Any update on this? I’m doing a 700size DDFP heli and I’m rather interested.
Chris Olsson mentioned (indirectly) that it’s difficult to match the thrust AND response time of the traditional VP set-up. So in order to have the best response time, the tail prop should be as small as possible. That is only just big enough to overcome the worst case main rotor/blade torque.
But how big is that torque? We should be able to calculate it, if we know the Lift/drag ratio on the main blades and the weight or max required lift of the application.
I’m not familiar with the heli blade airfoil, and we can’t directly use the usual airfoil tools due to the circular movement (low inner speed, high outer speed), but we may integrate or approximate our way out of that.
Another way may be to calculate the force of the pinion gear to the main gear by current, voltage, efficiency, rpm, and estimated mech losses. And from that calculate required tail rotor thrust.
I’ve attached a screenshot of my 700 heli with the traditional, belt driven tail rotor (light weight, low head speed build)
The total power is around 330W in hover, and most likely some 50-80W are for the tail drive and belt loss, so maybe 250W for the main blades. That equals 2.4Nm (steady hover, 1000rpm), or 280gr of thrust at the end of tail boom (90cm from the main shaft).
This is steady state hover, and we need to add thrust for climb performance, say twice the lift. If the lift/drag is assumed constant (it’s not), the required anti-yaw thrust increases to 580gr, which is still fairly low.
I have used a 2826mm (outside dim) motor with an APC slow fly 8x3.8 prop, that should provide some 700gr of thrust and is still fairly rapid in it’s spin up. Still working on the parameters before testing it out (can’t get the tail motor to spin/arm…)
test with pass-thru tail ESC signal: