Tell me what makes it fly

[addict]

To any one who can help me with this technical question about flight itself.
Bernouille's principle states that the faster the velocity of a liquid, the lower is the static pressure that it creates. This is the principle upon which the secret to flight was unlocked by humankind.

Therefore, because air flows faster over the cambered top of a wing, the pressure above is less than that beneath, thus creating lift.

My question is this: If a surface is moving into the wind has a cross section, in other words, a surface being hit by the wind, such as the cambered side of the wing, wouldn't that slow the velocity of the air down, making the speed slower than on the bottom, where no resistance is caused because it has no surface pointing into the wind?

Anyone familiar with the principles of flight, I would love to get your input.

[Andy W]

You can't slow "the air", you can only make localized adjustments in it's path around the object in question and set in place disturbances that can temporarily alter the momentum of the molecules you affect and those that follow behind them. For every action, ...

Think about what happens (to both your hand and the water around it) when you move your hand through water, especially angled relative to the direction of that motion..
..a

[raptor22]

This is a very volatile subject. It usually degrades into insults and a few people hating others and someone leaving ezone whenever someone brings it up. Oh, well. I hope it isn't someone I liked.

First of all, the principal is ANY incompressible fluid, not just liquid. Gasses act incompressible when flying.

2nd, it is not only the pressure difference that causes lift. YOu also have to look at the newtonian theory. It states that, since every action has an equal and opposite reaction, and since the wing accelerates air downward (this is important for both theories), the opposite reaction is lift.

Both theories precisely describe the lift in an accurate way, so it is generally accepted that it is both equally and you can use whichever one suits your convenience. Bournoulli is easier to use, but does not work with lots of turbulent flow or laminar seperation bubbles. However, its ease of use has made it popular.

As for your question, the air is actually accelerated ont he bottom as well. Just not nearly as much.

--Alex

[Viper Pilot]

How about a little quiz???

Here (http://observe.arc.nasa.gov/nasa/exhibits/planes/planes_quiz.html)

Then some aeronautic lessons (http://observe.arc.nasa.gov/nasa/core.shtml.html)

[Fantam]

How come my flat plate foam plane flies so well without an airfoil. It flies straight and level normal side up and upside down. No noticable angle of attack?

[raptor22]

That quiz bites. IT uses the wrong idea for lift (equal transit time), and the correct answer to the ball should be the "stays still" because he did not say in what conditions you were using. Therefore, scientifically speaking, you should assume it is in a "perfect" theoretical environmet; a vacuum and zero weight for anything.

--Alex

[Viper Pilot]

Oh, Oh . . . . here we go!!!

:) :)

[Viper Pilot]

That quiz bites. IT uses the wrong idea for lift (equal transit time), and the correct answer to the ball should be the "stays still" because he did not say in what conditions you were using. Therefore, scientifically speaking, you should assume it is in a "perfect" theoretical environmet; a vacuum and zero weight for anything.

--Alex

you failed the quiz, didn't you!!!

:D :D :D

[raptor22]

How come my flat plate foam plane flies so well without an airfoil. It flies straight and level normal side up and upside down. No noticable angle of attack?

IT has a little angle of attack to produce lift; just not noticeable. Camber is not required for lift, so you basically have a symmetrical airfoil.

Low RN airfoils (like for parkfliers) when properly designed tend to be very thin with little camber. THerefore, as the plane gets smaller and smaller the flat plate is more and more like the ideal airfoil and performs similarly to one.

--Alex

[raptor22]

you failed the quiz, didn't you!!!

:D :D :D

no. I just got the question about lift wrong (and the ball) because the answers are wrong.

--Alex

[raptor22]

Okay, so I gues since there were only 5 questions that gives me a D! not a failure!

THe answers were still wrong, though. If they were right, I would bave gotten an "A"!

--Alex

[raptor22]

OMG! that was a NASA site! I'm disappointed in NASA

Go to some other nasa sites, and they will specifically prove the "lift" question wrong as a "common misconception" of lift!

--Alex

[slipstick]

So what you're saying is that even NASA can't be certain how lift works !

I feel a lot happier about my own level of ignorance now ;).

Steve

[addict]

Thank you to all who answered. I am however, now not only still completely ignorant but also confused. Hate to sound like I'm whining, and I'm sorry if it seems that way, but this question has lingered in me for a long time now.

Andy W. Thanks for your answer, and the example of your hand in water. However, it seems to me that in your example, it is the angle of your hand's attack that causes the lift. Does this mean that the incidence of your wing is what causes lift? If so, then the shape of the top of the wing should not make any difference, because you could accomplish the same result with a flat bottomed wing, regardless of the shape of its top. Surely there is more to it than that. Otherwise, I would not be hearing so much about the shape of the top being cambered. Am I misunderstanding you? Please say so if I am.


Raptor 22 and Viper Pilot: Indulge this layman while I try to learn from the masters. If there are multiple theories which can be used to accurately explain flight, with Bernouille's being the simpler one, then bear with me while I try to learn them one at a time.

Raptor 22: My question was actually provoked from its dormant state in my mind, by a visit to your beautiful San Diego this past Monday. I visited the S.D Aerospace Museum, where Bernouille's theory is used to explain the essential element of lift (flight). The museum explains it as I did: that the cambered surface of the top of the wing, creates a higher velocity of airflow over it than the flat bottom. Since Bernouille's principle states that the higher the velocity, the lower the static pressure, this means more static pressure on the bottom of the wing, thus creating lift.

Assuming that we all agree that my reiteration of the principle is essentially correct( as simplistic as it may be), then my question (also simplistic) still stands: with a zero angle of attack(0 incidence) on the wing, how does a cambered surface allow faster airflow over its surface than a perfectly flat one? After all, the cambered surface has some area facing into the wind, while the bottom does not. Wouldn't that create more drag on the top, and cause more pressure on the top than the bottom?

