"P" factor question

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  • "P" factor question

    If you are sitting in a tailwheel airplane, engine running at power...not moving. Is there asymmetric thrust? Or, does the plane have to move forward before the thrust is asymmetric?


  • #2
    I say no.

    Without motion, the airflow is perpendicular to the prop disc.

    Comment


    • #3
      Originally posted by Terry Carraway View Post
      I say no.

      Without motion, the airflow is perpendicular to the prop disc.
      I agree. P factor comes with forward motion.

      Comment


      • #4
        Originally posted by Bruce Gorrell View Post
        P factor
        Hate to get involved in this discussion, made me look back thru some old stuff to see what I'd written in Warbird Notes about it: Talked about it in #44:

        ----------------------------------------------------------------------------------------------
        #44 “TORQUE” VE “P-FACTOR” TERMINOLOGY

        (1) Definition of torque – (Aerodynamics for Naval Aviators): “

        (2) Definition of torque – (Webster): “a twisting or wrenching effect or moment exerted by a force acting at a distance on a body”.

        (3) Definition of P-factor – (Aerodynamics for Naval Aviators): “

        Torque is the reason that the left tire’s tread on a Mustang invariably wears faster than the right one. Did’ja ever notice that? Well, take a quick look at one the next time you see a Mustang parked at an airshow, many times you’ll find it to be so! When I start a take-off in a Mustang, it’s with the stick fully to the right and full right rudder before I ever add the throttle. Initially the ailerons aren’t going to do anything, but as they start to gain effectiveness with an increase in airspeed, they’ll counteract the effects of torque. Until then the Mustang definitely will “tromp” or “twist” downwards on its left side. That’s torque! Also, even without wind, it’ll also want to swerve off towards the left side of the runway! That’s P-factor, a different thing from torque. That attempt to swerve towards the left is a common characteristic of our American-built airplanes!

        I’m able to demonstrate torque in a Mustang, Corsair or airplanes like that; however I cannot in a Swift or even a T-6! The latter two simply don’t possess adequate power to exhibit any readily noticeable amounts of "torque" effect to the student.

        Yet to write:
        An abrupt change in angle-of-attack? I.e., raising the tail during the take-off roll. Precession force.

        Len Dolny in the back seat of a TF-51 over at STP’s Holman Field, that balked/rejected landing with the left wingtip dragging the ground.

        Elevators have twice the area of the rudder; any fool can raise the tail before enough rudder is available to control yaw! Expression out in west Texas about “getting a 90 degree change of scenery without ever moving your neck”!

        Raise the tail abruptly? Well, maybe you deserved what you got from that rash act of yours!

        In the end analysis, all the above discussion attempts to precisely define the terminology or the forces for the flight instructor to enable him to fix the correct terms in his/her student's mind. After all is said and done, however, what we‘re really looking for here is the innate ability of the pilot to apply a copious amount of controls BEFORE needing them to keep the airplane from abruptly turning towards the left (or for the Limeys in the Sea Furys, to the right!).

        ----------------------------------------------------------------------------------------

        Lot's more there on the Swift, it and our Staggerwing could sure teach'ya what the rudder's for!

        best, randy





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        • #5
          Originally posted by Jeff Hartmann View Post
          . . . tailwheel airplane, engine running at power...not moving. Is there asymmetric thrust? Or, does the plane have to move forward before the thrust is asymmetric?
          Hi Jeff,
          Asymmetric thrust due to so called “P” factor? My opinion: No. Not if the airflow entering the propeller disk is parallel to the propeller axis or rotation.

          However I’d venture there is a turning force induced by the spiral airflow pushing the vertical stabilizer to the right even with a stationary aircraft. You could do an experiment with an airplane on a giant treadmill.<GD&R>

          I think some, due to either misunderstanding or oversimplification, lump several independent turning forces together into the “P” factor pot.

          Regards,
          Tom Charlton
          "The aeroplane has unveiled for us the true face of the earth." - Antoine de Saint-Exupery

          Comment


          • #6
            Thanks guys, Instructing years ago in Citabrias I was able to demonstrate P Factor, slipstream , and torque. (wing wash in and offset fin). I just got thinking about P Factor sitting still and wondered if there would be any. Obviously not until there was some forward motion. Slipstream on the tail most likely. I watched a newbie (demonstrate gyroscopic precession) in a Cessna AG Truck, he started to take off in an empty airplane, popped the tail up and turned hard left. No damage (except ego) and dirt packed between tire and rim.

            And how the h#ll do you change the font and size on this page...it keeps getting smaller, or I'm getting tired. I have tried the bar...doesn't seem to do anything.
            Last edited by Jeff Hartmann; 07-09-2018, 00:42.

