Just wanted to share some insight on the importance of allowing blade flapping for dual heli configurations (i.e. tandem (chinook) and side-by-side (V-22 like)) . Dual heli configurations generate yaw moments by tilting one rotor in one direction and the other in the opposite direction to use the thrust vector horizontal component from each rotor to make a moment about the yaw axis. So for the chinook, differential lateral cyclic is used to yaw the aircraft.
However many RC rotor systems are very stiff in flapping. Most of the 2 bladed hubs have a stiff rubber like collar around the spindle that keeps the blades from flapping and provides agile response characteristics. This is counter productive for dual heli’s. Dual heli designs rely on the blade flapping to tilt the thrust vector and create the moment for yaw motion.
As many of you may know, @picoflug has a couple of tandems that he flies with ardupilot firmware. I just found out that he designed and had a custom hub built for his tandems (shown below)
As you can see, it has flapping hinges. This type of rotor hub will do much better in a dual heli as it is more effective in letting the blades flap and tilting the thrust vector. There should be much better yaw control with this hub. It will also put less stress on the airframe from rotors trying to twist the fuselage when yaw inputs are made.
Bill is absolutely right, the blades need to be able to flap up and down. (For this) soft blades are even better.
You can’t fly a tandem with a stiff head and hard blades.
The problem is the different servo deflections.
For nick (pitch) you need very small ways, for yaw very large.
I have already helped many tandem pilots with this, most of them have far too little deflection at yaw.
Soft blades are hard to get, on one of my tandems I have wooden blades.
VARIO’s Chinook tandem has an overall soft rotor head, and they have soft blades.
very interesting, I would advice this brand (maybe you already know them) when looking at “softer” blades: https://derblattschmied.com/
Top quality finishing and balancing, lightweight and very flexible span-wise. Still very stiff on torsional axis.
We use them with positive results from 600 class to 2.5m rotor diameter.
Hi, this 3-Blade-head (and the former too) from SAB are used by many tandem-pilots. It is good, you schould use the soften red rubber dampener. I use them on my smal tandem.
@MamboJambo From what I understand of intermeshing configurations, there is a requirement for both differential cyclic and differential collective to control yaw. @PittRBM could give you his experience with that. So if the configuration uses Differential cyclic then yes, more blade flapping would be beneficial to providing yaw control.
