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BigBad

Mon Jun 30 2014, 02:33PM

Registered Member #2529 Joined: Thu Dec 10 2009, 02:43AM
Location:
Posts: 600

Dr. Slack wrote ...

Now there's general agreement that bigger props are better, what happens in the limit of an infinite length blade? I'm a bit nervous when equations seem to say 'longer is better, without limit' that some other effect has been ignored.

It makes a difference right now because the downwash speed is reducing as you increase the rotor size.

But the rotor first has to carry its own weight, so as you add rotor eventually the vehicle tends to become pretty much all rotor and then adding extra rotor makes very little difference, because the weight and the lift go up together and the downwash speed stops changing.

So when you include rotor mass, the efficiency hits a limit with infinite rotor length. But at the moment, presumably the mass of the rotor is small relative to the battery and payload.

Uspring

Mon Jun 30 2014, 03:51PM

Registered Member #3988 Joined: Thu Jul 07 2011, 03:25PM
Location:
Posts: 711

Ash wrote:

But where are we in terms of Lb/Hp and Lb/ft^2?

Just plugin the numbers

The only value that isn't known precisely is the Î¶. From what I've seen in Patricks measurement his Î¶ is at the lower end of the range. That might be due to losses in his motor. The table quoted does not contain the motor efficiency.

EDIT: Udo, as I understand it, the power conversion of the prop isn't what's relevant. What's relevant is the subsequent losses due to V (air velocity).

All losses are captured in the Î¶. Where they come from is interesting, but to calculate thrust from rotor power, one needn't know.

Ash Small

Mon Jun 30 2014, 05:52PM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

Uspring wrote ...

Ash wrote:

But where are we in terms of Lb/Hp and Lb/ft^2?

I'm hoping Patrick can provide these figures. I looked back through the thread, but couldn't find all the necessary data.

Uspring wrote ...

"All losses are captured in the Î¶. Where they come from is interesting, but to calculate thrust from rotor power, one needn't know."

I still think we're looking at this differently. I'm looking at this by visualizing the 'equal and opposite reaction' (Newton's third law?) that opposes the 'action of the prop'. This 'reaction force' is equal to the 'action force' of the prop.

I'm still struggling to get my head round the power equation. I'll look at it again later. I'm not saying some of the maths here isn't flawed, but I do think I'm visualizing the problem reasonably well.

Patrick

Mon Jun 30 2014, 09:11PM

Registered Member #2431 Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639

Ash Small wrote ...

Uspring wrote ...

Ash wrote:

But where are we in terms of Lb/Hp and Lb/ft^2?

I'm hoping Patrick can provide these figures. I looked back through the thread, but couldn't find all the necessary data.

i can get you these numbers : kg per w (thrust) and kg per m^2 (disc loading).

Ash Small

Tue Jul 01 2014, 10:35AM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

Patrick wrote ...

Ash Small wrote ...

Uspring wrote ...

Ash wrote:

But where are we in terms of Lb/Hp and Lb/ft^2?

I'm hoping Patrick can provide these figures. I looked back through the thread, but couldn't find all the necessary data.

i can get you these numbers : kg per w (thrust) and kg per m^2 (disc loading).

That would be ideal. I can convert Watts to Hp, etc. Then we can see where we are on that Wikipedia graph.

EDIT: With reference to comments regarding blades of infinite length, and drag losses from 'long blades', I personally expect to run into 'manouverability limitations' before these 'long propeller limitations' come into play.

For example, if we look back at 'Atlas', the 'human powered helicopter' I linked to earlier in the thread, which doesn't quite have an infinite length prop, nor quite achieves zero losses, it's pretty easy to see, I think, that manouverability has already reached zero, as near as makes no difference. The 'very large' disc area, however, obviously doesn't have anywhere near 'infinite drag'.

I have mentioned earlier that it's always a 'trade off' between efficiency and manouverability. I'm reasonably confident we can reach the efficiency figures we're after, what I'm not sure about is whether the aircraft will be manouverable enough to be of any practical use in 'real world' scenarios.

Patrick

Tue Jul 01 2014, 08:17PM

Registered Member #2431 Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639

for all of the recent flights made on my you tube channel "Test Pilot Mafia", ive been using the 10 x 4.5 MR APC prop. ill post the data here in a minute.

AUW = 1.62 kg
hover time = 8 minutes
1.62 kg / 3 fans = 540 g/fan
at 6,200 RPM --> 540 g / 96 W = 5.62 g/W

disk area = PI x R^2 so 10" = 78.5 in^2

pics

Ash Small

Tue Jul 01 2014, 08:57PM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

Thanks. Just doing the conversion.

2.18 Lbs/foot^2

9.247 Lbs/Hp

This is where we appear to be, based on those figures provided. Please feel free to check my maths, etc.

I'll try and plot a log/log graph as suggested by Neil when I get a chance. That should make extrapolation a lot simpler.

Unless anyone else beats me to it

Patrick

Tue Jul 01 2014, 09:12PM

Registered Member #2431 Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639

im thinking of going to 14 inch props.

pics to follow;

10 x 4.5 prop multirotor, APC

14 x 5.5 prop multirotor, APC.

Given the above thrust stand and data sheets:
0.089 Hp x 746 W = 66.4 W @ 6k RPM, (for the 10x4.5 MR prop)
7.4 A x 11.32 V = 83.7 W
83.7 W - 66.4 W = 17.3 W lost in the electric motor and ESC.
17.3 W / 83.7 W = 21% loss from battery to wind conversion.

Ash Small

Tue Jul 01 2014, 10:44PM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

So if you go to 14" props and decide you still have plenty of manouverability, but still need longer flight time, will you consider going bigger still?

I think you want less pitch if you increse to 14", by the way

Patrick

Tue Jul 01 2014, 10:58PM

Registered Member #2431 Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639

Ash Small wrote ...

So if you go to 14" props and decide you still have plenty of manouverability, but still need longer flight time, will you consider going bigger still?

I think you want less pitch if you increse to 14", by the way

they dont make them with less pitch. and 14 is already huge!

APC multirotor props :

i could go with foreign child-slave-labor made products though...

EDIT:
(16 inch would be the maximum, even then i dont know if i could support a duct around it.)

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