When performing eights on pylons I know that groundspeed (GS) reduces as we fly into the wind and increases as we fly with (or away from) the wind. This is the direct cause for the need to constantly change altitude (PA).

My question is in two parts:

  1. As I descend into the headwind, why does my indicated airspeed (IAS) increase while my GS is reducing (and vice versa when flying with tailwind)? I have made a chart that shows XX GS = XX PA. When I look at my actual GS, I then look at my chart and find that I am at the correct PA for that GS but my IAS seems to spike in the descent and plummet in the climb.

  2. Should I use the throttle to hold my IAS as it was in the entry?

  • 3
    $\begingroup$ It's been 50 years since I did eights on pylons, so take this with a grain of salt. As I remember, I did them at a constant power setting insofar as the throttle position was concerned. The rpm would vary because I was in an airplane with a fixed pitch prop. The change of IAS was due to the change in pitch of the airplane and thus to be expected, so I would say to leave the throttle alone after the initial power setting. Any pitch change will cause a change in airspeed if the power is not changed, so an increase when pitching down is to be expected, and vice versa. $\endgroup$
    – Terry
    May 13 '17 at 23:21
  • $\begingroup$ Eights on pylons are a completely 'eyes outside' maneuver, apart from occasional safety checks on altitude and airspeed. I'm not sure why you're trying to hit exact values, but assuming that you enter at a reasonable altitude and speed I wouldn't worry about them any more and rather focus on flying the maneuver correctly. $\endgroup$
    – Pondlife
    May 15 '17 at 14:59

Answering the easy question first:

No, you should leave the throttle alone once you've begun the maneuver.

As for the other question:

Without seeing your technique, I suspect your IAS is changing because of your climb/descent rate

I don't know how quickly you're climbing and descending, but if you're trying to hit specific speeds at various points during the maneuver, it's likely that you're going to end up flying mechanically and abruptly. An eight-on-pylon should ideally be flown with your eyes mostly scanning outside, and only occasionally coming inside to reference your altitude and speed.

If you're trying to hit pre-computed numbers rather than looking outside and keeping the pylon on the lateral axis through smooth application of the controls, then my guess is that you'll be climbing and descending more than you need to, and you'll be doing it more abruptly than you need to, which may lead to large changes in IAS.

Have a look at the Airplane Flying Handbook, page 6-17:

With prompt correction, and a very fine control pressures, it is possible to hold the visual reference line directly on the pylon even in strong winds. ..... It is important to understand that variations in pylon position are according to the apparent movement of the visual reference line. Attempting to correct pivotal altitude by the use of the altimeter is ineffective.

(emphasis mine)

This part of the answer may be entirely incorrect, because I don't know what your flying technique is. If you can give us more information, we can give better answers.


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