Air waves

Model Airplane News, May 2001

Dave Gierke's write-up in the March 2001 issue is terrific, and it clearly involved a tremendous amount of hard work.

One of the things I've always felt about plain-bearing engines compared with more sophisticated engines is that in the latter, the extra sophistication seldom pays for the extra weight. As a consequence, simpler, cheaper engines often have a higher power to weight ratio than ball-bearing ones. I've taken Dave's figures and calculated torque/ounces of weight, and his figures confirm what I've often thought. The top two engines in this respect are the Tower .40 and the Thunder Tiger GP-42, at 6.1 and 5.6. The bottom one is the SuperTigre GS .40 at 4.7.

I think the tendency to use high-performance engines is more frequently seen where there is a limit on displacement in competitive situations-like when setting speed records-and other similar racing situations and pattern flying when there is a displacement limit. Sport and scale fliers really need the best power/weight ratios, regardless of displacement. It's like the old Detroit maxim: there's no substitute for cubic inches!

So why would we ever go to the complication and cost of ball-bearing engines when the simpler and cheaper plain-bearing engines really give better performance absolutely as well as better performance per dollar? And if we need more power, we can just go to a larger displacement.

Thanks for the best article on the subject I've read. It answers all the questions that anyone could ever have in this area.

ALAN C. BROWN

Watsonville, CA

How did Dave Gierke make his dyno? I assumed that the photo of the "break-in" stand and the dyno were switched in the article, but I couldn't figure out how the torque was measured. [email]

RICHARD OUTTRIM

Richard, yes; the photo captions were inadvertently swapped in the article. The dynamometer in question has been in development since 1969 when I decided that I needed to improve my engine performance for RC pylon racing.

Currently, I measure torque by voltage. I'll explain: the engine and its mount are attached to a rotation rod that allows the assembly to have a limited rotation within two ball bearings contained in pillow blocks. At the opposite end of the mount, a pendulum weight is attached to the rotation rod; it offers resistance to the rotation generated by the engine's torque reaction as it's running.

By attaching a plywood calibration wheel to the pendulum end of the shaft (it has a string wrapped about its circumference and ends at a weight container), I can determine torque by adding weights of known value and multiplying this by the known radius of the wheel (T = F x r). To make things simpler during a test, I've wired a potentiometer in series with a battery, and the voltage is monitored by a digital voltmeter. The pot's output shaft is "geared" to the rotation rod by a loop of Kevlar string (similar to that of an old-time radio tuner), so its motion is increased about 3:1. Therefore, as the engine's torque reaction rotates the rod-finding its static equilibrium point-the pot indicates the rotation as a change in volts. Later, I convert this change to torque. I use 36 volts DC because it gives me a good voltage difference per unit of torque, especially with small engines that have limited output; other voltages work well also.

This whole thing sounds more complicated than it actually is. I've had many requests for information on the dyno, so I'll detail its fabrication and operation in Volume 2 of my book, "Two-Stroke Glow Engines," which should be published sometime in 2003.

DAVE GIERKE

MICRO ELECTRIC RC

Quiet, simple electric power for small models has become practical, and the size and weight of radio systems has shrunk dramatically. Slow, quiet, lightweight RC models have become practical for the first time.

For those who, out of curiosity or necessity, may be interested in this new light RC stuff, our club, the DC Maxecutors, has found some things that work pretty well. Practical and affordable airborne RC plus electric motor systems that weigh a total of 2.5 to 3.5 ounces are now available and can provide 5 to 10-minute flights.

This micro gear works nicely in 4- to 7ounce models that have wing areas of between 140 and 300 square inches, with wingspans of 24 to 36 inches. The resulting wing loadings yield nice, slow flight capability; pretty easy to fly in fairly restricted areas.

Many kits and free-flight designs are suitable for conversion to micro RC, including most of the new Dumas and many of the older Guillow, Seaglen, Comet and Megow kits. And, when they're scaled up 2 to 2.5 times, most of the great free-flights, like Walt Mooney's Peanut designs, have proven to be excellent electric RC subjects.

Successful conversions we have seen include: Guillows 24-inch Nieuport 11 and 24-inch SE-5 kits; Golden Age Seaglen 30inch Cessna C34 kit; Hurst Bowers' Lincoln All-Purpose and Velie Monocoupe (scaled to 30-inch span); Mike Midkiff's Junkers and Brewster Buffalo; and my 30-inch Handley Page W8b, 26-inch Dornier Libelle, 30-inch Bleriot Canard and 36-inch Grumman Skyrocket.