Which is the proper speed to be maintained during a Forward Slip? Near Vfe or Vref? Which one will let you lower with the shorter ground distance and why?
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asked Jun 16 at 13:33
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$\begingroup$ What type of aircraft are you flying? $\endgroup$– Robert DiGiovanniJun 17 at 13:00
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$\begingroup$ General Aviation, a Cessna 150 for example. $\endgroup$– Ygor MontenegroJun 25 at 23:52
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$\begingroup$ Don't ever slip at the landing speed. That speed is calculated to be 30% above the stall. When you slip the stall speed increase could be greater than 30%. $\endgroup$– wbeard52Jun 26 at 17:00
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$\begingroup$ Keep in mind that in many aircraft the ASI is completely unreliable in a slip, typically reads way low. $\endgroup$– quiet flyerJun 27 at 19:08
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3 Answers
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Vfe is quite fast to be flying on a landing approach slipping or not, although I suppose there's nothing stopping you from doing that if you need to dive that steeply. You still end up with excess energy and will land long. What matters in the end is your energy state as you get to a few feet above the ground crossing the threshold.
A moderate speed safety margin above landing flap approach speed is sufficient. Keep in mind that a calibration error is created when the pitot is angled off the airflow and the airspeed will read a little lower than when straight.
If I'm approaching at 70kt and start slipping to lose altitude, I push over a bit to bring the airspeed up to 75 or 80 kt during the slip to have some safety margin against stall/spin. As I get close to the surface, I'll ease the nose up while keeping the slip in to bleed off the excess speed, then straighten out, more or less in one motion.
If I really messed things up and find myself too fast/long as the flare is coming up, I'll keep the slip in place right into the flare, straightening just before touchdown.
In the end, you slip at the lowest speed that is safe, only going faster because you need to dive more steeply, and are aware that you will arrive at the threshold with excess energy that still has to be dissipated.
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For a constant rate of descent, the slower of the two airspeeds will shorten the ground distance. This is because you are covering less horizontal distance for the same altitude lost.
answered Jun 16 at 14:57
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1$\begingroup$ IMO, this answer isn't really helpful. It's factually correct, true. However the core tenant, that there is a constant rate of descent, is misleading as that would not be the case with a faster dive in a slip. $\endgroup$ Jun 26 at 11:52
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$\begingroup$ @KennSebesta, sometimes my brevity is intended to highlight a question that is not well thought out. If the OP cares enough to bite, both the question and my answer evolve through our engagement. This one just withered on the vine… $\endgroup$ Jun 26 at 15:11
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First and foremost is safety. There is no general answer because each aircraft model has its own safety margin when flying cross controlled.
Seek expert advice, and always try the slip at altitude first.
Consistent speed is important for judging approaches because aircraft kinetic energy is proportional to the square of velocity. Excessive speed can really butcher an approach.
With the Cessna 172 I was lucky to find that forward slips could be done at my favorite approach speed: 65 knots. That is exactly 1.3 of the flaps up stall speed of around 50 knots.
This enabled a forward slip with out any great change in approach speed. Just used it when a little high. Rounding with excess speed can have one floating down the runway for an uncomfortably long time.
answered Jun 26 at 20:03