MR Arms. Rectangle CF, Aluminum, or 25mm CF Round Tubes for AUW 18-20lbs

[jfro]

Contemplating building a hex or x8 in the 850-900mm range with AUW of 16-20lbs. Most likely using 3515 400kv or similar sized motors w/ 14" or 15" props.

I'd like to get opinions on pro's and cons of what make a stong frame with the least amount of potential vibrations as far as the construction of the arms go.

On my current x8 with 21.5mm arms, I have had to spend a fair amount of time leveling the motors on the round arms. If I overtighten them, the ends of the tubes will crush/ crack. Consequently it seems to me that the rectangle arm, either carbon or aluminum would be easier to keep the motors level.

As to strength, I have no idea what is stronger.

Thanks in advance for any advice.

 

[jfro]

Was this a dumb question?

 

[maxwelltub]

No good question. I believe it's better aerodynamics. Carbon core uses this design and works great.

 

[jes1111]

In terms of aerodynamics a square tube is the worst followed by a round tube. The best would be a tear-drop shape. The arm not only disturbs the airflow but also is a source of vibration - as the blade passes over it there is a "beat" effect (which is why vibration frequency is higher when using a tri-blade prop). Tear-drop shape would be expensive to produce, so round tube is the best compromise. If you are crushing round CF tube then it is probably unidirectional core with maybe one layer of decorative weave on the outside - proper woven or wound tubing will do better - or make a plug in plastic/wood/whatever so the clamping doesn't distort the tubing.

 

[jfro]

jes1111 I've leaned to not tighten the round arms down so much. However, this means I have to check alignment more often on the x8.

I've thought about plugging the ends, but I run my 3 motor wires through the tube and on an x8, that's 6 wires. Already difficult enough running wires through without dis-assembling the frame. Plugs would probably make that impossible.

I have noticed both Carbon Core rectangle arms and UAV aluminum rectangle arms are well received by their users. Maybe the disturbance isn't significant as I see some good video footage off both.

I'm currently mulling over putting the 3515 400kv motors on my 21.5 arms or
1. switching to 25mm round cf tubes.
2. Switching to carbon cores rectangle arms
3. Switching to aluminum rectangle arms....

Anybody know if these are "unidirectional core with maybe one layer of decorative weave on the outside" or are they the right type ?
http://www.quadframe.us/collections/aluminum-arms/products/25mm-carbon-tubes-arms

 

[gtranquilla]

Any mechanical or structural engineer will tell you that the tube configuration is recognized as the optimum configuration to maximize strength to weight ratio of the motor arms. While a tear shaped tube will create less drag in it optimum orientation to wind, your MR orientation changes which makes it impractical. The square tube has no real advantage over the cylindrical tube when using the following method.

First, scribe a straight line the full length along the outside of the motor arm cylindrical tube using a right angle tool such as a section of aluminum right angle from your local hardware store. This ensures an absolutely straight line that will be parallel to the cylinder center axis. (an old Estes model rocketry construction trick from the 60's where they used a door frame or a cupboard drawer edge). The remaining alignments at the hub and motor ends should be simple to figure out after this first step.

 

[jfro]

I do the alignment a little different. I use a small level on the side (vertical) of the two baseplate edges for the top/bottom motors.

The scribe line would probably work, but I don't really want to put a score in the arms... Maybe a ink line on both ends, but for now, I just take a small level with me and make sure the x8 is level, then check the motors....

 

[HoverRound]

I'm curious if anyone is using much smaller diameter solid CF rods instead of larger hollow tubes. I understand that there might be concerns about where to run wiring but I would think it could be easily adhered to the bottom of a 0.25" rod. What are people's thoughts on this?

 

[maxwelltub]

I didn't clarify in my first post. Carbon core uses a rounded rectangular tube. So similar to a tear drop but does't taper. alignment is not an issue on these arms, and I believe they are better at handling prop wash then a round tube.

 

[jfro]

I didn't clarify in my first post. Carbon core uses a rounded rectangular tube. So similar to a tear drop but does't taper. alignment is not an issue on these arms, and I believe they are better at handling prop wash then a round tube.

I have been contemplating the CarbonCore frame, or at least the arms as I like the idea of the rounded rectangle shape. My other thought is the aluminum arms from Vulcan UAV or their complete frame. Either of these arms will keep the motors on plane in regards to alignment. I'm guessing the Carbon core are always in alignment as they won't bend, just break, while it's possible the aluminum rectangle arms could get bent a bit. However, with the cost of aluminum arms, it wouldn't be a big issue to swap out an occasional bent arm.