Slips and Indicated Air Speed

Discussion in 'Flight Following' started by WDD, Aug 3, 2020.

  1. WDD

    WDD Line Up and Wait

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    So, I was flying with a club CFI for a new member check out on Saturday, and I was demonstrating a slip to him.

    Now, I was taught that you slip with low wing into the wind. (If you have a slight cross wind from left, you yaw right and dip left wing.

    He then imparted wisdom to me that slipping to the right resulted in a lower indicated airspeed than if you flying into the wind, and slipping to the left caused the indicated air speed to be optimistic, and you'd be flying slower than it shows you are. And that one should always slip to the right. This happens not because of the effect on the Pitot tube on, but the air flow over the static port, and because the C 172 only has one outside static port.

    From my perspective, it does not make sense that slipping would effect the static port - it was designed to not be effected by air speed. But it seems that the Pitot Tube will however be affected because you have air hitting the side of the tube more than front on, reducing the pressure and thus reducing your Indicated air speed. Thus, your indicated air speed would be lower than actual. The effect would the same left or right of relative wind direction. In a nutshell, a slip in either direction would not cause an optimistic air speed reading, but a lower reading than actual.

    What am I missing?
     
  2. midwestpa24

    midwestpa24 Pattern Altitude

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    A slip in any airplane can impact your airspeed readings, as the pitot tube is no longer directly into the airstream, as well as the potential effect on the static port. What is that effect, who knows? The key is to not entirely trust your airspeed in a slip, period.

    As to which direction to slip, what you were taught is what I teach. The primary reason is so on your crosswind landing, you aren't having to switch rudders for the crosswind correction.
     
  3. Lindberg

    Lindberg En-Route

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    Your IAS will be wildly inaccurate while slipping. Ignore it.
     
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  4. Larry in TN

    Larry in TN Pattern Altitude

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    When you are in uncoordinated flight, the airflow is at an angle to the fuselage. This creates higher pressure on one side and lower pressure on the other. If your static port is only on one side of the airplane, the static pressure will be falsely high or low.

    This is fixed in many GA airplanes, and all larger airplanes, by having a static port on each side connected by tube. In the middle of that connecting tube is a T-fitting which connects to the rest of the airplane's static system. This allows those unequal pressures to balance out and provide an accurate static pressure to the instruments.
     
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  5. Tantalum

    Tantalum Final Approach

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    I don't think you're missing anything actually. It's just not worth overthinking - the airspeed indicator does indeed use the static air line as well, and depending on the orientation of these things around the airframe (pitot vs static) any uncoordinated flight (of which a slip is a dramatic example) will result in a less-than-accurate airspeed reading

    ^basically, yes.
     
  6. Lindberg

    Lindberg En-Route

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    What about the pitot?
     
  7. dmspilot

    dmspilot En-Route

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    Slipping results in relative wind hitting one side of the fuselage, why wouldn't it affect the static port?
     
  8. RoscoeT

    RoscoeT Pattern Altitude

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    These two sentences are unclear, but be aware that the wind direction has no impact on your true airspeed or ground speed while slipping left or right. I say this because some pilots think that if you slip in the direction that points the nose more into the x-wind, that your ground speed will be lower. This is false.

    It is also untrue that you should always slip one way or the other. As others have mentioned, depending on the airplane, you may see different indicated speeds slipping left or right. So what? Both speeds are likely inaccurate. You don't need an airspeed indicator to tell how fast you're flying. Slipping forces you to get in touch with that seat of the pants, feel the airplane pilot sh|t. :D
     
  9. flyingron

    flyingron Touchdown! Greaser! PoA Supporter

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    Airspeed doesn't make you stall, AOA does. If you keep the AOA reasonable, the detrimental part of the slip would be an excessive descent rate. That's easy to correct, lessen the slip (or get rid of it entirely).

    As for the static port, it depends on the aircraft. It could affect an aircraft with only one port on the side of the fuselage. Those with them on the pitot mast or with multiple ports are less succeptible.

    As for direction of bank, for losing energy (the forward slip), it is really unimportant.
    For the slide slip where you are trying to combat drift while maintaining longitudinal alignment, you'll want to slip into the wind as that is the whole point.
     
  10. Dan Thomas

    Dan Thomas Final Approach

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    That CFI is right in one thing: slipping can mess up the IAS, but he's wrong in another: a slip to the left in a 172 will decrease the IAS, not increase it. The airspeed indicator is a differential pressure gauge, measuring the difference in pressures from the pitot and static port. The static port's pressure is normally the same as ambient pressure, or real close to it, but slipping to the left will ram some air into it and push the airspeed indication down. The pitot being off-alignment with the relative airflow will aggravate that. Airplanes with static ports on both sides see much less IAS variation with slips.
     
