8:1 ratio

Again, shallower slope scarfs are recommended for highly stressed areas, such as free standing spars, but in many cases, such as lightly bent stringers, planking and other elements where the loads aren't as localized, then steeper slopes are permitted and more importantly save materials.

The aircraft industry is the most conservative building industry in the world. I'm not surprised they'd have issues with many boat building practices. These are the same engineers that are still using 1920's designs and 1930's foil sections, even though they know full well there are much better design arrangements. The Beechcraft Starship, is a classic example of this. Better in every regard to the King Air it was designed to do battle with, yet the conservative industry couldn't get their head around it. If it wasn't for guys like Rutan everything would look like a DC-3.

 

Absolutely true. Except for the experimental aircraft industry, the old line manufacturers are still stuck in 1945. Difficult to tell a new Cessna from one built in 1945. Meanwhile Burt and other small designers and builders turn out planes with performance light years ahead of the big guys.

 

Agreed Tom, but when you talk with conventional aircraft pilots about experimental, they all act like it's a redheaded step child. They begrudgingly acknowledge the advancements and/or innovation, but the industry as a whole and individuals in it, act like they don't exist or are fairly inconsequential.

 

talk about the pot calling the kettle black.

 

@PAR and Tom:

Airplane folks tend to be conservative because experimental aircraft can easily play experiments with pilots and passenger's lives, which is not appreciated by many people. A big majority of airplane users (be it pilots or passengers) need a reliable airplane to get them from point A to point B with a good probability of success.

In case of a major structural failure in a boat, you still have a life-raft option, followed by an additional option of a PFD, followed by your swimming skills and stamina capability. If it happens to an airplane during a flight, you have none of the above.

Plus, if an experimental or an acrobatic airplane falls apart and kills a pilot and/or everyone else on board, most people will say "well, it was an experimental plane, poor fellas knew the risks they were taking". If the same thing happens to a commercial or a GA airplane, the whole model range might get a bad reputation, with all the economical consequences for the manufacturing company.

Cheers

 

NVIC 7-95, Guidance on Inspection, Repair and Maintence of Wooden Hulls, suggests 1/12 slope on glued scarf joints used on wood structural members for small passenger vessels.

 

Quote from Wiki:

“Where scarfed joints are used in the restoration of vintage aircraft most developed countries will only issue an airworthyness (sic) certificate if all such joints have used an angle no less than 1:8"

The document extract states the plain scarf is the simplest and strongest of the glued scarf joints. Does anyone know what is meant by the "efficiencies" given for joints of various angles?

 

daiquiri

I don't think we were advocating that commercial manufacturers of light aircraft market planes on the cutting edge. Liability and safety concerns would not allow it. My latest experimental flight was in a Pietenpol which is a stick and fabric job only slightly advanced over the Wright Brothers Flyer. Nearer the other end are the Lancairs and similar which are well proven and have performance well beyond the updated ones from the 40's. The Light Sport designation has resulted in some nice planes far more sophisticated than the Cessnas and Pipers that make up the majority of planes on the flightline. Some really nice planes are coming out of Europe too.

As in boats, for an afternoon fun flight, I'd as soon go up in a J3 or T Craft as the latest and greatest. Going cross country or needing to get somewhere fast is another matter and a Lancair or turboprop LongEZ would be a much better choice.

 

Slavi, are you suggesting we not hand out PFD's for everyone aboard and stick with well founded 500 D/L ratio gaffers? Aircraft crew and passengers could just as easily be handed a parachute with instructions embroidered on the cover in several languages.

Boats place people in just as hazardous a situation as aircraft, but yachts and commercial craft have adopted considerable innovation in the last century, unlike a large portion of the aviation industry. I have regular rides in a PT-17 and enjoy the crap out of it in the 95 degree summer, but having my second ride in a Long EZ last year, I can quickly tell you which I'd prefer if it wasn't 95 degrees outside. My point was the industry is conservative for conservative sake, more so then safety. They just don't like or are willing to accept, change except in avionics packages. They're still flying gaffers, just with digital displays.

 

Have you guys checked out the sailplanes coming out of Europe? Most are not in the experimental cat, although the "UL" cat is increasing. The newest designs are lightyears ahead ot the power gang. If anybody wants to see first hand, well back seat, I am always looking for an excuse to go flying!
Oh, but it would cost you a beer!

Bob

 

Around 1964, it seems, standardised test methods for wood and wood joints were enacted. And in that particular setup, An 8:1 scarf was standardised. Reading between the lines a bit, it seem to be the finest scarf that would reliably fail at the scarf joint on that particular setup. Samples with finer scarfs tended to fail outside the the joint so frequently that it was a headache for the testers. So almost all the existing datasets are based on 8:1 tests. These tests almost all involve axial loading. Didn't stumble upon much bending and no torsion, but I was just scanning the stuff for a couple hours.


As a side note, spruce is almost always used in multispecies tests and it clearly has some unique properties as far as cyclical load life expectancy and the way stresses get distributed. I think it has do with the stress/ strain relation in tension and in shear being similar in spruce. It seems to behave more like an isotropic material, and that suggests it would work better with glued joints since the glue is isotropic. Just a guess as to the mechanism. Maybe someone here can expand on it.

 

Well, there are only a few things that get us going as well as the scarf ratio topic! Like us old guys nodding over our medications in our rocking chairs, threads approaching senility have a tendency to wander off topic, but this one has literally become blue-sky!

 

Well AK, being means of transportation heavily influenced by the necessity to be as light, streamlined and resistant as possible, ships and airplanes have always been kind of distant cousins from structural, propulsive and fluid-dynamic point of view. So yes, a side-drift, but circumscribed to the family house.

 

FWIW, the RINA (Italian) rules for classification of pleasure yachts state that, when scarfing is necessary, following ratios are the minimum required:
- 6:1 for keels (4:1 is allowed if scarfs are both bolted and glued)
- 5:1 for solid-wood hull planks
- 8:1 for plywood planks
- 6:1 for grown frames
Many very fine wooden boats have been built in Italy under RINA rules, so I guess these values work fine when scarfing is done properly.

Incidentally, getting back to boat's cousins , as far as I remember 10:1 is the minimum commonly accepted for homebuilt wooden airplanes' structural parts.

Cheers!

 

Thanks for the interesting information Slavi! I formed a lot of respect for Italian engineering when I was In Europe supporting the ITER fusion power project, practical but not afraid to be innovative.

The RINA numbers are almost exactly what I have been using, which are based on experiments to see what I could get away with, plus a sensible safety factor.

The only one I would take issue with is the 5:1 ratio for solid-wood hull planks; since these are stiffer than equivalent plywood planks I would use at least as long a scarf to reduce stress at the ends of the joints on a severe bend, although it would be fine for an almost straight plank run. However, these represent a minimum.