Undefined airfoils result in unrealistic flight performance

[jayeye2011]

The Stearman does not fly correctly. I suspect part of the problem is that the airfoils are for all intents and purposes are totally undefined. From the stearman.tmd file: // airfoils <[airfoil][AirfoilHorizontalStabilizer][] <[float64][Cl0][0.0]> <[float64][Cd0][0.0052]> <[float64][Cm0][0.0]>

<[airfoil][AirfoilVerticalStabilizer][] <[float64][Cl0][0.0]> <[float64][Cd0][0.005]> <[float64][Cm0][0.0]>

<[airfoil][AirfoilRoot][] <[float64][Cl0][0.0]> <[float64][Cd0][0.005]> <[float64][Cm0][0.0]>

<[airfoil][AirfoilTip][] <[float64][Cl0][0.0]> <[float64][Cd0][0.005]> <[float64][Cm0][0.0]>

Come on - get serious. Wings should not have a 0.0 coefficient of lift except for hypersonic aircraft. The Stearman 75 uses a NACA-2213 airfoil. But I bet you could use a Clark-Y airfoil since it was a 1930's aircraft. The flight performance you should be trying for

Model 75 PT-17 Kaydet Seats | 2 (tandem) Length: | 24 ft. 9 in. (7.54 m) Wingspan: | 32 ft. 2 in. (9.81 m) Wing Area: | 298 sq. ft. (27.7 sq. m) Height: | 9 ft. 8 in. (2.95 m) Empty Weight: | 1,931 lb. (876 kg) Loaded Weight: | 2,950 lb. (1,195 kg) Fuel | 46 gal (gravity fed, 4-7 gal not available in flight) Max Speed | 117 knots (134.6 mph, 216.7 km/h) Service Ceiling | 13,200 ft (4,023 m) Cruise Speed | 83 knots (95.5 mph, 153.7 km/h) Max Range | 438.8 nm (505 mi, 812.7 km) Fuel Burn | 12-13 gal/hr Climb | Roughly 800 fpm The heavy Model 75 PT-17 was fitted with a nearly 500 lb, seven cylinder 220 HP Continental R-670-5 radial engine pulled by a Sensenich wooden propeller. Most still-airworthy Boeing-Stearman’s now have higher-performance metal propellers, like Capt. Walker’s, which has a 102-inch diameter ground-adjustable McCauley propeller. “Climb performance with an airplane that grosses out very nearly 3,000 lbs and has only 220 horses up front is going to be leisurely. You don’t have any choice.” Budd Davisson source: https://disciplesofflight.com/boeing-stearman-model-75-aircraft-profile

[jayeye2011]

http://www.aerofiles.com/airfoils.html Boeing bought Stearman Conventional Aircraft: Wing Root Airfoil Wing Tip Airfoil Boeing 75 PT-13 Kaydet (N2S) NACA 2213 NACA 2213 Boeing 75 PT-17, PT-27 NACA 2213 NACA 2213 Clark-Y (WACO YMF-5 USES) http://airfoiltools.com/airfoil/naca4digit?MNaca4DigitForm%5Bcamber%5D=2&MNaca4DigitForm%5Bposition%5D=20&MNaca4DigitForm%5Bthick%5D=13&MNaca4DigitForm%5BnumPoints%5D=81&MNaca4DigitForm%5BcosSpace%5D=0&MNaca4DigitForm%5BcosSpace%5D=1&MNaca4DigitForm%5BcloseTe%5D=0&yt0=Plot Since there are no polar plots at airfoiltools.com for a NACA 2213 I would try substituting either a Clark-Y (more aerobatic) or a NACA 2412 http://airfoiltools.com/polar/details?polar=xf-clarky-il-1000000 http://airfoiltools.com/polar/details?polar=xf-naca2412-il-1000000 I would just use the 2412 since its cross-sectional profile looks closer to the 2213

[jayeye2011]

The NACA 2213 is not a symmetrical airfoil so there should be some positive value for Cl0

[jayeye2011]

Clark Y wing seems to work better. Here is the tmd file with Clark Y with tweaked incidence angles to get close to 83 knot cruise speed. 20230209 clark_y stearman_tmd.zip

[jayeye2011]

Actually, the Clark-Y is probably not correct as this makes the Model 75 perform like a WACO YMF-5. Probably a 2412 is closer but using a symmetrical airfoil while not really correct is probably not any worse and it is your choice.

[jayeye2011]

Doing some research it turns out that the Spitfire used a NACA 2213 for the wing root section, the wing tip section was different airfoil. https://www.aerosociety.com/media/4953/the-aerodynamics-of-the-spitfire.pdf

So here is the polar plots from an freeware X-Plane 11 model of the Spitfire by domhenry model. source: https://forums.x-plane.org/index.php?/files/file/68133-supermarine-spitfire-mk1/

I believe that this data was determined using XFOIL, a free program that computes the aerodynamic performance of subsonic airfoils. Note however that I have not actually run XFOIL to confirm this is the case. relevant links https://web.mit.edu/drela/Public/web/xfoil/ https://www.youtube.com/watch?v=N4j-5Ypfyho https://www.youtube.com/watch?v=4A-tzoU8YEw https://www.youtube.com/watch?v=S1e1GniH1TI https://www.youtube.com/watch?v=N4j-5Ypfyho

X-Plane Airfoil Maker is a handy program for generating the polar plots using point data from XFOIL. If you do not have X-Plane you can download a free demo version of X-Plane which includes both the Airfoil Maker utility and Plane Maker utility programs. X-Plane 10 or 11 or 12 will work since Airfoil Maker has not changed but I would get X-Plane 11 demo since this version had the most freeware aircraft models that can be used to gain insight into their specifications. Note that the demo is only limited in terms of how long you can actually fly in the flight sim but is not time limited as far as using the utility programs for analysis. https://www.x-plane.com/desktop/try-it/older/ Youtube Videos on how to use X-Plane 11 Airfoil Maker https://www.youtube.com/watch?v=UgGIF2QkWWM https://www.youtube.com/watch?v=DmkuqJuFwWI https://www.youtube.com/watch?v=F1cj2_f9gtM

[jayeye2011]

Well, deciding to just tweak the airfoil specs, I started off with the DomHenry Cl0, Cm0 and Cd0. I first corrected the left and right stabilizer incidence angles so all elements had the same -0.01047 rad value. Then via trial and error correction I finally got 83 knots using a Cl0 of 0.31. This was 83 knot, 39% throttle, 6000 feet, 0% elevator trim level flight as shown in screenshot.

        <[airfoil][AirfoilRoot][]
            <[float64][Cl0][0.31]> 
            <[float64][Cd0][0.007]>
            <[float64][Cm0][-0.04]>
        >
        <[airfoil][AirfoilTip][]
            <[float64][Cl0][0.31]>    
            <[float64][Cd0][0.007]>
            <[float64][Cm0][-0.04]>
        >

here is the tmd file (includes removal external reflection on cockpit windshield)

20230211 83kt stearman_tmd.zip

[jayeye2011]

I think by increasing Fuselage and Engine Nacelle Cdx to 0.45 this is slight better. 20230211 Cl0+Cdx stearman_tmd.zip