Free Flight "The Pylon Effect" causes turn.

[Texas Buzzard]

I am attempting to ask if anyone remembers why a F.F. with a taller pylon turns more than a shoulder winged F.Fs.

Years ago I saw a few diagrams that showed a swirling draft of air from the prop, when viewed from the aft end this air swirlred in a clock-wise direction.

Simpson who was in the USAF won FAI several times and he used what looked like a piece of 1/8th balsa ( 5' x 9") for his fin. This fin was just ahead of the stab and could be moved up or down thru' a slit. He controlled the rate of turn by the placement of that fin (up or down) - the ship had a low pylon.

Some people discussed the "p" effect....I never did understand that.

Can we have some discussion of factors that cause a contest F.F. to turn?

[JetPlaneFlyer]

Buzzard,
The reason the up and down adjustment of the fin causes a turn is due to the helical prop wash. If the fin is above the fuselage the prop wash forming a helix around the fuselage hits it from the port side (assuming conventional prop rotation). This side force causes to model to yaw to port.
If you slide the fin downward so it protrudes from the bottom of the fuselage then it experiences the opposite effect, the prop wash hits it from the starboard side, so the model yaws starboard.

'P' effect is caused when the aircraft is flying at high AoA. When the aircraft flies nose high the prop axis is no longer aligned with the direction of flight. This means the air stream approaching the prop is coming from the bottom and therefore the downward moving blade has a higher air speed than the upward moving blade. Because the thrust of the prop is proportional to the square of the blades airspeed this means one side of the prop disk (the starboard on a conventional prop) produces more thrust than the other side which yaws the aircraft (port yaw for conventional prop).

Of course none of this answers the point about pylon mounted wings 'turning' more than other designs. To be honest I never considered that a pylon wing mount (all other things being equal) did tend to turn (under power) more than a shoulder wing. The only thing I can speculate is that a wing that is 'in' the helical prop wash (low wing, mid wing, shoulder wing) tends to create a rolling torque which opposes the motor reaction torque due to the fact that one wing is expiriencing 'up' wash from the prop, and the other sees 'down' wash. A pylon mount wing however, where the wing is perhaps out of the prop wash, or at least seeing only the horizontal component of the helix, won’t produce this opposing torque so the model may tend to roll (port for a conventional prop rotation) more than a shoulder wing design. I'm not really convinced by this argument though; I'd suspect that the effect would be too small to notice.

Steve

[vintage1]

Thereare so many effects..


Torque, P effect and the counter to torque when the rolling slipstream bounces off the various surfaces.

My Smeed 'pushy cat' has a pusher prop that has the lower part of the slipstream hitting the tail. And a high mounted tailplane. It tends to be neutral or to turn slightly RIGHT under power..

Normally with the upper part of the slipstream hitting the tail you get a left yaw from that as well as the prop torque. To an extent, putting the wings in the slipstream will counter the torque.

I would say therefore that a low mounted engine and a high mounted wing out of the slipstream will tend to exacerbate left turns.

Certainly my latest model..sort of slightly spitfire-ish low winger seems to behave completely differently to the standard sort of high wing vintage stuff.

As to P-factor..well I am always sceptical about that. It has its genesis in high power tail draggers swerving on takeoff when the angle of attack of the prop was massive - like 25 degrees or more..once in the air, the angle of attack relative to the AIRSTREAM is very low..probably no more than 5 degrees and less with downthrust. So I discount it as part of the inflight equation.

That is almost always bound up in the spiralling airstream - which creates the torque in the first place, and if straightened out will create an opposite reaction to that torque. Plus the yaw causes by assymetric fins: Fin in the top of the prop blast=left yaw. In the bottom = right yaw.

A pattern plane with thrust line, wing and the average area of the tailplane all more or less in line tends to fly very straight. If it ends up needing side or downthrust, it is to my way of theinking deficient in its aerodynamics to start with :D


A high wing plane with high power and with the prop blast top hitting the fin - thats very contest modellish - is going to want to turne left a LOT. But thats OK, because that helps contain the nose up moment you are going to get from a naturally stable plane that is flying faster than the glide. Instead of a loop, you get a tight left hand spiral.


