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-   -   Selecting Electric Power Systems - (https://www.wattflyer.com/forums/showthread.php?t=18521)

AEAJR 12-17-2007 09:17 PM

Originally Posted by arniep (Post 302538)
I think 75 watts per pound should do it. I am not a speed demon and actually enjoy slow flight with the ability to do some maneuvers.


What motor did you finally choose, or are you still thinking about this? If you are still looking for advice, let us know.

You never said how much the plane weighed or what battery you were planning to use, so I will take some guesses. I will assume a 1 pound plane and 3 cell lipo is your target pack. So a 75 watt set-up should work.

Take a look at the combo 061 on this page.

This is a 90 watt package

I have one of these in my Mountain Models SmoothE and am very happy with it. You will need a 3 cell Lipo or 8 cell NiMh pack that can handle about 10 amps. I use a 3 cell 1250 15C pack from FMADirect.

mred 01-02-2008 01:16 AM

Need some help with motor replacement
I need a little help with a motor. I have this plane, Ultrafly Outrage-3D that I am in need of a motor. It calls for an Ultrafly B/06/10. I had it in my shopping cart at Tower, but got side tracked for a few days and when I came back, it was gone. I mean gone forever. It's not even listed on the Ultrafly site as a motor they make anymore. I guess it was on sale to git rid of the few they had left and now I am out of a motor. I don't know what to replace it with because I don't know anything about the motor to begin with. While I have been flying for awhile, I am kind of new to elect. planes and only have a couple now. Love my Super Sportster with a brushless motor in it. Way over powered, but love it to death. 450 Watt motor with a 4000mah 3 cell LiPo battery with a 9x6 prop and boy does it fly great. I'm not insane for power on all my planes though, just some of them. Anyway, any help sure would make my life easier. Thanks.

AEAJR 01-02-2008 03:47 AM

I presume you read the first post and the one just before yours. So what help do yo uneed?

You know about watts and watts per pound. How can I help you. I don't track every plane and motor made.

adwb 01-08-2008 09:49 PM

missing link help please
Please can any one tell me how to use this link in the 1st post in this thread
thank you

"First I recommend you read this article for background
information. It will help you understand and use the rest of the resources
listed below.

Sizing Electric Power Systems -

AEAJR 01-08-2008 11:06 PM

Thanks for letting me know about this. I think that site just closed down. Try this:

http://www.ampaviators.com/index.php?option=com_content&task=view&id=41&Itemi d=27

I updated the link in the first post.

Bob Vollaro 01-11-2008 05:50 PM

I attended my first club meeting last night. The topic was"how do I know what system to use" I'm new to RC so I'm trying to soak-up as much knowledge as possible on power systems. AEAJR's intoduction and related web sites was excellant! I'll pass it along to members (Flying Tigers AMA club #3937):) The sport needs more of this type Info. for folks just getting into Electrics. THANK YOU AEAJR

AEAJR 01-11-2008 06:56 PM

Glad to help. I have been approached by a number of people who want to repost that. Feel free to distribute. Tell them to post questions and comments here so those questions and answers can add to the knowledge captured in this thread.

Mike Anderson 01-18-2008 03:23 AM

Watts / Lb at the Extreme
I'm coming late to this party but thougt that an article I wrote for our Model Aeronautics Association of Canada (MAAC) magazine last April, would fit rather nicely here:

Extreme Electric Flight
Mike Anderson©

In July 1987, Keith Shaw proposed a few new rules of thumb for electric flight in his Electric Sport Scale article published in Model Builder magazine. For sport aerobatics you needed 40 to 60 watts per pound from the battery and you’d need 70 to 100 W/lb for "good" aerobatics. In the electric flight world of 1987, the best motors were DC (brushed) with cobalt magnets and could deliver 75% of the battery’s power to the prop. Big, high current batteries were 1750 mAh NiCd cells that get you a 3 and a half-minute flight at 30 amps. Now fast-forward twenty years.

The latest generation of LiPoly batteries are rated to deliver 50C currents for short bursts of 3 to 5 seconds and with capacities around 5000 mAh, that means 250 amps to the motor. The latest AC (brushless) motors can deliver 90% to 95% of that power to the prop. Nowhere do these two facts work best together than at the extreme performance end of electric flight; F5B, AKA racing gliders.

Visit the FAI website (www.fai.org/aeromodelling) and you’ll soon discover that F5B competition rewards planes, pilots and equipment that climb the fastest and fly the fastest. Pilots fly laps between pylons with the motor off, but need extreme power to climb to height again to re-enter the course. They can make no more than 10 climbs in 200 seconds to about 400 to 600 feet altitude and they want to be going FAST when they enter the course, snap off their motors and fold their props in tight for a gliding speed run. When that’s all over they climb one last time to glide before making a spot landing exactly ten minutes later.

Wings and tails are hollow-moulded in composite structures of carbon fibre, kevlar & fibreglass, producing a thin, tough wing that can slice through the air like a Samurai sword. Fuselages are moulded in different combinations of the same materials. An empty airframe weight of 650 grams would be considered heavy.

At the last F5B world championships held summer 2006 in Pitesti, Czech Republic, world class F5Bers were pulling over 250 to 300 amps for 3 second bursts out of 16 NiMH cells that were delivering power in the 500 W/lb range. Climb rates were close to 10,000 feet per minute and velocity entering the course would be in the 130 to 150 MPH range. Many teams showed up with dozens of 16 cell packs, many with the intention of flying the pack once then trashing it, knowing that the abuse of a single flight would render them useless for further competition. Several competitors simply pushed too hard and had packs either venting or popping cells in flight. Speed controllers were overheating and planes were seen trailing smoke.

At this level of competition, F5B has become the Grand Prix of aeromodelling sport. The FAI will be considering rule changes to reduce the competitive pursuit of power-at-any-cost, by installing a watt-minute limiter on every plane. In its simplest terms they are fixing the size of the gas tank. You can burn all your watts at once, or pace yourself over the eleven climbs. The result is expected to be power levels that don’t cause catastrophic equipment failures that can lead to personal injuries or property damage.

The FAI is also considering allowing LiPoly batteries. In the Fall of 2006, LiPoly battery cells started hitting the market with incredibly low internal resistance – the 5000 mAh, 50C, 250 amp types. Low internal resistance means voltage stays high under 50C loads and less internal heat is generated. These batteries will deliver high amps without overheating, weigh half as much as the 16 cell NiMH packs and deliver better climb rates because of the weight reductions.

At about the same time, motors have been coming out, specifically design for this event, that don’t even reach their peak efficiency of 94% until they are pulling 190 amps! Their efficiency doesn’t drop off much even at 250 amps. The challenge is to prop the motor to keep below the 60,000 rpm limit and keep the current in the 200 to 250 amp range, otherwise you’re not making the best use of the motor.
Speed controller development hasn’t been standing still either. Castle Creations offered an ‘experimental’ 300A ESC in January 07 and were sold out in days.

Combine these developments and we will be looking at planes flying at 700 watts per pound this summer. They will climb at 11,000 feet per minute, have almost 5:1 thrust-to-weight ratios and reach 150-mph motor-on velocities then gently land at slow speeds. Motors & batteries won’t be dangerously hot and participants & spectators alike will be safer. Welcome to the new world of extreme electric flight.

Mike Anderson 01-18-2008 03:46 AM

Watts/Lb in real life
Just for fun . . .

A Spitfire with a 1096 kW Merlin weighing about 6750 lb flew on . . . . 162 W/Lb.

An Me109 was pretty much the same.

A Zero was closer to 145 W/Lb.

peterboshoff 01-22-2008 06:29 PM

I am starting the other way round.
I have an EMAX 4030-10 brushless motor (5000gm thrust) with an ELF HOBBY 100A Brushless controller and a 5000 mAh 5 cell Lipo battery - but it don't want to work - Can't get the motor powered up - or the ESC programmed - Get "music" but nothing else - other servos connected with the receiver does not work either - Same receiver on another ESC and another motor does work - Does some ESC's only work with certain brushless motors?? IF I can get this monster working I will decide what plane to design. (Close to quarter scale, I presume - once I have learned to fly, though.)

Mike Anderson 01-23-2008 12:30 PM


Originally Posted by peterboshoff (Post 342513)
I have an EMAX 4030-10 brushless motor . . . Can't get the motor powered up

Assuming this ESC has a BEC, can you operate the Rx on battery power & disable the BEC to see is the BEC is the problem?

