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Power Systems Talk about motors, ESC speed controllers, gear drives, propellers, power system simulators and all power system related topics

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Old 10-17-2013, 01:44 PM   #1
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Default Please advise on 8 motor power system...

Well, yes, I just cannot help myself...

I have started engineering on a 6 foot semi scale Hughes H-4. And I want to run all 8 motors, no fakies. I have decided to go brushless, and equip each motor nacelle (right word?) with an ESC and a LiPo. Just run signal wires through the wings. Or I will mount a big battery in the nose, and all ESCs there too, running 24 wires through the wings to the motors. I still have to calculate with balance and weight. Using the Ivan P building method, I'm aiming at 3000 grams AUW.

OK, so power-wise, I would like to be at 700 Watt, since it's a float plane. Hey, let's go nuts, take 800, divided over 8 motors, makes 100W each. I'm good at math, aren't I? Now I want to run a 4" 4 blade prop, because that would be most scale, but this is not a must. 3S has my preference, although 2S would be an option. At 3S, 100W is 9A, at 2S 13,5A. No big numbers there.

Here are my first guesses at what to buy:
http://www.hobbyking.com/hobbyking/s...dProduct=31690
http://www.hobbyking.com/hobbyking/s...dProduct=32551
http://www.hobbyking.com/hobbyking/s...cro_Motor.html

Please feel free to comment,
Thanks,

Hugo

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Old 10-17-2013, 02:04 PM   #2
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Your motor choice has a way too low KV for a 4" prop. Look for a motor with a KV of 2000 to 2600.

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Old 10-17-2013, 02:08 PM   #3
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You are right, I posted the wrong kv version. Should have posted this one:
http://www.hobbyking.com/hobbyking/s...cro_Motor.html

Same thing, 1800kv. I think that should be plenty as I want to go 4-blade.

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Old 10-17-2013, 07:27 PM   #4
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Originally Posted by HugoW View Post
Well, yes, I just cannot help myself...

I have started engineering on a 6 foot semi scale Hughes H-4. And I want to run all 8 motors, no fakies. I have decided to go brushless, and equip each motor nacelle (right word?) with an ESC and a LiPo. Just run signal wires through the wings. Or I will mount a big battery in the nose, and all ESCs there too, running 24 wires through the wings to the motors. I still have to calculate with balance and weight. Using the Ivan P building method, I'm aiming at 3000 grams AUW.

Hugo
Yeah, it would be a mess, but putting one battery in the fuse, and running wires to each motor would add weight.

Perhaps one way to go, is to put one good LiPo battery in each wing panel, with the motor and esc wiring for each motor all contained in the wing. All the ESC's can be then connected to just one ESC control cable going to the receiver. Methinks one good LiPo battery would weigh slightly less than four smaller LiPos of equal total amp hours. Plus, it would be a LOT easier to charge one battery per wing, rather than four.

Please note that if your ESC's have BEC's, they must be disabled by pulling the red wire from each ESC servo cable. Just push in the tiny tab, pull the red wire out, and cover with shrink tubing. That way, if you want to go back to single ESC use in a different model, it's easy to put the red wire back.

One hundred watts per motor would be about right for 3000 grams. Don't know how that "Little Prop" would translate to actual power pulling the model forward though.

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Old 10-17-2013, 10:48 PM   #5
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3000 grams is around 6.6 lbs, I dont think 8 motors with 4" blades is going to give you enough power/thrust

http://www.hobbyking.com/hobbyking/s...cro_Motor.html
This is a very nice powerful micro motor, I have recommended it many times, Barry uses them on his multi motor planes with a 7x6 apc Prop on 3 cells. I use this motor at 20 amps all day long and they love it. this motor has thick windings in it, so it will take the higher amps.

This 4 blade prop should work well as its a SF prop, its 6.5 x 2 as per customers that have purchased it, you will have to modify it a little, maybe by just drilling all the way through it and using a nut to hold it on, the motor shaft on the 1800 kv motor i belive is 3mm and this prop is 2.3mm dia. the props can be cut down a little and rebalanced.
http://www.hobbyking.com/hobbyking/s...de%20propeller

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These things have great promise with a bit of engineering, chop off the front dome of the spinner and bore out the plug to fit a prop adaptor.
Use a two bladed spinner and cut out two extra slots then mount on a CF2805 or bigger and hold on tight! sounds great and loads of bite from those paddle blades.
Have one mounted on a 21'' span Sea Fury I am just finishing and really looking forward to getting it into the air,


use turnigy 25 amp esc and 1300mah 25 to 50 C 3cell lipos, or you may want to use 1- 2200mah lipo for 2 motors, you will need at least a 25amp esc. with a 1800kv motor. because you will be drawing about 15 to 20 amps, and be around 150 to 200 watts per motor assy.
http://www.hobbyking.com/hobbyking/s...ontroller.html

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Old 10-17-2013, 11:39 PM   #6
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http://aviationtrivia.info/Hughes-H-4-Spruce-Goose.php

Small 4 blade props are really hard to find. the 4 blade 6.5 x 2 prop may do it. you might be better off using a 2 blade prop, like a APC 6x5.5



you may need to buy or build a thrust stand to test the power/thrust of your power system to see if you have enough thrust to fly your plane with.

http://www.bphobbies.com/view.asp?id...=V786359&img=l


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Old 10-18-2013, 12:39 AM   #7
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just an idea, you may want to make the motor, esc and battery a modular system and have it slip into the nacelle and lock into place with a spring loaded pin.


