At one point, Through Gates of Splendor describes Nate Saint gliding in his Piper PA-14 Family Cruiser at 45 mph; the plane's stall speed is 44 mph. Earlier in the book, he flew at 45 mph. Is it normal and safe to fly or glide only one mile per hour above stall speed in a small airplane? When gliding, he started at 3,000 feet and "throttled all the way back [and] pulled flaps"; could "pull[ing] flaps" have lowered the stall speed? I don't remember something like this being described when he flew under power at this speed, nor do I remember the book mentioning his altitude.
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1$\begingroup$ Stall speeds are specific to a configuration, such as flaps up or flaps down (or also various intermediate flap settings), as well as power-on and power-off (more so in some aircraft than others). Stall speed also varies with aircraft weight, so to say that this aircraft stalls at XX mph is a simplification -- often speeds are published at a maximum weight, so in lighter cases you'll stall slightly slower. Also, speeds may be published (and described) in MPH or in Knots. Until you know exactly what case the "44 mph" value describes, saying that 45 mph is 1 mph above stalling may be dubious. $\endgroup$– Ralph J ♦Jan 25 at 6:02
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$\begingroup$ @RalphJ the 44 mph stall speed is with flaps down. Would "pull[ing] flaps" put them up or down? $\endgroup$– SomeoneJan 25 at 6:10
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$\begingroup$ Indicated airspeed is not a true representation of the actual stall speed. If you practice stall in an actual aircraft, often it will stall at an indicated speed well below the published stall speed. $\endgroup$– Mike SowsunJan 25 at 15:27
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4 Answers
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could "pull[ing] flaps" have lowered the stall speed?
Yes, it absolutely would. ("Pulling" means "lowering", especially in the context of the PA-14 where the flaps were controlled by a simple mechanical lever that you actually "pulled" on to lower the flaps.)
You'll find evidence in support of this reduction in stall speed if you peruse any airplane flight manual (so long as said airplane has flaps.)
A reduction in the neighborhood of 5-7 mph would seem typical for light airplanes.
Keep in mind also that distance from ground, and smoothness of air, are two factors that hugely affect the safety (or lack thereof) of flying very near the stall speed. Also the stall characteristics of the aircraft. If you are just going to feel a gentle burble over the tail, which know you can stop by just easing the stick/yoke forward a tad, and if you fail to do that the nose will just gently start to drop a bit with the wings staying level, and you are high above the ground, and there's no turbulence, then there's very little danger in flying right up to the "edge" of the stall.
Btw, stall speed also varies according to the square root of the gross weight. Published stall speeds are for a specific gross weight-- likely the maximum gross weight allowed. Unless the author specifically stated that the aircraft was flying "1 mph above the stall speed", one should be careful about jumping to conclusions based on a published stall speed corresponding to one specific gross weight.
Throttling "all the way back", on the other hand, will typically very slightly increase the stall speed in a single-engine prop plane of conventional configuration. But the published stall speed is likely for the power-off configuration anyway.
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1$\begingroup$ Thank you! The 45 MPH stall speed is with flaps down, but the other factors affecting the stall speed could definitely have made it more than a 1 MPH difference. $\endgroup$– SomeoneJan 25 at 17:27
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As usual, lift $L$ generated by a wing can be expressed by:
$L=½\rho V²SC_l=W$
where $W$ is the airplane's weight, $\rho$ air's density, $V$ speed of flight, $S$ wing surface and $C_l$ lift coefficient. $C_l$ has a typical trend like shown in the following picture (source - $A$ is the aspect ratio):
As visible, $C_l$ increases proportionally to the angle of attack $\alpha$ until the wing can't get anymore and it just stalls: we reached the maximum lift coefficient $C_{l_{max}}$ that can be extracted from the wing before it stalls.
If we now rewrite the previous lift equation in respect to $V$, we see how the stall speed changes:
$V= \sqrt{\frac{W}{½\rho S C_l}} \Rightarrow V_{stall}= \sqrt{\frac{W}{½\rho S C_{l_{max}}}}$
From the latter equation it can be seen that the stall speed depends not only on the wing configuration (via $C_{l_{max}}$) but on density and weight as well. In particular, a heavier airplane implies higher stall speed i.e. the need to fly faster to stay away from stall; and also flying higher (lower $\rho$) has the same effect.
So stall speed is not one single value but it depends on the flight conditions.
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It is possible for any aircraft to fly at one knot above the stall speed and maintain altitude. It does however require a lot of thrust to overcome the induced drag generated from having such a high angle of attack. In a glide, where you don't have to maintain altitude, flying that slow can easily be done. The stall speed represents the point where the wing can no longer fully support the weight, but anything above that is fine.
I wouldn't say it is recommended practice though. Flying so close to the stall is dangerous if you don't have enough altitude to recover. It'll also be uncomfortable for passengers. For perspective, in the PA28 I fly, the stall speed with flaps extended is 44 knots, but we land at 60-65 knots. We also have a "safe slow cruise" configuration which has two stages of flap extended and gives us a speed of about 75 knots. Basically, we don't hang around near the stall speed unless it's for practice.
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In gliders, yes.
When thermalling in a glider, it's routine to fly very close to stall. This minimizes the sink rate, minimizes the turning radius, and maximizes the extracted lift.
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$\begingroup$ I’d hesitate to fly that close (1mph) above stall since the slightest turbulence, which is to be expected in thermic conditions, could lead to a stall or more likely a spin. In a large thermal it would be typical to fly at minimum sink which will be well above stall speed, or perhaps a little below that to achieve a tighter turn radius to stay in a strong core. $\endgroup$– FrogJan 25 at 18:37
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$\begingroup$ @Frog it all depends on the glider's stall characteristics. Stalls only lead to spins if the plane is badly coordinated, is skidding to the inside, and the stall is not immediately recovered. When I'm flying, I am frequently at the ragged edge 1kt above stall. My glider has very gentle stall characteristics and I respond to the wing buffet within ms of the onset. I certainly understand others flying differently, but my thermalling got much better once I learned not to have a knee-jerk fear of slow flight. $\endgroup$ Jan 26 at 5:46