Does an uncoordinated turn increase stall speeds? When you do cross control for crosswind landings in GA aircraft like C172, should you be worried about higher stall speeds during flares?
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asked Jul 1, 2017 at 18:32
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$\begingroup$ If you are taking flight lessons have your instructor demonstrate a cross-control stall at altitude. You'll be amazed at how much the higher the stall speed is than normal. $\endgroup$– PugzJul 1, 2017 at 20:22
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1 Answer
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First question, yes. For two principle reasons: instrument error and a change in relative wind across the wing. The increase in indicated AS would be small.
Second question, not a factor. During the flare, you are close to the ground, and the AOA is mostly controlled by feel, sink rate and all the other things that factor into that complex maneuver. So if the speed were slightly higher, it probably wouldn't make a difference.
But let's talk practicality. When landing a C172 in a direct crosswind of 20k or so, I would add 5k to my speeds, and if the winds were gusty I would add accordingly (like 1/2 the gust factor approximately).
It is better to focus on having a stable approach with good AS and pitch control, than it is to fret over a potential 2k indicated stall speed differential. Besides, in ground effect the numbers change as well.
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$\begingroup$ Thank you for your reply. One follow-up question. What about during turns in the traffic pattern? Let's say you overshoot the final and use greater than 30 degrees of bank without stepping on the rudder to get back to the center line. Does this uncoordinated turn increase the stall speed? $\endgroup$ Jul 1, 2017 at 19:56
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2$\begingroup$ @lemonincider Yes, that is the classic formula for a stall-spin on turn to final. If you find yourself in that situation, GO AROUND! $\endgroup$ Jul 1, 2017 at 20:08
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$\begingroup$ First, all turns should be coordinated, such as base to final. Second, if you need more than 30 deg bank on final, you are likely exceeding the crosswind capability of the aircraft. I teach a rather tight final, but if you are turning a longer final, I would do coordinated maneuvering to get you on the centerline, and then begin your uncoordinated final approach slip. If you get messed up on shorter final, it's go-around time. $\endgroup$– mongoJul 1, 2017 at 20:22
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$\begingroup$ Yes that's for sure. But what I'm asking is not if you get into a stall or spin when your turns are uncoordinated; when you bank too steeply, you can fall into a stall or spin because of the resulting increased load factor. I'm well aware of that. What I'm asking is rather whether uncoordinated turns themselves affect the stall speed of an aircraft. $\endgroup$ Jul 1, 2017 at 20:22
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2$\begingroup$ An uncoordinated turn changes indicated stall because of instrument error, and the relative wind change across the wing. A load factor, such as from a turn can also change the stall speed, but I initially didn't address that due to the assumption that you were concerned about final and flares. A crosswise flow will reduce lift, and in some aircraft it can be 40%. With a reduction in lift, the stall speed increases. The crosswise flow will exist in uncoordinated flight, whether you are turning or not. If you are turning there will also be a load factor which will impact the stall speed. $\endgroup$– mongoJul 1, 2017 at 23:32