Mr._Ryan wrote on Jan 31^st, 2011 at 10:20am:



Interesting question..  Next time you practice landings, try to notice the "d" factor (upside-down "P")..

Even at a low power-setting, like short-final, you'll get yawing opposite normal P-factor when you pull to idle... and since there's almost alway SOME sort of X-wind, you're already working the pedals.. and by the time the "d" factor fades, you'll prob need a little right rudder as you're near (or on) the ground..  And once on the ground, the trim (built-in, or by trim control) (some PA28s have rudder-trim), set to account for slipstream roll at cruise might require some rudder-work until you slow down a bit.

** I made up "d-factor" **   

DaveSims wrote on Jan 31^st, 2011 at 7:41pm:


Ditto!

This is sound advice. Brett is 100% correct, but touchdown is not the time to be pondering all that, obviously. You should be looking at the end of the runway, and "stirring the pot" as needed to touch down and roll out parallel to the runway centerline. Or just to stay on the runway. 

But regarding p-factor in general: in the aircraft you are probably flying now, it's really only worth considering when you are at low airspeeds at high power settings. In such situations, whatever they might be, I'd offer one of the best instructor tips I ever got:

"Pretend your right (throttle) hand is connected to your (rudder) foot, and they always move together (during low airspeed/high power flight)"

Thanks to everyone first of all for your replies, they are appreciated. I've gotten to thinking more about exactly what happened and more specifically what my question really is, so allow me to clarify:

I believe what happened is that I didn't kick out the crab fully from a slight crosswind. The ball was centered (something I'm always aware of), and what happened was that I landed not pointing directly down the runway. Then what happened was, when I went to straighten myself out by using the rudder, the plane started dancing a bit and my instructor was quick to tell me to knock it off with the rudder pedals.

So while I understand what happened, it got me to thinking (and this is what I originally was wondering): if I do land going with the airplane oriented perfectly straight down the runway, will it continue to go perfectly straight after touchdown, or should I expect to have to use some rudder input to keep going straight down the runway?

Mr._Ryan wrote on Feb 2^nd, 2011 at 1:37pm:


The reason why they started putting nosewheels on airplanes was so that they would tend to straighten themselves out after touchdown- that is, begin rolling in the direction they were moving when they touched down, even if the nose was pointed slightly askew. There's more mass in front of the mains than behind, so if the plane touches down crooked, that greater mass will come towards the center as it pivots on the first wheel to touch down (which, we hope, is the upwind wheel).

But it's important that the direction mentioned above be on the runway centerline, or at least parallel to it, so some interplay of aileron (for drift left or right of center) and rudder (to keep the airplane's centerline aligned with the runway centerline as much as possible) is needed most of the time, when there's a crosswind.

Ideally, when there's a crosswind,you want to touch down in a perfect sideslip, totally negating the effect of the wind, so that the trike gear doesn't have to help you. But nobody's perfect, and in fact, sometimes you can only sideslip so much in certain conditions, and you will end up "crabbed" a little as the mains touch down.

This is why most taildraggers have a lower demonstrated crosswind component than nosewheel planes of the same weight, etc.  With a taildragger, the greater mass is behind the mains, so if it touches down crooked enough, the back of the plane tries to come to the center the long way- that is, around the outside. The result is that you will either roll off the runway or wind up whipping around in a "ground loop." So in taildraggers, there may be quite a bit of stick and rudder movement during that critical moment of touchdown/rollout, and it better be right!

And of course with any plane landing in a crosswind, you have to follow through with continued input as you roll out. The deflection of rudder and ailerons should increase as you slow down, because as the airflow diminishes, these surfaces become less effective, but may still be needed. I forgot that a few times landing nosewheel airplanes, but only once flying taildraggers!!