P-factor and slippery airstrips

Matt Dearden

Matt Dearden

English born professional pilot, writer, blogger and columnist. Currently flying the Pilatus PC-6 Turbo Porter in Papua, Indonesia.

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17 Responses

  1. sp769 says:

    Thank you for posting, great blog and very interesting.

    • Matt Dearden Matt Dearden says:

      You’re welcome. It was good to write this one as it forced me to read up on it all and refresh my memory. That take-off is one I won’t forget in a hurry, that’s for sure!

  2. Matt,
    Do you ever wheel land the PC6?

    • Matt Dearden Matt Dearden says:

      Not on the short airstrips but on the longer ones sometimes I will, especially with a forward C of G (e.g. when empty). I also find it’s easier to control the roll-out on badly constructed tarmac airstrips with lots of undulations. With a three pointer those undulations can throw you off line, requiring some quick rudder/brake work. With a wheeler landing they don’t and it’s more controllable. This is all in my opinion however and it’s an often discussed topic among PC-6 pilots.

  3. Butch Walker says:

    great article Matt! I’ve just discovered your blog, love it!

  4. Anonymous says:

    umm. p-factor as you describe it only happens as you’re moving forwards though (unless i’m misunderstanding): the faster you move forwards while “nose-high”, the more pronounced the effect is; the extremes are: standing still (no p-factor, though, especially on a taildragger, there’s still “something”, see below), respectively: chopper (rotary wing) moving forwards at almost 0 deg angle to the plane of rotation of the “propeller” (choppers need to have an asymmetric (cyclic) pitch because of this, otherwise the blade going into the wind will have much more lift than the one running away from the wind, which would obviously roll the chopper)

    still something: there’s still corriolis-type (?) effects: the, airflow from the propeller is not “clean”, it is “twisted” by the prop, which is significant so close behind the prop, the effect is that the actual flow on the rudder, as pushed by the prop, will actually be considerably sideways, and also one of the wings will actually get slightly more lift than the other (this is most obvious to glider pilots when towed by powerful tow planes, especially on the takeoff roll); so if you have no grip, all these combined with torque and gyro-effect will probably be enough to turn you slightly as you push the gas forward on such a big engine. I wouldn’t underestimate the gyro effect though, i suppose you spin that prop somewhere around 3000rpm on takeoff, that means very nearly a big bad gyro in front, it doesn’t take much of tail lifting to make it precess, i would think

    oh, did i mention what a great blog this is? please keep writing, and thank you. that, and gopro should consider “lending” you some gear ;). great stuff

    • Matt Dearden Matt Dearden says:

      Hi there and thanks for your comments.

      You’re quite right about p-factor increasing as you speed up along the take-off roll; I probably should have mentioned that in the article, so thanks for clearing that up for anyone reading this.

      The swirling prop-wash does indeed also have an effect on things, although with the Porter not as much as the torque and p-factor in my experience. Prop RPM is 2000 at all times with the Porter when flying.

      The gyro effect is noticeable if you deliberately pick the tail up once rolling and you do have to press a little harder on the right rudder to keep things straight but as I mentioned, we tend to depart from the shorter airstrips in the same nose-high attitude as when sitting on the ground, so it’s less of an issue.

      All you really have to know though is, that you need rather a lot of right rudder trim (i.e. full) and pedal to keep things straight but it’s always good to understand the theory as to why.

      Cheers! And if GoPro want to “lend” me anything I’m all ears ;o)

    • Anonymous says:

      right, i keep forgetting it’s a turboprop. weird (wonderful) things (re rpm).

      thanks again, theory is all good and fun, but it’s different to have the “hands on view”, so thanks for that

      cheers, see you around

  5. Anonymous says:

    Great live example for tailwheel training briefings

  6. If the P-factor gives more lift to the right side of the prop disk, then why doesn’t the gyroscopic procession mean you get a nose-up moment from it? Shouldn’t the force on the right side move 90 degrees to act on the bottom of the disk?

    • Matt Dearden Matt Dearden says:

      Hi,

      Good question and I see what you’re getting at but I’m fairly sure that because P-factor is not a force acting on the prop disc but rather is thrust produced by the prop disc, it does not produce any gyroscopic procession.

      Even if it did, because this would be basically forcing the tailwheel onto the ground, you would be hard pressed to notice it compared to the other forces in play.

      Perhaps someone with more understanding of gyros could add something further to this discussion?

      Cheers

  7. Anonymous says:

    May be that is why Pilatus decided to set a 2° angle off the centerline of the transversal axe; so it would help to minimize this force…

  8. Johan Prins says:

    Oh, and – on my Zenair CH701 STOL, I have to apply opposite rudder compared to take-off, when I go to a steep descent with throttle far back.
    Very nice article; everyone (I mean *everyone*) should fly taildraggers at some time in his/her pilot’s career, maybe even continue to do so.

  9. Johan Prins says:

    I forgot: can you apply reverse prop pitch after touchdown on a Porter?

    • Matt Dearden Matt Dearden says:

      You certainly can! Rarely used though unless you’ve made a bit of a hash of the landing and find yourself hurtling towards the end of the runway with both wheels locked..