I'm working on a conceptual design phase project for aerobatic aircraft (for university) and I can't seem to find any statistics about key aerodynamic properties (such as in the question), so I hope somebody here does know something about this subject?
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1$\begingroup$ Related but not identical $\endgroup$– KoyovisDec 11, 2017 at 12:10
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$\begingroup$ If it's NACA 0012/0015 it will be in Abbott & Von Doenhoff, Theory Of Wing Sections. $\endgroup$– KoyovisDec 11, 2017 at 13:47
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$\begingroup$ Do they usually have flaps? Then with or without flaps extended? $\endgroup$– user7241Dec 11, 2017 at 17:52
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2 Answers
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First, don't take the two-dimensional value from an airfoil chart and use it for the full aircraft. Subtract 15-20% for wingtip margin and fuselage influence. Wingtip margin means that the wing is less loaded towards the tips to keep a lift margin for aileron control and to avoid a tip stall.
Next, aerobatic aircraft have lower maximum lift coefficients than GA aircraft. Their wing loading is lower, so they can afford this. A symmetric airfoil mounted at zero incidence has proven to be the best choice, and while some older aerobatic aircraft use cambered airfoils, the camber creates a distinct disadvantage in transitions from normal to inverted flight and vice versa.
And last, the maximum lift coefficient depends on quite a list of parameters. If you need a value for the calculation of structural loads, make sure you add enough strength to cover the increased lift possible with high pitch rates.
If you need a figure for a single-seater piston-powered aerobatic aircraft without flaps for the estimation of minimum speed, use a lift coefficient of 1.3.
answered Dec 11, 2017 at 20:34
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$\begingroup$ Actually, engine thrust angle and the fuselage angle create a lifting body, add to the lift component and is considered to cancel the asymmetric span lift. So using a CL based on wing area alone comes close enough to real world results to be commonly used for theoretical performance figures. The one adjustment usually made is increased drag due to tip vertices and aspect ratio. An elliptical wing is considered 100% efficient while a rectangular wing is about %90. Using the mean chord width also negates changes in Reynold number for a tapered wing. en.wikipedia.org/wiki/Elliptical_wing $\endgroup$ Dec 11, 2017 at 22:02
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$\begingroup$ @jwzumwalt why is there an asymmetric span lift? $\endgroup$– user7241Dec 12, 2017 at 3:03
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$\begingroup$ @jwzumwalt: May I politely disagree? Please read this answer for my take on the efficiency of elliptic wings. $\endgroup$ Dec 12, 2017 at 20:24
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$\begingroup$ It looks to me like you are attempting to refute real world wind tunnel tests by NASA (Previously NACA) Technical Note 2249 and reproduced on pg 75 of the "Aerodynamics for Naval Aviators" with personal philosophy. I'll go with the wind tunnel tests... $\endgroup$ Dec 12, 2017 at 21:24
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$\begingroup$ @jwzumwalt could you still enlighten me on the asymmetric span lift? Is that due to propwash? (A)Symmetric with respect to what? $\endgroup$– user7241Dec 13, 2017 at 3:43
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The primary textbook for many (possibly most) University aerodynamic classes is FREE (Aerodynamics for Naval Aviators). This book explains all theoretical and practical knowledge needed to have a well rounded understanding of aerodynamics. It explains all the formulas needed to design an airplane in the real world. It does not get into the bizarre out of touch academia :)
Page 75 discusses span lift. https://www.faa.gov/regulations_policies/handbooks_manuals/aviation/media/00-80T-80.pdf
To directly answer your questions regarding aerobatic CL. Most purely refined aerobatic airplanes will use a symmetric or modified symmetric airfoil. The exact coefficient of lift depends on shape of the leading edge, chord width, and Reynold number (~speed vs chord width).
The 1.3 given above would be close to typical - perhaps a little low, but it depends on how rounded the leading edge is and the design speed of the aircraft. Airfoil books I have show very thin sharp symmetric airfoils at 1.2 and very fat ones at 1.6. Most are 1.4 to 1.5. But remember, the exact same airfoil will show minor differences due to wind tunnel peculiarities, and now most "testing" uses computer fluid dynamic simulation!
An example of a popular aerobatic plane is Extra 300 (symmetric airfoil) often used in Red Bull Competition https://en.wikipedia.org/wiki/Extra_EA-300
You can study various CL at
Other good books I use
https://www.scribd.com/document/322901165/the-science-of-flight-w-n-hubin-1992
https://www.scribd.com/document/322676208/design-for-flying-david-b-thurston-1978
https://www.scribd.com/document/24571577/aircraft-flight-testing-design
An excellent basic series of how2 design aircraft was published in the EAA "Sport Aviation" magazine from February 1990 thru February 1991 primarily by John Roncz.
answered Dec 12, 2017 at 16:32
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$\begingroup$ Airfoil data is not the same as wing data. $\endgroup$– KoyovisDec 12, 2017 at 21:55
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1$\begingroup$ ...although for this wing with no washout and constant airfoil, 3D data would be pretty close to 2D. Just tip losses and wing/body interference to account for. +1, I like the navy book. $\endgroup$– KoyovisDec 12, 2017 at 22:27
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$\begingroup$ Except for service ceiling, we are generally most concerned with stall and maxCL while landing. It turns out that all factors including ground effect pretty much cancel each other out and an aircraft will stall while landing so close to the projected wind tunnel data for an airfoil that seldom any other computation is needed. The standard wind tunnel test section is 3ft long and the resultant data is mathematically adjusted/predicted for full scale applications. $\endgroup$ Dec 13, 2017 at 0:39