Scioly.org

Most people under estimate the importance of stiff motor sticks for the rubber flight events. The motor stick needs to be a significant part of your weight budget to avoid these flexing problems. From my experience, 20 to 30% of the total planes weight. This is NOT the place to skimp.

Besides bracing, there are two other ways to get there for about the same weight, but you have to build with this in mind to start.

The easiest is a little counter intuitive.
- Instead of using stronger (and at the same time heavier) wood, use LIGHTER (and weaker) wood, but just a lot more volume wise. If you do a little digging on the internet about the mechanics of bending you'll find an important property of a stick to prevent bending is its moment of inertia. The other important property is the stiffness of the material. They multiply together, higher is better.
- Now here's why its counter intuitive.
-- In the direction of bending, that moment of inertia of a rectangular beam goes up as the THIRD power of the rectangle dimension in that direction.
-- So example.
--- First stick, hard balsa, say 12 lb/cuft, height of stick one arbitrary unit.
--- Second stick, soft balsa, lets say 2/3 the density at 8 lb/cuft. For the same weight stick, you can now have a height of 1.5 units.
--- The second stick has a lower stiffness, in proportion to its density (more or less) of about 2/3. BUT, its moment of inertia as increased by 1.5 cubed or 3.375!! Multiplying the stiffness by the moment of inertia you have doubled the stiffness of the original motor stiffness!!
--- You could take this further and use say 6 lb/cuft wood. If you keep the stick the same weight, it will be 4 times stiffer than the original stick!
- Now that is a one dimensional case, but you can do it in both dimensions if you are careful. Use lighter wood, increase both cross section dimensions to get back to the same weight, and you will have a stiffer stick than your original.

The other alternative is harder to do, but just takes this idea to the extreme.
- Make a hollow motor stick. For a round hollow tube, the stiffness goes up as the FOURTH power of the diameter. With this you can save weight AND increase stiffness over a stick made of solid, heavy balsa.
- As an example, why do you think bicycles are made with hollow tubes? For the same weight of metal, that hollow tube is FAR stiffer than a solid rod.

Jeff Anderson
Livonia, MI

I will second Jeff Anderson's recommendation to use adequate cross section motor sticks. My teams are using the Freedom Flight kit with a few modifications and the supplied motor stick with cross section dimension 1/4"x3/8" and 6 lb/cu ft to 8 lb/cu ft is plenty stiff. We have flown a maximum torque of 1.5 inch ounces when testing the three dime bonus option with no motor stick bending issues. The airplane climbs great even with this high torque. We have observed motor stick twisting issues with torque in this range, which necessitated changes in the left wing washin.

1st place in Texas was 1:24 with 1 dime bonus, fairly low ceiling 20' ish (not good at estimation), Freedom Flight Kit with modifications.

The PA state WS was won with a flight of 2:50. The ceiling is peaked, so the maximum height depends on how tightly you can circle, but this flight was made to an estimated height of about 24 feet. It was a scratch built plane with flat wing and dropped stab, twin endplates angled 15 degrees from vertical on both, and used a repitched flaring Ikara prop. Second place was 1:18 with 2 dimes on board (design not known), which just edged out a FF plane which flew 2:22 to a third place finish. Both the 2nd and 3rd place teams have qualified for the national tournament.

The Michigan State WS 1st place was 2:42. The ceiling height is 21' 3.5'', length 151'11'', with 66'. The 2nd place was 1:50 with one dime bonus (= 2:45). Since the team flying without dime received more wing coloring bonus (162 x 0.1 = 16.2; the one flying with one dime received wing coloring bonus: 110 x 0.1 = 11), so they end up won the gold.

Here I raise a question: Is it really worth it to try flying with dimes when you consider the effect of the 10% wing coloring bonus? The technical difficulty of flying a plane without dime to 3:00 is about the same as flying with 3 dimes up to 1:12 ( both will get 180 sec time). But the 10% non-technical wing coloring ( even a three year old boy can do it) will give the team flying without dime 180 x 0.1 = 18 sec time, but only will give the team flying with 3 dimes 72 x 0.1 = 7.2 sec time. The team who challenges themselves to fly with 3 dimes payload would loss 10.8 sec because of the wing color bonus.

That's an interesting point you bring up and something I hadn't even noticed until now.

I wonder if that was the intention of the rule makers though, as it seems like anyone using the bonus would be at a sizable disadvantage when the rules should be encouraging people to try the bonus. (10 seconds is a lot when you're pushing the limits of the model)

I think I'll submit an FAQ or maybe this would be better as a rules clarification? Hopefully it gets answered before nationals.

DoctaDave wrote:I wonder if that was the intention of the rule makers though, as it seems like anyone using the bonus would be at a sizable disadvantage when the rules should be encouraging people to try the bonus. (10 seconds is a lot when you're pushing the limits of the model)

I think I'll submit an FAQ or maybe this would be better as a rules clarification? Hopefully it gets answered before nationals.

I wouldn't expect anything to change, especially with how the rules work this close to nationals. And this isn't really a problem; a bonus encourages teams to experiment with different setups, but it isn't known whether it is a more competitive approach in all cases. I'll bet the percentage for the bonus was decided using an equation and rounding to nice looking numbers.

I agree with the post from SO2016 above. It would be more fair if the colored panel bonus were to be applied after (and on top of) the dime bonus. I coached four high schools for Wright Stuff this year and agree that the challenge of attaining 3:00 or the one, two or three dime-bonus equivalent of 3:00 is very nearly equal. I make this statement based upon data from about 350 test flights completed this year by my four schools. With the scoring as-is, the colored panel bonus is a substantial penalty when flying with dimes.

SO2016,

Could you please post the remaining results from the Michigan State Championships yesterday if you have them? I am wondering how the Holt HS and Haslett HS teams placed.

Thanks,

Brian T.

Brain,

Holt HS teams placed at 5th, and Haslett HS teams placed at 8th. Detroit Country day: 3rd, Grand Rapid Christian HS: 4th, Troy HS: 6th, Saline HS 7th.

SO2016,

Thanks very much. Sorry, I should have asked this too. Which teams are representing Michigan at the National Championships?

Yours truly,

Brian T.