We all know that curved shape on the top of an aerofoil causes air passing over it to flow at a higher speed than on the underside of the wing. This faster air flow creates a low pressure zone above the wing. The lower velocity air under the wing is high pressure. Therefore lift is generated.

Why then does a fully symmetrical wing generate any lift at all? Why are the lifting forces not cancelled out?

Also, why do aeroplanes with totally flat wings, some of the foamies for instance, with no aerofoil section to speak of fly at all?

Anyone answer these form me?

Regards,

Dave

AOA

Because that expanation of why planes fly is extremely oversimplified. Angle of attack gives the wing lift. Even flat bottomed wings with a lot of curve on the top have an angle of attack which gives zero lift and yes, even reverse lift for inverted flying.

How is this for a grabber? I have a full scale pilots license,and have always been told that it is vacuum on the top of the wing that gives it lift. Now if that is so ,how does a spacecraft maneuver in outer space where there is total vacuum[no pressure]? Or are the old ideas to simple
as to how a plane flies? Or this one,if a tree falls in the woods and there is no one around to hear it,there is no noise.

Crasherboy, they use maneuvering jets. The wings don't work in a vacuum.

Esc, there's two things that give the wing section a different path length on the top compared to the bottom. Camber is one of them and angle of attack is the other. A symetrical wing is no longer symetrical when it has a positive angle of attack to the oncoming air.

Also the bedtime story about the lift coming about as a result of the different path lengths is just that. That old theory also relied on the idea that when the air split apart that the top had to race to the rear to meet up with the same air that it was split away from. However that's been shown often that it's not the case. It's far more complex than that. Yes there is a difference in pressure from the bottom to the top in a wing that is developing lift and there is also a redirecting of the direction of airflow from the leading edge to the trailing edge. The pressure differential and this deflection of the airflow being tied together. If you look at the pressure differential it adds up to the amount of lift created. If you also look at the difference in momentum created by accelerating the air into a downward flow it also adds up to the lift that is created. But they are different faces of the same thng.

To play and wonder google up Foilsim and go play with it. It's truly an informative and easy to use tool for studying airfoil basics and flow around different shapes. You can even generate an "upside down flat bottom" section by altering the camber value to a negative amount and then see how your upside down wing still generates lift after all.

Mmm, thanks for replying guys, but I am not sure that I get this at all. It's obvious that my original post was a gross oversimplification and that something else is actually going on during flight, that is what I was trying to get at.

Your examples of angles of attack do not quite fit in with a lot of models. Take for example my 3D profile planes which have no angle of attack, the wing section and tailplane being exactly parrallel. The wing section is fully symmetrical with exactly the same camber on both sides. This section is chosen for several reasons, e.g. maneouverabilty and invered flight. How then does it create lift that is not cancelled out by the inverse side of the aerofoil?

Crasherboy, spacecraft do not "fly" in outerspace but rely on the main engines for thrust and much smaller thruster nozzles for attitude changes. It is only when, for instance, the shuttle enters atmosphere that the control surface can have any effect on direction at all.

Regards.

http://www.amasci.com/wing/airfoil.html

Control line stunt airplanes are usually set up with symmetrical airfoils and the wing and stab set 0-0. When you watch one fly around level it looks like it is flying level. However many have the bottom of the fuselage parallel to the 0-0 line and when they are inverted it is clear that they are flying around slightly nose up, enough to give the wing enough lift to keep them trucking along.

The true angle of attack on those stunt jobs is so small that you will have a tough time seeing it unless you really look against some referrence .
Airfoils will still be a mystery for many, as long as those old "fact" books are still around .

What most don't see, is thaall of those "highly specialized shapes and thousands of scientifically designed wind tunnel researched etc.,etc., are nothing more than shapes which under finite conditions work a bit better or worse than one with a slightly different finite condition.
Quite a few full scale aircraft were used at flying speeds and loads which kept the wing on the ragged edge of simply not working.
On our models the airfoils are very forgiving--because the angles they work in, are so low.

Actually, AOA is the angle of the wing to the airflow. What you're describing there is called incidence. The angle of the wing in the fuselage is the angle of incidence. Same name for the angle the stabilizer is built or trimmed into the fuselage.

The airfoil isn't parallel to the airflow when a symmetrical airfoil is generating lift (right side up or upside down). A symmetrical airfoil be it a flat plate or any profile, starts generating lift as soon as it's pitched away from it's zero lift line. If it's flying perfectly level, it won't be creating lift either above or below the airfoil. Pitch it either noseup or nosedown and it'll instantly start generating lift.

Rock,

To your post #10, I'll stick by Jim T's description. Except for some very small, intentional stab trim angles built into the models of a very few CL stunt fliers of National and World Class caliber, stunt models are rigged with 0-0 incidence (decalage, whatever you wish to call it.) To get a lifting AOA, the wing needs a very slight pitch "up" in the direction of the required lift. The entire model makes that pitch rotation. Elevators and interlinked maneuvering flaps contribute, yes, to achieve that net "positive" AOA.