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Old 10-18-2013, 02:24 AM   #8
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Originally Posted by CHELLIE View Post
3000 grams is around 6.6 lbs, I dont think 8 motors with 4" blades is going to give you enough power/thrust
Hi Chellie
You're right. Just ran the numbers through www.motocalc.com. It will take a motor that will turn an 4X3 propeller at 30,000 RPM to get a lousy 10 ounces of thrust, per motor. So, that would be 80 ounces of thrust for all 8 motors, that on a model that will weigh 110 ounces or so. It will take about 26000 RPM on that 4 inch prop to hit 100 watts per motor. Problem is, the prop efficiency really drops off with reduction on prop diameter.

At a more realistic 20,000 RPM, 50 Watts per motor, and 4 ounces of thrust per motor, the model likely would never get off the ground.

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Old 10-18-2013, 10:43 AM   #9
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Thanks for all the input. I will have to do some serious thinking, as a 7" prop will simply not fit. On the inner motors, they will hit the fuselage, and between each nacelle there is only 4.7". Hmmmm... I am not bound by available props, though, I can make any size myself. I use two 2-blade props, saw and glue them together to make a master. From this I make a mold, and I can cast fiberglas props from that. Would increasing to 4.5" pitch make a positive contribution? I could go up to 5" diameter, but it would look weird as the tips would overlap. It is mechanically possible because the motors are placed slightly behind each other.



I think the one battery per wing / four motors is a more sensible idea, thanks. The wings will be in 3 parts anyway, one central with all motors and two tips. I think. For now.

This is not an easy puzzle!

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Old 10-18-2013, 11:12 AM   #10
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Hi Use this prop, cut it down a tad to 5" and balance it, its a SF prop with big paddles, it will give you the thrust that you need, test it on a thrust test stand, if you have enough thrust, your good to go if not try it at 5.5"and it will be a lot easier than making your own props, you will need a prop that is a SF prop, Slow Fly,

http://www.hobbyking.com/hobbyking/s...de%20propeller

you will need at least a 3 cell, 4000mah lipo per wing, thats 1000mah per motor and at least a 25C or larger on the C rating. thats 2 - 4000+mah lipos, one battery per wing.

http://www.hobbyking.com/hobbyking/s..._3S1P_45C.html

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Old 10-18-2013, 11:51 AM   #11
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If you have not yet build the plane, i would advise you to space out the motors more, it wont be that noticeable, and give yourself at least space for a 5.5" to 6" prop, or you may not be able to acquire enough thrust for the plane to fly, there is going to be a trade off, not all scale planes are able to be down sized and yet fly,

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Old 10-18-2013, 12:24 PM   #12
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Originally Posted by CHELLIE View Post
... not all scale planes are able to be down sized and yet fly,
Yeah, scaling down from 320 foot 11 inch to 6foot has it's challenges...

Here I shifted the left wing motors, 1/2" for the first and 1" at each spacing. It does look very different from the right wing...



Choices, choices, it it was easy it wouldn't be fun, right?

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Old 10-18-2013, 06:05 PM   #13
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Originally Posted by HugoW View Post
Thanks for all the input. I will have to do some serious thinking, as a 7" prop will simply not fit. On the inner motors, they will hit the fuselage, and between each nacelle there is only 4.7". Hmmmm... I am not bound by available props, though, I can make any size myself.

Hugo
FYI, there are some very high centrifugal (Centripetal?) forces on the props that we use in our models. If one of your home made props ever shed a blade, the off balance result would do serious damage to your model. It happened to me once, some 15 years ago.

Did some crude calculations back then, the centrifugal force on the prop hub of that 1000 watt motor was over 900 pounds.

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Old 10-18-2013, 06:14 PM   #14
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Thanks for the advise, I'm not really afraid of such a failure. I make these together with a friend who makes them for his F5d racer as well. I haven't calculated, I fear that will far exceed your numbers. So that is not the problem, at this point. Problem is I scale this thing down about 54 times... I've drawn a 10 foot span version, too, that is already easier. But way to big to handle in transport for me.

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Old 10-18-2013, 07:40 PM   #15
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Originally Posted by HugoW View Post
Yeah, scaling down from 320 foot 11 inch to 6foot has it's challenges...

Here I shifted the left wing motors, 1/2" for the first and 1" at each spacing. It does look very different from the right wing...



Choices, choices, it it was easy it wouldn't be fun, right?