Viper Pilot: thank you for your participation. Thanks also for pointing me to the quiz, but I still know too little about the subject to actually "get it" and be quizzed on it. If you disagree with Raptor then maybe you could answer my question for me: how does a surface with some area hitting the wind ( a cambered surface) have more wind velocity than an area with none (the flat bottom)? Of course this assumes that there is no incidence built into the wing.


Thank you all,
addict

[raptor22]

Does this mean that the incidence of your wing is what causes lift? If so, then the shape of the top of the wing should not make any difference, because you could accomplish the same result with a flat bottomed wing, regardless of the shape of its top.

I visited the S.D Aerospace Museum, where Bernouille's theory is used to explain the essential element of lift (flight).

Assuming that we all agree that my reiteration of the principle is essentially correct( as simplistic as it may be), then my question (also simplistic) still stands: with a zero angle of attack(0 incidence) on the wing, how does a cambered surface allow faster airflow over its surface than a perfectly flat one? After all, the cambered surface has some area facing into the wind, while the bottom does not. Wouldn't that create more drag on the top, and cause more pressure on the top than the bottom?



Thank you all,
addict

Answers are in the order of your paragraphs.

In a symmetrical wing, yes that is completely true that the AOA is important for lift. And it is true that the top needs not be curved for flight, but it can help the wing develo more lift by keeping it form stalling as easily.

Ahhh, that explains everything. I do not have the best opinion their explanations, as they are wrong. In fact this is the same explanation that is incorrectly used by the FAA because it is easy to explain. I had a hard time keeping my mouth shut in ground school because of it.

This (http://www.grc.nasa.gov/WWW/K-12/airplane/bernnew.html) is an excellent link to a nasa site that will give you a very good explanation of why what you have learned is wrong and the two theories of flight.

I am assunming you read the target for that link here. Basically, a cambered airfoil has a curved mean camber line. So, while a symmetrical airfoil cannot produce lift at zero AOA, the cambered airfoil can still produce a net turning of flow.

--Alex

[raptor22]

Since you seem to be intersted in camber, the effect of camber is to move the airfoil polar upwards. The polar is a graph of an airfoil's drag versus its lift produced.

A symmetrical airfoil (zero camber) has the same amount of drag produced at negative lift as positive lift, so it flies the same inverted as right side up. The minimum drag is produced at zero lift, so little to no camber is best for airplanes that will fly fast so do not need a very good lifting airfoil but rather low drag ati low AOA's.

When you add camber, like I said, it shifts the polar up. This means that the airfoil can produce more lift and has that same minimum drag at whatever amount of lifting capability is required. However, it will produce more drag at high speeds.

I have added a generic polar graph to show you what I mean. The gray line is a symmetric arifoil, while the blue line is the same airfoil with camber added.

[Ollie]

"Many ask the simple question 'what makes an airplane fly'? The answer one frequently gets is misleading and often just plain wrong. We hope that the answers provided here will clarify many misconceptions about lift and that you will adopt our explanation when explaining lift to others."

http://www.allstar.fiu.edu/aero/airflylvl3.htm

[Viper Pilot]

Welcome back, Ollie. I hope all is going well for you.

As usual, your input is always very informative. Thanks for the excellent link.

Viper

[Foamyfoam]

http://travel.howstuffworks.com/airplane.htm

[pimmer]

I suspect that the rediection of air against all airfoils is more aptly described by the coanda effect. That is, the tendency for a fluid to follow a surface. Check this link out

http://jnaudin.free.fr/html/coanda.htm

It gives a good illustration. Even a symmetrical airfoil will exhibit this effect at any angle of attack to the fluid stream. This is an even better link describing the effect in terms of wings:


http://humane.sourceforge.net/published/coanda_effect.html

[Ollie]

Lift is complex to understand.

Only one part of lift to understand:
"One answer is that the Bernoulli principle is easy to understand. There is nothing wrong with the Bernoulli principle, or with the statement that the air goes faster over the top of the wing. But, as the above discussion suggests, our understanding is not complete with this explanation."

Only second part of lift to understand:
"The lift of a wing is equal to the change in momentum of the air it is diverting down. Momentum is the product of mass and velocity. The lift of a wing is proportional to the amount of air diverted down times the downward velocity of that air."

The third part of lift is to understand:
"This tendency of fluids to follow a curved surface is known as the Coanda effect. From Newton’s first law we know that for the fluid to bend there must be a force acting on it. From Newton’s third law we know that the fluid must put an equal and opposite force on the object which caused the fluid to bend."

The fourth part of lift is to understand:
"bound vortex" or "circulation"

Lift is complex, not simple to understand!!!

Please, read all parts:
http://www.allstar.fiu.edu/aero/airflylvl3.htm

[johnsloper]

I used to teach sailing.

We didn't tell them much about bernoulli. We just said pull it in until it stops luffing then ease it out just a bit. For the advanced, we'd say trim in until the tell tales, 6" yarn every 5 feet, on the leach (T.E.) start flowing forward, then ease it out to where they flow 95% of the time backwards.

In the afternoon, as the wind would build, and they'd become overpowered, we'd have the students decrease their a.o.a. by easing the sail or steering closer to the wind. Eventually the boat is trimmed where the front third of the mainsail is luffing (bubbled towards high pressure side). Add a bit more wind and it's time to shorten sail (head to wind, ease mail halyard, shorten sail at root, retighten halyard, increase a.o.a. via heading or boom angle).

[flieslikeabeagle]

I'm not an aerodynamicist, but I think I have a contribution to make in simpler language than some of the very informative posts above, in regards to the question of why the top surface of an undercambered wing has anything to do with making lift.

Imagine putting a drop of water on the top of a tennis ball suspended in the air. As we all know from experience, the drop of water will run down the side of the ball, all the way to the lowest point on the underside of the ball, and only then drip off. Why didn't it drip off halfway down the side of the ball, at the point where the balls surface first curves away from the vertical? There are two reasons, first surface tension sticks the water to the ball, and second, a moving fluid has a tendency to stick to a surface. Its called the Coanda effect, IIRC.

