after wrecking the airplane electronics i got in crashes i decided to put in some stuff i had laying around rather then buying more stuff. The thing is the esc is meant for a boat and the motor is for a car, the motor is a [link=http://www.hobbyking.com/hobbyking/store/uh_viewItem.asp?idProduct=15195]motor[/link] and the esc is a 120 amp turnigy boat esc. The prop is a 7x5 by landing products. total weight of the plane plus a 3700 mah 25c 2s batt is about 1000 grams or 2.2 lbs

in case the link doesn't work motor is a 3650 2800 kv inrunner.
All numbers given are static.

amps at max throttle 51.87
the watts were 387.9
voltage didn't sag below 7.35

amps at just below half throttle 22
watts were 170

after 5 min of mostly half throttle with a few burst of full motor temp was around 115 at the end bell and esc temp was only 85-90 depending on where i temped it.

the plane is a mini magister by multiplex so im assuming it falls under the glider/trainer area.

Based on my calculations (2.2 x70 =154) i have enough power to fly at half throttle and enough power at full throttle to go near vertical.

CG was maintained at 2.65" behind the leading edge of the main wing by gluing nickles to the tail of the plane and putting the battery as far aft as possible.

so the question is do you guys see any problems with the flight? I know it is out of the ordinary but it seems like it should work.

Wind hasn't been cooperating so i haven't had a chance for a test flight and the plane ended up in this state from trying to fly in 15+mph winds in the first place so im trying to learn from that mistake.

The stock prop is a 12x8 with a gearbox so I imagine you'll have trouble with it going all the way down to a 7x5. The weight of the components you've used might be an issue too.

I have the mini mag stock prop looks more like a 6x4 for a brushed 400 motor also there was never a gear box

Stock prop is only 5 by ? id say 4 or so. it glued directly to the shaft. have a pic but I'm on my phone and don't know how to do it from here.

7.4 x 2800kv = 20720

A 7x5 prop spinning at 20720 produces about .765 HP or about 569 watts.

To get that wattage you have to push 76A through the motor which is TOO much, so you must not be spinning the motor at a proper speed at full throttle.

( Remember that any time you spin the motor it is ALWAYS trying to hit the KV rating of 20720 or so RPM. )

The motor is rated at 750 watts and 50A so you are not effectively using the motor.

I would switch it out for a better suited motor and battery combo.


Then I'd take the electronics, get a Great plains gearbox and motor mount, and put it in a .25 sized electric plane.

[link=http://www3.towerhobbies.com/cgi-bin/wti0001p?&I=LXMLK0&P=7]Gearbox - Click me![/link]

[link=http://www3.towerhobbies.com/cgi-bin/wti0001p?&I=LXLCL4&P=ML]Mount - Click me![/link]


If you have more than one identical battery, hook them up in series to create a single 3700mAh 25C 4S pack ( this is done merely with a connector, you don't change anything on your batteries ).

Select a pinion gear ( provided with the gear drive ) to give you about 13,000 RPM's at full throttle and then install a 12x7 or 13x7 prop

This gives you a nice larger electric with plenty of power and speed and uses your cast off electronics effectively.

You can separately get additional 4S packs to power this combo for additional field flight time and charging.

20720 assumes 100 efficiency which I know is not the case with any motor, especially a hobby king motor. So 20720 x .80 is 16576. I do have 2 3700 mah packs and several 4s 4000 mah packs but they will not fit.

Nope, you're confusing electrical efficiency with KV RPM's they are not the same thing.

Your motor will still run at it's KV rating even with a low efficiency. The big difference will be that more current is demanded from the battery to do that and much of that may be lost as heat.

20720 is actually a bit conservative since LiPo packs start off producing far more than their rated voltage and tend to operate around their rated voltages in normal operation.

As I said, all of this is ill suited to that plane, you should pull the electronics and install are more suitable motor and ESC.

Then put the motor into a bigger plane with a gear drive.

