which is more stable, symetrical or semisymetrical i.e. clark Y?

[Joel K. Scholz]

Working on an aircraft that is being designed for beginners. It is currently using a swept wing, with positive dihedral. I chose to use a symetrical airfoil because it stalls straight ahead. I feel I could reduce power requirements by using a higher lift airfoil i.e. Clark Y or eppler 197. Would I gain or lose stability?

[BMatthews]

All else being equal the cambered airfoil will require a larger stabilizer than a symetrical airfoil. But in practice the difference is very slight and as long as your tail length and stabilizer area are more or less "normal" you won't see any difference. Only on models with extreme setups will the pitching moment of the airfoil really show a problem.

For example a buddy of mine had a Great Planes low wing model with a huge chord. Sort of like a low wing Lazy Bee. I don't remember the name of it but I think it may have been the Slo Poke. Anyway, when flown at normal speeds there was no problem and the model had normal pitch stability. However when put into a dive the pitching action of the airfoil would take over and try to make the model tuck into an outside loop. It was very unnerving to fly and an example of some bad design choices IMHO.

But as long as you don't follow such an extreme example you should be fine.

Oh, and I'm not sure if the choice of the ClarkY or similar will let you fly it at a lower power setting or not but it will certainly allow the model to fly slower thanks to the fact that the airfoil will allow for a higher lift coefficient before stalling.

[Sparky Paul]

Joel, "stalls straight ahead" is more a feature of the trueness of the wing to no warps, equal washout on each side than the type of airfoil.
A modest amount of sweep won't make much of a difference, but a lot will require an unrealistic amount of washout to prevent the usual swept-wing tip stal.
Unless there's a pressing need for sweep, it's an added doo-dad that can interfere with good controlled flight.
As the plane is a trainer, a Clark-Y would be a better choice than symmetrical, for the more forgiving flight qualities.. adequate lift, modest drag, and ability to fly slower than an equivalent sized symmetrical wing.

[Joel K. Scholz]

Thanks Guys. These are helpful comments.

[Texas Buzzard]

which is more stable, symetrical or semisymetrical i.e. clark Y?

When you say "stable", may I assume that you are thinking about behavior as related to angle of attack & lift cooeficients & does it stall abruptly.

As a general rule an airfoil with a larger radiused leading edge ( more blunt) will have a softer stall. They also will maintain lift to a higher angle of attack.

To answer your question: a Clark Y as we modelers make them, can be twitchy heading into gusts - winds make them like a more forward C of G.

Personally I LIKE the semisemetrical over the symetrical airfoils - they get best of both worlds.

True symetrical airfoils ( especially those under 12% thickness) can be the least stable in pitch.

But use what works for you.

[macboffin]

One of my nit-pickin' hates ; "Semi-symmetrical Sections".
There are none! A section is either symmetrical or not!
How about "Half dead corpse". Half dry water. etc.

[slipstick]

One of my nit-pickin' hates ; "Semi-symmetrical Sections".
It's what we call "communication" ;). Everyone you know will readily understand what is meant by the term. If instead you talk about a "cambered airfoil with a low camber relative to its thickness and some lower convexity" most people will not have the faintest. I generally do pedantry as well as most but deprecating a useful, well-understood phrase because it's theoretically "incorrect" is too much even for me.

I guess that makes me merely semi-pedantic ;).

Steve

[MCarlton]

Remember, a section like Clark Y or indeed any other flat bottom section can be set at a lower incidence angle than normal with quite nice results, you get a nice flat cruise and then, throttle back and add a couple of clicks of up trim, and there, you've slowed everything down and it still flies well. Cut price variable geometry!