The air flowing over the top surface of an undercambered wing does the same thing as the water droplet on the ball - it doesn't "drop off" the wing at its highest point, but rather follows the curve of the wings top surface all the way to the TE, and only then "falls off". This is assuming laminar flow, of course.

Due to the fact that the top surface of the wing has been curving downwards before it hits the TE, the air flowing over the top of the wing is yanked downwards too, and as it leaves the TE it is flowing downwards as well as backwards. The reaction force to that pushes upwards on the wing - Newton and his "every action has an equal and opposite reaction" law again.

So even in a cambered flat-plate wing, as used by some parkfliers and indoor planes, both surfaces of the wing contribute lift. It is my understanding that the top surface actually generates most of the lift, in fact.

-Flieslikeabeagle

[flying4fun]

Heres what i know about how flat plates fly and a bit about airfoils with camber:

The way flat plate airfoils fly is by varying the angle of attack (the angle of the wing compared to the oncoming airflow). If you could pause a 3d plane with a flat airfoil in mid-flight, and examine it very closely, you would see that the wing is not completely parallell to the incoming airflow. If it (the model) were in level flight, the wing would actually be at a positive (wing leading edge pointed towards the sky) angle of attack. Now, a "real" airfoil (one with camber) produces lift in a different way. An airfoil with camber produces lift by deflecting air downwards at the trailing edge, which produces an upward force on the wing (Newtons 3rd law). So airfoils with camber do not need to have a positive angle of attack to stay in level flight, only to climb (increasing the angle of attack on a wing in increasing amounts will keep on producing more lift and more drag. But at a certain angle of attack (different on every wing) the drag will overcome the lift produced, and the wing will stall).

Alex

(yes i cut and pasted from another thread. Why? Because im lazy!!!) :D

[raptor22]

Heres what i know about how flat plates fly and a bit about airfoils with camber:

The way flat plate airfoils fly is by varying the angle of attack (the angle of the wing compared to the oncoming airflow). If you could pause a 3d plane with a flat airfoil in mid-flight, and examine it very closely, you would see that the wing is not completely parallell to the incoming airflow. If it (the model) were in level flight, the wing would actually be at a positive (wing leading edge pointed towards the sky) angle of attack. Now, a "real" airfoil (one with camber) produces lift in a different way. An airfoil with camber produces lift by deflecting air downwards at the trailing edge, which produces an upward force on the wing (Newtons 3rd law). So airfoils with camber do not need to have a positive angle of attack to stay in level flight, only to climb (increasing the angle of attack on a wing in increasing amounts will keep on producing more lift and more drag. But at a certain angle of attack (different on every wing) the drag will overcome the lift produced, and the wing will stall).

Alex

(yes i cut and pasted from another thread. Why? Because im lazy!!!) :D


Only thing is, flat plate foils (or any thoer symmetrical) also produce lift in the same way as as a cambered foil. They just do it at a higher angle of attack. And cambered foils do not only need AOA at climb; they also use it at slower speeds and in turns. Also remember, a climbing airplane has the wings producing the same lift, but the power is pulling it upwards.

--Alex

[flying4fun]

Alex,
Thanks for clearing that up.

Alex

[Foamyfoam]

Only thing is, flat plate foils (or any thoer symmetrical) also produce lift in the same way as as a cambered foil. They just do it at a higher angle of attack. And cambered foils do not only need AOA at climb; they also use it at slower speeds and in turns. Also remember, a climbing airplane has the wings producing the same lift, but the power is pulling it upwards.

--Alex
I tend to disagree. On a flat plate, lift is generated almost entirely as a result of air hitting the under side of the plate, due to newtons laws. This 'lift' is different from the pressure differentials created on a cambered airfoil, or for that matter, a symmetrical one. This is largely due to the shape of the leading edge of the plate. On a proper airfoil, the leading edged is curved. This allows the stagation point to move down with incresing AOA up to the stall angle. On a flat plate, however, this is no the case, at any positive AOA, the stagation point would be on the underside of the plate, there is no one point where the airflow splits. Yes, there may be laminar flow on the upper surface of the wing, however, it is only for a small range of AOA.

[raptor22]

I tend to disagree. On a flat plate, lift is generated almost entirely as a result of air hitting the under side of the plate, due to newtons laws. This 'lift' is different from the pressure differentials created on a cambered airfoil, or for that matter, a symmetrical one. This is largely due to the shape of the leading edge of the plate. On a proper airfoil, the leading edged is curved. This allows the stagation point to move down with incresing AOA up to the stall angle. On a flat plate, however, this is no the case, at any positive AOA, the stagation point would be on the underside of the plate, there is no one point where the airflow splits. Yes, there may be laminar flow on the upper surface of the wing, however, it is only for a small range of AOA.


That is where you are incorrect. The stagnation point and kappa circulation (thought I'd add that in) act EXACTLY like they do in a "real" airfoil.

IF what you are saying was the case, then a flat plate craft would be unstallable.

--Alex

[Foamyfoam]

That is where you are incorrect. The stagnation point and kappa circulation (thought I'd add that in) act EXACTLY like they do in a "real" airfoil.

IF what you are saying was the case, then a flat plate craft would be unstallable.

--Alex
Again, i disagree. I simply cannot visualise a steady, laminar flow over the top of a flat plate at positive AOA. Instead of being unstallable, i think the flat plate is always stalled at positive AOA. Thats why i said earlier that lift is produced by newtons laws. Surely you cannot say that a flat plate is even remotely as efficient at providing lift as a cambered or non-cambered airfoil, the reason being that there is not a pressure differential. This are my thoughts, I, however do not claim to be an expert, im just a 10th grader.

[Foamyfoam]

Furthermore, a stall occurs when the seperated airflow results in a wake of turbulent air on the upper surface of the airfoil, this is almost always true on a flat plate. I agree, Raptor, that there is a stagation point, however, in the case of a flat plate, the stagation point does not serve a productive purpose.

