wieght comparison IC to Electric

[Basil]

I have never been involved with IC engines and need some advice......

I am trying to find the weight of a tank and fuel that would be used for a 61 motor. This is to help plan a build in electric what is on plan as a 61 motor.

I am aware that the choice  is rather open to choice  or what space dictates.   Many thanks Bas

[Nigel Sharp]

Just go onto the Web and search for the technical info for a typical .61 two stroke or 4 stroke depending on motor type on plan.

 

They usually give the weights. Esp OS

 

Then it's down to the tank capacity. But plan balances CofG when empty.

 

Electric has the battery weight constant. So you can balance with the battery in as if the ic was empty.

 

For example my Loaded Dice 1 was for a .61 two stroke with 12 oz tank. And tuned pipe.

 

I have a 5050 580 kv motor 80 amp esc and a 6s 6200 battery in place of ic.

 

The weights combined are very similar to the ic setup

 

Balanced at the designers location with aadditional at most 30g lead at motor mount.

 

Bonus is that the balance point does not shift during flight as would with fuel consumption.

 

 

[Neil R]

Here's my attempted sums from when I electrified an old Acrowot:

 

Irvine 61 = 700g with muffler, fuel = 8 oz = 227 cc x 0.79 = 180g, mount = 50g (guess), tank = 60g, 4.8V battery = 120g, servo = 45g; Total = 1155g.

Motor = 332g, Rack Mount = 92g, Lipo + bullets = 700g, ESC = 62g; Total = 1186g (2.6 lbs), so pretty much unchanged!

 

(Noting that the ESC can replace the weight of the Rx battery and throttle servo).

 

This was for a 5055 580kv motor, and a 5s 5000 mAh Lipo, which I measured at 1250W (~190W/lb). I think my intent was to replicate the Irvine, but the final result was notably more powerful (and still too heavy really). 

 

[Nigel R]

Opinions / experience :

 

Little weight difference once you get to 0.40 size or bigger.

 

A .61 engine would need about 900W of electric to be a straight swap.

 

[Geoff S]

I agree that with modern electric systems (brushless motors and LiPo batteries) there's little difference.  However, with heavy brushed motors and NiCad batteries electrically powered models were much heavier than the equivalent glow. In fact when I built my electric Lazy Bee the wing had extra panels to increase it area to keep the loading similar to the glow powered versions.

[Cuban8]

1 hour ago, Geoff S said:

I agree that with modern electric systems (brushless motors and LiPo batteries) there's little difference.  However, with heavy brushed motors and NiCad batteries electrically powered models were much heavier than the equivalent glow. In fact when I built my electric Lazy Bee the wing had extra panels to increase it area to keep the loading similar to the glow powered versions.

The Lazy Bee! a blast from the past there..............................Had a mate who went LB crazy, built all sorts of variations on the original theme as many did back in the day - all very nice but I could never see the attraction myself that so many people had to the design that in my eyes, was an odd little model.

Still going strong I see on Andy Clancy's website 👍

[Outrunner]

Don't forget that a model designed to be electric powered can have a much lighter structure than one with an engine shaking everything about.

[Basil]

Thank you to all those that contributed, Bas

[Don Fry]

Tank size, I use the formula, 

2 stroke, cu.inch x 1.5, ie, a 61 needs a 9 oz tank.

4 stroke, cu inch x 1.0, ie, 61 4st, a 6 oz tank. 
I don’t go smaller, if the formula suggests a tank size that doesn’t exist, go to next size up. 

[Konrad]

Basil, I’m sorry I’m not clear which way you are going. Are you going from a 10cc system shown on the plans to an electric conversion. Or are you going for a electrical system shown on the plans to 0.61 cid engine. By the title I thought it was the former.

 

Don, for 2.1 hp plus 2 stroke 0.61 cid engines I used 12 to 16 oz tanks. 

Edited April 25 by Konrad

[Konrad]

If I'm constrained by the landing gear I will go with 3 or 4 bladed props to try to harness the torque of the electric motor.

[Andy Gates]

I usually work differently from the method being used by the OP.

 

Reason being - the build does not need to be so heavy as electric does not produce the vibrations of IC.

My builds of converted machines usually weigh in at around 2/3 of the original. Block balsa being replaced with curved thin sheet over formers, thicknesses being reduced throughout, birch ply (1/8" normally) in stressed areas only - firewall & UC.

 

I then use the rule of thumb 100w/lb to work out power requirements.

