Apex (oz14248)

About this Plan

Apex. Radio control sport delta model.

Quote: "Tired of the same old thing? Why not put a little excitement in your next R/C project and give this .25 powered sport delta a try! Apex, by Steven Ellzey.

Here we go again. I do seem to have a taste for what many call unusual models. Unfortunately, most unusual models are also somewhat complicated. In an effort to correct this oversight. I decided to come up with a simple. lightweight, and yet unusual model.

A delta wing is one of the simplest configurations available (it has no horizontal tail) and the small wingspan makes for a stronger wing. Putting simple and delta together sent me down the road to what would end up being the Apex.

My first pass at this idea was basically a foamboard wing with a box fuselage. While this very simple model flew fairly well, it did have a few problems. It was a bit on the heavy side and it lost quite a hit of speed in a tight turn. I knew the weight problem would be solved by the use of a built-up balsa wing, but preventing the loss of speed in a turn would, however, require the use of some slightly more modern technology.

The loss of speed in a turn is due to what we aerodynamic types call induced drag. This problem is not limited to models of course, even delta wing fighters have the same problem. The high induced drag is due to a strong vortex that forms along the leading edge of the wing when it is at a moderate to high angle of attack. This vortex then flows down the leading edge of the wing The suction it generates creates quite a hit of lift, which is good, and quite a bit of drag, which, needless to say, is bad.

Well, some bright chap thought of just bending the leading edge of the wing down and letting the suction help pull the wing along, thereby reducing the induced drug. Armed with this theory, a great many people (myself included) ran off to their wind tunnels to find out if this idea would really work and, to everyone's surprise, it did. At this point the great many people wrote a great many papers discussing the effects of what is now called a vortex flap.

Simplicity dictates the use of a fixed vortex flap, which has a few drawbacks. In straight and level flight, the drag created by the vortex flap reduces the top end speed a bit, plus sustained inverted flight is not very easy or pretty. However, when it comes time to maneuver, the use of the vortex flap increases the capability of the model dramatically.

In the vein of keeping things simple, every piece of wood (with the exception of the hatch sides) can be cut out using a straight-edge, and the wing is built on a flat surface. All of this makes for fairly fast.

A few words of warning should be passed on at this time. First, this model is for experienced pilots only. This plane is designed to be very responsive and can get into and out of trouble a dozen times in less than twenty feet. Next, I know a few modelers who feel if the plans call for a 25 size engine, then a 40 should be just about right. Do not try that here! The model simply cannot handle the added weight and torque of a bigger engine.

If I did not scare you off with the last couple of sentences and you want a plane that is a real kick to fly, let's start cutting wood.

Construction: Wing: Construction of the wing begins by cutting to length the six 1/16 x 4 in sheets of balsa that form the bottom of the wing. Notice that they do not go all the way across the wing, but only to the line where the vortex flap starts. These sheets are then butt-glued together so that they resemble a six-tier wedding cake or a Picasso Christmas tree. Next, using carbon paper under the plans, draw a line at the inside edge of the vortex flap, two more lines that are each 1-1/8 in off of each side of the centerline, the outline for the elevons and the outline for the landing gear and pipe mounting plates.

Cut out the elevons and set them aside. Replace the balsa in the landing gear and pipe mount area with 1/16 plywood. Even if you do not plan on using a pipe you should put the mounting plate in place so that you can change your mind at a later date. Now glue 1/4 in square balsa to the sheeting at the leading edge; then glue the spars in place, centered on the glue joints of the wing sheeting..."

Direct submission to Outerzone.

Scan by MarkD, cleanup by Circlip.

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