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Old 12-11-2012, 10:19 AM
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Picking the “right” airfoil for model aircraft

Very few people who build model airplanes would have a clue how to pick an airfoil for their design based on “real” airfoil data (real meaning empirically collected test results from the aerospace industry) and this includes the airfoil specifications displayed in RealFlight. This is further complicated because some data on real airfoils won't directly apply in the modeling realm, as data is collected off of much larger scales than the average model and while the airfoil can be scaled the air particles it is moving through can’t be. Most airfoils, however, will scale down comparatively. What this really means is that in comparison, if one airfoil is tested to stall sooner than another airfoil, then the stall speeds may be much different when scaled down but both airfoils should still stall in the same order.
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Old 12-11-2012, 10:21 AM
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Most modelers have to learn from experience, knowing that the subtleties between one airfoil and another close to the same shape will make very small differences. So here I will review the basics on airfoil design and selection, sticking to generalities that are easy to observe. The attached image was borrowed from a Paraglider Design Handbook. I chose to include it specifically because it is fairly basic and without any extra info that most people don’t need. Airfoil geometry can be summarized by just a few parameters. These are maximum thickness, maximum camber, position of max thickness, position of max camber, and leading edge radius. A reasonable airfoil section can be made, given just these parameters.

Common airfoil design terms (simplified):
Leading Edge - the foremost edge of an airfoil section.
Trailing Edge - is its rear edge
Chord (or Chord Line) - is a straight line connecting the leading and trailing edges of the airfoil and the value C represents its length.
Mean or Camber Line - is the line defined by the vertical mid-point of the airfoil at every point along the chord.
Camber - while the calculation of Camber is more complicated than I want to get into, it can be thought of as an average measurement of the distance between the Mean and the Chord lines divided by the Chord.
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Old 12-11-2012, 10:23 AM
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The most common families or groupings of airfoils are:
Symmetrical – Top and bottom are exactly the same. This creates an airfoil without Camber, that also produces zero lift at a zero angle of attack. All other airfoils are cambered and will produce aerodynamic lift due to their shape moving through the air.
Semi-Symmetrical – Like the picture shown above, the top and bottom are similar but the airfoil has a slight camber.
Flat Bottom – Very uncommon in RC models, airfoil bottom is flat from leading edge to trailing edge. This shape provides a fairly high camber and results in high lift, but can be structurally weak and hard to effectively use on models.
Modified Flat Bottom – Flat from main spar to trailing edge. This is actually a semi-symmetrical airfoil, but it’s the shape most people think of when you say flat-bottom. This shape increases the camber of the rear section of the airfoil moving the center of lift aft, giving more gentle stall characteristics.
Under-cambered – Mostly used on gliders (and WW I aircraft), the bottom closely matches the top with just some space in between. This shape produces the most lift of any airfoil, but also the most drag.
Reflexed – The trailing edge turns up slightly causing the camber to change to negative toward the aft of the airfoil, giving a positive twist to the lift (positive to the front and negative to the rear). This type of airfoil commonly found on flying wings without a horizontal stabilizer.

It should be noted that airfoils which have the same basic shape within a family, will also have common flight characteristics. Any differences between them will be small and on the order of landing a little faster and not an immediate loop into the ground on take-off.
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Last edited by dhk79; 12-11-2012 at 10:47 AM.
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Old 12-11-2012, 10:25 AM
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Rule of Thumb for Airfoil Selection:
Choosing an appropriate airfoil family for any given design is usually simple. If the plane is to be a precision aerobat then a symmetrical airfoil is most appropriate because it flies the same in any given attitude. A true aerobat should always have symmetrical tail surfaces too. While flat (slab) surfaces, which are symmetrical sections by definition, work well for tail surfaces up to a point they aren't as good as a true airfoil.

If the plane is to fly slowly or carry a heavy load (but is not intended to do aerobatics) then a semi-symmetrical or under-cambered airfoil should be considered.

For secondary trainers, sailplanes and sport aerobatic biplanes a semi-symmetrical is common choice. If the biplane is intended to do precision aerobatics, however, then a fully symmetrical airfoil should be used.

True flat-bottom airfoils are generally a poor choice for any design. They are next to impossible to trim properly because they are extremely speed sensitive. It is possible to trim this trait out, but it means spending hours tweaking the wing incidence, decalage (angle difference between the upper and lower wings of a biplane), and engine thrust.

Modified Flat bottom airfoils are used for powered aircraft that are willing to make the compromise of having more drag in exchange for slow flight or high lift capabilities. They do not penetrate the air well but can stay aloft at very low speeds.
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Old 12-11-2012, 10:26 AM
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Airfoil Thickness
How thick should the airfoil be? Wing thickness is a compromise between speed and lift. A thicker wing has more drag but more lift and is capable of slower flight. Thicker wings also tend to "bounce" around more in the air because they can't cut through it as easily. A thinner wing produces less lift but is faster.

One other thing to note is that as wings get thicker they also become stronger. If a wing is thick it is easy to build it strong using conventional construction techniques. If the wing is thin then more exotic techniques are required to prevent the wing from breaking in flight.

