Rotaryphile

I am scratch designing and building an Edge 540, for powering by 10 to 15 horsepower engines. Am planning on using the scale airfoil, but have never seen a model that uses the very blunt (scale) airfoil, with maximum thickness at about 15% chord behind the leading edge. Such airfoils are used on most of the full-scale unlimited competition aerobatic monoplanes, such as the Edge, Extra, and Sukoi to good effect. I am wondering if there is a reason why modelers reject that type of airfoil in favor of a more conventional airfoil with maximum thickness about 30% chord behind the leading edge, typical of pattern model airfoils. I have used very blunt airfoils on competition fun-fly models with good results - they seem to have higher maximum lift coefficients and the additional drag seems barely noticeable.

willembad

I am no expert in any area of aerodynamics, but I believe there is two main reasons:

1) Airfoils don't scale well because the different copies will have to operate at different Reynolds numbers.

2) The sharper leading edge on the model airfoils causes it to have a cleaner stall. A lot of flying these days are happening during the stalled condition, but the sharper airfoils make for a cleaner transition. Most of the latest aerobats are very lightly loaded and need the help stalling.

Just my thoughts.....

Willem

AFSalmon

My Feedback: (3)

This topic sort of came up in another post in this forum. Seems moving the peak thickness forward in some applications can cause a more abrubt stall characteristic but maybe a wide range of usable AoAs. Now this is the first time I've heard of full scale aerobatic airfoils having the peak thickness at about 15%! That seems very radical as compared to the typical 30% of the NACA 00xx foils. Are you sure this is correct?

Ollie

The E472 was designed by Dr. Eppler for full scale aerobatic aircraft. It has its maximum thickness at around 17% of the chord. It was tested at model reynolds numbers in the University of Illinois windtunnel by Dr. Selig, et. al.

Here is a quote from the airfoil comments in Summary of Low-Speed Airfoil data, Vol. III, page 32:

" -- Eppler identified four important factors of successful aerobatic airfoils: high lift coefficients both upright and inverted, a hard stall for flying snap maneuvers, high drag near zero lift to aid in speed managment and, low drag at high Cl's to improve maneuvering performance."

The comments go on to say that the E472 meets these requirements at model reynolds numbers above 100,000. Continuing the quote:

"Although the airfoil was not tested with flaps, Eppler did show computational results showing an extremely hard stall with positive flap deflections (trailing-edge down). While this may be useful for snap maneuvers, it could prove less than desirable when maneuvering at low speeds during landings."

My take on this is that inboard split flaps should be used on models employing airfoils with these characteristics if slow, safe landings in gusty conditions are a design objective. In any case the model must be flown at angles of attack well below the stalling angle of attack to avoid rude surprises during the landing approach.

shill

My Feedback: (22)

When fiberclassics designed their Extra 330s they tried the full scale airfoil first. It flew precision fine, but wasn't any good at 3d. It may have been the other way around, I'll see if I can find out for sure. Ether way, they went back to a more standard model airfoil for best all around performance.

Tracy Hill

AFSalmon

My Feedback: (3)

Shill,

I'd be curious to know which way the fiberclassics 330 wing acted using the fs airfoil. Look forward to your checking this out. Ok, from Ollie's inputs, sounds like 3D flying really does not want a forward peak thickness point but rather the standard NACA position of about 30%. I'm still curious however as to whether the standard NACA foil really is the best for a 3D aerobat model. Wonder if there's a better position maybe around 25% chord?

Always in search of knowledge! Mike

shill

My Feedback: (22)

AFSalmon,

I found the quote on the fiberclassics website.

"The first try with a full scale airfoil wasn't too successful, although it performed great freestyle maneuvers. For pattern all test pilots did't like the snap characteristics. Beginning of 2000 we decided to mill a new wing mold. Now the FiberClassics Extra 330s has a slightly modified pattern airfoil, as we use on our successful Giles G-200."

The airfoil on the fiberclassics G-200 and Troybuilt extra 260 are Similar, same designer. The Troybuilt comes to a shaper point. The hight point is further forward than normal. There are some pictures on the Troybuilt site in the 42% Extra section, that will give you some idea of the shape.

http://www.troybuiltmodels.com

The Troybuilt is my favorite plane. It snaps better than anything else I've flown. But you can't make it snap with elevator alone, at any speed or elevator throw, which is real nice for 3D. The FiberClassics is a very close second.

Tracy Hill