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How Airplanes Fly

Old 04-11-2009, 12:07 PM
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AEAJR
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Default How Airplanes Fly

Mike Parsons has opened this thread to a WattWiki entry.

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An outstanding article. Nice job!
Entry: How Airplanes Fly

Posted by: Mike Parsons

Text:
How airplanes fly - the basic principles of flight

The basic principles of why and how airplanes fly apply to all airplanes, from the Wright Brother's first machine to a Stealth Bomber, and it's actually not difficult to understand how airplanes get, and stay, airborne.

Aerodynamic forces

Essentially there are 4 aerodynamic forces that act on an airplane in flight; these are lift, drag, thrust and gravity (or weight).
In simple terms, drag is the resistance of air (the backward force), thrust is the power of the airplane's engine (the forward force), lift is the upward force and gravity is the downward force.
So for airplanes to fly, the thrust must be greater than the drag and the lift must be greater than the gravity (so as you can see, drag opposes thrust and lift opposes gravity).
This is certainly the case when an airplane takes off or climbs. However, when it is in level flight, the opposing forces of lift and gravity are balanced. In a descent, gravity exceeds lift and to slow an airplane drag has to overcome thrust.
The picture below shows how these 4 forces act on an airplane in flight:

The thrust is generated by the plane's engine (propeller or jet), gravity is a natural force acting upon the airplane and drag is the normal friction caused by the plane moving through air molecules. Drag is also a reaction to lift, and this lift must be generated by the airplane in flight. This is done by the wing of the airplane...

How wings generate lift

A cross section of a typical airplane wing will show the top surface to be more curved than the bottom surface. This shaped profile is called an 'airfoil' (or 'aerofoil').
During flight, air naturally flows over and beneath the wing. Any given 'parcel' of air gets split in two as it hits the leading edge of the wing, and both halves of that parcel actually meet up again at the same moment, as they come off the trailing edge of the wing. So because the air moving over the top of the wing has more distance to cover (because of the curvature it is forced to follow) in the same amount of time as the air passing below the wing, it has to move faster.
If you're having trouble following that, look at the picture below showing a parcel of air hitting a wing. Arrows A and B is air getting split at the same moment, and meeting up again at the same moment.
Faster moving air is lense dense than slower moving air, so this results in a lower air pressure on top of the wing, and a higher air pressure below the wing.
In addition to this pressure gradient, some of the air that passes beneath the wing is deflected downwards. This causes an opposite upward force (Newton's 3rd Law of Action & Reaction) that acts upon the underside of the wing, effectively pushing it upwards.
This combination of reaction and pressure gradient generates the lift needed to hold the airplane up as it flies. And the faster a wing moves through the air, so the actions are exaggerated and more lift is generated.
Above, the movement of air over an airfoil

However, a direct reaction to lift is drag, and this too increases with airspeed. So airfoils need to be designed in a way that maximizes lift but minimizes drag, in order to be efficient.
A crucial factor of lift generation is the Angle of Attack - this is the angle at which the wing sits in relation to the horizontal airflow over it.
As the angle of attack increases, so more lift is generated - but only up to a point until the smooth airflow over the wing is broken up, and so the generation of lift cannot be sustained. When this happens, the sudden loss of lift will result in the airplane entering into a stall, where the weight of the airplane cannot be supported any longer.


Airplane control surfaces

For an airplane to be controllable, control surfaces are necessary.
The 4 main surfaces are ailerons, elevator, rudder and flaps as shown below:

To understand how each works upon the airplane, imagine 3 lines (axis - the blue dashed lines in the picture above) running through the plane. One runs through the center of the fuselage from nose to tail (longitudinal axis), one runs from side to side (lateral axis) and the other runs vertically (vertical axis). All 3 axis pass through the Center of Gravity (CG), the airplane's point of balance.
When the airplane is in forward flight, it will rotate around each axis when movement to any control surface is made by the pilot. The table below shows the appropriate actions:
ActionAxisControlled by RollLongitudinalAilerons PitchLateralElevators YawVerticalRudder
The following sections explain how each control surface effects the airplane...
Ailerons


Located on the trailing edge (rear) of the wing, towards the wing tips, the ailerons control the airplane's roll. Each aileron moves at the same time but in opposite directions ie when the left aileron moves up, the right aileron moves down and vice versa.
This movement causes a slight decrease in lift on the wingtip with the upward moving aileron, while the opposite wingtip experiences a slight increase in lift. Because of this subtle change in lift, the airplane is forced to roll in the appropriate direction ie when the pilot moves the stick left, the left aileron will rise and the airplane will roll left in response to the change in lift on each wing.
The ailerons are controlled by a left/right movement of the control stick, or 'yoke'.

