"Combo planes"

Discussion in 'Hangar Flying' started by Pete, Dec 12, 2004.

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  1. Dec 12, 2004 #1

    Pete

    Pete

    Pete

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    Rag&tube/aluminum planes like the Bearhawk and wood/rag&tube designs (Legal Eagle, Pietenpol) seem to be abundant, but I am interested in examining a plane with an aluminum/composite combo construction. Anyone know of such a design?
     
  2. Dec 12, 2004 #2

    orion

    orion

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    Depends on where the components you mention are used. The logical application of laminate and aluminum are: Composite for the body and aluminum for the wings. One such airplane is the Glastar, where you have the composite body with the aluminum wing. But the Glastar also uses a steel tube internal frame, thus it is not purely what you ask for.

    The Glastar's laminate fuselage in the cockpit area is more a fairing than primary structure. Aft of the cockpit though the steel frame ends, so the tailcone and empennage uses the composite as a primary struture material. The German Symphony (certified version of the Glastar) is a bit different as the steel tube framework does not end at the the cockpit but continues aft, all the way to the tail.

    Combining aluminum and composite in any other way though may run into a few problems so it is not a combination that you generally see in the industry. One issue to deal with in an airframe application is the difference in the coefficient of expansion of the materials being used. This is especially the case when using graphite. Graphite is very stable in the presense of temperature variations while aluminum can of course change dimensions quite a bit. The change in dimensional stability can lead to unsightly surface distortions or even areas of localized high stress, which can initiate cracking or even failure of the structure.

    Fiberglass and aluminum also have this issue, although their expansion coefficients are a bit closer.

    Finally, when using graphite with aluminum, there is also the very serious issue of galvanic corrosion, so it is quite rare to see both those materials in one airplane. It can be done, but care must be taken to keep any component from making a direct conductive contact between the two materials.
     
  3. Dec 13, 2004 #3

    Pete

    Pete

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    It seems that with hotwiring, a composite wing bolted to a tube and cloth or aluminum structure might be nice.

    The Bearhawk, for example.
     
  4. Dec 13, 2004 #4

    orion

    orion

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    Keep in mind though the structural considerations of any such endeavor. Hot-wired foam covered with glass is heavy - it doesn't sound that way but I know of many projects where builders ended up being very surprised at the final weight.

    Even airplanes such as the Ezes, despite their rather small size, end up with relatively heavy wings, especially if the builder is not careful with the process.

    If you want simple construction, get the Bearhawk fuselage, and mount a set of Cessna 172 or 180 wings to it. It'll be lighter and faster to build.
     
  5. Dec 14, 2004 #5

    Pete

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    Interesting, I hadn't thought of the foam sandwich as that heavy. I have hotwired a sample wing in foam before, and it seemed that one could assemble wings very, very quickly using this method.

    Not sure about wood, but I figure most of the cuts are crosscuts and a good miter saw is all that is needed? Or do the cut joints have to be sanded smooth? If that's the case, then the Pietenpol looks fun. Wood does scare me a little since I'm a cruddy carpenter - I can do framing and stuff like that, but don't ask me to build a wood box or anything with fancy joinery.

    I like the Bearhawk, but I dread working with sheet metal (having done just a bit before) and I hate solid rivets even more. It also seems like the required tooling for a plans-built aluminum plane is more expensive - brake, shear, sliproll (?).

    Do you know offhand if tube fuselages can be TIG'ed? I think I've heard that MIG is out because of hardness issues - the focused heat creates a harder, more brittle zone that ends abruptly or something like that.

    BTW, thanks for the advice so far. :p:
     
    Last edited: Dec 14, 2004
  6. Dec 14, 2004 #6

    orion

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    The issues of a foam based composite wings are many. Weight is only one. You would also have to design a structure that would allow itself to be mounted as the airframe requires, you'd have to figure out how to incorporate the fuel tanks, and of course how to run the electrical and control system. Yes, the foam based wings do sound easy at first but it's only after you get into the details of the construction that you figure out that it probably would have been easier to actually build molds and go the long way.

    Wood can be an alternative but like all other candidate materials, it too has issues that need to be considered before you commit. However the issue of accuracy is one that you'll need to pay special attention to regardless of the construction method you choose. You may want to try building a few precision projects that are less critical (such as RC airplane kits) in order to get practice being accurate in your work and assembly techniques.

    Regarding aluminum, if you're looking at a kit, most components come already formed. It is unlikely that you'll need to go out and get heavy equipment to manufacture the parts and assemble the airframe.

    For a plans built airplane though, some level of fabrication tooling would be required. There are however alternatives that you could build at home, which are much cheaper. However you can also go to places like Ebay and find pretty good deals on used stuff.

    Regarding the tube welding, yes, 4130 can be TIG'd, although if you ask this question of ten people, you'll most likely get ten answers. Overall though, I've seen people do beautiful work with TIG, MIG and gas, so I guess the logical choice is whatever you're comfortable with. If you pay attention to the needs of the material and the weld, the choice of method does not seem to be the real issue.
     
  7. Dec 14, 2004 #7

    Craig

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    Methods and madness

    Pete -
    I'll second Orion's comment about TIGging 4130 - works a charm. You get a very nice bead, lots of strength, little or no distortion. Having said all that, I must confess that my fuselage is all oxy-acetylene welded!

    Wood is good - even with spruce, it is cheaper than metal, and much easier on the pocketbook for working tools. Yes, for most projects, a good miter saw with a planer blade works great. For some cuts, you may need a bandsaw. And fitting the plywood skins takes a table saw and a really sharp planer.

    I've rarely sanded a joint - usually, the sharp planer blade or a wood plane makes an extremely smooth cut. Plus, you really don't want the sanding dust absorbing the glue instead of the wood grain itself.

    If you go the wood route, be sure to use a good epoxy varnish on the interior sides of everything! On ply skins, mask off the glue lines and varnish away, before installing. Be sure to not varnish the contact surfaces on either piece - the glue won't stick as good as it should if you don't do this.

    I'm not good on composites, tho I've done my share. And ply or glass over foam is much heavier than you would think. The reason it works on RC models is that the skin is balsa or monocote, not aircraft grade waterproof plywood. The glue in the ply is what contributes to the weight, although the foam can become quite heavy as well. I didn't study calculus, so I can't calulate the volume of a wing, but with foam weighing 3 to 5 lb/cu.ft., it is bound to add up!

    Aluminum can be fun, especially with a kit like RV puts out. You have no need of brakes or shears, and you do use pull rivets in places where it is difficult or impossible to use hard rivets. Still, I do like to use those little rascals - makes you feel you have really accomplished something when done with the rivet gun for the day.

    Whichever method you choose, and whichever material, the whole process is gonna be a GREAT learning experience for you.

    Keep in touch here on the forum, let us know how you are coming along.
     

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