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Thread: Ideas on cheap building and design

  1. #1
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    Ideas on cheap building and design

    This is an extension of my thread on Cantilever Spars.

    I am rebuilding my Bowers Fly Baby, built in the 60's and from this experience have some firm ideas about building and designing in wood and homebuilding in general.

    Optimized wooden structures built carefully are wonderous light. Bolts, metal fittings, 1/8 wire and turnbuckles are heavy. My tail feathers are so light and even overbuilt according to the plans. But when attachment hardware and all the fittings are added they almost double in weight.

    In my design all tail feathers will be one piece and cantilever with the stabilizer sitting below the rudder as in modern practice.

    All hardware will be stainless in wood structures.

    All bolts in wooden structures will be bushed.

    Wings will be simple geodesic wooden structure like the Cygnet. That wing has root fittings, strut fittings and captured hinge nuts. No compression tubes and expensive drag wire assemblies and no large expanses of plywood.

    All washers will rest on 1/8 macarta pads bonded to the wood. Bolted assemblies crush wood. The oversize washers are too small.

    Fittings are required anywhere there is large compression loads.

    If anyone wants more input on wooden design in light of my experience rebuilding a 40 year old plane please ask.

    One note the aircraft primary structure is sound, rot free, and not a single glue joint has failed from original construction with resorcinol nail clamped construction. A few added repairs with epoxy have shattered in shear. All joints were checked with a magnifying glass and a razor blade probe.

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    Registered User Jan Carlsson's Avatar
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    Re: Ideas on cheap building and design

    I saw on a Boeing Stearman wing, had alu fittings with alu bolts, (very light) a few bolts had been replaced with steel (AN) bolts and sure they ware rusty. (iron - alu - moister) if using steel bolts they should, and the holes in the wood be oiled with line oil.

    Jan

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    Registered User bmcj's Avatar
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    Re: Ideas on cheap building and design

    Interesting. When bolting to wood, are steel fittings necessary or can you use aluminum (assuming that the aluminum is still stronger than the wood you are bolting to)?

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    Registered User djschwartz's Avatar
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    Re: Ideas on cheap building and design

    If you are bolting an aluminum fitting to wood then aluminum bolts might be OK. But the typical application is a steel strap or weldment. In that case an aluminum bolt would not be a good idea. Also, aluminum has more corrosion, fatigue, and abrasion issues than steel so you'd need to be careful in making any substitutions even if the stress loads appear to be within the capabilities of an aluminum bolt.

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    Re: Ideas on cheap building and design

    Either metal works with proper technique. Stan Hall recommended epoxy the fittings in place. Extra care at sealing the wood surface and prepping the fitting is mandatory.

    I plan on bonding thin macarta or phenolic pads between fittings and wood structures. Many Fly Baby owners always coat bolts in varnish before installation and any time they are inspected.

    On the Fly Baby website Drew Fidoe did a detailed writeup regarding what he found when rebuilding his plane after a hangar collapse. That article convinced me to do a detailed inspection of my airframe. He noted that cad plated fitting did real well. My original builder chromed some pieces but this really concerns me from what I've read.

    This Old Plane
    Last edited by Canuck Bob; June 19th, 2009 at 10:58 AM. Reason: forgot link

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    Re: Ideas on cheap building and design

    Boxed structures- I opened and reinforced my fin tail post to repair damage caused by tailwheel spring fitting and side loads.

    The enclosed area of the post showed evidence of moisture staining. Checked out sound but I added an Ash bearing block to fill the internal void to transfer loads to a higher fitting.

    When building boxed spars all voids will be filled with foam insulation board or expanding foam before closing the box.

    All internal bearing blocks will be hardwood, quarter sawn white oak, hard maple, or hickory. Softwoods elongate easily with small side loads. 100% Birch plywood pads would be acceptable I just love working with solid wood when I can.

    I beam spars would be prefered when appropriate.

    Wax is also one thing that seals very well. I will wax all fittings after wing is done prior to covering, NO wax around unglued joints though.

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    Re: Ideas on cheap building and design

    Wing spar bearing plates in many designs are glued on rectangular blocks the fit the spar. These blocks should actually taper on the longitudinal axis to spread the load over a long area. I'm quessing the abrupt change in strength causes a sharp stress concentration along the weaker edge of the bearing block(?).

    The pictures of the compression failures found in wood wing Citabrias are down right scary. They formed in perpendicular failure lines directly beside the bearing plates in parallel straight lines. A couple of wings failed due to this.

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    Registered User Dan Thomas's Avatar
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    Re: Ideas on cheap building and design

    Quote Originally Posted by Canuck Bob View Post
    I plan on bonding thin macarta or phenolic pads between fittings and wood structures. Many Fly Baby owners always coat bolts in varnish before installation and any time they are inspected.
    I'd wonder about micarta between fittings and wood. Micarta was often used as a self-lubricating anti-chafe material and as bushing or bearing blocks. It's still used to make control cable pulleys because of its antiwear properties. The advent of nylon and then UHMWPE made it obsolete. In any case, I'd be wary of sticking it between a fitting and the wood, especially if the bolts are loaded in shear. The micarta will allow slipping of the fitting and chafing of the bolt. Not good. In shear applications, whether bolted or riveted, the friction at the interfaces, created by the pressure of the fastener, is part of the strength of the joint.

    Dan

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    Registered User Dan Thomas's Avatar
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    Re: Ideas on cheap building and design

    Quote Originally Posted by Canuck Bob View Post
    Wing spar bearing plates in many designs are glued on rectangular blocks the fit the spar. These blocks should actually taper on the longitudinal axis to spread the load over a long area. I'm quessing the abrupt change in strength causes a sharp stress concentration along the weaker edge of the bearing block(?).

    The pictures of the compression failures found in wood wing Citabrias are down right scary. They formed in perpendicular failure lines directly beside the bearing plates in parallel straight lines. A couple of wings failed due to this.
    The plate stiffens the spar along the area of the plate. The spar cannot flex nearly as easily through this area, so the loads pile up at the ends of the plate and the spar suffers.

    Dan

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    Registered User Dan Thomas's Avatar
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    Re: Ideas on cheap building and design

    Quote Originally Posted by Canuck Bob View Post
    When building boxed spars all voids will be filled with foam insulation board or expanding foam before closing the box.
    Any closed structure should be vented a bit to allow moisture out and pressures to equalize as altitude changes. Foam in a box provides tight little spaces for moisture to collect and inhibits air circulation.

    Dan

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    Registered User PTAirco's Avatar
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    Re: Ideas on cheap building and design

    Quote Originally Posted by Canuck Bob View Post
    Either metal works with proper technique. Stan Hall recommended epoxy the fittings in place.
    If you epoxy the fittings to the wood, you'll never be able to inspect the holes in the wood for elongation or cracks. Removing the fitting is likely to cause damage.
    "Aeronautical engineering is highly educated guessing, worked out to five decimal places. Fred Lindsley, Airspeed."

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    Re: Ideas on cheap building and design

    Quote Originally Posted by PTAirco View Post
    If you epoxy the fittings to the wood, you'll never be able to inspect the holes in the wood for elongation or cracks. Removing the fitting is likely to cause damage.
    I was noting Stan Hall's work. Personally I don't think I would bother. On large fittings I would be cocerned with the different expansions of the dissimilar materials. Also replacement of fittings would require butchering the wood.

    However Mr. Hall was a highly regarded expert and designer of homebuilts and sailplanes in particular. The Cherokee was his design.
    Last edited by Canuck Bob; June 19th, 2009 at 03:41 PM. Reason: misspelled Stan

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    Re: Ideas on cheap building and design

    Quote Originally Posted by Dan Thomas View Post
    Any closed structure should be vented a bit to allow moisture out and pressures to equalize as altitude changes. Foam in a box provides tight little spaces for moisture to collect and inhibits air circulation.

    Dan
    Good point. I'm actually designing with I beams or open C chanell beams. I like the idea still of the foam and feel proper construction with this method requires eliminating the pockets. The solid glue face for the spar web would eliminate ply buckling concerns and allow the use of naturally oriented ply webs, 8' lengths.

    Vent holes still make visual inspection impossible and this is a concern to me.
    Last edited by Canuck Bob; June 19th, 2009 at 03:41 PM. Reason: clarification

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    Re: Ideas on cheap building and design

    I like to eliminate all mechanism within the wing. My aileron controls on the Fly Baby are a little sloppy from age and will be refreshed to tight tolerances. A real pain when inside a covered wing. It has too many control horns and pivot points that add up sloppiness quickly with age.

    I am going to use some adaptation of Steve Wittman's hidden hinge flap/aileron control system. Basically concentric tube hinges mounted to the rear wing spar and external to the wing.

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    Registered User Dan Thomas's Avatar
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    Re: Ideas on cheap building and design

    Quote Originally Posted by Canuck Bob View Post
    However Mr. Hall was a highly regarded expert and designer of homebuilts and sailplanes in particular. The Cherokee was his design.
    I thought the Cherokee arose out of John Thorpe's Sky Scooter:
    Piper PA-28 Family (Cherokee / Arrow / Archer / Dakota) aircraft - Information and pictures of the Piper PA-28 Family (Cherokee / Arrow / Archer / Dakota)

    Dan

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