Rudder


The rudder is located on the back edge of the vertical stabilizer, or fin, and is controlled by 2 pedals at the pilot's feet.
When the pilot pushes the left pedal, the rudder moves to the left. The air running over the fin now pushes harder against the left side of the rudder, forcing the nose of the airplane to yaw round to the left.

Elevators


The elevators are located on the tailplane, or horizontal stabilizer.
Like the ailerons, they cause a change in lift when movement is applied; moving the elevator up (pulling back on the yoke) will cause the airplane to pitch its nose up and climb, while moving them down (pushing forward on the yoke) will cause the airplane to pitch the nose down and dive. Elevators are linked directly to each other, so work in unison unlike ailerons.

Flaps


Flaps are located on the trailing edge of each wing, between the fuselage and the ailerons, and extend outward and downward from the wing when put into use.
The purpose of the flaps is to generate more lift at slower airspeed. This enables the airplane to fly at a greatly reduced speed without the risk of stalling. Flaps also generate more drag which slows the airplane down much faster than just reducing throttle power.
Although the risk of stalling is always present, an airplane has to be flying very slowly to stall when flaps are in use.

So all these factors are why and how airplanes fly. Radio control model airplanes can of course be more simple - for example, just have rudder and elevator control, or perhaps just rudder and motor control. But, the same fundamental principles always apply to all airplanes, regardless of size, shape and design.
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Old 04-11-2009, 01:51 PM
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Mike,
Unfortunately the section on 'how wings generate lift' is incorrect (both parts).
The first theory presented is sometimes called the equal time of transit theory and is based on the unfounded assumption that air separated at the leading edge must for some inexplicable reason meet up again at the trailing edge; this is totally incorrect. The theory also neglects the obvious fact that symmetrical airfoils and flat plates fly just fine and that planes with 'normal' airfoils can fly upside down.
Also the bit about deflection of air downwards (which is closer to the truth but still not the whole story) neglects the effect of the upper surface of the wing. This theory can be called the skipping stone theory' and is at best a partial truth. If this theory was true then a stall (which is the separation of air flow from the upper surface of the wing only) would not effect lift at all.

The correct explanation of how a wing works can be found here: http://regenpress.com/. Unfortunately the truth about lift is quite complex and difficult to grasp hence the invention of simple but wrong explanations such as those presented.

Steve
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Old 04-11-2009, 02:20 PM
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As well as debunking the incorrect lift theories NASA have a pretty simple page on how a wing really works some of which could perhaps be incorporated into a corrected version of the new WattWiki page: http://www.grc.nasa.gov/WWW/K-12/airplane/right2.html

The explanation on the NASA page does not go into the same depth as the one I posted earlier but it's easier to grasp.

Hope this is constructive..

steve
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Old 04-11-2009, 04:43 PM
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Mike,
A couple more observations, which hopefully do not come across as ‘nit picking’...

The Aerodynamic Forces (thrust/drag/lift/weight) section fails to address how a glider maintains forward speed when it has no engine to provide thrust???

Also the idea that for a plane to climb there must be more lift than weight is possibly missleading. In fact in a steady climb the wing actually produces less lift than in level flight because the prop itself provides some lift. Taken to the extreme in a vertical climb the wing produces no lift at all, the prop (or jet or whatever) produces all the lift. The wing only needs to produce more lift than the plane 'weighs' when some acceleration is involved producing positive 'g' force. This could be as result of a turn or pulling out of a dive or pulling up into a climb.

