Difference in Airfoils
 

After looking through and asking several questions about a second plane, I have found that many second planes are all semi-symmetrical airfoil, but still have a lot of dihedral and wing area to keep the stable and slow. My real question is how much does an airfoil affect flight performance? I trained on a World's Model Happy Fly 20 which has a symmetrical airfoil, but is still a high-wing, trike gear plane. I have had about 16 flights so far, and nine of them have been solos. Should I still get a typical second plane, or would it be too easy, and I would just outgrow it too quickly?

RE: Difference in Airfoils
 

The difference is that a symmetrical, or semi-symmetrical airfoil will not provide as much lift as a flat-bottom wing. Therefore, a flat-bottom wing will produce more lift at slower speeds, allowing you to fly more slowly (which is often a good thing for novices). The disadvantage of a flat-bottom wing is that when trying to perform maneuvers (like a split "S") as you generate speed in the down-line, the plane will try to "Balloon" when you level out because the excess speed is creating too much lift.

The Dihedral is for stability. Think about the kites you used to fly as a kid. There was always a string tied between the ends of the cross member to "bow" it toward the back. This, essentially, is dihedral If you look at the first pic below, you'll see how that bow on a kite works the same way that dihedral does, and vice versa.

When one side dips, the air can hit it more squarely, whereas it will deflect off of the other side. This creates a self-righting tendency.

By the same token, if you look at pic 2 you will see how dihedral will make it difficult to fly upside-down. Now, the self-righting tendency is not only gone, but you have the opposite effect. As one wing dips, it LOSES it's ability to catch air, while the other wing GAINS ability, so the wing wants to return to it's upright position.

To answer your next question, I would say that if you started on a semi-symmetrical wing, you have skipped the first step, and now you are passed that. Move on to a more advanced plane or you will probably get bored quickly.

You sound like a great candidate for a 4-Star 40.

RE: Difference in Airfoils
 

"semi-symetrical" is actually a mis-nomer. Either you are symetrical (ie, 0 camber) or you have some camber. Some airfoils have a lot, some don't have much. As Minn said, most trainers use a fairly high lift airfoil with quite a bit of camber. They also tend to be set up such that increasing airspeed results in a change in pitch trim, ie ballooning, which also means it will try to automatically pull out of a dive.

Stability is more a function of dihedral as Minn said (for roll stability), and the relationships between the wing and the tail (incidences and areas mostly) (for pitch and yaw stability).

There are some very acrobatic and maneuverable planes out there with cambered airfoils, though most acrobatic planes have symetrical airfoils to make inverted flight easier, and allow them to do maneuvers inverted almost as well as upright. And you can make a very stable airplane with a symetrical airfoil as well. So the airfoil is only a small part of the story.

That said, I looked up the Happy Fly, since I'd never seen one. I think Minn's recommendation is right on. You should be fine with any of the low wing sport planes like the 4* series, the super sportster, or the others mentioned in the many threads on "second" or "third" planes on the forums.

RE: Difference in Airfoils
 

If you just like to build, and have the budget, by all means go ahead and get a 4* or Super Sport, or any of the dozens of intermediate models that are available. You can't go wrong with any of them.

That being said, with nine solo flights you are a long way from exhausting the possiblilties of your trainer. The Happy fly 20 has no dihedral, and is more of a shoulder wing than high wing. The proportions are more like a "stick" than a typical trainer. If you move the cg aft a little and increased the throws a little you would have a very capable aerobatic machine. With the cg at the aft limit, it would even be a little wild. You may begin to master that airplane after a hundred flights. Get an experienced model pilot to give you a demonstration with your Happy Fly and I think you will be amazed.

To address your general questions:

Airfoil - The only advantage of a symetrical airfoil is that it flys upside down with the same angle of attack as right side up. It is not quite as efficient as a cambered airfoil but is pretty well standard on aerobatic airplanes. The difference in flying characteristics between symetrical and cambered is not much. The "ballooning" tendency mentioned by MinnFlyer is more a function of cg position than airfoil shape. A forward cg is very stable in pitch and will pitch up sharply with speed increase, whereas a rearward cg is less stable and will not pitch up much (balloon).

Dihedral- Dihedral causes an airplane to roll away from a sideslip. If a wing drops due to a gust, or control input, the airplane will slip toward the low wing and if it has dihedral will roll back toward level. In aerobatic flight, in maneuvers that involve a sideslip, dihedral requires aileron input to prevent the roll. This is most apparent in knife edge flight where the plane is in a constant sideslip, and if it has dihedral effect it will try to roll out. This is sometimes referred to as "roll coupling".

Performance- With any airfoil, a light wing loading provides better performance and slower landing speed. Landing speed is not effected by dihedral and cg position.

Tricycle landing gear- The gear configuation only affects the way it taxies. Going to a tailwheel configuration gets rid of the drag of the nosewheel and results in a little better performance.