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Jan 03, 2016, 08:20 PM
It must have a machinegun
Scaledown's Avatar
Thread OP
Help!

Help with rubber motor size


I've just built my first rubber powered plane since I was 10 years old. Its a 36 inch Rainbow from Easy Built Models.
I'm still doing low powered trimming flights.
The motor is a single loop of 1/4" rubber about 24" long. I found a table that said the maximum safe turns is about 1300. I've got it up to 650 turns and all I get is an extended glide. I had to remove all traces of down thrust just to achieve that. It lands still with probably 200 turns on the motor. I wasn't expecting a vertical climb, but I recall doing better when I was 10 years old.
Should I be using two loops?
Do I need to wind it to the maximum just to achieve any climb?
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Jan 03, 2016, 10:22 PM
Registered User
gossie's Avatar
A single loop of 1/4 = 2 strands. Nowhere near enough.

6 strands is what you need of 1/4 for good flight.
Don't forget to lube it. I use brake grease from the local auto store. Comes in a tube.

Motor as suggested will take 1000 turns okay.

FWIW I have a SENATOR of 32in span with a 30 gram motor, 1/4 X 6 strands and stretch wound can get close to 2000 turns on it.

Stretch winding is to have someone hold the model or use a stooge, pull the motor out X about 4 or 5 times the length and wind.
Best in a blast tube.

Motors don't last forever though.
Jan 03, 2016, 10:29 PM
B for Bruce
BMatthews's Avatar
I know it came in the kit but that much rubber is only good for holding the nose block in place. You simply won't have enough power to get any sort of decent flight from it.

You're going to need to buy some rubber from the local hobby shop or from one of the online sources. And since rubber is perishable when hung on a wall in a shop and left for who knows how long I'd strongly recommend that you buy from an online supplier that rotates their supply more often and likely stores it in a cool dark place while it is in his possession.

Since I just saw that you're located in Auz you'll need to wait for one of your neighbours that flies down your way to post up a good source for rubber in your neck of the world.

With the rather zippy spinning prop that comes in the kit I'd suggest that a good motor for your Rainbow is going to be 6 strands of 3/16 wide rubber with the loops you form from it being about 1/4 longer than the distance from the rear peg to prop hook.

You make this motor up by forming it from the strip around a couple of good size nails in holes in a handy board then tie off the end with a square knot followed by a close in overhand stopper knot in each tag end. As you start to draw the knots tight lubricate the rubber with some saliva as you pull tight. Otherwise it won't tighten up enough.

So it's obviously actually one extra long single loop but you fold it over so it's in thirds to form a "3 loop" motor with "6 strands".

Now with the slack due to the length you'll need to alter a few things.

First off you'll want to make sure your nose block is a snug fit in the nose or that you use a pin to secure it from falling out when the motor tension fades to nothing.

Second is that you're going to want to get some ground handling gear to aid in winding and handling the model. Most important for now is some sort of stooge to hold the model. Sometimes you might have a handy buddy around to hold the anchor peg. Or if you're like most of us and your buddies are all busy doing something else then you'll want to make up some sort of holding stooge. This relies on using a rear motor peg made from brass or aluminium tubing so you can slip an anchor wire through holes in a "U" shaped yoke at the top of the stooge and through the tube anchor and that holds the model while you wind the motor. There's any number of possible options for stooges from something that slips into the rear bumper of your car boot/trunk to some snazzy high dollar camera tripod modified with a new yoke for the head and secured to the ground. What you come up with will depend on what tools you have in the shop and what you're comfy with working with.

So on to the rubber lube you'll need. Go down to the drug store and buy a bottle of the thickest liquid hand soap you can find, a similar size bottle of glycerine and a small bottle of castor oil. You can use these to make up a fine rubber lube that is easily washed off if the rubber gets dirty or if a strand breaks and needs to be tied. The mixture you want to make up is 1 part soap to 1 part glycerine. Then to these add what would amount to roughly 10% castor oil. You don't need to make up the WHOLE BATCH at once. A couple of ounces of the final mix will last for a few years unless you leap into rubber flying in a wholesale manner.

That's likely enough of a flood of information for now. But the rubber, lube some form of stooge to hold the model is the priority at the moment.

But pretty quickly you will need to get some sort of winder. The popular Sig or similar small plastic 5:1 winder can be used for flights with fewer winds for your early flights. But if you start looking for a more powerful climb with more turns the torque needed to wind the motor is going to totally overpower the plastic gears and/or your ability to turn the super short handle those winders come with.