[Adrian Page]

Ever notice propellers have pitch? They have airfoils too but I think if you eliminated pitch and kept the airfoil, your prop would produce close to zero thrust. It seems to me that pitch in a prop is pretty much the same as AOA in a wing. I think the airfoil makes about the same contribution to lift in a wing, as it does to thrust in a prop....some.....but not very much.

Adrian

[raptor22]

Again, i disagree. I simply cannot visualise a steady, laminar flow over the top of a flat plate at positive AOA. Instead of being unstallable, i think the flat plate is always stalled at positive AOA. Thats why i said earlier that lift is produced by newtons laws. Surely you cannot say that a flat plate is even remotely as efficient at providing lift as a cambered or non-cambered airfoil, the reason being that there is not a pressure differential. This are my thoughts, I, however do not claim to be an expert, im just a 10th grader.

At small RN, it is very nearly as effecient is my point. Lift is produced by newtons law in all airfoils, but nearly all of this lift is from downwash produced by aiflow over the TOP of the wing. This is the same in a flat-plate foil. It produces lift in exactly the same way.

Flat airfoils DO have the same pressure differential as a "standard" airfoil when not stalled, and they do not stall at only a few degrees AOA like you seem to beleive. They are capable of respectable AOA before stall and a decent lift coeffecient.

Thin airfoil theory states that a infinitely thin airfoil has nearly the same properties a "real" airfoil with the same camber line. So, to designing a low-RN airfoil can be simplified by producing a theoretically perfect (infinitely thin) flat plate with approximately the required charicteristics. A little thickness is then added till the "ideal" airfoil is produced. This technique is used for props, etc.

Why would the flat plate airfoil be so close to the final result of a "real" airfoil if it was so inferior? It wouldn't be.

Once again, why do you experience a defined stall with a falt plate foamy if it is arlready stalled? You wouldn't.

--Alex

[raptor22]

Ever notice propellers have pitch? They have airfoils too but I think if you eliminated pitch and kept the airfoil, your prop would produce close to zero thrust. It seems to me that pitch in a prop is pretty much the same as AOA in a wing. I think the airfoil makes about the same contribution to lift in a wing, as it does to thrust in a prop....some.....but not very much.

Adrian

Very true! Even heavily cambered wings produce relatively little lift at zero AOA.

--Alex

[ShamelessMale]

So a question for the Newtonian people that argue that a wing creates lift by "turning the air". We can calculate the lift created by the wing by measuring the effect of the wing on the air, yet HOW IS THE LIFT FORCE TRANSFERRED FROM THE AIR TO THE WING!!!!

[raptor22]

I see where you are going with this. Newtonian vs bournoulli....

I am not either, as neither explanation is the ONLY correct way to describe lift.

By pressure.

Or, by an energy direction change, as in the newtonian theory. I cannot explain it in a direct force sense. That is like "how does the flow acceleration from a jet engine produce thrust?"

--Alex

[Foamyfoam]

So a question for the Newtonian people that argue that a wing creates lift by "turning the air". We can calculate the lift created by the wing by measuring the effect of the wing on the air, yet HOW IS THE LIFT FORCE TRANSFERRED FROM THE AIR TO THE WING!!!!
Just as all forces are transferred. In using newtons priciples to describe lift, it is simply the airflow hitting the underside of the airfoil at a certain velocity at positive AOA. The result would be a force directed upwards.

[raptor22]

Hey, see this post from markdrela (MIT aerodynamic professor and designer of some really cool stuff":


One of the most important characteristics of a glider airfoil is "speed range". This means that the Cd must be very low near the bottom of the operating Cl range. The Cl can go as low as 0.1 in a fast upwind glide, and this is where airfoils differ a great deal.

Compared to a modern thin low camber section, something thicker like a Clark Y will have roughly 100% more drag at this Cl. A flat-bottom Gentle Lady airfoil might have a 200% or more drag. In practice, this means that gliders with these airfoils cannot fly fast at a reasonable glide angle, and hence do not have wind penetration ability. The thicker airfoils can have narrower chords and proportionately higher wing loadings which will help to overcome this drawback, but only partially.


The thin high performance sections he states are so good, as thin low camber sections, are subject to thin airfoil theory; stating that a flat-plate-type airtoil with the same mean camber line are roughly equivelent. I would hardly say that very high performance glider sections are "constantly stalled".

--Alex

[raptor22]

Just as all forces are transferred. In using newtons priciples to describe lift, it is simply the airflow hitting the underside of the airfoil at a certain velocity at positive AOA. The result would be a force directed upwards.

You also have to remeber air directed down by the TOP of the wing. The airflow hitting the bottom produces relatively little amounts of lift!

--Alex

[Foamyfoam]

You also have to remeber air directed down by the TOP of the wing. The airflow hitting the bottom produces relatively little amounts of lift!

--Alex
Im only describing the newtonian forces' contribution to lift. I am aware there are other factors be it 'Bernoulli or newtonian.' In the picture, i am trying to demonstrate my earlier point about the flat plate stalling at a relatively small AOA. However, this discussion is largely irrelavent, our shock flyers fly because of the tremendous power to weight ratio. Its flying on the prop rather than the wing. You may disagree but it is true to a large extent.

[vintage1]

There seems to be a misunderstanding that Newton and Berbouilli descrbe something different: They don't.

The relationship between lift, pressure and the velocity of the airflow and the momentum change of it is constant. Newtn gets the answer through calculatin momentum change: Bernouli by calculating pressure differential.

Bernoulli makes teh maths simpler, But the assumptions it depends on break down as turbulence comes in to play. Newton is far more complex, but makes less assumptions.

If ou think of Newtonian as the most pure and accurate analysis, and Bernoulli as a quick and dirty way to get to nearly the same answer or an unstalled wing, you are not far off.

