Sloping Off - March 2006

A Slimmer's Guide to Depron
by Mike Roach

Until a month or two ago I worked almost entirely with 3mm Depron, a light, easily worked material that is ideal for park fly and indoor models of about 30” wingspan.  At about that time, Pauline at Flitehook introduced me to the thinner grades, which are nominally 0.5, 1 mm and 1.5 mm.  They have the same properties as all other Depron: a shiny side formed during the moulding process and a definite grain, just like balsa.  The weight of the thinnest sheets is a bit mind-boggling: let me relate them to the published weights of balsa of the same nominal thickness (data from the Waltham Chase Aeromodellers website):

For a sheet 36 x 3”, all weights in grams

Thickness         Depron             Contest grade balsa      Medium grade balsa
(mm)                                        (5 lbs/cu ft)                   (10 lbs/cu ft) 

0.5                   3.5                   no data                         no data
1                      6.4                   4.4 (1/32”)                   8.9 (1/32”)
1.5                   9.6                   8.9                               17.7
2.0                   6.5                   13.3                             26.6
3                      6.2                   17.7                             35.4
6                      17.4                 28.4                             70.9

For a comparison, a 36 x 3” sheet of 100 gsm copier paper would weigh 6.75 grams: a sheet of the more usual 80gsm paper would weigh 5.4 grams. 

CONCLUSIONS

  1. The thicker sheets of Depron are lighter than the equivalent thickness of balsa, even of the highest quality, but the very thin sheets are relatively slightly heavier than the very best quality indoor grade balsa.
  2. The equivalent weight of paper is a bit of a shock!
  3. The reason for the anomalies in the Depron weights lies in the production process, which forms a harder (and presumably heavier) skin on both surfaces.  The skin thickness does not seem to be related to overall thickness, so obviously the thinner sheets contain disproportionately more “crust” than the thicker ones.  Incidentally, it seems to me that the “grain” of the sheets is determined by the orientation of the skin.  It is impossible to curve 0.5 Depron in the one direction without creasing the skin, but in the other, it curves very easily.
  4. The 0.5 mm and 3 mm sheets are obviously the best bet for model-making, and copier paper is not necessarily the lightest material to use for fairings, cockpit covers and the like.

WHAT’S THIS LEADING UP TO?

As luck would have it my 2-oz Sopwith High-Speed model dropped from its roost in the ceiling and bashed up its tailplane, rudder and wings.  This is the model featured in the August 2005 AMI and is build to “reduced-Cal” rules.  The wings just needed a dab of glue, but the tail feather were a mess and rather than build stick and tissue replacements, I just cut them out of 1.0 mm Depron, sprayed them lightly with Ford Ivory from a can, added some lines for the internal structure with a felt tip pen and stuck them in place.  The whole job took no more than 10 minutes: changing the controls from pushrod to closed loop took a whole lot longer!  After a very successful Indoor session at Calshot, by very kind invitation from Southampton MFC, I took the process a stage further and re-winged the little plane, converting it from a no-cal to a fully detailed model.  The change of material added about ? oz to the flying weight (no matter how light sheet Depron is, tissue paper and air is lighter!).

That was using 1mm sheet.  Up to this point the large sheet of 0.5 mm Depron had sat in my model room getting slightly foxed round the edges.  It seemed very fragile and I felt it could hardly support its own weight without creasing.  However, to take the process a bit further and in the interests of research, I cut the fuselage sides for an even smaller Sopwith, the Bee.  This tiny aircraft was only 16ft 3” span, so at the same scale as the High-Speed, it is only 14” span. To cut a long story short, the model only needs some Falcon hardware and a little PU05 motor to complete it.  I’m hoping for a flying weight of less than 1.5 oz, which will give it the same wing loading as its 24” span 2 oz cousin.  It is built almost entirely from 0.5 Depron, with only the wing ribs and tail from 1.0 sheet and the cowl former from 2mm sheet.  The U/C legs and interplane struts are from 1/32 basswood.  The weight of the bare model is only 20 grams.  As a comparison, Al Backstrom’s Peanut model of the same aircraft weighed 27 grams with prop and rubber; Joe Fergusson’s version, which flew at the last Indoor Nats, weighed 20 grams without the motor.

These little balsa and tissue models are quite tricky to make and need very careful handling.  My Depron version was simplicity itself to put together and there lies the attraction of the material: it is very easy to make models with and although they are not quite as “scale” as a proper built-up wing they fly beautifully and repair very quickly.

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