Electric Conversion

[Richard Acland]

I recently built a Stolp Starlet from plans This is a 72 inch span model which is powered by a 70 four stroke using a 11x7 prop.

 Without blinding me with science I am thinking of converting this to electric power. What size electric motor, battery and speed controller set up would give me the same power output as a 70 four stroke.and what sort of flight duration could I expect.

I am familiar with LI Po batteries and charging as I have an electric powered glider.

 

[Adrian Smith 1]

All depends on the AUW of your model and the type of flying style you are aiming for.  There are formulas on this Forum that should guide you although George at 4-Max can advise you. My 74" aerobat is powered by a Dualsky GA3500 motor using a 120A HV ESC on a 20x10 prop, with 2 power packs of 6S wired in parallel, However, I suspect this may be overkill for you as I am not familiar with the Stolp Starlet. 

[Graham Bowers]

Think it best if you add a bit more detail. Weight of the midel and a photo to get the ambuance and guess what sort if flier it may be. Googling for what you might have adds uncertainty. Other than that, start by deciding what propeller you need and use 100 watts per pound as a first pass. There are calculatirs available and a free version of ecalc on the hacker website will tell you what havker kit you would need. Sorry this is all one paragraph, am away and on my phone.

[Dickw]

12 hours ago, Richard Acland said:

I recently built a Stolp Starlet from plans This is a 72 inch span model which is powered by a 70 four stroke using a 11x7 prop.

 Without blinding me with science I am thinking of converting this to electric power. What size electric motor, battery and speed controller set up would give me the same power output as a 70 four stroke.and what sort of flight duration could I expect.

I am familiar with LI Po batteries and charging as I have an electric powered glider.

 

As others have said, more detail of the model would help. In the meantime, what rpm would you expect on that 11x7 prop with your 70 four stroke? Without any "blinding science", duplicating that rpm on that prop with electric would at least fly it, but I am sure we can do better once we have more detail.

 

Dick

[Tom Gaskin 1]

My go-to 70 equivalent is an AXI 4120/14, 60A esc on 5 cells and a 13.5 by 6 prop. This is a 5055 size motor of 660kv, that is good for a kilowatt (about 1.2 hp)

 

Tom

[kc]

The Stolp Starlet full size seems to be a slow flying type of plane ( VW engine in some ) so for scale flying the model might use a very low power setup.  Look at 4 Max recommended setups and Customer model s for a similar weight and size.  Maybe the setup used by a Telemaster 6ft and 6 pound at 500 watt might work OK for scale flying ( half the power and much cheaper than Tom Gaskin's setup! )   But it all depends on weight and how far forward you can get the Lipo to achieve CG balance.   With electric the weight is further back than glow where all the weight is right up front.

[Richard Acland]

Thanks for the replies. I have checked the weight of the model with the engine removed and it comes to 5lb 5oz. It will not be subject to aerobatics just a bit of leisurely flying 

Edited August 25, 2023 by Richard Acland

[Shaun Walsh]

What battery do you propose to use?

[Richard Acland]

5 minutes ago, Shaun Walsh said:

What battery do you propose to use?

I dont know, that is why I was asking for advice on a set up.

[kc]

The question really is how much weight does it need to get CG right bearing in mind Lipos are long and thin so weight is a fair way back.  A lash up with  borrowed Lipos and a few rubber bands or tape to secure might be a lot of help.   Then how could you get that size lipo inside the model and change after flight.  Will the lipo fit behind  the existing bulkhead and still get CG correct?

[Shaun Walsh]

4 minutes ago, kc said:

The question really is how much weight does it need to get CG right bearing in mind Lipos are long and thin so weight is a fair way back.  A lash up with  borrowed Lipos and a few rubber bands or tape to secure might be a lot of help.   Then how could you get that size lipo inside the model and change after flight.  Will the lipo fit behind  the existing bulkhead and still get CG correct?

Exactly, work out what size battery will fit then select motor and prop to give around 500-600 watts of power.

