Just curious really. I have no desire to get a drone. Though a helicopter would be nice. I would sell it and retire early.
Achieving high altitude is a problem with the rotary wing. Air density is not enough to allow it to fly. But assume the drone will only fly at high altitudes, couldn’t the rotor blades be made bigger, wider or longer? To get more of a bite on the low density air?
There would be more mass to move with a bigger rotor, but most of the energy used is to push through air creating downward thrust. If the air is thinner, then a larger rotor should not require more energy to spin than a smaller rotor in denser air.
Moving a bigger mass requires a more powerful engine. More powerful engines require more fuel. Both result in more weight. More weight requires a more powerful engines, which requires more fuel…etc. The opposite would be a glider, which can soar for hours with 0 fuel on board. Helicopters are beneficial for many reasons, but high altitude and duration are two areas that fixed wing drones excel.
To take an extreme example, some NASA engineers are working on a drone that could work on Mars. There, gravity is moderately lower at about 3/8 * g, but air pressure is much lower at less than 0.01 Earth’s atmosphere. The link doesn’t go into the details about the aerodynamics, but generally it looks like the prototype Martian drone has huge rotors that spin at relatively high speeds.
I’m guessing that, if operated at 1 atm, the rotors would be so draggy that they would barely spin (if the motors didn’t simply stall or burn out).
I’m talking about a ‘drone’ that does not need to take off at low altitude and attain great height.
One that is designed for height and never goes to low altitude.
Just one that operates at altitude. So. Yes the rotors would be bigger, or wider or dig deeper. But since there is less air density this should not take any more work on the part of the power plant. Or very little. Downward thrust, is downward thrust.
You can design a helicopter to work at high altitudes. It will require more wing area which means longer or more rotor blades, and a bigger power plant. That means a fuel will be a larger percentage of the gross weight, and payload will be a smaller percentage. If you’re talking about a combustion engine vs. batteries you’ll want a turbine engine will be better at maintaining efficiency at high altitudes. If you’re talking about batteries you’re pushing the limits of sustainable flight.
More area, more drag. But that’s in comparison to the same helicopter at a lower altitude. It may be possible to use air foils with less drag at that altitude because he is considering that the aircraft never has to operate at lower altitudes.
There’s also the issue that the speed of sound decreases with altitude, so simply revving up will stop paying off when the rotor blade tips start to go supersonic sucking up efficiency.
Up to a point you could increase area, pitch or both, although pitch will also reach a point were efficiency goes south as the airfoil begins to stall; and that would be probably sooner than later.
So you are left with increasing area as the best option, that can be done either by keeping the same diameter and increasing the chord or number of rotor blades or by making them longer (well, you could combine both things) As the aspect ratio, the ratio between the wingspan and the chord, decreases efficiency again drops, also the more blades you add again efficiency drops. Then of course as you make the blades longer the tips move faster so you are going to get into the first problem, breaking the sound barrier at the tip, sooner.
As you can see you have a bag of tricks to play with, but as altitude increases the tricks stop working and at some point you just run out of them.
FWIW, I’ve flown a drone at almost 4500 meters of altitude in the same configuration as I flew it at sea level. It felt like the proverbial lead sled.