So, the single bladed propeller is ~ 15% more efficient than the two bladed propeller. It swings 'round without the bother of an additional blade creating messy vorticies ahead of it. Fitting single bladed props to a quadcopter drone has been done, a simple google will demonstrate this. Single bladed props deployed on a multicopter drone already have advantages. They will pack away nicely, by folding the only blade inward and there's half the chance of damage.
Now, what if we push it further? Moving the motors inward. The whole machine becomes much more compact, but the propellers are now within range of interference with one another, the rotors COULD be separated vertically, but that's boring, and they're operating in downwash. The precise control afforded by brushless motors means that with adequate computational power (which is light, cheap and small nowadays) even though control inputs demand different speeds from each rotor, the whole set of four (6-8) can be throttled independently, while subtle adjustments ensure they never run into one another. Now, the whole thing can be smaller. Combine the single bladed propeller with the bs0 intrinsically unbalanced brushless motor <link> and you save weight.-- bs0u0155, Jan 05 2016 Intrinsically Unbalanced Brushless Motor http://www.halfbakery.com/user/bs0u0155 [bs0u0155, Jan 05 2016] assumptions busted https://www.youtube...watch?v=CmhwROMBK8E [pashute, Jan 07 2016] Underactuated swashplateless cyclic pitch helicopter rotor https://www.youtube...watch?v=aEPf0QHVuMMAs mentioned in my anno. [notexactly, Jan 11 2016] By moving them in, you lose the control leverage and you also lose the lack of turbulent interference.-- pocmloc, Jan 05 2016 How are they throttled independently yet manage to not contact eachother if operating in radial interference?-- RayfordSteele, Jan 05 2016 //How are they throttled independently yet manage to not contact eachother if operating in radial interference?//
Well, brushless motors, or stepper motors as they used to be known, can relay exactly where they are in their arc. The motor controller can also command exactly the degree and speed of rotation (given enough perimeter magnets). So, say rotor 1 needs 1000 rpm and rotor 2 needs 1200 rpm, the motor controller just needs to organize rotor 2 so that it is in front of rotor 1 at the beginning of the interference zone. All you need to do is speed up one rotor and slow the other for PART of its arc and you can get the order right going into the interference zone. Then the motor controller has the other, say 300 degrees, to modify the rotor speed so that the AVERAGE rpm matches the commands for each rotor.
If one rotor needs to average 1200 rpm, then it can pulse to 1300 for 5 degrees or so to ensure clearence, then slow down to 1195 rpm for the rest. With 4 rotors all sharing the same space, it's a MUCH harder problem. But it's just an on-line computational problem that's right up a microprocessor's alley.-- bs0u0155, Jan 05 2016 //By moving them in, you lose the control leverage and you also lose the lack of turbulent interference.//
An empty Boeing has way more control leverage than, say Concorde of the same weight, but keeping the mass centralized gives better roll rates, for example. Anyhow, why don't we make the booms telescopic, there, now it can be a regular single bladed prop drone, then tuck in for that small window.-- bs0u0155, Jan 05 2016 //single bladed...more efficient
So, remove the single blade and get maximum efficiency.-- not_morrison_rm, Jan 05 2016 //All you need to do is speed up one rotor and slow the other for PART of its arc//
But you're doing that several hundred times per second (or at least several tens of times per second). That's going to put a much greater load on the motors, and therefore increase power consumption, no? Even with energy recovery, I think you'd still have problems.-- MaxwellBuchanan, Jan 05 2016 Ok, so it looks like RPM is in the 10-50,000 RPM range.
So, 50k is ~ 600/second. Make it easy, lets have 3 pairs of magnets for 6 steps. That's 3600 switches per second. This is ~250uS for computation and switching time. Now, power FETs switch in the 2-5us range, so no problem there. CPU clock speeds mean nano seconds per cycle, so no problem there.
//That's going to put a much greater load on the motors, and therefore increase power consumption, no?//
err... working that out is above my pay grade.-- bs0u0155, Jan 05 2016 And it's below mine. Who can we ask?-- MaxwellBuchanan, Jan 05 2016 //tuck in for that small window// good point. All the blades could be briefly stopped in the inward-pointing position for ballistic entry of even smaller windows.-- pocmloc, Jan 05 2016 Or rotate through perhaps 30º, then feathering for the return.-- pocmloc, Jan 05 2016 //That's going to put a much greater load on the motors, and therefore increase power consumption, no?//
Yes, but maybe even worse is that the motor will need to be much more powerful (larger and heavier). Normally the prop is accelerated to full speed over many revolutions. To have the slightest hope of this working, you'd need your prop to be super light weight, but it will also need to be stronger than a normal prop to handle the rapid acceleration.-- scad mientist, Jan 05 2016 //rotate through perhaps 30º, then feathering for the return.//
Hmm, if you had sophisticated blade angle control, hard to do with something weighing a few grams and spinning that fast, but you briefly change the angle of two flexible blades. One gets a coarser pitch and flexes up, the other briefly reverses and flexes down, missing each other. You could do this mechanically with a cam, to occur at the same place in every rotation... that is over the drone body where any prop downdraft is wasted anyway.-- bs0u0155, Jan 05 2016 // Or rotate through perhaps 30º, then feathering for the return. //
That's an oscillatory motion that's not coupled to a rotary or other continuous motion (apart from the overall motion of air), meaning high power consumption.
The oscillatory component of [bs0]'s idea will still result in higher power consumption, but not much higher than a constant-speed rotor, because most of the thrust comes from the constant-speed component.
[scad mientist], this oscillatory rotary motion superimposed on continuous rotary motion has been done, for a slightly different helicopter rotor purpose: [link]. Acceleration from zero to full speed within one rotation shouldn't be necessary; just sync the blades the same as during full-speed operation, right?-- notexactly, Jan 11 2016 [notexactly] That MAV is pretty cool, I wonder if they could do the same thing with single bladed props and an increase in RPM to compensate for thrust loss? Folding the blades back would make it extremely small.. for [pocmloc]'s ballistic entry point. Would be an extremely capable way of getting a grenade exactly where someone doesn't want it.-- bs0u0155, Feb 05 2016 random, halfbakery