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1/4 scale pup electric conversion

Old 03-26-2016, 06:50 PM
  #1  
sabreflyer
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Default 1/4 scale pup electric conversion

Hi all, I am new to electric and in the middle of building a 1/4 scale pup to come out at around 15lbs. It was destined for a 120 fs but I want to fit it with an electric motor. I have an Axi motor 232 kv and some 10s flight packs. This is supposed to be equal to a 140 ic size. I could do with the extra weight of the 10s pack at the front but could I use this set up using an appropriate prop or would it be too much?

I am really grateful for any help you can offer. I started researching motor combos but after an hour or two of finding contrary info I have found a link here and it certainly looks a helpful site.
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Old 03-27-2016, 02:35 AM
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kyleservicetech
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Originally Posted by sabreflyer View Post
Hi all, I am new to electric and in the middle of building a 1/4 scale pup to come out at around 15lbs. It was destined for a 120 fs but I want to fit it with an electric motor. I have an Axi motor 232 kv and some 10s flight packs. This is supposed to be equal to a 140 ic size. I could do with the extra weight of the 10s pack at the front but could I use this set up using an appropriate prop or would it be too much?

I am really grateful for any help you can offer. I started researching motor combos but after an hour or two of finding contrary info I have found a link here and it certainly looks a helpful site.
We need the wingspan, wing area, and which AXI motor you've got. As far as having "To much Power", that really isn't an issue, since an overpowered motor can be dialed down by just putting on a smaller diameter prop. If the selected motor/batteries is way to high powered, you will have issues with to much weight.

There are a lot of very good computer programs available for helping in your decision. The one I use is www.motocalc.com, free for 30 days, then $39.

You enter in the wingspan/wingarea/motor/prop/battery pack, and click on the "Opinions" tab. Motocalc will respond with a general opinion on how your model will fly. Motocalc is pretty close, if the motor specifications are accurate. And, with programs such as motocalc, you can enter a range of propeller pitch and diameters, and the program will spit out a whole bunch of results. So, if you key in a 16 to 18 inch diameter prop, and a 10 to 12 inch pitch, motocalc will spit out results for 16X10, 16X11, 16X12, 17X10, 17X11 - - - and so on. Each with a predicted opinion on how it will fly.
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Old 03-27-2016, 02:59 AM
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Assuming the Axi is the equal of a 1.40 ci 2-stroke its CAPABLE of appx 30% to 50% more power than a 1.20 4-stroke.


However... you can moderate the actual power by prop selection. electric motors attempt to deliver the power that is demanded. If you don't demand it by putting on the larger prop load (that it is rated for) it will just deliver the power needed to turn the prop you installed.

Also.. the structure needed to deal with the cyclic stresses of a piston engine is more than what is needed for as much as double the power electric drive.

So with proper support for the (typically shorter) electric motor being spaced away from the glow power firewall, you should be fine.

I expect your biggest issue to be achieving proper CG since WWI fighters typically have short noses, the brushless motors are much lighter than the glow engines they replace and getting the battery far enough forward can be a problem.
I've had to add lead to the cowl of every WW1 fighter I have... except the Albatross which has a longer than typical nose.
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Old 03-27-2016, 05:44 AM
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My 1/4 scale pup uses a 295KV motor....20X10 prop and 6 4000 mah lithium cells.
It weighs 15 pounds and has stellar performance with this combination.
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Old 03-27-2016, 03:18 PM
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Hi all, and thanks for your replies.

The wingspan is 77" and the chord is 15" so that would work out at 77x15x2= 2310 wing area. It should come out at around 15lbs in weight (so same as yours, Ron).

The electric motor I have is the Axi 5325/24. It is a 232kv motor.I have been running this in a 2m aerobatic model on 10s 5000ma and a 20x13 prop and 90 amp esc. I wondered if I could use the same combination and just swap the prop if needed? I could use the extra weight of the 10s pack up front for the C of G.

Again, many thanks for your help.
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Old 03-27-2016, 04:14 PM
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I think it looks like you are in business sabre. Bear in mind, easy battery access is all the rage these days. If it too big of a PITB to get too, it detracts from the overall experience. I think this is why kyleservicetech enjoys his A123 cells so much. I am under the impression that he never removes them from the plane. So yeah, do your best to accommodate a reasonably accessible hatch into your build.
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Old 03-27-2016, 05:10 PM
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Camel is easier to do a hatch for than the pup... the camel's gun mount and hump generally are provided as a molded piece which makes a great hatch.

(hint for hatch location...)

These WWI bipes have you working right by the prop when connecting the batteries... be careful.
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Old 03-27-2016, 09:57 PM
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Originally Posted by sabreflyer View Post
Hi all, and thanks for your replies.