Most sport designs have airfoils in the range of 14% to 16% thick. These airfoils have proven to be safe with few or no bad habits at reasonable wing loadings and can slow down nicely to land. I normally use airfoils from 12% to 18% depending on the aircraft and drop down to about 10% thick for an extremely fast model.
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Old 12-11-2012, 10:27 AM
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Selection Trade-offs within airfoil families:
A thinner airfoil flies and lands a little faster and is smoother, but a similar one that is a little thicker can perform aerobatics in a tighter volume.

The leading edge radius takes the lead role in stall characteristics. A sharp (small radius) leading edge typically has a shallow stall angle. That means it will stall sooner than a blunt leading edge. Therefore, a smaller leading edge radius will allow sharper stall maneuvers, and a wide one will give a gentler stall.

Stall Characteristics:
In addition to the leading edge radius, washout can be used to improve (lessen) stall characteristics. Washout simply means the wing is built with a twist (either physically or by airfoil selections) so that the wing tips are at a lower angle of incidence and stall later than the wing root thus keeping the ailerons effective longer in a stall. Washout, however, limits aerobatic capabilities so it’s a trade-off. A better option for aerobatic aircraft is to sand the leading edge such that it becomes more blunt toward the tip.
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Old 12-11-2012, 10:31 AM
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Final Comments:
Anyone who managed to read and absorb most of this should realize that while RealFlight has about 1500 airfoils available, it only takes around 20 or so to handle most situations.

Next up – Picking the “right” Prop.
Carefully match the power plant and propeller to the airframe instead of matching the propeller to the power plant alone. All airplanes have a maximum airspeed at which they will fly smoothly. If the engine has more power available after this speed is reached you won't see more speed, but the model will begin to buffet or worse.
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Old 12-11-2012, 01:28 PM
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I smell a sticky!
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Old 12-11-2012, 03:12 PM
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Sticky it!
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I used to like my planes bouncy...but now i am thinking super-cheap to re-build is better...
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Old 12-12-2012, 12:09 AM
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Quote:
Originally Posted by Kris... View Post
A very valid point jeffpn can a mod please delete all are posts sorry Doug.
You can easily delete your own post.
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Old 12-12-2012, 07:43 AM
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Doug,

What does the camber value actually tell me? How is that value used? It is obvious how the fowler flaps are used in commercial jet wings to create an under cambered wing for huge lift. Other than the Stortch(& WWI aircraft) have you seen it used in RC planes?

I have a friend building a true slope soaring glider, he is using a fence like extendable spoiler device on the top chord to kill lift. Is it possible to model that in RF? Great thread... sticky vote. and the prop guide too!

Last edited by 12oclockhigh; 12-12-2012 at 07:52 AM.
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Old 12-12-2012, 12:34 PM
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Camber is a value that can represent the comparative aerodynamic lift of the airfoil. This is the lift that an airfoil will generate at zero relative angle of attack. A higher camber means more lift (also drag, but we won't worry about that now). A fully symmetric airfoil has no camber and therefore will not generate any lift at a zero angle of attack.

Look at it this way - A trainer with a high camber can fly level to the ground and stay up, but a sport plane with a fully symmetric wing needs to maintain a positive angle of attack in order to stay up.

In R/C most under cambered wings are wire-cut foam (or injection molded), as they'd be a real pain to try to build out of balsa. You'll find them on some gliders or slow-flyers.

You can model a spoiler in RF's physics, it's just another control surface. It takes some work in the 3D model to make it look pretty, but it can be done.
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Old 12-12-2012, 01:28 PM
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I read this and scratched my head... the Alpha 40/60 and several other trainer designs are flat bottom wings. I found them good flying planes. Now the minute you want to do anything other than flat level flight.. they go screwy.

Quote:
Originally Posted by dhk79 View Post
True flat-bottom airfoils are generally a poor choice for any design. They are next to impossible to trim properly because they are extremely speed sensitive. It is possible to trim this trait out, but it means spending hours tweaking the wing incidence, decalage (angle difference between the upper and lower wings of a biplane), and engine thrust.
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Old 12-12-2012, 01:41 PM
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Not, really. A flat bottom wing can fly without problem but will often need design tweaking to get it set up right. I probably wasn't clear enough with my statement - if you are designing a new aircraft, a flat bottom wing would probably not be the best choice to select unless you really need the slow speed flight capability. In the case of existing models, someone else has already done the design tweaking and you'll probably only need to adjust trim. Additionally the speed sensitivity can be a big issue and for this reason most trainers are under-powered. If you ever try putting a bigger engine on a true flat bottom trainer, you'd find it real pain to fly.

Since we're talking about a simulator here, give it a try and let us know what you think.
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Old 12-14-2012, 11:08 AM
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We did some research and development airfoil structural testing yesterday. You might be interested in the early compiled video results before the FAA and NTSB report is issued.

http://www.youtube.com/watch?v=fXD8a...ature=youtu.be

Do you think that if we had picked a thicker NACA airfoil the results would have improved?
.
.

Last edited by 12oclockhigh; 12-14-2012 at 11:10 AM.
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