Steve

Last edited by JetPlaneFlyer; 04-11-2009 at 08:52 PM.
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Old 04-19-2009, 02:11 PM
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As no one has commented i propose to modify the WattWiki page to correct the error regarding lift theory..

This is what i have in mind to replace the existing 'lift' section:


How wings generate lift

False theory

First let’s get one of the most popular, apparently plausible but incorrect theories of lift out of the way


The Way Things Work [Macaulay 1988]

This theory may be called the ‘Equal Transit Time Theory’. It works on the premise that all airfoils must be more curved on the top than on the bottom, and so have a longer path over the top. The theory also assumes that the air that’s divided at the leading edge of the wing must again meet with the same ‘piece of air’ when it gets to the trailing edge thereby having an equal transit time. Based on these two false assumptions the theory predicts that the air moving over the top of the wing will have higher velocity and so according to Bernoulli’s principal the faster moving air will have a lower pressure and this lower pressure creates lift on the wing.
This conveniently overlooks the obvious facts that symmetrical airfoils and flat plates can fly quite well and planes with ‘normal’ airfoils can fly upside down. I’ve actually heard believers in this principal argue that planes cannot in fact fly upside down, despite the evidence of their own eyes! Equal transit theory also does not consider ‘under cambered’ airfoils where the bottom surface has a similar or even longer path than the top yet these airfoils generate most lift of all. The assumption that air at the trailing edge must meet up with the same piece of air it was separated from at the leading edge has no foundation and in physics can be shown to be false in a wind tunnel (air over the top arrives before the air on the bottom!)

The way wings really work

Newton’s third law of motion states that for every action there must be an equal and opposite reaction. Image a cannon firing a cannonball as force is applied to the cannonball accelerating it in one direction, the cannon recoils in the opposite direction with equal and opposite force.


A wing in level flight turns the air in downward direction which required downward force, the equal and opposite reaction creates a force pushing the wing upward, this is the force known as lift.
It’s quite easy to imagine how a wing moving through the air at an angle to the airflow will push downward on the air with its lower surface, turning the air downward and increasing its pressure, but this is only half the story. The upper surface of the wing also does at least half of the lift production. Air flowing over a surface has a tendency to follow the surface even when the surface curves or is inclined away from the direction of flow. This is called ‘surface attachment’ or sometimes after the man who discovered it the ‘Coander effect’. This means that the air flowing over the top of the wing tends to follow the wings surface deflecting it downward and lowering its pressure.

The flow turning theory also works perfectly well for flat plate wings

If the angle of the wing to the airflow is too great surface attachment of the upper surface breaks down and this greatly reduces lift produced by the wing (and increases drag).. this is known as a stall.
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Old 04-19-2009, 04:12 PM
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I hadn't noticed this thread before, and I guess I'm somewhat surprised nobody else has commented yet. At any rate, I approve of your proposed revision. I'm a few weeks away from graduating with a degree in aerospace engineering, so hopefully my approval has some merit.
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Old 04-24-2009, 06:40 AM
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Hi. So the Newton explanation is basically the wing works in such a way to redirect air downards to create enough momentum to keep the plane up. I've been reading around and came across the Bernoulli approach to the same thing that talks about circulation around the wing and the trailing vortexes rotating in the opposite direction (to follow the conservation of angular momentum), but am having trouble unifying the two together. In particular, the part where "the circulation around the wing generating the lift" is the same phenomenon as the trailing vortexes throwing air down to provide enough momentum to keep the plane up...it's not two effects accumulating to produce lift...it's the same effect approached in different ways and I'm having trouble merging the two together. Maybe someone could help me out?

Maybe the Bernoulli approach also belongs in there? Either way it seems the core of the thing is at the Euler equations which take into account everything which I do not understand at all. http://www.av8n.com/how/htm/airfoils.html

Also, maybe it should be added that lift is NOT generated by air bouncing off the bottom of the wing producing higher pressure. THis would be flying using drag, rather than lift.
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Old 04-24-2009, 07:12 AM
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Originally Posted by DKNguyen View Post
In particular, the part where "the circulation around the wing generating the lift" is the same phenomenon as the trailing vortexes throwing air down to provide enough momentum to keep the plane up...it's not two effects accumulating to produce lift...it's the same effect approached in different ways and I'm having trouble merging the two together. Maybe someone could help me out?
The missing link in your analysis is that circulation and the trailing vortices are the same thing with two different names. It's essentially one huge vortex, shaped like a horseshoe, which follows the wing for a portion of its path and trails backwards to infinity at the wingtips. "Circulation" is just the portion of the vortex which is attached to the wing.