The wingspan is 77" and the chord is 15" so that would work out at 77x15x2= 2310 wing area. It should come out at around 15lbs in weight (so same as yours, Ron).

The electric motor I have is the Axi 5325/24. It is a 232kv motor.I have been running this in a 2m aerobatic model on 10s 5000ma and a 20x13 prop and 90 amp esc. I wondered if I could use the same combination and just swap the prop if needed? I could use the extra weight of the 10s pack up front for the C of G.

Again, many thanks for your help.
With that huge wing area, you're not going to need a lot of power to fly what looks to be a very slow flying airplane. The AXI 5325 motor has a winding resistance of 0.045 Ohms, which is rather high for this size motor. Motocalc indicates this AXI motor is going to run very hot with a 20X13 prop and a 10S LiPo.

A quality motor of this size should run in the area of 85% to 90% efficiency. Using Motocalcs numbers with the 20X13 prop, you're only going to get around 60% efficiency. Motocalc indicates the model will fly OK, as long as full power isn't applied continuously to the motor.

Motors such as the Hacker A60-7S motor would work well in your setup. But, with the 10S LiPo battery pack, it would likely be way overpowered. As an example though, with the A60-7S motor, motocalc predicts an efficiency of 91% while pulling the same Amps out of your LiPo pack. This means much more power turning the prop, versus making the motor hot.

FYI, as a comparison, the Hacker motor has a winding resistance of half that of the AXI motor. That is one of the items that causes lower efficiency of the AXI.

Below is the motocalc prediction with the A60-7S motor:

MotOpinion - Quarter Scale Pup
Sea Level, 29.92inHg, 59F
Motor: Hacker A60 7S V2; 215rpm/V; 1.2A no-load; 0.026 Ohms.
Battery: FlightPower 3700 (20C); 10 cells; 3700mAh @ 3.7V; 0.0037 Ohms/cell.
Speed Control: Generic; 0.001 Ohms; High rate.
Drive System: 20x8 (Pconst=1.31; Tconst=0.95) direct drive.
Airframe: Quarter Scale Pup; 2300sq.in; 242oz RTF; 15.2oz/sq.ft; Cd=0.075; Cl=0.45; Clopt=0.57; Clmax=1.24.
Stats: 152 W/lb in; 138 W/lb out; 18mph stall; 26mph opt @ 79% (25:04, 92F); 30mph level @ 88% (21:52, 95F); 1519ft/min @ 40.8; -366ft/min @ -9.1.
Power System Notes:
The full-throttle motor current at the best lift-to-drag ratio airspeed (42.9A) falls approximately between the motor's maximum efficiency current (40.4A) and its current at theoretical maximum output (680.7A), thus making effective use of the motor.
The voltage (34.5V) exceeds 12V. Be sure the speed control is rated for at least the number of cells specified above.
Possible Aerodynamic Problems:
The diameter (20.0in) to pitch (8.0in) ratio is greater than 2:1, which will result in reduced propeller efficiency at flying speeds. An appropriate smaller diameter, higher pitched propeller would improve this.
Aerodynamic Notes:
The static pitch speed (52mph) is within the range of approximately 2.5 to 3 times the model's stall speed (18mph), which is considered ideal for good performance.
With a wing loading of 15.2oz/sq.ft, a model of this size will have very sedate flying characteristics. It will be suitable for relaxed flying, in calm or very light wind conditions.
The static thrust (369.6oz) to weight (242oz) ratio is 1.53:1, which will result in extremely short take-off runs, no difficulty taking off from grass surfaces (assuming sufficiently large wheels), and vertical climb-outs. This model will probably be able to perform a hover or torque roll.
At the best lift-to-drag ratio airspeed, the excess-thrust (144.9oz) to weight (242oz) ratio is 0.6:1, which will give steep climbs and excellent acceleration. This model should be able to do consecutive loops, and has sufficient in-flight thrust for almost any aerobatic maneuver.
General Notes:
This analysis is based on calculations that take motor heating effects into account.
These calculations are based on mathematical models that may not account for all limitations of the components used. Always consult the power system component manufacturers to ensure that no limits (current, rpm, etc.) are being exceeded.
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Old 03-27-2016, 10:11 PM
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If motocalc is saying 60% efficiency... the prop is most likely too big.
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Old 03-27-2016, 10:22 PM
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Originally Posted by fhhuber View Post
If motocalc is saying 60% efficiency... the prop is most likely too big.
Yup
To get a motocalc prediction of 90% efficiency out of the AXI motor, you need to put a 16X8 prop on it, and reduce the current to 40 Amps, and Watts down to around 1400 or so.

