In working with my contest free flight models I seem to have reinforced what I might have read about but did not act on become an issue. Let me explain, I have read that to build a competitive balsa free flight model wood selection is important, heavier wood is needed in places where strength is really important, while other parts of the model can utilize lighter wood. Hoping to build lighter models for short kit models I have order lighter density balsa for the balsa needed to finish the model.
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| Embryo Model Wrinkles from Too Light of Wood |
My first big struggle with using too light of wood came when I used too light wood for the longerons in the fuselage of an embryo model. After breaking the fuselage during construction, I could not get the model covered without sags and wrinkles either. More recently I had the wing fold in my Wilbur old time rubber model just after launch. In asking online what normally causes this, everyone told me using too light of balsa for wing spars. The structure for the wing is minimal on this model so I can see the importance of adequate strength in the spars.
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| Wing Folded |
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| Inspecting the Damage |
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| Replacement Spars |
To find heavier wood, I purchased several strips from local hobby shop. I weighed the strips and used an online balsa density calculator. This wood was really heavy. The lighter of the 1/8” square strip was almost 13 pound balsa and other strips were over 20 pound per square inch, the lightest strip was used.
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| Really Heavy Wood |
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| Not as Heavy |
In doing the repair instead of splicing pieces I removed all the covering from the center section and replaced the two spars all the way to the wing panels. Using acetone on the Duco glue joints the broken spars were easily removed. My experience has been that when I splice balsa with another piece doubling it most likely there will be a break in the balsa right next to where the balsa has been doubled. Maybe on a larger piece splicing at an angle would work satisfactorily. I am also repairing a Witch Hawk 500 wing and went a fair distance back from the breaks in replacing with new spars.
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| Replaced Spars |
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| 1/4 mil Mylar Before Tissue |
With tissue covering there is also the issue of the covering sagging in times of higher moisture like early morning flying. No doubt the model takes on more weight and the tension tight tissue gives the model structure is less. To help combat this problem in the center I am covering the center with ¼ mil mylar and then covering with tissue over that.
Update 8-25-2018
I finished up covering the center section mylar and tissue then made some flights in really humid conditions. The center section remained tight, tip sections and stab sagged. Someone mentioned the tissue is still taking on water weight.
Bill Kuhl
http://www.ideas-inspire.com
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