Originally Posted by Keyrigger
I have seen one picture of a Vario Airwolf with two mini thrust motors installed. If you were to gain enough thrust to act on the heli to increase its flying speed, it would eventually fall out of the sky because of blade procession/recession. Basically, you would fly faster than the surface area of the rotor blades could generate lift and it would only generate it on the side that rotates forward. They will look cool but don't set your hopes up too high. Nice to have the room to work on such a large heli. Take care.
While this is a real world hazard called "Dissymmetry of lift", I doubt scale RCs are likely to suffer from this. RC Helis are so overpowered underweight that many hazards that full size helicopters have to be cautious of don't apply to RC. The rotor disc load just isn't anything close to full scale helicopters.
I'll spare the calculation, but at 1500 rpm, an 800 size heli as a blade tip speed of over 300 mph. Let's say you have the heli traveling at 100 mph, using the EDF thrusters, with only a small amount of blade pitch (1 degree), the advancing blade side would be traveling at 400 (300+100) mph, and the retreating side at 200 (300-100) mph (relative to the wind). So yes, the advancing side is producing much more lift. BUT the heli still has another 12 degrees or more of pitch it can use to properly compensate for blade speed, and what is even better, a FBL controller will automatically correct for this when it senses the heli want to to roll to the retreating blade side. Actually because of the 90 degree precession inherent in spinning masses (the rotor), the heli will want to roll to the retreating blade side and also pitch up.
As you lower the headspeed, and put on weight (scale fuselage), you will increase the risk of retreating blade stall, but I am completely confident that 100 mph is a safe speed for 800 size scale birds. In fact, without doing the calculations, I would even estimate that 150 mph would be just fine.