okay let the debate begin...newton or bernoulli

[VNugget]

I beg to differ. Since a wing is curved at the top, the air going over the top must travel a greater distance than the air at the bottom. I base this on the shortest distance between two points is a straight line. I may have missed the class when they changed that principle. The diagram shown is nice, but what it can't show you is that the air going above the wing is moving faster than the air at the bottom of the wing.

The reference to Bernouli is that he said that a high pressure and a low pressure are created from the difference in the speed of the air going over and under the wing.

I take it you didn't care to scroll down a bit and take a look at the PHOTO (not diagram) that shows pulsed smoke streamlines over an airfoil, with the pulses representing "locations" (molecules, if you will) in the air that started out at the same time at the same place along the flow axis. (I guess it's my fault I didn't make it exactly clear which figure I was talking about.)

Clearly, the the parcels do not reoconnect at the same place. The distance travelled doesn't mean jack with regard to speed, unless you introduce a requirement involving time.

And regarding the other quote, he didn't say anything about speed.

 

[Mr. Cole]

We had a similar discussion recently that I participated in, along with VNugget and some others. I agreed with Iflyifrvfr's explanation, up until the equal transit theory. Other than that I think the jist is right, and Avbug posted a very thorough explanation. One night while reading a post from Vnugget asking the question of what causes the air to accelerate, the thought of the wing as the bottom half of a Venturi, or as Avbug called it an inefficient Venturi, seemed obvious all of a sudden. Of course I hadn't discovered anything new, but figuring it out for myself was a satisfying experience, thanks to the discussion happening at the time.

Regarding the question of downwash and its role in lift, I also agree with Avbug in that it isn't really one or the other. In fact the greater the downwash, the greater the induced drag. If I'm not mistaken the downwash is caused by two related effects. The first is the wingtip vortices, which impart a downward velocity component to the free airstream passing the wing. If we had an infinitely long wing we could eliminate the wingtip vortices. But there is also another type of vortex, that created by the air trying to move from the higher pressure area to the lower pressure area not around the tips, but over the leading edge. When the sum of the vortices are added to the free airstream, the result is a relative wind of the wing sections that is inclined to the relative wind of the free airstream.

The lift vector of each section is perpendicular to the local relative wind, but the local relative wind of each section is inclined to the relative wind of the total wing, as measured some distance ahead of the wing. Since we measure lift and induced drag not in reference to the local wing sections but the entire wing, the induced angle between wing relative wind and section relative wind causes the section lift to be resolved into a wing lift component parallel to the overall relative wind, but also a induced drag component perpendicular to it.

If we could create a wing that was infinitely large and eliminate the vortices, then there would be no vertical velocity components added to the free stream, the relative wind of the sections and the overall wing would be the same, and induced drag would be zero. In a way this is what ground effect represents. Because of the ground plane the vertical velocity components due to the vortices are reduced, the wing section relative wind approaches that of the overall wing, reducing the angle between the two, and the lift vector of the section is closer to perpendicular as measured by the wing AOA, and induced drag is reduced.

Dave

 

[Jeff Helgeson]

Marcone

Neither.

Gutenberg is the correct answer.

Because, without the printing press, we would not have pilot certificates, AFMs, FARs, AIMs, company manuals, or the single most important thing that makes flight possible.


Money, and lots of it!


See? If they ever go to a paperless cockpit and a cashless society, look out! All aircraft will instantly fall directly to earth.

Along these lines of thinking, we would never get off the ground if it weren't for Marcone. Without the radio, we would not have our air traffic system.

 

[Lead Sled]

okay aside from possible spelling errors, which contributes to lift more... newtons laws or bernoulli's theory?

There are some other viable theories as well...

The most promising one is known as Passenger Fear Induced Lift or PFIL. The lift comes from all the pax grabbing onto their armrests and pulling. It is easy to see. The engines and take off roll are simply there to put enough fear into the pax. On takeoff, as the noise and vibration picks up, they all grab hold of their armrests and off you go.

