AB Bob

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Hello all you smart folks,
I'm quite new to R/C aircraft.
I still fly flat bottom winged airplanes.
I understand that with a flat bottom or semi-symetrical wing, that lift is generated by the air on top having to flow faster than that on the bottom.
But, how is lift generated with a fully symetrical wing? [sm=confused.gif]
Thanks in advance.
Bob

da Rock

The wing doesn't fly parallel to the ground, doesn't fly "level". The front is up a degree or two, and TAA DAA....... it's not longer symmetrically aligned to the airflow, and winds up working like an asymmetrical profile.

Dr1Driver

What da Rock is talking about is called AOA - Angle Of Attack, and his description is perfectly corect.

Lnewqban

One of the good things about these airfoils is that they lift as much inverted as upside up, always presenting certain angle of attack to the air thru which they move.

No AOA = No lift (For symmetrical airfoils)

For example, some vertical tails have symmetrical airfoils, but since they are perfectly aligned with the airstream (AOA = zero), they do not generate any lateral force (sideways lift), unless the rudder is deflected (which makes the airfoil non-symmetrical or cambered at that moment).

For any airfoil type, the slower the wing moves thru the air, the bigger the AOA must be, so the lift force remains the same.

See the schematic and explanation posted in here:

http://www.rcuniverse.com/forum/fb.asp?m=8002743

MTK

Bernoulli plays a role with the flat bottom airfoils but total lift is not the result of Bernoulli alone. Bernoulli effect is woefully incomplete in explaining total lift in an airplane

Air is surprisingly massive....a 100 foot cube of air weighs, what? What would you guess? Better yet look it up. Point is that the amount of reaction up force from moving a large amount of very massive air back and down can be quite large. Newton's second law of motion is the key player here

The reaction force that any airfoil in the horizontal position (travelling in free air) generates is more related to air "wetting" the surfaces and being thrown back and down. That's wing downwash that some have heard about and is what really generates lift. In the flat bottomed example, Bernoulli accounts for maybe 25% of the lift an airfoil displays, and that drops to just about zero for a symmetrical airfoil.

Suggest that one look up the subject in NASA's website to get the real skinny on lift.

Dean Pappas wrote a good piece on the subject that appeared in MA a couple years ago. Probably a good read for anyone wanting more

gboulton

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Stick and Rudder: An Explanation of the Art of Flying : Wolfgang Langewiesche

The very first bulleted point on the back cover, under "What's in Stick and Rudder?"

"* The invisible secret of all heavier-than-air flight; the Angle of Attack. What it is, and why it can't be seen. How lift is made, and what the pilot has to do with it."

An extremely enjoyable and informative read, written BY a pilot FOR pilots, SPECIFICALLY to get you around all the technobabble and scary math bits. It is the Bible of how and why airplanes do the things airplanes do.

Lnewqban

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

"The real details of how an object generates lift are very complex and do not lend themselves to simplification. For a gas, we have to simultaneously conserve the mass, momentum, and energy in the flow. Newton's laws of motion are statements concerning the conservation of momentum. Bernoulli's equation is derived by considering conservation of energy. So both of these equations are satisfied in the generation of lift; both are correct. The conservation of mass introduces a lot of complexity into the analysis and understanding of aerodynamic problems...................The simultaneous conservation of mass, momentum, and energy of a fluid (while neglecting the effects of air viscosity) are called the Euler Equations............If we include the effects of viscosity, we have the Navier-Stokes Equations .............To truly understand the details of the generation of lift, one has to have a good working knowledge of the Euler Equations."

Lnewqban

I second that; great reference[sm=thumbs_up.gif]

Some portions of that book can be read in-line at:

http://books.google.com/books?id=zla...page&q&f=false

MTK

Yes that's right...Euler's math and Navier Stokes math on fluid dynamics are keys if one wants a mathematically rigorous answer.

To the original poster, Bob, your question is a very simple one that countless others before you have asked. The answer is extremely complicated for a rigorous treatment.

I oversimplified the rigorous answer on purpose to give you a snap shot of the concept....it's pushing air back and down from the wing.

