Mini/Micro rubber conversion - RC Groups
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Dec 03, 2001, 01:38 AM
FASST flyer
KatManDEW's Avatar

Mini/Micro rubber conversion

I've been introduced to indoor flight and I've picked up a couple (few) small (tiny) rubber powered kits. Anywhere from 18 inches clear down to a 12 inch Walt Mooney Peanut scale jobbie.

Any suggestions for power setup's for these massive airframes? (motors and gearboxes)
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Dec 03, 2001, 10:57 AM
Fixed Wing Fanatic
Jim Walker's Avatar
Probably a KP00 geared motor and a 5-4 gunther prop on 4-6 120mah nimh cells. GWS micro reciever without case, JST connectors, and replace antenna with ultra fine relay wire wrapped around balsa stick and coated with dope. Wes tech LS-3.0 servos and the lightest esc you can find. Make sure your wing loading is 4oz sq/ft or less to get the speed envelope necessary to indoor flight.

Bottom line, lighter is better.....
Dec 03, 2001, 11:00 AM
in persuit of low wing loading
Gordon Johnson's Avatar
Depending on whether the plane is 12" or 18" and monoplane or biplane, your motor choices will be to the small end of the spectrum. One of the smaller will be the KP-00, followed by the Kenway KR-1 (available in low and high voltage versions). These two are less expensive and do not require special speed controls. Next up would be the Astro Firefly (4:1 and 16:1 ratios) and the WesTech DC5-2.4, which is more powerful than the Firefly, but has more current draw. These last two are coreless motors and so require high frequency ESC's, like the JMP-HF9.

You can get these motors from a variety of sources:

If you get the Kenway I recommend you get it either from Cloud9 or Dave Lewis (Homefly). Todd has the older version which is not ball bearing and requires the motor to be mounted backwards. The newer version also has a much better prop mounting system.

You are going to have to approximate the thrust you need based on a projected weight for the model. John Worth (RCMicroflight) recently published some guidelines. Dick Miller (Flying Models magazine) also has some widely used rules of thumb. I'll try to find both sources and post summaries of their rules of thumb within the next day or two.

You should also realize that with planes this small, especially the 12" spans, you are going to be looking at the lightest equipment. For the small ones you will probably need to use either the Westech linear servos or go with some sort of actuator. But, you are in the right place, that's what this forum is for.

I'd also suggest a subscription to RCMicroflight. A subscription allows you to peruse all the past issues on-line. There is a wealth of information there, as well as static tests of some of these motors that is quite helpful.
Last edited by Gordon Johnson; Dec 03, 2001 at 11:24 AM.
Dec 03, 2001, 11:34 AM
Balsa Flies Better!
One other guy to talk to is Tom McAnn at Sky Hooks and Rigging. Check his website- there's lots of stuff. His micro receivers help shave some vital grams- I have two- they work fine as long as nobody's nearby.

Dec 04, 2001, 07:44 PM
FASST flyer
KatManDEW's Avatar
Great information. Thanks!

The 12 inch I have is a Walt Mooney Andreason BA4-B biplane. Also have a 18.75 inch DARE Spad 13-C and a Dumas 15.5 inch F-4F Wildcat.

I don't know how to arrive at projected weights. I figured I could go with the lightest electronics, like those mentioned, but it the powerplants that worry me. That's where I've been having trouble with all my electrics.

I am subscribed to RCMicroflight.

Do the Sky Hooks receivers have trouble with other transmitters nearby? There are always a ton of them nearby when I fly indoors. I already have some E-Cubed antennas so can I use them? Are the built-in ESC's on the RX72-HYB and RX72N-HYB high speed for use with coreless motors?

Should I use Lithium batteries?

Thanks again. I want to start building one of these but I figure I need all the hardware first so I know how/where to mount it.
Dec 05, 2001, 05:47 PM
Fixed Wing Fanatic
Jim Walker's Avatar

Yes, the Skyhooks and Rigging recievers are single conversion with minimal filtering to achieve the absolute minimum in weight and so are very prone to glitching indoors around other transmitters. If most of your flying is indoors at fly ins, get the new GWS R4P reciever with the JST connectors and change out the antenna wire with 19" of small diameter magnet or relay wire. With these modifications you only pick up a couple of grams of weight compared to the Sky+Rig reciever and you don't have to sweat interference at indoor meets. Not to mention saving about $70.........

The size and weight of airplane you're talking about can't handle the weight of lithiums. I don't think Tadirans are available any more and even the lightest of these is over budget for a 12" span plane. Lithium Ion's are even worse. If you are going to use a firefly motor, DC5 coreless or a geared N20, I'd say your best bet would be 4-6 120mah nimh cells. They only weigh 3.2 grams each and will provide up to 1.5 amps of current which should be plenty for the motors I mentioned.

