Hydraulic Main Gear Cylinder Setup

10-12-2009, 01:47 PM

Planemakers | Hydraulic Main Gear Cylinder Setup

Hi Guys,

Here is a little article about the set-up, care and feeding of your hydraulic main gear system.

First, for ease of explanation and typing, I am going to call the curved aluminum bar that is bolted into the fiberglass flanges attached to the underside of the mid-deck that the main gear hydraulic cylinders are attached to the “Dog Bone”

This is how I install the hydraulic main gear cylinders for the first time:

First: Install the rod end bearings into the hydraulic cylinders with more than enough threads up inside the stainless steel bar. Put an AN970-3 fender washer with the 3/16” hole drilled out to ¼” under the jam-nut. You’ll see why in a few more steps.

Second: Stick the hydraulic rams through the holes in the gear pockets from the inside and pin the rod ends to the back side of the gear swing arms.

Third: Pull the hydraulic cylinders inward until they are fully extended.

Fourth: With the dog bone in place, raise the hydraulic cylinder up to meet the dog bone. If you need to trim the dog bone back so the cylinder “ears” straddle the dog bone while fully extended then pull down the dog bone and trim it to fit. Every single one of these that I have done have required trimming.

Fifth: When the hydraulic cylinder straddles the dog bone while still being fully extended, mark the dog bone and drill the hole for the bolt that will hold the hydraulic cylinder in place.

Sixth: When you put the hydraulic cylinders in for the last time, set them up so they are the correct length then turn out the rod end one or one and one half turns. This will allow the hydraulics to push the gear leg into the thrust angle tightly but only just a little. Once it moves that extra thread it will bottom out on itself and not keep trying to push the dog bone up through the mid deck. Or if things are not perfectly aligned, it will try to twist the dog bone off the mid-deck.

Seventh: Support the hull and raise the gear, with the hydraulic cylinders in place, up until they hit the underside of the upper wing skin and then tape a spacer about 3/16” thick in that area. If the gear leg bottoms out in the gear pocket before the wheel hits the wing skin then put the 3/16” spacer in the gear pocket. Raise the gear up again until the tires (or gear legs) hit the spacers and find a way to hold them there.

Eighth: Go inside the aircraft and measure the distance from the back side of the fender washer in the first step and the face of the hydraulic cylinder. It will be about an inch.

Ninth: Take some 1 inch diameter nylon rod and cut it off to that dimension. Drill a one half inch diameter hole down the center. Take off the fender washer, put the nylon spacer sleeve over the rod and replace the washer, jam-nut and rod end.

Now when the gear goes all the way up, the hydraulic cylinder will bottom out on the nylon spacer before the wheel hits the wing skin. This will keep all the hydraulic stress inside the cylinder and the dog bone will only need to hold up the weight of the wheels and not fight against the 1000+ PSI of hydraulic fluid trying to pull the wheel through the wing skin.

Tenth: Make sure your gas struts are in good shape. When you are flying in turbulence, the gear will try to bounce around. If the gas strut is weak or dead all the bouncing will get transferred through the hydraulic cylinder to the dog bone.

If you have any questions about this feel free to ask.


John J

10-13-2009, 09:06 AM

Fred Lohr

that is great john, thanks. just in time for me to redo that area since my dogbone pulled loose. It should probably be part of routine maintenance to change those gas shocks every 2 years or so. I'll bet that is how my problem developed cause those shocks came with my kit in 1997 and have been in use since 2004. Weak shocks leads to constant pull on the dogbone with the gear up. I have some replacements that i had purchased but not yet installed. do you have a source for these and the part numbers to order for maingear, nose gear, water rudder and canopy?

10-19-2009, 08:43 AM

Planemakers | Gas Strut Numbers

Hi Guys,

Fred asked me for the part numbers and supplier for the gas struts so here they are:

Landing Gear: FEN 11K - H9510 110 lbs

Water Rudder: FEN 343 - H9518-25 25 lbs

Old Style Canopy: FBN 22G - H9516 36.3 inches long 90 lbs

New Style Canopy: FAN 22J - H9515-100 26.3 inches 100 lbs

Supplier: Orr & Orr



I hope this helps,

John J

08-07-2016, 08:24 PM

tthunne | Gas Springs

When ordering replacement springs take the part numbers off the springs themselves. As well as there being 2 different canopy springs the same is true for the landing gear.