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Old Jul 18, 2016, 10:50 PM
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converting from rubber power to electric

Hi from rcbobcat55 I don't know for sure if this is the place to discuss this subject or not. If it isn't then please let me know where to go for this info. I recently acquired a Jetco Lark old time free flight model kit and wish to convert it to electric. I have a bunch of umx nano bricks and motors and am looking for suggestions as to a power system for it. Is there a chart listing the relationship of rubber strands to motor size? I have a bunch of out runners which range from 5 grams in weight upward and am sure one of them will do the job but how do you determine which one? Any help would be appreciated as I have several other mall rubber powered designs which I wish to convert in the future. Thanks much-Bob M..
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Old Jul 18, 2016, 11:06 PM
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Have a look at the Pixie in my Blog, looks about the same size. the electrics are now old and out of date but you may get some ideas.

Colin.
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Old Jul 19, 2016, 03:13 PM
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I've never seen a conversion from rubber to electric. At 27 inch span the final model needs to be around 2 to 2.5 oz to fly in a manner that it would fly at as a rubber power model. Anything more and it'll have the glide of a greased manhole cover. Even at 3 oz it'll be flying much faster than it should.

Normally you'd fly a model of this sort with around 10 to 14 grams of rubber. And at 14 grams it would be pretty much a hot rod. But this means that your electric conversion needs to be light. I'd say you will need to hold the weight of your motor timer and motor driver and power source down to 25 to 28 grams or the model just won't fly like you expect.

The good news is that to get the model to fly in a nice sport model like manner you will only need around 30 watts per lb of model weight to get a nice climb. And at 2.5 oz that's only 30 x 2.5/16= 4.7 watts. On one cell that means just a touch over 1 amp will do the trick. So you can run one of those small and light 150mah cells. And obviously you'll want to use your lighter 5gram motor and look for a really light timing and ESC package to run the motor. One that is able to run on a single Lipo cell.
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Old Jul 19, 2016, 04:21 PM
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There are several RTF micro planes available for about $100. Once you break the airframe, simply move the electronics to your Lark.
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Old Jul 19, 2016, 04:53 PM
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I think I'd be inclined to try one of old brushed UMX motors e.g. from a P51. They're good for about 60-75 grams which should be plenty. And a brushed motor has the advantage that you don't need an ESC/brick (unless you're going for RC not FF) just a simple timer (can be as low as 4 components).

Steve
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Old Jul 19, 2016, 08:02 PM
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Hi from rcbobcat55 Thanks much for all the replies and suggestions. I will most likely be using a umx champ brick driving a brushed power unit out of a champ or a bl brick driving a 5gram out runner. Unless I really get carried away with the tissue and dope I should be easily able to achieve a weight of 2 1/2 oz. I built several larks and ranger 28 models in the 1960's and they all flew well. I lost two Ranger 28's on oos flights. My uncle was an aeronautical engineer and showed me how to get hundreds of extra winds out of stretched and lubed rubber motors. I even lost several AJ Hornets using stretch wound motors. Now we can put rc in most of the old models such as the Ranger 28 and 30 and keep them from flying away. I have a Flyzone playmate foamie which I fly off my side deck when it is calm out and most of the flights with it are in the 20 minute plus range. I had an OOS flight with it the other day and found it in my friend's yard about 1/2 mile away-- I was so busy enjoying the thermal that I let it get out of range.The power system from it might be ideal for the Lark. Anyways, thanks for all the advice-I will keep you posted on the final results-Bob M..
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Old Jul 21, 2016, 05:50 AM
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Oh, I was assuming an RC conversion. My bad.
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Old Jul 21, 2016, 12:40 PM
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Hi trisquire from rcbobcat55 Your suggestion was a good one. I have been buying up wrecked umx airplanes for a couple of years and the equipment from them is the perfect size and weight for this kind of model-thanks again for the suggestion-Bob m..
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Old Jul 24, 2016, 09:07 AM
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You may have to shorten the nose by half or more to accommodate the motor weight. Sounds like a fun project... Enjoy... Robert.
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Old Jul 24, 2016, 11:22 AM
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Hi from rcbobcat55 Thanks much Robert for the suggestion-I will lay all the parts out on the plans and see what needs to be changed to achieve the recommended cg. I will keep the blog posted on the progress-next up will be an old A1 glider design with umx guts for rudder/elevator controls, then a Ranger 30. I love flying off my side deck and picking up thermals over my garden and garage. (my side yard is about 100 ft wide and over 300 ft long with only a garage to get in the way. It is, of course, surrounded by 50 to 60 ft high trees.Thanks again-Bob M..
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