I'm not sure you really want to have a basic "How Planes Fly" article that gets into vorticity, though. It's a very complicated subject, so it needs some simplification to be used as an introduction. The discussion above seems like a good compromise to me. It gives the basics (redirecting air downward) but avoids getting into all the complicated details. The bit about the Coanda effect should probably be removed, as it's a bit misleading. I'll have to think about it and see if I can find a simple way to get some more detail in there.

Finally, planes do fly because of drag. Drag and lift are the same quantity, pointing in different directions. The wing has a small drag coefficient no matter what the air is doing to it (similar to friction - its effect changes with speed but it's just a simple number associated with a given design), but most of the drag on an airplane is induced drag. Induced drag, just like lift, is a result of pressure differentials. The wing actually just makes one big force perpendicular to itself. Because the wing has an angle of attack, this force is pointed mostly up (lift) but also slightly backwards (induced drag).

Last edited by aramid; 04-24-2009 at 07:34 AM.
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Old 04-24-2009, 09:26 AM
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Maybe I was a bit vague in my description of my problem. Yeah, I understand how the vortice is one big horseshoe that loops from one wing to the other (though I have a harder time figuring out what it looks like for something like a propeller or heli rotor).

To rephrase what I was trying to say earlier...
I am having a hard time reconcilling/unifying these two aspects of lift:

1. Circulation causes the air flowing along the upper surface of the airfoil to be faster than the air flowing along the lower surface of the airfoil. Faster air exerts less pressure perpindicular to it's stream, so a higher pressure is produced below the wing than above the wing. THis pressure differences manifests as lift and pushes the wing up.

2. The vortices produced by circulation are shed at the wingtips and thrown downwards, basically throwing air downwards producing an upward force called lift.

To me, they seem like two separate phenomena that would accumulate to produce lift even though I know there is only one mechanism at work. Which is the one producing the lift? Which is the one causing the other? The first approach doesn't account for conservation of momentum, but the second approach does not seem to account for how longer wings are needed for heavier loads (it seems to imply all the lift is produced near the wingtips where air is thrown down in the trailing vortex). That is...unless the pressure difference in #1 "sucks or pushes" the airfoil upwards which in turn produces the downard trailing vortex.

The link I pointed to basically said this:
"It doesn’t matter whether you consider the vorticity to be the cause or the effect of the descending air — you can’t have one without the other."
Which I can appreciate seeing as how in electromagnetics you can't really have an electric field without a magnetic field. And although, I study it in school, I don't know the quantum physics that is at the root of it all. BUt I guess the human mind is more inclined towards the mechanical rather than electromagnetic, so although I'm fairly satisifed with just taking the EM explanation "as is" for the time being, I am not satisified with doing the same with the mechanical aspects of airfoils.

Either way, it still seems that if the trailing vortex shed at the wingtips is the air thrown downards to produce an upward force, it still implies that the lift vector is focused at the wingtips, which I'm pretty sure it's not, or else longer wings wouldn't really make a difference.

I was also having a hard time unifying induced drag. I had gotten as far as the wing force vector was pointed backwards slightly. But then there's also the talk about the shed vortices at the wingtips. From my understanding, planes need these vortices throwing down air towards the ground to fly. But there is induced drag at the wingtips from air leaking between the pressure difference above and below the wing forming another vortex (I am unsure if this is the same vortex that is being shed to produce lift, or if it is a separate vortex that reduces the pressure difference at the wingtips or produces turbulence resulting in less lift or induced drag, or both). Whether or not induced drag is a lump term for a few different forms of drag, or whether it is a single drag force being approached in different ways, I do not know.

