Small-Field Flying
Paul Bradley | [email protected]
Converting an existing rubber-powered FF model to an electric-powered RC aircraft
Converting rubber-powered FF models to small-field RC models is a common practice within the small-field-flying community. I have mentioned doing this on more than one occasion and I’ve had the privilege of highlighting the conversion projects of some fine modelers in these pages.
Reader Ivo Lucchitta sent me a note on this subject and suggested that the steps to perform such conversions may not always be obvious to the builder. Rubber-powered FF aircraft are generally built with lightweight structures that go along with their relatively low wing loadings. When we add things such as a motor, batteries, radio gear, and movable control surfaces, some adjustment to the original model structure is in order.
When contemplating such a conversion using a kit or set of plans, you have a number of options:
- Add a little structure for the landing gear (if the model has landing gear).
- Add some structure for a motor and its mount.
- Add some structure to the wing to allow for a higher flying weight.
- Modify the flying surfaces to allow for the intended movable surfaces.
What happens if you have an already built model in your inventory that you would like to convert to motor-powered RC? That was the situation for Ivo. He had a Comet 54-inch rubber-powered Taylorcraft that seemed to be a perfect candidate for conversion. Since the model was already built, some of the “normal” conversion steps were not as convenient.
Ivo shared a photo of his model as it waits for conversion and a little refurbishing. Having built a Comet 54-inch Taylorcraft many years ago, I can understand Ivo’s desire to take advantage of the existing airframe. The big airplane is an excellent conversion subject when building the model initially, but converting an existing airframe presents different challenges.
Several things come to mind. The landing gear will need some beefing up and provisions will need to be made for mounting the motor and other RC-related equipment. Since Ivo wants his Taylorcraft to be electric powered, a provision for accessing the flight battery pack will be necessary. The layout of the model’s structure makes some of these adjustments slightly challenging with an existing airframe, but certainly not out of reason.
Stripping off the tissue covering, a step already deemed necessary for renewing this nice model, will provide great access to the large, spacious airframe. That should make it easy to add structure to strengthen the landing gear.
Since the Taylorcraft is a high-wing airplane with wing struts, making the struts load-bearing should allow the existing wing structure and fuselage attachment points to carry the flight loads of the heavier RC model. Using spruce or basswood for the struts and adding some load-distribution structure at the attach points should be all that is necessary.
The biggest challenge I see would be modifying the nose structure to make it easy to access the motor and flight battery. The Taylorcraft uses a nice layout of shaped sticks to generate the form of the cowl. In a rubber-powered model the cowl does not have to be removable because the motor is wound by pulling it out through the nose plug.
For RC conversion it would be best if the nose of the fuselage—represented by the cowl area—could be easily removed. That would give good access to the motor, the corresponding mount, and the speed control. I would be inclined to cut off the nose at the cowl line and add some structure to allow the cowl to be a removable unit. I would retain the covered framework of the cowl structure to keep the classic look of the stick-and-tissue model. Several rare-earth magnets would be ideal for retaining the cowl.
I think the rest of the conversion—things such as control surfaces and mounting the RC gear—would be straightforward, especially with the room available in the Taylorcraft.
I'm sure Ivo has some good ideas for how he will handle the conversion. I look forward to seeing his completed project. I will keep after him for pictures so we can all enjoy his challenge and the results. Thanks for the teaser, Ivo.
Before leaving the subject, a source for laser-cut reproduction kits of the excellent Comet 54-inch rubber-powered models is Tom Martin Radio Control. Please see the source listing at the end of this column for contact information.
Pat Tritle’s 54-inch Aeronca Chief (electric from the start)
It is interesting how things go in this world. At roughly the same time I received the note from Ivo regarding his Taylorcraft, Pat Tritle sent word that he had just completed a 54-inch Aeronca Chief for electric power. That's a coincidence since Comet has a 54-inch Aeronca Chief as a stablemate to the Taylorcraft.
For those of us who don't want to make the conversion decisions, Pat's model provides a great alternative. It is designed from the ground up for electric power and radio control. Pat's model retains that wonderful look of a covered, built-up structure and has the structural design details already worked out.
Pat wrote: "The first step out of the norm was to fatten up the typical 12% thick airfoil to 14%. Because this type of airplane often requires up-elevator trim for cruise power settings, I set up the horizontal stabilizer at 1° negative.
