This is a story about a landing gear modification to the Murphy Rebel. It was conceived to allow more aggressive, and ultimately safer backcountry exploration. In my opinion, this mod takes the Rebel from a very capable plane which is near the upper end of its structural ability in off airport excursions, to that same aircraft now having the strength and design to take much more punishment and handle the events that will occur in that environment!
Right up front I'd like to say that adding a set of Alaskan Bushwheels is probably the best bang for the buck as far as upgrading off-airport capability. Even with this gear mod, those tires absorb probably 85-90% of landing loads. I've seen this in videos shot from behind and through the gear looking forward. Even in hard landings or on rocks, the suspension rarely moves and those tires are soaking it all up. However, those large diameter tires do impart a force on the suspension and its associated mounting areas. This mod is about strengthening those areas and suspension components while attempting to find a balance between performance gained and the addition of weight to the aircraft.
When I decided to learn to fly in 2007, I also knew that I wanted to build the airplane that I would be flying. Living in Alaska provides incredible opportunities for both scenic and purpose-driven trips. Based in Wasilla, Alaska, in ten minutes I could be in places that would be a state park in other states; here it was just another place! This, coupled with many friends with various taildraggers, convinced me that some sort of off-airport aircraft would be perfect.
Enter the Murphy Rebel. The Rebel is an offering from Murphy Air and is advertised as an "off airport" aircraft. Built as designed, it has a maximum gross weight of 1,650 lbs. Builders commonly build the plane with an 0-320 and come in around 950 lbs, leaving a somewhat typical (for this size aircraft) useful load of 700 lbs.
Since the purpose of this article is to describe a complete upgrade of the landing gear, it's important to understand the original gear components.
The Rebel was originally designed as a bungee gear aircraft and later a Cessna-type spring gear option became available. Luckily, in 2007 I found a great deal on a 1992 bungee gear kit that had never been started! It took almost 4 years to build.
All kit components are aluminum with the original bungee design main gear legs comprised of 1.25" x .125" wall square tubing. Other members of the gear are various sizes of aluminum tubing with the axles (also aluminum) sandwiched between plates. These plates connect the axles to the suspension.
Based on my experience, I would guess that what Murphy really means when they say "off airport" is a fairly smooth grass runway and that's just fine for many folks. However, that's not what I wanted to use my plane for!
Fast-forward to 2012. I'd been flying my Rebel for almost a year and had also been on skis in the winter. In June, a sudden engine stoppage (another story) caused me to decide to land on a paved road. I had a fairly high sink rate and when I landed, it pretty much wiped the gear right off the bottom of the plane, along with some other associated damage.
Faced with a very involved rebuild that included opening up the fuselage to replace gear components, I had the idea to upgrade/replace it.
Since the venerable Super Cub has been tried and tested here in the off-airport environment, I decided to investigate adding Super Cub gear components to my Rebel. I'd already made some good friends at Airframes Alaska, home of all things Super Cub. These guys build it all from individual components to complete aircraft, as well as manufacture the amazing Alaskan Bushwheels. Although they already made most of the landing gear parts and pieces I would need, I was going to need some help coming up with Rebel specific parts that would not only attach and support their gear, but also work with Murphy's original design. That design incorporates the gear attach point as well as the wing strut attach point in the same fitting.
I didn't want to change the wing incidence which was "set" by this fitting. I worked with their engineer to design this fitting and then picked the brain of several A&Ps including another well known local Cessna guy. I did this because I knew that changes were going to be necessary in outer skin and floor thickness to accommodate the new loading.
I readily admit that while parts of my gear upgrade were engineered, others were simply "best guess" by persons I considered ultimately qualified to make that guess. The Rebel fuselage is very similar to the Cessna with bulkheads and exterior skins carrying loads. My "best guessers" made their living repairing this style aircraft. In my case, I tend to overbuild, if anything and this probably made my plane heavier than it had to be. Remember, we're experimental here! Read on to see what was involved in creating a much more robust and dependable landing gear system!
Note: This modification, although not incredibly complicated, is quite involved and requires strict adherence to proper building methods and safety. Use careful measurements of the structure and/or existing components that will remain in place during the build to ensure that you build your bird back to its original configuration. This modification is not endorsed by Murphy Aircraft and is simply a documentation of my own experiences. Should you choose to modify your aircraft, proceed carefully and at your own risk!
First off, I have to say that this project is not for the faint of heart— especially if you are doing it after your plane has been finished, whether you're in repair mode or just have decided to make a gear upgrade. Best case scenario is doing it during the initial build.