On approach, since they're all relaxed after inflight meals and a drink or two, you need flaps and perhaps even spoilers to make sure you get enough lift. If you find yourself below the glideslope, you might even have to add a burst of power to frighten them enough!

That's why flying cargo is so much more dangerous. (Note the cargo 747 accident last week.) It really takes quite a pilot to scare a cabin full of mail!

That also explaines how helicopters are able to takeoff vertically - those things scare the heck out of me and I'm fearless.

However, now that I think about it, I see the flaw in that theory as well...

It's not Newton or Bernoulli or PFIL or any other theory; it's $$$ MONEY $$$ that makes airplanes fly.

'Sled

 

[nosehair]

Often as not, ignorant flight instructors try to explain lift by stipulating that two air molecules begin the journey around the wing at the leading edge. They postulate that the two molecules must reach the trailing edge at the same time. Therefore, they reason, the molecule that must move the farthest in that time must go the fastest. The faster the airflow, the lower the pressure and bingo! We have lift.

The problem is that the two molecules will never meet again, and no requirement exists for either one to arrive at the trailing edge, or any other point in space or time, at the same moment. The neophyte instructor's theory falls flat.

Yes, the distance along the curvature of the upper camber of the wing is greater (on a non symmetrical airfoil), but it is irrelevant with respect to the creation of lift, and downwash. Upper camber exists to form half of an inefficient venturi in concert with the free airstream, and it is this venturi effect which causes an increase in velocity over the wing. It is the acceleration of this airflow downward, along with acceleration (a change in speed and direction) imparted by the wing and angle of attack, that accounts for lift.

Yes, avbug, us ignorant, neophyte instructors have been using that "simplistic" description since...oh, the beginning of "modern aviation"...or, the introduction of nosewheels on training airplanes.
First of all, that's the way it is taught in FAA publications, secondly, it is a simple,easy-to-understand description of bournelli's theory. Sometimes, it's better to over-simplify to get the concept, then go back and clean-up any important details that were not exactly accurate.
I think the problem is that most instructors don't follow-up a basic conceptual description with a more accurate detailed explanation of aerodynamics.

Your description has not taught me anything. You say "it is this venturi effect which causes an increase in velocity..." You are making a statement of fact, not an explanation in terms that the student understands.

You have to explain "the venturi effect". Isn't that like all the molecules of air (or water) have to speed up to get through the restricted space?..and don't they all end up more or less together at the other end of the venturi?

So, there's a lot more to this story than we get into with a beginning student, usually. And that is usually enough. If you know the aerodynamics in the PHAK, that is enough for pilot certification. Even though it may not be precisely accurate.

It is also healthy for flight instructors to discuss, among themselves and interested students, the finer points of the lift theory, but healthy discussions don't "put down" persons who either don't know, or choose to simplify.

Remember: K.I.S.S = Keep It Simple Stupid

 

[Dizel8]

Actually, the fear concept holds some truth, although I have recently learned, that an even bigger factor might be noise. The below, from a recent flight test, does some what validate said point:

"New design - BN2-XL

Undaunted by technical realities, the design team at Pilatus Britten - Norman has announced plans for the BN2-XL, promising more noise, reduced payload, a lower cruise speed, and increased pilot workload.

We spoke to Mr. Fred Gribble, former British Rail boilermaker, and now Chief Project Engineer. Fred was responsible for developing many original and creative design flaws in the service of his former employer, and will be incorporating these in the new BN2-XL technology under a licensing agreement. Fred reassured BN-2 pilots, however, that all fundamental design flaws of the original model had been retained. Further good news is that the XL version is available as a retrofit.

Among the new measures is that of locking the ailerons in the central position, following airborne and simulator tests which showed that whilst pilots of average strength were able to achieve up to 30 degrees of control wheel deflection, this produced no appreciable variation in the net flight of the aircraft. Thus the removal of costly and unnecessary linkages has been possible, and the rudder has been nominated as the primary directional control. In keeping with this new philosophy, but to retain commonality for crews transitioning to the XL, additional resistance to foot pressure has been built in to the rudder pedals to prevent over-controlling in gusty conditions (defined as those in which wind velocity exceeds 3 knots). An outstanding feature of Islander technology has always been the adaptation of the O-540 engine, which mounted in any other aircraft in the free world (except the Trislander) is known for its low vibration levels. The Islander adaptations cause it to shake and batter the airframe, gradually crystallise the main spar, desynchronise the accompanying engine, and simulate the sound of fifty skeletons fornicating in an aluminium dustbin.

