Originally Posted by
OldLurker
Maybe this doesn’t make sense, but here goes: one thing that puzzles me a little, perhaps only because of my own ignorance, is that for pushing air
downwards (ie VTOL) development efforts have focused almost entirely on what are effectively propellers – one or two big unshielded rotating wings, or sometimes lots of small ones – whereas for pushing air
backwards (runway takeoff), jet propulsion is increasingly the norm, on smaller and smaller aircraft, now down to the VLJ category.
Of course the military have played with jet propulsion to push air downwards, with vectored thrust as in the Harrier, or with lift jets or fans, but
AFAIK they’re few, and vectored-thrust has mostly gone away from VTOL towards improving manoeuvrability of fighters.
There's a couple of key differences between lifting an aircraft vertically with "blowing air" and pushing it forward via "blowing air". Lifting an aircraft requires generating enough thrust to completely lift the aircraft and then add control margin on top of that. Pushing an aircraft through the air just requires overcoming the drag of the airframe and wings. So the lift demands are an order of magnitude higher for VTOL flight. There is a relationship between thrust/disc area and the ability to turn horsepower into thrust. Low disc loadings (large area for a given thrust) is far more efficient than high disc loadings at turning horsepower into thrust. Classic example diagram below:
So using jets, etc. to lift the aircraft requires FAR more power than using a large rotor does.
Another key difference is the speed of the air entering the rotor/fan/jet... In a helicopter, the velocity of the air flowing in "inflow" is relatively low. In an axial flow fan or jet, it's quite high. This affects the design each system and it's hard to optimize the same thruster for both near zero inflow and very fast inflow. There's plenty more to it, but these are some big ones.
This discussion is basically the genesis of the tilt rotor: large enough rotors to be near the helicopter end of the hover efficiency plot and rotors that are too large to be optimal propellers, but good enough. The somewhat undersized rotors mean hovering requires more power installed than a normal helicopter, but that power then enables much higher cruise speeds as an airplane.
The intro sections of the NASA XV-15 monograph discuss a lot of attempts at working around the physics of the above chart. Worth a read to those interested.
https://history.nasa.gov/monograph17.pdf