If you don't ask, you will never know. So here goes...

Can someone explain me how does airfoil work? In other words, if someone talks about a fully symmetrical airfoil - what does this mean? I know it is probably a stupid question, but I don't know it and if I don't ask I never will.

Korps,

Your question is not stupid at all, since it is something as hard to understand and see as it is hard to explain.

Made it very simple:
An airfoil is the transversal shape of a surface that is forced to move thru a mass of air at certain velocity.
Some of that force is converted into a force named lift, that is pointing in a direction perpendicular to the velocity of the wing.
When that lifting force is bigger that the weight of the model, the thing goes up; when it is equal to the weight, the model flies horizontally; when it is smaller, the model descends.

The lifting force is proportional to the area of the wing, to the angle of the airfoil respect to the velocity, to the density of the air, and to the square of the velocity.

There are many shapes of airfoils, and each has a different purpose.
Each has a different coeficient of lift (CL), which is proportional to the AOA, and this CL affects the proportions of lift to velocity, area and air density described above.

For high lift, the shape is curved in order to force more air downwards for the same wing velocity (high CL): this is called a cambered airfoil. Those are not good for high velocities.
For aerobatic maneuvers, the shape is straight or symmetrical (zero camber or curve) (low CL), in a way that the wing lift the same upside down. These are also good for high velocities.

The discussions and therories about how the phenomena happens are endless; here are some good references:

http://en.wikipedia.org/wiki/Lift_(force)

http://en.wikipedia.org/wiki/Airfoil

http://www.rcuniverse.com/forum/m_79..._1/key_/tm.htm

Regards!

A simple airfoil has a flat bottom and a curved top surface. This means the air has to travel a little further over the top of the wing to meet up with the air that's going across the bottom. To do this the air over the top has to speed up (it's got further to go) and by speeding up its pressure drops. Now there's less air pressure on the top of the wing than on the bottom and this is what the wing feels as lift. But a symetrical wing section has the same curve top and bottom which means that if it's facing directly into the wind then it can't make any lift because the air is moving at the same speed over the top as it is over the bottom surface. To get that type of airfoil to provide lift it has to be angled up very slightly. This is called the angle of attack. It only needs a very slight angle of attack for a symetrical wing section to provide enough lift to fly level, maybe 1 degree or even less depending on what the wing loading is. The beauty of a symetrical airfoil is that the model is just as happy flying inverted but will still need that slight angle of attack by giving a tiny bit of down elevator.

Edit...it seems the previous post was sent while I was typing .

So that is why a Trainer plane for instance (which has a flat bottom wing), will fly more stable and more slow - because it generates more lift in level flight? But for this same reason it will not fly or not fly inverted well because it will be "pushed" down?

If you then have a symetrical wing, the plane will have "equal" lift inverted as up and therefore is good for aerobatics? But at low speeds and level flight it will not generate as much lift - therefore stalling earlier?

Do I have this right[] Or am I misreading here.

The simple answer to this part of your question is that a fully symetrical airfoil will have the same opposing shape on the bottom side as that on the top side.

Another term you will hear often in modeling is 'semi-symetrical airfoil' this just means the bottom shape while faily close to the top side shape is slightly different.

John

"So that is why a Trainer plane for instance (which has a flat bottom wing), will fly more stable and more slow - because it generates more lift in level flight?"

A flat bottom wing is a cambered airfoil, which happens to have the bottom surface flat, just for construction convenience.
As you say, this cambered airfoil has a high CL, generating lift at slower velocities; hence, it can fly slow enough for training.
Now, the stability of the model has nothing to do with the airfoil shape; the trainers are stable for other reasons.

"But for this same reason it will not fly or not fly inverted well because it will be "pushed" down?"

The cambered airfoil ("concave" shape) can fly inverted, but it needs to compensate for the "convex" shape with higher AOA and/or higher velocity.

"If you then have a symetrical wing, the plane will have "equal" lift inverted as up and therefore is good for aerobatics?"

Yes.

"But at low speeds and level flight it will not generate as much lift"

The wing must generate the same lift force, in order to equal the weight of the model.
For the same wing area, the symmetrical airfoil (lower CL), needs higher AOA and/or higher velocity than any cambered airfoil (higher CL).

"- therefore stalling earlier?"

Not necessarily. Normally cambered airfoils tend to stall at lower AOA's than symmetrical airfoils.
For any airfoil, the stall depends only on the maximum reacheable AOA, which is called critical AOA.

Ah I understand

Thank you people. This has always annoyed me for not knowing. I'm sure there are a lot more in depth discussion regarding this, but now I understand the concept and theory behind this.

Keep in mind that dihedral will have as much or more to do with holding a plane inverted as the airfoil does.

Minn, the under-cambered type - never seen it before. Which types of planes has that?

Many of the early aircraft designs featured the under camber sections. In models, it was more popular with free flight designs from the 30's and 40's, and still seen today in old timers flow with radio. The MEN Trainer also used a similar section in a rudder, elevator and motor control design.

The MEN Trainer would also fly around inverted for as long as you wanted, with just the rudder and elevator controlling the model. Dihedral make no difference as to whether or not you can fly a model inverted. Airfoils of any design will fly inverted if you have enough angle of attack and enough power to overcome the drag inherent with creating lift....the same as when they are right side up.

Sorry, but I have to disagree. Dihedral plays a much bigger roll in inverted flight than does the airfoil.

A kite is bowed so that if it tips to one side, air deflects off the rearward side, whereas the air hits the forward side with a straighter (thus more powerful) force which levels the kite out again. A flat-faced kite would never fly.

Dihedral does the same.

Picture having a metal plate that is bent in the center and pivoted on a bracket. Now drag that bracket through a pool of water. The plate will never be able to swing forward.

This is the exact same principle as how dihedral works (in fact, this IS dihedral)

This is why trainers often have a LOT of dihedral - to take advantage of its self-righting characteristics.

So you are saying that you can not fly an airplane inverted because it has dihedral? Really?

Because I have yet to see the model that will not fly inverted, regardless of dihedral or airfoil when it has adequate power. Most models have more than enough engine to maintain sustained flight, with enough excess to make turns and fly around the field.

This includes all models that have ailerons and includes all that just have rudder and elevator for flight controls.

No, I am not saying that I cannot fly a trainer inverted, but thanks for the insult.

What I am saying is, an inverted plane with a lot of dihedral will fight to upright itself. It is a lot more difficult to fly inverted than a plane with little or no dihedral.

Regardless of airfoil.

So what did you suggest here?

What I am implying is that your implication is very misleading.

Your statement, "Dihedral make no difference as to whether or not you can fly a model inverted." implies that a plane will fly as easily inverted as it will upright.

CAN a person fly a high-dihedral winged plane inverted? Yes.

Can a beginner (who is most likely to be flying this type of plane) do it? Doubtful.

You can also stand on a beachball in a swimming pool, but it's not as easy as it sounds.

I'm not a beginner, yet I have some difficulty flying a trainer inverted. I can do it, but it's not as easy as it is with my, say Skylark 70 or the Venus II.

Now I know why.

CGr.

Mike, do you have any video of you doing that?

Thanks for the displays, that helps explain dihedral

Jon

Yes, it can be seen on that TV show, "Bone-headed Americans injure themselves on Home Video"

I usually have beginners flying their trainers inverted long before they learn to land. I teach them basic acrobatics early on for two reasons:

1.) - that is why most of them want to learn to fly in the first place.

2.) - It is more stressful than flying around in circles, teaches them what to do in unusual attitudes, and makes them more relaxed when they start doing approaches to the runway.

Now if your trainer has ailerons, flying inverted is only slightly more difficult than flying upright. If it has rudder and elevator only, it becomes slightly more interesting, but still not difficult with a couple of minutes of instruction.

The goal (mine at least) is to turn out a pilot that can fly anything. I don't consider the job finished just because they have solo'ed and can take-off and land. I have spent years working with different pilots, as they upgrade their models, their skills and techniques. It is rather pathetic how few pilots reach their full potential, I know that I haven't. Often, they are too proud or too embarrassed to admit that they need more instruction, and quite frankly too often the available instructors do not have the skills or knowledge either. In that case the only option may be to read more and experiment at the field to put theory into practice.

Another problem is that there are often too many students and too few instructors. You do not have to be the greatest pilot to help with instruction. Assisting with basic field procedure, radio, engine, tank setup, and just talking to new students is an enormous help to a harried instructor.

I am not sure how standing on a beach ball in a pool of water is relevant, nor the pretty pictures of plates pulled through water. But whatever.

You da man, I guess.

CGr.

My point is, you said, "Dihedral make no difference"

Point a trainer straight an level and take your hands off the sticks. Will it continue to fly straight and level? Yes

Now flip it inverted and take your hands off the sticks. Will it continue to fly straight and level? No

Dihedral makes a difference.

That's why they put it there.

When I take my hands off the sticks, then I am not flying the model.

But the dihedral is still there! That's what this discussion is about.

I don't know if they are properly named, but a wing formed by a cambered plane is known as a Jedelsky wing.

Now, a flat plane, like the one used in foamies, operates on the principle of a symmetrical airfoil; only that it is a less aerodynamic shape and produces more drag.
However, for slow velocities, like in 3-D flight, drag is not important.

No the discussion was about the effect of airfoils until you posted:

I stated:

I made no reference to the degree of difficulty or the skill level required. But it is a very low bar.

Now if we were discussing free flight models, you might have a point though the effects of dihedral and the effects of the airfoil are completely different. But with radio control (as in this is a RC forum), you are grasping at straw.