"Super" Seven / "Plus" fuselage stretch?

Hey everyone. I'm still relatively new to the world of RVs (in fact, this is my first post here on VAF) so please forgive me if the answer to my question is well-known.

This past weekend, at the Arlington fly-in, I saw an RV-6 "Plus" for the first time. This is an RV-6 that has about a foot added to the length of the fuselage, at the baggage compartment (where the airplane has a straightforward and pretty constant rectangular-box cross-section with no tapering). In this "stretched" RV-6, the panel and seats are in the same place relative to the wing and landing gear, but the aft end of the cabin (the bulkhead, the slider canopy rail, and the spot just aft of the flaps trailing edge where the lower/outer edge of the rectangular-box fuselage first becomes rounded as it starts to transition into the tailcone) is all about one foot further aft, relative to the wing and the seats. This allows for two small rearward-facing seats for kids, or a lot more baggage. (And there's a Rocket-style IO-540 in the nose, for balance). I chatted with the owner. The fabrication is not complicated, since it's just a rectangular box. Apparently the only really tricky part was getting a slider canopy that's a foot longer than normal. (The manufacturer makes the canopy with a few extra inches of material, then trims that off before shipment. This builder managed to order a canopy right out of the machine, no trimming, and that got him an extra 6 or 8 inches of length as well as an extra inch or two in height. The remaining extra length came from an extra-wide skirt at the aft edge).

When I got back home, some Googling revealed this (do a Ctrl+F for "RV-6 Plus"), this, this, and this. Apparently there are a few of these "Super Sixes" and "RV-6 Plus" out there.

My main question is... Has anyone done this to an RV-7? I've been searching for "Super Seven" or "RV-7 Plus" but found nothing. It would be a really cool airplane.

Right now I own an RV-6 (which I bought from the builder a few months ago) but I'm already thinking of maybe getting a -7 in a few years, and/or maybe building one someday. If at all possible, I would like a taller canopy (because in all the airplanes I flew before the RV-6, I had an easier time seeing over the nose during landing), so at the very least, I would investigate this possibility of getting an un-trimmed canopy and/or building a wider skirt so that the top of the canopy is a couple inches higher than usual. But adding a fuselage stretch would also be worth looking into, it seems to me.

What do you guys think?

Welcome to VAF!!!!

AirShowFan said:

Hey everyone. I'm still relatively new to the world of RVs (in fact, this is my first post here on VAF)

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Bernardo, welcome to VAF

Yes, there have been stretched 7s. And 7s with 540s in them.

Steve Barns would be a good person to talk to, he did a super 6.

Steve is a regular here.

You are going to have a few issues stretching a -7 vs. the -6.

The -7 kits come pre-punched, that means there are holes in the bulkheads, skins, ribs, etc.

If you are going to stretch a -7, you will be making a LOT of parts over a standard kit. Doable, but a lot of extra work.

Call Me Crazy

....but wouldn't you just build a 10? Same engine, same outcome, less hassle?

Cam
RV7a Dreaming

hydroguy2 said:

Super-7 link

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Gorgeous airplane, but... I don't think it has a stretched fuselage. The fuselage stretch is the mod that I'm most curious about, not so much the IO-540. (I have seen lots of Harmon Rockets. In fact, my first RV experience was a ride with Canada's Team Rocket ). But it was interesting to learn that the IO-540 gives you better gas mileage, according to that Super-7 site.

camwebb777 said:

....but wouldn't you just build a 10? Same engine, same outcome, less hassle?

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But aerobatic, and with a bubble canopy, and a tailwheel, and slightly smaller overall. And, potentially, with a less-powerful engine. (I don't think the IO-540 is necessary to make the fuselage stretch work. Mounting a normal engine towards the front of a long Harmon nose, and maybe adding a fuel tank in there and calling it unusable if you have backseaters, might be enough for CG purposes).

N941WR said:

The -7 kits come pre-punched, that means there are holes in the bulkheads, skins, ribs, etc.