The hot setup for a winder on the cheap is one of the better style old carpenters egg beater style hand drills. It'll need modifying before you can use it. Do NOT just slip a wire into the chuck. It can slip out far too easily and ruin your model. When you get a good drill post back here and we can guide you through the mods to it. But for now keep your eyes open at yard sales, flea markets, boot sales or whatever you call those places with lots of "stuff" that we all love to scrounge through for deals. I've posted a picture of one of the better style drills below. Note how you can see the driven gear in an opening in the frame with a bearing area for the shaft ahead of the gear. This is the style you want to find. There's a lot where the shaft and gear are both ahead of the bearing and the shaft is held in with a small pin. Avoid that style like the plague! You want the style like this one with the bearing between the chuck and driven gear. Extra points if the one you get has an idler gear up top by the handle. The idler aids in keeping a better gear mesh and smoother operation. It's well worth it if you can find one like that.
Jan 03, 2016, 10:31 PM
B for Bruce
BMatthews's Avatar
I called for less rubber than Gossie in my reply because those plastic props do not have enough pitch. The size of motor Gossie is suggesting will spin that size too fast and use the power up too fast. The flight will be impressively fast mind you. But the extra speed will showcase any trim issues and it might well be too fast right into the deck. So that's why I offerred the idea of 6 strands of 3/16 instead.

Now if you get to where you carve your own prop so it has more diameter and pitch? THEN I'd fully agree with the stronger motor.

Hey Gossie, where around your area can he buy rubber? No point in suggesting North American suppliers when he's practically in your back yard.
Jan 04, 2016, 04:01 AM
Registered User
gossie's Avatar
Good post BM, as in good info.
3/16th would be fine, or how about even four strands of 1/4??????
Can't help myself going for MORE POWER.

Perth is over 2000 miles from me, but there are people there that fly rubber jobs.
Will see if I can seek someone out?????????? They fly at Meckering, about 130 K from Perth.

The OP looks to be a pretty good builder........Just take a look at his blog pics.

Don't let a potential Free Flighter escape guys. So few of us now flying REAL models that we build ourselves.
Jan 04, 2016, 04:56 AM
Diesel Danny
danny mz's Avatar
I just did a search, somebody recommended this place in Perth as very good and I found that they sell 3/16" rubber - it's under the free flight/accessories listing. http://shop.acercmodels.com/aircraft...ht-accessories

You're right gossie, dont let them get away

regards * Danny M *
Jan 04, 2016, 08:16 AM
Registered User
Pat Daily's Avatar
If you go to freeflight.org and look at their technical library (free) there are several articles that may help you with a rubber ship--they certainly helped me get back into rubber free flight.

Here is one of the articles by my flying buddy Don Srull.
http://www.freeflight.org/DigestOnli...matingSize.pdf
Jan 04, 2016, 08:25 AM
It must have a machinegun
Scaledown's Avatar
Thread OP
Thanks for the advice everyone.
I hadn't thought about trying Ace Models. Most hobby shops don't carry anything that isn't bind and fly these days.
I have a 10:1 winder. I need to work out how to hold the back end of the motor. This model has a wire hook instead of a peg.
I read somewhere that people use Armorall as a lubricant. Is it good enough?
Jan 04, 2016, 10:05 AM
Registered User
Pat Daily's Avatar
Armorall works. I prefer Dow Corning Dow 33 silicone paste.

FAI rubber is available from http://www.faimodelsupply.com

Don't know if there is a vendor in Oz that carries it.
Last edited by Pat Daily; Jan 04, 2016 at 12:42 PM.
Jan 04, 2016, 12:17 PM
B for Bruce
BMatthews's Avatar
OK, this is starting to get a bit more techincal. But it may help Scaledown if he wants to buy a slightly bigger bulk pack, such as a whole pound of rubber.

My own preference after trying the wider and more narrow strip widths is that I would rather use more strands of narrow rubber than a few of the wide stuff. I found that the motors last longer and have less chance of the little edge nicks that form from winding turning into edge tearing which leads to broken strands.

So.... along that line I'd like to switch from 6 strands, or three loops, of 3/16 to buying 1/8 rubber and using it to form a motor with 4 or 5 loops of 1/8. The 4 loops will give a slower and more gentle power burst which would likely be suitable for small local park flying so it doesn't get up TOO high. The 5 or even 6 loops of 1/8 would be more for bigger fields out in open areas where a little more drift isn't the end of the world.

