Jazz 20 (oz11963)

About this Plan

Jazz 20. Radio control sport aerobatic model. For 20 power.

Quote: "Get a flying start from Gordon Rae's 47 in compact sportster. Jazz Twenty.

The modern generation of light-weight sport twenty engines, as typified by the OS 20 FP, are excellent power units. Along with plenty of power and economical fuel consumption, they can be throttled down to a reasonable speed. With these thoughts in mind, the concept of a compact model with a grown up performance looked to be an attractive proposition to supplement my usual gliding activities.

One aspect of power flying that I enjoy is having a model with a wide flight envelope that successfully meets the compromise of low and high speed flight. From take off to aerobatics, through low & slow fly-bys, both upright and inverted, and then finishing with a nicely lined up and executed landing, all with a carefully manipulated throttle, makes for a satisfying flying session.

For those of you raring to go, you can skip the next bit, but if you are interested in the 'why and how' and contemplate an original creation of your own, then the following notes may be helpful.

Getting down to the drawing board focuses the mind onto the various parameters required to achieve the intended performance. With Jazz Twenty it was decided to use a low wing configuration from the beginning, so we had a starting point for the layout. If one requires a particular performance, the wing loading is one of the major decisions to be made, so it is useful to consider the all up weight of the various items required and then go for the target loading.

It is also useful if you've got a good eye for line and proportion, as it is nice to generate a pleasing shape. If you can do this and be a little different, you are well on the way to producing something you can be pleased with.

Feel the weight: Taking into account the 'scale effects' disadvantage of a smaller model, together with the low speed flight characteristics I required, a wing loading of below 18 ozs./so. ft. was deemed desirable. A check on the van ous components' weights will give an idea of the wing size necessary to achieve this loading:

Engine/silencer/mount/ tank/prop & spinner: 9 oz.
Fuselage/tail unit/ undercarriage etc: 16 oz.
Wing and all Solarfilm covering: 9 oz.
Radio gear (Fleet mini/ 500 mAh battery): 8 oz.
Plus possible 2 oz for ballraced engine: 2 oz.
Total = 44 oz.

One could safely use a 270 mAh bat tery and get rid of another 0.25 or 0.5, but with four servos, I prefer the extra security of the 500 for longer flying sessions.

Given the weight, we can now decide on the wing area for our chosen wing loading.

Wing Area required: 44 (oz)/18 x 44 = 352 sq in.

Now to the all important wing in more detail. Structurally, two hefty 1/4 in sq hardwood spars, together with leading edge sheeting and central panel spar webbing to take care of bending and twisting loads in the form of a 'D' box, are used in the conventional way. This, together with a sufficiently wide, stiff trailing edge from which the ailerons can be cut, provide the main load bearing structure, integrated with the ribs and capping.

The planform, together with a symmetrical 16% section at the root and 13% at the tips, combine with the curved tips to provide the necessary aerodynamic washout and tip vortex control at higher angles of attack. This provides the required low speed stability, enhanced still further by not having the ailerons extending into the critical trailing edge tip area where it would adversely upset the local angle of attack. The planform is not critical in the high speed/low angle of attack situation.

The aspect ratio of the wing is chosen to give a sufficiently high roll rate, together with reasonable glide/low speed performance, ie lower AR = faster roll rate; higher AR = better low speed/glide performance.

Aspect Ratio: Span 47 in divided by mean chord 7.75 in = 6.1 to 1.

While on the subject of control surface effectiveness, one should note that a sealed (gapless) aileron hinge line will also improve roll rate. In fact all control surfaces hinge lines should always be sealed for best performance.

To control the wing, the tail has an area of just over 25 percent of the wing area and a moment arm of 11 in (TE to LE), with an elevator of 35% of tail area. The fin and its associated area is a generous 9%) of wing area, with around 50% of that as rudder to give good bite for take off, stall turns and, with enough power, knife edge flight etc. The nose moment is chosen to achieve the desired centre of gravity position, tak-ing into account the weight of the engine mount and fuel tank.

The two wheeled undercarriage is chosen for its weight advantage over the trike and is swept forward from the correct wheel position to the strong point of the engine bulkhead for attachment and in so doing gives it that distinctive rakish look. The overall package of Jazz Twenty has worked out very well, with an air-frame weight of less than the original estimate, giving a wing loading of under 16 oz/sq ft. The use of standard radio gear in place of the mini I used woul still come within the target figure..."

Jazz 20, RCM&E, January 1995.

Direct submission to Outerzone.

Supplementary file notes

Article pages, thanks to RFJ.

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