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I'm looking at the Aeronca C-3 Collegian, which had a Clark Y airfoil, and I'm finding the following:

  • Wing area: 142 ft²
  • Empty weight: 466 lbs
  • Payload: 470 lbs
  • Stall speed: 38 mph

When I do the math for that wing area, a gross weight of 936 lbs, and a stall speed of 38 mph, I get a coefficient of lift of of about 1.8; that's higher than I see from either simulations or wind tunnel tests.

I'm inclined to trust these figures, since they line up with pilot reports on the C-3, and the airspeed range would be very narrow if the stall speed were higher; could it be that distortion of the fabric results in a later stall? The little I could find on the subject doesn't suggest any such effect. What's going on here?

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One contributor with a lightweight airplane like the C-3 is thrust. At a high angle of attack, a significant fraction of the engine's thrust is directed downward, lightening the loading that the wing's lift is working against. Even at idle, this allows slowing further before the angle of attack becomes critical -- making the coefficient of lift appear higher than it is.

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    $\begingroup$ That's not a bad thought. If I say 250lbs thrust total, which seems reasonable for a 35 hp engine, the vertical component at 18° angle of attack is about 80 pounds, which reduces the Cl needed to about 1.6, which fits the data I know better. $\endgroup$ – JasonFruit Dec 20 '18 at 20:48
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Sometimes the stall speed listed is the indicated airspeed at stall, where there can be significant calibration error and the A/S will read 3-5 mph low. If it doesn't say 38 mph CAS (calibrated airspeed) you can't really be sure if it's corrected or not.

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Keep in mind that the stall speed is not always given for the max takeoff weight. The official aircraft manual (if exists) lists effects of takeoff weight on stall speed.

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