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Ash Small

Tue Jun 24 2014, 07:48PM

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

Ok BB, I understand now, a mis-understanding.

When I talk about 'peripheral losses' I don't mean 'tip losses'.

I'm talking about the 'drag losses' at the circumferance of the accelerated column of air. This is where the 'peripheral losses' are, in the 'eddy currents' and 'vortices' at the interface of the accelerated column and the surrounding air.

This is why I use analogies with 'drag coefficients' of cars, etc'

I've not actually got on to the 'prop design' bit yet, I'm still trying to 'identify' the losses, if you like.

I don't mean 'tip losses', I mean 'losses from the periphery of the accelerated column of air'. I believe it's important that we understand these things first, before we try to minimise them. I hope this clears up any mis-understanding.

EDIT: It's these 'drag losses' that dominate over everything else.

EDIT EDIT: I've also noticed the similarity with the 'propeller equation' and realised that it 'looks like' the 'same maths'.

EDIT EDIT EDIT: Udo provided the equations, for which I'm extremely grateful

Carbon_Rod

Wed Jun 25 2014, 10:17AM

Registered Member #65 Joined: Thu Feb 09 2006, 06:43AM
Location:
Posts: 1155

Some chainsaws offer a 2.8hp brushless motor, ESC, and battery for under $200:

Just need 4 of them.... and some really big rotor blades.

Patrick

Wed Jun 25 2014, 10:12PM

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

Carbon_Rod wrote ...

Some chainsaws offer a 2.8hp brushless motor, ESC, and battery for under $200:

Just need 4 of them.... and some really big rotor blades.

yeah but a chain saws a bit over kill.
i was thinking of a glow-electric engine. Evo 40NX glow engine, with a coupler to a BC3536 outrunner.

like here:

(motor)
and here:

(glow engine)

Patrick

Fri Jun 27 2014, 06:18AM

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

Ive made a 4 minute test flight, i guess i should upload to youtube the onboard video.
so im at 3.94 V per cell after 4 minutes of flight. thats looking like a 8 minute flight, with a 6.6Ah battery.

im real tired, its 2am here, soon to bed. bt if i do the math right, 6.6Ah = 8 minutes in hover.
then, 30 / 8 = 3.75 x 6.6 = 25Ah of battery.
and, 3.75 x 510 grams = 1.9kg of battery.
this for 30 minutes or so of flight?

yikes, i could lift that (4kg), but then id lose some duration from greater all up mass too. so im thinking itd be 20mins at best, i not even going to try that with this machine, shes just to valuable.

Ash Small

Fri Jun 27 2014, 08:18AM

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

Patrick, I suggest we try to do the maths for your existing assymetrical tri-copter, and then 'scale it up' (maybe without the ducts) and do the maths for the 'largest practical design', but base everything on your existing design, because it seems to be a 'reasonable compromise' in the first place.

I'm not in any rush for the figures, though

Patrick

Fri Jun 27 2014, 08:37AM

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

i do have an inflight data logger, so tomorrow ill make a full duration flight down to 20% on the batteries, then well see the amp hours put through the machine.

im thinking of a brushless motor, with 3 phase schottky bridge rectified, and a glo engine, to produce 800-900 watts
(500 W electrical, after losses) with the glow carb cotrolled automatically via a PID loop and servo. This would hold 10.5V min 12.5 max the problem is there would have to be something like a battery or ultra-capacitor to slow the volt change with load, to prevent high frequency load-throttle oscillations. and im sure there would need to be a roll off frequency on the V detector... like an SMPS.

Ash Small

Fri Jun 27 2014, 09:28AM

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

I think we should see what the maths does as we scale everything up, and reduce 'downdraught velocity'. I'm expecting to get significantly longer battery life, but we'll at least get some figures and then compare 'diesel-electric' to 'battery' power.

Uspring

Fri Jun 27 2014, 09:48AM

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

this for 30 minutes or so of flight?

Flight time scales up linearly with battery size only if the battery mass is an insignificant part of total aircraft mass. Generally you'll have a time of flight dependency like this:

TOF ~ mb / (ma+mb)^3/2

ma being the aircraft bare mass and mb the mass of the battery. This will go through a max at mb = 2*ma. What's the mass of the copter w/o Lipos?
For lots of batteries, the capacity/weight ratio of the batteries becomes increasingly important. It looks like the NCR18650s are better than yours in this respect.

Ash Small

Fri Jun 27 2014, 10:08AM

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

Yes, but efficiency goes up with prop size, and there is LOADS of room for improvement.

Ideally, you want reduction boxes on those motors. (or reduction belt drives, or something)

It will increase overall mass, but it will increase efficiency more.

Uspring

Fri Jun 27 2014, 12:16PM

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

Yes, but efficiency goes up with prop size, and there is LOADS of room for improvement.

I didn't mean to imply that prop area doesn't matter. The equation just allows to calculate how ToF changes by just adding batteries and keeping everything else the same.

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