I have apparently had my brain eaten by my 18mo old son.... he's worn me OUT the past 2 days (mom had dental surgery) and I can't for the LIFE of me remember this IMPORTANT little factoid....

There are 3 "basic" types of wing profiles (at least for trainers): "flat bottomed" (again, lost my brain to a toddler); semi-symetrical (where it's ALMOST the profile along the top of the wing as the bottom); and symetrical (top and bottom of wing have the same profile). The semi and the symetrical have rounded edges on the top and bottom (i remember that much.....)

Here's where the STUPID question comes in: I know that "flat bottomed" wings are HIGHLY stable, largely "self-correcting" and the standard type for trainers - characterized by LONG glide paths (usually...). But how do the semi and symetrical wings "act" in the air? I'm sure they're easier to stall and probably a little trickier on landing b/c of not as much glide, but what are the REAL characteristics in flight?

I told you this was a stupid question..... maybe it can help some REALLY new people Why I didn't write this into my "little red book" of "important crap to remember" I have NO idea...... [:@]

Thanks for the patience [&o]

I am by no means an aerodynamic expert but semi-symetrical wings generally perform much better inverted than flat bottom wings in my experience. This makes the plane perform outside maneuvers better. Symmetrical wings do even better yet.

I suppose it is true that flat bottom airfoils do produce more lift for a given surface area or they wouldn't be used so much on trainers. I don't think that tells the whole story though. Some semi-symetrical and fully symetrical wings provide a lot of lift and the planes glide very well. In some cases, almost trainer like. I assume this has to do with the specific characteristics of the airfoil, wing area, weight of the airplane (wing area and weight = wing loading), drag on the airplane, etc.

For example, a 4* and GP Rapture use a semi-symmetrical airfoil and the Tiger-2 is symetrical. In my experience, all glide well, land gently and don't have any "surprises". This tells me that the general shape of the airfoil is one factor but there are definitely others as well.

Like I said, I'm no expert though. This is based on what reading I have done and my own experience & observations.

First, 18 month olds are a piece of cake compaired to 18 year olds. You may not agree now, but by the time you have cared for both, you will agree[8D]

The airfoil has nothing to do with self corecting. Dihedral is what tends to right the plane back to level flight, at least as far as keeping the wings level with the horizon.

A flat bottom wing will have lift at zero degrees angle of attack. A semi symetrical will have some lift, but more with a sligh angle of attack. A full symetrical has no lift until the angle of attack increases a bit.

I crashed and killed my 4*60. Semi Symetrical wing. I bought a Pheonix fun star as a quick replacement. With my 4*60, I would get the plane centered on the runway and slowly increase speed, keeping on the center line and it would lift off in a shallow climb, like a full size plane untill I pulled a bit of up elevator then it went into a steep climp. The fun star on the other hand would have happly eaten up the full 500 ft of runwan and never lifted off if I handn't cranked in a bunch of up elevator to change the angle of attack. It turns out that the Fun Star sets with about 2 degrees of incidense when on it's wheels, so I had to push down on the spring of the tail wheel to get the angle of attack high enough to get lift. What a susprise. Landing on the other had was a pleasure as the plane keep the low AOT when it touched down. The 4*60 would ballon if the speed was to high when the wheel touched down and the taill dropped..

As pointed out before, the closer you get to a symetrical airfoil, the eaiser it is to fly inverted. Remember that the airfoil doesn't know if it is inverted or upright. A flat bottom wing's lift is to the top, or arched side. If the side is inverted, the lift, is still there, driving the plane down. It takes a high angle of attack to keep a true lift to keep the plane level. As the symetry increases, the lift decreases at zero AOT and the the fully symetrical all lift is due to AOT.

Hug and kiss the kid and help mom get better soon. Just remember your mon and dad went through the same things with you. You are just paying your dues for being raised by your parrents

Don (a four time grand dad who still remembers when he was young)

oh dear GOD please don't make me think about "Damien" as an 18 year old I'm depressed enough as it is b/c I can't fly my Senior Falcon until I either figure out how to clean it where it is (TOTALLY enclosed -> don't ask, I didn't build it, I inherited it from my Father-In-Law and HE can't even remember why he did it ! The '70's... what can you say...[&:] )

Thanks for the good info on the wing. If I understand you correctly, flying straight and level "hands off" is best for "flat bottoms" and semi+symetricals demand a little trim on the UP elevator to keep the AOT above zero (just a little, not overkill )

I don't EXPECT to fly inverted any time soon, but you NEVER REALLY KNOW until the spam hits the fan I've got a hard enough time keeping the plane in the air w/o getting inverted and "brain baffled" having to remember that pulling UP means you just plowed a row of potatoes [:@] ( I saw a guy do this recently in his BRAND NEW Hanger 9 P51 PTS..... I felt nauseous [:'(] )

Not necessarily, I have a Big Stik 40 which has a symmetrical airfoil and it is very difficult to stall it. If you try really hard you just get the nose mushing down before she starts flying again. It will also slow down to a walk for landings.
I think tip stalling the wing has more to do with the shape of the wing cord than the airfoil symmetry.
I learned on an Avistar (semi symmetrical) and it was really easy to fly. I also built a kadet LT 25 for my son (he is 8 and flying!) and it has a flat bottom. It is really stable and self correcting due to the dihedral, but the flat bottom makes it hard to fly in windy conditions.

Cheers,
Ray

Nope

Flying straight and level has nothing much to do with the airfoil, flatbottom or not. And semisymmetricals don't demand any kind of trim from the elevator if the airplane was designed correctly. The size of the horizontal tail, how far back it is, and it's angle of attachment to the fuselage (the angle of incidence) are all designed to adequately control the pitch of whatever airfoil the designer chose for the airplane and to provide adequate pitch stability. The designer designs that horizontal tail to work adequately and efficiently, and having to trim the elevator suggests he didn't do the design correctly. Yeah, you might have to trim for a speed other than normal cruise, but thats an exception. Models and fullscale planes have a design speed where trim isn't needed.

There isn't a best airfoil for flying straight and level. The major difference in your three types of airfoils is how much weight they can carry and how efficiently they do it.

Wing loading is a major factor in the flight characteristics you're attaching to profile shape and dihedral is another.

You really can't attach such general flight characteristics to profile shape. If you want a good understanding of how models and full scale airplanes fly, "BASICS OF R/C MODEL AIRCRAFT DESIGN" by Andy Lennon is an excellent book. It's cheap, covers EVERYTHING and does it very, very well for guys who don't want a degree in aeronautics but who do want to understand it all. You'll find out one thing for sure. That there are a number of important things and not a single one of them "does it all" on how the airplane flies. The book might also catch your boys attention when he starts reading. And until then, it'll give you answers to some of the questions he's sure to ask about airplanes. He hasn't started asking about everything yet has he. Wait until he learns the words "why" and "what".

The book by Andy Lennon is good for fundamentals, just don’t take some of his stuff too serious, and be aware that some the equations are wrong.

There is only thing one can generalize about airfoil shape if we constrain the foils to flat, semi, and symmetrical. The only generalization is the symmetry of positive vs. negative angles of attack (AOA.) It is not possible to make any other conclusion, including stability, wing loading, any lift to drag assumptions, stall characteristics, etc. without knowing the exact airfoil shape, size of the chord, and the speed of flight.