I wrote another post recently that caused me to think about the stability of pylon planes..I concluded that by having a very high wing, and a long large tail, they could run with a very rearward CG and very little decalage at all..using the pendulum effect to get stability, and that would make them far less sensitive to power on stalls and zooms. The downside being that to get the CG low the motor is also low, and that means the model will yaw and roll left a lot with the wing out of the slipstream and the tail in the top of it.

I guess the successful model was one in which the exact amount of power, and all these effects combined to give the perfect spiral climb and the perfact large circle glide.

I did bery little contest free flight..a friend made one, and I remember it being very hard to trim..finally we zeroed in on a setting that wasn't a series of power on zooms and stalls or a spiral into the ground..and it then flew way. The game, we decided, wasn't worth the candle ;)

[Texas Buzzard]

Jet Plane Flier and Vintage 1 have come through again with super posts - and they stuck to the subject......thanks guys. I'd susspect both of you have some gray hair for you can't have that much knowledge stored between your ears without spending years actually doing these things. :rolleyes:

When I wrote the original posts I was thinking about a few free flights that I built and flew in the past. For the most part, I built 3 different sizes of Ron St. Jean's "RAM ROD", Class A (Mayhew's) "Zeek" and the 1/2 Zero by a guy from L.A. I had a Gastove by Michael Gaster of the UK which I didn't build. It climbed and glided to the left. It was typical pylon but had an extra long tail moment arm.

The Zero had a thrustline up high and it was very close to the underside of the wing. The tail was down low. It, I think, was that way to absorb higher powered engines that were coming out....it climbed well.In 15 sec.it made one right turn. :confused:

Back to RAM ROD - my favorite. I flew that plane with significant (?3 or 4 degrees?) right thrust and 5 degrees downthrust. It was forced to climb to the right. The right tip panel was warped positivly (washin) to conteract the right turn under power. For glide turn I tilted the stab...right side high for right turn in glide. In the late 50's and into the sixties RAM RODS were very popular. The pylon was "normal"-sized. :) :)

The best FF guy in Austin,Texas was a super good builder and a smart one too. He liked the Goldberg "Zipper". and variations of it. He wanted a fast climb and with some of his variants he could get almost a vertical climb with maybe two turns on the way up. The Zipper looked like a typical pylon job from the 1940's. It was! Harold's Zippers climbed to the left and glided to the right. He had on set of plans published in about 1964 for a Class A FF and called it the "Nipper". Had a thin sorta' Clark Y ( 8% possibly), for fast climb. He was looking for thermals. I know he reduced the size of the fin some, he said so the plane could "find" a place to allow the dihedral to take over....I understand that to mean he cut the fin down just to the poijnt that a Dutch Roll was about to start....got it?

Thanks guys for prodding my mind. Maybe I should say thanks for waking me up. :) :p ;)

[gyrocptr]

Re: the more specific question "...why a F.F. with a taller pylon turns more than a shoulder winged F.Fs."

To my understanding, the very tall wing pylon interacts with the spiral prop-stream to help control the aircraft turn during the power-on portion of the flight. The wing "pylon" was originally a tall "cabin". The tall "cabin", low thrust-line combination was found to be the most resistant to the deadly, left-turn-power-on spiral (into the ground). Carl Goldberg minimized the "cabin" to the profile, pylon. The forward pylon, positioned close to the low thrust-line engine interacted with the spiral prop-stream to help generate more right-roll/right-turn during the power-on phase.
To reduce the need for down-thrust, designers started moving the engine up the pylon. This did not affect the "pylon"-effect (the interaction with the spiral prop-stream still generated right-roll under power), it did lower the vertical fin relative to the spiral prop-stream and reduced the left-turn interaction between the spiral prop-stream and vertical fin. Carl Goldberg designed a very-high-thrust-line FF model (? FAI Viking): low fuselage; lifting-stabilizer; vertical fin extending below fuselage; wing mounted up on a tall pylon; engine mounted at the top of the pylon near the wing, with upthrust. Nearly vertical power-on phase with a quarter-roll transition into the glide circle.