Help with Power System for Harlequin

I'm planning to build Peter Millers Harlequin (RCME March 2012) and convert to electric. Originally it was flown using a .35 two-stroke.

It weighs 4 1/4 lbs so working on 100W/lb it will need 425W. Also, I have a 3S 3300mAh lipo which I would like to use.

Can anyone advise me on what power system I should buy and the best place to buy from?

Thanking someone in anticipation

Geoff,

To get .35 equivalent power you'd really want well over 500W which

is probably pushing it a bit for a 3S 3300mAH.

I use a 'Wasp' 3542-1250kv motor/3s 3300/APC 9x6 for around 420W

very successfully in a 3.5lb 45" span aerobat - but this is equivalent

more to a .25. This sort of power level is much better suited to your Lipo.

A Harlequin has ground clearance for a bigger prop so the 1000kv version

on a 10" could be better if you want to use this size motor.

A Harlequin would probably fly quite nicely but fairly sedately on this power.

The Wasp was from BRC Hobbies but is a generic Chinese motor

available in similar guises from 4max, Giantshark, Hobbyking, Robotbirds etc.

Bear in mind that identically spec'd motors from different suppliers don't

always behave the same! - so do use a Wattmeter to check your setup.

Thanks Richard - your suggestion would probably be ok as I'm a beginner and this will be my first aerobat

What would you recomend to get the best out of the plane at a later date ie 500W+.

Hi Geoff,

I'd recommend you go up to a 4S/3200 - 3700mAH Lipo to get 500W+.

You'll also need a different motor from the one mentioned earlier or you'll

find you need to go down to a very small prop to keep the current within

limits.

There are many suitable motors, but one that springs to mind is the

Emax BL2820-07, 919kv from BRC Hobbies at just under £20.

Use a 60A ESC for plenty of 'headroom' so that cooling isn't as critical

and an 11x6 or possibly a 12x6 APC-E prop.

I reckon a 4S combo like this would work superbly in a Harlequin.

I really don't think you need to move to 4S yet. If you pull 500W on 3s that's 45A. that's a 13C battery. I can't think of many batteries that won't supply 13C....

Higher current will mean more heat (so losses will be a little higher) and you'ld need a bigger ESC than if you went for more cells but it's not a massive problem.

NTM 3536 (1400kv) motor, 10*6 prop (that's what I used to run my 35s on so it should fit the plane nicely!!!)

**LINK**

3s battery, 3300mAh, 400 watts input (average if 20/30C batteries), 1.7kg static thrust, pitch speed 61mph. And a max prop speed of 15,000rpm (about right for the glow too).

accourding to

**LINK**

what's not to like?

Be aware that in my experience Ecalc under-estimates current draw by about 5% so you would want to ensure sufficient headroom with the above set-up. I typically run Turnigy plush ESCs (which are the same as HobbyWING pentium) so I'd go for the Plush 60 on the above set-up....

Sorry, I can't help with electric flight, I'm an IC guy. But please do a build log, I'd like to see this build compared to my Funfly build which I did.

Have fun!

CS

It's no surprise to see the difference in opinion. I've found different instructions with regard to all the aspects of deciding on a power set up. I've found a variation from 100W to 180W per pound for aerobatic aircraft.

I'll try to do some calculations ( it's all new to me ) to investigate the suggestions before I make a decision

If anyone has converted the Harlequin I'd bee very pleased to hear what they did and if it was successful.

Another good and rough guide is :

take the 2S IC size (35)

multiple by 2, then add a zero on the end. In this case it would be 700watts.

i had a Galaxy Magician, which is 30-40 size and that flew well on 600 watts, infact i could fly her around at 3/5 throttle. Keep in mind that 70% throttle is roughly half the max amps pulled. I would rather fly around at 70% throttle, than at full throttle all the time if you only have 450 watts (using the 100 watts / lb rule).