You'll just have to go to the Sky+Rig web site and check the specs of their hybrid reciever/ESC. You're looking for 120khz or more frequency if you wan't to make sure and not kill your coreless motor prematurely. Even so, you're still stuck with the glitching in crowds problem.

I'm not familiar with E-cubed antenna's. I'm sure range is not a problem, so it's really just a question of weight. Would magnet/relay wire weigh less? You're going to have to challenge every .1 gram on this plane if you want it to fly well.
Dec 05, 2001, 06:47 PM
Registered User
jberg's Avatar

Originally posted by KatManDEW
Should I use Lithium batteries?
Jim is right. The only option I see is to use 2x CR2 Lithium primary photo cells since the 450mAh Tadiran cells are out of production (unfortunately).

Normally you throw the CR2 away after use. They seem to be much cheaper in USA than in Germany, here they are around 6-8?(~US$) apiece. Some people have tried to recharge them with different success (and yes, it is dangerous, you might burn your house etc. etc.)
Dec 05, 2001, 10:09 PM
FASST flyer
KatManDEW's Avatar
Why JST connectors on the receivers? Are these the connectors on the Wes Tech servos? I've heard poeple talk about not using connectors anwhere but between the battery and ESC. Just solder/hardwire everything else.

Why LS-3.0 and not the lighter LS-2.4's?

The E-Cubed M72 is at Todds Models on his receiver page. It weighs 0.6 grams and it's wrapped on like a 5 inch cocktail straw.
Dec 06, 2001, 06:09 PM
Fixed Wing Fanatic
Jim Walker's Avatar
I guess I was thinking from a money standpoint. If money is no object for you, then by all means get the LS-2.0 servos. The JST connector version of the GWS reciever is much lighter and does match the connectors on LS servos.

Yes, if don't mind voiding warranties, eliminate all the connectors except the the one between the battery and the ESC. That will definitely get you the lightest possible set up barring actuators and conversion boards. You could even go that route and save more weight. I remember reading in RC Microflight about a 3 channel (two actuators and an ESC) system that weighed less than an ounce including batteries. There's even another level to go to if you want to join the cutting edge in the quest for lightness. The Aeronutz across the ocean are using voltage amplifiers so they only need two secondary cells to power everything.

If you are happy with only rudder and throttle control, another gentleman on RC Microflight made a plane that weighed less than half an ounce powered by a throttled CO2 motor and a broadband super regen reciever that weighed less than a gram.

It's a question of money and time. How much do you want to spend, and how much time are you willing to spend making subminiature components from circuit diagrams?
Dec 06, 2001, 08:03 PM
in persuit of low wing loading
Gordon Johnson's Avatar
If you decide to eliminate the tiny JST connectors and just solder the connections on, you will save approximately 0.1g to 0.2g per male of female JST connectors. So, you can count up how many you would have and estimate the savings before you start soldering.

BTW, FMA says they are coming out with a lighter dual conversion receiver in late February. But, no one knows how light yet.
Dec 06, 2001, 08:10 PM
FASST flyer
KatManDEW's Avatar
The LS24's were only $4 more apiece than the LS30's. I got a few of each.

Tom McAnn convinced me that the RX72N-HYB receiver does not have trouble indoors and with multiple transmitters in the area so I got some RX72N-HYB's. It's single conversion but so are all the micro RX's I know of except for the FMA's. Tom said the RX72N-HYB is narrow band where as the other Sky Hooks RX's are wide band and that is why they have problem with nearby transmitters. The RX72N-HYB is really about the same weight as the GWS but the RX72N-HYB has a built in ESC which will save a gram or thereabouts.

I would like to go 4 channel but I started thinking that if I want to use one servo for aileron's it would be most easily acomplished with a single rotary servo instead of the linear ones. If I go 3 channel I will need a bunch of dihedral in the wings, right?

Any suggestions on redesigning the rubber kits with working control surfaces, and hinging them? What about the type of covering? I can't find any specs on the KP00 and Kenway motors. One of them is available in low or high voltage. How does that relate to cell count? Any suggestions on motors for my larger 18 inch rubber kit (biplane)?
Dec 08, 2001, 01:19 PM
Registered User
<<Any suggestions on motors for my larger 18 inch rubber kit (biplane)?>>

A DC-5 2.4 geared 4:1 (10 tooth pinion and 40 tooth gear) with a Gunther push on prop is a GREAT power plant.

Dec 08, 2001, 03:10 PM
Just an average RC'er
Jim McPherson's Avatar
Originally posted by KatManDEW
If I go 3 channel I will need a bunch of dihedral in the wings, right?

I wouldn't say "a bunch" of dihedral is needed. Especially on those rubber kits, they should already have the dihedral that you need built in, they don't have any control surfaces in FF so the designers build them with dihedral for stability.