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Mar 26, 2018, 06:01 AM
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Selecting motors for a CL-415, 76"


I want to build a (dual engined) CL-415.
Wingspan 76".
Weight approx 4.5Kg (10 lb).
Seems to be a Clark-Y profile (flat base).

This is a seaplane thus aerobatics are not called for

Now I am trying to find a suitable motor set (2 motors) and prop. A four blade prop (scale!) would be ideal, but I wont mind a 3 or even 2 blade one if price or availability is a problem.

Looking at the hacker website (in german, sorry) I get the impression that the A30-10XL (or 12XL) would be the best choice.

A30-10XL: recommend for plane weight: 2.5Kg, 440W, 7000 rpm, 14x7 APC-E, 3s, 900kv

But those engines are not cheap, at €95 that is quite expensive.

I usually fly gliders so I am a bit at a loss here with motor selection, so if anybody could confirm (or reject) this selection that would ease my mind

Secondly, as these motors are quite expensive I looked for some cheaper, but similar ones. Of course I ended up with a Turnigy, the 3542 which at €25 is a lot cheaper.

3542: 10 turns (like the A30-10XL), 730W, 1185kv, 3s-4s (Max power is probably quite a bit lower on 3s, probably A30-10XL range, guessing!!)

Is the price difference worth it? 180 vs 50 euro's for both engines is not make or break, but I would need good reasons to spend the extra money.

Is the 3542 a good replacement? or is the A30 the better solution?
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Mar 26, 2018, 05:04 PM
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scirocco's Avatar
You need to provide probably the most critical piece of information - what diameter props will fit with enough clearance from the structure?

You CANNOT make any valid assessment of motor suitability until you know your maximum prop diameter and have decided what voltage to run.
Mar 27, 2018, 01:46 AM
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That shows my ignorance I thought that the diameter and speed were interchangeable. And hence the diaper would not matter all that much (within a reasonable range).

I just measured the clearance, and its 18 cm, = 7". But 7" is very close!
To have a little wiggle room 6.5" would be better. But I do not believe 13" props exist... do they?

So the max diameter would be 12".

For batteries I have a collection of 3S lying around, so that would be my preference. However I also think that the power requirements can be more easily met with a 4S, if so, I am willing to add those to my collection.

(Btw I just found the book "Electric Motor Handbook" online and am now reading the relevant chapters)
Mar 27, 2018, 05:26 AM
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scirocco's Avatar
13" is a common size and in 3 blade Master Airscrew has a 13x8 and a 13x12, available in opposite rotation which is not essential but a nice to have. And there are 12 and 13" APC-E 2 blade in a wide range of pitches.

If we plan on about 60W/lb per side, you should get excellent two motor performance, yet still have enough power to fly a couple of level circuits and an approach should one side quit. Directional control 1 motor out we can't really address here.

So at about 10lb, we want a motor and battery combo that takes around 600W to spin up to a 13" prop, with pitch and rpm to achieve at least 60mph pitch speed. IMO, 600W is into 4S territory both because you'd need 70-80A ESCs to cope with the ~60A current required for 600W on 3S and because motors in the about 170g-200g range that is conservative for 600W tend to have slightly lower Kv better suited to 4S for 12/13" props.

I'm going to use a pair of 4S 4000 as an initial power source assumption, largely because I know that a similar amount of total energy in 6S 5000 form flys a 80" Pawnee sportily for over 7 minutes.

Motors around 200g with Kv about 750 rpm/V need about 630W to turn an APC-E 12x8 x2 at about 9000 rpm for a nice solid 68mph pitch speed or about 720W for a 12x10. The example motor I used in ECalc is the Turnigy propdrive v2 4248-750 @ 231g.

Motor weight and Kv are the key more so than brand.

You could consider 3S with Kv about 1000 - 1100 rpm/V eg Hacker A30-8XL and the same 12x8 or 12x10, but the motor selections seem thinner to me and you'd be pushing everything harder.

The reason I've looked more at 2 blade props is that 3 blade props of the same size run slower due to the higher load they put on the motor, and thus pitch speed sags. This will be a relatively highly loaded model and you need good thrust at a solid 2.5 to 3 times stall speed just as much as static thrust.

