right or wrong????

[FearlessFreep]

I am certainly no expert in the science of aerodynamics or fluid dynamics.

I do know that ther are a few important aspects that must be taken into consideration; mostly Reynolds Numbers, boundary layer and seperation of flow. When you are working in the smaller scale a lot of funny looking devices can actually produce Bernoullian lift instead of just the Newtonian lift that one would think was the only producer of lift. If there is no seperation of flow a camber can be effected with just a flat surface with a small angle of attack.

The diamond shaped airfoil on the F-104 has to do with supersonic flow.

Absolutely symmetrical airfoils operate because a small angle of attack is applied in level flight to cause a difference of pressure between the upper and lower surfaces. If the angle of attack is zero there is absolutely no difference in pressure so no lift is produced.

This is all what I have been taught or been exposed to by research or conversation. I offer no guarantees to it's accuracy.

The thing to do is get a serious aeronautical engineer discuss all of this.

It is all interesting conversation though.

 

[willbav8r]

Kinda goes to show we really don't know squat about the explanation of lift..........thank god for those Wright bros.

 

[WrightAvia]

right or wrong????

People that portend Bernoulli's Principle is the only factor regarding lift, haven't flown next to a caravan.

Whether the caravan is loaded or not, the plane has a nose high deck angle in level flight and the cord line of the wing is not perpendicular to the horizon.

Also, if you look at a caravan from the front, you'll see that there is quite a bit of "cant". The engine is canted to the right and it is canted down. One reason is to offset left turning tendancies, the other is to align the center of thrust, with relative wind.

Lift opposes weight, no matter how it is derived...even when skipping a stone on a pond of water.

 

[A Squared]

Principle of Equidistant Transit Time, which is the foundation of Bernoulli's Theory, how could a symmetrical airfoil develop lift?

This again. Seems everyone wants to "prove" that Bernoulli is wrong because of the equal transit time thingy. Seems they believe that if the equal transit time is wrong, than Bernoulli *must* be wrong. The thing is, you won't find the equal transit time concept in any serious aerodynamic text. I've read a bunch. No serious aerodynamicist believes the air on top has to met the air on the bottom at the trailing edge. The only place I can find it presented is in the FAA deplorable "flight training handbook" If you're basing your arguments about science on that, well, that's a little foolish.

Have you ever studied photos of an air foil in a wind tunnel with pulsed smoke streams? It's very interesting, because it allows you to compare air flow velocities. What it shows clearly is that the air over the top of the airfoil flows faster than the air below the wing. and it's not just because of the longer path (another false premise) If we take 2 adjacent air molecules that part company at the stagnation point, one going over the wing and one going under the wing, it's true, they do *not* meet at the training edge, far from it, the one over the top actually *beats* the one going under the wing.

So, if your "debunking" of Bernoulli hinges on the equal transit fallacy, then think again. Bernoulli's principle isn't based on the idea of equal transit times.

As far as the symmetrical airfoil, again, you have to look at the larger picture of the total airflow around the airfoil. A symmetrical airfoil generates no lift with no angle of attack. As soon an angle of attack is introduced, the stagnation point moves down and "under" the wing. That air flowing "over" the wing is indeed traveling a longer path than the air moving "under" and it's doing it much faster than the air over the top. The overall flow pattern looks almost identical to the flow pattern around a cambered wing.

I can only shake my head at those who propose "Newton" as an alternative to "Bernoulli" as if somehow Newton's laws of motion were antithetical to Bernoulli's principal. Folks, they are both intertwines pieces of the picture. It's sort of like having 2 factions arguing over what makes an automobile move, one group says that it's combustion, one group says it's friction, neither grasp the whole picture.


Yes, folks, Newton's third law is involved (so is the second for that matter) in keeping an airplane in the air. Yes, air is displaced downward, it has to be. Newton's laws demand it, and they haven't been repealed; modified a little by Einstein, but not repealed. Whatever force keeps an airplane in the air, it can only happen by accelerating air downward, there's no other way. No one with any understanding of the situation suggests that an airplane *doesn't* displace air downward as it flies. That's a given, it doesn't negate Bernoulli's principle. There's a tendency for people to view air like a stream of water shooting out of a hose and bouncing off a piece of plywood. The thing is, airflow is not some discrete stream. You can't just think of it in terms of only the air that is touching the airfoil. An airfoil affects (and is affected by) the air for a relatively large distance above, below, in front of and behind it. Most of us grasp the concept that the what happens to air that is up to a half wingspan or more from the airfoil, drastically affects it behavior (ground effect) yet, we can't look beyond the air that we visualize as beating against the wing like a garden hose. The lift is created by the airflow of all air in within many feet of the airfoil. Bernoulii's principle, Coanda Effect , and Newton's laws all are part of what's happening.

