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May 01, 2021, 02:27 PM
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Thread OP
Discussion

Converting my Butterfly to Electric


I have a Great Planes Butterfly. It has a .10 glow on it now. I am going to convert it to electric. I have picked out a SunnySky X2814-900KV turning a 9x5 or 9x6 prop. What do you all think?

Plane specs https://www.greathobbies.com/product...od_id=GPMA0090

Here are the MotoCalc opinion

MotOpinion - Great Planes Butterfly
500ft above Sea Level, 29.92inHg, 57F

Motor: Sunnysky X2814-900KV; 900rpm/V; 0.7A no-load; 0.052 Ohms.
Battery: Kokam 2100SHD (20C); 3 cells; 2100mAh @ 3.7V; 0.0114 Ohms/cell.
Speed Control: Generic Brushless ESC; 0.006 Ohms; High rate.
Drive System: APC 9" Propellers; 9x6 (Pconst=1.11; Tconst=1) direct drive.
Airframe: Great Planes Butteryfly; 916sq.in; 62.4oz RTF; 9.8oz/sq.ft; Cd=0.05; Cl=0.64; Clopt=0.76; Clmax=1.24.
Stats: 34 W/lb in; 30 W/lb out; 14mph stall; 19mph opt @ 56% (53:41, 71F); 20mph level @ 59% (48:07, 72F); 399ft/min @ 14.1; -122ft/min @ -4.3.

Power System Notes:

The full-throttle motor current at the best lift-to-drag ratio airspeed (12.7A) falls approximately between the motor's maximum efficiency current (11.9A) and its current at theoretical maximum output (102.2A), thus making effective use of the motor.

Possible Aerodynamic Problems:

The static pitch speed (51mph) is greater than 3 times the stall speed (14mph), which might make take-off or hand launching difficult, and is inefficient in flight unless high speeds are intended.
Pitch speed can be decreased by using a lower pitched and/or larger diameter propeller, a higher gear ratio, a lower cell count, or some combination of these methods.

Aerodynamic Notes:

With a wing loading of 9.8oz/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 (23.9oz) to weight (62.4oz) ratio is 0.38:1, which will result in medium length take-off runs, and no difficulty taking off from grass surfaces (assuming sufficiently large wheels).
At the best lift-to-drag ratio airspeed, the excess-thrust (15.3oz) to weight (62.4oz) ratio is 0.25:1, which will give good climbs and acceleration. This is a good in-flight thrust to weight ratio for a basic trainer.

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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May 01, 2021, 06:36 PM
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scirocco's Avatar
Quote:
Originally Posted by billkater
I have a Great Planes Butterfly. It has a .10 glow on it now. I am going to convert it to electric. I have picked out a SunnySky X2814-900KV turning a 9x5 or 9x6 prop. What do you all think?

Plane specs https://www.greathobbies.com/product...od_id=GPMA0090

Here are the MotoCalc opinion

MotOpinion - Great Planes Butterfly
500ft above Sea Level, 29.92inHg, 57F

Motor: Sunnysky X2814-900KV; 900rpm/V; 0.7A no-load; 0.052 Ohms.
Battery: Kokam 2100SHD (20C); 3 cells; 2100mAh @ 3.7V; 0.0114 Ohms/cell.
Speed Control: Generic Brushless ESC; 0.006 Ohms; High rate.
Drive System: APC 9" Propellers; 9x6 (Pconst=1.11; Tconst=1) direct drive.
Airframe: Great Planes Butteryfly; 916sq.in; 62.4oz RTF; 9.8oz/sq.ft; Cd=0.05; Cl=0.64; Clopt=0.76; Clmax=1.24.
Stats: 34 W/lb in; 30 W/lb out; 14mph stall; 19mph opt @ 56% (53:41, 71F); 20mph level @ 59% (48:07, 72F); 399ft/min @ 14.1; -122ft/min @ -4.3.

Power System Notes:

The full-throttle motor current at the best lift-to-drag ratio airspeed (12.7A) falls approximately between the motor's maximum efficiency current (11.9A) and its current at theoretical maximum output (102.2A), thus making effective use of the motor.