            Comment


            • #7
              Originally posted by Jeff Hartmann View Post
              And how the h#ll do you change the font and size on this page...it keeps getting smaller, or I'm getting tired. I have tried the bar...doesn't seem to do anything.
              CONCUR!!!!!!!!!!!!!! Now then, THERE'S a real mystery!

              best, randy

              Comment


              • #8
                Originally posted by Tom Charlton View Post

                . . .I think some, due to either misunderstanding or oversimplification, lump several independent turning forces together into the “P” factor pot.

                Regards,
                Tom Charlton
                I'm talking off the top of my head here (or maybe some other orifice) but it seems to me the TORQUE reaction is due solely to the acceleration of the rotating parts inside of an engine and not the reciprocating parts, so whichever engine has the heaviest crankshaft is going to have the greatest torque reaction.

                I do not know this for a fact, but I suspect the Merlin crank is far heavier than the crank of a radial engine of similar power, so the Mustang, Spitfire, Hurricane et al are probably, as a group, the bad boys of the bunch.

                [The preceding opinion is based strictly on a SWAG.]

                - TT
                Tom Tyson-A&P

                Pilots without Mechanics are just Pedestrians with fancy watches . . .
                ( . . . and Mechanics without Pilots are Unemployed.)

                Comment


                • #9
                  Originally posted by Tom Tyson View Post
                  . . . seems to me the TORQUE reaction is due solely to the acceleration of the rotating parts inside of an engine . . .
                  Hi Tom T.
                  Talking out –my-, ah, other orifice:
                  Perhaps best to not think in terms of acceleration but TORQUE in a steady state, constant RPM?

                  so whichever engine has the heaviest crankshaft is going to have the greatest torque reaction.
                  - TT
                  With that reasoning you’d want to include the rotating “mass” of the propeller. Typically its contribution to the inertia of the rotating mass would –outweigh- that of any crankshaft. Seems to me.

                  I’d postulate that:
                  The –mass- of the rotating parts, non-accelerating, constant RPM, is irrelevant. The resistance to turning the propeller against air molecules is everything.

                  To prove it out I’d put our Mustang in orbit aboard a Falcon 9 Heavy. Crank that sucker up! Now, since the ailerons ain’t gonna work use our STCd gas thrusters to prevent our Mustang from rolling –only- during the acceleration phase then, when at a steady state hi RPM, turn off our STCd gas thrusters. Little rolling moment other than a small amount of TORQUE to overcome internal friction. Oh and look: we’ve had to retard the throttle way-back to keep the RPM down to max RPM.

                  Oh by the way, you want TORQUE? Strap yer self into one of them there sling-wing machines<g>.

                  Regards,
                  Tom Charlton

                  "The aeroplane has unveiled for us the true face of the earth." - Antoine de Saint-Exupery

                  Comment


                  • #10
                    Originally posted by Jeff Hartmann View Post
                    Instructing years ago.......... demonstrate P Factor............and torque.
                    Good on ya! Last night (anyway, in the middle of it) I was able to find some stuff that I 'd written years ago about students considering their instructor as a "font of knowledge" and the disservice that IP could/would do to that that student by using the two terms interchangeably. THEY ARE NOT! I wish now that I would'a copied whatever it was so that I could re-produce it here.

                    Now back to that question of how do you change the fort, this was h--- proofreading my typing!

                    best, randy

                    Comment


                    • #11
                      Tom T, I'll agree with Tom C here. The rotating mass includes the prop, and the prop's contribution to the total moment of inertia is far bigger than that of the crankshaft. Once the rpm is stabilized, the torque reaction is coming only from blade drag.

                      However, it's pretty common practice to lump all the turning effects (actual torque, P-factor, spiral slipstream, precession due to pitch, unequal wheel rolling resistance, etc.) under the simple rubric of "torque".

                      Comment


                      • #12
                        Originally posted by Ralph Jones View Post
                        pretty common practice to lump all the turning effects (actual torque, P-factor, spiral slipstream, precession due to pitch, unequal wheel rolling resistance, etc.) under the simple rubric of "torque".
                        Concur! "How-sum-evah", the IP should bea able to ascertain that the student clearly undertsnds that those effects, while commonly referred to as "torque", are, in acuality, are far separate/distinct.

                        best, randy

                        Comment


                        • #13
                          Originally posted by Randy Sohn View Post
                          I wish now that I would'a copied whatever it was so that I could re-produce it here
                          Ah, found it:
                          -----------------------------------------


                          Anyhow, back to the serious stuff. Over the years, I’ve seen this maneuver taught in two or three ways. And I’ve taught different ways of doing it, depending upon the objective. So I guess we have a consensus among flight instructors of the maneuver’s validity, but not of the terminology.

                          (1) One method would be while in straight and level flight to pick a point on the horizon and enter a turn away from it for some amount of turn and then, without stopping, reverse the turn to pass through the original point in the opposite direction, then again reverse the turn and so on, while all the time coordinating the flight controls (especially the rudder and ailerons). (2) Another variation or method used is to pick a straight road or a point on the horizon. Then precisely hold that point while initiating a bank (takes opposite rudder), then reverse the bank to an equal amount in the opposite direction. This is repeated over and over while using the flight controls to precisely maintain that point (especially the rudder and ailerons). This variation especially lends itself to getting the student ready for the aggressive use of the rudder in acrobatic flight.