And of course after Scaledown learns to carve his own props which will dole out the power more gently and over a longer run time he can add a couple of more loops to the motor to get up closer to the 6 loops of 1/4 equivalent motor.

Howzatt fellas? And you're right. With so few of us we all gotta help out.
Jan 04, 2016, 01:29 PM
Kansas is windy.
pburress's Avatar
I'll add a few tidbits to all this good advice. As a general rule, an appropriate motor size is a mass of rubber that's 1/3 the weight of your model without rubber. Personally I keep it simple and weight the models without rubber then multiply by .3.

For a motor lube you can use STP Son of a Gun (rather than Armor All) from the local hardware or auto parts store. My current preference is Dow 33 since it lasts longer on the motor and won't splash the fuselage as much since it's thicker than the Son of a Gun.
Jan 04, 2016, 03:02 PM
B for Bruce
BMatthews's Avatar
There is actually a fairly wide range of motor weight to model weight ratios that work. Much depends on how we're looking for the model to fly.

Serious performance larger models use up around 1:1. The big unlimited size rubber jobs being examples of this. Massive sized but incredibly lightly built models with around a 50 inch wingspan, 300 square inches of area weighing 4 to 5 oz loaded up with 4 to 5 oz of rubber that turns a huge 18 to 20 inch folding prop. They really are something to behold.

On the other end of the scale we have something like Coupe d'Hiver models where the model is 70grams and the motor is 10 grams maximum INCLUDING rubber lube on the rubber. That's around 12 to 13% rubber to model ratio.

What this range of options does is allow us to tailor the motor weight to the sort of flying we want to do. We can enhance the glide at the expense of the motor power for starters. And if done right what we get is a nice long gentle climb to a longer duration cruise and powered descent. Indoor scale rubber models are a good example of this. Particularly the ones flown in lower ceiling sites. VERY long and "skinny" motors to keep the energy mass of rubber but used in a format with very low torque and around a bazillion turns.

My own preference is to run my old timer models at right around pburress' recommendation of .3. I give up a bit of climb at this ratio but the motor weight doesn't overpower the glide.

But if I were to use the models for smaller field sport flying I might just drop this ratio down a little to enhance the length of the cruise. I wouldn't take rubber away. Instead what I'd do is regroup the motor so it's the same weight but refolded to have a couple less strands by making all the other loops a little longer. That way I can keep the same amount of energy but just change how it comes out.

If the field is a lot smaller I can also make a lighter motor with the fewer strands. The run time will be shorter but the model won't want to drift as far. A good range of model to rubber weight for flying in a more "compact" area with control over the altitude reached is probably down around 20% of the model weight.

The Coupe d'Hiver class of model shows that we can go lighter. But it sure does make us work at making the rest of the model SUPER efficient. Save that sort of stuff for later.

Scaledown, the "issue" you're facing is that plastic kit prop that tends to have a pretty low pitch to diameter value. As I posted up above this tends to let the power all come out in a burst. Which is why I suggested a smaller cross section of motor than the others. So read and pay attention but be warned that you may not find a "proper" size motor much to your liking when it's spinning the particular prop you have.
Jan 04, 2016, 04:34 PM
Diesel Danny
danny mz's Avatar
Quote:
Originally Posted by Pat Daily View Post
FAI rubber is available from http://www.faimodelsupply.com

Don't know if there is a vendor in Oz that carries it.
The place in Perth that I mentioned appears to list Tan Super Sport (Re-badged as Peck). Although they indicate prices per metre, I have found with other vendors that asking for a quote for a whole box can really bring the price down to sensible levels.

* Danny M *
Jan 05, 2016, 10:00 PM
It must have a machinegun
Scaledown's Avatar
Thread OP
I bought some Peck 3/16 rubber, fitted 3 loops last night and tested it this morning. The power is much more impressive. My trimming skills are less so. I have a bit of gluing to do.
Fun trying though.
Jan 05, 2016, 10:19 PM
Registered User
gossie's Avatar
Quote:
Originally Posted by Scaledown View Post
I bought some Peck 3/16 rubber, fitted 3 loops last night and tested it this morning. The power is much more impressive. My trimming skills are less so. I have a bit of gluing to do.
Fun trying though.
Tell us what it did wrong?

We may be able to help???????????

It looks a very nice model, so should fly well.


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