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J-4 project-3.14159265359


Registered User
Down low in the hills of Vermont USA
I am tired of reading, time to start writing here.
I am pulling a J-4a out of storage, I have had this plane since the early '80s. I started work on it but then a much easier Aeronca project came along followed by life evolving.
Way back then I collected this plane after a wind storm flipped it onto it's back. The plane had gone into non use as a trainer in the late '70s due to the instructors not liking the feel of it. If anyone had actually inspected the aft 4' of the fuselage they would not have questioned why this plane did not respond well. One would think it would need more than one tube connecting the tail post. The plane then sat till a tie down failed during a storm. The plane tried to solo but did not do to well.

Me, I have been working on aircraft since the early 70's, A&P and Pilots license since early 80's but have split my decades between aircraft restoration and building road race and rally cars.
I am a self employed fabricator, machinist and welder.
I live by the motto, If it ain't broke, Modify it!
Hence why this project will be in the experimental section of the forum.

Since the J4 never received much of any development post war I want the freedom to make changes similar to other Cubs have received over the decades. In many ways I am not looking at anything radical, just allot of little changes.
A few thoughts of where my mind is at.
Power, probably an O-200 with consideration of an Aeromatic prop. Engine will not be stock.

Fuselage, some structural changes such as I have no interest in ½ .028 upper tubes going to the firewall.
The birdcage has yet to be rebuilt so all the tubes will be replaced with changes such as extending the baggage area aft.
A few other dimensional detail changes will be done as well.

The landing gear I have is visually good but the oleos are frozen. I might as well build theses all new and doing so will be looking into some dimensional changes as well as better dampening in the hydraulics.

Wings, part of me wants clipped wings, part of me wants flaps and altered tips. My interest is more in cruise efficiency with the ability to get in moderately tight places, especially if the engine is out.
There may well be some composite sections in the wings as well, TBD.

Empennage, strap hinges and ball bearings will be used. Airfoil shape in vertical and Stab. Balanced and larger elevators, not determined about size of rudder yet.

Controls will utilize ball bearings as well as engineered plastics rather than steel on steel bushings. Essentially all stock parts with lower friction mountings.

Questions to follow,
Regards, Charlie
Welcome, where in VT

I have about a thousand hrs in 4 different J4As, all with C85 strokers, 100hp. About 800 of those hours on floats. Only things I would change would be remove the old baggage bag and metalize it to the floor with a hump for the elevator cables (easy ) The only other thing would be an adjustable stab instead of trim tab on elevator. It's an amazing ride the way it came from Piper, higher AOA then any other Piper, front and rear tank = 25gls of gas ( 5hrs ) Baggage is rated for 105lbs with rear tank full, cruiser ailerons, and lands slower then a J3 or other non flap cubs. I like the oleo gear but it's weaker then J3 type gear, I cracked an axel on skis. If I didn't leave it on floats I would build a J3 type gear for it. Best mod you can do is keep it light

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I think clipped wings will hurt the goal of getting in and out of tight places and cruise efficiency. If that is your goal I would go to a square wingtip and consider extending the wing.
Thank you Glenn, I would love to sit a learn a bunch from you, especially before I get to deep in decisions. I have learned allot reading what you write.
I am in East Wallingford, local to KRUT. I am prone to get up into the Daks at times, was at SLK for lunch last Sat.
The baggage floor is to be flat, balsa-composite construction. Pushpull is planned to be under the floor with the bellcrank aft of the floor area. Some areas will be heavier than original, others will be lighter. I hope the light parts dominate :smile:
My friend Dave has one sitting at Shelburne if you need to look at one or island Bobs NK17. Bob also has a J4 project for sale, and one for sale on floats in central NY.
As to push/pull rods, all the cables in a J4 are hidden. Everything on a 4 were faired, even the stab, with all the fairings it was a 95mph plane with 65hp

