Engineer’s Toybox

Just in time for the holiday season, here are two hobbies that have very specific motor requirements. 

—edited by Stephanie Gooch

Long Flight, Limited Ceiling

Indoor electric-motored, radio-controlled (r/c) aeroplanes are catching on as a hobby. They provide the fun of piloting without being reliant on decent flying weather, or a local space approved for model aircraft. While not all park flyers can bring their models indoors, and not all indoor models can be flown in the park, there is a good blend of technologies that can allow both objectives to be satisfied, depending upon wing loading and available flying sites. There are divisions for scale models, duration models, pylon racers, acrobatics and helicopters, but the most interest is in what is called the ‘fun fly’ model. With all the creative minds entering into model airplane design, particularly in indoor electric r/c, other types of events are sure to follow.

Why Indoor Competition?

What makes indoor electric r/c flights so interesting? Technology might be one answer to that question. Although DC electric motors have been around for quite some time, it has only been in the last few years that major advancements have allowed for high-efficiency, light-weight motors and small, light-weight rechargeable batteries that make possible high performance in indoor electric free flight. 

In the earlier stages of this sport, indoor models weighing less than one ounce were achieving flights in excess of ten minutes. For this type of outcome, it was necessary to research available motors and match the proper gear ratios to the most efficient propeller and battery. Hobbyist Bob Wilder explains that, “I found it important to perform tests to determine the maximum thrust while using the minimum current draw from the batteries. Therefore, I analyzed a variety of combinations of different motors with different gear ratios and different propellers.” When flying for duration, wing loading must be reduced, which also helps to increase flight times. 

The Rules

There are just a limited amount of rules for the r/c duration competitions. If interested in such an event, the overall weight of the model must be kept to an absolute minimum—not to exceed 100 grams. Secondly, you need to have enough thrust to fly the airplane successfully, which would be approximately 25%-30% of the total weight of the plane. There are other factors, one of which would be the size of the building being used for the event. A larger building would naturally enhance the performance of the flight duration because it would allow the plane to fly in larger circles. Larger circles means fewer inputs to control direction changes, which consume power with every input.

According to Wilder, the man who holds the record for indoor electric r/c flight duration, “The real bottom line in producing an airplane that will fly inside for several hours is thrust versus current draw.” That means that the size of the battery and the size and efficiency of the electric motor are key. 

The Motor

Some of Wilder’s earlier r/c aeroplanes models , when he first began to win awards, (with duration as high as 2 hours and 34 minutes) were built using Maxon Motor components. “I tried a lot of different motors and Maxon is one of the best,” he comments freely. Even when he tried another motor brand, he’d typically use a Maxon gearbox. “A lot of other companies are selling their own motors with a Maxon gearbox, too,” he explains.

There are two categories of motors that are typically used for these applications; iron-core and coreless motors. Wilder asserts that through much testing, coreless motors generally out-performed all other motors. Known for their high efficiency, Maxon motors provided the low current consumption needed in competition to provide extended battery life. Low inertia allows for an extremely low mechanical time constant, which is ideal for the fast accelerations needed during take-off to get the plane into the air. Additionally, the motor’s ironless core eliminates cogging and provides smooth rotation, even at the lower speeds required for landings. Because the motors have no magnetic saturation problems, linear speed-torque constants are available for simple, accurate control. Finally, Maxon motors and gearboxes are very quiet.

The Battery

As far as batteries are concerned, the most popular for indoor flyers is the rechargeable nicad battery. The nickel metal hydride battery is probably next in line. The benefit of the nickel metal hydride is that they have an amp-hour rating that is twice that of the nicad for the same size battery. A disadvantage is that there are fewer available sizes of nicads. Lithium batteries, too, offer a higher amp-hour rating, but are available in few sizes and expensive as well. 

The Receiver

Radio receivers for indoor electric r/c have been difficult to find because of the young age of the sport. Currently, one of the more popular, very lightweight receivers is a hand-made model produced in (understandably) limited numbers. Weighing 3.6 grams, the receiver is not narrow band, so several adjacent channels must be kept clear or interferance may cause problems. 

Developments in this hobby are worth studying by those involved in the creation of miniature flying reconnaissance RPVs. Or perhaps engineers on those projects would want to consider applying their expertise to indoor flying as a hobby. But competition might not be easy—Wilder’s latest record is at 4 hours 17 minutes and 57 seconds.

For more information:

Circle 345—Maxon Precision Motors, or connect directly to their website via the Online Reader Service Program at www.RSLeads.com/?112df-345

Bob Wilder welcomes correspondence at 1005 Hidden Oaks Court, Colleyville, TX 76034 or rjwmaw5@flash.net

(model steam locomotives...)