Last edited by DKNguyen; 04-24-2009 at 10:09 AM.
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Old 04-24-2009, 12:19 PM
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Originally Posted by DKNguyen View Post
.....
Originally Posted by DKNguyen View Post
To rephrase what I was trying to say earlier...
I am having a hard time reconcilling/unifying these two aspects of lift:

1. Circulation causes the air flowing along the upper surface of the airfoil to be faster than the air flowing along the lower surface of the airfoil. Faster air exerts less pressure perpindicular to it's stream, so a higher pressure is produced below the wing than above the wing. THis pressure differences manifests as lift and pushes the wing up.

2. The vortices produced by circulation are shed at the wingtips and thrown downwards, basically throwing air downwards producing an upward force called lift.

........
Bear in mind I’ve no formal education in aerodynamics; I’m just a model flyer with an interest in the physics involved.....


Circulation and vortexes are not two aspects of lift, they are both results of turning air downward.


Taking on your first question:...

Circulation
When air is turned down it creates a low pressure on the upper wing surface and a high pressure below, this is purely down to the forces involved in accelerating the air.

Now comes Bernoulli.. If the pressure on the upper surface is lower than the pressure on the bottom surface then according to Bernoulli the air on top of the wing must also travel faster that the air on the bottom.. (Note carefully that it is the pressure difference that leads to the velocity difference, not the other way around)
So we have the air on top going faster than the air on the bottom. If we now subtract the overall ‘freestream’ air velocity (i.e. the planes flying speed) from both top and bottom local airspeeds this gives us an effective circulation around the wing with the air flowing backward on the upper surface and forward on the lower surface. Because the speed of the circulation is caused by and is therefore in direct proportion to the pressure difference (Bernoulli), then the speed of circulation is also directly proportional to lift..


Now question 2...

Vortexes
Vortexes at the wingtips do not produce lift, in fact they decrease lift. Wings in a wind tunnel that have their tips joined to the tunnel wall have no tips and therefore no tip vortexes but they produce lift more efficiently than any real wing that has tips.
Tip vortexes are a bad thing and if they could be eliminated they would be.. High aspect ratio wings and wing tip fences/winglets etc are two methods of reducing tip vortexes but in the real world where wings must end ‘somewhere’ tip vortexes are unavoidable. Tip vortexes are a ‘by-product’ of lift not the cause of lift.

3D vortex flow is a pretty hard concept to grasp which is why I did not drag it into the WattWiki article...


You also mention induced drag.....

Induced drag is a product of the 3D airflow.. The vortexes rolling up around the tip from the bottom to the top of the wing must be balanced by a flow of air downward. This downward flow is called downwash and it means that the airflow over the wing is inclined downward.. Because lift is the force that acts at 90 Deg to the airflow direction then the downward inclination of the airflow leads to a rearward ‘tilting’ of the lift vector.. Lift therefore starts to pull the wing backward creating extra drag, commonly known as induced drag. If tip vortexes could be eliminated there would be zero induced drag but in the real world all we can do is minimise it by the tip vortex reducing measures I noted in the last paragraph.


This is all good stuff but would trying to cover it in the original article would obscure the simple ‘lift is a force due to turning airflow’ principal.

Steve

Last edited by JetPlaneFlyer; 04-24-2009 at 01:50 PM.
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Old 05-06-2009, 08:50 PM
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Crickey guys, I've been building and flying models for the past 50+ years, (on and off, mostly on).

But if some beginner asked me, "Basically, how does a plane fly ?", the last thing I would do is go into a load of technical detail, (some of it still argued as to right or wrong).
Well actually I couldn't go into too much technical detail as I don't know it or feel I need to know it.
I accept there are modelers who thrive on this sort of thing, but they have probably been in the hobby a while, or technically minded regarding aerodynamics to start with.

What I worry about some times is putting the 'average Joe' beginner off with too much information that wont necessarily get them flying a trainer, or answer why it's power stalling and crashing.

Quite a few times on the forums it has been obvious that a lot of beginner, (and some others), haven't grasped that lifting airfoils generate more lift as they go faster. They seem to think that throttles are full on for flying and off for landing. And then wonder why the plane is always climbing, (or stalling).

