Author Topic: Thought on electric settings (Read 4698 times)

Randy Powell · « **on:** October 19, 2015, 02:22:10 PM »

Showing my newbie self here:

I know some guys that have set up electric to run high rpm, low pitch. Maybe 10,200rpm and a 3.5 or 4 pitch prop. I've seen electrics set up to run in a more conventional way. Maybe 8500 to 9200 rpm with a 5 or 6 pitch prop (how I have been going). Anyone tried a low rpm, high pitch prop sort of setup? Maybe mid 7000 rpm and 7 or 8 pitch prop? Pros and cons? I imagine it would be more load and probably need more battery. Thoughts?

Wolfgang Nieuwkamp · « **Reply #1 on:** October 21, 2015, 07:34:12 AM »

Pro: Lower tip speed => less noise
Contra: Lower reynolds number => lower efficiency
Pro: less inertia => less hinging, but maybe 3 blade is necessary at low revs
Contra: New motor with lower kV required.

When pitch, diameter and rev/min are adjusted so that the same current is drawn (= same power), then the same battery and controller can be used.

One way to experiment with different blades is to use folding props. I do this on my variable pitch system (limited to 2 blades)

Tim Wescott · « **Reply #2 on:** October 21, 2015, 09:11:25 AM »

It'll need more torque, to make power at the lower RPM. All else equal, a motor's torque capability goes with weight, so you'll need a heavier motor. For the same kV you'll need more current at a lower voltage, so you may need to juggle ESC and batteries around.

Wolfgang Nieuwkamp · « **Reply #3 on:** October 21, 2015, 09:35:40 AM »

The higher torque is created by more windings on each pole, so that you have higher torque with the same current. That is why you need a lower kV motor, at the same weight, same power.

Tim Wescott · « **Reply #4 on:** October 21, 2015, 10:35:20 AM »

Quote from: Wolfgang Nieuwkamp on October 21, 2015, 09:35:40 AM

The higher torque is created by more windings on each pole, so that you have higher torque with the same current. That is why you need a lower kV motor, at the same weight, same power.

Part of my day job is selecting the correct motors for tasks, so this is based on direct experience. For a given motor design, you can change windings to your heart's content, but as long as you pack the windings into the slots efficiently, the constant for that motor's mechanical design will be how much torque you can generate for a given amount of power burnt up in the motor windings. This is, in fact, so well recognized that it's been given a name and some manufacturers sell motors by it: the motor size constant. https://en.wikipedia.org/wiki/Motor_constants.

That's because -- assuming that you started with a motor whose windings filled the slots -- when you put more windings on each pole you have to use finer wire to make it fit. Consequently, the total resistance in the armature windings will go up roughly as the square of the number of turns. The power lost to heat in the windings is equal to current squared times resistance (the phrase "I²R losses" is fairly common in power-design circles). As long as the windings are thoroughly packed into the available space there is basically no win between two motors with the same mechanical arrangement and different windings -- more torque = hotter motor. In fact, when you go to buy motors for industrial purposes you'll often get data sheets for one motor frame with many different windings, which all share the same motor size constant.

Randy Powell · « **Reply #5 on:** October 21, 2015, 10:43:46 AM »

I wasn't so interested in how to do it as if there is any benefit to it. I've seen guys using IC engines running this way and was just wondering if there were any benefit to doing it this way with electric.

Igor Burger · « **Reply #6 on:** October 21, 2015, 11:00:10 AM »

Quote from: Tim Wescott on October 21, 2015, 10:35:20 AM

That's because -- assuming that you started with a motor whose windings filled the slots -- when you put more windings on each pole you have to use finer wire to make it fit.

That is true, but since you run it al lower RPM, it means lower iron loses. The question is if you save more o less. All depends if your motor runs at its best efficiency, if it is overloaded or underloaded. For exmpale if you run it only little loaded so that loses in iron are say 10 time higher then copper loses, you will certainly benefit to run is slower at 1/2 RPM :- ))

Igor Burger · « **Reply #7 on:** October 21, 2015, 11:20:17 AM »

Randy, it is only fraction of equation. You must count with prop diameter (and also blade count, blade width) etc etc.

There is aproximate equation that speed stability (how much model slows down at higher drag) at constant rpm depends on delivered power. It means if you have the same power for both aplications, and you go up with pitch while keeping the same diameter you will go down with power and clearly you will have lower speed stability. To compensate that, you must go to larger prop, to load the shaft. Such prop is typically more efficient, but it also does not pull so well stalled. Because larger part of blade is over its critical AoA. You can tell it if you try to keep running piped model with low pitch prop and jurrassic bubling ST60. Difference in static thrust is large, but both will reach the same speed, so the in flight thrust is the same.

I wrote somewhere in some article that folloing props are _aproximately_ equivalent "powerwise" :
2 blades 12”x6”
2 blades 11”x4”
2 blades 11”x6” with 50% wider blades
3 blades 11”x6”

So advantage of higher pitch is little lower consumption, and disadvantage lower static thrust (could be visible in critical situations) however reason why I like relatively low pitch (motor running over 10 000) is that while power = torque x rpm, lower pitch at the same rpm make lower prop torque (moment) to airframe what make trimming easier. For example that 11"x4" prop has 1/3 moment to airframe compared to those other 3.

Wolfgang Nieuwkamp · « **Reply #8 on:** October 21, 2015, 11:30:12 AM »

Igor,

should it not be 11x7 or 11x8 instead of 11x4?

Randy Powell · « **Reply #9 on:** October 21, 2015, 11:30:38 AM »

Thanks, Igor. That makes sense.

Igor Burger · « **Reply #10 on:** October 21, 2015, 12:05:05 PM »

Quote from: Wolfgang Nieuwkamp on October 21, 2015, 11:30:12 AM

should it not be 11x7 or 11x8 instead of 11x4?

why?