If anyone is interested in a corollary, take a look at Jack Norris' concepts on propellers. Mr Norris explains how a propeller works without the rigorous math. The rigorous math has been around for only about 70 years BTW, and Mr Norris indeed is stepping on the shoulders of giants in his explanation. And if one thinks the subject of lift of a wing is misunderstood, one should consider how misunderstood the rotating wing is

Lnewqban

Here is the real thing, Bob,...........and in slow motion:

http://www.youtube.com/watch?v=6UlsA...eature=related

Very important:

Note the stagnation point (point at which the incoming airflow separates to flow up or down the airfoil) moving below the leading edge as the AOA increases.
As Da Rock has pointed above, the air does not "see" a symmetrical shape when AOA is present.

Trinut

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A 100 x 100 ft room holds about 620 pounds of air, at .062 lbs/ft^2 and 100ft x 100ft = 10,000 ft^3

A 5 lb model helicopter hovering over a scale large enough to catch all the downwash would still read 5 lb the moment it breaks ground.

An airplane wing does the same thing, creating downwash equal to the weight of the plane. It's just harder to run underneath the plane with a big scale as it is taking off to proove it ....

Jim Thomerson

Most control line stunt airplanes are set up 0-0-0 on thrust, wing and stab. If you look carefully, when one is flying level, either upright or inverted, you will see that the nose is pointed slightly up, giving just enough angle of incidence to support the weight of the airplane.

Lnewqban

I am not sure what the weight of a 100 foot cube has to do with symmetrical wings, but I would revise that air density and math.

A cube of 100 ft sides holds 1 million cubic feet.
Air at 60 degrees weights 0.0763 lb/cuft
Air at 80 degrees weights 0.0735 lb/cuft

Trinut

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Thanks for fixing that, a cube has 3 sides!. I think people would guess a room full of air weighs less than it does.
So a room, 10 x 20 x 8 foot cieling is 1600 ft^3 x .075 lb/ft^3 = 120 pounds

Jim Thomerson

F = MA, so it is not just how much air, M, an airfoil moves, but also how much it accelerates it, A.

AA5BY

I'm a carpenter and we carpenters love wedges and the forces they produce and I say lift is created because the wing at aoa creates an uphill wedge of dense air the wing finds it easier to climb over compared to pushing out of the way. Its my way of splainin it and I'm sticking to it.

DEVILDOG

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An easier approach to the mans question is. The faster air moves the less pressure it has. So as the air moves over the top of the wing it moves faster. The air under the wing is slow moving wich has HIGH pressure. This high pressure therefore lifts up the air plane.

A vacume sweeper actually doesn't suck the dirt into itself. The motor just spins really fast inside creating extremely low pressure and the room air is very slow moving air with very high preesure and dirt is really, kida, sorta, sorta kinda, pushed into the vacum sweeper.

A symetrical wing as they said goes into the relative wind or the direction that you are flying at a small angle of attack which really makes the upper wing a taller air foil that in return would require the air to move faster over the top and etc etc etc.


Not a rocket scientist but I did stay at a holiday inn express last night

mikewoodez

Wow, quite the informative thread. That's not to say that I haven't drooled over the thousands of previous posts about how an airfoil works, but I digress.

As someone who has been around airplanes for just about his entire life, I thought I had tackled the idea of lift early on; how wrong I was. The more you guys (rcuniverse) explain it, the more interested i get. Please allow me to explain: at the age of 4 years old, I was flying what is called a Long Eze, which if you dont know, is one of the coolest airplanes out there. It is a Canard Design with a pusher engine. My grandfather built it from a set of plans, an order of foam, and a stick to it attitude that seems to be missing from most of todays youth. Flying it, not just holding the stick and seeing what it does. Had my first landing at 7, and flew front seat (P.I.C) at age 14. I have been flying models for about 15 years (just so you know, Im only 24) and have been an aviation ADDICT my whole life. That being said, I still find the idea of lift incredibly elusive. I have always flown by "feel"(imagine my instructor's reaction when i could execute the greasiest of greased landings, without knowing how, why, or when I should do the things he said I should be doing). I can fly any rc that is put in front of me, purely from "feel": I KNOW when I need to coordinate a turn with rudder, I can "feel" when an airplane is getting close to a stall, and i can almost "see" when something Im flying has reached the ends of its performance envelope. Helicopters? Flew my first chopper for 6 months without ditching it, and that was only due to an errant puppy interested in the whirly-gig above its head(it got loose, Im a good rc-er).