                          Evidently, to a certain number of instructor pilots, the above aileron/rudder coordination exercises (especially #2, the one that holds the reference point) that we all give our new students are called “dutch rolls”.

                          Well, we’ve got some news for you, chum! Those aren’t “dutch rolls”, they’re simply plain old garden variety “coordination exercises” or “coordination rolls”. And, undeniably, they’re extremely useful for teaching coordination or for quickly evaluating an aircraft’s handling qualities! I’ve used them from the very first time I took my first lesson in an Aeronca Champ right up until the present. When we were aviation cadets in “Bevo” Howard’s USAF T-6 school, we were taught them from the very first day of our flight training. BUT THOSE ARE NOT DUTCH ROLLS! You copy that? “Sorry Charlie” but no cigar, those are NOT dutch rolls! No big deal, you say? Well, OK, but you need to realize that when you use an incorrect term it’s teaching your student something completely wrong. It’s sort of like the media using the term “Piper Cub” for every airplane less than a medium sized jet. And, besides perpetrating a falsehood, it can later kill him/her! And if you don’t think or realize that a dutch roll can easily become lethal, look up the Braniff/Boeing 707 (N-7071) flight training accident involving the tossing of a couple of pylon mounted engines off the wings in the fall of 1959.

                          So you say, “well then, just what IS a dutch roll?” In test pilot school we were provided with a detailed description of the phenomenon. While that description seems to be far more technical than required for this discussion you can look it up in your copy of “Aerodynamics for Naval Aviators“ or any other reliable reference book available. I’ve never been able to induce a satisfactory “dutch roll” in any straight-wing training type airplane. This roll/yaw coupling phenomenon is usually found only in swept- wing types. One might ask where the term “dutch roll” originated, most seem to think that it probably found its genesis in the rolling motion of a speed skater on the frozen ice canals of Holland. At any rate, I can attest that the recovery from this potentially violent and lethal maneuver is completely non- intuitive! In fact, I’d describe it as the antithesis of coordination. It is a good portion of the reason for the development of yaw dampers (when first developed they were often referred to as dampeners) on modern day jet transports. It is also the reason why, if one experiences a yaw damper becoming inoperative on the Boeing 727, that the overriding priority is to extend the spoilers and descend NOW to a cruise altitude in the twenties.


                          Stuff yet to write:

                          Research into some possibilities for erroneous use reveals that it might have had it’s origin in a Cessna approved Part 141 student pilot training syllabus in which it appears that the term was used incorrectly. This according to Steve Barber. Also, a FAA source tells me that his instructor also used the term incorrectly years ago in a Piper Warrior while teaching him to fly. So, no wonder huh? It’s sort of like that line in that old song by Johnny Cash, “bad news travels like wildfire!”

                          One also mentioned that “since swept wings are a relatively new aviation development”. Yup, like the Germans weren’t conducting research with them about six or seven decades ago!

                          Any instructor hopes that his student will later proudly recall him as a font of knowledge regarding the things he taught, imagine that same student’s disillusionment and disappointment years later if he learns that his instructor was just plain wrong. One instructor says he feels “it’s shorter to say “dutch roll” than “coordination exercise”. Well, that might very well be, however it’s also shorter to call an aileron a flap, but no one that I know does! Also, he said “aerobatic pilots in particular have a long tradition of giving their maneuvers colorful names”. That’s certainly true and some of them certainly are descriptive, the “torque roll”, “top hat”, “humpty-bump” and others immediately come to mind here. One instructor who’s always used the correct term told me that it most likely was because a school that included “dutch rolls” in their literature or syllabus could ask for higher fees from students. Oh, if it were only that simple!

                          --------------------------------------------

                          best, randy

                          Comment


                          • #14
                            Pure P-factor, no spiral slipstream effects, no torque reaction, no gyroscopic precession, requires wind across the prop disc. That makes the blades' angles of attack dependent upon instantaneous rotational position. With no outside air motion, wheter wind or forward speed, the blades' angles of attack are the same all the way around.

                            It might be easier to visualize in the case of retreating blade stall in a helicopter. Excessive forward speed stalls the rotor blade which is traveling in the opposite direction at the moment.
                            Hate is a self-built dungeon in which a man imprisons his own soul.

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                            • #15
                              Originally posted by Jeff Hartmann View Post
                              And how the h#ll do you change the font and size on this page...it keeps getting smaller, or I'm getting tired. I have tried the bar...doesn't seem to do anything.
                              You can adjust your browser's zoom-level with <CTRL><+> and <CTRL><->. <CTRL><0> takes you back to 100%.


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