 

They fly really nicely after a diet as above.

 

Example - i train people using a Ripmax WOT trainer - approx 6lbs is the spec.

Rebuilt the fuselage to drop weigh, binned all  the heavy covering.

Now flies at 4lbs, it will not stall or spin, flies backwards in a breeze and generally flies gently until you open the throttle up.

[Basil]

I am going from 61 IC to electric.Bas

[Dale Bradly]

A regularly seen issue with the converting a model designed for i.c. to electric is weight distribution: The i.c. engine is the heaviest part of that powertrain, and mounted at the front. With electric, the battery is the heaviest part, even though the total powertrain might wash up similar.  This if you intend on the obvious answer of putting the batt in what was intended to be the tank bay, the original designer didnt plan for this eventuality, and thus the weight distribution ends up out of whack, and you end up with a church roof to get it to balance and thus the electric version is heavier. If you're building from scratch, then you can plan for this in advance with batt location, and/or weight removal back aft.

[Don Fry]

4 hours ago, Dale Bradly said:

A regularly seen issue with the converting a model designed for i.c. to electric is weight distribution: The i.c. engine is the heaviest part of that powertrain, and mounted at the front. With electric, the battery is the heaviest part, even though the total powertrain might wash up similar.  This if you intend on the obvious answer of putting the batt in what was intended to be the tank bay, the original designer didnt plan for this eventuality, and thus the weight distribution ends up out of whack, and you end up with a church roof to get it to balance and thus the electric version is heavier. If you're building from scratch, then you can plan for this in advance with batt location, and/or weight removal back aft.

To quote Jon Harper, it’s only wood, usually, with hacking, the battery can be shifted forward. 

[Basil]

For the model in mind I can place the battery almost under the CG, well not quite .

Bas

[Don Fry]

That suggests a lot of lead. 61 motor, about 20 oz, electric motor a lot lighter, 

Lead to balance it, the difference between those two weights. Battery on CG, does not do anything to balance it.

[leccyflyer]

As Don says, you don't want the battery on the CG, you want it in front of the CG to compensate for the lighter weight of the electric motor compared to the IC engine. Putting the battery further forwards will reduce the need for carrying lead ballast in the nose.

[Basil]

For the Optica build the motor is to the rear of the CG, Battery to the frontish( Lipo more wt than a motor)

Sorry did not say that before Bas

[Don Fry]

First you need a defined CG from plan. Then you work out the leverage of the components of the ic system centered on the CG. 
Do the same for the electric components, less the power battery. Then you can predict its location, so  as to achieve the same CG, by putting in where the leverage numbers tally with the CG of the ic system.

 

[Nigel R]

IME, it's normally about as simple as shoving the lipo as far forward as possible, up against the old firewall. I've not yet had a model that won't balance like this. But. 

 

If needed, you can

build a box on to the firewall to mount the electric motor (you're probably doing this anyway, unless using threaded standoffs)

hack a hole in the old IC firewall;

poke the lipo forward, through the hole, into the motor mount box.

 

The lipo is a heavy component so moving it around makes a big difference to CG.

 

 

[Don Fry]

1 hour ago, Nigel R said:

IME, it's normally about as simple as shoving the lipo as far forward as possible, up against the old firewall. I've not yet had a model that won't balance like this. But. 

 

If needed, you can

build a box on to the firewall to mount the electric motor (you're probably doing this anyway, unless using threaded standoffs)

hack a hole in the old IC firewall;

poke the lipo forward, through the hole, into the motor mount box.

 

The lipo is a heavy component so moving it around makes a big difference to CG.

 

 

Yea, but it’s Optica he wants to balance, engine on the back. 

[Shaun Walsh]

On 25/04/2025 at 17:00, Konrad said:

If I'm constrained by the landing gear I will go with 3 or 4 bladed props to try to harness the torque of the electric motor.

Better to use a 2 blade prop and a higher Kv motor.

[Konrad]

Agree I like the two bladed props better and there are more options when it comes to matching the power to the airframe. 
 

I lost an earlier post where it was mentioned that the torque of an electric motor often allows a larger prop compared to a 2 stroke of the same power. As a result one often can use a power system of a lower power when making the conversion from 2 stroke to electric as the larger props are often more efficient. 
 

If starting from plans, I often end up with a much lighter airframe and lower powered than that spec’s on the plans. For an airframe that often exhibit better performance than originally designed. 
 

Key is that the electric motor often allows a prop better suited for the airframe.