Rebel aircraft construction builds forward and aft off of a square-shaped structure called the carry-through. It's made of three pieces of 2" x 1" aluminum square tube that are sandwiched between 3/16" thick plates at the bottom corners and then riveted with 3/16" rivets. These plates form the wing attach fittings at the top corners and the gear and strut attach fittings at the bottom. Across the bottom to complete the square are two 2" x 2" square tubes that are riveted on both sides of the vertical carry-throughs.
Replacing the fittings
To start this gear mod the builder must have this carry-through exposed, at least at the lower half. Before the project is over, fuselage panels forward, under, beside, and behind the carry-through will be removed either for access or replacement or both. The goal here is to replace the existing gear and strut fittings with new ones that will fit the cub gear and keep the same wing incidence. The new fittings, forward and aft, are made of 4130 steel and are the same thickness as the originals, 3/16". Note: In this upgrade there are several places where I'm adding steel pieces to the aluminum ones. Anywhere you install steel against aluminum it's a good idea to paint or use some epoxy chromate on their surfaces to avoid problems in the future.
You'll need a work table similar to the one used during original construction which is 16' x 4'. Shorter, if you remove the tail cone as I did. You'll use this table or some modification of it throughout the build. I say modification because at some point you'll need to have some of the gear components in place that extend below the top of the table. These pieces are spaced narrower than the table as well. I'm referring to the rear gear attach fittings and the front ones as well as the cabane vee which I'll get to later. I opted to just build an extension, not as wide as the table, onto the front end. This let me slide the fuselage forward to work on the front section. For the aft fittings, I cut holes in the table so that they could protrude through as needed.
When you look at the photos of the exposed installed components, you'll notice a 1/2" aluminum spacer between the aft bottom fittings and the carry-through. The cub gear is 1 1/2" inches wide at the attach point whereas the Rebel's gear is 1". Thus the need for the spacer. You can also see a 1/2" spacer that's welded on the inside of the upper ear of the aft lower fitting. This allows the wing strut fitting (which is 1" wide just like the original gear) to fit exactly as it did with the original gear components. Another thing you'll notice is the new set of ears on the inside of the fittings. This is to accommodate the cub cabane vee that the Rebel does not have. More on that later.
The new elements here are the redesigned steel fittings, the 1/2" aluminum spacer under both legs of the aft lower fittings, Delrin spacers along the bottom of the carry-through to help make a solid unit, and crush blocks inside the carry-through in the areas where the fittings attached. Because of the increased thickness at the fittings we chose to through-bolt from fitting to fitting. This necessitated having the crush blocks inside the tube. I used hardwood blocks; you could also use delrin if you wanted to but some type of crush block must be installed.
Note: The Rebel kit comes with two 2"x2" square tubes that are as wide as the carry-through assembly. These tubes are installed along the bottom of the carry-through. If building the spring gear version, the builder will place one aft and one forward of the vertical carry-through components. This completes the square. For bungee builders, Murphy allows one of the tubes to be split and a half attached to each side. This saves a little bit of weight. This is what you see in my photos.
Thickness upgrade
Another part of the gear upgrade is increasing the thickness of most of the components in the floor and side area. The original bottom skin is .020; you must increase it to .040. Likewise, the stock floor skin is also .020. It gets bumped up to .032. This is all part of the "box" that was created to hold the new gear. Inside the Rebel floor are many "C" channels sandwiched between the floor and outer skin that give it strength. These are made of .020 aluminum and they were all upgraded to .025. The exterior side panels that go from the bottom of the plane to the door sill were also upgraded. Each one is twice its original thickness, which was .020 in most cases. The inner fuselage panels at these areas are upgraded from .020 to .025. An exception is the channel that forms the door sill. It's hard to remember but I think it was originally .025. I made mine .032. Also, most builders choose to install several vertical .020 channels on the inside of the side pieces of the plane to add rigidity. Just like the floor channels, these were increased to .025. In my case, I'd already increased the thickness of many of these panels during the initial build anticipating future abuse in that area. Many were still usable.
Now we'll move rearward to the aft gear attach area. The original Rebel gear uses a one inch square aluminum tube 4" long to hold the gear attach fittings. There is also a fuselage bulkhead positioned on either side sandwiched between the fittings and the 1" tube. The tube is riveted to the .020 bottom skin and the aluminum fittings were riveted to it on both sides.
