Adding Elevator Trim

By Chuck Baynard
June 2009

[Webmaster's note:  One of the first things we learned when we started to fly was how you had to crank on the elevator trim system every time you changed power settings or airspeeds.  I get a lot of shocked looks from folks at Fly-Ins when I point out that my Fly Baby doesn't have a trim system... just a metal fixed tab on the tail.  The Fly Baby is so light on the controls that, once the fixed tab is set properly, the control pressures never get very high.  I can fly touch-and-goes all day never feeling that I'm fighting the stick.

However, as Chuck points out, the trim pressure on a Fly Baby does change during cross-countries (due to the fuel burning off).  This is never more than a slight irritant...but it IS an irritant.  He wanted to be able to adjust for the pressure change, and developed this nifty little trim system to let him do so.

My main point is that a trim system is practically mandatory for most airplanes, but not on Fly Babies.  Don't think you have to add a trim system just because the Cessna or Piper you learned to fly on had one.  Chuck's system is great in that it can be easily added after construction...if you decide you really need one, after all.  -- Ron Wanttaja]

I completed my Flybaby (N7627C) and flew it for the first time in September, 2008.   The project took eight years, and I thoroughly enjoyed the building and learned a great deal along the way.  Flying a plane that you have built yourself is extremely satisfying.  I have about 65 hours on my baby now, and feel right at home.  It’s a real blast to fly, kind of like a flying motorcycle.

With a fixed trim tab on the elevator, my plane flew hands off in cruise, but I found that after about an hour in the air, forward pressure was required on the stick to keep it from climbing due to the reduced fuel load forward of the CG.  Additionally, when throttling back on base, aft pressure was of course required to glide at a slower speed.  Stick pressures are so light on a Flybaby that this is little more than an annoyance, but there is something truly sublime about an airplane that will fly itself in all flight regimes, so I thought I’d look into adding adjustable trim.   

Since I didn’t want to cut into my beautiful fabric job to install an aerodynamic system in the tail, I decided to develop a “spring system” inspired by one in a friend’s Piper Pawnee.  The beauty of this approach is that the system can be entirely installed in the cockpit area, and doesn’t call for any alteration of  the elevator controls or cables.  The system sort of rides along on the existing walking beam located just behind the pilot’s seat at Sta. 5. 

The trim system consists of three main components or assemblies.  The crank box which is mounted on the port fuselage side under the throttle, the Sta. 5 walking beam mount with its cutout for the extended walking beam “ears” located just behind the pilot’s seat, and the two directional pulleys that lead the cable from the walking beam mount to the crank box.  (see fig. 1 for a picture of all components).

The crank assembly ( fig. 2) is simply a 1/8” plywood box with spruce framing sized to hold a 1976 Datsun pickup truck window regulator.   You can find new aftermarket regulators on the internet for about $20 – just saw off the part you don’t need.  I’m sure any number of regulators would work, the Datsun one just looked to be about the right size, and was. ( fig. 3)

There is room next to my left leg to crank the handle, and it is low enough not to interfere with the throttle (Figure 4)

The walking beam mount constructed of ¼” ply has two aluminum brackets riveted to the bottom which bolt it to Sta. 5, and a piece of aluminum  “L” at the front that is lapped over and screwed into the forward vertical face of the Sta. 5 lower horizontal.  As can be seen in fig. 5, the horizontal surface of the mount is cut out to allow extensions or “ears” to protrude through the top of the mount.  The 1/16” trim cable dead ends at the ears, is routed through two pulleys on the mount itself, and then goes to the side of the fuselage.  From there, it goes through two additional pulleys to align it with the crank box.  (see fig. 6).  The walking beam ears are simply two straps of .071 chromoly that are sistered to the walking beam on either side.  They are drilled to accept the clevis pins that capture  the ends of the two elevator cables, and have a large center hole that allows the center welded-in tube of the walking beam to pass through them.  There was enough room inside my original walking beam mount to allow the two new ears to mate against the sides of the walking beam itself without having to alter the mount.  The top of the ears are drilled to accept the AN-3 bolt that captures the end of the trim control cable.  I suppose you could construct a new walking beam with extended ears, but this approach worked fine without having to alter any preexisting work.  As can be seen, there are two pulleys on the mount itself that provide a fair lead to the ears, and then turn the cable about 90 degrees to direct it toward the fuselage side.  I made aluminum mounts and constructed covers for the pulleys by means of the small forming buck in fig. 1 to insure that the cables wouldn’t jump out of the grooves.  The 2” diameter, A-123 phenolic pulleys came from Aircraft Spruce – about $7.50 each. 

The third component, for lack of a better term, consists of the two directional pulleys mounted on the Sta. 5 vertical that route the cable to the crank box.  Since the lower of the two mounts stands off more and is less supported than the other mounts, I made it out of chromoly – the others are aluminum. 

The springs are the final touch – you’ll see that there are two. (fig. 7)  The horizontal (trim) spring creates the tension that raises or lowers the nose when you crank the crank.  (You need a spring in the system as opposed to just a solid cable so that you can override a  trimmed setting to dive or climb from the trimmed position – think about it a minute.)  The vertical spring’s only function is to take up any slack in the cable when the nose is trimmed way down and the stick is pulled all the way back.   This is a safeguard against the cable jumping the pulleys, perhaps not really necessary with the pulley guards in place.  I did not put an opposing spring in the system for nose down trim, although this could be added.  Instead, I have set the existing fixed tab on the elevator to provide the nose down force just necessary for level flight when getting very low on fuel – this proved to be a very slight deflection of the tab, so not much drag.  (Or at least not enough for me to go to the trouble of figuring out how to mount a second trim spring)  I tried a couple of springs before I found the one that was just right to serve as the horizontal trim spring – it came from Home Depot.  The one that proved to be just right has a pull of about 3.6 lbs, the first one I tried at 2.1 lbs was too light.   The vertical spring can be very light, as again it just takes up the slack when there is no pull on the cable.  Given a Flybaby’s light stick forces, the trim spring is only required to exert a moderate pull and is easily overridden even when the aircraft is trimmed full nose up – an important consideration.  

For a Flybaby that has a belly panel like mine, this is a very doable project.  I was able to work from a creeper below, and then from the top by sitting backwards in the cockpit on the floor.  By the way, the Pawnee system utilizes a cable that runs directly from the crank box all the way back to a spring on the elevator horn, but it’s so tight back there in a completed Flybaby that I thought working with the walking beam was the way to go.  The walking beam approach is necessarily more complex, but there’s plenty of space to work in, and you don’t have to cut into anything.    

How does it work?  Like a champ.  As the fuel burns off, just give it a crank forward, and it settles back down to level flight.  Crank it back a turn or two when you reduce power for base, and it comes down hands off at 65 mph likes it’s on rails.  Don’t you love an airplane that will do that?

Disclaimer:  The foregoing is provided only as information that may be of interest to Flybaby owner/builders.  I am just a builder, not an engineer.  If you intend to utilize a system  based on this approach, I would suggest having it inspected by an A&P with Inspection Authorization, or at least an EAA Technical Inspector.  

Chuck Baynard
June 2009

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