Spectre (oz8018)

About this Plan

Spectre. Radio control sport pusher model, for .25 engines.

Quote: "The last time you were at the flying field did you notice how much all of the airplanes looked alike? They either look like boxes with wings, or, as my father puts it, like pregnant guppies. Well, I decided that my next airplane would not fall in either of these categories, and from that decision came the Spectre.

I had a couple of goals in mind when I designed the Spectre, The first was to have a stable yet very maneuverable airplane, and second was for the airplane to have the appearance of a jet fighter, few of which look like pregnant guppies. My design philosophy was fairly simple: To design an airplane that was big enough for a 25 size engine, the radio, and little more. Even though the Spectre turned out small, there is still quite a bit of room inside. This is because you can access the entire volume inside of the fuselage, unlike conventional airplanes where everything in the fuselage behind the wing is empty, save a pushrod or two.

I used a pusher configuration mainly because it would look better. It allows a long tapered nose and a round squared off back, which is the way most jet fighters look. The wing was swept for looks and to push the center of pressure of the wing further aft, which helps balance the rear mounted engine. The wing was not swept to decrease drag. As a matter of fact, sweeping the wing on a model does not decrease drag. Wing sweep is used on full scale airplanes to decrease drag near the speed of sound where an airplane runs into compressibility effects. One problem with a swept wing is that they are more prone to tip stall. This is due to a span-wise flow of air which tends to make the tip stall first.

To counter this, the wing on the Spectre is twisted 4° leading edge down. This makes the tip fly at a lower angle of attack and stall after the rest of the wing. The leading edge is sharper at the root than at the tip so that at a high angle of attack and low speed, the air will separate at the root before it does at the tip. Since the airplane is short coupled and has a large amount of lateral area in front of the center of pressure, a large vertical fill area is required to provide good stability. This area is split over two fins rather than having one very large fin, This also has the advantage of tying the fins into the fuselage side where there is plenty of structure to fasten to.

One thing notably different in the wing structure is that the ribs are not constantly spaced; the reason for this is as follows: Wing ribs have basically two functions, one to give the wing its airfoil shape, and two, to stabilize the

wing skin from buckling. So, at the tips, where the loads are small, there is very little stress in the skin; therefore, few ribs are needed to stabilize it. At the root, however, the loads are at their maximum so more ribs are needed to keep the wing skin from buckling, and since the wing depends on the skin to carry part of the load, this becomes important. The leading edge extensions (those triangular pieces on the side of the fuselage in front of the wing) are functional. They increase the lift of the wing at high angles of attack by generating a large low pressure vortex that flows over the wing thus lowering the pressure over part of the wing.

At this point you're probably asking if all of this works. To which I would answer: It works beautifully. The Spectre exceeded my expectations on all counts. She will fly straight as an arrow from one end of the field to the other hands off, turn very tight, any can roll faster than any airplane have ever seen. The Spectre always stalls straight ahead, and when iz comes time to land, the controls remain effective all the way to stall..."

Supplementary file notes

Article pages, text and pics , thanks to hlsat, JHatton.

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