I admit to not reading the Wiki 'How planes fly', and I came across this thread by accident, but felt I had to add my point. Not too sure what that point is now, other than, remember who you may be trying to answer the question to. If it is a total noob who just want to read a simple answer to why his model flies the way it does.................are you answering him in a way he will understand and benefit from?

Sorry for a bit of soapboxing guys, I'm not anti-technical, just realistic. (Sometimes )

Ray.
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Old 05-06-2009, 10:07 PM
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Ray,
How much simpler do you want it than "planes generate lift as a reaction to turning air downward"? Which is what I modified the Wiki article to say and which is technically correct. Please take the time to read it before criticising.

If you prefer an even simpler explanation that is wrong then i cant help you.

Out of curiosity; what would you tell someone who asks "how does a plane fly"?.....Witchcraft perhaps?

Steve
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Old 05-07-2009, 10:23 AM
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My apologies for intruding.
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Old 05-07-2009, 01:15 PM
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Originally Posted by eflight-ray View Post
My apologies for intruding.
Ray,
Maybe I was a bit abrupt in my reply, it a fault of mine for which I apologise.
Criticism and comment is more than welcome but it may be more meaningful and better received if made after reading the article itself?

I appreciate that any theoretical talk is a turn off for some folk, that's fair enough. Folk who dont really care what keeps a plane in the air probably wonít be visiting the Wiki section titled 'Fight theory' anyway; but if they do visit then finding a presentation on flight theory should not be unexpected.

As to the relevance of theory to people who only want to fly model planes.. It's very true that many of the best flyers, far better flyers than me, probably have very little idea about what keeps a plane in the air. On the other hand to get a pilot's licence for full size aircraft you do have to learn flight theory and sit an exam, so maybe there is something in it?

Steve
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Old 05-07-2009, 03:14 PM
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I am a supporter of this watt wikki idea, but I think it has to be more basic and less controversial. A new student will never wade though all of this, not on his own anyway.

Maybe we should also eliminate all the "new" theories of flight that cause disagreement between more experienced modelers.

I'm not saying make it incorrect, just make it simple and basic. Every aspect doesn't have to be answered on the first lesson.
Flight theory for a private pilots licence is a lot simpler that what is posted on these forums.

I've included a picture from a private pilots course, I don't necessarily agree with it, but it's fine for initial steps into lift.

Paul
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Old 05-07-2009, 03:53 PM
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The actual Wattwiki article (unlike this thread) is really very very basic and as simple as can be... have you read it?

The diagram you attached might be simple but it's also incorrect. I donít think there is any merit deliberately presenting a falsehood regardless of how simple or appealing that falsehood may be?.. I more than a bit shocking that this stuff is being given out as part of a PPL

The 'correct' theory is not new. The theory of lift was fully understood long before the Wright brothers put it into practice. The incorrect theories sprang up sometime after.

Steve
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Old 05-07-2009, 04:06 PM
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Originally Posted by JetPlaneFlyer View Post
The actual Wattwiki article (unlike this thread) is really very very basic and as simple as can be... have you read it?

The diagram you attached might be simple but it's also incorrect. I donít think there is any merit deliberately presenting a falsehood regardless of how simple or appealing that falsehood may be?.. I more than a bit shocking that this stuff is being given out as part of a PPL

The 'correct' theory is not new. The theory of lift was fully understood long before the Wright brothers put it into practice. The incorrect theories sprang up sometime after.

Steve
The diagram is different from your wikki article, How?
It covers both Newton and Bernoulli and cites both as the cause of lift.
How is yours different?
I'm not trying to be disagreeable, I truly don't see any difference.

Paul
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Old 05-07-2009, 04:26 PM
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Originally Posted by pd1 View Post
The diagram is different from your wikki article, How?
It covers both Newton and Bernoulli and cites both as the cause of lift.
How is yours different?
I'm not trying to be disagreeable, I truly don't see any difference.
Paul
It's different because it actually shouldn't contain Bernoulli's theory. That theory does have a place in calculating lift, but not in the way it's shown in your flier or known to most people. That application is called the 'equal transit time fallacy' and is oversimplified, misleading, and incorrect. The wiki article mentions it in order to debunk it, not to support it. The flier also shows Newton's law incorrectly, by bouncing air particles off the bottom of the wing, which is another oversimplification and ignores the contribution of the upper surface of the wing, which should be generating suction and pulling more air particles down (plus the resultant lift arrow in that diagram should be pointing backwards, not forwards - that's where most of your drag comes from).