"The stabilizer incidence was against my better judgment at the time because of the fatter airfoil, but it proved to work well since the model trims completely neutral at cruise power. With the downthrust set up as it is, a click or two of power can be added to climb or power reduced slightly to descend—exactly what I was after! The whole idea behind the change was to set the model up to fly at scale speeds, which in that scale are slow!
"The model spans 53.8 inches with a flying weight of 14.9 ounces. Power is a Himax 2812-850 outrunner motor with a Thunderbird-18 ESC, an APC 9 x 6 slow-fly propeller, and energy from a Thunder Power 1320 two-cell Li-Poly battery pack. The four-channel control operates four servos."
Watch for a construction article in Model Airplane News. When it is published, Pat indicated that laser-cut kits will be made available.
Tiny solar-powered models — Ralph Bradley
Let the sun shine, let the sun shine in... well, maybe not in but on, in the case of a small model developed by Ralph Bradley. A while back I had the opportunity to show you a nice solar-powered small-field flyer that Ralph developed called the Sol Mite.
That model has a 30-inch wingspan and is relatively small by solar-power standards. When developed, Ralph had hoped he could one day build a solar-powered small-field flyer that was more in his preferred size range: tiny.
As luck would have it, Spectrolab, the company that developed the solar cells he used, has come up with cells that are even more efficient. The cells Ralph received are improved triple-junction gallium arsenide with efficiencies of approximately 31% under terrestrial conditions. With only two of the new cells in hand, Ralph developed a solar-powered model with an 8-inch wingspan that weighs 9.5 grams. I’ve included a couple photos of it for your review.
As was the case with his other solar-power effort, this new model is directly solar with no onboard buffer battery. If the sun does not shine, this little aircraft does not run.
Equipment for the airplane includes:
- a 900 MHz Plantraco 38-gram receiver
- magnetic actuators from Nick Leichty
- a 4 mm pusher motor
- a Plantraco Carbon Butterfly propeller
- a homemade 7.1:1 gear drive that uses watch gears
To fly the model you orient the wings to the sun and plug the receiver into the solar cells' connector. With the motor running, launch and you are off. The model has nice power overhead with the two solar cells and the 4 mm geared motor.
The only issue Ralph has run into so far with the model is making sure the sun is high enough in the sky when flying (on a calm day only). If the sun is too low, the power can brown out when the model turns away from the sun. That causes the receiver to reset and the motor shuts off. As long as the sun angle is adequate to keep the solar cells delivering power, all is well.
I have the feeling that as the demand for alternate power sources continues to climb, we are likely to see some nice solar-cell developments. Perhaps one day they will be good enough to allow solar-powered indoor flying. When that day arrives, Ralph notes with a big smile, we’ll need someone on the building dimmer switch to control altitude!
FMA Direct and Cellpro Revolution
FMA Direct was an early player in the world of Li-Poly battery technology as it applied to the model-aviation industry. Along with offering quality Li-Poly cells, the company has been a source for related products such as innovative chargers and battery-care equipment.
In the interest of continuing to be a valued source of products for people who fly electric-powered models, FMA Direct has announced a new working arrangement with WorleyParsons Advanced Energy Solutions of Australia. The Advanced Energy Solutions group, based in Singapore, has focused on portable energy systems since 1997 with micro turbines, renewables, fuel cells, and advanced batteries as part of its scope of work.
The companies’ combined engineering resources will help ensure that the latest developments in Li-Poly battery and charger technology are made available to the RC modeling community. The products will be marketed and sold under the brand Cellpro Revolution.
It is nice to see FMA Direct continue to work toward advancing the state of the art in electric power. We all benefit when the manufacturers make such efforts to deliver better products to help us fly our models.
Here we are once again at the end. As always, don’t be bashful about sending me a picture or two of your latest project. In addition to the E-mail address in the column header, you can reach me via regular mail at:
32238 Spinnaker Run Magnolia, TX 77354
MA
Sources:
Reproduction kits for the 54-inch Comet series: Tom Martin Radio Control 20805 S. Prospect Ave. Belton, MO 64012 (816) 331-8807 [email protected] www.tmrcsailplanes.com
Solar cells: Spectrolab 12500 Gladstone Ave. Sylmar, CA 91342 (818) 365-4611 Fax: (818) 361-5102 [email protected] www.spectrolab.com
Li-Poly cells and chargers: FMA Direct 5716A Industry Ln. Frederick, MD 21704 (800) 343-2934 Fax: (301) 668-7619 [email protected] www.fmadirect.com
Transcribed from original scans by AI. Minor OCR errors may remain.
