Replies to This Discussion

Permalink Reply by Joe Harrington on November 2, 2013 at 7:10pm

John,

Have you thought about mounting the servo vertically (stand it on its end)? I will be using the MGL servos with the dual stick option and this is what I was planning on trying so the servo would not protrude outside of the centre console. I do not have my servos yet so I am not sure this would work. There may be not enough clearance from the rudder cables(?) and/or the unit may still have too much y axis dimension to remain in the confines of the console(?).

Joe

Permalink Reply by John Austin on November 2, 2013 at 7:41pm

I was looking at that today - I think there's not enough clearance below the cables if mounted vertically over the raised heel rest area, but there might be fore or aft of that area. I'll look at that again tomorrow!

I thought about mounting it horizontally right behind the firewall with a longer pushrod. The lower nosegear bearing support plate is right underneath, however, and that complicates mounting. I may look at some sort of mount to bridge over that area and allow a central mounting position behind the firewall.

John

Permalink Reply by John Austin on November 3, 2013 at 3:17pm

Joe,

I looked again today and did some careful measuring, etc. At first, it looked like standing the servo vertically just behind the heel rest might work:

Alignment was pretty good with the torque tube arm, and with a neutral rudder there was good cable clearance. However, with full left right rudder, the left cable came down and hit the servo arm! I really don't want the servo motor protruding into the pilot's floor area (if mounted horizontally in center console) unless absolutely necessary. I looked again at putting the servo on a "bridge" mount - this time placing it above the nosewheel steering pushrods:

As you can see, I rested the servo on a plate resting on the pushrods just to get an idea of the dimensions for the mount and the geometry between the servo and the torque tube arm. Looks like a 7" wide mount bridging over the pushrods would work well. It needs about 3 1/2" height above the floor to clear the pushrods. It can be about 80mm deep so it would fit between the pedal tubes on the floor and the upturned lip between the tubes and the firewall. 

One could make it much narrower and put it between the pushrods, but I think the installation would be simpler and more accessible to simply straddle the pushrods and make it wider. I checked the path a servo pushrod would have to take between the rudder cables back to the torque tube arm and there appears to be no problems. Obviously, with the wide bridge, the servo can be mounted so the servo arm and the torque tube arm are in the same plane, so that eliminates any alignment issues. I've got an email in to Caleb just to be sure there are no issues with drilling the torque tube arm for the servo pushrod bolt (as long as edge distances are respected, of course!). Alternatively, one could "share" the existing AN5 bolt that connects the torque tube arm to the elevator pushrod, but that's a pretty long arc of movement and I think about 1/2 way up the torque tube arm would be a better location for the servo pushrod.

I'll update when I get either a mock-up or a mount fabricated.

Thanks for your input.

John

Permalink Reply by Joe Harrington on November 4, 2013 at 11:18pm

Thanks John, this will be very helpful when I install mine. It'll be great to see what you finally come up with.

Joe

Permalink Reply by John Austin on November 6, 2013 at 4:10pm

Joe,

I fab'd a mount to bridge over the nosewheel steering pushrods (also goes over my brake and fuel lines, too). I made it of .065 and used an .040 "L" across the back of the mount to further stiffen it and made .065 gussets that are riveted to the vertical sides and will be riveted to the flange that protrudes up from the aircraft floor. This should make it extremely rigid laterally. The servo bolts to a mounting flange I made (also .065) that is riveted to the mount. The rectangular servo gearbox also rests on the horizontal portion of the bridge, so it seems "locked-in" and can't flex the mounting flange (the pushrod forces will be fore and aft, not side-to-side, anyway!). The pilot's right rudder pedal comes very close to the edge of the mount, so I made a scallop in the edge facing the pedal to ensure clearance:

Since this topic's original question about pushrod alignment is moot, later, when I actually install the mount and servo in the plane, I'll likely close this thread and continue the follow-up in my other topic about Autopilot Servo Installation with Dual Sticks.

John

Permalink Reply by John Austin on November 8, 2013 at 3:03pm

As I pointed out earlier, the pushrod alignment issue has become moot since my pitch servo's arm and the torque tube arm are going to be aligned in the same plane, so I'm going to close this thread to further comments and continue it in the more appropriate thread, Autopilot Servo Installation with Dual Sticks. My servo mount did require further bracing and that will be covered in my latest post in the previously-mentioned thread.

John

N750A

P.S.: Although the pushrod alignment is moot for my installation, I stumbled across a Trio Avionics servo installation manual which said that the pushrod may be up to 8 degrees laterally out of the plane of the servo arm and that the elevator pushrod hole location should provide between 1.5 and 2.4 inches of travel.