Rubber motor question

[davidchoate]

THANKS GUYS.

[Sai1704]

I, too, have a doubt under the same topic. How does the thickness of the rubber thread affect its power delivery? What thicknesses are used for various sizes of models and props?

[WIDDOG]

First of all you ask a very interesting question. The rubber motor is definitely important. The rubber that comes with the kit is not efficient enough for a well working rubber powered model airplane. The thicker the rubber the more torque and speed the motor will have. However, The longer the motor the less torque and speed the motor will produce. I believe by far the most popular thickness of rubber is 1/8 inch thick. Also I believe that 3/16 is the "New" 1/8 inch thick rubber. The Guillow's 900 Series kits instructions include a recommended motor length and thickness. If you tell the forum what specific model you are building? I'm sure that someone on this forum would know the best rubber/prop combination.

[zoomie]

If you tell the forum what specific model you are building? I'm sure that someone on this forum would know the best rubber/prop combination.

That's good advice, WIDDOG .

Also would be helpful to know the distance between the prop shaft hook and the rear motor anchor.


zoomie

[Sai1704]

Thanks Widdog. I am planning on changing the rubber for both my lancer and Hellcat. And by coincidence, I just got 100 feet of 1/8" competition rubber. Lots of models, lots of flights with this one, then . But does the size of the prop also affect this, like how in electric RC, the prop size must correspond with the motor?

[WIDDOG]

Hi Anirudh,

By coincidence this evening was the first time I was successful at carving a prop from Balsa wood. I'm just a Intermediate leave model builder/flyer. There are members in this forum that could really answer your questions. I don't know much about the "Set Up" for the Lancer. As far as the Hellcat a 500 series kit goes... IMHO they are hard to fly. This is the best I could do with my 500 Series kit. http://youtu.be/aUMBwxJsBCo.

I really like the 900 Series kits. They are designed for rubber power. I have had better success with them.

For me I recently learned that I personally prefer Free Flight model building/flying. I'm not very good with RC or video games. So I'll leave the RC Questions to the RC members.

[davidchoate]

i did'nt know that the distance between prop and peg was an issue. my Javelin has 2 peg holes. I never tried the closer one. I have to make a new fuse for it anyway, and may choose to put 1 dowel at an spot not so close, but not so far from the prop just to see what happens.

[zoomie]

i did'nt know that the distance between prop and peg was an issue.

David, the prop/peg distance helps you calculate the length of the motors that you make.

For a simple replacement motor, here's an example: lets say the P/P distance is 8 in., and you want to make the replacement motor a little longer, say 10 in.; multiply 10 X 2 to get the rubber strip required, and add an extra 2 in. per end of the rubber strip for tying the knot and trimming off the loose ends afterward. Thus, the OAL of your rubber strip would be 24 in.


zoomie

[stx44]

Heres some observations with regards to rubber- Ive been using 3/16 in three forms:

Kit rubber bands- an 8" band can only take about 150 turns before it fails, so about 18 turns per inch. They also create a massive, but short lived burst of power.

Guillows strip rubber is infinitely better. It can probably take about 25 turns per inch safely, and creates a more gradual power release.

Tan 11 rubber- I'm currently using Pecks, and I tested it to failure the other day, and it took 80 turns per inch to fail. It also creates much less torque than the Guillows strip- this has pros and cons- longer run time, but a single loop wont run props greater than 6" comfortably.

Using my test bed (a 900 series bird dog) the kit rubber took 160 turns to fail, guillows took about 250, and the Tan 11 took 600 turns to fail.

The other thing Ive notices is that the Guillows tends to "knot" much quicker than the Tan 11 - by that I mean when it is relaxed it stil has a partial wind curl to it.

FYI I use armourall car/vinyl spray for lubricant.

[Steve Blanchard]

Rubber length and prop hook to peg distance does not always help calculate the length of the motor, but you should be making the finished motor at least 1.5 to 2 times the distance between the prop hook and the motor peg. Sometimes the distance of hook to peg is shortened to better balance the finished model but the motor can remain the same providing the fuselage is open enough accept it. There are many ways to keep the motor from bunching up inside the plane as it transitions from power to cruise to glide. The combination of wing span, prop diameter and weight determine the base settings for choosing the rubber motor thickness and length. The prop size is usually around 1/3 the wingspan. If you have a 16" model generally you would want to use a 5.5" to 6" prop. If that model weighs in at around 20 to 30 grams then you would start with around 2 loops of 3/32 (I say 2 loops of 3/32 instead of 1 loop of 3/16 so you can braid or back-wind the motors together to fix the bunching mentioned earlier). Make it 1.5 times the hook to peg. If this is not enough power then instead of jumping straight up to 2 loops of 1/8, just swap out one of the 3/32 loops for a 1/8 loop. Now you have a motor made of one loop of 3/32 and one loop of 1/8. There is a lot of testing needed to become proficient at this but once you get the feel for it, it becomes second nature.

I hope this helps (Probably just raised more questions, but that's good. That's one of the draws of this hobby)

Steve

[zoomie]

I hope this helps (Probably just raised more questions, but that's good. That's one of the draws of this hobby)

Steve, that's excellent info to know - thanks for sharing it .

Question for you: at what overall length does a rubber motor have to be before braiding it becomes a necessity? 1.5 X P/P? 2 X P/P? I ask because I feel some fliers here won't be making up long motors (as for FAC competition) too often, and would prefer shorter and simpler motors for casual sport flying most of the time.

Thanks,


zoomie

[Steve Blanchard]

Hey Zoomie,

Sorry it took so long to respond, been sick. The braiding becomes necessary when you find that the CG gets thrown off after the motor runs out and begins the glide phase of the flight. It's really all about trimming for all phases of the flight. The best thing to do is get the motor into the plane at the length you are most comfortable with, wind it up and let it run out as is you were flying it but hold it either in the winding stooge or in your hand. Then check the CG. You should notice that if it becomes out of balance it would more than likely be at the tail. Because you cannot be sure this would happen in the same manner every time you would either shorten your motor (but this changes the amount of power as well) or you braid or add back winds OR you can use a spring tensioner at the prop and then you don't need to braid. Anyway, these are all things that are best learned through hands on tutoring at the flying field or viewing and talking about others' models in person so you can really see how all works. I would encourage anyone on this board to find a free flight club in their area even if it's a bit if a drive and spend some time with other modelers. It really cuts down on the mysteries and speeds up the learning process.

Steve

[zoomie]

Steve, thanks very much for your reply, it answers some questions I've had for a while.

Oh yeah...get well soon!


zoomie