PBN will not disclose the technology they applied in preserving this effect in the XL but Mr. Gribble assures us it will be perpetrated in later models and sees it as a strong selling point. "After all, the Concorde makes a lot of noise" he said, "and look how fast that goes." However design documents clandestinely recovered from the PBN shredder have solved a question that has puzzled aerodynamicists and pilots for many years, disclosing that it is actually noise which causes the BN2 to fly. The vibration set up by the engines, and amplified by the airframe, in turn causes the air molecules above the wing to oscillate at atomic frequency, reducing their density and creating lift. This can be demonstrated by sudden closure of the throttles, which causes the aircraft to fall from the sky. As a result, lift is proportional to noise, rather than speed, explaining amongst other things the aircraft's remarkable takeoff performance. In the driver's cab (as Gribble describes it) ergonomic measures will ensure that long-term PBN pilots' deafness does not cause in-flight dozing. Orthopaedic surgeons have designed a cockpit layout and seat to maximise backache, en-route insomnia, chronic irritability and terminal (post-flight) lethargy.

Redesigned "bullworker" elastic aileron cables, now disconnected from the control surfaces, increase pilot workload and fitness. Special noise retention cabin lining is an innovation on the XL, and it is hoped in later models to develop cabin noise to a level which will enable pilots to relate ear-pain directly to engine power, eliminating the need for engine instruments altogether.

We were offered an opportunity to fly the XL at Britten-Norman's development facility, adjacent to the BritRail tearooms at Little Chortling. (The flight was originally to have been conducted at the Pilatus plant but aircraft of BN design are now prohibited from operating in Swiss airspace during avalanche season). For our mission profile, the XL was loaded with coal for a standard 100 nm trip with BritRail reserves, carrying one pilot and nine passengers to maximise discomfort. Passenger loading is unchanged, the normal under-wing protrusions inflicting serious lacerations on 71% of boarding passengers, and there was the usual confusion in selecting a door appropriate to the allocated seat. The facility for the clothing of embarking passengers to remove oil slicks from engine cowls during loading has been thoughtfully retained.

Start-up is standard, and taxiing, as in the BN2 is accomplished by brute force. Takeoff calculations called for a 250-decibel power setting, and the rotation force for the (neutral) C of G was calculated at 180 ft/lbs of backpressure. Initial warning of an engine failure during takeoff is provided by a reduction in vibration of the flight instrument panel. Complete seizure of one engine is indicated by the momentary illusion that the engines have suddenly and inexplicably become synchronised. Otherwise, identification of the failed engine is achieved by comparing the vibration levels of the windows on either side of the cabin. (Relative passenger pallor has been found to be an unreliable guide on many BN2 routes because of ethnic consideration).

Shortly after takeoff the XL's chief test pilot, Capt. Mike "Muscles" Mulligan demonstrated the extent to whch modern aeronautical design has left the BN2 untouched; he simulated pilot incapacitation by slumping forward onto the control column, simultaneously applying full right rudder and bleeding from the ears. The XL, like its predecessor, demonstrated total control rigidity and continued undisturbed. Power was then reduced to 249 decibels for cruise, and we carried out some comparisons of actual flight performance with graph predictions. At 5000 ft and ISA, we achieved a vibration amplitude of 500 CPS and 240 decibels, for a fuel flow of 210 lb/hr, making the BN2-XL the most efficient converter of fuel to noise after the Titan rocket. Exploring the Constant noise/Variable noise concepts, we found that in a VNE dive, vibration reached its design maximum at 1000 CPS, at which point the limiting factor is the emulsification of human tissue. The catatonic condition of long-term BN2 pilots is attributed to this syndrome, which commences in the cerebral cortex and spreads outwards. We asked Capt. Mulligan what he considered the outstanding features of the XL. He cupped his hand behind his ear and shouted "WHAT?" We returned to Britten-Norman convinced that the XL model retains the marque's most memorable features, whilst showing some significant and worthwhile regressions. PBN are not, however, resting on their laurels. Plans are already advanced for the Trislander XL and noise tunnel testing has commenced. The basis of preliminary design and performance specifications is that lift increases as the square of the noise, and as the principle of acoustic lift is further developed, a later five-engined vertical take-off model is also a possibility".