The above is just one approach - other options can be explored.

Just for contrast, for a Deuces Wild, sport twin I plan 200g Scorpion 3026/890 and 4S with 11x10 props both to ensure ground clearance and get good speed, approaching 80-90 mph but I'm deliberately seeking an overpowered hot rod. Should see over 800W per side
Mar 27, 2018, 06:49 AM
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Thanks a lot scirocco, using your input I am now getting some results with eCalc and am (slowly) learning "what goes where" and "what happens when" in eCalc. (I did not get any usable results earlier, seemed like random to me )

I do have some electronics background, so I concur that 4S makes more sense. I expected as much, and will do so.

I will play a bit more with eCalc.
Mar 27, 2018, 09:27 AM
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I think I might have found a good combo:

Plane weight: 4500g
Motors: 2
Wing area: 51dm^2
Battery: LiPo 5000mAh - 40/50C (4s/2p) (€35)
Controller: 60A
Motor: Turnigy SK3-4240-620 (€23)
Propellor: Master Airscrew 13x8 - 3 blades

eCalc results:

68% throttle, 5500rpm, 20A, 285W, 88% eff, 1937g thrust (x2), 56km/h, 12.8min
82% throttle, 6500rpm, 33A, 486W, 89% eff, 2700g thrust (x2), 72km/h, 7.7min
100% throttle, 7644rpm, 55A, 765W, 88% eff, 3741g thrust (x2), 85km/h, 4.6min

Anybody care to confirm or find fault?
Mar 27, 2018, 01:30 PM
An itch?. Scratch build.
eflightray's Avatar
I would assume your figures are from calculations, the real world can be slightly different, (better or worse).

My only comparison is my B-25, she weighs 9lb -11oz, but is bigger at 93" span, so wing loading will be lower. She flies on E-Power (Emax ?), GT2826 710kv motors, 13 x 6.5 APCe props, and 3s per motor, (25A 300W 6800rpm per motor). Works out at approx 60W/Lb. Still gets flown.

The motor may no longer be available, but it may give some idea.

It first flew almost 25 years ago, using brushed motors, belt drives, 12 x 8 wood props, and 28 Nicad cells, (14 per motor), and weight 2lbs more

Ray.
Mar 27, 2018, 03:52 PM
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scirocco's Avatar
Quote:
Originally Posted by rcrien
I think I might have found a good combo:

Plane weight: 4500g
Motors: 2
Wing area: 51dm^2
Battery: LiPo 5000mAh - 40/50C (4s/2p) (€35)
Controller: 60A
Motor: Turnigy SK3-4240-620 (€23)
Propellor: Master Airscrew 13x8 - 3 blades

eCalc results:

68% throttle, 5500rpm, 20A, 285W, 88% eff, 1937g thrust (x2), 56km/h, 12.8min
82% throttle, 6500rpm, 33A, 486W, 89% eff, 2700g thrust (x2), 72km/h, 7.7min
100% throttle, 7644rpm, 55A, 765W, 88% eff, 3741g thrust (x2), 85km/h, 4.6min

Anybody care to confirm or find fault?
ECalc is not triggering an alert, but your pitch speed is only 2.2 times predicted stall speed. IMO that will translate to a sluggish feeling model that requires nearly full throttle just to stagger around the circuit. By choosing the 620 rpm/V motor on 4S you are capping max rpm below 8000, putting yourself into a corner with limited viable prop choices if you do find it sluggish on 13x8x3, ie 12x12x2, 12x10x2, perhaps 12x10x2 but that might be a bit marginal on power given your draggy rise off water needs.
I'd suggest looking instead at the 4240-720 which is ~9500 rpm max with 12x8 APC-E 2 blade. Other viable props include 12x8x3, 13x8x2 (70A ESC for these), 12x10x2 and even 11x10 for a lower max power option.
The 12" props with some extra clearance should run a bit quieter too
Mar 27, 2018, 11:15 PM
homo ludens modellisticus
Ron van Sommeren's Avatar
Quote:
Originally Posted by rcrien
... (Btw I just found the book "Electric Motor Handbook" online and am now reading the relevant chapters)
More reading



Quote:
Originally Posted by scirocco
While an absolutely critical part of the system ...
Kv is actually the item one should choose last.