[Adrian Page]

I think the post was "tell me what makes it fly."
The answer is "angle of attack and thrust". Fancy airfoils do not make planes fly.(They do make planes fly better).
Many of the posts in this thread are worrying about efficiency. That is an interesting topic but flying is flying, efficient or not. Wallowing through the air held up mostly by the prop is still flying.
Try lowering the front gear leg of trike geared plane until it sits on the runway with the wing at a negative angle of attack. You will find that the plane will taxi at 60 miles an hour but it won't take off. Notice that you still have an airfoil moving through the air at high speed. Replace that airfoiled wing with a flat plate wing. Adjust the front gear leg so the wing has a positive AOA. Try to take off. Now the plane will fly!
Look at this way - If you could only have one...AOA or an airfoil...which would you choose?

Adrian

[Ollie]

I think the post was "tell me what makes it fly."
The answer is "angle of attack and thrust".
Adrian

Think?

Rockets with thrust and no angle of attack (no wing).

DS with no thrust (faster than 200 MPH).

[Adrian Page]

Ollie,

Sorry....I thought we talking about wings.

Adrian

[Ollie]

OK, for wings.

DS and wing and with no thrust. Record flying 249 MPH.

http://www.sloperacing.com/results/ds-speeds.htm
http://www.charlesriverrc.org/articles/flying/markdrela_ds.htm

[Foamyfoam]

That is amazing. Mind boggling. It must take incredible skill and experience.

[raptor22]

It does. Keep saying to myself that I ought to build a slope wing to try it.

--Alex

[Ollie]

http://www.rcgroups.com/forums/forumdisplay.php?f=126
http://adopt.specificclick.net/adopt.sm?l=50593388&sz=pop&
http://www.thehelix.com/soaring/articles.htm
http://www.rc-soar.com/tech/craig.htm
http://www.shredair.com/album/dsfest.html
http://www.northcountyflyingmachines.com/
http://www.bowmanshobbies.com/jw.html

[Adrian Page]

OK, for wings.

DS and wing and with no thrust. Record flying 249 MPH.

http://www.sloperacing.com/results/ds-speeds.htm
http://www.charlesriverrc.org/articles/flying/markdrela_ds.htm


Fine. Cross out thrust and type in gravity.

Adrian

[Ollie]

No, don't cross out thrust. Energy from fuel, thermals, flapping, DS, ect. All via flying energy.

The energy comes from position, velocity and forces like lift, thurst, weight, drag, for flying.

[vintage1]

Anything will fly if you kick it hard enough.
:D

[Adrian Page]

Good one Vintage! Although I'm not convinced that a cat produces any lift.

Adrian

[Adrian Page]

Does any one have a definition of what constitutes flight?

Adrian

[raptor22]

Travel of an object which is not physically supported by the ground.


--Alex

[flying4fun]

Does any one have a definition of what constitutes flight?

Adrian

"Falling with style" :D ;)

Alex

[Ollie]

Read all:
http://www.allstar.fiu.edu/aero/airflylvl3.htm

"Many ask the simple question 'what makes an airplane fly'? The answer one frequently gets is misleading and often just plain wrong. We hope that the answers provided here will clarify many misconceptions about lift and that you will adopt our explanation when explaining lift to others."

"The figure also shows how the downwash appears to an observer on the ground watching the wing go by. To the pilot the air is coming off the wing at roughly the angle of attack. To the observer on the ground, if he or she could see the air, it would be coming off the wing almost vertically."

"The amount of air pumped down for a Boeing 747 to create lift for its roughly 800,000 pounds takeoff weight is incredible indeed."

The lift force is by downwash reaction supported by the ground!

[Ollie]

http://www.pilotfriend.com/general_interest/potty%20aircraft/spinning%20wing.htm
http://www.cit.gu.edu.au/~anthony/kites/rotor/how_fly.htm
http://www.rcgroups.com/forums/showthread.php?postid=432329#poststop

[Ollie]

Insect flight:
"Not long ago insect flight seemed to defy the conventional laws of aerodynamics. In a typical aircraft the wing's camber (or shape) and its angle of attack create an area of low pressure over the top of the wing-otherwise known as lift. Conventionally speaking, insects can't generate enough lift to stay in the air. And yet they do. In 1994 Charles Ellington, a zoologist at the University of Cambridge, and his colleagues built a large, slow-motion insect model for wind-tunnel tests. Confirming the group's theory, the experiment revealed a microscale vortex sticking to the wing's leading edge during the downstroke. The swirling produced low pressure over the wings, generating copious volumes of lift."
http://www.berkeley.edu/news/media/releases/99legacy/6-15-1999.html
http://www.pulseplanet.com/archive/Sep97/1440.html
http://eaton.math.rpi.edu/workshops/AppliedMathDays/2003/Miller.h
http://www.efluids.com/efluids/gallery/features_newscientist.htm

You can choose simple thoughts or complex facts about flying.

[Adrian Page]

Travel of an object which is not physically supported by the ground.


--Alex
If I drop a rock it travels without support of the ground. It just doesn't seem like flight to me. I think we need a horizontal component.

Adrian

[raptor22]

I think its just ballistic flight.

--Alex

[flieslikeabeagle]

Ollie wrote:

The lift force is by downwash reaction supported by the ground!

A small correction: the lift force is the reaction to deflecting all that air downwards, no doubt about it, but the ground isn't involved (except when flying extra low in ground-effect, when the ground helps trap high-pressure air under the wing, aiding lift).

Newton's law (action/reaction) does not require the presence of something to push against. That's why rockets work perfectly well in outer space, with nothing to push against but hard vacuum.

If you haven't watched the movie "Pushing Tin", starring John Cusack and Billy Bob Thornton, it is very apropos of this discussion. A pivotal point in the movie hinges on the downwash from a 747 as it comes in for a landing.

I can't believe we're arguing about the definition of flight. "When you hear the sound of one hand clapping, then you will understand the meaning of flight, little grasshopper!"

