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Post  balogh Fri Dec 17, 2021 1:24 pm

Lo and behold, a local club member advertises this late COX  CL plane for sale...the engine has snap starter and slit cylinder so it is probably a late model.

Any idea which model it is?

Which COX CL bird is this? Fb_img10
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Post  Ken Cook Fri Dec 17, 2021 1:43 pm

Looks like the EZ-Flyer P-39 Airacobra
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Post  balogh Fri Dec 17, 2021 1:47 pm

Thanks Ken, indeed..I was wondering why the clockwise prop rotation, that rises a precession moment, that tends to be pulling the plane inwards into the circle?
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Post  batjac Fri Dec 17, 2021 3:32 pm

Actually, it's the opposite. A clockwise turning prop ( as viewed from the front) will tend to turn the nose outward. Realistically, I don't know why Cox didn't end up putting left handed props on all of their control line planes, since Cox put spring starters on all their RTFs. The only Cox plane that I have that came with a left handed prop is a NIB PT-19.

The Screwy Mark
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Post  balogh Fri Dec 17, 2021 3:40 pm

Mark,

My RC planes with counter clockwise spinning props ( viewed from the front) have their firewalls offset by 2..4 degrees to the right to keep the planes fly straight..but maybe the offset is not for balancing the precession moment of the prop, but to balance the aerodynamic moment of swirling air acting on the left rear side if the plane...
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Post  batjac Fri Dec 17, 2021 4:17 pm

Okay, you're making me go back to my student pilot study days, here.  The three forces affecting yaw on a plane due to prop rotation direction are asymmetric thrust (P-Factor), slipstream rotational velocity (airflow spiraling around the fuselage following the direction of rotation of the prop) creating a yawing motion due to air impinging on one side of the fin/rudder, and gyroscopic precession. A fourth factor, torque, which relates to roll, I will leave out for now.  P-Factor is more of a factor on planes in a pitching up moment (I won't go into why unless some masochist wants further theory), and Rotational Velocity is more a factor of propeller pitch and airflow velocity.  Being a taildragger pilot, I am quite familiar with these effects.  But I just love flying conventional gear planes (tricycles are for kiddies...).

I may be wrong, but I've always figured that on our small models, P-Factor is relatively negligible, and Rotational Velocity is the main thing that is turning our planes in on the circle with right handed props.  Yes, the right offset we build into our planes accounts for this.  And, whelp, I guess that's all there is to say about it.  It works.  I know most Cox RTFs had right offset but some didn't.  I can't say with any authority what percentage of the planes did have the offset.  The most recent plane I played with, the Hyper Viper, definitely did have lots right offset.

Gyroscopic precession?  I'm not sure how much that affects our little planes on takeoff if you don't make a rapid pitch up.  Yes, if you pitch up the plane, the precession makes the plane want to yaw 90 degrees to the right.  But in level flight, the yawing motion for precession is negligible, I'd think.  Now, the gyroscopic precession from the plane turning in a circle?  When I first thought about that, it seemed that since the plane was in a constant left turn, there would be a constant upward force exerted by the propeller gyroscopic precession.  But I'm not smart enough to figure out just how much.

So, after writing all this, I'm not sure what my point was.  But I know I was making one of some kind....

The Conventional Mark
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Post  Ken Cook Fri Dec 17, 2021 7:55 pm

I don't subscribe to the reverse rotation theory. I fly some extremely powerful planes, speed, combat and stunt. The only planes I ever had even offer a suggestion of turning into the circle was my speed ships. Aside from that, I never use rudder offset, no engine offset and no issues with planes coming into the circle. Low line tension or no line tension can and usually is a result of inaccurately built wings. There could be a a unnoticed twist or flaps misaligned, improper tip weight, etc. The plane now flies into the circle towards the pilot or flies level and has a tendency to want to fly across the circle in maneuvers.

Another trimming technique I use is to move my leadouts as far forward as I possibly can to the point that the plane offers little to no pull on the handle. I firmly believe Cox used it as a marketing tool. The planes generally ended up splattered and anything they could do to recapture the public was used. If the writing on the box suggested it could fly the entire stunt pattern believers feel that it must be able to do that. What wasn't explained is that the test pilots were very skilled.
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Post  balogh Sat Dec 18, 2021 1:13 am

Wow, Mark and Ken,  thanks a lot for your detailed explanation on the yaw motion and its influencers.

I only very dimly remember my kinematics studies 40+ years in school and this discussion  helped me recall some theories I learnt.

Some of my planes are more, some others are less yaw-sensitive to the firewall offset, it may have something to do on how tall their rear fuselage section is I.e. what moment the swirling air behind the prop will place on them.
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Post  batjac Sat Dec 18, 2021 4:31 am

Actually, now that I think about it, I do not believe it is P-Factor, slip stream rotational velocity, or precession that makes the planes turn into the circle.  I do not think it is a yaw problem at all.  I am remembering some cussing and thinking I did a long while back.  After a plane came in at me, I decided that it is the force that I did not want to talk about.  The reason that the plane flew in at me was because of the torque from the propeller rotation.  With a right-handed prop, the torque will cause the plane to roll into the circle, and with a left-handed prop the torque will roll the plane to the outside. If the plane is flying at speed, the airflow over the wing will keep the plane level. When the plane is just taking off and at low/no speed, there is no stabilizing force on the wings to keep them level, so torque effect is much stronger.  I think this was the point I was setting out to make in the earlier post, but I got so hung up on yaw theory that I got off on the wrong track.

The Revisionist Mark
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Post  balogh Sat Dec 18, 2021 5:11 am

Thanks Mark, this was the thinking that made me ask about the left hand prop on that plane...but now it is clear that the left hand prop wants to pull the plane out if the circle, and the right hand prop tends to pull it into the circle. Hence the rudder angle and/or firewall offset on some COX planes.
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