[Shaun Walsh]

  Here's an example using a 4s 4000mAh battery and this motor https://www.hacker-motor-shop.com/Brushless-Motors/Hacker-Outrunner/Hacker-A30/A30/A30-12-XL-V4-kv700.htm?shop=hacker_e&SessionId=&a=article&ProdNr=15716108&p=6514 with a 12x8 prop  

Battery Load: 9.52 C Voltage: 14.30 V Rated Voltage: 14.80 V Energy: 59.2 Wh Total Capacity: 4000 mAh Used Capacity: 3400 mAh min. Flight Time: 5.4 min Mixed Flight Time: 8.1 min Weight: 456 g   16.1 oz

Motor @ Optimum Efficiency Current: 27.20 A Voltage: 14.35 V Revolutions*: 9018 rpm electric Power: 390.2 W mech. Power: 334.9 W Efficiency: 85.8 %

Motor @ Maximum Current: 38.09 A Voltage: 14.16 V Revolutions*: 8605 rpm electric Power: 539.6 W mech. Power: 458.6 W Efficiency: 85.0 % est. Temperature: 52 °C   126 °F   Wattmeter readings Current: 38.09 A Voltage: 14.3 V Power: 544.7 W

Propeller Static Thrust: 2755 g   97.2 oz Revolutions*: 8605 rpm Stall Thrust: - g   - oz avail.Thrust @ 0 km/h: 2755 g avail.Thrust @ 0 mph: 97.2 oz Pitch Speed: 105 km/h   65 mph Tip Speed: 494 km/h   307 mph specific Thrust: 4.45 g/W   0.16 oz/W

Total Drive Drive Weight: 826 g   29.1 oz Power-Weight: 188 W/kg   85 W/lb Thrust-Weight: 0.92 : 1 Current @ max: 38.09 A P(in) @ max: 563.8 W P(out) @ max: 458.6 W Efficiency @ max: 81.3 % Torque: 0.51 Nm   0.38 lbf.ft Climb Capacity: 2517 m   8258 ft

Airplane All-up Weight: 3000 g   105.8 oz Wing Load: 60.0 g/dm²   19.7 oz/ft² Cubic Wing Load: 8.5   est. Stall Speed: 37 km/h   23 mph est. Speed (level): 87 km/h   54 mph est. Speed (vertical): - km/h   - mph est. rate of climb: 7.8 m/s

Edited August 25, 2023 by Shaun Walsh

[Graham Bowers]

1 hour ago, Richard Acland said:

Thanks for the replies. I have checked the weight of the model with the engine removed and it comes to 5lb 5oz. It will not be subject to aerobatics just a bit of leisurely flying

What a sweetie 😉

 

I more or less agree with the other comments however add the following:

 

5 and a quarter pounds with no engine will probably weigh around 7 and a quarter pounds with a motor, ESC and Lipo, so look for 700 Watts or so. It's iterative, so see what weight the powetrain comes out at, and adjust total weight accordingly.  (I compare electric motor+ESC+Lipo versus engine/silencer+throttle servo+tank+1/2 a tank of fuel + RC battery*) *Depends if you propose to run the RC kit off an ESC BEC, separate BEC  or dedicated battery, however probably a minor consideration in the grand scheme of things.

 

Round figures, Prop diameter, pitch and speed need to deliver 700 watts or so.

 

The prop selected for an IC engine has to match it's torque curve. You'll have more latitude to run a bigger pitch prop with electric  if you wish, with a motor of appropriate Kv.

 

The practical consideration of CoG is valid and that may be a major factor in deciding on the Lipo weight. However whatever weight you decide may be realised by a variety of Lipo capacities and voltages.

 

Practical next steps, if I were you. Decide on your desired flight time, and have  a play around with the Hacker eCalc tool.  I like 10 mins on IC to be honest, but accept less with leccy as it's a compromise. I've made some gross assumptions with the eCalc tool, but at least it shows the principles. I have hacker kit in one model, however, as they say, alternatives are available 😉

 

https://www.ecalc.ch/setupfinder.php?hacker&elevation=300

 

 

 

 

 

 

[Richard Acland]

Thanks for all the helpfull replies. Certainly gives me something to go on. It all looks a bit more complicated than I envisaged but I have a whole winter to sort it out.

Edited August 25, 2023 by Richard Acland
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