Electric R/C Flight

By Heather Mardon

Part 3: Converting an IC Kit

Hi,

Rather than plough into brushless motors this month I’ve decided to cover the conversion of an IC model kit to electric power. Don’t worry we’ll get to the brushless motors in a month or two.

There are several factors to consider when doing a glo – ‘E’ conversion. Obviously some kits convert better than others, the objective was to find a model where most of the original parts could be used to save on time and money. For this exercise I settled on the Airsail ‘Volksplane" model. The reasons for choosing this kit were as follows.

  • Reasonable wingspan (51", 1.29m)
  • Simple Design
  • Nearly all balsa construction
  • Designed for about a 0.25 glo motor
  • And of course, it was cheap!
airsail.jpg (6K)

After weighing all the kit parts I came up with an estimated empty weight of about 1kg (35oz). Remember the ‘empty’ weight is the value without the motor, batteries and speed controller. Once these figures were estimated I was able to feed the numbers into my trusty Motocalc software to see if the motor I had in mind was up to job.

I was planning to use a motor/gearbox combination called an ‘ND10’. This is a Speed 600 sized motor but has Neodym magnets and exposed replaceable brushes. It also incorporates an all metal 2.3:1 gearbox.

The optimum setup seemed to be 9 x CP2400SCR cells driving an APC 11 x 7 prop. According to the predictions this should draw about 24amps and turn the prop at about 7300rpm full throttle. The thrust to weight ratio was about 0.62 : 1, seems like it should fly!

On to the Building

This is not a review of this kit so I’m only going to go over the area’s were I deviated from the plan.

Fuselage Changes

The first mod was to cut a hole in the former that sits above the landing gear, to allow room for the battery pack. Mounting rails were glued in place along the full length of this cavity to hold the battery plate. The battery plate was made from lite ply and then extensively lightened using my softbore tools.(There would have been more photos of some parts of the construction but my digital camera ate them, or at least that’s my theory!). After looking at the ABS plastic molding supplied for the forward upper fus decking I decided to ditch it and make it out of balsa. A few stringers and some 1/16 sheet later and a few more precious grams were saved.

tb-Fuselage.jpg (7K)

Motor Mount

tb-motormount.jpg (8K)

Obviously the supplied glow motor mount was of no use so Ian went to work and came up with this little number made of lite ply and epoxied to the firewall (After some large holes were made in it)

Tail Feathers

The supplied parts consisted of solid 6.5mm sheet for these items which then had to be sanded to shape. I cut the supplied wood into strips and used it to fashion built up surfaces faced with 1/32 sheet.

This picture shows the rudder being built. The new tail surfaces ended up weighing about 40g verses 80g with the solid wood.

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Wing

This was built almost stock, the only deviation was to replaced the standard servo/push rod/bellcrank assembly with individual Aileron servos using Hitec HS-81’s installed into a reinforced rib. (Yes there was a picture once…….)

Under Carriage

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The supplied aluminium gear was cast aside in favour of a home brewed ply/carbon arrangement, which was about half the weight and gave an extra bit of ground clearance. Three layers of 0.8mm ply were clamped together with epoxy in a custom wood jig. Another layer of 10mm wide carbon laminate was then added to both top and bottom to increase stiffness.

 

Radio Installation and Access Hatch

A hatch was made in the cockpit area which attaches via a slot at the front and a magnet at the rear. GWS Nara pro BB servo’s provide for the tail waggly bits. The receiver is a standard futaba 8ch PCM. The main flight battery is secured via the black velcro straps that you can see in the picture. There is another access hatch under the front were another velcro strap holds the front of the battery pack. tb-hatch.jpg (9K)

Covering

This was with a product called Oz Cover (No prizes for guessing its origin) which is an iron on clear film. We used two different weights, the standard one on the wings and the ‘Lite’ product on the fus/tail. It has no backing to remove and needs a very high iron heat (the ‘lite’ needs a bit less). Once ironed on a heat gun can be used to take out wrinkles. After that it’s ready to be painted with enamels.

Flying

The model was completed ready for test flying at 2am on a Saturday morning in preparation of attending the North Shore Model Aero club electric flyin on Sunday the 24th of Feb. Final all up weight ready to fly was 1705g (61oz) , this was bang on the box weight for the glow version! The maiden flight went well and showed no shortage of power with a couple of loops thrown in to see what it could do. The first landing approach was a goody so Ian touched it down sweetly.

Since then we’ve had another 3 or 4 flights on it. Average duration is about 8min so all in all a worthwhile project and a pleasant easy to fly airplane.

That’s all for this month, next month I’’ll have event reports from NSMAC and maybe the Hamilton electric day as well.

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