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Hmmm, interesting. I hadn't thought of that. I wonder if it's possible to order those parts without the holes (similarly to how the builder of this 6+ ordered the canopy without the trimming). It might not be, if Vans uses the holes to mount the parts for some of the machining operations.

Mike S said:

Steve Barns would be a good person to talk to.

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I see he's in Santa Rosa. I'm planning a flight there for the airshow in a month. (I lived in California for 8 years and volunteered at that airshow as a photographer. Got lots of super cool airplane rides, including the aforementioned Rocket flight, as well as rides in an L-39, DC-3, C-17, etc. Looking forward to thanking the airshow organizers with a cool airplane ride myself now that I own an aerobatic airplane). I'll get in touch with him.

Thanks!!!

Hmmm,
So IF you stretch it behind the passenger comparment, how do you intend to fix the weight and balance? Also, how do you intend to verify the strength of the mod?

dedgemon said:

Hmmm,
So IF you stretch it behind the passenger comparment, how do you intend to fix the weight and balance? Also, how do you intend to verify the strength of the mod?

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The IO-540 will take care of the W&B issue. Most, if not all, Super 7's and 8's put a lot of led in the tail to offset the big engine
As for the strength question, that is why we took math classes and learned how to read.

dedgemon said:

How do you intend to verify the strength of the mod?

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That is a good and extremely important question.

(Firstly: Please keep in mind that this is all hypothetical. I just got an RV-6 and I'm planning on enjoying it for years before I start seriously considering the next airplane. And even then, I may opt to buy a flying airplane again instead of building. And if I do build, I'll probably want to make my first homebuilt as by-the-book as possible. So I'm thinking about this mainly for fun, not so much to base any real plans on).

I think I'd start out by talking with a lot of people who have designed RV mods, to try to understand how the most substantial structural mods for RVs have been designed and validated. Is common practice to do a ton of analysis? Or do most folks just say "I can show that my custom part is no less strong than the original part"? Like I said, I'm new to the world of RVs, and I've never even built a homebuilt myself, let alone modified one. But other people have modified theirs, and I would first investigate how they did it, in case the scope of my mod makes it too risky. Then, if I do decide to do it...

I would ask myself: What loads is the structure meant to react? Let's say I do two things: Add two rear-facing seats to the back of the current seats, and lengthen the tail boom. We could look at those things, one at a time.

The structure around the seats should be able to transfer the ~300lbs or so of weight from the seats to the landing gear and wing spar. So whatever structure transfers loads between the seats and the gear/wings needs to be (let's conservatively say) doubled in capability (in cross-sectional area, moment of inertia, buckling resistance, etc, as applicable). This structure is conceivably made up of anything that is aft of the firewall, forward of the aftmost cabin bulkhead, and lower than the canopy plexiglass. Specifically, it should primarily consist of the beams that sit above the belly of the airplane, as well as the side walls and their stiffeners.

I would also ideally want to beef up the wing root structure. Even if the airplane with 4 people remains below the MTOW, the fact that more of the weight is in the fuselage means a higher wing bending moment, by a factor of {weight of fuselage with four people and modded structure, divided by weight of fuselage of normal RV with two people plus bags}. If the weight does go above the original max-weight for the airplane, then either everything gets beefed up (at least the landing gear, wing spar, maybe wing skins...) or I recognize that so-much of the safety margin in the original structure got eaten up (so I should avoid high-G turns, bumpy air, and short-field landings, whenever I am exceeding the RV's normal max-weight). But I would try as hard as possible to avoid exceeding 1800lbs. Even if I still have lots of structural margin, the CG in a safe place for stability, and plenty of engine power for climb... a greater weight still inevitably means things like more fatiguing of the structure, a faster stall speed, etc.

