Efficient UAV Power

[chickenblender]

What's the best approach to building an efficient UAV power system? I've noticed that the sailplane folks tend to go with large-diameter, low-speed folding props -- but this seems like it would be good only for short climbs to altitude, not continuous use. So would a better solution be to use a smaller prop at a higher speed? Any advice or links would be appreciated.

Cory

[jetblackaircra]

Propellers are more efficient at lower disk loading (lower lbs of thrust per cubic inches of propeller disk area) Just like wings, the lower the wing loading the more efficient and the higher the aspect ratio the more efficient.

So, as a general rule, larger diameter is better. That said, props are usually designed to be run at a certain RPM, so if you get a prop that is too big for your motor/engine you'll probably be running it at an RPM below it's designed max efficiency.

There's a trade-off with any fixed pitch propeller as well... High speed versus high climb rate. The pitch of the prop determines where it will perform best. For instance, a 12X8 prop on a given airframe/powerplant combo might give you 500 fpm climb and a top speed of 43 MPH but a 12X10 on the same airplane might give you a 350 fpm climb and a top speed of 49. So, something like an 11X10 might be a good compromise. It will spin faster than the 12X10 to get a little more climb, but not top out at such a high speed.

So, get a prop that matches your powerplant, but is as large as you can handle as far as clearance issues (ground clearance, fuselage clearance when folded, etc.)

Experimentation is crucial, so plan on buying a few different props if you really want to optimize. Also, try out MotoCalc if you want a program that's easy to setup and get halfway decent estimates of performance. There are other programs that are good as well.

[chickenblender]

Excellent. Let me ask a follow-up question: Larger props require a slower rate of rotation, which (assuming an electric power system) requires a lower Kv motor or a lower voltage battery. Outrunners are typically offered with lower Kv's than inrunners, but I've heard that inrunners are generally more efficient. I know a lot of this is manufacturer-dependent, but should I be looking for a direct-drive outrunner or a geared inrunner for max efficiency? Or are there low-Kv inrunners out there that I'm not aware of?

Cory

[Unterhausen]

inrunner == gearbox, so kv isn't all that important. There is a tradeoff with weight of a gearbox, and the gearbox is a source of inefficiency and trouble. In general, if you can find an outrunner that works, you're better off. That said, if you need every last bit of efficiency, you probably are going to want to look hard at inrunners.

[matttay]

Chickenblender, how much are you trying to squeeze out? Most are flying airplanes with motor + props + ESC efficiency around 60 to 65%. It's fairly straightforward to move that towards almost 80% at a narrow region of operation. it's fairly difficult ot move to 85%.

For your initial design, what overall efficiency are you wanting? Does 75% versus 85% radically change your design decisions? If so, then you have a lot of experiments to conduct using equipment most don't have. If you just want to get to 75 to 80% and call it good, then you can probably get by with a lot less.

If 75 to 80% overall is good, then the plan is to spend a lot on a well made motor, lean towards higher voltages (3 or 4S) versus lower (2S), overspec ESC to reduce losses and operate for cruise at the efficiency peak of the motor. If you need to go beyond that, you'd want to understand how the prop unloads at cruise, understadn the motor + ESC interaction (some like more, some like less inductance), play with switching freqs, measure thermal rise of ESC to detect losses, etc

[jetblackaircra]

I don't know the specifics about small scale props, but on large scale (McCauley's single engine props) the max efficiency we ever experienced was probably slightly over 85% to 90% on just the prop, so I'd say you'd be lucky to get 75% on the whole system.

I'd say find a motor that is near your power requirements (over estimate the power needed if I were you) and then buy several props and a speed controller that will handle say one cell more than you plan on (in case you want to bump the battery voltage) then just experiment with it. Buy say 5 or 6 different prop size/pitch combinations and see what happens.

You'll spend way too much time trying to optimize and get that extra 5% efficiency than it's worth. with 5% efficiency increase in your powerplant you'll extend your flight time by what? 5%? I donno, I'd have to think about it, but I doubt it'd be too much more than that.

