I am looking at various designs and see a few different types of arms being used:
Flat cut carbon fiber sticks that join in the middle of the frame, sandwiched by CF plates and screws through them; used in FPV drones up to a quite large size (13+ in), at 10in and above they start coming with braces at the midpoint to rigidify them (see the MK4V2)
Carbon fiber hollow tubes that end on the edge of the frame, held by mounting brackets sandwiched between CF plates, sometimes folding; usually seen in bigger Ardupilot drones 450mm and up to massive sizes.
Less common, plastic arms of varied composition and configurations (hollow, trusses, etc) that if we exclude cheap toys are usually hinged for transportable low-performance camera drones like the Mavics.
I am wondering what are the pros and cons of each; which offers the best vibration dampening, strength, weight, cost; and so on. Why are flat CF arms prevalent in FPV drones while larger drones prefer tubes? What are the compromises?
Flatstock arms are fairly easy to make in one piece using water jet cutting. At smaller sizes increase in weight isn’t significant enough to justify switching to small and expensive tubes. They are also more durable making them ideal for crash prone applications like freestyle or racing.
Tubes provide the best strength and stiffness per unit mass short of custom variable cross-section arms made out of CF. They add a bit of compelxity as you have to align motor mounts to prevent yaw imbalance.
Injection moulded arms are good for mass manufacturing and facilitating complex folding mechanism like in folding DJI drones.
On the forum there is an example of a sub 250g quad that uses tubes for its frame as it is optimized for flightime.
Thanks. I did know that tubes are structurally far stronger than flat bars, but an issue is that the weakest link then becomes how they are mounted to a frame. The weight of the mounting bracket required to match the tube’s strength must go up quite quickly, so i was wondering how you start to figure out what the point is that tubes become preferable