Arguments which attempt to negate Bernoulli with Newton are so simplistic that they are absurd, and display a lack of understanding of the fundamental physical concepts

 

[fr0g]

If you only support the newtonian "action-reaction" principle of lift, then how do you explain the exponential loss of lift once a wing reaches the critical AOA. The bottom of the wing is still displacing air downwards just as it was before, and yet lift drops off dramatically - according to simple action-reaction calculations, a wing should produce most lift when it is at a 45 degree angle, since that is when the most air is being forced downwards. The AOA-lift curve would also follow a sine function, since when the angle is 0, there would be no lift, max lift at 45, and no lift again at 90. We all know that this is not how it really works. The top of the wing obviously plays a large role in creating downward moving air. A area of negative pressure above the wing (scientifically proven to exist) will not only exert a force on the wing in the upward direction, but will also exert the equal and opposite force on the air above it in a downward direction! so all of a sudden, there is newton again, creating downward moving air. However, it is only because of this low pressure area that it is created, and therefore both newtonian and bernoulli laws are really required on top of the wing. The lift from the bottom of the wing may in fact be purely newtonian, but it is a small percentage compared to the top of the wing - again because once the top airflow separates at the critical AOA, the majority of lift is lost.
LOL.. just my personal view, so take it with a grain of salt

 

[UnstableAviator]

http://www.grc.nasa.gov/WWW/K-12/airplane/foil2.html


That's a link to NASA'a "foil" sim. Its a pretty cool program for a dumb guy like me to visualize the airflow around a wing.

 

[dmspilot00]

I've always thought of it this way...

First of all, the "low pressure" above the wing does not "exert a force" on it. There's no such thing as "sucking." It is the higher pressure below the wing that exerts an upward force. I think of it as the high pressure coming both from the shape of the wing (Bernoulli) and the downward-deflected air (Newton).

 

[msw]

Lift: Rohr's Theory of Circulation?

I seem to recall from a college class long long ago, a discussion of "Rohr's Theory of Circulation" as it pertained to how an airfoil made lift. Have any of you smart guys ever heard of this? And maybe care to expound on it?

 

[rptrain]

A Squared is the man! Huge post of the day!

 

[Typhoon1244]

So let me get this straight.

Contrails are actually plumes of experimental gasses.

And "Bernoullian lift" doesn't exist.

And blacks in America were better off in the Fifties.



Wow, I'm learning some cool stuff from Flightinfo this season!

 

[SkyWestCRJPilot]

If it were all Newton then what's the point of flaps, slats, and vortex generators? What's the point of slotted flaps that allow high energy air below the wing to energize the air above the wing maintaining the boundary the layer and lift. According to Newton's 3rd law all that air below the wing coming above the wing in those sloted flaps is wasted lift. What's the point of spoilers which "spoil" the airflow above the wing thus reducing lift. If Newton's 3rd law were the only factor invovled all airplanes would have symetrical airfoils but that's not the case. Thousands of aeronatical engineers over the years must all be mistaken about their aeronautics then. Better let the engineers at Boeing know before they put a cambered supercritical airfoil on the 7E7. Like others have said, it's not so simple as just Newton or just Bernoulli.

 

[HerrJeremy]

Have you ever studied photos of an air foil in a wind tunnel with pulsed smoke streams? It's very interesting, because it allows you to compare air flow velocities. What it shows clearly is that the air over the top of the airfoil flows faster than the air below the wing. and it's not just because of the longer path (another false premise) If we take 2 adjacent air molecules that part company at the stagnation point, one going over the wing and one going under the wing, it's true, they do *not* meet at the training edge, far from it, the one over the top actually *beats* the one going under the wing.

So, if your "debunking" of Bernoulli hinges on the equal transit fallacy, then think again. Bernoulli's principle isn't based on the idea of equal transit times.

Doesn't the existence of wingtip vorticies disprove the "equal transit times" theory anyway?

 

[BeechSlapped]

Very good post by A Squared.

To add a bit more: I think we all agree that it is an undisputed fact that there is a difference in pressure between the upper and lower surfaces of an airfoil. This is the ACTUAL mechanical cause of lift because there are only TWO ways that a fluid can exert a force on a body: through pressure and shear stress. On smaller aircraft shear stress is low enough that it can be neglected. So it is that pressure difference, and that alone, which causes lift.


