DavidS wrote:
kwlf wrote: Rotorcraft and quadcopter drones typically deal with wind much better than fixed wing aircraft of comparable size.I am guessing they have no tail rotors?
No, some have tail rotors, but model helicopters are typically aerobatic and able to fly sideways at speed, which are not things that full-size helicopters are optimised for. They also have good mast clearance.
When I said they were good in the wind, I was thinking more of wind strength and gustiness rather than crosswind landings, which I’ve never thought to try. I did learn that it’s advisable to land full-size helicopters with a slight crosswind, chosen such that it lifts the blades over the boom as they slow down and start to flap.
Peter wrote:
I think the “lots of little engines” must be primarily an investor magnet operation, because the small prop efficiency is poor, the noise level is high, etc.
Usually the mantra is that it’s more efficient to generate thrust by accelerating large volumes of air a little, than by accelerating small volumes of air a lot. So for a given power and a single engine it is more efficient to generate static thrust using larger propellers/rotors. But if you can split the engine without losing engine efficiency, as you can with electric propulsion, you can also move large volumes of air by using multiple smaller propellers… provided that said propellers are not less efficient than the larger ones.
Googling “propeller efficiency Reynolds”, it seems that model aircraft propellers have efficiencies up to 65% for an 11" propeller (Brandt and Selig 2011) whereas our propellers are likely to be 80-90% efficient which I don’t believe improves significantly if you make them larger again. My single-seater aircraft takes 54" to 56" propellers. Looking at large electric aircraft: the EHang 184 drone has 8 × 1.5M propellers (59") and the Blackfly has 8 × 36″ propellers. The NASA X57 has lots of very small propellers, but these fold out of the way in the cruise, in which it uses two larger propellers.
So the question seems to be, “why do they use a lot of smaller propellers rather than one big propeller (or rotor)”, rather than “why do they use lots of small propellers” – because typically the propellers they are using are not all that small in aerodynamic terms. One answer might be redundancy. Another might be that they can blow air over the whole wing. Another might be simply that they are more compact and you can tilt or otherwise fit them round the airframe more easily. Another important one is that if you are depending on differential thrust for control, smaller propellers have less inertia and can change rpm faster.
I think I’m right in saying that the high frequencies produced by multiple small rotors operating out of phase, carry less well than the whump-whump-whump you get with large helicopter rotors.
I thought the advantage was that the many small propellors provided airflow over the wing greater than what would be available from the aircraft’s airspeed.
That’s true for some, but not all of them have wings.
Peter wrote:
But, aren’t you basically asking for the same discussion but without anyone questioning anything?
Not at all. It’s just that the topic is such that it is easy for everyone to come up with “cheap” points and the discussion simply weir off into that direction. And not just once, it will do that over and over again. There will always be someone new coming in to the thread who just has to point out that the energy density of current batteries makes electric airplanes doomed for instance, and 100 more who just have to explain in great detail how much they agree. I simply want to move away from such “discussions” as they are 100% uninteresting.
OK, but then we may as well debate the virtues of different types of unicorns.
According to that link, a unicorn can be captured only by a virgin, which is why they are incredibly rare nowadays, and my prediction is that Tinder will cause them to disappear completely.
What I find 100% uninteresting, in discussions about electric aeroplanes, is constant links, as though it were newsworthy, to the latest groups of venture capital harvesters who’ve just announced their prototype which could become a product when battery technology….
The Rolls Royce Accel makes it’s first flight
500+ hp and 300 mph
Hmm. Just looked up Archer Aviation (mentioned in th AOPA article), since they’re at my old base in Palo Alto. Seems very significant to me that the “leadership team” doesn’t include a CTO or VP-Engineering, or anything like that. It does include a fresh-from-central-casting “Chief People Officer”, and the two names at the top are both from a VC/investor background.
There MAY be some genuine attempts to build something that will fly and make money. But a lot more which are designed to separate investors from their $$$. Joby has raised $1.6B! But there again Theranos raised nearly $1B without ever having the slightest crumb of plausible technology.
Airbus with a hybrid helicopter. They call it an engine back-up system.
