External wing ribs, worth a second look?

[cluttonfred]

Here's a thought. A number of designs have used external wing ribs to stiffen the wing a simplify construction. Best known is probably the Thorp T-211 Sky Scooter, though I am partial to the Emigh A2 Trojan myself.



Is there a way to use external ribs for a truly simplified wing construction? I have in mind a jig in which metal sheets would be "flanged" along the edges or a pre-preg composite sheet formed in a similar way to make sections of a constant-chord wing. The modular wing sections would then be riveted or bonded along the flanges to create the wing skin and ribs simultaneously I could even imagine something similar with plywood sheets and external spruce cap strips.

Pros and cons? Feedback welcome.

Cheers,

Matthew

[martymayes]

I'm not convinced it would simplify the build of a one-off design. I can see were it might have advantages in mass production though.

Luscombe used the same concept on their 8 series metal skinned wing but they put the "ribs" inside the wing.

[rwanttaja]

Like Marty said, I can see it being useful in a factory environment where you can set up a press and run off as many feet of skin with rib flanges as you need. Much more difficult for a private user...imagine if you had to scratch-build an aileron like the smaller Cessnas use, the ones with the triangular stiffening.

If there were a commercial source with the appropriately-beaded sheets, there'd be a better chance. But most likely, it'd just be the kit manufacturer, and you'd be stuck if they went under. Similar if you were shooting for composite skins: You'd still need a mold.

As to whether it'd be simplified to the point where it has an advantage, hmmm. Didn't find a picture of a T211 wing without skin, but there was a closeup of a T211 wing on the IndUS site. It showed the spacing of internal ribs of about three feet in the aileron area. I think RVs have a rib spacing of ~10-12 inches (once you're away from the wing walk area). So...you're eliminating roughly 12-15 ribs per wing.

The question is, how much does that buy you? CAD design and CAM production gives you the ribs and punches the rivet pilot holes as well. How much time does it take to prepare and install an RV wing rib? I suspect the typical RV builder spends more time building his fuel tanks than the mere riveting-on of ribs. Pro-Seal. Ick.

However, this doesn't consider the OTHER implications: What additional structure must be added to the inside of the wing because of the external ribs? I strongly suspect cross-braces will be needed. Some of the time you save by installing fewer ribs might be used up by the additional work required inside the wing.

Anybody got a picture of a T211 wing without skin? Be kind of curious to see....

It's different with production-type aircraft; saving 100 hours per aircraft is significant over a 1000+ production run. But with one-of-a-kind aircraft, not sure there's a real advantage. The last factor is the quality of the design implementation. I'd certainly trust John Thorp, but conventional rib design is probably a lot less likely to be fouled up.

Ron Wanttaja

[Bill Berson]

The external flanges ( not really ribs) does allow for a full depth internal spar or multiple spanwise stringers which would be efficient in wing bending. It looks like the Emigh has an internal aux spar or stringer from the photo.

I assumed you are suggesting making 16"modules, something like the BD-4's original interlocking modules were made in fiberglass.
I would build the wing top assembly in one piece, then lower the whole top onto the lower skin and spar. Maybe use machine screws so the whole top could be removed, if needed, for maintenance or repair.

[cluttonfred]

Thanks, all, for your points, but I wonder if that's all there is to it? Neither the Thorp nor the Emigh seem especially slow or otherwise penalized compared to their contemporaries with the same power, in fact, the Emigh seems downright sprightly on a C-90 (specs from Plane and Pilot site). Am I missing something?

EMIGH “TROJAN”
1948–51

STANDARD DATA: Seats: 2; Gross weight: 1,450 lbs.; Engine: 90 hp Continental. PERFORMANCE: Top speed: 130 mph; Cruise: 115 mph; Landing speed: 48 mph; Range: 550 nm; Service ceiling: 13,000 ft.

[Victor Bravo]

The external "ribs" would sure eliminate spanwise flow problems, but at the cost of a fairly large bucket full of of intersection drag.

Many airplanes have internal ribs and full-height spars, accomplished easily by splitting the ribs into fwd and aft sections.

The external ribs might possibly reduce oil-canning, or keep a better airfoil shape, but that is probably all about rib spacing as opposed to internal/external.

External ribs are far more damage-prone, and thus would have to be built with a thicker gauge metal than otherwise necessary.

If you used external ribs, thinking that it would give you more room inside for fuel tanks, you would need to put in a few internal ribs to contain the tanks from moving, making an equal number of the external ribs redundant. Leaving those external ribs off (because the load was taken by your internal fuel tank restraining ribs), would look terrible.

To paraphrase the character Tony Montana in "Scarface"... "Say goodbye to my little laminar flow!" You ain't gonna get none of it with external ribs. So fuel efficiency, speed-per-horsepwoer, etc. suffers to a significant degree.

All in all, IMHO, put your creative energy into something else in the design of your airplane, and not trying to accomplish external ribs.

[martymayes]

Thanks, all, for your points, but I wonder if that's all there is to it? Neither the Thorp nor the Emigh seem especially slow or otherwise penalized compared to their contemporaries with the same power, in fact, the Emigh seems downright sprightly on a C-90 (specs from Plane and Pilot site). Am I missing something?

Well, one time I flew side by side with a Emigh Trojan with my Cessna 120 and we were evenly matched on top speed. I know how fast my C-120 was and it wasn't as optimistic as the numbers P&P obviously quoted from a sales brochure or something.

[Flyfalcons]

I wouldn't want to own something that ugly. Aesthetics do play a major role in aircraft selection.