  11. Capt. Geoffrey Thorpe

    Capt. Geoffrey Thorpe Touchdown! Greaser! PoA Supporter

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    That would be incorrect. Particularly when the port is only on one side. HUGE difference right to left - Cessna 120 - slip to the left IAS can go to 0 because the static port acts more like the pitot. To the right, the port is "shadowed" by the fuselage, IAS goes up.
    Ummmm, ain't that the whole point of a slip?
     
  12. flyingron

    flyingron Touchdown! Greaser! PoA Supporter

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    Well, everything in moderation. Nice thing about a slip, is if the descent rate gets too much, it's a lot easier to cancel it than retracting flaps.
     
  13. Larry in TN

    Larry in TN Pattern Altitude

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    It would be affected as well.
     
  14. Capt. Geoffrey Thorpe

    Capt. Geoffrey Thorpe Touchdown! Greaser! PoA Supporter

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    I will point out that most of my time is in aircraft without flaps which leaves the forward slip as your weapon of choice if you want to come down like you are in a Navion with gear and flaps out... :)
     
  15. MauleSkinner

    MauleSkinner Final Approach

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    the only absolute in aviation is that there are no absolutes in aviation. Always be highly suspicious of anyone who uses “always” or “never” in a sentence. ;)
     
  16. luvflyin

    luvflyin Final Approach PoA Supporter

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    Slipping into the static port increases the pressure on it. Airspeed indication is the difference between static pressure and pitot pressure. So more pressure on the static reduces the difference. That would give the same result as decreasing the pitot pressure.
     
  17. jsstevens

    jsstevens Final Approach PoA Supporter

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    Go up to a safe altitude and try it. Keep power, bank angle, etc all the same. Switch sides and see what the ASI says.
     
  18. SoonerAviator

    SoonerAviator Final Approach

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    I thought the point of a slip was to maintain alignment with a particular track by using the controls to account for crosswind/drift (just like a crab). A forward slip is where the point of the maneuver is to introduce a more aggressive rate of descent in addition to the course alignment.
     
  19. Daleandee

    Daleandee Pre-takeoff checklist

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    I Never gave it much thought as I'm more using the reference of the nose on the horizon in a slip. With up to 30º of flaps available slipping isn't something I do very often for altitude loss. But sometimes it's just plane fun!

    In my airplane with the engine idled, full flaps, and a good slip, it will fall from the sky like a refrigerator!
     
  20. Capt. Geoffrey Thorpe

    Capt. Geoffrey Thorpe Touchdown! Greaser! PoA Supporter

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    According to the FAA, maintaining a ground track in line with the runway with the nose pointed at the runway is a "side slip", beyond that is the forward slip where the nose no longer aligns, but you still maintain the track. On the other hand, if you don't have a pedal all the way to the firewall, it ain't much of a slip.
     
  21. Shepherd

    Shepherd En-Route

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    Most people have it right.
    Succinctly: Don't bet your life on indicated airspeed in a hard slip.
     
  22. nauga

    nauga Administrator Management Council Member PoA Supporter

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    Pitot tubes are less sensitive to misaligned flow than static ports.
    Note I did not say *in*sensitive, just far less.

    Nauga,
    flushed
     
  23. flyingron

    flyingron Touchdown! Greaser! PoA Supporter

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    Unlike a crab, the airplane in a slip is no longer in alignment with the regular wind. The ground track is immaterial. The wind is now blowing at an angle to the aircraft.
     
  24. chemgeek

    chemgeek Pattern Altitude

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    What's the purpose of the slip? To lose altitude, probably doesn't really matter much which way. To correct for crosswind? Low wing into the wind. Airspeeds won't be 100% accurate, but so what? They won't be egregiously off, just leave ample margin. I usually slip no less than 1.3 Vso indicated. Never come close to a stall. YMMV. Refer to the POH.

    The advice to always only slip in one direction seems arbitrary and uninformed.
     
  25. Tarheelpilot

    Tarheelpilot En-Route

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    I always slip into the crosswind so it’s less delta to align with the runway. I can’t say it’s the right way. I think it’s easier. If someone wants to do it in a more challenging way I guess that’s fine. Whatever. Don’t crash is my advice.
     