What Im trying to say is that you should be commended or your forays into learning how things ACTUALLY fly, because some of us who could fly the rivets off of a Cessna do NOT understand HOW it works.

disclaimer: after reading my own post, i realize i may come off as an airplane snob; not so. You guys (RCU) are the lifeblood of what I hold deareople who hold true to the innate fascination of flying.

Keep it up you guys, these threads are IMPORTANT.

Michael

gboulton

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Mike,

vertical grimmace

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I would like to comment on the "feel" thing as well. When you get a hold of a very well built, good flying control line airplane, you get to experience this. This is one of my draws to CL to this day. I know what my plane is doing all the time and I am "flying" it. Maybe a harken back to "stick and rudder" flying. While I do get the a good fell for my RC aircraft, it is not quite the same as CL. It is great to be able to feel the lift and stall as opposed to just seeing it.
Also, I ordered that book, just to add it to my collection. Looks quite informative.

daleflysrc

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How simple could this be !! How complex can one make it !! Just read "How we invented the Airplane" By Orvil and Wilbur Wright !! After all, they were just a couple of bicycle mechanics who patented controlable flight !! and along the way disproved everyone else's airfoil/lift theroy !! I think its a whole 78 pages

kmeyers

Bob
How did you know I was smart?
Anyway here it is.
It is simple :
Air Presure is created by colisions with molecules in the air against the wing surface.
All planes flat bottomed and semi or fully symetrical are held up by more colisions (presure) on the bottom then the colisions (presure) on the top.

The fact that there are so many variations is the different performance that we look for.

You are very welcome
Ken

SushiSeeker

Although the Shuttle is also a lifting body, it is a great example of this principle and the AoA is so pronounced you can actually see it.

(The shuttle is also proof that with a sufficient AoA, even bricks can fly!

tomfiorentino

I don't know about all of this guys and I'm not an expert...but...I don't think either a bernoulli or newton orientation to this is wrong. They are probably different ways of looking at the same thing. Bernoulli explains the pressure differentials that account for downwash. I don't think you can have one without the other.

Anyway, having said that I can more readily understand lift from pressure differential. One post referenced Bernoulli accounting for 25% of the lift...I don't know if that was to illustrate a point or if it that was an actualreference to something authoritative.In any event, from what I have read, moving any kind of airfoil through the air creates negative pressure (relative to atmospheric) on BOTH the top and bottom surfaces because the air is accelerated over both the top and bottom. But the negative pressure on top of the wing is much greater than the bottom. That's why they say the top of the wing does most of the "heavy lifting" (no pun intended). In fact, it's only with very extreme angles of attack that bottom pressure becomes positive (relative to atmospheric) but even then, short of a stalled wing, the top is more negative. The point is, I don't understand a wing being"pushed"up when in almost all flying circumstances bottom pressure isnegative. I'll put my flame suit on...but I like what Ihave heard about the wing being sucked up instead of pushed up. I know that isn't totally correct though...

But before I get torched on that commentplease consider that we all know a flat bottom airfoil creates lift at zero angle of attack when theoretically pressure along the bottom of the wing would be as close to atmospheric as you can get (I know there has to be some degree of acceleration along the bottom...I'm just saying).

Just my two cents I guess. I'm seeing this thread a little late but I am vacationing down in Virginia Beach right now. I keep making trips over to Oceana Blvd where the Naval Master Jet Base is and watching the jets go in and out. They fly some 200 practice flights out of there per day...formation....touch and goes...it's great and I think Imissed my calling. If the Wright Brothers could only see one of those!!!