Okay, as long as we're being ridiculous, how about "three dimensional motion in a fluid, by a method or process requiring that the fluid have non-zero density" ? That covers lighter-than-air flight, heavier-than-air flight, squids and undersea gliders, and excludes rockets and ballistic motion.

-Flieslikeabeagle

[Adrian Page]

It also covers hoola-hooping underwater.:)

Adrian

[flieslikeabeagle]

LOL. I'd say it doesn't - hula-hooping does not *require* a non-zero density of fluid around you, as specified in my suggested definition. You could hula-hoop on the surface of an airless planet, as long as it had gravity. This is a case of ballistic motion, not flight.

-Flieslikeabeagle

[Adrian Page]

D'oh!
....on further reflection, it should be possible to hoola-hoop in a gravityless vacuum. :D
Your definition is pretty good but "falling with style" is pretty darn close....at least as it pertains to airplanes. It wins hands down for economy of syllables!

Adrian

[flieslikeabeagle]

Heh heh! :)

Got to agree with you on the gravity-less hula hooping. I confess that for some obscure reason I was thinking of skipping rope when you mentioned hula-hooping. Skipping rope actually requires gravity. :)

There's just something terribly wrong, though, with allowing a line from a movie sponsored by the all-powerful Empire Of The Mouse to define flight. And I think, in the movie, the context was actually "That's not flying! That's falling with style!"

I guess we could settle for Supreme Court Justice Potter Stewart's 1964 definition of obscenity, "I know it when I see it.". But that is so subjective, and so unsatisfying to the techie mind...just the sort of thing a Supreme Court Justice should *not* be using for a definition!

-Flieslikeabeagle

[vintage1]

How about 'using the properties of a non-zero density and mass gaseous fluid to achieve a non ballistic and sustained path clear of the ground'

That covers lighter than air flight and all flight using moving surfaces to generate lift. It even covers rockets if you bend the definition to mean that the rocket efflux is 'using the properties of a non-zero density and mass gaseous fluid '

[flieslikeabeagle]

Vintage1, I'd leave out the word gaseous - squids fly underwater with their fins, and if their density is too close to that of the water around them to count, recently there have been several man-made undersea gliders, used variously for ocean research and by military interests. Those are definitely heavier-than-water, and fly exactly as an airplane does in air.

Don't forget those little toy compressed-air powered rockets that use water as the propellant!

But if you leave out the word "gaseous", I mostly like your definition.

I still like the "capable of motion in three dimensions" part of my earlier definition, though. If sustained straight-line flight - with no maneuverability - counts, there is evidence that a number of people beat the Wright brothers to the first powered flight.

I think we're converging on something pedantic enought to satisfy the nit-pickers!

-Flieslikeabeagle

[vintage1]

I don't call flying in water flying.

Submarining or planing maybe.

Don't forget those little toy compressed-air powered rockets that use water as the propellant!

Backed up by compresssed air mate! There's the gas!

First decide whether squids and sharks are flying or not.

[raptor22]

I do. It uses the same dynamics as incompressible flow flight. I think "fluid" would be more exact.

--Alex

[raptor22]

Plus, those little rockets could also fly with a plunger to power the water instead of air! They would still fly.

--Alex

[borneobear]

Lift is the force that holds an aircraft in the air.


Too bad this thread died out nearly 2 years ago. It was an interesting read. ;)

[vintage1]

Don't be silly. Everyone knows the secret of flight is to remove the ground from under the airplane faster than it can fall.

Just as the secret of levitation is to throw yourself at the ground, and miss.

[BMatthews]

.....with a zero angle of attack(0 incidence) on the wing, how does a cambered surface allow faster airflow over its surface than a perfectly flat one? After all, the cambered surface has some area facing into the wind, while the bottom does not. Wouldn't that create more drag on the top, and cause more pressure on the top than the bottom?....

I can't find it at the moment but there were some pressure diagrams provided for a sort of "Clark Y' airfoil that was set up for a low or zero angle of attack. In that diagram I seem to remember that the air at the leading edge is indeed at a higher than ambient pressure and that this small zone did have some spread over the first 5 or so % of the upper surface. But from that point on the pressure went to lower very quickly. Also due to the very low angle of attack there was a lower than ambient "bubble" on the lower side just behind the leading edge. This bubble being perhaps 10% wide with the rest of the lower surface being a higher than ambient pressure. These zones of high and low pressure alter a lot as the wing section is swiveled through various angles of attack.

You can play with this pressure mapping yourself if you call up Foilsim and input various airfoil shapes and then play with the angle of attack. The one in foil sim isn't as clear as the colored pressure gradient mapping of the pictures I saw but if you take a bit of time to figure out the graph of the pressures you'll understand it.

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

You'll need Java loaded in your computer to see this run but it's well worth the trouble to get it going. VERY educational for a lot of various airfoil and wing related knowledge.

[Ollie]

There is a problem with air. The air is invisible untill you make it visible with smoke, cloud, etc. There are two kinds air associated with an airfoil, the flow and the boundary layer. The air boundary layer near the airfoil is almost stuck to the airfoil surface. The boundary layer shears the air untill the air reaches full speed of air flow. It is very important for the thickness of boundary layers for model airfoils. The thickness of the boundary layer includes laminar, turbulent, separation bubble and full separation (stall or bubble burst). Especially the airfoil separation bubble sticks to the airfoil making the flow. As the bubble change the airfoil coefficients of drag , lift and pitching moment.

[Ollie]

That's more.

There is a very nice picture along post #39 Attached Thumbnails and Fig. B. It is a flat airfoil at a small AOA and streamlines.

However, it doen't show a seperation bubble on the top of the leading edge of the airfoil. This separation bubble allows the streamlines to go around the sharp leading edge without zero radius and infinite acceleration.

BTW, which have you seen a separation bubble with your own real eyes?

That's why I think it is a problem with air because the air is invisible untill you make it visible.

[nmasters]

Do photographs count? :)

[Ollie]

Sure, photographs count.

