Postby jander14indoor » April 21st, 2016, 12:48 pm
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