The next question is why is there a pressure difference? Well, this is due to the variances in flow velocities along each and every part of the airfoil surface. Here is where it becomes complicated and here lies the souce of the confusion. The problem is that everyone has been trying to give a simple explaination as to why the flow speeds up and slows down along the airfoil. This requires use of formulas known as the Navier-Stokes equations which are far beyond the scope of this board. These equations use conservation of energy to solve for the pressure, velocity, and density of the flow at a given point. They are nearly impossible to solve unless one uses a Computational Fluid Dynamics computer program. This is how it is done today. How did they do it long ago? They basically stuck an airfoil section in a wind tunnel and measured the lift and drag empirically.

Anyway, in conclusion, all you Newton desciples out there need to realize that the downwash produced by a wing is simply a BYPRODUCT of lift, it is not the reason FOR it. However, using this downwash will give a fairly accurate result if you want to calculate lift, just remember that the physical mechanism is pressure differential.

Finally, let me throw this out for thought and discussion. The physical reason for thrust from a jet engine is the sum of the internal pressures and NOT the high-velocity exhaust from the nozzle. This is merely a byproduct of thrust.

BeechSlapped

 

[mar]

The Curve Ball

For MSW--

Believe it or not I'm still living out of boxes from my last move, thus all of my aerodynamic texts are unavailable.

But there is a theory of lift based on circulation and explains why the curve ball curves and the tennis/golf ball slices.

Basically the threads/fuzz/dimples all energize the boundary layer and create (redistribute) the low pressure areas.

There were experimental applications to aircraft (rollers in the wings) to help energize the boundary layer but I think they found vortex generators to be cheaper, lighter, more reliable and easier to work on.

As for Bernoulli vs. Newton....well, it's like my pitch vs. power answer: It Depends.

Or maybe it's just all PFM.

 

[VivaZapata]

So let me get this straight.

Contrails are actually plumes of experimental gasses.

And "Bernoullian lift" doesn't exist.

And blacks in America were better off in the Fifties.

Yes Typhoon1244, Welcome to the "*Bizarro World".


*Seinfeld

 

[A Squared]

frog,

Yes, that's an excellent analysis. I've used the same argument in previous discussions. The trouble is the "bernoulii is a fraud" folks generally lack the bacic high school math skills to even grasp what you are saying, let alone understand that it neatly trumps their position.

Herr Jeremy

>>>>Doesn't the existence of wingtip vorticies disprove the "equal transit times" theory anyway?

Yeah, sure, if it needs disproving. That's one of many things that disproves it. No-one actually beleives in it. For some bizzare reason though, many of the arguments presented by the "bernoulii is a fraud" crowd center are based somehow on the equal transit time concept, as if it was relevant.
The only place you will find the "equal transit time" concept is in children's science book written by someone who doesn't understand lift themselves, and in the FAA Flight training Handbook (sad commentatry on the FAA) A few years back, there was a guy who was making a lot of "bernoulii is a fraud" noises, he even managed to get his pap published in an aviation magazine or two. His argument was this: The air going over the top has to have the same transit time as the air under the bottom, so if we measure the length of the top surface and the bottom surface, we can calculate the difference in velocity between top and bottom. Then if we plug those numbers into Bernoulii's equations, we can show that it can't possibly generate enough lift to keep the airplane up. He was right, the numbers didn't work out. They wouldn't, couldn't, his basic premise for his entire argument was dependent on the "equal transit time" fallacy. That has been known not to be true for at least as long as folks have been putting pulsed smoke streams in wind tunnels (1920's maybe?) If you didn't catch that his numbers were based on the "equal transit time" fallacy, they sounded good. If you did notice this, it was clear that his numbers were completely meaningless, just a bunch of computational diarrhea.

 

[Typhoon1244]

...a bunch of computational diarrhea.

I'm going to incorporate that into my everyday vocabulary!

 

[HerrJeremy]

on the topic of wing aerodynamics.....I was noticing a RJ on the ramp a few weeks ago(later found out it was a Canadair). On the bottom of each wing there were 5 fin-like protrusions. Are they there to help the wings' efficiency, or are they just housing for some mechanism of the wing?

 

[Typhoon1244]

on the topic of wing aerodynamics.....I was noticing a RJ on the ramp a few weeks ago(later found out it was a Canadair). On the bottom of each wing there were 5 fin-like protrusions.

Those are aerodynamic fairings for the flap hinges and actuators. They smooth the airflow over what would otherwise be a chunky, irregular object sticking out of the bottom of the wing.

You see the same thing on most modern jetliners. (The fairings on the 747 would make decent boats...they're huge!)

 

[GeorgeTG]

from the "horse's" mouth

What is Lift

cheers
George