Widening a Corben Jr Ace.

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Tench745

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I have asked these questions in one form or another in multiple places. I'm asking again here in hopes that some of the more experienced members can give me some solid direction.

Setting the stage:
For a number of reasons, I bought plans for and started building a Corben Jr Ace.
The Corben Jr Ace is 33 1/8" between tubing centerlines in the cockpit. At that width it is considered very narrow for the 2-seater it's supposed to be. My dad and I, who have pretty narrow hips, just managed to squeeze into a standard width Jr Ace, but closing the door was hard. By contrast, the Pober Jr Ace is 39" between centerlines but, as I understand it, is a different aircraft in many construction aspects. I would be interested in looking over the plans for one and seeing what exactly is different, but that's not the focus of this thread. I'm mostly mentioning it to avoid confusion, and clarify that while I am aware of the design, it's not the airplane I am building.

Widening the Corben Jr has been done before. I am aware of at least one built to 36" wide. The owner of that aircraft suggested adding another couple inches to the width would make it a more comfortable 2-seater. Finding succinct/specific information about the changes that were made when widening the aircraft has been a frustrating, largely fruitless endeavor for me up til now. I know I could just make the cross-tubes any length I want and call it wider, but I don't know how wide I can push that without needing to up wall thickness, or tubing size, modify the gear geometry, etc.

I know changing the width will snowball into a number of additional changes to systems and parts just to make them work with the new geometry.
For that reason I will be building a mock-up to test whether a wider fuselage is even necessary for my wife and I to fit and to experiment with the comfort of different widths.
I have already come to terms with the fact that I might just be building a wide single-seater. Nonetheless, I want to do my due diligence and explore the option of widening before I write it off entirely.

So, here are the questions:
1) Does anyone have experience widening a Jr Ace, or another similar aircraft?
1A) If so, how much was it widened and what structural changes were necessary?​
2) If I wanted a professional to do a structural analysis of the changes, how would I go about finding one, and what might that analysis cost?
3) To reduce the effect widening the fuselage would have on the wing/struts, it would make sense (to me) to widen the top of the cabane structure the same amount as the fuselage. I can better explain this idea if anyone wants to explore it with me.
3A) What would be the best way of accomplishing that?​
3B) Is widening the cabane to preserve geometry worth the effort, or is it easier to adjust the wing struts, aileron cables, and anything else that will change relation to the wider fuselage?​
 

Tench745

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Not very familiar with the structure, but is the cabane above the cockpit one central "keel" tube or is it two "longeron" style tubes?
Neither really. The forward cabane and rear cabane are independent of one another. The cabane V is simply welded to a 1" cross tube which the wing mounting brackets bolt through.Cabane Detail.JPG
 

TFF

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I would widen the fuselage, let the cabane and wing struts change with it. If you raised the wing a small bit the angles of the cabane will end up close to what they were. You will have to chase dimensions, but that’s the beauty of tube fuselages and wood wings, there is lots of wiggle room. There are people who have mismeasured worse than what you are trying.
 

Little Scrapper

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Nobody is going to give you a yes/no answer. What’s known for sure is that builders have a long history of widening the Corben. You won’t find many specific examples of people who lay out a plan of attack on how to to do this using a actual Corben. This is a old school airplane built by old school folks for decades. They don’t hang out on forums and many have passed away from old age. You just need to commit and execute the mission. I have all the sets of plans for reference including the Pober Jr Ace. There’s not much difference, these are just old school tube a fabric fuselages with minor differences. If you want to widen it you can. But for a true useable two seater it’s not the most desirable design. The most useable open cockpit two seaters are tandem designs.
 

Victor Bravo

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IMHO take a look at widening it out to 39 inches (size of the Taylorcraft, a little more comfortable for medium length flights), and putting a traditional proper "upper cabin superstructure" above the fuselage to mount the wing. This will only require two longeron tubes, four upright tubes, and five diagonal tubes (or maybe six if you want to use an X brace over the cockpit instead of a single diagonal). There are guys on this forum who can verify the tube sizes and load paths. Chances are you can simply use the same size tubes as the main fuselage longerons for all the superstructure parts and be pretty close.

Your wing struts and wing span can be exactly the same, and you'll use little less expensive wing spars (because all four spars will each be a foot or 18 inches shorter).
 