Wolfgang Nieuwkamp · « **Reply #11 on:** October 21, 2015, 12:10:09 PM »

Because you state that they are "power wise equivalent". At the same rpm, I can imagine that a 12x6 and an 11x7 consume the same power.

Igor Burger · « **Reply #12 on:** October 21, 2015, 12:11:47 PM »

That is clear that powerwice, but how you got that equaivalence?

Howard Rush · « **Reply #13 on:** October 21, 2015, 12:24:32 PM »

Quote from: Tim Wescott on October 21, 2015, 10:35:20 AM

The power lost to heat in the windings is equal to current squared times resistance (the phrase "I^{2R losses" is fairly common in power-design circles). As long as the windings are thoroughly packed into the available space there is basically no win between two motors with the same mechanical arrangement and different windings -- more torque = hotter motor. In fact, when you go to buy motors for industrial purposes you'll often get data sheets for one motor frame with many different windings, which all share the same motor size constant.}

Speak up, Tim.

Wolfgang Nieuwkamp · « **Reply #14 on:** October 21, 2015, 12:27:24 PM »

Igor,

Well, on a static measurement device they produce more or less the same thrust at the same rpm.

Igor Burger · « **Reply #15 on:** October 21, 2015, 12:30:14 PM »

But we cannot fly 7" pitch prop and 4" pitch prop at the same rpm

Tim Wescott · « **Reply #16 on:** October 21, 2015, 12:33:43 PM »

Quote from: Howard Rush on October 21, 2015, 12:24:32 PM

Speak up, Tim.

Fixed. I shouldn't use formatting wingwazzles without checking my work.

Dennis Adamisin · « **Reply #17 on:** October 21, 2015, 02:03:04 PM »

Quote from: Randy Powell on October 19, 2015, 02:22:10 PM

Showing my newbie self here:

I know some guys that have set up electric to run high rpm, low pitch. Maybe 10,200rpm and a 3.5 or 4 pitch prop. I've seen electrics set up to run in a more conventional way. Maybe 8500 to 9200 rpm with a 5 or 6 pitch prop (how I have been going). Anyone tried a low rpm, high pitch prop sort of setup? Maybe mid 7000 rpm and 7 or 8 pitch prop? Pros and cons? I imagine it would be more load and probably need more battery. Thoughts?

Hi Randy
When I set up the 6S test bed using a Strega ARF, the first prop used was a 13x6.5. It settled in around 8000 RPM and worked OK in good conditions. However, at 80 oz the Strega had a knack for pushing through the prop when the wind came up. Later tried a 13x4 at 11k and it handled the wind much better, but at that speed it sounded like an unmuffled 60. It was also at 99% of APC's rated RPM limit. Subsequent development led to the thin 13x4.5EP and the 13x5.5EP that work exceptionally well in the mid 9k range. Focus was on flight performance, so I do not have any meaningful data on trade-off with battery usage - other than to say it had excess battery capacity with any of the props tried.

I have some of the 12x6.5 WEP's that run well at 82-8400 RPM, but they are a bit heavy. Let me know if you want to try a dozen or so!

Barry Robinson · « **Reply #18 on:** October 22, 2015, 09:35:37 AM »

Hi Guys,
My approach is working well for me and might be worth sharing. I am using a Turnigy Aerostar 720 Kv motor with a new shaft fitted o I can use a 5 mm collet drive from Hacker. The Turnigy shaft is 6 mm, so I remove this and turn a new shaft using ground bright steel 6 mm bar and turn the front down to 5 mm for the collet drive. I am using my own production 3 blade wood prop which is 12" x 7" and is turning at between 7800 and 8000 revs. The Model fly's at 5.34 seconds a lap and pulls really well. I use a 30c - 3200 ma battery and in flat calm, lands with about 13% left in the battery. In wind it is normally about 20%.The ESC is running at 45 degrees centigrade internal temperature and I am using the Igor Burger Timer system and Jeti 66 ESC.
Before this I was using MVVS and Plettenberg motor sat 950 Kv and 11.5 x 6 3 blade wood props and 9000 revs. These went well, but no where near as good as the 12 x 7 set up. I tried the 12 x 7 on the Plettenberg and while it worked well, it was outside it's best rev range and the ESC was working at 65 degrees centigrade. Hope this helps or provides interest.
Forgot to say I use 4s batteries.

Barry Robinson · « **Reply #19 on:** October 22, 2015, 04:02:13 PM »

Quote from: Barry Robinson on October 22, 2015, 09:35:37 AM

Hi Guys,
My approach is working well for me and might be worth sharing. I am using a Turnigy Aerostar 720 Kv motor with a new shaft fitted o I can use a 5 mm collet drive from Hacker. The Turnigy shaft is 6 mm, so I remove this and turn a new shaft using ground bright steel 6 mm bar and turn the front down to 5 mm for the collet drive. I am using my own production 3 blade wood prop which is 12" x 7" and is turning at between 7800 and 8000 revs. The Model fly's at 5.34 seconds a lap and pulls really well. I use a 30c - 3200 ma battery and in flat calm, lands with about 13% left in the battery. In wind it is normally about 20%.The ESC is running at 45 degrees centigrade internal temperature and I am using the Igor Burger Timer system and Jeti 66 ESC.
Before this I was using MVVS and Plettenberg motor sat 950 Kv and 11.5 x 6 3 blade wood props and 9000 revs. These went well, but no where near as good as the 12 x 7 set up. I tried the 12 x 7 on the Plettenberg and while it worked well, it was outside it's best rev range and the ESC was working at 65 degrees centigrade. Hope this helps or provides interest.
Forgot to say I use 4s batteries.