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Old Mar 27, 2015, 06:55 AM
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advise for the correct power/weight setup?

i have a 6.5lb. pattern plane im setting up for electric. my clearance is limited to around a 13 max 2 blade prop. this does not really give me the power to weight ratio that is required. i was thinking 3 or 4 blade prop to solve this problem. am i missing something? im getting close with a 4 blade but realize this is not really the ideal set up for a pattern plane. power 46 with 10.5x8 4 blade gives me around 140watts/lb. any ideas? thanks, Walt.
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Old Mar 27, 2015, 04:03 PM
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Walt,

are you looking for an e-power plant for the bird or are you trying to work a setup using the E-Flite Power 46?

If you have the motor in hand and/or installed, your options are a little more limited. If on the other hand, you are looking for a 46 sized e-power setup, then there's really no problem.

Also, when you say 6.5 lbs, is that an AUW estimate? What would be useful is to know how much the model weighs absent motor/ESC and LiPo but with the finished airframe complete with radio including radio battery and optional voltage regulator if needed (2s lipos should have one unless you are using HV servos). BTW, I strongly recommend a radio battery rather than a BEC.

David
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Old Mar 27, 2015, 04:34 PM
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While I await some answers, I'll go ahead and break it down a bit. For spirited pattern performance, what you are looking for is between 1200 and 1400 W of power on a 46 size model.

Assuming the AUW is indeed 6.5 lbs flight ready which essentially means "wet". The main factor to an electric model's weight is the LiPo so this is why it is important to know how much the model weighs absent power plant and "fuel". Knowing that, we can gauge how much an e-power setup will add to the airframe/guidance weight and see if we hit our target or power to weight.

That said, your typical spirited performance on a 46 size sport pattern model will require a 5s setup. Knowing that, the power requirement is met with a nominal current draw of 75A (18.5V x 75A = 1388 W). Meeting this power input can be achieved with different types of motors and props. But..., in order to keep the model flying on a "glow like" prop and performance, you will need a higher Kv motor. E-Flite motors are nice but they are generally aimed at large diameter/low pitch 3D type props. For a sport/pattern model, a motor such as the Electrifly Rimfire 46 (42-60-800) is much better suited as it has an ideal 800 Kv. In comparison, the E-Flite Power 46 has a lower 670 Kv. This difference alone changes the prop required to get the motor up to power. The Power 46 might require something like a 13x8 whereas the Rimfire 46 can achieve the same output on an 11x7 or 12x6.

My approach is to prop the motor up allowing it to reach close to peak power (or peak current draw) at full throttle knowing that it is not a setting that can be sustained as it would with a glow engine. Full throttle allows maximum power in up-lines and for momentary level flight but in order to prevent the motor from overheating and burning up, throttle has to be managed. The Rimfire 46 is rated at 60A continuous and 100A peak (1850 W!) so if you prop it to reach 75A peak you will have ~1400 at FOT. This is performance similar to that you would have with an OS 55AX.

As for weight and flight time, it really comes down to how you fly. You can go light using much throttle management and a 3200 pack or aim for all out performance at a weight penalty and use a 4600-5000 pack. All 5s.

I hope this helps,

David

P.S. I just used the Power and Rimfire 46 as motor examples but you could use any other brand that has similar specs. The goal is find a motor that can sustain current draw and have the correct Kv for the prop you would like to run. 800Kv on an 11x7/12x6 gets you in the 55AX type space.

P.S.2 three and four blade props are an alternative which can be used for fun (better on scale models) but they are really not required for a proper 46 sized e-power plant.
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Old Mar 27, 2015, 06:35 PM
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Walt,

a really useful tool to check power setups is one such as Scorpion Calc. I don't recall who wrote this software but it is used by several manufacturers and "branded" by including their series of motors and specs.

Anyhow, I poked with Scorpion Calc a bit as they are a slightly better quality motor than the E-Flite/Electrifly offerings.