Tench745

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Nobody is going to give you a yes/no answer. What’s known for sure is that builders have a long history of widening the Corben. You won’t find many specific examples of people who lay out a plan of attack on how to to do this using a actual Corben. This is a old school airplane built by old school folks for decades. They don’t hang out on forums and many have passed away from old age. You just need to commit and execute the mission. I have all the sets of plans for reference including the Pober Jr Ace. There’s not much difference, these are just old school tube a fabric fuselages with minor differences. If you want to widen it you can. But for a true useable two seater it’s not the most desirable design. The most useable open cockpit two seaters are tandem designs.
I don't expect a yes/no answer. In an ideal world someone would come in with pictures and tubing sizes they used to widen their own airplane; I do not expect this, but I can dream. Once the lockdown is over here in NY and if I choose to pursue a widening I will definitely be soliciting opinions from my tech councilor and all the other old timers I can find.
IMHO take a look at widening it out to 39 inches (size of the Taylorcraft, a little more comfortable for medium length flights), and putting a traditional proper "upper cabin superstructure" above the fuselage to mount the wing. This will only require two longeron tubes, four upright tubes, and five diagonal tubes (or maybe six if you want to use an X brace over the cockpit instead of a single diagonal). There are guys on this forum who can verify the tube sizes and load paths. Chances are you can simply use the same size tubes as the main fuselage longerons for all the superstructure parts and be pretty close.
Your wing struts and wing span can be exactly the same, and you'll use little less expensive wing spars (because all four spars will each be a foot or 18 inches shorter).
I talked with Bill Woods back in 2017 when I was first looking at building an Ace. 38-39" is the width he suggested would make it a comfortable 2-seater. That was, understandably, all he wanted to say on the subject. Then he showed off the 42" wide prototype "Senior Ace" he had in his shop. With the Senior design he had gone to a cabin style superstructure as you suggest. So far as I can remember, he intended to use a stock set of wings. As with so many things on that visit, I wish I'd taken pictures.
I have both my wings essentially finished, so any material savings from shortening them would be moot at this point.
On another note, my tubing order came in today, so I can start in building tail surfaces while I explore fuselage options.
 

Victor Bravo

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If you already have completed wings, then leave them as-is. The distance and geometry between the upper and lower fuselage spar and strut mounts, and the mid-span spar-strut mount can stay the same. Only the distance between the left and right spar root fitting mounts will go from 6 inches to 39 inches. Your finished wingspan (and overall aspect ratio) will increase, but the bending loads on the spars should be the same as it was. The load transfer on the carry-through tubes between the wings will have to be figured out as a new component of course.
 

Riggerrob

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To simplify landing gear construction, have you considered keeping the same width between lower longerons .... and only increasing width between upper longerons?

Will bulging doors help much?

How about staggering the seats by 6 inches or so? .... just enough to allow the passenger's left shoulder to fit behind the pilot's right shoulder .... The 6 inches can be adjusted merely by swapping seat back cushions (3 inches thick). Remember to secure cushions with Velcro - or stronger - to prevent them from blowing out when flown solo.
 

TFF

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I believe you can fly it solo from the center, and if flying two up, you share the stick. There is no room for moving one seat back in a design like this anyway. I do see my head hitting the back cabane struts, wide or not. That would require some thinking.
 

Victor Bravo

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Do you mean Fred Rosenhan, who would later make Rosenhan wheels and brakes? If so, the beautiful Salvay Skyhopper he built is now owned by a friend of mine and stored in the hangar next to me... and it's looking for a new home :)
 

Bill-Higdon

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Do you mean Fred Rosenhan, who would later make Rosenhan wheels and brakes? If so, the beautiful Salvay Skyhopper he built is now owned by a friend of mine and stored in the hangar next to me... and it's looking for a new home :)
Does the Skyhopper still have 2 compasses in it, bit of a story there. Yes Fred & his brother Cort are the ones I'm talking about last time I saw Fred was in the mid 90's.
 

Victor Bravo

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I will as the current owner if it has 2 compasses in it, or (more likely) if he ever saw two compasses in it and removed one :) I'm not sure if he bought it firectly from Fred, or from someone who bought it from Fred, but it is definitely the one Fred Built. Gene Salvay even saw it, or knew about it, and talked to my friend about it. Good solid airplane, been hangared or garaged 100% of the time for however long I've known about it. Bob has a Comanche 260, just restored a Miller turbo Twin Comanche, and had restored a Cessna 150 to modern IFR standards for the kid who soloed it but didn't want to keep flying. Before the ink was dry on the student pilot license (15 minutes after his shirt was cut) the son took off in the 150, his Dad in the Skyhopper off the left wing, and me in a Taylorcraft off his right wing.
 
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