Image (oz2645)

About this Plan

Image. Radio control model. Trainer for .60 power.

Quote: "Give Turnaround pattern a try with this large, easily constructed model. For two or four stroke power. Image, by Bob Aberle.

Early last year I decided to explore the new turnaround pattern competition. The proper name for this event is the FAI R/C Aerobatics (F3A). Much has been written on this subject and if you are interested in the specific rules you might want to look up the details which appeared in the March 1984 issue of Model Aviation (page 114). I doubt if my busy schedule will ever permit me time to become an expert at any particular facet of our R/C hobby. However, I have always felt that to be credible in my writing and design work it is important to constantly try new things. The basic theme in my articles is to get you to try new things, don't just lock into a single phase of this wonderful hobby. So, in this case, I will give you some pointers on my choice of a design intended as an entry point into turnaround pattern competition.

The single, evolving trend in this new event appears to be a large model. Conventional pattern models over the past ten years have averaged 600-700 square inches of wing area. New turnaround pattern models are now showing up with wing areas in the order of 800 square inches and more. For this new design, I settled on a wing area of 828 square inches. This somewhat odd figure resulted because of my insistence on using the entire 36 inch length of balsa. Hence, the 72 inch total wing span (36 inches X 2 wing panels). The aspect ratio was selected at 6.25:1 (wing span divided by the average wing chord) which is a compromise of current design trends. So with the balsa wood length and the aspect ratio as design constraints, my new Image wing ended up with an area of 828 square inches.

Being quite large, weight would definitely have to be another design consideration. I selected 7-1/2 pounds as a target figure for this model. At that weight, the wing loading would work out to 20.8 ounces/square foot which is a very light loading. For reference purposes, my Hobby Barn Curare (October 1980, FLYING MODELS), weighed 9 pounds and had a wing area of 695 square inches, which yielded a wing loading of 29.8 ounces/ square foot. That model flew (actually it is still flying) well with an O.S. MAX .60 engine and landed gently, despite the weight. But the trend in the new turnaround event is for lighter wing loading.

To achieve lighter weight, I decided, as many modelers have, to drop the use of retracts (landing gear). A fixed gear was employed on this design which probably saved a half pound of weight and certainly makes the model easier and faster to build. Another popular innovation in these large pattern models is to install separate aileron servos out in the middle of each wing panel. The idea being to operate the ailerons at the center of their span where extra control force can be imparted (can also help prevent aileron flutter). To make the aileron linkage installation and adjustment easy, the servo actually hangs out in the slip stream. Does that add drag? Yes, but the overall effect is negligible when considering a model of this size.

As a further innovation I chose to estab-lish the engine thrust line. wing and stab centerlines, all on the fuselage reference line (FRL as it is called in the aircraft industry). This gives you a 'zero, zero, zero' configuration of engine thrust line, wing, and stab. You obtain certain advantages out of this set-up but unfortunately, as will be explained later, there are some drawbacks as well which must be addressed.

Simple construction was the final overall criteria for the Image. Everything I design must be simple because I only have a month or two to dwell on a subject befopre moving on to the next magazine project. An Aberle design has to be easy to build with a minimum expenditure of time. This model is in the 'Image' of a true pattern design, except that it builds like a Quicky 500 (oz6868) racer. Total construction time was just under six weeks including drawing board time..."

Direct submission to Outerzone.

Update 17/11/2012: replaced this plan with a clearer version, courtesy of theshadow.

Update 03/06/2016: article pages, text & pics added, thanks to RFJ.

Supplementary file notes

Article pages, text & pics.

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