Possible Aerodynamic Problems:

The static pitch speed (51mph) is greater than 3 times the stall speed (14mph), which might make take-off or hand launching difficult, and is inefficient in flight unless high speeds are intended.
Pitch speed can be decreased by using a lower pitched and/or larger diameter propeller, a higher gear ratio, a lower cell count, or some combination of these methods.

Aerodynamic Notes:

With a wing loading of 9.8oz/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 (23.9oz) to weight (62.4oz) ratio is 0.38:1, which will result in medium length take-off runs, and no difficulty taking off from grass surfaces (assuming sufficiently large wheels).
At the best lift-to-drag ratio airspeed, the excess-thrust (15.3oz) to weight (62.4oz) ratio is 0.25:1, which will give good climbs and acceleration. This is a good in-flight thrust to weight ratio for a basic trainer.

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.
I think you are significantly under propping the motor and at 34W/lb will end up with marginal performance, even for a motor glider, well below what would be expected from your 0.10 glow.

As long as it fits, using a 12x6 prop will about double the power, still at a modest 60W/lb, and still well within the capability of the motor and battery.

IF you can't fit 12", any increase in diameter will be highly beneficial. Also consider a 3 blade prop if you are diameter constrained
May 02, 2021, 02:17 AM
Registered User
Thread OP
So, would it be better to go with a higher KV motor if I don't have clearance for a larger prop?
May 02, 2021, 06:34 AM
Registered User
scirocco's Avatar
Quote:
Originally Posted by billkater
So, would it be better to go with a higher KV motor if I don't have clearance for a larger prop?
9" is super small for a 99" span model but if 9" is indeed the max diameter you can use and a 9x7x3 doesn't absorb enough power, you could get more power by using higher Kv or higher voltage.

However, 9*7*3 should use about 200W, adequate at 4lb and any weight you can shave will just be a bonus. For example, 3S 1300 mAh is plenty big enough to deliver 200W

Or, think laterally and use a larger folding prop and hand launch / belly land.

I'd suggest that in assessing max prop diameter assuming you keep the gear you need very little ground clearance for the prop, as hand launch should be feasible.

If you went with 4S and 9x4.5, you'd expect about 240W. A 1000mAh pack would be big enough.

You'd get the same outcome with the X2814-1250 on 3S with 9x4.5

My personal opinion - if you can't fit 11", before spending on either a new motor or new battery, get a 9x7x3. That will do the job and is about equivalent to 10x7
May 02, 2021, 02:00 PM
Registered User
Thread OP
Ran the numbers again. Using a 10x6 I can get a hair over 50w/lb. 10 is the very largest I can get unless I modify landing gear to raise nose of plane. Folding prop isn't feasible due to size of motor mount location.

MotOpinion - Great Planes Butterfly
500ft above Sea Level, 29.92inHg, 57F

Motor: Sunnysky X2814-900KV; 900rpm/V; 0.7A no-load; 0.052 Ohms.
Battery: Kokam 2100SHD (20C); 3 cells; 2100mAh @ 3.7V; 0.0114 Ohms/cell.
Speed Control: Generic Brushless ESC; 0.006 Ohms; High rate.
Drive System: Generic 10x6 Prop; 10x6 (Pconst=1.31; Tconst=0.95) direct drive.
Airframe: Great Planes Butteryfly; 916sq.in; 62.4oz RTF; 9.8oz/sq.ft; Cd=0.05; Cl=0.64; Clopt=0.76; Clmax=1.24.
Stats: 51 W/lb in; 43 W/lb out; 14mph stall; 19mph opt @ 53% (48:28, 71F); 20mph level @ 56% (44:26, 72F); 575ft/min @ 20.7; -122ft/min @ -4.3.

Power System Notes:

The full-throttle motor current at the best lift-to-drag ratio airspeed (19A) falls approximately between the motor's maximum efficiency current (11.8A) and its current at theoretical maximum output (99.8A), thus making effective use of the motor.

Possible Aerodynamic Problems:

The static pitch speed (47mph) is greater than 3 times the stall speed (14mph), which might make take-off or hand launching difficult, and is inefficient in flight unless high speeds are intended.
Pitch speed can be decreased by using a lower pitched and/or larger diameter propeller, a higher gear ratio, a lower cell count, or some combination of these methods.