  1. Decide your peak power requirement based on the weight of the model and how you want to fly it.
  2. Pick a preferred cell count (voltage) and pack capacity for how to deliver the power.
  3. Pick a prop that will a) fit on the model and b) fly the model how you want - often as big as will fit is a good choice, but if high speed is the goal, a smaller diameter higher pitch prop will be more appropriate.
  4. Look for a size class of motors that will handle the peak power - a very conservative guide is to allow 1 gram motor weight for every 3 watts peak power.
  5. Then, look for a motor in that weight range that has the Kv to achieve the power desired with the props you can use - a calculator such as Ecalc allows very quick trial and error zooming in on a decent choice. For a desired power and prop, you'd need higher Kv if using a 3 cell pack compared to a 4 cell pack. Or for a desired power and cell count, you'd need higher Kv if driving a smaller diameter high speed prop compared to a larger prop for a slow model.

The reason I suggest picking Kv last is that prop choices have bounds - the diameter that will physically fit and the minimum size that can absorb the power you want. OTOH, combinations of voltage and Kv are much less constrained - at least before you purchase the components.

So Kv is not a figure of merit, in that higher or lower is better, it is simply a motor characteristic that you exploit to make your power system do what you want, within the constraints you have, eg limited prop diameter if it's a pusher, or you already have a bunch of 3S packs and don't want to buy more, and so on.

Minor lay-out changes by RvS
Mar 27, 2018, 11:20 PM
homo ludens modellisticus
Ron van Sommeren's Avatar
Quote:
Originally Posted by rcrien
... I thought that the diameter and speed were interchangeable. ...
Minor changes in setup can have huge effects:
Motorcurrent is proportional to pitch¹, voltage², Kv³ and diameter⁴.
Power-drawn is proportional to pitch¹, voltage³, Kv³ and diameter⁴.

Quote:
Originally Posted by rcrien
... A four blade prop (scale!) would be ideal ...
2,3,4,5 blade hubs, RH and LH, adjustable pitch, replacement per blade.
For electric, IC and folder. 5,1" to 31,3" diameter.
Ramoser Varioprop - RCG

Vriendelijke groeten Ron
Last edited by Ron van Sommeren; Mar 27, 2018 at 11:26 PM.
Mar 27, 2018, 11:30 PM
homo ludens modellisticus
Ron van Sommeren's Avatar
Quote:
Originally Posted by rcrien
... I do have some electronics background ...
Do you have (access to) a watt-meter? DC clampmeter?

Without a watt-meter you are in the dark, until something starts to glow
Mar 28, 2018, 01:34 AM
Registered User
Thread OP
Quote:
Originally Posted by scirocco
ECalc is not triggering an alert, but your pitch speed is only 2.2 times predicted stall speed. IMO that will translate to a sluggish feeling model that requires nearly full throttle just to stagger around the circuit.
Ah, you picked the exact point on which I was uncertain. But I did not know that the pitch indicator was useful for that. I was looking at 68% throttle just to get of the grass (water) which seemed a bit high for me. I was still a little in the dark about what the pitch number was good for. Now I know!

Thanks scirocco!
Mar 28, 2018, 01:37 AM
Registered User
Thread OP
Quote:
Originally Posted by Ron van Sommeren
More reading
A good thing that I like to read
We did away with TV some time ago now, and it sure feels like I gained a whole extra life!
I'll be checking out your links one by one

Thanks.
Mar 28, 2018, 02:56 AM
Registered User
Thread OP
I do have some meters, yes. But the EM2/RDU combo seems quite nice to me. In-flight recording?, yes please!
And at those prices.. unbelievable.. after all I am a sucker for measuring.. ordered one..
Mar 28, 2018, 03:29 AM
Registered User
Thread OP
Quote:
Originally Posted by eflightray
It first flew almost 25 years ago
25 years for a model? wow that is seriously old.
I did rebuild one of my earlier planes (Graupner Mosquitto) recently, but none survived that long. Though I did sell some planes, so they did not all crash (in my hands).


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