The NOTAR could hardly be called jet exhaust. It has a fan that delivers airflow that directs air for the easier torgue-wise yaw control, and to drive coanda effect vents on the side of the tail for anti-torque control. The fan could be considered a rotor in the same sense as a tail rotor is. But as far as vertical lift rotors, the vast majority of all helicopters only have one.
At first guess, it appears that the cost of multiple rotors would be prohibitive. It would require multiple engines, or multiple transmission links, either of which would be expensive and add weight to the craft. These are just wags though.
The one I was thinking of used turbine exhaust ducted through a directible nozzle - similar to this(although the one I saw was a more modern military prototype, I think)
Ok, those were pretty cool, and more designs like that have been tried over time. Not exactly what would usually be called a jet exhaust, but in some designs there probably is an actual nozzle using hot exhaust to provide propulsive force, so it would qualify in that sense.
Another novel use of turbine exhaust was the Djinn that use the turbine output to drive tip jets on the main rotor. This minimized the torquing effect from the main rotor requiring less power to be directed to the tail rotor, and increasing the stability of the craft. Unfortunately it’s an efficient means of driving the main rotor. The US military tried similar approaches.
Yeah - terminology slippage on my part, a bit - although if it’s a turbine engine with a nozzled exhaust providing thrust (even though the primary purpose of the engine is driving the rotors), I’m not going to lose sleep over having called that a jet.
Models do it with an electric motor running each rotor. Due the small size, these can run at fairly high rpm without the rotor tips going supersonic. The high rpm allows good power density in the motors which drive the rotors directly. Large rotors would require large, heavy, high torque motors or heavy gear reductions.
Then there are the batteries. A few minutes duration is acceptable in a model…not in something lives depend on.
So, basically, electric power is out for a full scale helicopter. You want light and reliable, so that probably means turbines. Turbines don’t change speed very quickly. This means you need collective control on all rotors so that they can run constant speed…and that means heavy bearings and linkages at each hub.
Next is the fact that these become unstable unless ALL the motors and speed controls are working correctly. Unlike multiple engine fixed wing aircraft, this means each additional power plant increases the odds that one will fail and cause a crash. The V-22 Osprey links the two rotors in case of single engine failure, but this is a heavy and complex part of the design.
Helicopters have to have a lot of heavy parts just to work at all, so anything that adds additional weight or complexity is not going to be favored by designers.
For what it’s worth, here’s the first manned multicopter (or so they claim). But nowhere near “full size” by the OP’s criteria.
You’d think he’d have worn a crash helmet that actually fitted :rolleyes: