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I know this question is somewhat general and could vary from plane to plane so I will elaborate on the plane in question.

The specific plane I am curious about is the Mooney Mite which has a metal center section but wooden wings and wooden rear half and empennage which at the age most of these planes are is prone to issues. It seems like a great little plane.

My question is can one be built out of aluminum to the same specs (maybe with a bigger engine) and still fly properly or will the weight change (CG change) affect the plane too drastically?

Likewise could a Piper Cub or similar be made of all metal (again with a bit more power up front) and still be an effective plane?

For the record I do not currently have plans to build this plane (although it was available as a kit plane at some point in history. I am aware of the certification issues with making alterations like to an existing plane. I was asking more about if it was possible from an aeronautical standpoint.

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  • $\begingroup$ I feel obliged to say that if you do decide it's a good idea to do this, check with the regulatory authorities first. It's quite likely that the major overhaul you're proposing could affect the airworthiness of the aircraft in question, and the CAA or FAA would have a lot to say about the result. In the UK, the CAA has a "owner builder" programme to help you get the result certified as airworthy again -- other countries might not be so accommodating. $\endgroup$ – Landak Apr 15 '15 at 23:04
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Ed Swearingen used to say that it made little difference if you build an airplane from wood, aluminum or carbon epoxy - in the end the weight would not be too different if the parts are designed well.

Note that many aircraft in the 1930s used the combination of an aluminum frame and fabric covering. Especially the control surfaces of many aircraft, but also the fuselage, wing and tail surfaces of the Vickers Wellington were built this way. Also, many gliders before the advent of fiber-composite structures used a welded steel-tube frame covered with fabric for their fuselage. Since glueing works better with wood, the fabric cover needs to be sewed to the ribs and stringers, so suction forces will not lift it up over an extensive area.

Key is the limited dynamic pressure: If you do not increase operating speeds, exchanging wood for aluminum will be straightforward. Only the torsion box of the wing would need closer attention, since the low weight per area of plywood cannot be recreated with aluminum sheet - the wall thickness and the resulting buckling strength would be too low. You would be well advised to add more stiffeners and ribs!

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  • $\begingroup$ I understand that more ribs etc would be needed I was not necessarily asking about a part for part wood for aluminum exchange but more a general shape/airfoil design exchange. $\endgroup$ – Dave Apr 15 '15 at 18:07
  • $\begingroup$ @Dave: All right, I just wanted to make sure that I am not misunderstood. If designed well, the metal structure should reproduce the wooden shape faithfully. $\endgroup$ – Peter Kämpf Apr 15 '15 at 18:10
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By Armchair Aviator's - https://www.flickr.com/photos/fun_flying/6156194821, CC BY 2.0, https://commons.wikimedia.org/w/index.php?curid=22627615}

Yes the Mooney Mite can be constructed out of aluminium, but will have to be fully re-engineered. Wikipedia states:

The Mite is constructed mainly of fabric-covered wood, with a single spruce and plywood "D" wing spar.

Fabric cannot take any structural stresses, all are absorbed by the internal wooden frame construction. The fabric is not part of the structural design and is only there to provide an aerodynamic shape.

By contrast, aluminium aeroplane construction uses the Stressed Skin design, where the skin not only defines the external shape but also is loaded with compression, tension and twisting stresses. One could say that in an aluminium wing, the only function of the spar is to keep the skin apart so that it can do its structural work. In a wood/fabric construction the spare takes all of the load and is therefore of larger volume.

Stressed Skin construction is most effective if the skin sections are far apart: with larger aircraft. GA aircraft are generally of Stressed Skin construction nowadays so me might safely assume that the smaller Mite might be constructed in aluminium with little or no weight penalty. If properly re-engineered.

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