This creates a condition where each aft gear attach point is on its own as far as strength goes. This just would not provide the strength we needed in this area. I envisioned a "box" which contained all the gear attach points and supporting structure with all the components connected together. My solution was a 1 1/2" piece of 4130 steel tubing placed all the way across the floor where the new gear would attach. 1 1/2" because that's the width of the cub aft gear at its attach point. I bolted 4130 gear attach fittings on either side of this piece with the existing bulkheads again sandwiched under the fittings.
The location of this steel tube is determined once the front fittings are installed (at least temporarily) by bolting the gear on at the front and then setting up the location for the aft fittings. It's very helpful at this point to have the aircraft on a table and to have the new bottom skins temporarily cleco'd in place. Ultimately, the 1 1/2" tube gets riveted to the bottom and inner floor skin. Because the actual floor is 2", I used a 1/2" Delrin spacer between the tube and the floor skin. As with the original construction, it makes sense to have all pieces cut and fit before any final riveting!
At this point the gear fittings, their components, and the rear tube are all temporarily in place and you've probably have made, fitted, and temporarily installed all channels inside the floor area. Be sure and get those beefier channels that support the seat rails back exactly where they go!
Interior and exterior panels
Now it is time to start making and fitting those interior and exterior fuselage panels. Many of the pieces being replaced are simply flat shapes that, if removed carefully, can be used as a pattern. You may have to reproduce oil canning creases on several of them and this is easily accomplished using a brake. With pieces that have bends in them, I've had great results by pounding them flat and using them as a pattern. Make the new flat piece and then measure for the bend, including the radius. A trip to the brake and you're good to go! After all pieces are in place and you're happy, it's go time with the riveter! When you have finished riveting all of these pieces together, you'll have created a super-strong "box" that will not only mount your new gear correctly and safely, but will be strong enough to take much more punishment than the original design ever could!
One additional component that will have to be fabricated and installed is the cabane vee. This is an integral part of the super cub landing gear as it provides the upper connecting point for whatever type of shock absorber you decide to use. As mentioned before and visible in the photo at the beginning of this article, I built my Rebel with the bungee suspension. With the bungee Rebel, the suspension components form a sort of "cross" from the axles to the upper mounting points that are bolted through the bottom skin onto the lower carry-through. In the center of this "cross" is a point called a "dog bone" at which they all meet and on the pieces that go to each axle, a bungee/slide component exists on either side. That is, one aluminum tube slides inside another, captured by the bungees. There's no provision for rebound.
The cabane vee for the Rebel is wider than the standard cub one because the Rebel is 44" wide. It bolts to the extra set of ears on those lower fittings and spans the width of the fuselage. It is a one-off piece that can only be built once the front fittings are in place due to the possibility of small differences in final dimensions between the fittings.
Along with the vee comes the task of deciding which suspension to use. Luckily there are several great choices out there; several that weren't available at the time of my build. The standard cub suspension uses a "hydrosorb" which is a piece that combines hydraulic movement (like a shock absorber) with bungees. It has some rebound capability although not much. I believe rebound capability is important. Another option and the one I used is Burl's Alpha-Omega Suspension System (AOSS). It uses polymer discs inside a metal tube to provide dampening as well as rebound. I love it and it has worked well. It is, however, quite expensive and there are less expensive choices available today. Notably, there are several companies making simple systems using hydraulic shocks with springs and they are lighter and cheaper than other systems. I'd look at them if I ever did another one of these for myself.
Whatever suspension setup you choose, you'll need to have a custom length shock strut made. This is the piece that goes from the shock-absorbing unit to the axle. It is the final component.
While I'm on the subject of the cabane vee, I'd like to give a shout out to my friend Paul Gray who was instrumental in the planning and building of the components for this mod. Paul was working with Airframes Alaska back then and he bent over backwards to help make it all happen. He's a first class fabricator/welder who now has his own shop, SKUNKWORX.
To wrap this all up, let's talk about performance, cost, and weight.
In the area of performance, obviously it is hard to quantify changes or improvements. The goal was to build a structure that would mount and use supercub landing gear components safely and reliably. I believe I've achieved this. The 31" bushwheels along with their wheel weigh 41 lbs each and that's quite a moment arm to have out there bouncing around! I'm sure I've had several hundred landings since the build and many were not pretty! I've not found any evidence of deformation or degradation in any area. I'd also like to mention that the aircraft's stance is widened in comparison to the stock gear. I believe this wider stance adds important stability while landing and talking off. Additionally, the cub gear legs are longer which increases prop clearance and angle of attack when taking off.