On a typical large-scale plane like you're describing, you would not be using the ESC's BEC anyway - that would be too unreliable, unwise & unsafe. . . . once you learn to fly that is . . . probably not in the first year as a new pilot.

gfdengine204 03-28-2008 09:13 PM

I'm getting it, but.....still not quite there yet.
I've read this thread a few times, and I think I am starting to understand better. However, I'm curious about one thing: kV. I get the watts per pound, and volts X amps to get the watts, but how do I know what kV motor to use for a particular plane? Or, does the kV follow along with the watts (meaning, the more watts the motor puts out, the lower the kV)?

For example, say I am building a simple foamie, and I have all the TowerPro motors to choose from at www.headsuprc.com. I can always send Jeff at HeadsUp an email, and say "I have this particular plane, what motor combo do I need?", but I would like to learn to figure it out myself. Hopefully I am making sense here.

It's much simpler when I need a motor conversion (say, to replace a .40 glow engine) and go to a website like HobbyCity that has motors listed as direct replacements, but still, I would like to be able to figure this out on my own.

mred 03-28-2008 11:45 PM

The KV is the RPM the motor wants to turn for each volt applied. So for a 1000KV motor it will turn 1000rpm with 1 volt or 10,000rpm with 10 volts applied. The higher the KV rating, the faster it will turn and the smaller prop it wants for it to stay in the Max amp range of the motor. The lower the KV rating, the slower it will turn and the bigger prop it can turn. So if you want a fast plane go for a high KV motor and if you want a big prop with a lot of pull, go for a lower KV rating. For something like a powered glider or 3D type plane you want a big prop for thrust to help it climb or hover, but if you want a fast plane then you will get fast flat line speed, but nowhere near the clime thrust of the bigger props. Hope this helps a little.

gfdengine204 03-28-2008 11:56 PM

Originally Posted by mred (Post 384097)
The KV is the RPM the motor wants to turn for each volt applied. So for a 1000KV motor it will turn 1000rpm with 1 volt or 10,000rpm with 10 volts applied. The higher the KV rating, the faster it will turn and the smaller prop it wants for it to stay in the Max amp range of the motor. The lower the KV rating, the slower it will turn and the bigger prop it can turn. So if you want a fast plane go for a high KV motor and if you want a big prop with a lot of pull, go for a lower KV rating. For something like a powered glider or 3D type plane you want a big prop for thrust to help it climb or hover, but if you want a fast plane then you will get fast flat line speed, but nowhere near the clime thrust of the bigger props. Hope this helps a little.

Thanks mred,

I get that the kV equals the RPM per volt; that's the easy part. I guess what I don't get is how to select the right kV value for the plane I want. So if I have, for example, 4 motors that fall in the Watt value I have determined for that particular weight plane, I select the higher value kV if I want more speed, and the lower if I want more thrust?

Also, does the amount of thrust ever come into play here? When I look at the HeadsUpRC pages, it groups the motors by thrust levels, not by Watts.
I really hope I am not making this more difficult than it need be; I'm just having a tough time wrapping my head around it.

mred 03-29-2008 12:17 AM

OK, take two motors that are the same exact spec, except for the KV rating. The low KV motor will turn a bigger prop for things like 3D where you have slow speed , but require a lot of thrust. The lower KV motor would be the choice for this type of flying because it can turn a big prop slow and gives lots a hovering ability. The higher KV motor would still fly a 3D plane, but you would need to use more throttle to hover and would not pull out as fast or not at all.

Now go to a sport plane that is used mostly for sport flying and some stunt playing around and you want a faster, higher pitch prop for speed. This is the high KV motor. You can fly something like a Super Stick on this type of motor because you are not really going to be flying 3D type stuff and you are not worried about hovering with it.

So, if you want high thrust 3D type flying, go low KV. If you want sport fast flying, go high KV motor.

Also, the bigger the motor the slower the KV will be. When I say big, I mean size of motor, not watts. A big motor has more mass to turn, so it has to turn slower where a small motor has less mass, so it can turn faster.

Does this make any sence to you ????

gfdengine204 03-29-2008 12:20 AM

Yeah that makes sense, I think.

So, say I build a flat foamie that i have no intention of 3D flying. Just to putz around the field. I can use a higher kV rated motor since I am not worried about hovering, right?

