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The Physics Of Propellers (And Electrical Motors).

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Patrick

Tue Oct 30 2012, 12:43AM

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

The purpose of this thread is for others who are interested in the physics of flight to comment on propellers and the forces needed for flight, mostly energy in vs useful work out...

As BigBad stated:

BigBad wrote ...

For a quadcopter making the blades longer would greatly improve efficiency.

For hovering or going slowly, jet engines suck balls wrt efficiency. For long flights, everything else being equal, you want to throw a large amount of air down slowly (which is what big long wings do), rather than a small amount of air down fast (which is what jet engines do).

It's because energy goes as 1/2 m v^2, whereas the thrust goes as mv, where m is the mass, and v is the downwash speed; so if you work it you want m as big as possible and v as low as possible, so that mv = vehicle weight

The same would apply to DeSeversky ionocrafts, or lifters as theyre called...

OK Ive been looking at these AR Drone props specifically. They look odd to me, like the pitch is more than just steep, it looks like they made a prop specifically for there planned mass and rotational speed... Ive been told that props develop 80% of theyre thrust from the outer half of the radius, while 20% come from the inner radius.I wonder if the following pics show that Parrot corp tried to impliment BigBads point.

My heavily modded AR Drone...

Prop root and trailing edge are heavily pitched on the inner most radius.

APC "normal" propeller. (11" diam, 7" pitch.)

Does anyone see what i mean or is all this just trivial?

klugesmith

Tue Oct 30 2012, 02:03AM

Registered Member #2099 Joined: Wed Apr 29 2009, 12:22AM
Location: Los Altos, California
Posts: 1716

Nice pictures there. I've seen that extra-wide chord near the hub of rotors such as those on cheap Picoo-Z model hellicopters.

Patrick wrote ...
... I've been told that props develop 80% of their thrust from the outer half of the radius, while 20% come from the inner radius.

Duh!

The outer half of the radius sweeps 75% of the total area, while the inner half sweeps only 25%.

You would get that 80/20 ratio exactly, with uniform areal loading of the disk,
if you allot _no_ thrust to the innermost 1/4 of the total radius.
Perhaps the innermost 1/16 of total area is occupied by a rotor hub, blocked by a powerplant, etc.

Patrick

Tue Oct 30 2012, 02:18AM

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

klugesmith wrote ...

Nice pictures there. I've seen that extra-wide chord near the hub of rotors such as those on cheap Picoo-Z model hellicopters.

Patrick wrote ...
... I've been told that props develop 80% of their thrust from the outer half of the radius, while 20% come from the inner radius.

Duh!

A = Pi r^2 was implied from bigbad and i, didnt think it needed to be explicitly stated. Yet the AR drone and your pico Z thingies, seem to load the disk non-uniformly... thus possibly shifting the 80-20 ratio to favor the inner radius more... As BigBad suggests avoiding the high tip speed, and looking for high-mass low-velocity air stream, means the prop root area.

2Spoons

Tue Oct 30 2012, 03:58AM

Registered Member #2939 Joined: Fri Jun 25 2010, 04:25AM
Location:
Posts: 615

No, I think what is being said is a large slow prop is better than a small faster one. The prop root doesn't contribute much thrust as the air speed of the blade goes to ~ zero at the centre. This is also why the pitch increases towards the hub - you have to have material there to support the blade tips anyway, so it might as well be shaped to get some thrust.

I would imagine there would be a lot of optimising of tip speed vs drag vs lift vs airfoil profile vs airflow rates - a complex multivariable system

Sulaiman

Tue Oct 30 2012, 04:27AM

Registered Member #162 Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140

AFAIK, the pitch of the blade should be such that
the difference in airspeed between the front and back of the propeller
is the same for all radii,

so the tip has a very shallow angle as it is the fastest moving part,
and the angle gets steeper as the centre is approached.

I am definitely not an expert , but there is plenty of info. on the net,

I believe that propellers are optimised for one particular airspeed
and optimum engine torque/rpm,
I do know that it's not simple math !
and there are a host of compromises (materials, turbulence, resonance, efficiency ........)
e.g. hobby aircraft engines run at high rpm, and its cheaper, more reliable and efficient
to use a small prop. rather than add a gearbox.

On a similar topic,
I remember when (during the 'cold war') USA got pissed off with Toshiba Japan
for selling a many-axis milling machine to the Russians
as it enabled the manufacture of submarine propellers that do not cavitate.
Even now, as a subsidiary of Rockwell USA we are not allowed to repair cnc machines capable of six or more axes !
So I guess that the best design info. for propellers is marked in big red letters
and if you read it a UAV or black helicopter will appear ..... silently and efficiently ;)

Meatball

Tue Oct 30 2012, 04:57AM

Registered Member #2401 Joined: Mon Sept 28 2009, 04:25PM
Location:
Posts: 74

^^Not to mention that the outer half is sweeping over that large percentage of displaced air with a higher velocity than the inner velocity sweeps.

Patrick

Tue Oct 30 2012, 05:03AM

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

2Spoons wrote ...

No, I think what is being said is a large slow prop is better than a small faster one. The prop root doesn't contribute much thrust as the air speed of the blade goes to ~ zero at the centre. This is also why the pitch increases towards the hub - you have to have material there to support the blade tips anyway, so it might as well be shaped to get some thrust.

I would imagine there would be a lot of optimising of tip speed vs drag vs lift vs airfoil profile vs airflow rates - a complex multivariable system

well to take your point to the extreme, embry riddle developed the maple leaf UAV, a single wing rotating with prop or turbine...

BigBad

Tue Oct 30 2012, 02:38PM

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

You want the highest aspect ratio for best efficiency, but you're constrained by the biggest disc that will fit your box; so you can calculate the aspect ratio to give you the lift you need, based on that disc size.

As someone else has pointed out, the extra chord in the picture is because the wing is going more slowly there, and they're trying to get more lift; but it costs drag; I think that bit of the wing is less efficient.

Patrick

Tue Oct 30 2012, 05:55PM

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

BigBad wrote ...

...and they're trying to get more lift; but it costs drag; I think that bit of the wing is less efficient.

this is what i want to figure out in this thread, intead of just guessing.

And yes BigBad 10 inch diam. is looking like my max workable sized prop, but michigan's UAV is a quad with 7" diam 3 bladed prop, they have twice the battery mass and half the flight time as my tricopter machine.

I think most of you on this forum will enjoy this clip!!!
No human pilot at all, autonomous pilot and course map is unknown, with the goal changing place after each attempt...

Carbon_Rod

Wed Oct 31 2012, 06:08AM

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

RC prop calculators are common, and are usually badly written too:

Such a small maze.... yawn... gridSLAM + MDP for the win...
=)

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