I feel like I'm preaching to the choir or maybe nmaster is the preacher. Show us, please.

[nmasters]

I don't think I'm qualified to preach to anybody. I'm just an art school dropout with a nice little technical library.


Well there don't seem to be very many pictures of laminar separation bubbles on the web. A google image search for "separation bubble" (http://images.google.com/images?sourceid=mozclient&ie=utf-8&oe=utf-8&q=separation%20bubble&sa=N&tab=wi) turns up mostly drawings and CFD. Here's a selection of links:


Okay, every scratch builder who goes looking for airfoils will run across this one eventually. It's a smoke flow picture of a bubble on an Eppler 387 (http://www.ae.uiuc.edu/m-selig/uiuc_lsat.html) at Re=100,000:


This is a separation bubble on a horse trailer (http://www.engr.uky.edu/~fml/gallery/gallery/truckandtrailer3.jpg). Really :cool:


This one shows an animation of a bubble on a turbine blade (http://www-g.eng.cam.ac.uk/whittle/current-research/hph/lp-turbine-mjb/lp-turbine-mjb-3.html) .

This one shows some wind tunnel tests of sails (http://www.wb-sails.fi/news/Stallpics.HTML) .

This is leading edge cavitation on a hydrofoil (http://www.lhm.mw.tu-muenchen.de/gd/forschung/bilder/kav_fluegeltest.gif) . Not quite the same thing but cool nonetheless.

This one shows a sphere in a water tunnel (http://www.efluids.com/efluids/gallery/gallery_pages/wake_page.htm) with laminar separation and a wire to trip the boundary layer thereby delaying separation. Not a bubble picture at all but a great illustration of how tripping the BL can reduce drag.

-- Norm

[saucerguy]

The question was primarly eccentric since it's dealing with raw theory and is quite valid.

You can place a square block as the leading edge, leaving the lower section flat. Yes, it won't make a very nice wing, but it will produce lift. "leaveing technical talk out with this post for clarification for anybody just walking in with no background in this". Raw theory dictates this will work as asked from the original post in 2004 and I'm not sure why they had to ask it in the first place unless this is day one in their learning endeavors.

[kf2qd]

The definition of lift that we use has been simplified in order to make it usable outside the lab. Various objects fly because the way they interact with the air creates a force we can manage and manipilate that we call lift. It is a mixture of vacuum (on the top of the wing) and the acceleration of air in a downward direction (a helicopter demonstrates this more obviously than does an airplane, though both are doing the same thing). A flat plate , because of it's angle of attack, deflects air downward and creates a slight vacuum over some portion of it's upper surface. The 2 combine into the quantity we call "LIFT"

Or maybe these things fly because they don't know any better... (or some heroto undiscoverred anti gravity concept out there in the 12th ot 11th dimension..)

[ghoti]

Addict and others:
Newton’s 2nd law of motion is usually stated incorrectly as F=MA.

In fact Newton said, “force is equal to rate of change of momentum”. Momentum is mass times velocity and so F=M times dv/dt and that then becomes, by the rules of calculus, F= V dm/dt +M dv/dt. The right side has two terms not one. The term dm/dt (rate of change of mass with time) is usually zero and drops out of the equation but not always. A hovering aircraft has no velocity so in that case the describing equation is F=M dv/dt where dv/dt describes the momentum change of the air being forced downward..

As I read it, Bernouli’s equation accounts for perhaps 5% of the lift and the momentum change of the air accounts for the remaining (majority) part of “lift”.
My $0.02 worth. Bill

[biber]

Bernoulli and Newton are two correct independant descriptions of the situation of a lifting surface.
Bernoulli's law can be applied correctly to this thing, aswell as the momentum change thing can be.
Both don't contradict nor exclude each other.
Each is one possible description of the phenomenon.
Don't mix the word description up with explanation.
Physics is nothing about explanations,
it is all about describing the physical world.
There is no problem to describe a lifting surface under different aspects,
the relationship of dynamic and static pressure along streamlines,
or the relationship of momentum changes to netto forces.
So Bernoulli accounts for the same 100% of it aswell as Newton does it.
The omnipresent questions of the how and why may not be validly applicable, however.
Or could you tell me if the lift causes the momentum change
or rather the momentum change causes the lift?
But perhaps one can say, each one implies the other.

biber

[marfish]

I just got Profili and have been "playing". I've been having fun. :)
Maybe I'm just reiterating what someone else has already said, but actually, I have read on these forums and in other scientific papers that the air ahead of the leading edge gets compressed and forced up, pushing the LE down. Then as the air uncompresses over the top of the wing and rushes down the back to the TE it creats lift on the TE(hence the nose-down moment) I challenge you to fly a Frisbee upright with absolutely NO SPIN. Can't be done. ;)
I see a well-shaped airfoil doing two things:
1. Holding the air molecules apart in a defined path that both streamlines their reunion and creates the pressure gradients.
2. Blending the seperation of air molecules into a nicely proscribed curve to minimize the drag as much as possible. Bending the air up and over the top, thereby drawing the air downward in the aft part of the path and getting down-wash(lift) counteracts the up-wash at the leading edge and ads some additional lift, probably due to uncompressing while in transition. It's better than none at all for shaping and smoothing purposes. I admit that camber is good for positive lift at lower AoA and generally lower drag than non-cambered floils. But even a symmetrical wing can produce mounds of lift at very high C/L and respectfully low C/D. Look at the NACA 0010 and a host of others. The AoA will generally be a bit higher, but that's another study.