I appreciate your oppinion on the matter. The number you are coming up with is unloaded rpm. Eagle tree logging will show u that a motor rated at a certain kv will not reach that kv under load. Also this motor is rated up to 4s so 14.8 x 50 (the max rated amps) and that is where 750 watts comes from. On 2s you would need to pull 100 amps to pull close to 750 watts. So I am effectively using the motor with a 7x5 on 2s. Go to a 8 x 3.8 slowfly and amps go in excess of 50 amps. A fully charged 2s lipo will read 8.4 v. But under a load they sag to 7.4 or even lower rather quickly.

Although ill suited, these electronics work just fine. Flew today for solid 6 min motor came down at 120 esc was 97 as were the batts. Tons of vertical power. It also does a little better in the wind with the added weight. I'm happy with it. Couple sketchy landings where the prop shaft hit the ground but nothing happened with the 5 mm shaft. 1/8th inch shafts seem to bend easily.

Correct, but the motor still TRIES to reach it's KV rating and that has nothing to do with it's efficiency.

Anything less than the KV rating represents power loss to the prop or wasted as heat.


Correct.

Incorrect for the very reason you stated; that to run close to 750 watts you DO need to pull 100amps from the motor, something your ESC and batteries cannot attain. That's the point.


In a properly configured system, the voltage output from the battery will start at around 8.4v and run that way for well over 1-2 minutes, then drop to around 7.4v or slightly below and stay that way when the plane unloads in the air.

If you are getting more sag than that, your battery pack is not up to the task.

It is still an ineffective use of what you have. You are using LESS than 30-40% of the motor's capacity at MAX.

Your motor can produce far more power than what you are pulling from it, and you are yanking the excess weight this represents around in the air on the plane. A smaller motor would likely do better, weighting less and running more efficiently.

While you may not destroy your electronics ( only prematurely use up the batteries ) given what you are currently doing, you came here seeking advice which you received as to what problems exist with your reused electronics.

That being the case why come here in the first place and ask if you were already dead set on doing things as you've done?

i came to share and see what people would say. Like i said i appreciate your opinion i just don't necessarily agree with it 100%

Esc is a 120 amp with a 240 amp burst ratting and it sees amp draw in excess of 100 amps in my boats so it can handle a good sized load. 3700 mah 25 c is good for 92.5 amps so with me only pulling 50 amps at WOT they are just fine.

Do you have an eagle tree logger? At full throttle about 1000 mah is used every minute after 2 min 40% of a 5000 mah battery is used up, if your batteries are still giving you 8 volts after 40% ( more in the case of the 3700 mah packs) then you have some amazing cells.

If the motor was more efficient energy wouldn't be lost as heat, or due to resistance which would result in higher rpm under load. So really efficiency and KV actually are directly related


Why would i want to pull 100 amps from a motor only rated to handle 50? As mentioned earlier the esc is up to the task, i know the batteries aren't. Im having a hard time understanding why pulling 50 amps from a motor rated at 50 amps is not effectively using it. I think we can both agree that on 4s 41440 rpms is too much, while a gear box is a nice idea there simply isn't room for one, nor is there room for a 4s pack big enough to handle a 50 amp load.

If you only saw how hard I was climbing yesterday you'd know i wasn't lugging around extra weight.

I have 2 2s 25c 3700 mah, 1 2s 20 c 3700 mah, a 2s 20c 3200 mah , 3 2s 20c 5000 mah pacs and a 2s 25-50 c 5000 mah battery which is also why I like my setup because i can utilize all of those batteries and have plenty of time at the field.


How will pulling 50 amps from a battery rated at 92 amps constant prematurely use up the battery? Its only 54% of its max rating, well within a safe limit.