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I think clipped wings will hurt the goal of getting in and out of tight places and cruise efficiency. If that is your goal I would go to a square wingtip and consider extending the wing.
This is what I want for opinions, part of my thought would be a bit shorter for drag reduction without to being to short increasing wing loading. Obviously flaps increase weight.
Glenn, I intend to make the Stab trimable. It will be interesting to determine how to allow the movement while keeping the fairing.
....Hence why this project will be in the experimental section of the forum....
This part of your project raises questions. Since you are an A&P you should know about the Experimental certification regulations. IF not you should read up on them since that which you are proposing will generate a non flying pile of parts if you do not follow the regulations. Do it correctly please so as to not butcher an old classic.
To this point my experience has been keeping certificated aircraft legal while the homebuilts I have done were plans built planes from scratch. I actually do have a muddy spot as far as building a legal experimental from a certificated plane. I have many reasons for wanting this project to migrate but it is possible that it should not. There are a number of details in this project that were easy to get approved a few decades ago but seem very difficult to do, document wise in more recent times.
Charlie, we might know each other by sight. I'm part of the crew out of KLEB and live in Barnard. No experience with the J4 but lots of tube and fabric time. If I can help out let me know.
I would be more than happy to meet up over there, I pass through that area monthly when headed over to the Cannan Track.
I will also be out at Oshkosh for the week as well.

The FAA is really hard core regarding taking a certified aircraft into the experimental category. They are adamantly against it. Even after you weld up half a new fuselage, clip the wings and everything else, you may get stuck in the experimental exhibition category which is VERY restrictive and will also kill any resale value. You might call Charlie Becker at the EAA and talk to him, or if you can get the right guy in your local FSDO, talk to them. Make sure they are going to approve your plan before you put all that time and effort into it.
Just my 2 cents

Hope it helps

Good information, I am starting to work on my approach to the legal aspects of this build and how to achieve my goals, as well as if I can achieve my goal.
Part of me wonders if I should just build a fully new fuselage from raw materials and that way I am sure there is no hidden rust in the structure.
As for the wings, one goal may be to have composite LE and top skin, since my wings are wood spar, wood ribs I tend to feel I am best starting with fresh spar blanks since the top skin will not need any original ribs under it.
So I am listening, I agree I need to get my ducks in a row before doing any non reversible work. At this time the full plane is drawn in 3D, these drawings started over 20 years ago. I am running out of revisions so it is time to make final decision about the build.
Recently I have printed out full size prints of the tail feathers and have fabricated the control horns and ready to build out the rudder and elevators.
Should I decide to build a fuselage from scratch I might well get that built faster than sectioning the original one. I do not have all the tubing in stock but one truckload would have the material here soon enough.

Thank you for the critique, it is needed.
I wish the guys in the FAA that I have worked with in the past had not retired. I new a few that are real straight shooters and new me and where my mind is at.
From page 12 of the above AC 20-27G.

e. Converting a Type-Certificated Aircraft to an Amateur-Built Aircraft. Thepractice of performing alterations, repairs, and rebuilding of previously type-certificatedaircraft for the purpose of obtaining an experimental amateur-built airworthinesscertificate is not authorized under § 21.191(g). Such maintenance actions properly fallunder 14 CFR part 43, Maintenance, Preventive Maintenance, Rebuilding, andAlteration. You will not receive credit for these actions toward fabrication or assembly.We will not accept applications for airworthiness inspections on such aircraft.

(1) This policy has been in effect since 1952 under section 1.74-3 of theCivil Aeronautical Manual 1 (CAM 1), which specifically states that “structuralcomponents of other aircraft may be used [for amateur-built aircraft]; however, it is notintended that this provision be used to avoid obtaining approval of major alterations toaircraft previously certificated in another category….”

(2) You should use the normal supplemental type certificate process formodifications to these aircraft and they should be kept under their existing maintenanceprograms to ensure continued airworthiness.
So, how have others built up from the bones of a production plane to experimental in the past, or have they been stuck into Exhibition?
I think I am seeing the path I need to go, Build a replica. Since there are clearly questions in any near 80 year old steel tube structure I am best off in not using any major part of the exiting plane but to build all the major parts from scratch.
This will leave my existing plane as a restorable structure and allow me to build a homebuilt that will comply with regulations.
Back to researching my path.
I haven't attempted what you propose myself as I've also had some "good guys" at the FAA to work with. All of them and I are retired now, which as you have stated makes things a bit more difficult.

