Scioly.org

Polar:

We have a column "Rubber Width" in our log sheet, but it is expressed as g/in. The important thing here is that a given "width" as measured in mm or inches may have a different g/in due to density or thickness variations. The stored energy will be proportional to the g/in, not the width.

In this year's case, since the mass is not controlled by rule, perhaps it is less important (in terms of getting usable rubber made), but it is still an important parameter, more so than a dimensional width.

In addition, read the notes on this simple rubber stripper. It cannot strip off a tiny amount, so be sure to get stock rubber widths that you will strip off 25% or more. It should give reliable results when you do that. You may find you need some very thin shims for adjustment.

When stripping rubber, you want to first cut blanks a bit longer than you need, all the same length. Then weigh each blank, and order them by weight (and this will tell you how much your density is varying). Now, with short samples, set up your stripper to give the percentage you want with your heaviest strip. For example, if your goal mass (g/in * length of blank) is 72% of the current blank strip, then weigh some 3" pieces, strip them, weigh again, and determine the percentage. Adjust stripper until the percentage matches your goal percentage. Keep careful data on each 3" sample strip, so you know how much the percentage changes for each shim setting. Strip a LOT of 3" samples!

Now strip your heaviest blank, and weigh it. How close did you get? If needed, make adjustments. If your next blank is same mass as first one, strip it, weigh it, log it.

Now as you get to lighter blanks, you will want to shim out the blade, based on your logs. So on a lighter strip, with the same goal g/in, you will need to cut slightly wider (slightly higher percentage) to get the same final mass.

It is a lot of work up front, but once you get some reliable data amassed, you will find it straightforward to hit your target g/in without wasting too much rubber. After stripping a bunch of samples, you will see what the incremental shim is to change a certain percentage.

Only in America do we have units such as "g/in". You can do g/cm, but then trading data becomes difficult!

Coach Chuck

Coach Chuck,

Ok, I get that the motor width may have variable width which is why it is not reliable. So just to confirm, it is recommended to experiment with different widths, but measure the rubber in terms of length and weight?

Thanks

Yes, I think that is a reasonable way of putting it. It is easily converted to g/in too. The g/in is a characterization of the sliced rubber, while length and weight is the same thing but relative to the whole loop. Both ways of looking at the data may be useful in different aspects of optimization.

To make things simpler for experimentation, you may want to tie the rubber to a consistent mass (say, 2g), and then record the length. Then the length is directly proportional to the "width" in terms of g/in. After trying a set like this, you could move on to another fixed mass (3g?) and again vary the "width", recording length.

The g/in representation is helpful when trying to duplicate your optimum rubber.

Coach Chuck

okay so basically we have one rlly crappy plane and its not meant to be good, it was soley for practice on winding and launching for us freshmen (even though i dont really need that.. but others do so we made a plane for that). anyways, its turning right. it has never turned left. im assuming its because the stabilizer from the back is tilted so the left side is down, but i dont know if thats even the case. maybe its the way we are winding? any suggestions?

Assuming the prop is turning CCW when viewed from the front (which must be the case unless you built a prop and it is backwards), then winding will not cause turn in wrong direction.

Tail should be tilted so right side (facing from the rear) is down. Look also at any yaw of the tail group. Vertical fin may need a little left at the rear. Finally look at the prop hanger, make sure it is straight, not twisted to the right.

If all that is OK, then look at your wing and stab for warps or twists. If left wing leading edge is high, or right wing LE is low, it could cause it to roll right, resulting in a turn right. Same with stab.

Coach Chuck

coachchuckaahs wrote:Assuming the prop is turning CCW when viewed from the front (which must be the case unless you built a prop and it is backwards), then winding will not cause turn in wrong direction.

Tail should be tilted so right side (facing from the rear) is down. Look also at any yaw of the tail group. Vertical fin may need a little left at the rear. Finally look at the prop hanger, make sure it is straight, not twisted to the right.

If all that is OK, then look at your wing and stab for warps or twists. If left wing leading edge is high, or right wing LE is low, it could cause it to roll right, resulting in a turn right. Same with stab.

Coach Chuck

alright cool its the tail tilt thats what i thought. but our plane technically doesnt have to turn left right
i mean we are gonna make it turn left but as a practice plane for freshmen next year we can just use a crappy plane from this year that turns right so i wanna get the same concept to them without having them destroy anything

I have seen one or two turn right over the past few years. However, the torque of the prop wants to naturally turn it left, so when you make it turn right, it is fighting the prop, which may result in more drag and poorer times. But, even at Nationals there have been a few turn right.

Coach Chuck

coachchuckaahs wrote:I have seen one or two turn right over the past few years. However, the torque of the prop wants to naturally turn it left, so when you make it turn right, it is fighting the prop, which may result in more drag and poorer times. But, even at Nationals there have been a few turn right.

Coach Chuck

good to know, thanks

Anyone know if there is such thing as black or grey mylar?

Hi, I'm kinda new to this event and am just reading up on what materials I need for the plane. It says to use a 15:1 ratio winder, isn't that kinda just like a gear box? Would it be OK to make said 15:1 ratio winder from my own supply of gears? Or should I buy a commercially available one?

Thanks,
JavaScriptCoder