123 (oz16924)

About this Plan

123. Free flight sport model for rubber power. Wingspan 18 in.

Quote: "Fowler-type flaps and leading edge slats double the area, and increase lift five-fold - all without trim upset. In this rubber-powered experiment, speed reduces to one-fifth. RC possibilities are limited only by your imagination. 123, by J Lowrie McLarty.

The 123 is the result of happenings of several years ago. The UCs and RCs seemed to be landing faster and faster. Even the rubber-powered models were flying further. This experience had nothing to do with my increasing age, of course!

In an effort to create a slower flying aircraft, Fowler-type extended trailing edge flaps were built onto a Senior Falcon RC. A previous experience with similar flaps on a Sniffer FF had shown the need for 10° positive angle on the stabilizer to offset the airflow effect of the flap. The movement to the Falcon elevator was therefore set for maximum at the servo and at the control horn. In flight, this movement was not nearly sufficient. Using enough flap depression to have the aircraft fly slower would upset it to such an extent that it would alternately look at the noonday sun above and the cornfield below. Altitude loss was considerable. During one of these gyrations, a post, then a rock pile, was hit - although a cornfield was available!

So, I decided to go back to basics. Basics to me means glider and rubber-powered design with flight trials. The goal was to have an aircraft which would safely fly fast or slow, and require little trim.

The stabilizer was placed very high on the fin to be well clear of any air affected by the wing, and whatever was hanging out from it. There did not appear to be much to gain by duplicating the flap arrangements on the newest jets. I noted that, although some jet airliners have leading edge extensions and a series of trailing edge flaps, they require a great amount of stabilizer movement. The extent of this movement can be seen by looking at the long slot in the fin. I was determined to find some configuration that would not require large changes or critical settings for the flying surfaces.

Fairly heavy 10 inch span gliders were built. I used flat wings - no airfoil sec-tion. The fuselage was about 1 x 1 in cross section. A wood screw in the nose gave balance and some center of gravity change when desired. Various leading edge and trailing edge flap widths were hinged. Heavy copper wire was used for ease of adjustment. No dihedral appeared to be required. High-wing and parasol positions were used. Very promising results were obtained.

Leading and trailing edge depressions of equal width and up to 600 were tried. Stabilizer trim needed from straight to flapped wing was only a few degrees. The difference in speed between hand-thrown flaps-up and flaps-down was very good. It seemed 10 to 1. However, it was probably about a 3 to 1 difference. I felt quite happy about the small amount of trim needed and was ready to try a basic rubberband flyer when an idea struck. Such a happening can be fatal!

Many projects are not completed because a 'better' idea makes obsolete that which has been previously done. The large aircraft designers are faced with this, of course, though most resign themselves to doing the best possible within a certain calendar time. Modelers are usually not faced with such a deadline unless entering contests. Fortunately, the idea that struck was easy to try and worked very well with a small amount of experimenting. The 123 is the result of this idea.

The wing is split through its center in a horizontal plane (thus yielding an upper and bottom portion). Then the lower portion is split again - approximately in half, longitudinally in a vertical plane. The pictures clarify this. This arrangement doubles the wing area. For this reason, the difference in lift is 2 to 1. Because the new-found area is depressed, the difference in lift coefficient is about 5 to 1. This combination, with the increased drag of depressed portions, results in a difference in forward speed of at least 5 to 1 in flight. For a rubberband model this is very, very good.

It was decided to develop a wing which would incorporate an aileron, so that the design could be used in the future for RC. Refinement of the relative areas of the forward and rear wing panels resulted in slow and fast flight modes without changes in trim or stabilizer settings. One benefit of having ailerons rather than depressed panels is that the wing tips are approximately 100 more negative, relative to airflow, than the rest of the wing. This means no wing-tip stall, and I expect full aileron control at all speeds when ailerons are used.

Early trials of the rubber model were good but not outstanding until an airfoil section replaced the flat wing. The correct fin area was very important. Optimum stabilizer area is not known. The 123 has a stabilizer 25% of the unfolded wing..."

Direct submission to Outerzone.

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