cessna spin

[Genghis the Engineer]

Interesting.

With Flaps10, I wouldn't expect the normal Vs of about 40 kn to be significantly reduced.

With 40° bank, a quick back of envelope sum suggests a stall speed of about 52 kn, which gives a reasonably sensible margin below your chosen speed of 75 kn IAS.

But, heedm's suggestion makes sense. If you were flying initially into wind by attitude, and turned downwind continuing to fly by the picture out the window (and particularly ground references) and not by reference to instruments, then you could find yourself fairly rapidly below Vs due to the inertia of the aircraft. Combine that with a turn, where in an aircraft with moderate aileron drag like the C150, and a spin entry is not an unreasonable possibility.

Bear in mind that if you were that low, then the horizon is not a hugely useful attitude reference because it's position keeps changing due to surrounding landscape.


Considering your question about the use such aircraft are put to in a flying school, this aircraft would have been certified almost certainly to FAR-23. The worst case in there is a 3 kn/s decelerated case in turning flight. Let's look at your case...

At 75 kn / 40° bank, your turn rate will be 29s/per 360° [turn rate in rad/s = g.tan(bank angle)/V], so you could go through 180° in 15 seconds (rounding off). So you could change from 75 kn (into a 20 knot wind) to 75-(20*2)=35 kn airspeed in 15 seconds. The mean deceleration rate (it'll actually follow a sine function) will be 40 kn / 15 = 2.7 kn/s - which is getting into the right ballpark.

Working out the sine function is easy enough, at 29°/s, it comes out at an airspeed of 55+20cos(12.4t). [Note I'm making one major approximation here, that your aircraft's inertia is too high to allow the airspeed to match - this is obviously untrue, but bear with me]. To find the rate of change in airspeed at any point, differentiate this with respect to time.

Plugging these into a spreadsheet (and differentiating numerically), it suggests that you hit the stalling speed of 52 knots at 7½ seconds (or roughly broadside to the wind) whilst decelerating about 4½ kn/s. This is right on the edge or possibly beyond what the aircraft is certified to have acceptable handling at.

I haven't taken into account the relatively low intertia of the aircraft, but equally I haven't taken into account that you may have inadvertently not increased power enough to hold the turn, pulled back on the stick slightly - or many other things. Analysis of that depth requires a lot more work, and I charge for it !

But basically, flying medium turns, at that speed, by reference to ground features, in a strong wind, does appear to give reasonable potential for an accelerated stall in the turn - which is quite likely in any aeroplane to cause a spin entry without prior stall warning, particularly when you may well have been decelerating at or beyond the certification limits for the aeroplane.

G