Push Rod Question????

[Ed]

How Ya Doing, Good People;

I have viewed many photos here of profile planes, I have seen several with the push rod on the outboard side. Is this a personal preference, design or trim issue. How does it work? do the leadouts go though the fuse. What are the benefits of either?

Ed Vega      P.S. I noticed my profile no longer says New Pilot

[Leester]

Which side of the fusalage the pushrod exit's usually depends on how you mount the bell crank. If you have the control arm of the bell crank towards the outboard most will exit the push rod out the outboard side and then the opposte if you mount the bell crank with the arm facing the inboard. If you mount the BC facing the inboard your UP line will be the forward line facing the outboard and the back line is your up line. Either installation and your leadouts exit the wing in the same way. Having the BC facing the inboard is suppose to have an advantage but someone better qualified will have to explain that.

[Peter Nevai]

Hola,

The location of the bellcrank has a lot to do with the location of the pushrod exit. Especially on a profile plane that has no fuse to hide the rod. If the belcrank pivot is near the center of mass and is mounted in the traditional fashion the arm of the bellcrank ends up on the outboard side. You can mount the bellcrank inverted and then the pushrod ends up on the inboard side.

Each has it's advnatages and is more a personal preference than anything else.

[john e. holliday]

Most of the profile kits i have built since the early 50's has shown the bellcrank mounted inboard of the fuselage, hence the pushrod is/has to be on inboard side of fuselage.  Of course most were built up wings, so it was easy to move the bellcrank outboard when it was desired.  Mostly it is more pleasing to have the pushrod inboard for looks.  Also the bellcrank arm for the pushrod can point inboard or outboard, depending on how close you want the pushrod to the fuselage side.  Same goes with the control horn, it can be on top or below the elevator depending on how straight a pushrod run you want.  In my opinion it does not matter if the front leadout is up or down, because I have them close together, approximately 1 inch spacing.  DOC Holliday

[L0U CRANE]

Ed,

One thing RCers do that makes a lot of sense - they generally put the on/off switch on the side away from the exhaust.

Almost all profiles have the engine outboard, and our models are 'juicier' than RC models (we use at least some castor in the fuel.) So, the hole for an inboard pushrod is on the "dry" side of the fuse. Should reduce fuel soak into the wing root area. That also goes with the reversed - UP line forward - bellcrank...

Why Up line forward? The prop acts like a gyroscope. Tilt the 'axis' up, and the gyro tries to turn its axis to the direction 90° 'later' in the direction of rotation. For most models, that means that UP input tries to swing the nose outward, and DOWN tries to swing it inward.

The control lines take different loads when we give a control motion - the load shifts toward the side we need to pull against the air loads on the moving surfaces. With UP line forward, that has a tendency to pull the nose in, IOW, to oppose the gyro effect of trying to turn the nose out. And vice versa... So, there's less tendency for the model to change its attitude in yaw (left/right).

Bellcrank position is NOT important for trimming a model to fly well. LEADOUT position is! The pull aims through the leadouts toward the CG, wherever the bellcrank is. Putting the bellcrank in line with the angle the lines make when they reach the tip guides can reduce bending the leadouts, and the friction that comes with that.

It is also important to mount the bellcrank pivot for strength - which usually means near the fuselage centerline and maximum rib depth wing chord location.

To get the UP line forward, you can either turn the bellcrank over from the traditional positioning (pushrod inboard of the pivot), OR put the elevator control horn above the stab/elevator chord line. ...Btw, that puts the flaps horn under the wing chord line.

Done right, a model will not swing in yaw on sharp control inputs, and the controls will remain blissfully free for a long time. This is a very simplified description. I've played with some calculations to try for a complete cancellation of the gyro effects, and for where leadout guides should be with a given CG location. I think it works, for me, anyway...

[Paul Smith]

I ALWAYS put the pushrod outboard.

No matter what the design, I put the bellcrank mount on the outboard side so it can be braced against the whole fuselage.

Any weight that can be moved outboard should be moved outboard, thereby minimizing tip weight.

ps: I put a DuBro ball link on this worn out old bellcrank so I could get a few more planes out of it.

[Ward Van Duzer]

Paul,

Never, NEVER "cheep" on control components!  He'll get you sooner or later...


Ward-O