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Power (wattage) and prop question...

Old 04-15-2013, 02:14 AM
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Beemerider
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Default Power (wattage) and prop question...

I just replaced a motor on my 2m sailplane. I'm running a 1300kv and I got what I'm told are unusual if not incorrect power #'s when I checked it with my wattmeter on two different folding props.

Used a 3s2200 20c fully charged

With a 9X5 I measured 230 watts and 23 amp current draw

8X4.5 I measured 245 watts and 26 amp current draw

The prop are from different vendors and are similar but not identical in design. I would have expected the numbers to have been reversed. I flew the plane with the 9X5 today and it did well, did not have the chance to swap props to compare performance in the air. Does this make sense?
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Old 04-15-2013, 02:39 AM
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kyleservicetech
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Originally Posted by Beemerider View Post
I just replaced a motor on my 2m sailplane. I'm running a 1300kv and I got what I'm told are unusual if not incorrect power #'s when I checked it with my wattmeter on two different folding props.

Used a 3s2200 20c fully charged

With a 9X5 I measured 230 watts and 23 amp current draw

8X4.5 I measured 245 watts and 26 amp current draw

The prop are from different vendors and are similar but not identical in design. I would have expected the numbers to have been reversed. I flew the plane with the 9X5 today and it did well, did not have the chance to swap props to compare performance in the air. Does this make sense?
You are correct, this does not make sense. Methinks one of those two props has their spec'd blade pitch value way out of kilter.
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Old 04-15-2013, 02:44 AM
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flydiver
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Sure. There is HUGE variation in props between vendors and 'models' Sometimes a smaller prop (on paper) will draw more amps. Some sizes are just flat better than other sizes in the same model line.

I know for a fact the APCe 10x5 is almost exactly the same as the GWS 10x6 HD in terms of amp draw, but they look really different.

Read Post #7 in this test by Dr. Kiwi on EMP props.
http://www.rcgroups.com/forums/showt...ight=EMP+props
[The 7x4.5 EMP was a real surprise - it draw more than the 7x6 EMP, yet generated less thrust at lower pitch speed... that's a lose, lose situation!]
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Old 04-15-2013, 04:28 AM
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fhhuber
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One a folder and the other isn't?

Different spacing between the blades in a folding prop hub changes the actual diameter and pitch vs that which is labeled on the blades.
APC sells their folding prop blades labeled for one hub width, but they have several widths available, since you need a different distance between the folding "hinges" for different spinner sizes.

************

GWS flexible "slow fly" blades will be more efficient and draw less power at low RPM (before they start to flex significantly) than the APC SF blades. Once you get the RPM up enough that switches... The GWS props can flex in a manner that causes them to demand more power than the stiff APC.
Blades look about the same shape and have the same pitch + dia labeled on them... but that's about all that's the same.

When one prop starts flexing and another doesn't... you can't expect any "rule" about comparing them to survive all cases either...
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Old 04-15-2013, 08:51 AM
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A little odd but not all that unusual. There are good reasons why some makes/types of props are preferred to others and one of the common reasons is that some props seem able to absorb a lot more power while producing a lot less thrust. Go with the one that looks more efficient .

Steve
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Old 04-15-2013, 02:18 PM
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Walt Thyng
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Before the e-power knowledge base became what it is today and before the advent of "E" specific props we had to test and measure the performance of every prop. I never did check for pitch accuracy, but often found seemingly anomalous results. It was not at all unusual to have a prop of less diameter or pitch draw more amps and produce less thrust. Those difference were often much greater than what you found. You have now learned why a "whatt meter" is an essential tool in electric flying (-:
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Old 04-15-2013, 02:27 PM
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Beemerider
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So is a standard prop (i.e. non-folding) more efficient than folding as far as thrust and power draw? And I guess the down side is more drag when throttle off?
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Old 04-15-2013, 03:18 PM
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Folding vs non folding doesn't make any significant difference in efficiency... if you have the folding prop's design hub diameter.
deviate on the hub diameter (typically for a bigger spinner) and you alter the prop and lower its efficiency, increase diameter, increase thrusts and increase the power demanded to turn it. Its just not the same prop at all.

There are usually other differences in a folding prop that will make it perform differently from a non-folder.

Each prop design is also most efficient at one rpm. Faster or slower and it won't be as efficient. Just like you wouldn't expect a Piper Cub's wing to work well at supersonic speed even if it would stay in one piece, you can't expect any prop to work well at all speeds.
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Old 04-15-2013, 03:53 PM
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The 9" has a spinner with the end open to allow airflow into the motor (aluminum--I guess most of them are), the 8' prop has a solid spinner.

Is there an advantage or disadvantage between either type of spinner? Other than what seems obvious to me?

Also--how much is the right amount of play or friction in the pivot point of the blade? How easily should they fold? It seems the spinners come with washers that fit on either side of the blade within the "ears" of the spinner to eliminate play.

It seems some of my spinners are noisier than others.
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Old 04-15-2013, 04:02 PM
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the wind needs to be able to fold the blades back.
Usually that means you need to be able to point the nose up and the blades fall back.

The idea is to minimize drag in glide.

I haven't done any experimenting with the spinners made to allow air to pass through for motor cooling.
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Old 04-15-2013, 05:20 PM
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Originally Posted by fhhuber View Post
the wind needs to be able to fold the blades back.
Usually that means you need to be able to point the nose up and the blades fall back.

The idea is to minimize drag in glide.

.
Also, if your model doesn't have a landing gear, the folding props reduce or eliminate prop breakage on landing.

If you use a folding prop, you will likely have to turn on the braking function for your ESC and motor drive system. If not, that prop will just keep windmilling.

Use a "Soft" brake if possible. I've had ESC brakes that turn on so solidly, it unscrewed the prop nut.
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Old 04-15-2013, 08:35 PM
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Originally Posted by Beemerider View Post
The 9" has a spinner with the end open to allow airflow into the motor (aluminum--I guess most of them are), the 8' prop has a solid spinner.

Is there an advantage or disadvantage between either type of spinner? Other than what seems obvious to me?

Also--how much is the right amount of play or friction in the pivot point of the blade? How easily should they fold? It seems the spinners come with washers that fit on either side of the blade within the "ears" of the spinner to eliminate play.

It seems some of my spinners are noisier than others.
Gotta love more air flow, I use headless or sometimes called "turbo" spinners, especially if your prop-ing your motor at the max.

As for the amount of play, I like just barely touching, but still loose enough to move with a strong breath. Someone else might have it different. IME, over time, of course start to move more freely. So a little tight shims is ok.

I use spinners from espirit.com and hobby lobby.com and valuehobby.com and a couple other, but I havent noticed any "different" noises, but I'm kinda hard of hearing too! Just wondering....spinner might be just a bit out of balance or maybe just different brand of prop. What brands you using?

cr
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