Clearly, how many Watts you need is also directly related to how you want to fly the model. 80-100 watts/Lb is good for trainers and scale aircraft. For sports/aerobatic, personally i'd be looking at 130-150 and for 3D 150-180 - which I can't imagine you'd be doing with the Harliquin.

Using either method (assming 130-150 W/Lb) gves you about 6-700 watts required. As you've mentioned on 3S this would be rather large on Amps drawn, so 4S might be better for duration.

I found that the 4000mah 25C 4S2P Rhino Lipo which is almost sqaure, fits in most places an 8oz IC tank would fit on a 30-40 size model.

I've had a good look around the internet and Richards suggested Emax BL2820-07, 919kv from BRC Hobbies may be worth a shot. The spec says it is suitable for 3S (440W) and 4S (560W). I could set it up using my 3S and if not suitable move on to a 4S

I also like the idea of being overpowered rather than on the edge so Bens NTM 3536 (1400kv) motor is also a good possibility

Both good options - perhaps I'll toss a coin !!!!

Geoff,

I'm no expert, but for a model of the Harlequins size and weight, I'd be looking at either a 4S set-up OR staying with 3 cells but using something in the 4 - 5000 Mah size range. The bigger the cell, the happier they are at running at higher current draws.

tim

For my own ammusement I thought I would try to calculate what a suitable setup might be.

I believe Peter had an OS40FS in his original, 0.64 bhp at 12000 rpm according to OS.

There are 746 Watts (ish) per horse power so the OS gives 0.64 x 746 = 477 Watts of output power.

Assuming our electric motor is 75% efficient we need 477 / 0.75 = 636 Watts of input power.

With 4S LiPo nominal 14.8 volts thats 636 / 14.8 = 43 amps.

Output power = torque (Nm) x angular velocity (rad/sec)

477 = T x 12000 x 2 x pi / 60

so the torque is

T = (477 x 60) / (12000 x 2 x pi) = 0.38 Nm

Motor torque constant, Kt, is torque divided by current.

Kt = 0.38 / 43 = 0.0088 Nm/A

Kt is numerically equivalent to Kv measured in volts/radian/second.

The Kv quoted for brushless motors is however expressed in rpm/volt

Kv = 60 / (2 x pi x 0.0088) = 1085 rpm/volt

So based upon that with a 4S LiPo you would need a 1085Kv motor large enough to handle 636 Watts and 43 Amps.

For example one of these might fit the bill.

I've not tried to apply this method to electric flight before so have no practical evidence to back it up but the result does seem to be consistent with the general consensus above.

Following SDF's calcs (which simialrly also equal IC size x2 and add a zero)

The PPO 3548-900kv gives 606 (upto 700 depending upon prop brand) Watts for 41A on a 4S lipo swinging a 12x6 prop.This would be a perfect fit. The Lipo I memtioned earlier is a 4000mah and using this set up I can get around 10 min flight times at 60-75% throttle for most of the flight.

SDF's calculations are very interesting and seem to me to be a very sensible approach to solving the problem.

I had been told that using the ' IC size X 2' method would give an overpowered set up. However taking the motor efficiency into account would appear to nullify the arguement.

I will be following this with interest, as an "electric" Harlequin will be next on the board when the Super 60 is complete.

What is your opinion with regard to the power system Anthony ?

SDF's calc's look spot on, and Christian has already found the purple power motor to suit, I like the purple power stuff, I have one in my Super 60. When I come to buy the motor I will get a 12 x 6 and a 12 x 5 prop and see what I get with my wattmeter.

Tony

That is my conclusion as well _ I think !!!!

So it's the RobotBirds 780W 1100kV motor with a 4S battery.

Any suggestions for the 70A Speed Controller ?

Almost there !!!!

Now I don't mind what you power my designs with, That is up to the builder and good luck to you.

However this thread has just confirmed my reason for preferring IC. Look at all the opinions and different ideas as to what to use. Everyone different.

Me? I like things simple.