This is a list of the Experimental Category sub titles: http://www.faa.gov/aircraft/air_cert/airworthiness_certification/sp_awcert/experiment/ They each have their own set of operating limitations. Some more restricted than others. Some require the FAA to reissue the certificate upon expiration. The FAA may or may not issue the certificate for a full year. There is also "Restricted Category" which also has sub categories and special limitations.

If I were in your shoes, I would give very serious thought about acquiring field approvals for your ideas. Run them by your PMI. Talk over the project with him, getting all of the ducks lined up in a row before you cut into the airplane. Understand just what structural substantiation would be required. What flight testing may be required due to aerodynamic changes. Some alterations may require "one only" STCs. There are regulations which cover all of these items. It's sort of like eating an Elephant, one bite at a time. If this becomes impossible from your perspective only then would I use the existing airframe for a three dimensional "drawing" from which to copy and build a totally new airplane which can be "Experimental-Amateur-built". When you're finished, sell the J-4 with it's title to someone who wants to bring it back to life. These birds are rare, as in there weren't very many built.

This is the Field Approval AC: http://www.faa.gov/aircraft/air_cert/airworthiness_certification/sp_awcert/experiment/

Somewhere I've seen a check list of items which the FAA inspector is allowed to approve and those which he must pass to another department. I can't find it quickly. For example, I do know that alterations to a fuel system must go to engineering. I had to do this for a "supplemental" fuel tank for which I was issued a "one only" STC. "Supplemental" and "Auxiliary" mean different things and have different regulations covering their approval. The descriptive word of the alteration can make or break the deal. This is one reason to sit down with the PMI and have a good talk.
This is what I want for opinions, part of my thought would be a bit shorter for drag reduction without to being to short increasing wing loading
For what it's worth, reducing the wingspan can actually increase the drag reducing the cruise speed. The smaller wing area will have to work harder to support the same weight. That will cause the nose to fly higher, increasing fuselage drag slowing the cruise speed.
For what it's worth, reducing the wingspan can actually increase the drag reducing the cruise speed. The smaller wing area will have to work harder to support the same weight. That will cause the nose to fly higher, increasing fuselage drag slowing the cruise speed.
That is something I am trying to decide on, my original thoughts were something like PA20 size wings which would be utilized at the lower gross weight of the J4. My curiosity that needs to be evaluated is how that size wing would react with the 700# reduced load.

Last night I started redrawing the fuselage as being how I would approach it from a clean sheet of paper rather than reworking an 80 YO structure. This is still just detail changes but some little changes make a big impact. Some of what I am leaning towards is better/ more efficient structure from the firewall back to the front wing spar and lift struts.
X bracing on top of birdcage.
Upper structure aft of the rear spar will now be similar to a PA18 allowing a more spacious baggage area.
The aft shape of the cabin doors is being reworked. Both for aesthetic reasons as well as making it easier for the over 60 guys to get in & out.
The use of wing tanks simplifying fuel management rather than the original fore-aft tanks. Fuel system and structure is being designed to support 160HP. I intend to build with an O-200 but design for future possibilities.
Mounting of the elevators and Stabilizer has more efficient load paths as well as utilizing a jack screw for trim.
Tail wheel will be a leaf spring back at the tailpost.

I am also making some changes in the control system.
First being the pushrod back to the elevator bellcrank will be run under the cabin floor, said bellcrank will be moved aft of the baggage area.
Aileron cables from the controls will run up the front spar rather than the numerous pulleys and bellcrank under the seats and up the aft spar.
Going to a true experimental build I might make a new control stick assy. There is weight to be saved and reduced friction available in a fresh design. This allows the original controls to remain with the original airframe.
Brakes will be toe brakes on both sides.
Many of these goals were in my original design but there were more design concessions with utilizing the original structure.

