The E-flight Airliner Retract Project By Don Giandomenico - October 8, 2007
I have owned a E-Flight Super Airliner for a little while now and have enjoyed the heck out of it. I really like the scale appearance of the jet along with the form and function of the twin ducted fans. However, for as long as I have owned the plane I have wanted it to have retractable landing gear. I can still recall people saying what I was thinking “Man that’s a really neat jet, wish they came with retracts”. Well the time had come for me to try to engineer a way to add mechanical retracts to the airliner without adding a bunch of weight.
I started out my project by searching for a suitable set of mechanical retracts for the airliner. This would be a challenge as there are not too many retract units set up for the 3 pound 10 ounce foam airliner. I needed to find a plastic bodied retract mechanism that could handle up to 4 pounds but would not weigh more than a few ounces. This would be key to the success of my project as a heavy plane would be no fun to fly.
After searching the web a bit I found just what I needed. Robart Mfg. makes a neat little set of mechanical retracts that fit the bill exactly. Their 600 & 602 mechanical retract units support planes weighing up to 5 pounds and only weigh a few of ounces combined. The 600 mains are a 90 deg. retract as well as the 602 steerable nose wheel unit. Both of these units feature glass-filled nylon construction and 1/8” wire struts. Unfortunately they are a little pricey at $55+ for the set but that is OK if they do the job.
To drive the retracts I would need a set of servos. Being a long time fan of the quality and value of Hitec Servos I knew right were to go. I chose to use a Hitec HS-225MG Mighty Mini servo for the mains and a HS-81MG Micro servo for the nose wheel. The HS-225MG is a metal gear servo that produces 54.2 oz-in of torque at 4.8V which is plenty of power to lift up the mains. The HS-81MG is a little smaller producing 36.1 oz-in of torque at 4.8V. Using metal gear servos will help prevent the gears from stripping if the retracts should mechanically bind or be jarred.
Now that I had my equipment lined up I was ready to start the build. I chose to document the airliners weight and center of gravity to use as a reference later. I installed the dual Electrifly 11.1V 2100 mAh packs into the nose of the airliner to correctly weight the plane as well measure the CG.
I set the airliner on my Great Planes CG Machine to measure where the CG was. The E-Flight manual shows the CG at 140 mm from the leading edge of the wing at the root (right at the wing saddle joint) and my airliner was real close at 138 mm.
The next step would be to weigh the plane. I used my digital cooking scale to weigh the plane and she came in at 3 pounds 9.75 ounces (57.75oz) with battery installed. It is worth mentioning that this model is configured with the suggested E-Flight brushless motors, speed controllers, servo size and battery size. Nothing at this point had been modified to the design of of this plane.
To get an idea of how much weight I would be adding to the plane I weighed the retracts and servos. The retracts with wire struts came out to 3-1/8 ounces and the servos came out to 1-3/4 ounces for a total of 4-7/8 ounces unmodified.
Part of what makes the airliner a very cool model is the scale-like gear it has. I mean how cool are “triple trucks” on the mains as it comes in for a landing. The spring supported struts are also a cool touch to the aluminum landing gear. Unfortunately getting all those wheels up into the belly of the aircraft would be impractical. This is where I would have to make some compromises but before all that can be worked on I would need to figure out a way to mount the retract units to the wing.
I started out by stripping the stock gear from the wing mounting blocks. The stock fixed gear wire would no longer be needed as well as the nylon straps holding them in. Notice the towel under the wing to prevent scarring of the delicate foam surface.
The stock landing gear position is pretty far back from the CG possibly to keep the airliner from rotating too soon on takeoff. I decided to move the gear a bit forward to allow clearance of the main wheels from the wing mounting screws on the belly. I moved the gear center line about 15 mm forward from where the old fixed gear wires supported the gear struts. (Existing mains were at 90 mm from trailing edge of wing, new gear position is at 105 mm.)
I temporarily placed the main gear retract over the support block that is glued into the foam wing and thought of a way to mount them. The wing is pretty thin at this location so countersinking the retracts in the wing would be a bit more difficult than I like. I am a big fan of the K.I.S.S. method (Keep It Simple Stupid) so I decided to choose a less invasive procedure on the wing blocks.
To keep the wings structural integrity as well as the wing blocks I decided to use additional plywood block mounts that will be mounted into slots morticed into the existing wing blocks. These plywood pieces will be mounted on end and allow the retract units to be screwed into them for support. I marked up one of the wing blocks with my proposed plywood locations. Using a Dremel tool and a cut-off wheel I cut some slots to the same depth as the existing slots in the block.
Using a screw driver I snapped out the pieces between the slots.
Using my Dremel tool with a high speed cutter tip I cleaned up the slots and fine tuned them to 34 mm X 6 mm. Keep in mind that the depth of the slots are no deeper than the existing ones. Notice that the forward slot lower edge is the same edge as the existing gear wire slot. This makes it easy to lay out where the front gear plywood block is to be located.
To fabricate the retract mounting blocks I used a high grade 1/4 birch plywood. I measured out four blocks that were 34 mm wide and 22 mm tall.
I cut out the blocks and sanded them to the exact size in preparation for installation. A fit test was in order and vertical height was evened to about 18 mm above the wing block.
I used a 30 minute epoxy to glue in the blocks making sure they stayed plumb and square while curing. I used a very small amount to epoxy the blocks as to not add weight to the wing. The main advantage of using this morticed joint design is that is is very strong and requires very little epoxy.
Because the retract units will be surface mounted it leaves a less than desirable square look to the mount. To clean up the appearance of the gear I decided to make a set of balsa blocks that will glue onto the front and back of the gear mounting blocks. Not only will these blocks clean up the look it will also help with the aerodynamics of the mount.
I used a small amount of epoxy to glue the balsa blocks to the front and rear of the gear mount. The exact dimensions of these blocks is not important to the operation of the gear.
