What good is a symmetrical airfoil?

Mitch G

I'm trying to understand what benefits does a symmetrical airfoil offers that a traditional (flat bottomed) airfoil doesn't?

Similarly, what downsides does a symmetrical airfoil have compared to a traditional airfoil?


Mitch

stuartaw

I think the advantage of a symmetrical is for aerobatics - it lets the plane fly pretty much the same either way up - very useful for 'pattern' flying.
As far as I know assymetrical airfoils usually give better lift: ie better lift to drag ratio, and/or more lift at a given speed.

Stuart

ChrisP

Agree with Stuart, plus......

The movement of the centre of pressure of symmetrical aerofoil is less than with cambered aerofoils. Please note that a 'flat bottomed aerofoil' is also a cambered aerofoil !

PROVOCATIVE STATEMENT :

This is why trainers with a symmetrical aerofoil can be EASIER to fly than the typical flat bottomed types.

If you fly one of the vintage type models (such as the Junior 60) with a heavily cambered aerofoil, you will find that these are basically one speed aeroplanes.

Ron Cichowski

Makes flying stabs work better too. Also useful in slope where most of the lift comes from the vertical component of the wind rather than the foreward motion of the airfoil.
Ronm

Flying

Simplified answer (Cos I'm a simple guy) A symetrical airfoil depends on the angle of attack (angle of airflow)to generate lift whereas semi symetrical or flat bottom wings will generate lift at at no or even negative angles of attack. This has advantages for some types of planes eg Aerobatic models when you want the plane to be able to fly well both ways up. It has disadvantages too as more power / speed is needed to get a given amount of lift.

I'm Flying

Mitch G

Hmmm.

The reason I asked about this is that I have an E-plane and it uses a symmetrical airfoil.
Well, I find it hard to trim out and find it quite sensitive to CG and motor thrust angle. This seems to make sense since generating lift is all about angle of attack with this kind of wing.

But, I didn't see these issues mentioned as negative aspects of a symmetrical airfoil in the replies received. So, is this just my experience? Or, are these some of the reasons all planes don't use symmetrical airfoils? Because, the replies I've read so far make it seem that symmetric is best(assuming you have a decent power plant).


Mitch

Sparky Paul

All wings (airplanes) are "sensitive" to the c.g. position. Move yours forward, or cut down on the control throw, either thru physically getting less throw or use dual rates/expo to desensitize the system near neutral.
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Sparky Paul http://www.angelfire.com/indie/aerostuff
PJB's Seriously Aeronautical Stuff http://www.networkone.net/~pjburke/index.html

RB

When you dive a plane with a flat bottom wing the plane automatically pulls out because the wing makes more lift the faster it flies. This is one reason trainers have this airfoil.

A symmetrical wing in a dive just continues the dive { assuming the cg and incidence is correct }.

Ray

ChrisP

This is the way you trim a model :

Step 1. Get the C.G. to where it is supposed to be. This determines the stability margin of the model.
Step 2. If the model does not glide properly, use elevator trim or change the incidence difference between the wing and tail. Do NOT change the C.G. as you will be changing the stability margin.
Step 3. If the model does not fly satisfactorily under power, change downthrust until it does. Do NOT change C.G. or incidences.

With great respect to Ray, all wings create more lift the faster the model flies. In fact irrespective of the aerofoil section a wing will generate 4 times as much lift if the speed is doubled (assuming that the aerofoil is not at the zero lift angle of attack).
Also symmetrical aerofoils do not 'continue to dive' in comparison to flat bottomed aerofoils. What is being talked about here is the stability margin of the model, not the aerofoil section.

The statement 'symmetric is the best' is also not correct ! No way would you build a floater type of sailplane with a symmetrical section. These models fly within a very narrow speed envelope to which the lift and drag of the aerofoil section is tuned.

A really good example of the use of varying aerofoil sections are planes like the Boeing 767. In cruising flight the aerofoil is almost symmetrical. On the landing approach, with all flaps out, the aerofoil is highly undercambered and - significantly - the flaps considerably increase the wing area in turn lowering the stall speed.

Lift is only a function of :
Air density (hot and high is bad !!)
Velocity
Wing Area
Lift coefficient of the aerofoil (form & angle of attack)

It's that easy.

If you want to know more, please buy Andy Lennon's excellent book on R/C aircraft design! It will tell you all you want to know.