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Old May 27, 2011, 07:17 AM
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Kv, prop pitch for 3D

I believe I understand the basic outlines of choosing a power system for electric planes.

My question is, what's special about power systems for 3D? In the general case, I aim for low RPMs and a large propeller, and hopefully a prop that has a lot of pitch relative to diameter.

I get the sense (from others) that this may not be the optimal approach for 3D. To wit, that it might be better to use more RPMs (more Kv or battery volts) and shallower props. But I haven't heard these other arguments presented in detail.

If 3D power systems are different from the more general case, I'd like to understand why that is.
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Old May 27, 2011, 07:48 AM
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Lower Kv motors are designed to give more torque that is required to turn larger props. By larger I mean both longer length as well as higher pitch. In general higher pitch is for speed, not post stall 3d aerobatics.

Using a longer prop length (with a smaller twist) is better for 3d as it takes many little bites of air as opposed to a larger twist that takes fewer larger bites. Because smaller bites of air are being taken with a smaller twist proper, higher Kv's are better as not as much torque is required. Think of it as resolution. You like pictures with more pixels for better resolution so in 3d you would like more little bites or air that are gained from using a prop with less twist.

In 3d prop blast keeps airflow over the control surfaces so you would like the most area covered in a consistant manner. This criteria is met with a longer prop with a lower twist. The longer prop also aids in hovering simply because it is longer and inherently adds stability.

If you are using larger props with higher twist than you have found that your motors are being stressed and are likely running hot or you are having low flight times to reduce the heat build up due to the extra power required to turn the higher twist prop. I'm betting that you have even let the magic smoke out of a few motors.

Hope this helps you out.

Enjoy your flying.
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Old May 27, 2011, 08:27 AM
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Ah, interesting. I'd kinda assumed lower Kv ratings for 3D but now you mention it, I'm thinking back to a few motor adverts that I've seen for Helicopters and these are always high Kv - OK so prop hanging isn't exactly the same but maybe there's a similar concept at work...?
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Old May 27, 2011, 08:57 AM
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Bingo.

In real world aviation a low prop pitch is termed a "climb" prop. Efficient for climbing...many smaller bites of air. As opposed to a "cruise" prop (higher twist or pitch) which takes fewer large bites of air and is efficient for cruise speeds.

In rc 3d the equivalent of a climb prop is argueably the best. (Higher Kv motor and lower twist).
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Old May 27, 2011, 09:06 AM
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Originally Posted by KE Spin View Post
If you are using larger props with higher twist than you have found that your motors are being stressed and are likely running hot or you are having low flight times to reduce the heat build up due to the extra power required to turn the higher twist prop. I'm betting that you have even let the magic smoke out of a few motors.
No, sir, and I'm proud of that fact -- I've never smoked a motor, or an ESC in flight, and I've deployed quite a few. I almost always scope things out with WebOCalc beforehand if there's any doubt.

Just wondering if the rules might be different for 3D, and you've given one possible rationale, so thanks for that.

My planes suffer no shortage of power or thrust in 3D, but I would like to have better control of hover height using throttle. Seems straightforward on the sim, but much less so in reality.
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Old May 27, 2011, 09:11 AM
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Like I said, it is argueable. Some guys like more pitch others don't. You are using lower Kv motors so you should generally be safe.

The problem really happens when a larger high pitch prop is paired with a higher Kv motor...then, heat and smoke.
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Old May 27, 2011, 09:39 AM
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If you look at the props, depending on power systems and battery voltage, most of us are running the 48" class planes on 11/5.5, 12/6, 13/6.5 and on the 55" class planes it's 14/7, 16/8, etc. You'll notice the pitch is exactly half of the diameter, otherwise referred to as half square. At least for the mid sized electrics, that seems to be the ticket.
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Old May 27, 2011, 04:36 PM
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The half square formula works well at least to 50cc size airplanes.

based on weight determine how many watts you will need, gennerally 160-200 watts per lb.

Pick a prop with a diamater roughly 1/4th the wing span.

14" prop on 57" wing span 18" - 19" for a 70"-72" wing span (good balance between torque and thrust)

Pick a motor in the correct weight range that will turn your selected prop at the Watts you want.

KV will depend on the battery voltage you intend to use. 3s,4s or 6s

Larger diamater props will be difficult to control P factor and torque roll in a hover

Higher pitch props will be harder to controll altitude in a hover and not as smoth in harrior.

small changes in throttle with a high pitch prop will show as quick changes in speed

Helicopters use high KV motors because the have a very high gear ratio. The actual rotor rpm is very low.

Should get you started,

Dennis
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Old May 27, 2011, 09:06 PM
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So nobody's recommending less than half-square, still, right? In any case, that's where I generally end up for my 3D planes, so I guess there's nothing to improve.
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Old May 28, 2011, 06:37 PM
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Personally, I match my prop to how I fly that plane. For low slow 3d stuff like my foamies, I like a larger diameter flatter prop that moves more air over the surfaces when flying slowly, as well as giving the throttle a less touchy response (slower acceleration). For bigger sky work with tumbles and such, I generally prefer a prop that is a bit higher in pitch, so I can quickly gain energy for tumbles and such, but that makes throttle more difficult to fly in harrier/hover type work. Typically I'm around 2:1 diameter to pitch, or slightly above on the pitch.

Then I pick a motor size and KV that will turn that prop at the desired efficiency and power (I prefer 200W/lb typically for 3d, at 85% efficiency or better), on the battery voltage I want to use. Kv is directly influenced by voltage, you can fly the same plane on 7.4V or 14.4V and get similar performance by using motors of different KVs.
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Old Sep 22, 2013, 09:17 AM
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Up north in Norway
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Nice post ! Just learned abit more about props !
As far as i can see from this post, it looks like i'm preaty spot on with prop size, but still, i'm haveing trouble to hoover it :/
Might have too much pitch and maybe to big prop?

sooo, i got a Yak 54 ep 1.4 meters.
Running on 5s 3700mah, 4250 KV840 motor, 80amp esc(i think it was), and 14x7E propeller.

I see others hoover this yak very nice indeed, so something isn't right here.. hmm
Any help on this would be great!

Thanks
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Old Sep 22, 2013, 10:46 AM
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Your pitch should be ok. The 14" prop is on the low end of length but you likely can't go any much bigger with the 840kv motor and 80a esc. I have noticed that smaller prop disc's and a nose heavy or overall heavy build make for tough hovering. How is your cg and all up weight compare to what is recommended for your plane?
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Old Sep 22, 2013, 10:49 AM
Honey bee king fan
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Quote:
Originally Posted by KE Spin View Post
yes, looks like that should work nicely.
hmm.. but it dosn't :P
Like i said, for hoover and harrier it's not to good :/
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Old Sep 22, 2013, 10:56 AM
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How does your battery setup and cg compare to the recommended setup? Are you using much larger batteries (extra weight) as compared to the recommended setup.
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Old Sep 22, 2013, 11:07 AM
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Up north in Norway
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I wouldn't say that. Throttlestore had some 3D revomended setup, but isn't on site anymore.

HK recomends this:
Flying weight: 2200~2500g
Motor: 3740 Brushless outrunner 500kv
ESC: 60A (Not Included)
Battery: 3000mah 22.2V 6S lipo (Not Included)
Propeller: 14x7

Havn't checked weight on mine yet, but i belive i'm around avarage.

Because this modell is built with the servo board wrong way inside (fab error), i hade to use some lead inside the cone.
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