kubark42
Well-Known Member
- Joined
- Oct 19, 2020
- Messages
- 90
I'd love some feedback on a design I'm working on. It adapts from the pylon in the self-launching AC-5M. The idea is to replace the draggy plate design with a streamlined pylon. The motors connects to the shaft pulley (not shown) with a belt. This is a pusher configuration.
This design is based on the Large Strut streamlined section from Carlson Aircraft (quick aside: what wonderful people there!) as the pylon. The motor is mounted halfway down and to the side. The overall dimension is about 60cm, with 40cm above the forward reinforcement. The prop is 120cm, and the total pylon mass is around 1.5kg (not counting the propeller, motors, bearings, etc...).
Because it's motor driven, there's very little vibration. It can't possibly be anywhere near as bad as the original two-stroke engine. So the the predominant excitation mode will likely come from the propeller blade sweeping past the pylon, at about 2800-3000RPM.
Structurally, the new design has a higher moment of inertia in the forward direction, so it will at least exceed the strength and stiffness of the original. However, it has a far lower moment of inertia laterally, which means it can potentially bend where the original could not.
The column also won't be as stiff in vertical torsion, which means that there's a possibility of a poorly damped harmonic oscillation. In particular, I have calculated the natural resonance frequency of the propeller/tube assm. and it's 188Hz. A two-bladed prop at 2800RPM is exciting the structure at 93Hz, which is almost exactly half the resonance frequency.
I don't really know how to go about checking for this being a problem, aside from building it and trying it out. And if it doesn't work, there's not much that can be done aside from finding a bigger streamline tube, or just scrapping the design.
I have designed it against the loads given in FAA 23.371 (gyroscopic & aerodynamic), and it passes with at least a safety factor of 1.7. However, there's nothing in the design guide for propeller/pylon oscillation, so that's a big unknown.
Questions:
This design is based on the Large Strut streamlined section from Carlson Aircraft (quick aside: what wonderful people there!) as the pylon. The motor is mounted halfway down and to the side. The overall dimension is about 60cm, with 40cm above the forward reinforcement. The prop is 120cm, and the total pylon mass is around 1.5kg (not counting the propeller, motors, bearings, etc...).
Because it's motor driven, there's very little vibration. It can't possibly be anywhere near as bad as the original two-stroke engine. So the the predominant excitation mode will likely come from the propeller blade sweeping past the pylon, at about 2800-3000RPM.
Structurally, the new design has a higher moment of inertia in the forward direction, so it will at least exceed the strength and stiffness of the original. However, it has a far lower moment of inertia laterally, which means it can potentially bend where the original could not.
The column also won't be as stiff in vertical torsion, which means that there's a possibility of a poorly damped harmonic oscillation. In particular, I have calculated the natural resonance frequency of the propeller/tube assm. and it's 188Hz. A two-bladed prop at 2800RPM is exciting the structure at 93Hz, which is almost exactly half the resonance frequency.
I don't really know how to go about checking for this being a problem, aside from building it and trying it out. And if it doesn't work, there's not much that can be done aside from finding a bigger streamline tube, or just scrapping the design.
I have designed it against the loads given in FAA 23.371 (gyroscopic & aerodynamic), and it passes with at least a safety factor of 1.7. However, there's nothing in the design guide for propeller/pylon oscillation, so that's a big unknown.
Questions:
- Am I overlooking anything of importance?
- Does the column likely need to be stiffer laterally? (By using stays (aka guy wires), it can be made effectively as stiff as the original design.)
- Am I overthinking the possibility of harmonic oscillation about the vertical axis?
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