IMAC (International Miniature Aerobátic Club) is the organization that grew out of the interest of flying scale aerobatics. The group was founded in 1974 with 97 charter members. Their intent was to emulate the IAC, which was dominated by biplanes at the time, so the IMAC initially called themselves the National Sport Biplane Association. In 1976 the National Sport Biplane Association became affiliated with the IAC and became IMAC.
In the next few years, membership in IMAC grew, and more model aircraft manufacturers began producing scale acrobatic aircraft. The Pitts still was popular, but monoplanes like Leo’s Laser and CAP 21s were also being built. At this time (early to mid-80s), most of the scale aerobatic models used in competition were 1/4 scale or less, meaning they had wingspans between 60 and 80 inches and engines ranging from 0.60 to 2.0 cubic inches running on model airplane fuel (glow fuel).
In the late 80s and early 90s, new high-performance mono-planes began to appear on the IAC flight line and also at IMAC contests. Extras, Sukhois, and CAPs became the hot ride of choice. Here is one major advantage of flying models over their full-scale counterparts—the price difference between a clipped-wing Cub and an Extra is a nonissue!
During the 90s every kit manufacturer was producing these hot rods in sizes from 1/6 scale to 35 percent scale (54- to 105-inch wingspans), with the larger aircraft powered by gas engines in the 2.4-to 6.0-cubic-inch range. These scale acrobatic aircraft were very popular with all modelers. This trend was helpful to IMAC. Formerly, the solely recognized form of model aerobatic competition involved “pattern” aircraft that appeared dissimilar to their full-size cousins- narrow, ultra streamlined, and unnaturally long moments.
For IMAC, the only aircraft requirement for classes above Basic is that it is a faithful scale model of a known aerobatic aircraft. With all the Extras, CAPs, and Yaks out on the market, many fliers already had what was needed to be competitive, so IMAC membership grew steadily. From the late 90s to the present, growth was not only in membership, but also in the size of the airplanes themselves. It’s not a shock to see models 40 to 46 percent scale on the IMAC flight line. These large scale models are powered by twin-cylinder gas engines that are 9.0 cubic inches (150-200 cc) and produce close to 25 hp!
You might ask why so big and will they get bigger? For the first question, size does count. Big tends to fly and present better than small. But size alone does not make you a winner. Many contests still are won, even in the upper classes, by smaller aircraft flown by highly skilled pilots. For the second question, I would say we have reached the max size. Typically, 40 percent aircraft weigh 35 to 40 pounds, and this combination of power, weight, and size perform the best.
NUTS and Bolts
What is involved technically and financially with an inexpensive smaller size aircraft and, at the other end, an all-out, spare-no-expense 40 percent ship? For the smaller aircraft I will use a 25 percent scale Raven that is made by EG models. This kit is one of the more popular ARF (almost ready-to-fly) kits. It has a wingspan of 73 inches and a ready-to-fly weight of about 11 pounds. ARF means the airframe comes pre-built and covered. All you have to do is some minor assembly, like installing the engine and radio gear. This Raven sells for about $500. The cost for a good 30cc, petrol engine is around $400. You also will need at least five high-quality servos, setting you back another $300. A simple $200 radio will work, but you are much better off spending the extra bucks for a basic computer radio ($400). So for a total of $1,500 you can be competitive in IMAC.
On the other end of the spectrum is a 40 percent (116-inch wingspan) Extra 260 from Composite ARF. This is a fully composite kitset. The cost of this kit is about $5000. You will need another $400 just for hardware, glue and other nickel-and-dime items. This aircraft will need one of the 150-cc twin-cylinder gas monsters ($2000-3000). You need the best servos you can get and many of them. Typically that means 2 for the rudder, two for the elevators, two on each aileron, and one more for the throttle. That’s 10 servos, and the best ones run about $250 each. Other miscellaneous items like a carbon fiber prop and spinner, servo leads, batteries, switches, etc. will add another $2000. The total for this big ride is a little under $11000, and that’s without the radio transmitter. Pilots who fly these 40 percent aircraft tend to buy the top of the line computer radio, so add $1,500.
This is where the scale models really excel. How would you like a thrust-to-weight ratio greater than i-to-i? I believe Wayne Handley’s Turbo Raven was the first and only full-scale aerobat to achieve this goal. With our models, 2-to-i is very typical, and some are close to 3-to-i! Can you say vertical? In the above example, the 40 percent CAP has a finished weight of about 38 pounds, and the thrust of the 150-cc engine would be close to 100 pounds. What this means is that our flying style is a little different than full-scale, with many of our figures flown at half power or less. Full power is used mostly for the vertical figures.
To fully experience the power these scale models have, come to an IMAC meet and watch the Freestyle event. We can pull some incredible maneuvers, including some really wild stuff called “3-i).” How about starting a torque roll at 10 feet AGL and letting it slide down to the runway (torque rolling all the way), touching the rudder on the ground, and then going to full power, shooting up like a bottle rocket? Or how about doing the “elevator?” This is a very high alpha maneuver where the aircraft is pitched up to about 60 degrees with zero ground speed and then descends to a landing. There are many more figures, and this style of flying could be the subject of an article itself.