Rubber Motor: Length or CG

[Mitch]

I have a question for you flyers with Rubber Power. What do you think is more important.

Length of the motor or placement of CG?

I have tried to increase the motor lengths in my models, but some people tell me to keep the center of the motor over the CG of the plane. My first flying Me109 (402) had a longer motor than on the plans. My second one was shorter than plans. I plan to modify my first Me109 and make 3 rear peg points and try to study the differences.

Maybe someone has some thoughts on this matter...

Thanks, Mitch

[David Lewis]

Motor length is a compromise guided by which benefit (long motor run versus low weight) you want more of. If your design naturally has a forward CG with little or no noseweight, then a long motor is likely to work well.

[Wildpig]

.... I plan to modify my first Me109 and make 3 rear peg points and try to study the differences.

Thanks, Mitch

Sounds like a good plan to me.

[Mitch]

That's about what I thought, but I think it will be fun to make an experiment. I have been making changes in every build so far... So I will go back to my original. My Me109 E, I built a year ago. I will have 3 motor mounts and 3 specific motors (for each mount) I have seen planes at contests and at the beginning of there flight they not flying properly. Probably due to the weight shifting as the rubber unwinds. I will wind each motor to the same turns per inch.

So we will have to wait until I have some vacation time. 3 weeks to go. Here is the test model:


I will remove the fuselage cross and add 2 more motor mounts. This model was built very close to Guuillow plans with the exception of moving the peg aft.
This plane will also have an adjustable nose block.

Mitch

PS: I forgot my camera on the ship, but the 1002 Stuka skeleton is finished. Building it for rubber power was much easier than adding all the "extras"... pictures next week. I am confident it will fly to some extent... If it flies well, I will then plan to add the dropping bomb!

[zoomie]

I have a question for you flyers with Rubber Power. What do you think is more important.

Length of the motor or placement of CG?

Mitch, CG is more important .

With your experimental 109, remember that you'll need to recheck CG and retrim as required every time you change from one rear peg/motor to another.

It would be ideal to have the center of the motor at the CG point, but is not always practical. Most scale models turn out to be tail-heavy and require additional weight in the nose to compensate. The farther forward the rear peg is located, the less nose weight will be needed.

A simple way to calculate rear peg location is to measure distance between prop hook and estimated CG. This distance X 2 for the peg puts the CG at the center of the motor, with the peg just behind the TE (usually). This can work well with short-nosed models as it helps to keep motor weight away from the tail. The same hook to CG distance X 3 puts the peg farther back and is often preferable for longer motors. I like to move the location just a bit forward to the first upright or fuse. former that I come to for a strong anchoring point.

With P.H. to CG X 3, the peg location often ends up at the second upright/former from the tailpost. If you check some different plans you'll find a lot of them locate the peg there as well.

There is a formula for peg location that is more precise than what I use, maybe someone will post it.


zoomie

[davidchoate]

I found the Cg affects the way it flies, and a longer motor does no good if the plane hits the ground before it unwinds.Mitc I was wondering when you would fly it. I like your details on your flights. They are helping me get better with my FF p[lanes.

[davidchoate]

I have a javelin. I like it because its easy to adjust things. My fist javelin flew well. I added a double loop motor and it eventually broke in half. I remade a fuselage and added strength. Its not going to break, but Its a struggle for it to climb now due to the weight gain. I am not going to make a3rd versin. I got another kit from Guillows very similar to the Javelin. I plan to make some modifications to it from the plan from what ive learned first hand.Its called a Lancer. I may try one more thing , and thats too make a flat hor. stab. instead of the airfoiled one, and increase the wing area abit. If this is a futile mistake, or an advantage I dont knw.If anyone has done this I am interested in your results.

