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Registered Member #3414
Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245
BigBad wrote ...
So far as I can tell, the maximum efficiency you can get is mostly to do with disc loading, everything else is an also-ran.
This is exactly what I've been repeating all the way through all these related threads, although the terminology I've been using is cross section/cicumferance ratio. The losses are all at the periphery. Larger prop means lower velocity of accelerated air 'disc loading' is just 'different terminology'.
BigBad wrote ...
Frankly if you make it bigger, so you have to disassemble it a bit to put it in a car, you should still do that.
Yep.
BigBad wrote ...
I mean, sure if you've got the wrong props on your bird, or too much or too little battery, or the motor is inefficient, then you'll get substandard hang-time, but once those are within reason in the correct proportions, the only thing that will give you more hang-time is reducing the disc loading, and it's the one thing you can always improve by making the props bigger, and rebalancing everything.
Exactly.
Patrick wrote ...
at this point, im wondering if a contra-rotating/coaxial is the way to go. this way i have the largest external dimension being the rotor disc.
Registered Member #2529
Joined: Thu Dec 10 2009, 02:43AM
Location:
Posts: 600
Patrick wrote ...
at this point, im wondering if a contra-rotating/coaxial is the way to go. this way i have the largest external dimension being the rotor disc.
Yes, although you can get the same disc area with a quad in the same external dimension, but the structure to rigidly hold the rotors in their relative positions may be slightly heavier, but it's mechanically much simpler; and you'd get much the same hang-time.
I mean 4 discs of half the diameter have the same area and width as one disk of the full diameter; so it makes no difference in area.
For example this guy gets an hour from his quadcopter:
Registered Member #3414
Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245
BigBad wrote ...
I mean 4 discs of half the diameter have the same area and width as one disk of the full diameter; so it makes no difference in area.
But, four discs of half the diameter has twice the circumferance in total, therefore the peripheral losses are twice as great as one disc with the same area, and the peripheral losses dominate virtually to the exclusion of all other losses, all other factors being equal.
(If the four rotors are close enough together, though, losses won't actually be twice, but if they are far enough apart not to 'interfere' with each other they will be.)
Se my earlier posts. Peripheral losses are the dominant factor here.
Registered Member #2529
Joined: Thu Dec 10 2009, 02:43AM
Location:
Posts: 600
Ash Small wrote ...
BigBad wrote ...
I mean 4 discs of half the diameter have the same area and width as one disk of the full diameter; so it makes no difference in area.
But, four discs of half the diameter has twice the circumferance in total, therefore the peripheral losses are twice as great as one disc with the same area, and the peripheral losses dominate virtually to the exclusion of all other losses, all other factors being equal.
(If the four rotors are close enough together, though, losses won't actually be twice, but if they are far enough apart not to 'interfere' with each other they will be.)
Se my earlier posts. Peripheral losses are the dominant factor here.
I agree that a quad won't have the same hang-time of a single main prop, but the link showed someone got over an hour, so it has decent performance.
On the topic of novel flying machines, I've got a VTOL rocket design I've been working on for a while. It's got a pumped biprop engine, vertical takeoff/landing (it hooks off/back onto a cable) and after reaching apogee it falls sideways prior to landing (aerodynamic braking greatly reduces the landing burn). I've got a basic layout diagram for it, but no detailed design as yet, so I haven't modelled the mass distribution to make it sure it will keep the correct attitude at each point in the flight. It's looking quite promising though.
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Novel flying machines