I checked the 4025-740 motor and here is what I found:
  1. 5s/Graupner CFK 12x11: 10.6K/80A/1469W
  2. 5s/APC 12X10E: 10.7K/76A/1398W
  3. 5s/APC 13x8E: 11.0K/69A/1272W
  4. 5s/APC 11x10E: 11.0K/66A/1220W
As you can see, this motor gets in the target range on a 12x10 delivering close to 1400W on 5s. Note that this motor has a 740 Kv so the higher 800 Kv Rimfire 46 would deliver the same power on slightly less prop and likely spool it higher. I'd venture an APC 11x9/11x10E would be at just about the same power level as the Scorpion but would turn in the 12.5/13K RPM range. An 11x10 turning 13K is serious power for a 6.5 lb model and more than comparable to say an OS 55AX turning an 11x7 in the 14.5K range.

David
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Old Mar 27, 2015, 07:41 PM
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wow! lots of great info. thank you. unfortunately it looks like i goofed up with the power 46 that i already purchased... to be honest at this point if i can get spirited SPORT performance ill be happy and use your vast knowledge, now mine, to purchase the PROPER e setup on my next build. i really want to build a bridi UFO so i guess i can DO IT RIGHT with that airframe. its a much nicer model anyway i think. not only did i get the wrong motor but was planning to run 4s batts. live and learn as they say. i was tiring of 3s performance in some of my other planes so i can recycle the 4s batts. just when i thought i knew what i was doing.... . thank you for the eye opening information. i will use it well on my next build. i am so in love with these older pattern style planes. i did not weight without power train. the 6.5 lbs. is with everything ready to fly on a 3600 4s batt. im getting about 850 watts of power at this point. im hoping i can fly her as a sport plane on that. well under your spirited pattern plane figures unfortunately... i had no idea the rimfire was so much stronger a motor than the eflite! wow! shocked!
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Old Mar 27, 2015, 08:40 PM
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Quote:
Originally Posted by walter3rd View Post

wow! lots of great info. thank you. unfortunately it looks like i goofed up with the power 46 that i already purchased... to be honest at this point if i can get spirited SPORT performance ill be happy and use your vast knowledge, now mine, to purchase the PROPER e setup on my next build.
Walt,

right, I suspected a Power 46 might already be in the mix. Knowing that and your all up weight including a 4s 3600, the thing to do now is to remove the battery and weigh it. Naturally, the model should weigh 6.5 lbs less the battery weight but that doesn't always work out () so it might be worth re-weighing the airframe without the LiPo installed - just to confirm.

Given the power 46, the ONLY way you will get proper power out of it, is to up the voltage. You will naturally also need to increase prop load but on 4s nothing good will come out of the airframe. You might wind up finding yourself in a tight spot and lacking adequate power, the model might not live long. Keep the 4s batteries for 25/32 size models and you'll be golden with that. For 40-50 size airframes we need 5s. For smaller 60 size classics we need 6-8s and with larger ~10 lb airframes we are looking at 8-10s. For example, EU-1A's do well on 8-10s while a model like the UFO should do well on 6s if built lightly and the proper motor Kv is used. Again, the Rimfire 60 is properly set at 600 Kv for a lighter 60 size classic.

Returning to your Power 46 setup, I highly recommend you pick up a couple of 5s batteries. The main problem with the Power 46 is that it has a spec max current draw of 55A with a continuous rating of 50A. In other words, this motor should not exceed 60A (they typically understate the motors capacity to prevent too many returns). If it could handle 6s then that would be no problem but with 5s being the ceiling, you can expect a peak power input of 18.5V x 60A = 1110W. This is fine on a 40 size 5 lb model but a little weak on a heavier larger 46 size model. Still, you really should get it going on 5s and prop it with something like a 13x8 or 13x10 to get it to max out at 60A. With 1100W if the model weight remains at 6.5 lbs, you'd still be looking at a power loading of close to 170 W/lb which is not bad at all. Note that number of blades alone does not affect power loading so going to a 3 or 4 blade prop will not increase power per se. It is a matter of overall geometry. True, a higher blade prop will load the motor more but it doesn't mean that you can't get the power output required from a 2-blade prop. You can even go to a 12x12 square prop and you'll likely approach the 60A peak at WOT.

In short, I'd strongly recommend that you use your 4s for other models and move this ship to 5s. If you are not satisfied with the Power 46 once you've loaded it up properly, you can always change the motor (at not too high a cost) keeping the LiPo and ESC unchanged.

