Pollywog airfoils work well for stunters, but there's no real advantage over more normal sections that don't have that tendency for the covering to lift away from the ribs. You may still hear these called Clark Y-type. Many full-size planes used "lifting' sections, so Scale and Carrier models often do too. Speed and racing CL models usually don't do much inverted or outside stuff, so they use "lifting' airfoils. For an aircraft flying slowly and carrying a heavy load a semi- symmetrical or under-cambered airfoil can be selected. The main reason I wanted to go undercambered is the scale appearance. Pollywog airfoils are a bit tricky to cover, since the paper, or whatever fabric, shrinks, and will try to lift off the ribs where the curvature is below a straight line or outward curve. Wingspan will be around 78', with a chord of 7.25'. This is why the undercambered airfoil is bad. So the smart designer chooses an airfoil that is not experiencing drag rise (not climbing the walls of the drag bucket) at zero CL.
#Undercambered airfoil free
If you've ever seen a Free Flight "undercambered" airfoil wing, you should get the idea. On another note, we all know that CL of the wing sections falls off kinda elliptically going out the span as the wingtip is approached. There's a story often heard that an early kit manufacturer changed the ribs in his kits to this shape so that he could print or die-cut more ribs on a sheet of balsa that fit into his kit boxes. Most pollywog sections I've seen were symmetrical, usable for stunt. Intro to Flight by John D Anderson is a great starting point for basic design parameters. Pollywog airfoils' shape drops below a smooth outward-bulging curvature in the back third or so of the wing's chord. In general to start a preliminary aircraft design problem you need to start with basic parameters such as configuration and geometric parameters which will contribute to GTOW, available power, target stall speed etc.
#Undercambered airfoil for free
It was, probably still is, popular for free flight sport type models, and other uses where you basically only need lift when the model is right side up. Instead, just about any airfoil with a normal looking top surface curvature and flat bottom surface, from near the leading edge back, has been called a Clark Y-type airfoil. I doubt many "true" Clark Y aifoils were ever used by modelers.
IT also shows how slats work.The Clark Y airfoil actually had a specific set of coordinates - heights of upper and lower surfaces listed by % distance back from the leading edge. one last thing and I am not sure it is talked about here but thin undercamberd airfoils can exhibit nasty stall characteristics without any aerodynamic warning.in fact I have read lots of accident reports, "I was in a 45 degree bank at 50 mph with 2 on board and it stalled"ĪNyway I thought the video was of interest to all here. I guess they work on really slow very low powered airplanes but as kitfoxs amd avids got bigger and bigger engines, it reared its ugly head as a drag producer. Since an undercambered airfoil achieves a higher CL at the cost of a higher CD, the flat-bottom airfoil will have an advantage in penetrating the wind. I think Trent said 4% of the airfoil which I think is only 2" aft of the wing leading edge.Īlso the undercambered airfoil has more drag than most airfoil even at 0 angle of attack. This is why it is important to get the VGs up front, if too far back they are in "dead" air and cant do any good. These results obtained are calculated using the thin airfoil theory so the use of the results are.
But for cambered airfoils the aerodynamic center can be slightly less than 25 of the chord from the leading edge, which depends on the slope of the moment coefficient. It is also used for reinforcing closed plywood structures. For symmetrical airfoils, so the aerodynamic center is at 25 of chord. IF you look at the flat or undercambered airfoil, at high angle of attack the airflow seperated pretty far forward on the wing. Aircraft fabric covering is a term used for both the material used and the process of covering aircraft open structures.
Flat Bottomed and Under-Cambered airfoils are the highest weighted total in decision matrix. Here is an interesting video circa 1938 about flat, uncambered and streamlined air foils. Decision Matrix used to determine best airfoil class.
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