As for lengthening the tail boom: The longer tail boom means a bigger moment arm from the tail fins/surfaces to the fuselage. You may thing that that would increase the bending moments during flight. However, the bending moments required for balance and for maneuvering are still the same as they were with the shorter tail. So what you really want are reduced tail forces, i.e. less deflection of the control surfaces, i.e. you'd probably find yourself applying a hair less force on the elevator and rudder controls in order to get the desired result. Also, you'd ideally want smaller vertical and horizontal stabilizers, or set the horizontals to a slightly lower angle of incidence, although neither of these things may be worth the hassle. In any case, though, given some control input (e.g. you pulling as hard as you can in order to get out of an emergency situation) or some high angle of attack or sideslip, you'd still see the same tail forces as a normal RV, and thus greater bending moments. If you'd like to preserve the margins in your RV's ability to withstand those, then you'd want to beef up the structure where the tail is joined to the fuselage, proportionally to the increase in the distance to the fins and control surfaces.

You'd also have to be more careful of tail-strikes, if you're in a tricycle airplane. Not an issue in a taildragger, but the taildragger's nose would come down a little bit, so, slightly less ground clearance for the prop during taxi, etc.

I'm just making this up off the top of my head while I look at a photograph of my RV-6 (my computer wallpaper) and try to imagine the impact of adding two seats and lengthening the tail, in terms of all the structure that is there to react/transfer loads from those features. I'm probably missing something. The first step in doing such a mod would be to investigate this more thoroughly, rather than just sit here at my desk and assume I've thought of everything.

The bottom line: Each piece of structure is there to react some load. If you know what those loads are (and what they can be in the worst case, e.g. high-G turn or rough landing or big gust) and how they'd be impacted by your mod (longer moment arms, heavier weights...), then you can resize the pieces of structure accordingly. But that' a big "if", especially if you don't work for Vans and don't have access to their loads models (which I don't).

Well, that was an enjoyable lunch break, but now I should get back to work... and then, to practicing landing on the grass with my instructor. I'm planning a trip to a grass strip this weekend, my RV has a nosewheel, and the last thing I need is this/this to ruin my weekend

N941WR said:

The IO-540 will take care of the W&B issue. Most, if not all, Super 7's and 8's put a lot of led in the tail to offset the big engine
As for the strength question, that is why we took math classes and learned how to read.

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All I needed to do to adjust the weight & balance was to relocate the battery and ELT back beneath the HS. Worked perfectly. You can't foul up the W&B unless you do it intentionally.

See you in OSHKOSH!!!!

DRBuilder said:

All I needed to do to adjust the weight & balance was to relocate the battery and ELT back beneath the HS. Worked perfectly. You can't foul up the W&B unless you do it intentionally.

See you in OSHKOSH!!!!

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That is great news! I know of one S8 that had to put pounds of led in the tail to get the W&B to work out.

So, it sounds like your S7 is a standard -7 with the big engine bolted on, is that correct? If so, I'm even more impressed by how strong these planes are!

DRBuilder said:

All I needed to do to adjust the weight & balance was to relocate the battery and ELT back beneath the HS. Worked perfectly. You can't foul up the W&B unless you do it intentionally.

See you in OSHKOSH!!!!

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Isnt' your engine mount shorter too in order to shift the weight aft, or am I dreaming this up?

The rv14 ?

Maybe the RV14 will be a stretched 7. Maybe we will hear about it next week.

N941WR said:

That is great news! I know of one S8 that had to put pounds of led in the tail to get the W&B to work out.

So, it sounds like your S7 is a standard -7 with the big engine bolted on, is that correct? If so, I'm even more impressed by how strong these planes are!

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That is correct. Other than modifying the engine mount, the airframe is completely stock.

Sig600 said:

Isnt' your engine mount shorter too in order to shift the weight aft, or am I dreaming this up?

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Yes, it is slightly closer to the firewall but still allows you to remove the L Mag without removing the engine.... just barely though!

Sig600 said:

Isnt' your engine mount shorter too in order to shift the weight aft, or am I dreaming this up?

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There really isn't much room for a shorter mount, Sig. On my -10, it's a close fit, so the -6 or -7 if not lengthened, would have to either be ballasted or have the battery shifted.

Best,