Then, in the case of all these TowerPro and similar motors, what makes me decide which one is the right one? Or could more than one be the "right" one?

mred 03-29-2008 12:36 AM

You could use more then one type of motor for a given plane, but for a foamie, you really don't want to fly very fast, so a lower KV rated motor would be better, or you could just use the throttle and fly it slower. They are not made to fly very fast, or they will start coming apart, unless you have one of the REALLY good ones, like a P-38 type foam plane or something like that. To fly something like Super Spoerster E you can fly faster and use a higher KV rated motor, but still can't get real carried away with speed. I am flying one now on a 450 watt brushless motor and can't open it up because something bad may happen. It's the same with foam planes. Go to fast and bad things start to happen. That's a big motor for that plane, but it sure is fun. You just have to watch what you are doing with the throttle when you fly something like that. It's up to you how you fly your planes and what kind of motor you use. Just don't be one of these guys that has an on off switch for a throttle. Each plane has many motors that will fly it. It mostly depends on how you want to fly and pick your motor from that.

TopSgt 04-10-2008 09:55 PM

Funster power setup
Hi all. Ihave a funster arf from hobby lobby with the recommended power setuo of ajax 292710 bl and ajax 45amp speed controller and dx 7 using the digital servos that came with the dx 7 and a high torque jr servo on the flaps also using the ubec from hobby lobby. This is a 5.5lb auw airplane with a 5amp 3s1p lipo.
Here is the problem--using the recommendeed 13x8e prop i get 320 watts with a frsh batterie dropping to almost 300watts. 13x10 same readings.
12x8 280watts 14x6 310watts 14x8 350watts Usind 14" prop have less than 1" clearance to ground--not good. Now what??? Do I need to go to a three blade prop?? If so what size?? Never used one before. Motor and esc only good for 3s so cant up the voltage unless I change both.
Help?? Thanks in advance. I figured from these forums that I needed 450-500watts. Baylor????????

mred 04-11-2008 01:56 AM

Unless I misread something, the motor can take 3 to 5 cells, but the ESC is rated for 2 to 3 cells, so you could change the ESC out for another, or go to the 3 bladed prop, or both.

That is not a high speed plane so somewhere around a 6" or 7" pitch is fine. If you change to a 3 blade prop, the dia. should go down 1". If you are using a 13" now, you can go to 12" and you should try for around 1 1/2" ground clearence if you can get it.

You don't need to worry about what power is you are running at as long as you are not going over the rating of the motor, ESC and the battery. If you have a 500W motor and the ESC can take 45 amps, then you need to adjust the amps to keep the current down so you don't go over the Watts the motor is getting. As long as you keep the Watts under 500, you will be fine. I would shoot for something like 450W with a 500W motor. It's not good to run them at their rating all the time. Also, with a new ESC you can go to 4 or 5 cells to get more rpm along with the lower pitch and it should pull the plane fine. You may have to play with props to see what you and the plane like, but a 12X6 or 7 will pull the plane good.

You could also replace the landing gear with something that will give you a little more ground clearence or, BP Hobbies has an ESC that can take 2 to 6 cells at 45 amps and is only $22.95 or you could use the CC if you have an extra $100.00 that needs a new home. There is a lot of ways to go. Just depends on what you want and what you can afford, but if you have another battery, the ESC is the only thing you will need to replace. The BP ESC does not have a BEC, so you will need to add that if you don't already have one, but that is only another $10.00 or so depending on what you want.

Take care and let us know how it turns out.


mred 04-11-2008 02:36 AM

OK, I did some more looking on your motor and they reccomend a 12X6 2 blade on 4 cells, or a 13X6 2 blade on 3 cells. If you want to get more ground clearence, you can use 4 cells and a 12X6 2 blade prop and it will give you a little more or you could stay with the 3 cells and go 3 blades and use a 12X6 prop. Either way this will give you some ground clearence and still have enough power to fly the plane OK, but I think the 4 cell will give you a little more power. It just depends on what you are aiming for.

