Kema 90 (oz10082)
About this Plan
Kema 90. Radio control sailplane model, for 3 channels.
Quote: "Kema. Try this traditional construction aerobatic soarer, by Keith Humber.
Having not designed an acrobatic slope soarer since my old 'Kema 84' I thought it was about time I came up with a more up-to-date design using more current design thoughts but still with conventional ply/balsa construction.
The wing is semi-symmetrical progressive sectioned, ie the tip is slightly different to the root. Using the sandwich method of rib construction, this produces a slightly differing section throughout the whole wing. The fuselage is 1/16 ply, rear boomed; this incorporates the wing and tail mounts so the incidence alignment is easy and it's a simple matter to obtain the correct relationship between the wing and tailplane when building same.
Let's get started. The boom is constructed first as a ply/ balsa box. Then 3/16 balsa pod sides are fixed to the boom, and the 1/2 inch sheet bottom glued in position with the laminated nose block rebated into place then carved and sanded to shape. The pod is lined with glass fibre laminate; this is optional - mine is lined thus but I am not really convinced that it is strictly necessary. Look on it as a form of insurance!
The wing is spruce sparred top and bottom with 1/16 sheet 'D' box leading edge and 3/8 sheet leading edge capping to the box and ditto the trailing edge (see the plan - all will be revealed!). All the rear sections of ribs at the open area of the wing are capped with 1/16 x 1/4 cap strips. The 3/8 sheet ailerons are shaped and hinged with either Mylar or Magic Tape used in the manner of the old control-line hinges of the fifties and sixties. Believe me, it works fine on a soarer and is cheap and neat. In fact, I now use this method on all my non-scale gliders. You occasionally have to replace the odd hinge after a heavier than normal landing but this can be done very quickly and easily.
The wing has approximately 1/2 inch dihedral under each tip; this is not really essential but I think that it improves the appearance of the wing and is worth the trouble. To my eye, straight wings always tend to look as though they have anhedral. There is no washout used at all.
Tail matters. The tail group is from 1/4 sheet, hinged again with Magic Tape - or Mylar, for those not totally convinced! The rudder has a bottom Mylar hinge mounted in a shaped balsa tailskid, pinned and epoxied. This is my solution to wing/elevator joiners. By hinging the rudder thus you can use a one-piece elevator with thin ply saddles. Cut both the rudder and the elevator to allow clearance and there you have it, a nicely balanced rudder (for stall-turns and spins) without the hassle of split elevators and joining wire bridge..."
Kema 90, Radio Modeller, October 1990.
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Article pages, thanks to RFJ.
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User commentsGreat to see the Kema 90 published by OZ. I built this model over 22 years ago; to me, it looked like a good plan-built alternative to the the excellent but ubiquitous Phase 6. The Kema 90 is really good design and I still fly it now; although it can't match more modern gliders for performance, it's tough, simple to build, docile with rates or exponential 'on', fully aerobatic and it looks good in the air. I would thoroughly recommend it to a beginner moving on from rudder/elevator to 3-axis control, and/or to an RTF flyer who would like to try an easy self-build.
In the light of experience I made three modifications: two to the wing and one to the rudder. The wing is held on by a single central bolt, which is concealed by the hatch. This is very clean arrangement but I found that, when landing on roughish sites with grass tussocks or clumps of heather, the wing would sometimes pivot and damage the central servo mounting. Occasionally the bolt would break. Also, the bottom of the rudder would sometimes catch on these obstructions, which pulled out the hinges. Eventually I removed the central servo and replaced it with two wing-mounted mini-servos, modified the wing and hatch so that the wing could be banded on, and increased the upward angle on the bottom of the rudder. With these mods, the model isn't quite so pretty and it's probably a little slower, but it hasn't sustained any damage for the last 20 years.
Thank you Keith Humber.
Simon Burch - 02/07/2020
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