Nacelle Redesign for Zero-Torque Static Condition

[StephenCarlson]

At least to offer as an option, the nacelles need to be redesigned so that the thrust axis is aligned with and intersects the axis of rotation, such that no torque is conducted to the servo except for overcoming P-factor and gyroscopic dynamics. The primary tradeoff here is that the hinge pin must be split, unless I displace the axis of rotation to be located under the motor, which has issues of being way too cumbersome and allowing the motor to have a long moment-arm which is itself non-zero static torque. Designs like the Eflite Convergence go this route, and I am not a fan of the aesthetics and structure that results. I like having the motor mass being as close to the axis of rotation as possible.

The reason for wanting to implement this is due to the fact that while 2.0 kg*cm of torque is suppose to be more than sufficient, a failure mode has manifested wherein the servo does not strip or burn out completely, but simply loses dynamic output torque capability. This results in transition from hover-to-forward flight absolutely failing, with the aircraft turning into a spinning and tumbling ball of buzzing propellers. I've crashed two prototypes multiple times due to this, and have had very sketchy transitions where it didn't result in a crash. I'm still researching the issue to be sure it is the servo alone that is the cause, but it seems likely.

The downsides of this implementation would be as follows: First, the hinge pin becomes much more complicated as it becomes two separate pins on either side of the motor. Nacelle width is increased. Getting the axis to rotate smoothly is going to be extra effort. Second, servo loading is reduced but servo failure is now masked, and should a servo completely fail, the failure mode is indeterminate. The previous mechanism fail-safes to hover, but this is only useful if the system is commanded to hover on such a failure, otherwise, should the system remain in forward-flight, it would become a ball of buzzing flopping propellers.

[StephenCarlson]

New Nacelle is almost finished, pending test prints and polishing.

[StephenCarlson]

Repeating an excerpt of some direct messages I've sent to a few folks:

I just finished designing the improved nacelle arms, which have the following features:

• Support for 3x6x2.5mm ball bearings with 3mm shaft. The old 1.8mm metal pin is also available.
• The tilt rotation axis for the motor is very closely aligned with the thrust axis of the motor. The torque effort by the servos should be very much alleviated compared to the old design.

I need to print these myself, and these are very much a "beta", but I want to get these out to everyone that I know is building a MiniHawk-VTOL before they encounter the tilt/yaw issues that I've had when the tilt servos cannot pull hard enough and the vehicle does unwanted behavior. Here is a link to the STLs:

https://drive.google.com/drive/folders/14OMxRNMnFPUckKqzTNQ1_S6gNBs8WnNN?usp=sharing

[StephenCarlson]

The new design has been test flown and is working perfectly. Pushed at 81abe56c47fa66ecf6d3cfdf3e897b6ba21a7ef4.

[StephenCarlson]

Crosslinking to #11; the new nacelle design solves wires getting smashed or fouling the movement.