

Yes, that's the type of thing (the NASA link from Steve) that the ED and Civil Aviation crowds need to be looking at. Good stuff.

We could probably also immediately find a dozen or more articles that contradict that one with some rigor. The beauty of the interweb...

I used the term '1/2 a venturi' as a way of discussing Beroulli's part of the lift equation, and I'm not the first. The article Steve linked for us refutes that nicely - however, there is no getting away from the laws of conservation of energy. A low pressure zone does exist above an efficient aerofoil because of it's shape and the resultant uniform acceleration of air over it.

I would add here that the laws of conservation of energy do not rely upon a venturi for their relevance or effect. They apply to relative movement between air masses having the same properties, and a venturi is simply another mechanical method of achieving that relative movement - as is an aerofoil.

Now before I am held up for contradicting myself, I agree that a brick moving through the air also generates low pressure zones and vortices around itself. We use our knowledge of aerodynamics to control all of those secondary effects of our production of lift in the interests of efficiency - which is why we present an aerofoil and not a brick to our airflow.

And, I like first and foremost that we can control an airflow to obtain the effects we desire - and, well... aviate with it.

I content myself with lift being a direct function of AoA, and that the other factors discussed in the generation of lift are a constant of and secondary to the lift event - except maybe for modification of the real-time effective AoA by entrainment of the surrounding airmass. (Coanda)

In other words the lowering of static air pressure due to it's acceleration over the wing (Bernoulli), and the separation and re-joining of that air mass (Coanda) - are both secondary to the wing impacting with then riding over that air mass (Newton).

Cheers Mic

Then I found this, and we're all up the creek... http://www.terrycolon.com/1features/fly.html

Last edited by Mic; 04-05-2010 at 08:19 AM.
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Old 04-05-2010, 08:38 AM
  #35  
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Originally Posted by Mic View Post
..... however, there is no getting away from the laws of conservation of energy. A low pressure zone does exist above an efficient aerofoil because of it's shape......
Nic,

Again I’d need to counter the assertion that the shape of the upper surface is directly responsible for the low pressure generated there. The 'airfoil' shape is really just there so that the wing better conforms to the curve of the airflow as it turns downward. The purpose being to keep the airflow attached, which reduces drag and delays stall.
The low pressure itself is generated by the centrifugal pressure gradient as the air turns downward when it passes over the wing. As proof of this a 'flat plate' wing works just fine and produces low pressure on top just like any other wing (albeit with more drag)


Notice here that I've not mentioned Bernoulli once because Bernoulli is not what directly leads to the low pressure. Pressure reduction on the top surface (and indeed increase on the bottom) is the direct result of the turning of the air and the centrifugal pressure gradient that is caused when air (or any fluid) turns. Bernoulli is a secondary effect i.e. the lower air pressure (caused by air turning) leads to higher velocity, as per Bernoulli.
http://www.grc.nasa.gov/WWW/K-12/airplane/right2.html
http://www.regenpress.com/

Quote from the second linked page:
"Description of lift as due to pressure gradients in curvature of above-wing and below-wing flows was given by Otto Lilienthal in his 1889 book titled Birdflight as the Basis of Aviation. Unfortunately that has been neglected for well over a century in favor of pseudoscientific descriptions"
The idea that the air has to accelerate around the 'longer path' over the curved top of the wing sounds awfully like the 'equal transit time' theory, which is probably the most popular misconception on how a wing works: http://www.grc.nasa.gov/WWW/K-12/airplane/wrong1.html

Steve

PS.. The circulation explanation for lift as discussed in your link is really a more advanced (and harder the grasp) way of looking at the flow turning principal. Circulation is also discussed on one of the links I provided previously.... Both turning and circulation are correct, just different ways to look at the same thing..

Last edited by JetPlaneFlyer; 04-05-2010 at 10:38 AM.
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Old 05-12-2010, 03:40 PM
  #36  
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Default Interesting thread

When I was in middle shool I did a science project on "How an airplane flies" I have been aware of the "Bernoulli Principle" for 30 years. I was always fascinated by it.(It seems it helps a chimney produce a draft when there is no convection). Just recently I got into this hobby and I wondered what was up with the airfoil of a wing, how can it fly upside down, what about symetrical airfoils? This thread has really inspired me to think about and study this science more. Thanks
VP

Last edited by Victory Pete; 05-13-2010 at 12:51 AM.
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Old 05-13-2010, 12:32 AM
  #37  
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Unfortunately, what everybody "knows" about airplane wings is totally wrong. This thread is a start in the right direction, but even it is full of misleading information. The link Mic posted, http://www.terrycolon.com/1features/fly.html, is the best introductory explanation I can remember seeing.
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Old 05-26-2010, 05:58 PM
  #38  
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I found this site, which is funded by NASA to be helpful.
http://www.allstar.fiu.edu/aero/airflylvl3.htm
VP
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Old 09-17-2014, 04:53 PM
  #39  
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Originally Posted by JetPlaneFlyer View Post
This is my favourite page on how wings really work: http://www.regenpress.com/

It does cover surface attachment (aka 'Coanda effect')

Steve
This link doesn't seem to work anymore. Is the article still available anywhere?

Thanks.
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