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2 Answers

Questions on Steep Spirals

Asked by: 3126 views Aerodynamics, Commercial Pilot, General Aviation

Hey all,

I'm working on commercial maneuvers in an arrow (PA28R200 Arrow II), and there are a couple things with steep spirals I'm having trouble/confusion with:

-When I practice steep spirals my bank rarely exceeds 30 degrees. The basic idea that several instructors have passed on to me is that we want to track a constant radius circle while maintaining best glide, period. Hence any variation in bank angle is only to correct for variation in ground speed. The PTS specifies bank angle not to exceed 60 degrees, I'm wondering should my maximum bank angle be greater? I've tried flying this maneuver with about 50 degrees of bank but have more difficulty trouble maintaining my best glide speed and keeping a constant radius. I guess my question is - what do you think about my choice of max bank angle?

-Second question: What are your thoughts on flying this maneuver low drag (prop aft, gear up) vs. high drag (prop fwd, gear down). My thoughts on both: With high drag I consistently have trouble completing all three turns even when starting from as high as 6500 agl! Obviously this is less of a problem with the low drag option. On the other hand, with the arrow the gear warning light will go off on me the whole time if I do this low drag. Since the PTS doesn't say anything is low drag vs. high drag a matter of personal preference?  

Along those lines, what starting AGL altitude would you use for this maneuver in an arrow?



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2 Answers

  1. Mark Kolber on May 11, 2015

    Since your CFI is prepping you for a checkride, probably with a specific examiner whose preferences are know, I would get most of my technique guidance from him.

    From my perspective, it’s been a while but when I taught steep spirals, I taught them with gear down, a 45° downwind bank entry, and a glide speed of 90 kts – a ground reference maneuver speed, not “best glide.” There’s nothing in either the PTS or the Airplane Flying Handbook that specifies the entry speed, only that once establishing the chosen glide speed, it be maintained within 10 kts.

    Essentially, it’s a gliding, lowering altitude version of turns around a point in terms of both purpose and execution.

    Having about 1,000′ available for each 360° rotation (on top of the rollout altitude) should be sufficient.

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  2. Kris Kortokrax on May 11, 2015

    The maneuver is a steep spiral. I assume that the word steep is used in reference to the bank angle, not the pitch angle.

    On pages 3-7 and 3-8 of the Airplane Flying Handbook, turns are broken down into shallow (less than 20 degrees), medium (20-45 degrees) and steep (over 45 degrees).

    The purpose is not to locate a place to land, but to spiral down to a landing over a suitable spot which has be reached as a result of a glide (or luck at having had the engine fail over a good spot). I would leave the gear up while spiraling. In a real life maximum effort, go to high pitch (low RPM) on the prop, but for practice, you can leave it in low pitch (high RPM).

    In this situation, I would not use best glide speed. I would use minimum sink speed instead. This will maximize my time in the air. I found an approximation for minimum sink speed in Bill Kershner’s Advanced Pilot’s Flight Manual. He suggests 1.2 times the stall speed (in CAS).

    The gross weight for the Arrow II is 2650. I found an empty weight of 1665 for an Arrow II. Adding two 170# people and full fuel, that would put us at 2293#.

    The gross weight stall speed in CAS with flaps and gear up is 71 wings level, 88 at 50 degrees bank and 100 at 60 degrees bank. The indicated speeds would be 60 wings level, 83 at 50 degrees and 97 at 60 degrees.

    At a weight of 2293, the CAS stall speeds would be 66 wings level, 82 at 50 degrees and 93 at 60 degrees. Indicated speeds would be 56 wings level, 78 at 50 degrees and 89 at 60 degrees.

    The minimum sink speed at 2293# would be 1.2 times stall speed of 66 MPH (CAS) yielding 79 (CAS). This would give 72 as an indicated airspeed for minimum sink.

    At a lower bank angle, your radius of turn will be greater and the time to complete a 360 will be greater. Using 90 MPH, at 30 degrees your radius will be 942′ and 45 seconds will be required to complete a 360. Using 50 degrees of bank the radius will be 456′ and the turn will take 21 seconds. These values are for a no wind condition, but the illustrate the basic difference between bank angles.

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