Quote:
I was tiring of 3s performance in some of my other planes so i can recycle the 4s batts.
That is a good plan but note that there is nothing inherent about 3s performance being dull. It really is a matter of finding the right combo of motor and prop to get spirited performance. 3s is ideal on smaller 2-3 lbs models but once you get into the 3.5 lbs airframes, 4s is a better way to go - assuming we're not talking WWI models.

Quote:
I had no idea the Rimfire was so much stronger a motor than the E-Flite! wow! shocked!
E-flight is a weird thing. One can't quite compare two motors like the Power 46 and Rimfire 46 and say the Rimfire is much stronger. They both can deliver about 1100W although it appears the Rimfire is wound to handle slightly higher current which DOES make it stronger all else being equal.

However, it is misleading to say the Rimfire is stronger than the Power in the same fashion as one might say for example of a Rossi 45 or YS 45 compared to an OS 46. With the glow engines the statement does hold true but with electric motors, it really comes down to how much voltage is being applied and what prop one uses to load the motor and bring the current draw up to produce power.

In other words, one can seek say 1600 W of power and whether one uses a Power 60 or a Rimfire 60 really comes down to what type of RPM, prop and voltage one wants to run. One can get the same 1600W of power from each but one might require a larger slower turning 3D type prop while the other will turn a smaller "classic" prop at high RPM giving performance more akin to a piped glow 60. So when it comes to e-power, Kv is a magic number which one must pay close attention to.

To finish the thought, note that the Kv also indicates what the maximum RPM the motor will turn all else being equal. Using the Power and Rimfire 46 examples:

Power 46 (670 Kv): 18.5V x 670 x 0.85 = 10.5K RPM (12.4K RPM no load)
Rimfire 46 (800 Kv): 18.5V x 800 x 0.85 = 12.6K RPM (14.8K RPM no load)

In the above, the 0.85 is a typical efficiency factor (the motor is not 100% efficient due to power lost to heat for example) one must take into account when the motor is loaded. Program like e-calc take this into account providing accurate numbers. The RPM numbers offered above are about the maximum RPM the motor can be expect to spool up regardless of prop. That is the RPM the motor was designed to turn at so the motor will draw more current in order to try and reach that figure.

David
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Old Mar 27, 2015, 10:57 PM
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i will do just that. ill get some 5s batts and see how much performance i can squeeze out of this plane with that setup. do you have a general figure for the watts /lb. for good pattern performance? thanks for all the wealth of knowledge and the education. Walt.
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Old Apr 01, 2015, 04:33 PM
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Well, you probably saw my setup of my Dalotel plane in another thread and I follow David.
I have a Robbe Roxxy 4250/06 800 kV motor in it on 5S (big fat 4200 mAh 5S lipo's) and a 11x7 prop running on +/- 12100 RPM max which really pushes the plane. It's the perfect setup.
This motor pushes maximum 1250 W before melting down, I push it up to +/- 950W to keep it healthy.

The AUW is +/- 6,2 pounds so this makes about 163 W/lb

This Dalotel needs 5S, I have some vintage sportflyers too in the same size and they fly perfectly with 4S, but 4S would not provide me enough energy for this model. It would fly with 4S, and it would fly rather good, but for this kind of plane you go no less than 'awesome'

I fly some Sebart 50e size planes too and they require a 6S setup. I need that power for those powered vertical lines without going too high on amps.

My advice: go for a 12x8 prop, look for a 700-720 kV engine that can push roughly 1100-1300 on 5S. Since you're plane is a little heavier and bigger than my Dalotel you need a little more proppower, so a bigger prop compared to mine and a motor which is a little bigger.

One important remarkt: be sure that your lipo's can provide the right amount of current without cracking to much. Higher C values wanted, or go for e.g. 4000 mAh than 3300 mAh.

Small comparison: I flew a Calmato trainer (returned back to the hobby late 2013 after a rather long hiatus) with those 5S cells, a 620 kV motor and a 12x8 prop.
It was a Calmato on stero´ds and it serves me very well for candy dropping etc.
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Old Apr 01, 2015, 07:19 PM
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thanks. walt
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