This build would probably be initially done utilizing original wings and then access what I may want to do as a next step. This will also get the plane in the air much faster.
Time will tell, allot of learning needs to be done still, like trying to learn who in the Portland ME. FAA I will be working with.
Couple things learned. The problem getting in and out is not the shape of the door. Island Bob had trouble getting his size 12s in and out of his J4, Bob can't leave anything alone. The problem is the seat and the front legs. We all have now done the island Bob mod to our seat. Remove 3.5" of tubing front to back in the center of the seat where the tubes are straight. Then cut the 2 front legs off and reposition them inboard a little so the they line back up with floor tubing. Easy in and out. Get rid of the seat sling also replacing it with 3/16 ply

Good info, I have been looking at my seat structure with questions in mind, granted my fuselage is not here yet.
My right leg does not work very well so some things need to be checked out as I proceed.
I am intending the floors and other plywood will be replaced with either foam- glass or balsa- glass panels. This would include the seating.
A little bit of where my project is at. Here are two images pulled from my CAD program of a general view of where this project is going.

As you can see this will not be trying to use an original Cub relicensed as an experimental. This is a clean sheet of paper design dimensionally similar but structurally optimized from the pre war J4A
The wings will be composite upper and leading edge, spars are composite. The lower surface aft of the front spar will be fabric. The landing gear will all be spring over hydraulic but utilizing considerably more modern damping to provide no bounce landings.
At this point I am fabricating small parts, essentially making the "kit" so next year I can weld up the fuselage in a few week session. Wing construction will follow, Design work is not finalized and FEA on composites is more intensive than tube structures for me.
More later,


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Just as a side note, while I understand you plan to go Experimental for lots of other reasons, I did get a field approval for a J-3 adjustable stab on a J-4 back in the 80s. I joined a group of guys that had bought a J-4 that had been wrecked and rebuilt a number of times. Discovered the plane had the aft fuselage from a J-3 married onto the J-4 complete with the J-3 tail feathers. Since the plane had been flying that way for decades we asked the FAA to grant a field approval based on known flight data, which they granted without a lot of fuss. (Not today's FAA!) I don't have a copy of the paperwork anymore, but one could get a copy by getting the 337s filed against N24699. I'm guessing this was around 1987 or 88.

-Cub Builder
That is some good useful information. My real J4, I have replaced the aft section of the fuselage due to rust, this was done in the early '80s. My tail feathers are not of use due to damage so if I restore that plane as well as build this one then changing the trim has great merit.
Results of doing my BOM spreadsheet is all the tubing going into the fuselage, Vert fin and stabilizers will weigh 89 Pounds. This does not include all the tabs and brackets.

I am going to install a new FEA program and go through the fuselage a few more times before I lock it down. I like my old trustworthy software but I think it is time to treat myself to something newer. Interested to see if it shows much difference in stress concetrations.
To touch on this thread a bit since I have not posted in quite awhile. The major drawings for this project were essentially completed back in March. Details keep evolving. This is a clean sheet design as a look alike of my original J4A. Dimensionaly this plane is true to the original except in height of the fuselage at the cabin which externally is reduced while the seating area remains similar. At this time this plane is drawn in a few versions, simple and light, the closest to original and would be O-200 powered, this being a Light Sport eligible version.
My desired build is with a more complex wing, generous double slotted flaps option of drooping ailerons and probably slatted outer leading edges. This version may well carry more power on the nose. A rendering of this was posted last fall.
Back in November construction began on small components first with control horns for the tail, Then the jack screw was built recently the control sticks have been built. All of these components are mounted on and pivot on ball bearings. Most every part of the control system is on bearings. Outer tail surface are strap hinges with Delrin bushings. I will soon start building the pedals soon.
The actual version I will build will be dictated in the next month with the outcome of my 3rd class medical which looks very positive. I am a believe it when I see it type so I remain conservative and cautious.
The day I see that I have my 3rd class I will place the material order for the fuselage. Wing design detail drawings will then be finished once I truly know what version will be built. No drawings are finalized till materials get cut.
More later.
I recently ordered material for the fuselage as well as the lift struts and the struts have arrived today. I just received the Bill of laden for the tube order so that should be here next week or so.
The actual construction of the fuselage will begin in July but small sub assemblies such as top of the cabin and firewall will be assembled as soon as my welding bench is cleared of customer work since right now there is a partially welded Elva Courier chassis in progress.


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Thanks for the update Charlie. I'm looking forward to your project updates. I don't have much experience around a J4 except for fixing some rusty tubes for a local guy.
Today my fuselage kit arrived :roll:
Well not exactly a kit and surly not a quick build version. But it's here.


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