Without getting into the complexities of vorticity and circulation, airplanes fly because air moving around the wing is redirected (not bounced, but smoothly redirected) downward. There is also a significant pressure differential between the top and bottom surfaces, far larger than equal transit time and Bernoulli's equation predicts, which can be used to calculate lift. This differential is a result of several factors, and is unnecessarily complicated unless you're actually trying to calculate lift for an arbitrary airfoil (one for which no lift coefficient data is available).

I've read through the Wiki page, and I think it's definitely not perfect, but it's a good introduction to the subject for beginners and doesn't contain any blatantly incorrect information (I really don't like the bit about the Coandă effect, because it's not normally discussed when doing aerodynamic calculations - the flow actually sticks to the top of the wing because not doing so would create a vacuum and pull the flow right back against the wing - but as a simplification used in an introduction to the subject, I can accept it).

Last edited by aramid; 05-07-2009 at 04:44 PM.
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Old 06-08-2009, 11:16 AM
  #19  
emanuelchristos
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Re the glider bit:
It uses gravitational potential energy, and converts it into kinetic energy (foward motion) kind of the same way a ball rolling down a ramp, while thermals and ridge lift move it upwards and give it more gravitational energy. Hehe its simple sience.
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Old 06-08-2009, 01:24 PM
  #20  
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Originally Posted by emanuelchristos View Post
Re the glider bit:
It uses gravitational potential energy, and converts it into kinetic energy (foward motion) kind of the same way a ball rolling down a ramp,
Yes, thats why i edited the Wiki article with the following line a few weeks ago:
On a glider forward thrust is a result of the plane flying on a downward slope (similar to freewheeling down a hill on a bike)
Feel free to expand on the article if you wish, there are still many areas that could be improved upon.

Steve
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Old 06-09-2009, 06:24 AM
  #21  
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True... sorry for that... I didnt quite make the connection in my mind about the wiki...
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Old 06-27-2009, 05:07 AM
  #22  
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Bernoulli's law is about air travelling through a venturi not air travelling over the surface of a wing. It's only used as a principle that air travelling over a convex curve will produce low pressure.
A glider flies foward because it has most of it's weight in the front but it loose altitude.
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Old 06-27-2009, 09:14 AM
  #23  
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Originally Posted by Huffy01 View Post
A glider flies foward because it has most of it's weight in the front ..........
Not sure what you mean by this

A gliders CG position is not really relevant to where it's forward 'thrust' comes from. A glider that has a rearward CG flys forward just the same as any other. For instance some free flight gliders have their CG near the trailing edge of the wing and they still fly forward. Canard gliders have their CG near the rear of the plane and they fly forward fine. If the CG is moved back so that the model is unstable then it wont fly at all, but that's another issue that would effect a power model just the same as a glider.

Here is a diagram showing the forces on a glider in 'steady state' flight.. note that forces are generated due to the glide ange.. CG of the glider does not get mentioned because it's not relevant. The diagram comes from 'Model Aircraft Aerodynamics' by Martin Simons (a good read)
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Old 06-27-2009, 09:33 AM
  #24  
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Originally Posted by Huffy01 View Post
Bernoulli's law is about air travelling through a venturi not air travelling over the surface of a wing. It's only used as a principle that air travelling over a convex curve will produce low pressure.
Yes, your right, Bernoulli never worked on wings or lift theory.

Air travelling over a convex surface does indeed produce a low pressure on the surface but the reason has nothing to do with Bernoulli.. It's simply down to centripetal acceleration.
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Old 06-27-2009, 10:10 AM
  #25  
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I've got this other "thingy","question" about angle of attack.
I recently bought a DVD "Performance tuning for glider's". There is a big emphasis on wing incidence, tail decalge and fuselage datum.
All the angle's are at zero degrees to each other for best performance.
Is the 4 degree's angle of attack just for full size ,passenger carrying aircraft and trainer models.
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