I also could be wrong, :p but believe that the air on the underside of a foil or plate is way more laminar and smooth than the upper surface is if the wing is in positive AoA. It seems that we try very hard to keep as much of the top surface airflow as laminar as we can and for good reason: DRAG. It's harder for the air to turn the "corner" as it were and rush back down the back side without being thrown off in the process, especially since the weight of the plane is pulling it down and away from the flow. The seperation from the boundary layer creates enormous drag and lowers the available lift. For the bottom, the weight of the craft helps the underside stay laminar, by pressing down on the air below.
Mark Drela's 6% thick "Super Gee" foils have MORE DRAG at "0" lift than when in substantial lift AoA(at RE= 500,000). It has virtually the same drag at "0" C/L as the NACA 0010 symmetrical foil(10% thick). But add a little AoA and the Super Gee (AG47c -03f) reduces drag a little while increasing lift. The NACA foil continues to increase drag with increased lift, but at a slower rate at higher C/L's. Carry the graph out past C/L of 1.0 and their L/D values are almost identical. :eek: So for a wing that needs good inverted performance, go with a high performing symmetrical or very low cambered foil.
This is all about lift and drag and what type of performance is going to be expected from the wing under normal flying. Isn't this fun??? :D
Lotsa lift,
Marlan

P.S. Adding snap flaps to the graphs was a huge eye opener. ;)

[Ollie]

"1. Holding the air molecules apart in a defined path that both streamlines their reunion and creates the pressure gradients."

I would say:
1. Holding the streamlines apart near the leading edge in a defined path (airfoil plus boundary layer) that both streamlines their reunion near the trailing edge except for the turbulence thickness. Two packets of air in streamlines that start near the leading edge, one over the top and the other under the bottom, but top packet of air wins and the bottom packet of air lags near the trailing edge and creates the pressure gradients.

[vintage1]

Bernoulli and Newton are two correct independant descriptions of the situation of a lifting surface.
Bernoulli's law can be applied correctly to this thing, aswell as the momentum change thing can be.
Both don't contradict nor exclude each other.
Each is one possible description of the phenomenon.
Don't mix the word description up with explanation.
Physics is nothing about explanations,
it is all about describing the physical world.
There is no problem to describe a lifting surface under different aspects,
the relationship of dynamic and static pressure along streamlines,
or the relationship of momentum changes to netto forces.
So Bernoulli accounts for the same 100% of it aswell as Newton does it.
The omnipresent questions of the how and why may not be validly applicable, however.
Or could you tell me if the lift causes the momentum change
or rather the momentum change causes the lift?
But perhaps one can say, each one implies the other.

biber


Do chickens cause eggs, or is it the other way round?

Are our minds the result of the physics of our brains, or is the physical world the result of our minds seeing that way?

Is GW Bush the cause of all the USA's problmes, or did the problems cause GW Bush?

When did you stop beating your wife?

I think we should be told..;)

[biber]

Now you lost me, Vint. :confused:

biber

[vintage1]

Now you lost me, Vint. :confused:

biber


A general comment on the difficulty of establishing absolute knowledge in a relativistic world..using language that relates to one area of knowledge to attempt to describe another.

Does the Newtonian movement of the particles 'cause' the pressure differentials that are described by Bernouilli? Or is it the pressure differentials that 'cause' the change in particle momentum?

Or is 'cause' the wrong word to use...does Left 'cause' Right..

Or is it that whenever we split something in half, 'left' and 'right' simply happen as a way of describing it?


YOU should know. There is a word to describe it in German. Weltanschaung.


Once upon a time the ancients considered the world was made up of four elements. Earth, Air, Fire and Water.

Today we scoff... but we too have Solid, Liquid, Gas and Plasma..identical in quality to what THEY called elements..

I suppose we say that things fly, because that is in the nature of the thing, and the air, and what we call flying.

Nothing MAKES anything fly. Flying is simply a question of the thing not hitting the ground for a protracted period. Throw anything hard enough and you will achieve flight.

The explanation of things with a high surface area to weight being somewhat better than a projectile, is in the end a complex interaction between a solid phase and a gaseous phase in the presence of gravity..or so we might say today..as post Newtonians..

At what point you decide to assume absolute knowledge and define the imnmutables (natural laws) , and then predicate the variables as down to a mechanism called 'causality' is entirely your choice.

Newton's question was not 'why do things fly' it was 'why indeed should they NOT?'..as he saw it as self evident and natural that planets did indeed fly for immensely protracted periods of time without hitting the ground....

And out of his 'explanation' which made gravity and mass immutables, came the assignment of properties to solid/gas boundaries, and gasses themselves.

However, it is imporatnt to realise that Newstons Weltanshaung is only one of many possible 'explanations' that fit the observed facts, and that indeed Relativity makes it only an approximation anyway, and that its sole value is actually not in its correctness, but in its efficacy.

Remember, in the limit we have no OBJECTIVE evidence for anything. There is no Observation that does not ultimately have an Observer, and he and many like him might be extremely mistaken in their Observations.

Today we might say that things fly because overall after integrating all the stochastic probabilities of state changes, it is exceedingly LIKELY that something defined as an aeroplane, will do what aeroplanes do. There is always a one in many zillions chance that it will vanish and reappear at the other side of the galaxy, but hey, "it just vanished...must have crashed over the other side of the canyon" is always a more acceptable alternative to a Newtonian..

"We are men, and we seek explanations"...and what stories we concoct out of the plethora of impressions we are subject to is the story of science.

It is, of couse, only a story, as is everthing else we think we know about anything. We have no absolute way of confirming even one item at all.

[biber]

Thanks for the clarification, Vint, I guess I got it. :rolleyes:

biber

[nmasters]

Do chickens cause eggs, or is it the other way round?

A'nt circular arguments fun?

http://yarchive.net/air/lift.html
http://web.mit.edu/2.972/www/reports/airfoil/airfoil.html
http://www.regenpress.com/

This is the Amazon page for "Stop Abusing Bernoulli!" (http://www.amazon.com/gp/product/customer-reviews/0964680629/ref=cm_cr_dp_pt/102-7573525-8121719?ie=UTF8&n=283155&s=books
) scroll down to Customer Reviews to get an idea of how impassioned well educated people can be with regard to their own favored way of explaining a complex phenomenon. This illustrates perfectly why I try not to get involved in origin of lift arguments.

--Norm