We now return to your regularily scheduled programming!

 

[VNugget]

Yes, avbug, us ignorant, neophyte instructors have been using that "simplistic" description since...oh, the beginning of "modern aviation"...or, the introduction of nosewheels on training airplanes.
First of all, that's the way it is taught in FAA publications, secondly, it is a simple,easy-to-understand description of bournelli's theory. Sometimes, it's better to over-simplify to get the concept, then go back and clean-up any important details that were not exactly accurate.
I think the problem is that most instructors don't follow-up a basic conceptual description with a more accurate detailed explanation of aerodynamics.

Your description has not taught me anything. You say "it is this venturi effect which causes an increase in velocity..." You are making a statement of fact, not an explanation in terms that the student understands.

You have to explain "the venturi effect". Isn't that like all the molecules of air (or water) have to speed up to get through the restricted space?..and don't they all end up more or less together at the other end of the venturi?

So, there's a lot more to this story than we get into with a beginning student, usually. And that is usually enough. If you know the aerodynamics in the PHAK, that is enough for pilot certification. Even though it may not be precisely accurate.

It is also healthy for flight instructors to discuss, among themselves and interested students, the finer points of the lift theory, but healthy discussions don't "put down" persons who either don't know, or choose to simplify.

Remember: K.I.S.S = Keep It Simple Stupid

There's a difference between imprecise and incorrect. If the reason for the change in speed of the flow is too complex for the student, then don't give it to him; just don't give him something that's completely wrong (as has been stated many times, with proff, already in this thread), like the "recombination" junk.

Anyone who puts that forth is simply ignorant of the facts. If you consider that a "put down," that's your own issue.

 

[Lead Sled]

I got a little time in those beasties about 30 years ago. You have now cleared up several questions that I have had all of those years. It also elegantly explains why the "Trilander" performed the way it did and why the Turbine Islander has done what it has done as well. Let those "aeronautintellectuals" out there debate Bernoulli and Newton all they want. What do they know? Someday, perhaps they too will come to their senses and realize that no one really knows the definative answer.

'Sled

 

[avbug]

Yes, avbug, us ignorant, neophyte instructors have been using that "simplistic" description since...oh, the beginning of "modern aviation"...or, the introduction of nosewheels on training airplanes.

Don't get upset merely because your daddy never removed the training wheels. When I was a "neophyte" instructor, I had long since been flying without the training wheel, and I never taught the antiquated and false concept that two molecules beginning at the leading edge of a wing have some innate need to spend time together at the trailing edge. I taught correct aerodynamics, and never had a student fail to grasp these basic concepts. Merely because you never understood or taught correctly, don't get upset. That's your fault.

First of all, that's the way it is taught in FAA publications, secondly, it is a simple,easy-to-understand description of bournelli's theory. Sometimes, it's better to over-simplify to get the concept, then go back and clean-up any important details that were not exactly accurate.

Because something is written somewhere, you should teach it. I get it. The FAA publications also cited ground effect for years as a "cushion of air" beneath a wing. Doubtless you taught that little nugget of misinformation, too...completely oblivious to the fact that it has nothing to do with ground effect. You probably never taught that ground effect is diminished upwash, and therefore a reduction in induced drag, due to the inability of the airflow in front of the wing to produce a local increase in angle of attack...which of course comes from reduced pressure above the wing owing to...you got it! Bernoulli's principle. More to the point, owing to downwash; air moves down in back of the wing, it moves up in front of the wing, local angle of attack is greater than the angle between the aerodynamic chord line of the wing and the free airstream. Reduce this local angle of attack, reduce induced drag, and wallah, ground effect.