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  26. luvflyin

    luvflyin Final Approach PoA Supporter

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    Arbitrary, uninformed and in IMO, irresponsible inasmuch as it came from a CFI. There is another reason besides losing altitude and crosswind landing to slip. Moving the cowl off to the side to see the sky directly in front of you and lower. Collision avoidance. I slip for a couple of seconds a few times every approach to landing I make and plan my descent to accommodate it.
     
    Last edited: Aug 4, 2020
  27. Ryanb

    Ryanb Administrator Management Council Member PoA Supporter

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    What happens if you’re flying a high wing? ;)
     
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  28. MauleSkinner

    MauleSkinner Final Approach

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    I was inverted.
     
  29. Huckster79

    Huckster79 Cleared for Takeoff PoA Supporter

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    It comes to knowing your machine and being able to “be one with it”... I don’t mean this tersely but if you don’t just instinctively know when your bird is happy or when you are about to make her do ugly things... without looking at so much as the ball... ya gotta go fly your bird more...
     
  30. SoonerAviator

    SoonerAviator Final Approach

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    Agreed, but what Ron said about side slip was that the "detrimental part of the slip would be an excessive descent rate"? What about a slip necessitates an excessive descent rate? Are we only talking about a forward slip or does that apply to any slip? I'm just saying that from the perspective of a descent (say on final), a normal side slip and a crab are probably fairly close in descent rate, right? One is aligned with the wind direction, one is aligned at an angle, but both when properly executed have the same ground track and similar descent rates.
     
  31. dmspilot

    dmspilot En-Route

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    Slipping will always increase drag due to the airflow pattern across the aircraft. The airflow pattern across an aircraft in a crab is the same as in normal coordinated flight (because a crab is normal coordinated flight).
     
  32. chemgeek

    chemgeek Pattern Altitude

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    Descent rate in a slip is controlled by both available power and the degree of slip. Descent rate can arrested, if necessary, by application of power or reducing the amount of slip, depending on the goal of the slip. Sometimes, "excessive" descent rate is desired, as in wanting to rapidly lose altitude to make a landing spot, etc. An AA-1A will slip like a greased anvil with full rudder deflection.
     
  33. flyingron

    flyingron Touchdown! Greaser! PoA Supporter

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    I was using the term "excessive" to mean "more than what the pilot desired" not some other quality of the descent rate.
     
  34. Shepherd

    Shepherd En-Route

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    Hey! Pampered rich airplane people.
    Back in the olden days, airplanes didn't have flaps. (or much of anything else)
    If you needed to get down in a hurry you pushed the appropriate rudder pedal as hard as you could, put in as much opposite aileron as the plane could stand, and brought the nose up as far as necessary to control airspeed.
    The problem here is that your airspeed indicator takes it upon itself to lie to you. Sometimes it lies a lot.
    So you:
    1. Practice this technique, often, so you develop a feel for airspeed based on sound and motion.
    2. Never, ever let the airspeed indicator approach your stall speed.

    Also back in the day an emergency descent in a Cub or similar aircraft was a spin. They don't teach that anymore.
     
  35. DoubleD

    DoubleD Pre-takeoff checklist

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    Sounds like a young CFI with some book learnin' and not much experience. Slipping away from a crosswind? Crazy.
     
  36. MauleSkinner

    MauleSkinner Final Approach

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    I dunno...kinda tough to slip from left base to Into the wind when it’s going to be a right crosswind on final.
     
  37. Capt. Geoffrey Thorpe

    Capt. Geoffrey Thorpe Touchdown! Greaser! PoA Supporter

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    And, don't slip hard into a low fuel tank. Now someone will come along to claim that per FAA regulation 23 whatever, something about usable fuel in any flight attitude, blah, blah, blah... But, once again, the laws of physics trump the laws of the FAA. Depending on the airplane, a hard slip, say to the left, while drawing on a 1/4 full left tank can result in silence if you try to go around.
     
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  38. flyingron

    flyingron Touchdown! Greaser! PoA Supporter

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    I usually exit the slip if I want to go around, the plane climbs much better.
     
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  39. Lindberg

    Lindberg En-Route

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    It's done all the time in gliders when slipping to land. Start slipping on downwind in the direction of the crosswind. Right aileron, left rudder, relax some of the aileron pressure and you turn left.
     
  40. MauleSkinner

    MauleSkinner Final Approach

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    Agreed...I probably wasn’t clear, but I was saying it’s kind of tough to only slip into the wind if you’re in a situation where the slip has to be the other way.