Hugo
The engine Spacing looks Great, and is not that Noticeable, Its just one of the trade off’s that you have to make when scaling down a real Plane to RC Planes, sometimes you have to make the wing a little wider for better lift, because RC planes fly slower than their real counter part, sometimes the rudder needs to be taller and wider as with the elevator assy for rc, just use the TLAR Method when Building,( That Looks About Right ) and Think RC Plane 47 Years in CL and RC building and flying and I still Crash from time to time LOL , Take care and have fun, Chellie

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Old 10-18-2013, 08:43 PM   #16
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The props look like each is far enough behind (or in front) of it's neighbor to allow the props to overlap. That would allow you the use larger props and space them out less.

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Old 10-18-2013, 09:39 PM   #17
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http://www.hobbyking.com/hobbyking/s...er_2840Kv.html

In the review section:
6x4 14750rpm 20A 9.75V 435g 90 km/h
4.7x4.2 19700rpm 15A 10v 336g 128
4.75x4.75 18750rpm 16A 9.88v 315g 136
5.2x5.2 16000rpm 19A 9.75v 345g 127
5.5x5.5 14500rpm 20A 9.62v 344g 122

I like the 4.75x4.75 part. 8x 315 grams makes 2520 grams, on a 3000 gram plane that is very good (it's a plane, not a 3D monster).

What do you think of that? I can easily smuggle in 0.75 inch extra diameter and still look scale.

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Old 10-18-2013, 11:26 PM   #18
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Originally Posted by HugoW View Post
http://www.hobbyking.com/hobbyking/s...er_2840Kv.html

In the review section:
6x4 14750rpm 20A 9.75V 435g 90 km/h
4.7x4.2 19700rpm 15A 10v 336g 128
4.75x4.75 18750rpm 16A 9.88v 315g 136
5.2x5.2 16000rpm 19A 9.75v 345g 127
5.5x5.5 14500rpm 20A 9.62v 344g 122

I like the 4.75x4.75 part. 8x 315 grams makes 2520 grams, on a 3000 gram plane that is very good (it's a plane, not a 3D monster).

What do you think of that? I can easily smuggle in 0.75 inch extra diameter and still look scale.

Cheers,

Hugo
Hi, I read some of the comments on that motor, and some were saying the bearing get rough after a few flights, i would steer clear of that motor IMHO.

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Old 10-18-2013, 11:30 PM   #19
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your going to need a lot of thrust to take off from water, you will need at least a 1 to 1 power ratio or more. I would try the 1800 kv motor the the 4 blade prop and test the thrust, your going to need at least a 5" prop, the thrust specs on the HK motor page are sometimes exaggerated. buy 1 of the 1800 kv motors and a set of the 4 blade props and do some of your own testing, the thrust test stand wont be that hard to make, the vertical and horizontal boards need to be the same size.

http://www.hobbyking.com/hobbyking/s...de%20propeller

http://www.hobbyking.com/hobbyking/s...cro_Motor.html


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Old 10-19-2013, 12:01 AM   #20
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Before I forget figure out what the wing loading will be at 3000 grams, 6.6 lbs, you may need to make the wing longer or wider if the wing loading is to heavy.

http://www.modelflight.com/wing.html

https://www.commonsenserc.com/page.p...g_loading.html

http://adamone.rchomepage.com/design.htm#calculate

http://www.airfieldmodels.com/inform...ng_loading.htm

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Old 10-19-2013, 12:20 AM   #21
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just to show you how powerful the 1800 kv micro motor is, here is my friend kosh with his plane with the 1800 kv micro motor, 7x6 apc prop and on a 3 cell lipo,


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Old 10-19-2013, 03:45 AM   #22
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http://www.youtube.com/watch?v=YgQcMbz4MHI

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Old 10-19-2013, 04:08 AM   #23
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Old 10-19-2013, 04:49 AM   #24
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You will be best off with all motors on a single pack.

Inboard 3 on both sides on the main throttle channel. Then mix the outboard-most motors on each side with rudder on a switch (disable the mix for normal flight) for taxi control. Rudder will be very ineffective for low speed taxi on water without use of differential thrust.

Weight of the wires from the motors to the central battery will be insignificant if you use the correct gauge wire. It is slightly better to have the extensions between the motor and ESC than between ESC and battery. But its easier to cool the ESCs if they are in the engine nacelles.

The main reason one battery is best is the reduced chance of "single engine out" or in the case of one batter each side losing all 4 on one side with the other 4 still at full power. Losing all on one side will almost certainly cause the model to spin and crash.
With matched motors and ESCs you should have all LVC's trip at or very near the same time. (do some testing and rearrange ESCs so they trip from inboard working out and "walking" from side to side)

Remember that when you lose one (or more) "engine" DO NOT increase throttle and do not pull up.
Set up for landing ASAP.
Avoid making a turn toward the side with more dead engines.
Turn 270 deg the other way if you need to make a turn toward the dead engine side. or make a descending turn to help maintain adequate airspeed.

Hopefully you have experience with twin and 4 engine bombers... plus a couple of seaplanes.
A Spruce Goose is not a good choice for a first multi-engine plane nor as a first seaplane.
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Old 10-19-2013, 04:50 AM   #25
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Originally Posted by CHELLIE View Post
Wow, can't imagine trying to keep eight engines all running at the same time.

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