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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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  • $\begingroup$ What type of aircraft are you flying? $\endgroup$ Commented Jun 17, 2023 at 13:00
  • $\begingroup$ General Aviation, a Cessna 150 for example. $\endgroup$ Commented Jun 25, 2023 at 23:52
  • $\begingroup$ 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%. $\endgroup$
    – wbeard52
    Commented Jun 26, 2023 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. $\endgroup$ Commented Jun 27, 2023 at 19:08
  • $\begingroup$ The purpose of a slip is to make drag, and the faster you go, the more drag you make. In a slip your aircraft is no longer in a "slippery" low-drag condition. So full rudder, and put the nose way down--the need to disappate the excess airspeed before flaring can be accomodated by picking a round-out point further away from the threshold. $\endgroup$ Commented Jun 3 at 16:09

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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.

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    $\begingroup$ 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. $\endgroup$ Commented Jun 26, 2023 at 11:52
  • $\begingroup$ @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… $\endgroup$ Commented Jun 26, 2023 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.

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Indicated Airspeed in a slip is unreliable, so forget trying to fly an airspeed. The simple and reliable way to control speed in a slip is to fly to correct pitch. What is the correct pitch? How about the same pitch you were using for the stabilized power off approach prior to the slip, (pitch for 1.3 Vso)? Then when you exit the slip your indicated airspeed will return to what it was before you entered the slip. Simple. If you are concerned about stalling lower the pitch slightly, but realize that you will be faster than 1.3 Vso when you exit the slip. I've used this method for 50 years and have never stalled... ;).

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  • $\begingroup$ In my concern, a forward slip should only be used during an emergency. If you are unstable, you should go around. In case of an emergency, what would you do? $\endgroup$ Commented Jun 4 at 17:37
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For a wings level, power-off descent first principles say that the flight path angle (assumed > -10 deg) is approximately the arcsin of drag over lift, D/L. For full-pedal trims, that small angle assumption may not apply. Furthermore, the equations of motion are more complicated. All 6 degrees-of-freedom have to be considered. For example, the resulting bank angle and side force will uniquely vary as a function of speed. FAA-H-8083-3C (p. 9-13) says the steeper your bank angle in the trim, the higher the descent rate (not necessarily angle). I would interpret that as not only higher drag but the lift vector getting more away from vertical. “…the point may be reached where full rudder is required to maintain heading even though the ailerons are capable of further steepening the bank angle…increase in airspeed increases rudder effectiveness permitting a steeper slip.” [emphasis added]
So, there’s a tradeoff unique to every configuration both in terms of the balance of roll and yaw control power available vs. speed and in terms of resulting rates of descent vs. forward motion. Sounds like the flight test method is a way to go (if you can get a good source of angle of descent data and account for winds on each run).

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