The reason it is not evident to onlookers, except that it looks different when the model is flying inverted, is that CL stunt models are capable of dozens of g's lift. Proportionally more, even, than the RC 3D and "Fun Flyers" since one objective in flying a high scoring CLPA pattern is to maintain velocity as nearly steady as possible.

With that capability at the extreme, 1.0 g flight clearly needs very little AOA - but it needs some!

Sorry about mixing up AOA and incidence, I should have known better than that! OK, da Rock's comments make most sense.

Whilst I realise that it is nigh on impossible to achieve absolutely 0 degrees AOA we must come close to it at some point during level flight - why then doesn't the near zero lift cause the plane to porpoise in flight?

Best to leave that for another day I think.

Thanks for your help guys.

Regards,

Dave

There is sort of a natural "anti-porpoise" that comes from the cg being appreciably ahead of the neutral point of the airplane. Pitch stability comes from the horizontal stab/elevator being adequately large and adequately far enough aft to "stabilize" the pitch. And that's what it does.

BTW, it actually isn't hard at all to achieve 0 angle of attack. Airplanes just aren't held at that pitch for very long, usually. But some of them very often do go past it on the way to an AOA that the pilot needs for his maneuvering.

There aren't too many reasons to pitch an airplane to fly at zero AOA. The problem with actually using that AOA for any positive benefit is because most airplanes are designed the way they are. Most airplanes are designed with cruise as the primary criteria of their design. That usually biases the layout to have the least drag at whatever that cruise speed is. So the designers layout the wing to the fuselage, the incidence of the wing, so that at the cruise speed desired, the wing will fly at the best AOA for that speed with the expected load, and the fuselage will be at the best angle of attack for reduced drag. So the AOI is not at the zero lift angle when the fuselage is at it's least draggy angle. So if for whatever reason you then try to fly the airplane at the zero lift angle of the wing, the fuselage winds up not being at it's best angle of attack for reduced drag........ so...........

About the only reason that stands out in my mind for wanting to fly an airplane at zero AOA (a fighter plane in WWII wanting to dive away from an engagement) would be that you're looking for the fastest dive. You don't want lift out of the wing. And you want the least drag from the whole airplane. Yet that might not happen since the fuselage probably is holding back the max speed.

Of course I know that they use rockets to maneuver in space,however have any of you wondered how that would work? There is a total vacuum there and what is there to push against. My idea was to make you all think. Are all the old ideas right?

I don't think space is a perfect vacuum, is it? It's probably pretty damn close though. I would think there are gaseous molecules floating about out there, e.g. Hydrogen which can be exerted on. Newton's 2nd Law of motion would still apply in this case. Not having to overcome gravity must also make this process much easier.

What's your take on this question then?

Regards.

Da Rock,

Excellent bit of information and insight.

Thanks.

For every action, there is an equal and oposite reaction, or something like that. F = MA The Force on the rocket is equal to the Mass x Acceleration of the particles (atoms, whatever) in the exhaust. A rocket is said to be 100% efficient when the rocket's speed is equal to the speed of the exhaust (I think that is correct, not a rocket scientist[8D]) so a rocket which is travekling through space faster than its exhaust is exiting is more than 100% efficient.

Imagine that you are floating in space, and that the vacuum around you is perfect. Imagine further that you have, right beside you, a large rock, with exactly the same mass as you (including your spacesuit). What would happen if you were to push the rock away from you as hard as you can? You and the rock would move apart in opposite directions. You would each have the same speed, relative to your initial speed.

The exhaust of a rocket is no different from that rock. It has mass, and is being pushed away at high speed. The rocket is accelerated by the resulting force, even in a perfect vacuum.

Back to airfoils & wings. A wing creates lift when the pressure on the top is lower than the pressure on the bottom. It is not correct to say that a wing also creates lift because it deflects the air downward. A wing with the same pressure on the top and bottom surfaces will not deflect air downward. We can't separate the lift generated by a wing into these two separate components. The old debate about "deflection" vs. "pressure difference" is nonsensical.


banktoturn

Relative to what? Fuselage? Tail plane? You may have 0ΒΊ relative to the fuselage or datum line, but flight has little to do with the fuselage. Incedence of the tail plane, engine thrust, trim settings are where the action is. You may have 0-0-0 for engine, wing and tail incedences, but at some time you're feeding in trim or have set the control throws to keep the model airborne. Otherwise it just follows a ballistic path. 3-D (IMHO) is just substituting the propeller airfoil for the wing and directing travel with large control throws.

Kill the engine and we'll see how much angle of attack your model requires to fly with a symmetrical wing.

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