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Ash Small

Sat Jun 28 2014, 06:15PM

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

Patrick wrote ...

Uspring wrote ...

All-up-mass (AUW) = 1.61 kg
battery mass = 0.51 kg
unlaiden mass = 1.1 kg
lets say this equates to 8 min of flight.

Here's a table of expected flight times for addittional battery packs:
The first comes from your measurement, the others are extrapolation.

1 Pack (0.51kg) 8min
2 Packs 10.6 min
3 Packs 11.5 min
4 Packs 11.7 min
5 Packs 11.7 min
6 Packs 11.5 min

As expected, im adding more and more mass for less and less flight time.

I think we pretty much covered this already. We need to increase prop size for increased efficiency

(or, more specifically, we need to reduce air velocity while maintaining the same thrust)

Patrick

Sat Jun 28 2014, 06:38PM

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

Ash Small wrote ...

Patrick wrote ...

Uspring wrote ...

All-up-mass (AUW) = 1.61 kg
battery mass = 0.51 kg
unlaiden mass = 1.1 kg
lets say this equates to 8 min of flight.

As expected, im adding more and more mass for less and less flight time.

I think we pretty much covered this already. We need to increase prop size for increased efficiency

(or, more specifically, we need to reduce air velocity while maintaining the same thrust)

yes, but we needed mre math to convince others, not just our hunch...

Ash Small

Sat Jun 28 2014, 06:46PM

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

Patrick wrote ...

yes, but we needed mre math to convince others, not just our hunch...

It's not a hunch, although my previous experience is with marine propellers.

I've never really done the maths properly before, though. Just chose the largest prop size that was practical, and worked from there.

Ideally, you want a low pitch angle as well. From what I was reading earlier, under ~12 degrees, something to do with 'stall'.. Apparently this is much more important for 'copters than for 'planes.

Patrick

Sat Jun 28 2014, 11:18PM

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

Ash Small wrote ...

It's not a hunch, although my previous experience is with marine propellers.

but i got to be able to explain things to professors and such. so i can justify one direction of thought instead of another.

and for funding, which is quite hard at the moment.

BigBad

Sun Jun 29 2014, 12:50AM

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

Wikipedia page on disk loading:

Efficiency effects of disk loading:

LiftEfficiency

Only, with professors, don't mention wikipedia, just pull down the references, and refer to them. They get sniffy.

If you use 17 inch props, you'll get 17^2/10^2 = 2.9 times the hover time than with a 10 inch prop assuming optimal pitch and motor stuff in each case.

It's almost EXACTLY the same as gliders. You know how gliders have those long thin wings and can fly for miles and miles without using any energy. It's like that, only your wing is spinning. You want the thinnest, longest wings possible.

Ash Small

Sun Jun 29 2014, 11:38AM

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

Now we're getting somewhere, that's the graph I've been trying to plot.

If you get ~3 times flight time going from a ten inch prop to a 17 inch prop, you'll get to ten times flight time long before you get to 1 metre diameter.

How did you get to the 2.9 figure for a 17 inch prop, BB?

EDIT: Although, without ducting, I'm still guessing ~1 metre for ten times flight time

I think I'm there. 32^2/10^2=10.24. my hunch appears to be correct. 32 inches gives 10 times flight time, according to this method. This gives us a 'ballpark figure' to work from.

BigBad

Sun Jun 29 2014, 02:35PM

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

It's probably not 100% accurate because the longer rotor will weigh a bit more, but it's also thinner, and the motors can be lighter, because you don't need as much power.

Also adding multiple disks helps too; 4 rotors of 10 inches is ~4 times more efficient than 1 rotor of 10 inches, because they share the weight (up to the point where the weight of the rotor and motors and the structure completely dominate the mass of the vehicle, and then adding more doesn't help because the disk loading doesn't go down any further).

Patrick

Sun Jun 29 2014, 02:41PM

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

BigBad wrote ...

Also adding multiple disks helps too; 4 rotors of 10 inches is ~4 times more efficient than 1 rotor of 10 inches, because they share the weight...

yes, from my thrust stand, a slower lower thrust prop as higher grams per watts, as that same prop accelerates, the efcicenccy number drops.

Ash Small

Sun Jun 29 2014, 02:43PM

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

Well, the prop could still spin at the same speed, but with reduced pitch, for example, or it could be geared down.

I'm 'sort of' basing this on a direct 'scale up' of Patrick's existing assymetrical tri-copter design, as that's a design that was reached by well documented method, and for which we have some figures.

Working on a ten inch prop giving 8 minutes flying time (a figure I believe Patrick mentioned), I've arrived at the following graph:

EDIT: This also seems to fit Dr. Spark's 5 minute flight time with 5" props

Uspring

Sun Jun 29 2014, 03:40PM

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

BigBad wrote:

If you use 17 inch props, you'll get 17^2/10^2 = 2.9 times the hover time than with a 10 inch prop assuming optimal pitch and motor stuff in each case.

Are you sure? I get a linear dependency of flight time on prop diameter.
The graph you quoted is interesting. I can't derive a relationship between disk loading and efficiency, which does not take into account the mass of the aircraft explicitly.

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