Which Airfoil Should I Use?

Symmetrical? Semi-symmetrical? Undercambered? ...What?

The field of fluid mechanics is populated with a lot of very clever people who really know what they’re doing. As amateurs in the field of RC airplanes, it’s sometimes difficult to get our heads around subjects such as airfoil types and which is the best for a given aircraft. Hopefully, this article will dispel some of the mystery. 

 This Vampire has a semi-symmetrical airfoil which provides a great lift-to-drag ratio. 

Here’s A Secret

The subtle differences in the shape of airfoils are really not that important for most model airplanes. Sure, if you’re going for an endurance record, you might want to think about it more, but in most cases fine margins are imperceivable. Thankfully have a lot more room for error. You want some proof? Here's a circle that flies. 

Categories

Airfoils come in a whole range of shapes and sizes, both in the aerospace industry and in nature - really, nothing is standard. It all depends on the application; the environment in which the airfoil operates dictates its size and shape. 

It is possible to boil types of airfoils down to into various categories. Here are eight of the most commonly used airfoils used on RC model airplanes (in no particular order):

• Symmetrical
• Semi-Symmetrical
• Flat Bottom
• Under-cambered
• Reflexed
• Flat
• Stepped

Getting more into the technical and mathematical side of airfoil design, here are two popular names that you may see when researching airfoil shapes. Reading up about them might be a good way to learn about the nuances of how designers create airfoils. 

• NACA airfoils - These airfoils map a four-digit code to an airfoil to indicate camber, chord and thickness specifications.
• Clark-Y airfoils - These are flat bottomed airfoil shape which is widely used. The thickness of this airfoil is always 11.7% of the chord. 

Which is best?

Let’s break down the main categories to give you a better idea of which airfoil is right for your airplane.

Symmetrical 

Mostly for aerobatic planes. Cuts through the air evenly. Easy to trim. Great for precision aerobatics.

Semi-Symmetrical 

For sport planes, semi-aerobatic planes and sailplanes. Excellent lift-drag ratio. 

Flat-Bottom 

For easy to build airplanes. They’re simple to manufacture, draggy (which is sometimes good for creating a stable flight experience) but are often sensitive to changes in airspeed which makes them difficult to trim.

Under-cambered

For small, lightweight airplanes. Provide lots of lift but are quite draggy. Good for slow flying models. You can under camber just the outer section of a wing to increase stability. 

Reflexed

For flying wings. The trailing edge is upturned slighting which provides stability. Check out this article on how flying wings fly to find out more. One way to add reflex is to simply trim the wing's elevons slightly upward. 

Flat

For simple models. Flown at a high angle of attack, flat-bottomed wings are probably the simplest to make but can perform fairly poorly. Aircraft like the FT Viggen use angled canards to help provide lift. 

Stepped

Again, for simple models that don’t fly all that fast. A Kline-Foglean (KF) airfoil is one such type that is commonly used due to its ease of construction. They work by displacing air in a vortex pocket behind the step.

How To Choose One

As mentioned previously, the secret of choosing the correct airfoil is not to overthink it. You shouldn’t have to agonize over the differences between closely related NACA airfoils - there will be virtually no perceived difference to your flight experience. The best way to choose an airfoil is to start by thinking about what experience you want to have and how you’re going to manufacture the wing. 

Choose the type you think best fits your needs and go for it. Test it out and then tweak something on the next version. Working through a trial and error procedure is one of the best (and most rewarding) ways to learn. 

What experience do I want?

What will be the wing-loading and power range of my aircraft?

What will the wing shape (swept, tapered, etc) be?

How am I going to manufacture the wing in the first place?

Aerodynamics Podcast Episode with Josh Bixler

If you want to learn more about aerodynamics and you’re interested in the subject, here’s a recent Flite Test podcast all about it. The episode features Josh and James and covers Josh Bixler’s approach to designing Flite Test airplanes with aerodynamics in mind. 

Further Reading about Aerodynamics

Here’s a short reading list of articles you should read next! 

Wings

Stalls

Wingtip Devices

Ground Effect

If you found this article helpful, remember to give it a like!

Article by James Whomsley

Editor of FliteTest.com

Contact: james@flitetest.com

YouTube Channel: www.youtube.com/projectairaviation