Problem is, AXI recommends a 20X12 prop and a maximum of 75 Amps for this motor. With this, motocalc predicts over 400 degrees F on the motor. Don't know how accurate motocalc is on their calculation, but to me it's a matter of concern.
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Old 03-28-2016, 12:30 AM
  #11  
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Recommended prop and max voltage often have nothing to do with each other...
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Old 03-28-2016, 12:42 AM
  #12  
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Thanks for the replies.
Is the motocalc suggesting that the axi motor prop and 20x13 and 10s will run hot? If so this can't be the case as this combo has been used a lot for f3a aeobatics with no problems. Or is it that this combo would run hot in this type of airframe? I was thinking that I have the combo on the shelf and so would save me buying new gear and the weight of the combo would be useful. However, if it might not be good set up then I guess l'd rather get it right first time and get a new set up. What do you think?
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Old 03-28-2016, 01:04 AM
  #13  
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The draggy bipe will make the motor work harder. It will never achieve the same airspeed as the F3A model.

The only way to know for sure what happens is to get some data from a wattmeter or one of the data logger systems.

The calc program is just making predictions based on data that was input. I haven't tried motocalc in a long time... it used to be more and more inaccurate as the motors got larger. I had it predict that a model I flew in Pattern competition wouldn't get off the ground.
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Old 03-28-2016, 02:33 AM
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kyleservicetech
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Originally Posted by sabreflyer View Post
Thanks for the replies.
Is the motocalc suggesting that the axi motor prop and 20x13 and 10s will run hot? If so this can't be the case as this combo has been used a lot for f3a aeobatics with no problems. Or is it that this combo would run hot in this type of airframe? I was thinking that I have the combo on the shelf and so would save me buying new gear and the weight of the combo would be useful. However, if it might not be good set up then I guess l'd rather get it right first time and get a new set up. What do you think?
The current pulled by these motors depends on the prop, motor, and flying speed. What worked well with your 20X13 prop on a fast airplane might not work well with the same setup on a slow draggy Bipe.

As an example, if the prop blade airspeed is say 80 MPH at full RPM, and the models airspeed is 60 MPH, there is a considerable reduction of how much power is required to turn that prop. Or, do the same thing with a model that flies at 35 MPH. There will be much more load applied to the motor shaft, even though it's the same motor, same battery pack, same prop.

So, do you need a new motor/prop/battery? Probably not. If you hang to small of a prop on a glow/gas engine, it will over-rev, and possibly damage it. Hang to small of a prop on an electric motor, and it may not get off the ground. It simply won't work if you put a 6 inch diameter prop on your motor. Won't hurt, just won't work. (Of course, hanging to BIG of a prop on an electric motor is a real no no. These motors are DUMB. Overload them, and they will put out fantastic power. Until you see the smoke trail from the motors windings burning up)

So, somewhere between that 6 inch prop and your 20X13 prop will be a propeller that will work well with your model. Running the numbers through Motocalc, with your AXI motor and 10S LiPo, a prop on the order of an 18X8 would be in the ball park. With the 18X8 prop, motor efficiency is around 85%, a decent number.

With the AXI, 18X8 prop and 10S LiPo, motocalc predicts 7200 RPM, with a blade pitch speed of 52 MPH. (Your model won't fly faster than 50 MPH!) Static current will be around 50 Amps, Watts is around 1800, and the important Watts per pound of airplane will be around 120 Watts/pound. Anything over 100 Watts per pound of airplane for a slow flying model is very good.

Predicted climb rate will be around 1400 feet per minute at 38 degrees.

Motocalc is only as accurate as the specifications provided with the motor. I've got a number of Hacker motors, and motocalc is always within 5 or 10% of the predicted Amps and Watts.

As far as larger motors go, again, motocalc is close with my Hacker A60-5S, A60-16M, and Rimfire 50 cc electric. If you should find that with the AXI and 18X8 prop is off by perhaps 20%, just adjust the motors KV up or down til the number match. Then, use motocalc with the new KV number to select a better prop.

This is where it's about mandatory to use a Wattmeter, to make certain you are not overloading your motor.

Last edited by kyleservicetech; 03-28-2016 at 04:17 AM.
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Old 03-28-2016, 02:41 AM
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Originally Posted by birdDog View Post
I think this is why kyleservicetech enjoys his A123 cells so much. I am under the impression that he never removes them from the plane. .
You're right, my A123 cells are permanently installed in their model. No fire risks, no worries about storage charge, and every A123 pack is identical in performance to every other A123 pack in my collection. Top them off after a days flying and they are ready to go next day, next week, next month. They hold 95% of their charge after a full year on the shelf. As an example, last fall, I replaced a 12S2P A123 pack in my giant Big Stick with a new pack, since the original battery dropped off in Amp Hour capacity.