I received your advice. While i see that you have been a member for a good amount of time and have several post it doesn't mean you are right. After all I was out shopping yesterday and talk was still cheap, unlike the option you suggested which would require spending 40+ bucks on just mounting hardware and then buying an additional plane. The whole point of using these electronics was because they were laying around (free so to speak), I didn't choose them specifically for this project but i saw a possible way to make it work and i executed it. It happened to work out. I could buy a smaller motor and esc for the plane for nearly the same price as the gear drive and motor mount.

So yes you gave advice i just don't consider it any good, but I do appreciate your thoughts on the matter and the time you took to respond to my post.

Maybe you should have titled the thread and your questions a bit differently then.


The use of CAPACITY has little to do with the output voltage.

LiPo's don't react like NiCD's and NiMH packs.

LiPo's output above their rated voltage for the first minute or two of operation then the voltage drops a bit and stabilizes around the rated voltage of the pack. There-after it decreases VERY slowly until you hit the voltage "Knee" on the curve where the cells drop off percipitously.

You should NEVER let LiPo's get to the latter point.

After 20-30% usage on my 5000mAh 14.8, 22.2, and other packs I still get very close to their rated voltage, But during those first minutes the voltage output is above that.


Wrong.

KV is merely a measure of how fast the unloaded motor spins.

It tells you nothing about the efficiency of the motor at all.

You can have a 2800 KV motor that is highly efficient and one that is terribly in-efficient, yet in given situations the actual observed RPM's can be similiar or not.

You keep missing the point.

You DO NOT - WANT - to run 100 amps, but because of the way you have things set up with your electronics, to get to the proper power levels and best efficiencies for your motor you would have to have it pull 100amps to get close to it's wattage rating which is rediculous.

You should be running the motor at about 80% of it's capacity under normal flight loads at high throttle. You're not anywhere close with your setup.


Your motor is rated for 50 amps at 750 watts output

So run the numbers

750 / 14.8v = 50A 41440 RPM

750 / 11.1v = 67A 31080 RPM

750 / 7.4 = 101 A 20720

Get it?

To get 750 watts or close to max power out of your motor you need to run a 14.8v pack.

OR you need to push 67A if you are using an 11.1v pack just to get the same...

Or you need to push 101A if you are using a 7.4v pack as you are now.

The problem is that the motor simply cannot spin fast enough to keep up with the KV rating using a prop, unless you go to a VERY small prop

20270 RPM is rediculously high for anything but a tiny prop or an EDF. Your motor doesn't fit what you are trying to do.


Which is why that motor is not really suitable for the plane.

To use an analogy:

It's like putting a huge dragster engine in a Volkswagon. The Volkswagon will seem powerful, and can go faster than ever, but it will never attain the speeds that dragster engine can produce.


Actually it is lugging around extra weight. You could do better with a smaller lighter combo better suited to your 7.4v packs.

You're equating climb ability with unnecesarry weight. While the latter can affect the former, the two are not equal.

2s is an extremely low voltage for most RC planes. You will normally only find this voltage used for the micro aircraft. Likewise the motors that use 2S packs are for the micro planes.

You are better off and taking any two of the matched packs and with a simple cable turning them into a 4S pack. You don't have to do anything to the batteries themselves. The adapter does the job.

Two of the 2S 5000mAh packs will power a much bigger .32 to .40 sized aerobatic electric plane with ease.

e.g my old 48" Diamante flies with 14.8v ( 4S ) 3600 packs or with 4S 5000 packs and hovers at half throttle.


To get the power levels that motor SHOULD run at, you would have to put a lot of wear and tear on your battery packs.

Your are not currently doing so, so your batteries are OK, but in the same breath you mentioned the voltage drop, which indicates that the packs may not be up to the task.


The quantity of time here and the number of posts are irrelevant...

What is relevant are the numbers. They don't lie.

You can pull up Motorcalc and verify what I've told you.


More non-sequitors.... You asked after all about "problems" with your setup.

I listed them for you.

While not optimal, I'm glad you think it worked out for you. I'm not contesting the latter.... I'm only addressing the questions you posed.

