Moter shaft breaking

[1234]

Hi coach me and my partner liked to build a two blade motor like the hurricane heli but we did not have that kit so we decided to only build two blade for both rotors from the ffm kit. We build a really good middle shaft but we didn’t add carbon fiber to it. We know it gives the shaft extra strength so it doesn’t break easily but we thought it wouldn’t be a big difference and was unnecessary so we didn’t add it in. We try flying it with a 12.5 inch rubber that is 2.3g. We stretched it to 5x its length and wind it for 115 times. The max torque was 1.6 and launch torque was around 1 torque. As soon as we let go of the helicopter the middle shaft couldn’t handle the force of the rubber band and broke in four pieces. My question is if we added carbon fiber would it significantly prevent it from breaking, was it the reason why our middle shaft broke because I do like the idea of making a two blade heli as it would give a 10% bonus.

[coachchuckaahs]

It is not clear what you are calling the "middle shaft". Please review the glossary of terms available at https://www.freeflight.org/science-olym ... resources/

I am going to assume you mean the motor stick.

The motor stick is the most heavily stressed part of the heli, but can also be the heaviest part. Getting a lightweight motor stick that is strong enough is an engineering challenge.

You do not discuss the design you used for your MS. Some kit makers use multiple pieces to build a motor stick from sheets of wood that are sized to easily ship and to utilize laser cutting efficiently. This year the FFM design does that, and they use lightweight wood so that the heli can be built to weight. However, without reinforcement, the wood alone in that configuration is unlikely to survive.

When custom building my preference is to use a single solid piece of wood. If done properly, this can be plenty strong and light. However, choose wrong and it will break like any other configuration. We use 6-8# wood, typically 3/16" thick, and cut the blank 3/8" tall. Several samples may be needed from different parts of several sheets to find one that is stiff enough and not pithy. We then use bracing, usually just on the opposite side from the rubber. This can be kevlar thread, or kevlar fishing line from a local source. Do not try to use monofilament fishing line, it will stretch. Kevlar is dimensionally stable. In a pinch you may use heavy sewing thread. The truss support is typically 1" or so out at the middle of the MS. This truss helps reduce the chance of buckling when the motor is wound tight.

Reinforcement can also be with carbon strips. I have found the truss easier for students to make/install, but it is one more thing to watch out for while flying, as it may be easy to tangle rubber into the truss string.

You can also geometrically reinforce by building engineered shapes such as a rolled tube of thin balsa. However, the straight stick selected from good-grained 3/16" sheet of appropriate weight with a truss has served us well, and is more forgiving than an engineered shape. If needed you can taper the height of the stick toward both ends to save some weight without reducing the buckling resistance.

Coach Chuck

[bjt4888]

Hi coach me and my partner liked to build a two blade motor like the hurricane heli but we did not have that kit so we decided to only build two blade for both rotors from the ffm kit. We build a really good middle shaft but we didn’t add carbon fiber to it. We know it gives the shaft extra strength so it doesn’t break easily but we thought it wouldn’t be a big difference and was unnecessary so we didn’t add it in. We try flying it with a 12.5 inch rubber that is 2.3g. We stretched it to 5x its length and wind it for 115 times. The max torque was 1.6 and launch torque was around 1 torque. As soon as we let go of the helicopter the middle shaft couldn’t handle the force of the rubber band and broke in four pieces. My question is if we added carbon fiber would it significantly prevent it from breaking, was it the reason why our middle shaft broke because I do like the idea of making a two blade heli as it would give a 10% bonus.

Congrats on building and testing a Heli. It sounds like when you mention “middle shaft” you are referring to the motor stick and yes, the carbon on all four sides of the motor stick is absolutely required. If you want some help with this step, watch the NFFS video on carbon application (see Resources).

Coach Brian

[bjt4888]

To clarify, carbon, and truss, is required on the Freedom Flight models fuselage (motor stick) as it is a thin lightweight piece of wood.

Coach Brian

[dbailey]

Speaking of "engineered shapes" has anyone experimented with making a rectangular tube by laminating thin sheets onto a cored out middle piece?

[bjt4888]

Speaking of "engineered shapes" has anyone experimented with making a rectangular tube by laminating thin sheets onto a cored out middle piece?

Interesting question. The balsa “box beam” is a common fuselage construction technique in older generation outdoor free flight airplanes. Box was popular as it is easy to build and relatively strong for its weight. Modern outdoor FF has shifted to the carbon tube fuselage. Spars for the very high speed climbing F1C airplanes are a type of box beam though. They are long thin tapered rectangular rohacell foam cores with woven carbon “sock” vacuum bag laminated all around. These are so strong that a 30” spar only 3/8” square cross section will hold a persons body weight with only the spar ends supported.

Brian T