Slopemaster (oz1031)

About this Plan

Slopemaster. Slope glider for RC. Sailplane.

Quote: "Sailplanes, in the world of aviation, are to power planes as sailboats are to power cruisers in the marine world. The true sailplane enthusiast intensely dislikes having to put an auxiliary motor on his ship - it stinks up the air and dirties up the plane. Sailboat enthusiasts are just the same.

The big difference, of course, is that sailplanes, although great sport, do not have a high degree of practical utility, whereas sailboats can cruise in seas that power cruisers would not dare to challenge. Also, sailboats can 'ride the wind' around the world -their cruising range is limited mainly by the provisions that can be stored aboard.

But the basic similarity, in the sporting world, is the skill required to make maximum use of natural forces -the wind, thermal currents, and gravity - without having to rely on man made power sources. And that's why sailplane enthusiasts have such a great kindred feeling for sailboaters.

A little over a year ago I was introduced to the silent world of R/C sailplane flying, after many years of participation in R/C power plane activities. Since that first introduction, I've become a dedicated enthusiast for sailplanes, although I've never lost my original interest in power planes. I find both fields have their own problems and, correspondingly, their own rewards.

The Slopemaster was my first sailplane design - and like all first designs, by the time I finished modifying it, it still looks like the first version but is considerably different in aerodynamic details. Here's the way the changes came-about.

First, as always, I wanted a comparatively small model. But the very nature of sailplanes is such that even a small model has a pretty good wingspread compared to power models. With a seven inch chord, a power plane would have an aspect ratio of about five - maybe six, and the span would be 35 to 40 inches. A sailplane should have at least an aspect ratio of eight - and preferably higher - so I compromised at nine - and that gave me a span of 63 inches - a whopper, for me.

I wanted a nice appearing model, and like the pod and boom design, but that created some construction problems. So I figured out a way to give the pod and boom appearance, and still use the old slab-sided construction technique. Just shape the side to look like a pod and boom, then add 3/16 square longerons which can be rounded off after the top and bottom sheeting has been added. Looks very authentic from ten feet away.

Since I didn't have a lot of experience in glider designs for R/C, I sought some expert advice. Whitey Pritchard had built and flown several, so I asked him what wing section to use, bearing in mind that all I wanted was a good all around model that could handle both light and moderately strong winds. I wasn't looking for something that would fly in a gale - I'm a Sunday flier, not a storm trooper! "Flat bottom sections are always good when it comes to general purpose gliders," Whitey opined. That satisfied me - particularly since they are so easy to build right on the bench.

So, I zip-zipped a modified Clark Y section, and built a conventional type wing - no taper (some time later, my aerodynamicist friend Bob Andris informed me that at the Reynolds number I was operating at, the constant chord wing is just as efficient as a taper wing) and with about 4 degrees dihedral. I was figuring that by keeping the dihedral down the effective lift would be improved. As later events proved, this was a mistake. Drawing upon some limited experience with a soaring version of the Double Feature (oz5932) twin pusher which I 'honked up' to fly as a glider, only to find out that the rudder area was insufficient, I made a pretty big rudder for the Slopemaster - or so I thought. Once again, later events showed that there's nothing like a flight test to prove the adequacy of a design... "

Update 24/08/2012: Thanks Ctan, wingspan now changed to the correct value 63in.

Update 17/06/2013: Replaced this plan with a clearer scan, thanks to theshadow.

Supplementary file notes

Article pages, text and pics, thanks to DougS.

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