Aerodynamic Notes:

With a wing loading of 9.8oz/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 (33.1oz) to weight (62.4oz) ratio is 0.53:1, which will result in short take-off runs, and no difficulty taking off from grass surfaces (assuming sufficiently large wheels).
At the best lift-to-drag ratio airspeed, the excess-thrust (21.8oz) to weight (62.4oz) ratio is 0.35:1, which will give strong climbs and rapid acceleration. This model will most likely readily loop from level flight, and have sufficient in-flight thrust for many aerobatic maneuvers.

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.
Last edited by billkater; May 02, 2021 at 02:14 PM. Reason: Add photo
May 02, 2021, 05:22 PM
Registered User
Thread OP
Something I just thought of. The listed AUW is with the glow motor and fuel cell. I should end up alot lighter
May 02, 2021, 05:27 PM
Registered User
scirocco's Avatar
To me that looks like excellent clearance for 10" and possibly even enough for 11" - I settle for 1" of ground clearance from the tips, but have gotten away with less.

But anyway, I think you're in good shape. With 10" diameter to play with, 10x6 IMO is a workable start point, but if you don't have the prop yet I'd suggest 10x7 APC-E is a better option that exploits your motor and battery better to gives you larger performance margins with no downside on flight time unless you run around at WOT. Master Airscrew has a 10x7x3 which could be worth a try also.

Don't worry about the pitch speed warnings in MotoCalc - it's just an unavoidable consequence of having to use a small prop and get your thrust from highish rpm and generous pitch. The higher pitch:diameter props will also work well at the very low cruise power you'll need
May 02, 2021, 08:31 PM
Registered User
Thread OP
Thanks for the reply

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May 04, 2021, 06:32 AM
FlyLikeAnEagle-LandLikeADove
u2builder's Avatar
Larger wheels to allow longer prop? Dubro makes some real light weight and pretty thin ones.

Make sure the motor and battery are heavy enough to get the proper CG.
May 05, 2021, 05:03 PM
Balsa Flies Better!
A buddy had one of these when we were back in grad school- great airplane- he had a tired OS .20 on his.

I'd say that 200 watts spinning even a 9" prop would be fine. One concern- I think previously noted- you may not lose much weight going electric. IIRC, that airplane could easily need noseweight, so unless you're downsizing servos as well, wouldn't count on much weight loss.

Sam
May 05, 2021, 06:02 PM
Registered User
Thread OP
I'm thinking of moving the servos as far to the nose as I can. And extending the nose a bit. Throw as much weight forward as I can


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May 05, 2021, 08:06 PM
FlyLikeAnEagle-LandLikeADove
u2builder's Avatar
I started to convert my Dynaflight Butterfly a couple years ago, but realized it was going to need nose weight and put it aside. I think the idea of mounting the electric motor on an extended mount/lengthened nose might be a good idea. Moving the servos forward would also help.
May 05, 2021, 09:28 PM
If it flies, I can crash it.
rocketsled666's Avatar
You could always just use a bigger battery.
May 05, 2021, 09:30 PM
Balsa Flies Better!
I think you guys may be a little weight obsessed. The airplane flies beautifully with a glow engine- if you're even close to the same weight with electric power- even if you need noseweight- it'll still fly just as sweetly. Note- I'm assuming that we're talking about the same airplane- my buddy's was built from a Craft Aire kit- 99" wingspan, essentially a powered Mark Smith 99" Wanderer if I've got it straight. If these ARFs are a lot heavier, then I may be off in my recommendations.

If you had an old brushed motor lying around- this would be a good airplane for it. The other option is just run a somewhat larger electric motor- often more efficient anyway. Nothing says you have to put on the largest prop the motor can handle.

Sam

Sam
May 05, 2021, 09:35 PM
Registered User
Thread OP
I'm not concerned about the weight of the plane. I'm concerned with getting the CG right.

Here is a link to the kit

"GPMA0090 Great Planes Dynaflite Butterfly Trainer Kit .10-.20,99 Remote Controlled Hobby" https://www.gravesrc.com/great-plane...-10-20-99.html

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