As for the cost of this modification, obviously it's been several years since I completed mine and many if not all components/materials have increased in price. One of the good things concerning this is that you don't have to buy new components. You could buy used ones if you wanted to save money. Or build your own! Any Super Cub gear leg will work! I spent approximately $2,200 on new gear legs, cabane vee, the new fittings/hardware, and aluminum. Powder coating and various gear leg extras (like the steps seen in the photos) added to my outlay but I'm giving you the basic price. Truthfully, I had much of the aluminum sheet I needed but I did have to buy some. Then you have the cost of whatever suspension system you go with and wheels and tires if you don't already have them. I'm thinking around $7,000. if you needed new everything.
Now the big one, the one everyone asks about: Weight. My plane's original weight was 1,050 lbs. As I mentioned before, many guys are building the Rebel with the 0-320 and coming in around 950 lbs. I was already heavier than a lot of guys, having the 0-360, 31" tires, metal prop, and substantial beefing up in the original build. My completed weight after the gear mod was 1,103 lbs. I estimate I lost about 35 lbs in weight from the old gear and associated parts/pieces. I didn't weigh it. I just tossed it! I used heavy duty 3" gear. A guy could save some weight by using different gear than I did and also a different suspension system (AOSS weighs 14 lbs total).
Conclusion
In closing I'd like to say that I've been pleased with my new landing gear in every way . The aircraft just feels different in the ruts and bumps than before. They're just non-events. With this modification, there's a confidence that is invaluable when you're out somewhere that would requite a long walk home! If I ever built another Rebel, this mod would be at the top of my list. In fact, interest has been high from many Rebel builders and I have marketed three versions of this mod, ranging from complete kits to certain specific components.
The Murphy Rebel is a great aircraft and is also undermarketed, in my opinion. My landing gear mod makes it a little better. I like to say it's a set of landing gear that happens to have a plane on it!
This is a living article in the Knowledge Base. If you have feedback on the accuracy or legitimacy of this entry, or would like to add more information, join the discussion below or email knowledge@backcountrypilot.org to volunteer your input. Suggestions and changes will be incorporated readily.
Inspirational job with excellent craftsmanship Craig! Thank you for the awesome article. I misplaced your email. I'll be up your way this summer. Let's get together! First beer is on me!
Very interesting article, well written. I am interested in a Rebel myself. I owned and flew a Stinson for the last 25 years but now am looking at going experimental. I will be up to Wasilla area this spring with our shooting club. We shoot at Birchwood right next to the airport. I would love to meet with you and look at your plane when I am up there. Send me an email to [email protected] if we can get together.
Shane
Hi Craig,
I am new to this forum and lucky I found your article. I see you have made kits for people but I live in Australia so it would not be feasible or cost effective to send over. Could you send me a list of what I would need to fix a spring version or change to the cub version. I have no preference either way but have damaged my spring Rebel spreading the legs and buckling quite a few sheets of aluminium. I wait your reply email thanks in advance.
Phil Sims [email protected]
4 Tramontana St,
Andergrove,
Mackay.
Queensland
Australia.
4740.
While this knowledge base is a compilation of information from various sources, some official in nature, it is not a recognized or acredited source of aviation training information, and thus should be considered entertainment. Please consult a FAA-certificated flight instructor or mechanic prior to putting any information found here into practice.
The more commonly seen spring gear option for the Rebel
This is the original bungee gear on the Rebel.
Gear completely wiped off the plane as well as lots of other associated damage
The new, much improved Cub gear. What a great look as well!
The table I used during the rebuild – it's the same design Rebel builders use during construction. It would be an understatement to say that it's disturbing to see your pride and joy dissected on the operating table like this.
New front fittings temporarily bolted in place. You can see the ½” aluminum spacer and the Delrin crush block along the bottom.
Redesigned fittings, front and back, cut from 3/16" 4130 chromoly with 1/2" spacer bushings.
The 1/2” spacer welded on the aft fitting that allows the wing strut to fit in its original place
Locating the steel tube for the rear gear fittings. You can see the narrow extension I built at the front to accommodate the installed gear leg.
Floor section with upgraded channels and seat belt attach channels in place.
The stock rear gear attach.
Rear fittings in their proper place without the bulkheads attached.
The completed rear gear tube assembly almost ready to install. Because we'd originally planned on a 2" tube, there's an extra set of holes in these fittings that weren't needed.
You can see the new, thicker fuselage pieces in place.
The cabane vee as made by Paul Gray of SKUNKWORX.
Powder-coated and installed gear leg with Burl's AOSS suspension system, using a custom-length shock strut for my Rebel gear leg geometry.
WWhunter 
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