Airfoil opinions wanted

[zbrubaker]

I'm new to designing my own aircraft, and being an engineer, I like to understand and design everything before doing any work.

I'm designing a series of park flyers and I want to figure out the best airfoil to use. I don't know much about renyolds numbers and such, but I do understand most of the basic airfoil principles.

I've considered an undercambered wing, but would like better stall resistance at higher AOA, so I've considered the venerable Clark-Y. I've also heard that there are some benefits to having some reflex? on the trailing edge of the wing to improve laminar flow. What I've come up with is a Clark-y with the back half of an Eppler 379. I'm attaching a pic of what I've come up with (top is my airfoil, bottom is Clark-Y)

I'd be interested to here opinions other than "aerodynamics don't matter at that small scale"

Zeke

[Sparky Paul]

Look at the park fliers out there now.
Mostly have a "single-surface" highly cambered wing, for lift and drag, to keep the plane slow and close for the environment of a a park.
A more "airfoiled" airfoil works best on a plane with more performance.. especially in terms of flight speed. The faster it goes, the more difficult to keep it in close.

[zbrubaker]

I thought about undercambered, but I'm planing on using the EDF50 for power, so I want to reduce drag as much as possible. I'm looking at 'outfield' rather than 'infield' parkflyer range.

[Ollie]

Listen to Paul. Take his advise. He really knows what he is talking about.

As an example of what Paul is talking about: The GM15 airfoil is only about 60% as thick as the Clark Y yet it has much lower drag, just as high a maximum lift coefficient and a far superior stall characteristic at park flier sizes and speeds. I suspect that even thinner airfoils will be more effective for park fliers but , to my knowledge, they haven't been accurately measured in wind tunnels. The aerodynamics of park fliers is closer to indoor rubber models than to full scale aircraft that the Clark Y was designed for.

See:
http://soaring.cnde.iastate.edu/calcs/frames.shtml
for an on line program that can compare up to five airfoils at a time by displaying their lift-drag polars.

[zbrubaker]

Hi Ollie,

Thanks for the input. I looked at the GM15, at it looks like it would be a great choice.

Is there a way to determine/estimate the angle (incedence) the GM15 should be set at?

Zeke

[Ollie]

The angle of incidence can be anything because the reference line is arbitrary. The longitudinal dihedral or decalage is the angle between the wing and horizontal tail. That's what counts. A good place to start flight testing is about 1.5 to 2 degrees of decalage and varies some with aspect ratio because of induced angle of attack and the zero lift angle of attack of the airfoil. The best decalage angle is usually determined by flight testing and depends a little on where you locate the CG in the acceptable CG range. The longer the tail moment arm in relation to the average wing chord and the larger the tail area in relation to the wing area, the more aft the acceptable CG range will be located. CG location and decalage are partly a matter of matching the adjustments to the pilot's preferences and flying style as well as the purpose of the plane. Decalage should be adjusted by observing power off glide. Thrust line should be adjusted to compromize a variety of throttle positions. In the words of Don Stackhouse,"Everything affects everything else." I would qualify that with "almost and at least a little."

Prototypes of a design should have the angles between the thrust line, wing chord line and tail chord line easily adjustable because flight testing is almost always required to achieve the desired results. Accurate data is usually not available to feed all the necessary equations to be able to calculate everything ahead of time.

Even in the world of full scale professional aircraft design, prototype flight testing and adjustments are required. That's also why trim tabs are popular features of full scale aircraft and why R/C transmitters have trim levers.

[Dick Huang]

Zeke,
Both Sparky Paul and Ollie have given very sound advice. I offer you some single surface airfoils to try: they are the "Simplex" series ranging in max camber 4% to 10% at 40% chord. You can find more data on these airfoils from "Airfoil Sections" by John Malkin.
Dick Huang;)