The original pack turned my Hacker A60-16M motor and 19X12 prop at 6500 RPM. A brand new pack turned the same motor/prop at 6700 RPM. And, yes, the old pack was 5 years old with 500+ flights on it, pulling 80 Amps at full power.

They can be charged at very high rates with a high powered charger. I'm recharging my packs in 15 minutes. The bad news, is that for these giant electrics with high powered A123 pack, they can't be recharged with a simple 120 Amp Hour deep cycle battery. It kills the lead acid batteries. Been there, done that. Three times in two years.

Right now, I've got 140 A123 cells in my various model airplanes.
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Old 03-28-2016, 01:04 PM
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Thanks for all these very useful replies.I am off on hols for a week so not sure what internet access will be like but will ponder your comments.
As far as a hatch goes, I have something planned which should work and hopefully will not be too much hassle to use but. I will look into the A123 packs.
I will catch up with any further replies when I get back next weekend. Thanks again for your help.
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Old 04-02-2016, 03:07 AM
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Motor prediction programs are at best a semi educated guess. If it is the opinion of the guy writing it that you need 1000 watts per ounce to fly, the program will reflect that in every combination. I know for fact that a lowly Eflite Power 160 on ten cells using a 20X10 prop works well..****calc says it will burn up the motor on 9 cells at a ridiculously high amp draw......which I know from tests conducted on my own setup, is way off base.
like 25% high. another program I have tried is similarly inaccurate.
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Old 04-02-2016, 03:23 AM
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Originally Posted by Ron View Post
Motor prediction programs are at best a semi educated guess. If it is the opinion of the guy writing it that you need 1000 watts per ounce to fly, the program will reflect that in every combination. I know for fact that a lowly Eflite Power 160 on ten cells using a 20X10 prop works well..****calc says it will burn up the motor on 9 cells at a ridiculously high amp draw......which I know from tests conducted on my own setup, is way off base.
like 25% high. another program I have tried is similarly inaccurate.
IMHO, a lot depends on the type of model being flown with a particular power setup. E-flite indicates their Power 160 can handle 60 Amps continuous and 78 Amps peak for 15 seconds.

Put that power system in a very fast streamlined model, and the current drops way off at high speed. Put that same power system in a big slow flying Piper Cub, and that is a much different application.

Motocalc suggests that the current pulled with a 20X10 prop and a 10S LiPo will result in a current level of 78 Amps. I've got three giant scale models, two with Hacker A60 series motors, and one with a Rimfire 50cc electric motor.

The motocalc predictions on the Rimfire 50cc motor and my 12S3P A123 battery pack, installed in a 20 pound 95 inch wingspan airplane are pretty close. Motocalc was within 5% on RPM and current pulled by that Rimfire motor. But as usual, motocalc, and other computer programs are only as close as the motor specifications provided with the motor.

This is the main reason it is almost mandatory to backup what ever predictions a particular computer electric motor program are with a wattmeter.

(PS, I've been flying electric models for 33 years!)
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Old 04-03-2016, 08:29 PM
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as I have often said....measure everything...if you don't measure it, you can't improve it.
another thing to consider as you said is the type of model, Bi-planes and tri-planes have a " sweet spot" where they like to fly. It takes very little power to get them there, but is takes heaps of amps and volts to make them go faster than they were designed to go. if it takes 45 amps to swing a 24X10 prop at 4800 rpm on 6 cells, it will take a few times that to swing the same prop at 6000 RPM. ...and after about 4800 RPM the prop becomes less efficient, the faster you turn it, the less efficient it becomes.
You can get lots of thrust at low amps...rpm requires volts ( my opinion) my 1/3 size DR1 has a maximum rpm of 2400, but the 28" prop gives more thrust than the model weighs, on 6 cells. 1/3 size Pup, 3400 RPM huge prop, 9 cells ( 35 pound model)
1/3 size TM...6 cells big prop, low RPM 4900 max. . all have relatively low amp draw, and consequently longish flight times ( 12 to 15 min. ) 103" Catalina...6 cells ....84" Albatross 3 cells. All have the ability to do some no so very scaleish type manoeuvres......since the most of the thrust is produced by the outer 25% of the prop, make the prop big enough to have at least 25% of it outside the diameter of the cowl.
AND build as light as possible. Since there is virtually no vibration in an electric model, lots of wood isn't necessary. These are all things we learn by chatting with one another, listening, and experimenting. Hopefully we grow as we go along. That's what makes the hobby enjoyable, and nothing beats the feeling of satisfaction you get when you look at a full scale plane, decide to build it, draw the plans, put it together, and on maiden it flies great. The only kit I have bought in the last 20 or so years is the Balsa USA 1/4 size Pup, which by the way is a good flyer, but a little porky in the wood department. ( again my opinion) . The Pup we are talking about in this thread, will fly just fine on the power system he has chosen.
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