[Steve Blanchard]

Mitch,

CG is the more important element for stable flight. Motor length can be increased to the point where you either have more weight in rubber than is logical or if you run out of room (surrounding room is usually the only factor in this, length no so much). Move the rear peg to a point that gives you the best compromise for nose weight and motor length. Remember that as you increase the length of your motor the power drops. If you start with a length that is 2X the length of the hook to peg and increase to say 5X (completely doable), then you will need to more than likely increase the cross section of the motor as well. This adds weight and may or may not put you over on the value of the longer motor. Keep in mind that you can increase the cross section by adding a loop of a smaller cross section motor as opposed to just doubling what you have or adding another loop of the same size. Many of my dime scale models run on 1 loop of 3/32" combined with 1 loop of 1/16". To get to the 5X the length you will need to have the room inside for it as well as a rear peg with a sleeve over it keep the rubber from binding up in the back. In short, CG is more important and the rest is a balancing act between weight and power.

Steve

[davidchoate]

I thought about combining the blue and the gray motors, but shyed away from the idea because of all the negatives heard about the blue motors. I also encounter many knotting/binding problems. Not only at the rear, but the whole length. At times.Usually after several flight attempts. I have been taking this as a sign to replace my motor. And also when I begin to get a severe amount of stretching, I wiil replace it. I ( from my experiences) have found that not long after any of these two symptoms appear that motor failure is expected. And it has been catastrophic at times to the models.

[Steve Blanchard]

David,

I'm not talking about the Blue and gray motors. Throw those out as soon as you open the kit box! They will only frustrate you and destroy your plane eventually. I was speaking about Tan rubber, either Tan II (if you have any) or Tan Super Sport.

Steve

[Mitch]

Thanks for all the input. I will be home in a few days and hope to get in some trim flights. I will do some experimenting with this plane as I believe it will be a good flyer and was built to Guillow's instructions. I will take what I feel is the best and have that as my new starting point for models.

I believe the Guillow's plans are a good starting point for the schoolyard flyer, but when you get into competition there is a new standard... Too be continued, Mitch

[davidchoate]

I bought some Rubber motor from my LHS. It is Guillows though. I am assuming its the same as in the kit? I am new to FF more or less, but have learned alot already. Where can I order tan II? I have some kits from other manuf. that say tan II included, but if I use it then I dont have it for that model. I notice that I get about 10 flights from the Guillows rubber before breakage, and yes, I have had catastrophic results. My Javelin snapped in half when the motor broke,. Why doesnt Guillows provide a quality rubber motor? I love their kits, but they are not any cheaper than any other co. who makes them.

[Steve Blanchard]

David,

Sadly Tan II is no longer being made. The rubber that was used to make it was actually used to make golf balls. Now that Golf balls are no longer using it, the manufacturer saw no purpose in continuing it's production. We have always been a cast-off type hobby, forever borrowing from others. Tan Super Sport is the new rubber of choice. It is on sale right now at A to Z (Closing) for a really good discount. You can also order it online at other web stores like Volare products (Shorty's Basement) and I believe that Easy Built Models sells rubber as well. Just Google Tan Super Sport and you should be able to find it. The most useful sizes are:

1/8"
3/32"
3/16"
If you can, you should also get some 1/16". It works great mixing and matching for the perfect power package on some of your models. I hope this helps.

Steve

[davidchoate]

Thanks. Yes I have heard of A to Z, and their going out of business. . I am going to see what I can get from them. Also I hear they got kits for dirt cheap.

[Mitch]

Thanks again for ALL the input. I figured that was the answer. Half the reason I posted this question was to get some interaction. I still plan to do some testing. I feel like this is a HS science experiment, when I know what the answer should be, but I will still go though the process of the experiment to see the results first hand and really learn some more on how the placement of the rubber affects the model in flight!

I plan to use 2 loops (4 strands) of 2x hook to peg and start with 2 motors. One shorter and one longer then Guillow Plans. If I notice differences, I will go with the third motor in the position that Guillow plans show. I plan to fly with a Peck 8" prop.

Mitch