But that wasn't in the FAA publication...which was wrong...and you taught it wrong, doubtless. And justify it on those grounds. Therein lies the difference between an instructor, who administers a syllabus, and a teacher, who teaches a student. You are clearly an instructor.

You truly believe it's right to teach incorrect information, then go back and correct the lies and mistruths you've told? What an increadbily innovative, though seriously misguided concept!

Your description has not taught me anything. You say "it is this venturi effect which causes an increase in velocity..." You are making a statement of fact, not an explanation in terms that the student understands.

I wasn't aware that this was teach nosehair day. How obtuse of me. But then, you're the instructor, albeit the "neophyte" instructor? I suspect from your rant that nobody teaches you anything, do they? After all, you're the instructor, you're in charge, and you have FAA publications to parrot.

You have to explain "the venturi effect". Isn't that like all the molecules of air (or water) have to speed up to get through the restricted space?..and don't they all end up more or less together at the other end of the venturi?

No, they don't. Nor should they...where molecules end up isn't important to the discussion of why the pressure drops, and that's the point. A venturi is something a student understands; every student has placed his or her finger over a garden hose, seen the effect in the velocity of the water coming out the end, understands that as a relatively incompressible fluid is pushed through a restriction, it goes faster. Every student has seen a football player run and tackle a dummy, or another player. Every student understands that a charging linebacker has a lot more energy in front of the player, than to the side. Every student may be shown this by having the student push hard against a wall, and then be toppled by finger tip pressure from the side.

Every student can then easily see that a velocity increase moving through the "venturi" above the wing, the restriction that is similiar to a finger over a garden hose, is "increased" in one direction...toward the trailing edge, but decreased in every other direction. The student can be shown that a certain amount of energy is available; as it's value increases in one direction...it's pressure value drops. The student can see this in a balloon that deflates and flies across the room, as it's drawn on a chalk board, with little explaination. I've never seen a student that couldn't grasp these things, though apparently you feel it's beyond the capabilities of the student. Perhaps you just instruct really stupid students? I find that hard to believe.

So, there's a lot more to this story than we get into with a beginning student, usually. And that is usually enough. If you know the aerodynamics in the PHAK, that is enough for pilot certification. Even though it may not be precisely accurate.

I find it hard to believe you just said that. Apparently you're one of those who feels that minimum standards are the rule...meet the minimum and we're all golden. I have never taught by the practical test standards because I consider them minimum standards. I teach to proficiency, which is not found in the practical test standards. A proficient student will far exceed any toleances found in the PTS.

Likewise, who would be foolish enough to suggest that "it's enough?" What a mediocre, lazy attitude.

Even tough it may not be accurate? Wow. If that statement doesn't define mediocracy, then what does? You teach just enough, even though it may not be accurate. Heaven forbid anyone I know falls into your clutches. A craftsman strives for something higher. Even a "neophyte."

It is also healthy for flight instructors to discuss, among themselves and interested students, the finer points of the lift theory, but healthy discussions don't "put down" persons who either don't know, or choose to simplify.

Put down? I stipulated that a particular theory often espoused is done so by the ignorant...and it is. Either you wilfully teach incorrect information, or you do so ignorantly. There is no inbetween, no middle ground. Either one is lying, or simply spreading misinformation in ignorance. So which is it? Your call.

Remember: K.I.S.S = Keep It Simple Stupid

Perhaps you ought to make that KISSAI for keep it simple stupid and inaccurate. You really feel your students are stupid? I'm betting you are or were one of those "neophyte" instructors who feels that instructing is "time building," and "paying one's dues." That students are a means to and end, and that there are few punishments greater than being forced to ride around with those simple students while they pay you to put your certificate on the line for their simple, stupid, butts? Close? No cigar?

That's sad.

 

[uwochris]

Avbug,
Please check your pm's.

Thanks,
Chris.