Take care...

Tom

Lnewqban

I agree with you, Tom.

The phenomena of lift is one, only that it has been studied, explained and understood from several points of view.
To me, it is all about transference of energy between the wing (moving plane) and the atmosphere, and as it cannot be created or destroyed, it cannot be transferred in more than one way.

We disagree about the following:

There is negative static pressure respect to non-moving air over the top surface: air is accelerated in a direction opposite to flight speed above airfoil's speed.

Positive static pressure to non-moving air under the bottom surface: air is accelerated in the direction of flight to speed under airfoil's speed.

No AOA for flat plane = no lift
Example: vertical fin in line with rudder = zero sideways lift

I have copied the following text and schematics from an old post:

http://www.rcuniverse.com/forum/fb.asp?m=8037340

1) Attached diagram #1:
Velocity distribution on the surface of the NACA 2412 airfoil at 6Âș angle of attack.
(as shown at http://www.mh-aerotools.de/airfoils/...tributions.htm)

“Increasing the angle of attack to 6Âș also changes the velocity distribution: The velocity on the upper surface increases, whereas the velocity on the lower surface is reduced. The enclosed area increases and thus the lift (-coefficient). The differences are more pronounced in the leading edge region, which (in this case) contributes most to the total lift.
The image also shows that the stagnation point (where the velocity is zero) is not exactly located at the extreme left of the leading edge, but instead has moved slightly back on the lower side. The streamlines passing along the upper surface have to turn around the leading edge, whose small radius of curvature accelerates the flow in this region rapidly (from zero to 100 in a split second). This causes the velocity peak close to the leading edge, where the flow reaches about 1.85 times the speed of the onset flow.”

2) Attached diagram #2:
I have created it while trying to understand how the airfoil affects the air through which it moves.
I believe the diagram applies to any airfoil presenting a moderate AOA to the air.

Assumptions:
Non-disturbed atmosphere around the airfoil is in repose (v=0).
Non-dimensional airfoil (Chord=1).
V: Velocity of the airfoil.
v: Velocity of the molecules of air.

Here are my debatable conclusions:

1) The x-axis shows the chord length of the airfoil, the y-axis shows the velocity given to the molecules of air close to the surfaces by the airfoil (v) respect to the velocity of the airfoil through atmospheric air in repose (V).

2) The point of first impact is below the geometrical point determined by the AOA, due to the angle of the up-wash, created by the proximity of the airfoil (it is not clear to me how this happens).

3) The molecules being hit by the airfoil at the stagnation point receive energy from the speeding airfoil, and they are forced to move either up or down (more molecules are continuously being compressed or crushed behind them).

4) For the molecules that follow the lower surface: they are accelerated from the original repose state of the atmosphere up to 92% of the velocity of the airfoil (they gain kinetic energy transferred from the airfoil). This means that they drag behind, that they slide under the lower surface, that they move respect to the atmosphere slower than the airfoil (and in the same direction). Then, from 50% of the chord up to closer to the trailing edge, the molecules decelerate up to 70% of V (they loose kinetic energy while transferring it to the air below (and not receiving much from the airfoil)).

5) For the molecules that follow the upper surface: they are accelerated from the original repose state of the atmosphere up to 85% above the velocity of the airfoil in about 1% of the chord(!!!) (they gain much kinetic energy transferred from the airfoil almost instantaneously). This means that they do follow a circular accelerated movement from the stagnation point while climbing up the round leading edge of the airfoil, acquiring a speed (in a direction about 135 degrees opposed to the direction followed by the airfoil) almost double the speed of the airfoil at a point beyond, but very close to the leading edge. Beyond that point, they still move respect to the atmosphere faster than the airfoil (but in opposite direction); however, that velocity decreases steadily up to v=V at 92% of the chord (the molecules decelerate, loosing kinetic energy while transferring it to the air above (and not receiving much from the airfoil)). From the 92% up to the 100% of the chord (trailing edge), the molecules move slower than the airfoil (and in the same direction), just like the molecules following the lower surface.