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look at the 5,000 RPM row on the 12 inch prop, its 733g and 5.13 g/w then look at the 7,100RPM on the 10 inch, its 723g and 5.15 g/w
Not much difference in prop performance there. But if you look at the numbers at 6.7g/W, there is a bigger lift for the bigger prop. It seems like the bigger prop looses efficiency at higher rpms.
Yeah, sure, it's basically a freaking big pile of battery, but that just means you need to beg, borrow or steal a big pile of battery from somewhere.
Assuming a F ~ P ^ (2/3) dependence, the additional flight of time with more batteries will be more than cancelled out by the extra power to carry them beyond some point. It turns out, that for just hovering, the mass of the batteries should be twice the mass of the copter without batteries for max flight time. I've taken battery capacity to be proportional to its mass.
Registered Member #3414
Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245
Uspring wrote ...
look at the 5,000 RPM row on the 12 inch prop, its 733g and 5.13 g/w then look at the 7,100RPM on the 10 inch, its 723g and 5.15 g/w
Not much difference in prop performance there. But if you look at the numbers at 6.7g/W, there is a bigger lift for the bigger prop. It seems like the bigger prop looses efficiency at higher rpms.
Yeah, sure, it's basically a freaking big pile of battery, but that just means you need to beg, borrow or steal a big pile of battery from somewhere.
Assuming a F ~ P ^ (2/3) dependence, the additional flight of time with more batteries will be more than cancelled out by the extra power to carry them beyond some point. It turns out, that for just hovering, the mass of the batteries should be twice the mass of the copter without batteries for max flight time. I've taken battery capacity to be proportional to its mass.
I think this is similar to what I concluded, but didn't post (as I wanted to think about it for a bit).
I think it's a fairly linear relationship, my figure of ~12 minutes flight time for four motors and props plus 1kg of batteries will also be equal to flight time for two motors and props, and 1/2 kg of batteries.
Payload will be double with four motors and props and 1kg batteries, but it's a linear relationship.
Of course, with the same payload, you can carry a bit more in bettery weight, but efficiency isn't really related to any of this. You need to use 'other methods' to increase efficiency, although Im still of the opionion that peripheral losses will decrease with more motors/props, due to the 'combined' increase in cross section/circumferance of the 'combined' column of accelerated air.
Of course. for the same payload, I expext flight times to by increased with a configuration along the lines of a heptacopter carrying 2kg of batteries, but then total mass gets up to over 7 kg. (could probably carry maybe an extra kg of batteries here, maybe even a bit more, which. I guess should increase flight time to ~20 minutes (maybe more), but by this time I imagine IC engines/ swashplates would probably be advantageous.
EDIT: that 'initial' 12 minute figure, from which everything else has been extrapolated is based on approximate 'hover time'.
Increasing prop diameter would increase efficiency still further.
Registered Member #2529
Joined: Thu Dec 10 2009, 02:43AM
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Posts: 600
Uspring wrote ...
Yeah, sure, it's basically a freaking big pile of battery, but that just means you need to beg, borrow or steal a big pile of battery from somewhere.
Assuming a F ~ P ^ (2/3) dependence, the additional flight of time with more batteries will be more than cancelled out by the extra power to carry them beyond some point. It turns out, that for just hovering, the mass of the batteries should be twice the mass of the copter without batteries for max flight time. I've taken battery capacity to be proportional to its mass.
This is a good point. And yet other helicopters can go for an hour or more.
Presumably the overall weight of the helicopter relative to the disk area is a critical parameter which gives the constants of proportionality for hover time.
Something is not scaling down right.
That raises the question, why is this particular helicopter heavier than the electric helis that fly for over an hour? Are the batteries heavy, is the rotor heavy, are the motors heavy, is it the avionics, is it the payload that is heavy relative to the disc area?
Registered Member #3414
Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245
BigBad wrote ...
Something is not scaling down right.
That raises the question, why is this particular helicopter heavier than the electric helis that fly for over an hour? Are the batteries heavy, is the rotor heavy, are the motors heavy, is it the avionics, is it the payload that is heavy relative to the disc area?
These are very good points, but I assume Patrick has chosen his components carefully, and that they are pretty 'state of the art'.
Personally, after seeing comparisons of a ten inch and 12 inch prop, I'd like to see some data for, say, a 30" prop, for example.
The disc loading for that human powered helicopter, 'Atlas', that I linked to above was extremely low.
I've been saying all along that the only way to substantially increase efficiency is to substantially increase prop size, which equates to a substantial decrease in disc loading. I've also explained my reasoning in considerable depth.
EDIT: also, another point I've not really pressed is the 'angle of attack' of the blades. Ideally, you want a 'wing section' with a zero degree angle of attack, as this creates minimum turbulence, with all the lift created by the reduced pressure on the upper surface of the wing. Maybe it's time to try the old 'balsa wood and tissue paper' type wings of the 'rubber band' powered models I remember building as a kid?
There is some drag associated with the increased surface area of the 'wing', but this reduces as the speed of rotation decreases.
Once again, though, this all becomes a trade-off between efficiency and manouverability.
NB: Patrick's original design was required to be small enough to fly through doorways, and be manouverable enough to fly around inside buildings, as part of a competition, with only a few minutes flight time required.
Maybe it's time for another visit to my local helicopter museum?
Registered Member #3414
Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245
Patrick wrote ...
Ash Small wrote ...
BigBad wrote ...
Something is not scaling down right. That raises the question, ..... is it the payload that is heavy relative to the disc area?
Personally, after seeing comparisons of a ten inch and 12 inch prop, I'd like to see some data for, say' a 30" prop, for example.
yes, ive chosen the components carefully, but i need to lift heavy insturments, for a long duration of flight.
My calculations above actually ignored the 'payload' completely. I was going to 'factor this in' after getting a better grasp of the basic principles, and how they scale.
Presumably the overall weight of the helicopter relative to the disk area is a critical parameter which gives the constants of proportionality for hover time.
Exactly. If you use the equations for thrust, assume a certain mass ratio between copter and battery weight and a certain ratio between battery weight and capacity, the hovering time will be proportional to the square root of the ratio of disk area to copter mass.
A somewhat exotic example: A 2 m^2 prop (60" diameter) with a heli weight of 2 kg and 4 kg batteries of the type
THe batteries are 3.3 Amp-hours, 3s, 11.1 Volts, 255g. (so theres two i use in parallel.)
Registered Member #3414
Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245
Uspring wrote ...
A somewhat exotic example: A 2 m^2 prop (60" diameter) with a heli weight of 2 kg and 4 kg batteries of the type
THe batteries are 3.3 Amp-hours, 3s, 11.1 Volts, 255g. (so theres two i use in parallel.)
might just hover for an hour.
Thanks for providing some maths to illustrate this, Udo.
Significantly increasing the prop diameter will significantly increase the efficiency.
I think a good starting point might be, as Meil pointed out, to start with the biggest prop that will reasonably fit in the back of an average car, for example.
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Joined: Thu Dec 10 2009, 02:43AM
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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.
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.
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.
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