That's right. It may not be worth doing for THIS plane.

It's sometimes not worth it to try to make the wrong choice work... when doing it over may actually be cheaper.

Think what you may, the numbers say what I've been telling you.

I merely pushed the figures through on your setup.

Download Motorcalc ( you can use it for the first 30 days for free ) and input the values for the motor, ESC, prop and battery pack then run an analysis on your plane... maybe the graphs will better help you understand.

Kudos for making it work. You don't have the best performing, most efficient or lightest setup in your plane, but it flies. That's the main thing and if you're happy with it you did good. If you ever want to customize a system to a plane to get the best performance you can, opjose is quite knowledgeable and will probably answer your questions again.

Hey I also have several 4s packs. Like 4 4s 4000 mah 20 c packs. And another 4 5000 mah 20c packs. I usually run a 4s 2p set up in my boats. And also as u noted I have the 2 2s packs that I use in series all the time anyhow. What kind of kv would be good for direct drive? Over in the boat world we like to work with a prop speed around 30000 k so around 1800-2200 kv is a typical motor selection.
I'm thinking I want a mid wing for my next plane. What would be some options to build a plane around the 4s cells I have and I don't see why I couldn't utilize my 120 amp speedo. Only thing is I'm more interested in the parkflyer size and it seems too keep cost down. Is it possible to get a plane servos and a motor for 200 give or take? how big a plane are we talking to use 4 s in? Wing span? And would it be foam? Not good enough to want to get away from foam. Stuff is surprisingly tough and easy to repair. I have a truck but I prefer to have the plane in the back seat vs in the bed.

You have a lot of options with those batteries. Shooting for an amp draw of 10-15% of your pack gives respectable flight times, so for your 4000 mah packs that would be 40-60 amps, using between 500-700 watts (allowing for efficiency losses). Given the rule of thumb of 100 watts per pound for general sport flying and 125-150 watts for aerobatics, you'd be looking at a plane as heavy as 7 pounds (a slow floater) or as light as 3.5 pounds (high performance). So pick a plane in that range that you like and then work out what motor you need. There isn't a general Kv rule for planes. That decision is based on what prop the plane needs at what rpm. I wouldn't worry much about going to balsa if there's a balsa plane you like. Yes, they incur more damage from major crashes, but most crashes are just nicks and bumps which they'll generally stand up to just fine.

Even a 4lb foamy will be a very large plane and perfect for those packs wired with a connector into 4S packs.

The 120A ESC will also work with this setup just fine.

Assuming the lowest batteries he stated

3700mAh 2S 25C x 2 = 3700mAh 4S 25C

3700mah x .15 = 555 watts continuous

555w / 14.8v = 37A draw continuous ( more in bursts ).

So exactly as I originally told him, he could get a .25 - .32 equivalent motor rated around 750w burst and put that in a .25-.32 aerobatic or 3D plane.

Assuming a typical 11K RPM rate with 13K unloaded

13,000 / 14.8v = 878 KV

So a motor rated for 40-50A, 880KV and 750watts is what he needs along with a prop that will allow the motor to spin around 10-11K at full throttle on the ground. A 12x6e prop or a 12x7e prop will produced the required draw from the motor.

Or if he wants to re-use his existing motor, he can go with a gear drive as I stated....

However given the cost of the gearing and mounts, it's cheaper for him to merely order a new motor that conforms to what I've stated, and a serial adapter for his packs. That's less than a $40.00 expense.

The motor comes with the prop nut, so the standoffs and spinners may be the only other things needed.

While the 120A ESC is overkill, it's use will not hurt anything as long as it is programmed to turn off braking and other car related functions.

Any plane that falls into the size catagory I mentioned will work fine, especially if it is suitable for electrification ( e.g. it has a battery hatch, etc. ).

$200.00 should be enough to get the motor, servos and plane from places like HK ( USA ) or BPHobbies.


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