[Blue Sky]

Sage advice from jetblack.
A few other points to consider.
A larger diameter lower pitched prop may be more efficient and give a better rate of climb but may not provide sufficient pitch speed to be able to cruise at a reduced throttle setting. The problem is exaggerated when using a plane that has low drag, another important consideration.
Another option is to use a catapult or bungee cord for launching so that you can cruise on a smaller motor. This avoids the need to throttle back for cruising and eliminates the compromise between rate of climb and cruise speed. It eliminates the efficiency losses in the controller and motor that occur at reduced throttle settings, and saves on space and weight.
It really depends on what you are doing.
If you are building a draggy camera plane that only needs to go up, snap a few pictures and come down again then a high rate of climb is important.
If you are going for long range then efficiency at cruise is critical.
-Dave

[icebear]

Very intetersing questions and answers! I agree with everything that has been said but just wanted to add some observations I have made while testing.

I have found recently that in my small electric UAV setups that I actually get better "milage" with a smaller less efficient motor. It is probably because it happens to be a well optimized motor/prop combo and the larger motor is less efficient at lower rpm.

There is a 5 oz difference in the two otherwise identical setups (395 sqin polyhedral flat bottomed wing), but this does not explain the large difference in range/duration.

1) AXI 2808/24 with APC-e 9x4.5 and 3S-2500 pack - max 30 min duration (approx 4,5A cruise) - AUW 40 oz. Cruise speed 40 kph.

2) AXI 2212/34 gold with APC 9x6 slowfly and same 3S-2500 pack - max 60 min duration (approx 2,5 A cruise) AUW 35 oz. Cruise speed 37 kph.

My inrunner MEGA 16/25/4 is also more efficient in the specs but is actually worse than the AXI 2808/24 in real life. (did not test geared though)

So, max efficieny at higher power levels does not always translate to efficient low rpm performance. I also agree that controller efficiency at lower throttle settings is important to test in real life situations.

/Icebear

[chickenblender]

Wow, excellent advice from everyone. I am making an AP-oriented ship, but it will have a low-drag sailplane planform as much as possible with the motor on a stalk over the wing (to keep the prop out of the camera's way). I appreciate everyone's advice and I tend to agree that 5% efficiency isn't worth a ton of expense and trouble.

Another practical consideration that just occurred to me is noise level, which would tend to favor an outrunner over an inrunner with gearbox. One of the places I like to fly is out in the sticks, and I'll bet there are some folks that would think hard about shooting :eek: at this thing if they were annoyed by it.

Cory

[Blue Sky]

This has always amazed me and helps to put efficiency in perspective.
http://www.rcgroups.com/forums/showthread.php?t=115093&highlight=stinger+91+miles
If you aren't familiar with the stinger, this thing is tiny and has NO room in the fuse.
OOPS! Edited to correct link. Stinger flying 91 miles!

[Prashanth]

"AXI 2808/24 with APC-e 9x4.5 and 3S-2500 pack - max 30 min duration (approx 4,5A cruise) - AUW 40 oz. Cruise speed 40 kph. "

Icebear could you please share more info/specs on your plane (may be a picture)using this power plant, I was looking at building something that can give me about 30 min and have this setup...

Thanx

[Airboatflyingshp]

You can run an out runner with a gearbox for big thrust see the MEC site.

[icebear]

Prashanth,

Here are the specs of my setup

Scorpio Supermiss with Miss2 wing - polyhedral 400 sq in wing
(available as "Miss2" from Hobby-Lobby - this is a remake but virtually the same design)

Motor: Axi 2808/24 with APC-e 9x4.5
JETI controller 30A + U-BEC 5volts
FMA FS-8 Rx
2x HS81 servos
3S-2500 Flightpower EVO lite pack
Camera Pentax Optio S5z
Autopilot: Picopilot + GPS
Total: 40 oz = 14-15 oz/sq ft wiingload

Here is a picture of the plane. Let me know if you need more info.

/Icebear