TopSgt 04-11-2008 04:47 PM

ED--First of thanks for your help. I have a 100amp esc that I had stuck in temp to run the same test so no problem. I obviously had a brain fart on the motor, because I thought it was maxed out at 3s. What had me concerned was the Magic Numbers that I have read on this and other forums--50w/lb to barely fly 75w/lb to fly good 100w/lb for basic aerobatics and 150w/lb for extreme aerobatics. 450watts is approximately 1 horsepower. My old funster(kit built and heavier) flew great with a K&B .61 at approx 1hp(20 years ago) What upsets me is that for the same money HL sells the opto 45amp esc that will take 5s. Ialso have two 4ah nimh 9.6volt packs. Sooo what I think I will do is run them in series with the 100amp esc. 19.6v into 500watts equals 26.3 amps so this will give me ample power but more weight and balance issues. Oh well this is fun right?? Thanks again for your help---Baylor Ps wil try the 12x7x3 when lhs gets it in.

mred 04-11-2008 10:57 PM

I think that 1hp is around 750W, not 450W, so if your motor is putting out 500 Watts, you should have around .66hp unless I made a mistake in my math again. With this plane, I would go with somewhere around 75 to 100 watts per pound to get a decent flight out of it.

Most people are running around 1300 Watts with a Senion Telemaster and that is around 100 Watts per pound. If yours is 5.5lbs auw then you should be running around a 600W motor running it at about 500 to 550 watts. If you can run 500 watts with your motor without it getting hot, then you should be good. They never told the watt ratting of this motor on the HP site, but that should give you a good flying plane and while it will not 3D, most people don't try that kind of flying with this kind of plane. However, I do know a couple that insist on trying to hover everything they fly.

From what I have seen, the watt per pound is a good starting point for any plane. At 90 watts per pound on your plane should give you a good flying plane. At least that is around what I get from the data you gave me. I over power my planes some times more to use the equipment I have then for all out flying, but I do enjoy flying them a lot with more power. Anything around 100 WPP will give you a good flying plane.

Hobby City sells a LiPo battery that is priced pretty good and has a quality cell made in Korea, which is where most of the major man. get their cells from. It's the Zippy-R battery and if I remember right it is 21C to 25C rated and about half the price of the other batteries, but from what I have been getting out of these, it is just as good. I have been running two for almost a year now and it reads the same as when I started flying with it. It's a 2200mah battery and it still takes the same to charge up and is flying my plane just as good as it did when I started. I have almost 100 flights on each battery and they seem to be holding up pretty good. A new battery from them would drop a good bit of weight from your plane and make it fly better.

I think I got most of this right, but I would check my numbers if I were you. I have made mistakes before no matter how much I try and get everything right. Take care and hope you have a blast with this plane.


mossman 05-18-2008 05:46 AM

Ive got a n E max motor 2215/25 brushless with a dualsky 2200mah Battery motor is new after crash replaced it at 3/4 throttle to max runs real rough then cuts out like power surging.

coul hgis be the ESC. Battery is new dualsky 2200 mah 3 cell 16 C E Shy ballance charger for 3 to 4 cells charging off car battery takes a few hours to charge.

Ive had two supercubs which i loved flying then went to glow scanner & boomerang flew ok but messy & after a couple of flights crashed Ive gone back to electric.


peterboshoff 05-19-2008 10:35 AM

Watts and Thrust
[quote=mred;391897]I think that 1hp is around 750W, not 450W, so if your motor is putting out 500 Watts, you should have around .66hp unless ...
Watts /pound or thrust /weight - how the two relate depends on the efficiency of the power system - you can use a lot of watts without getting much thrust - What can you expect in terms of thrust with 500 watts.
(If you fly a plane of 5lb with 5 lb thrust you will be able to fly it - right or wrong?)
Still trying to figure it all out on my own. I can measure my thrust with my own designed thrust meter.

mred 05-19-2008 05:23 PM

Watts /pound or thrust /weight - how the two relate depends on the efficiency of the power system - you can use a lot of watts without getting much thrust - What can you expect in terms of thrust with 500 watts.
(If you fly a plane of 5lb with 5 lb thrust you will be able to fly it - right or wrong?)
Still trying to figure it all out on my own. I can measure my thrust with my own designed thrust meter.[/quote]

If you have a 5lb plane and are using a motor with 5lb of thrust, you are way over powered for some planes and under powered for others. It depends on the type of plane you have and the kind of flying you want to do. If you are talking 3D, you are underpowered, but if you are talking trainer, you are over powered. I good pattern plane it would be great. You have to tell me the type of plane and how you intend to use it before I could even guess at what you need. Yes it will fly, but how is the question. As for the thrust, it all depends on how much power you have and what kind of prop you are turning. You must take into consideration the plane and how you intend to use it before you can pick your power system. Give us ALL the information and I am sure someone can give you some information that is close to what you are looking for. Cheers.

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