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Propeller Physics and VABs Program.

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Ash Small

Thu Dec 04 2014, 10:57PM

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

Patrick wrote ...

ok so lets start this way.

1,430 g AUW.
3 rotors, all 12in diameter, 3.8 pitch, called a "slow-fly" prop.

while moderately maneuvering, a drew 31 amps at about 10.7 volts. from A 3 Ccell lipo (12.6 fully charged, 9.9v discharged)
im also having a battery problem, but we'll deal with that later.

Thanks for posting the figures. I've had a drink or three, so won't look at it tonight, and I'm pretty busy tomorrow, so may not get round to looking at it until saturday morning. The maths isn't difficult, though, it's all posted above, if you want to look into it before then

Ash Small

Fri Dec 05 2014, 07:04AM

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

I was up early this morning and looked at the figures over a coffee. It only took a couple of minutes to work out the following:

331.7 watts

0.86 m^2

1.66kg/m^2 (disc loading)

232 W/kg

(Maybe someone can check my maths. Maths isn't my strongpoint, especially first thing in the morning)

The disc loading looks ok, but any improvement will still give increased efficiency. I've done a quick calculation on theoretical power required, if there are no losses, and came up with a figure of 32W/kg, so, from the above figures, 200W/kg is being wasted in losses. While we're never going to get close to the 'theoretical' efficiency, and the figures you supplied aren't for hovering in ideal conditions, it does give us a starting point. I think if we can get more data from your test rig, we should be able to get some realistic comparisons of different blade profiles, etc., and extrapolate from there. (It's still early here, I'll need to check the figures etc later. Do we have RPM figures as well? I think you posted some in the other thread when you were using the test rig. Also any data from the manufacturers would be useful regarding RPM, etc. I suspect that the props you used are designed for higher disc loading, and that significant improvements can be made with different profile blades.)

Patrick

Fri Dec 05 2014, 08:22AM

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

Im thinking that adding a third blade. Even with tip loss, the captured air mass, accelerated less may make up for it.

these current props are 12x3.8
there are 13x4.7, and 14x4.7 props too. I should probly dyno these too.

they did have lists of data, all categorized at rpm intervals, but theyve take it down from their site. ill get them as soon as i can.

Ash Small

Fri Dec 05 2014, 09:33PM

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

Patrick wrote ...

Im thinking that adding a third blade. Even with tip loss, the captured air mass, accelerated less may make up for it.

these current props are 12x3.8
there are 13x4.7, and 14x4.7 props too. I should probly dyno these too.

they did have lists of data, all categorized at rpm intervals, but theyve take it down from their site. ill get them as soon as i can.

Two blades spinning faster is the same as three blades, assuming you spread the load evenly. You could just be overcomplicating things. 'Copters with more than two blades have very high disc loading, like the V-22 Osprey.

I made a mistake in the maths I posted this morning, I added the disc areas first, instead of dividing the mass by 3. I'll correct it in the morning.

Patrick

Fri Dec 05 2014, 10:03PM

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

Ash Small wrote ...

Two blades spinning faster is the same as three blades, assuming you spread the load evenly.

but dont we want the same thrust from lower tip speed (higher mass)? or does having 3 not change the the mass near the tip exiting near the tip?

Ash Small

Fri Dec 05 2014, 10:44PM

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

Patrick wrote ...

Ash Small wrote ...

Two blades spinning faster is the same as three blades, assuming you spread the load evenly.

but dont we want the same thrust from lower tip speed (higher mass)? or does having 3 not change the the mass near the tip exiting near the tip?

Well, it affects the total mass, if nothing else. Then there's balancing the thing...

Shrad

Sat Dec 06 2014, 11:21AM

Registered Member #3215 Joined: Sun Sept 19 2010, 08:42PM
Location:
Posts: 780

what about curvy blades to add contact surface with the air? that's another mean of increasing moved air mass, no?

Ash Small

Sat Dec 06 2014, 01:20PM

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

Shrad wrote ...

what about curvy blades to add contact surface with the air? that's another mean of increasing moved air mass, no?

Curvy blades, as you call them, increase drag, if they have too much 'curve'. You'll get eddy currents behind the blade. With marine props, you can get a state where you get cavities of vacuum behind the blades. This situation is called 'cavitation' and is generally detrimental. It can also cause pitting of the prop, which can be very detrimental.

So called 'super-cavitationg' props used for offshore powerboat racing exploit this phenomenon, with the back of the prop running in air or vacuum to reduce drag on the back of the prop, but these are still less than 50% efficient, compared to a well designed ship propeller, which can be over 90% efficient.

This is something we need to consider here, in order to get an efficient design, the blades need to disturb the air as little as possible in order to reduce drag. The air should flow over the blade, without turbulence being created in the 'wake' of the blade. Any turbulence here represents losses.

At least, that's my understanding

Andy

Sat Dec 06 2014, 07:37PM

Registered Member #4266 Joined: Fri Dec 16 2011, 03:15AM
Location:
Posts: 874

Hi Patrick, just being doing something and have a couple of ideas.
Have a prop at the top with deep pitch slow rpm, then one,two,three below with faster and faster rpm but less pitch.
have a by-pass vent for the top prop that is pressure controlled, normal closed but as wind movement etc happens it gets forced open.
Have two gearboxes 2 motors and four prop would probable be effeinct.
sometimes birds fly in a "u" shape to go in level flight, maybe use fly-by-wire to program effeinct movements for x-z plane, but use the y plane differently.
Don't design the UAV for hovering but more effeinct for movement.

A couple of cents :)

Patrick

Mon Dec 08 2014, 02:29AM

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

one guy is using pipe, then cutting it off to make his own props.

composite parts.

pipe cut props.

odd one.

Someone says just about any prop can be 70% or so efficient. Since were approaching 100% as an asymptote, eventually theres no advantage in going all out for that last 1%. (im tipsy so this was probly said already) So lets find out what the efficiency of the APC 10, 11, 12 and 13 MR's are, then we'll see how much margin is at stake.

APC has all these numbers, just waiting for re-posting them.

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