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Uspring

Fri Jun 13 2014, 10:05AM

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

The question then becomes, 10 inch in diameter at the prop circumference. what diameter do i taper out to at what distance behind the prop face?

A more or less cylindrical duct as shown here

is probably good for a prop not at rest relative to ambient air. Slipstreams are less bent for props moving forward in air.
The shape of the slipstream is caused in front of the prop by the low pressure (suction), which draws air inward toward the axis of the prop. The inward component of flow doesn't change, as it goes through the prop. The higher pressure behind the prop causes the inward flow to bend outward again, but not before the slipstream has lost diameter.

A duct alters the shape of the stream and prevents its constriction behind the prop and leads to the desirable lower air velocity. It cannot do so abrubtly as e.g. with a very short duct. It has to bend the stream to parallel to the axis in a continuous way in order to avoid turbulences.
That being said, these thoughts come much more from intuition than education on my side and for sure, this is reinventing the wheel.

Ash Small

Fri Jun 13 2014, 06:40PM

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

Once again, I think we're back to where we were. The larger the prop, and the lower the velocity of the accelerated air, the the smaller will be the constriction of the slipstream, and the larger will be the diameter 0f the column of air under/behind the prop, and so the shorter the duct needs to be.

There is simply no alternative to a large, slow turning prop if you are after extended flight times, ducts or no ducts, although a longer duct will help.

I'm still of the opinion (and I'm wondering if your test rig can test this), that if you use three props as close together as possible, for example, whether or not this 'approximates' to one large prop, with an associated increase in efficiency.

My reasoning is that there should only be one 'slipstream', shared between the three props, and only one 'constricted' column of air, rateher than three separate slipstreams, etc. (at least, an 'approximation' of this. This is why I believe the 'heptacopter' layout should improve efficiency considerably, but this concept will be more problematic to test with your rig, I suspect.)

Patrick

Fri Jun 13 2014, 07:31PM

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

yep, this is looking like a dead stick.

Perhaps i should be looking at nitro-electric, like a diesel-electric train. then for sure i could get 30-45 minute flights.

BigBad

Fri Jun 13 2014, 09:02PM

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

I still don't buy the argument that the guy that did an hour+ long flight with an electric helicopter shouldn't be emulated.

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.

Patrick

Sat Jun 14 2014, 12:12AM

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

BigBad wrote ...

I still don't buy the argument that the guy that did an hour+ long flight with an electric helicopter shouldn't be emulated.

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.

i guess i should compare battery mass to payload mass vs duration of flight and all up mass. remember the whlole is to drag useful instruments through the sky. so far they're just using helis or quads with 90% battery and a keychain HD cam.

Ash Small

Sat Jun 14 2014, 12:55AM

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

So what is the maximun weight limit?

I seem to remember reading something like 12kg, but can't find the article/link.

Can someone please confirm this, or provide the correct figure.

This is the starting point I'd use, and then work back from there. Once you've done some calculations, you may find you don't need whatever the maximun legal weight is, although this would surely give the maximun flight time possible with the best power to weight batteries available.

I suggest that if you don't use an approach something along these lines, someone else will, and they will then have the advantage over you in the 'market place'.

You could always produce cheaper versions as well, with reduced flight time, but if maximum flight time is the goal, then you have to push everything to the limit, or the competition WILL be able to do better.

Patrick

Sat Jun 14 2014, 02:48AM

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

well around 6 to 8 lbs starts getting heavy and big for an "easy" to deploy drone.

the 12x3.8 inch prop did turn out to be the most efficient.

Ash Small

Sat Jun 14 2014, 03:14AM

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

Patrick wrote ...

well around 6 to 8 lbs starts getting heavy and big for an "easy" to deploy drone.

the 12x3.8 inch prop did turn out to be the most efficient.

Just out of interest, what are the figures for the 12x3.8 prop?

What is the weight of motor plus prop, and how many watt hours (Or whatever the correct terminology is, it's late here) do you get from, say, a pound of batteries?

Patrick

Sat Jun 14 2014, 05:34AM

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

THe batteries are 3.3 Amp-hours, 3s, 11.1 Volts, 255g. (so theres two i use in parallel.)

Watts up data logger, power harness, and two lipo batteries.

these are big motors.

props with there data tables. i could only get 2 rows on the 12x3.8 prop till the motor starting overheating, i couldnt even press it 500 more RPM with ou the current trying to go past 20 amps.

About the data,
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

4 blade? possibly, or am i crazy?

Ash Small

Sat Jun 14 2014, 09:03AM

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

Not sure if my maths is correct (I've been up all night, couldn't sleep), but I make it four motors plus a kg of batteries equals ~twelve minutes flight time (quick 'order of magnitude' type calculation.)

Maybe someone can check this.

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