Toobuilder

Toobuilder

Well Known Member
Hi all,

A -8 recently purchased by a friend has taken up residence in my hangar as we work out a few bugs. Though I have been watching the -8 series for several years, this is the first one I've ever sat in. Anyway, I have had the opportunity to fly the thing, so that experience has pushed me a lot closer to building one of my own (I know, you guys hear this all the time...).

As I fly it and evaluate things I like and don't like, I will be jumping in here to find out what can be "fixed" and what I must live with. First up is the cockpit seems a little tighter than it could be because the canopy rails and front roll bar appear to drive the shape of the upper sills. I'd like to do a one piece, side hinged canopy like the -4. I have researched the various mods relating to this, but they all seem to retain the "pinched" sills. If you eliminate the canopy rails and front roll bar structure, is there any reason you can not run the sills parallel to the fuselage sides, thereby gaining some shoulder room?

Thanks.
 
Howdy and welcome!

The sill width is defined by the center section, which is where the main spar transfers load to the fuselage. The center section is a box beam, and flares out at the top.

Part of the reason for the flare at the is to provide the required lateral stiffness for the fuse. You don't want the upper edges of the fuselage flexing in and out a bunch--that would invite a buckling mode in the longerons! :eek: The designer has 2 ways around this. Thin light material that extends inboard (what Van's did) or thick heavy material. Any material they extend inward pays a dividend of distance ^4. In other words, making the sills 1/2 as wide would require 2^4 (16 times!) more material.

One of the reasons I chose an 8 is because per occupant, the cabin is sooo much wider than the side by sides. There is really a lot of room there. You could certainly modify it, but as I tried to point out above, it was designed that way for a reason--any modification is a buch of time and involves a good bit of compromise.

Kind Regards,
Guy Prevost
 
Thanks Guy,

If that particular shape (of the sill) is a requirement to satisfy the designed loads of the fuselage, I'll accept that. What I find curious is that this odd shape (not parallel) just happens to coincide exactly with the need for the canopy rails and front roll bar. It's either brilliant engineering on Vans part (certainly likely), or what is more often seen in engineering, a combination of requirements driving the shape (buckling strength + rail location + roll bar mounting = sill shape). Further, the "flair" at the top of the upright above the wingbox appears to exist to pick up the expected loads of the roll bar, not transfer loads to the fuselage. I guess what I'm getting at is that if you take the roll bar and canopy rails out of the equation, would you still have this sill shape?
 
If you do the Showplanes mod (side hinged canopy), the kit includes narrowed canopy sills.
 
I'm guessing the real answer is someplace in the middle. I would suspect the needs of the slider drove the final dimension, but some material is desired there for lateral stiffness.
 
Welcome to VAF!!!!

Guy Prevost said:
I'm guessing the real answer is someplace in the middle. I would suspect the needs of the slider drove the final dimension, but some material is desired there for lateral stiffness.
Click to expand...

I agree, I have been looking into this for the same reason as Toobuilder (M. Robinson),

The rails pinch me right below the shoulder, all is fine when my arms are up on the side plates, but when I reach for the throttle, or the stick, it gets tight.

Reaching for both is almost impossible with the stock plates, the Show Planes setup makes it a doable thing.

In addition, the use of thicker stock would probably allow further narrowing of the side plates--------need to get an engineer to give definite values for the changes.

A couple of additional benefits to the Show Planes setup, no front roll bar in the pilots vision, and it can be propped part way open for ventilation while on the ground. The stock wind screen is really good at keeping any breeze from the prop out of the front cockpit. And, the access to the rear of the panel is better.

By the way, welcome to VAF.

Get to know the search function here, Show Planes would be a good place to start;)
 
Thanks for the warm welcome everbody.

I've looked at the showplanes site and even seen mention of the narrowed sill, but the pictures shown don't look all that different from the airplane sitting in my hangar.

Though I'm employed as an engineer for a very well known aircraft manufacturing company, my expertise is not structural analysis. While I do have a pretty good grasp of structural design, I am fortunate to have a variety of engineering people who are well versed in this area to make up where I am lacking. I'll lean on them and see what I can find out.

At any rate, like Mike S points out, there are some "less than optimal" ergonomics when both hands are on the throttle and stick. I'd like to improve that situation if possible.
 
Wing Box & F804 bulkheads (RV8)

The RV8 center section carries primary loads from the wings into the fuselage, and between the wings.

The F804 bulkheads and associated stiffeners carry shear force due to lift from the wings and disperse those loads into the fuselage. This is their primary structural role. Besides carrying the shear, they also provide shape and stiffness to the fuselage and help to support the cockpit rails.

The wing box itself (the "box beam" that Guy mentioned) has a system of caps and shear webs in it that work to carry bending loads due to lift. This is the primary role of the wing box. Besides carrying the bending loads, this wing box also has secondary structural roles: transfer shear into F804, and landing gear attach (8A), and provide stiffness for various forward fuselage parts.

Looking at the 8 as a puzzle, it looks to me like Van chose cockpit rail width based on the tightest point in the puzzle: all the way back in the rear cockpit. The rails are laid out to be parallel for the sliding canopy. This is just my guess, but sure looks like this is how the cow ate the cabbage here...

BTW: don't mess with any of the wing box and/or F804 stuff unless you know what the heck you're doing. This is PRIMARY structure. ;)
 
As I recall, the SP setup gives you about an inch and a half more room.

I have sat in two 8 with this conversion, and for me, it is the only way to go.
 
The Showplanes conversion requires the trimming of the 4 804 bulkheads on the top flange, the surface that lays horizontally, by about 3/4" on each. The result is Mike's stated 1.5" width improvement.

The Van's Spec pages show the -8 cockpit being between 24 and 27 inches wide...wider than the 2 seat side by sides (per person) by 2.5". Add in the Showplanes mod and you're talking by 4" per person--that even beats the -10 for shoulder room. Seems about as good as you're gonna get in an RV.

I'm tall as well as wide shouldered, so the elimination of the roll bar/increased viz, and the tip over made it an easy call for me. The other -8/8As I've sat in put the roll bar right at my eye level, and I had to duck to slide closed. I wasn't convinced I would have any cushion left by the time I got the fit right, so ...hello Showplanes!

No regrets (despite Danny King's constant chastising :rolleyes:;))...

Joe
 
Well, after a very cursory review of some pictures of the structure, an engineer (a "real" engineer, unlike me) indicates that the cockpit sill should be able to be made parallel to the skin once the roll bar and slider rails requirements are removed. I simply have to duplicate the column load capability with the new sills. If the new pieces provide any real advantage in extra shoulder room vs. weight or not remains to be seen, but it's looking good enough to do a paper exercise.

I'll let you all know what we come up with.

Thanks
 
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