Learjet 35A (oz7484)

About this Plan

Learjet 35A. Radio control scale model. Twin ducted fan model for 2 x .65 engines. Wingspan is 76-1/2in (or 83-1/2in with tip tanks). Scale is 1/6.

Quote: "Mark Frankel has been designing and building models of the Learjet for several years. This presentation that appears here, and follows in the April and May issues, is the result of his latest achievement. It is truly outstanding and was 1st in Non-Military Sport Scale at Toledo 1988. Learjet 35A, by Mark A Frankel.

The generic name for all high wing lightplanes is "Piper Cub" due to the enormous success of Bill Piper's product. To date, only one other aircraft has achieved the same degree of recognition - the Learjet. To most people 'Learjet' is used to identify all corporate jet transports.

The original Learjet was designed by Bill Lear in the late 1950's. It. was conceived to he a small, high performance aircraft based on the Swiss attack fighter, P-16. The initial design, known as the Model 23. was powered by two General Electric J-85 turbojets, the same engine that was used in the supersonic Northrop T-38A. The Model 23 possessed the highest cruise speed and cruising altitude of any business jet. in fact, its performance was so spirited that it displayed a higher initial rate of climb than the afterburner equipped North American F-100.

In spite of its superb performance. the Model 23 offered extremely limited cabin space and, on occasion, it exhibited some ferocious aerodynamic qualities such as mach tuck and deep stall. Through a series of product enhancements, the Learjet became more spacious, longer ranged and easier to fly, while retaining most of its original performance.

In the early 1970's, Lear introduced a new turbofan fan design known as the Model 35A. This aircraft was powered by two Garrett AiResearch TFE731 turbofans which offered exceptionally good fuel efficiency with low noise. The design became a great success. With nearly 700 delivered to - date, it represents the most popular business jet ever produced.

Unlike the Piper Cub, however, the Learjet is not all obvious choice for a radio control subject. It violates my first principle of twin jet design which is: Never - I mean never, build a twin that has its engines displaced from the aircraft's centerline. It also runs afoul of my second principle which states: Never - I mean never, build a twin with a high wing loading. Nevertheless, if you have ever seen a Learjet climb out from a local airport, or even seen one parked on the ramp, you can understand why I disregarded principles 1 and 2.

After abandoning my principles, I was left with the weighty issue of what scale to work in. Usually, ducted fan model size is dictated by the amount of thrust available. The obvious power plant at that time was the Turbax I fan from Jet Hangar Hobbies with a K & B 7.5 engine. This seemed a perfect match for installation in the short nacelle. The K&B 7.5 would use a Mac's Products Wizard Pipe which could be completely concealed within the nacelle. Thus avoiding the age-old problem of a Tong tuned pipe hanging out of a short tailpipe - a sight guaranteed to offend any scale modeler.

The Turhax/K&B propulsion package is light, compact, and it had worked well in an earlier twin design of the A3J Vigilante (oz6042). Also, Tom Cook was having excellent success with these fans in his early F-4 Phantoms. However, the Turbax diameter is slightly over 5 inches and it dictated the use of a large scale to provide enough nacelle cross section to carry the fan.

At 2 in = 1 ft (1/6 scale), I had a fuselage that was over 8 foot long, while the nacelle was still not quite large enough. Worrying about my ability to transport a monster fuselage, I decided to do the only honorable thing - cheat. I settled on the 2 in = 1 ft scale as the largest practical size, but I had to expand the nacelle size (slightly) to house the ducted fans. While I was at it, I decided to add some wing area to relieve the anticipated wing loading problem. My original weight estimate was 20 pounds dry; however, I considered 22 pounds acceptable.

Throughout the entire process of drawing plans, building the fiberglass tools, and finally building the model, I could not help but wonder whether a model of this size would fly. Remember, in the early 1980's almost all successful ducted fan designs were small single engined examples weighing 9 to 10 pounds. Even large twins, such as Cook's F-4, weighed well under 20 pounds and were dimensionally smaller than the airplane that I hoped to build..."

Update 27/08/2017: Added the missing part #3 of the build article, thanks to davidterrell80.

Supplementary file notes

Article in 3 parts.

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