Is IC best for larger models?

[Nigel Heather]

I've always thought this.

 

As the size of the aircraft increases the size/weight of the motor/battery/ESC needed increases exponentially.

 

With smaller models an electric setup is lighter than an IC setup, but there must quickly come a point where it is heavier.

 

And as you go larger still the extra weight of the batteries becomes more and more a factor, 12S at 5000mAh weighs around 1.5kg (3.3 pounds in old money).

 

I notice that when I go to shows like Wings and Wheels, hardly any of the planes, and especially the big ones use electric.

 

Is there a rule of thumb where the electric setup because heavier than the equivalent IC setup.

 

But I do imagine, that for all the stuff we typically fly at the local club, electric is fine.

 

Cheers,

 

Nigel

 

 

Edited July 27, 2023 by Nigel Heather

[PeterF]

53 minutes ago, Nigel Heather said:

I've always thought this.

 

As the size of the aircraft increases the size/weight of the motor/battery/ESC needed increases exponentially.

 

With smaller models an electric setup is lighter than an IC setup, but there must quickly come a point where it is heavier.

 

And as you go larger still the extra weight of the batteries becomes more and more a factor, 12S at 5000mAh weighs around 1.5kg (3.3 pounds in old money).

 

I notice that when I go to shows like Wings and Wheels, hardly any of the planes, and especially the big ones use electric.

 

Is there a rule of thumb where the electric setup because heavier than the equivalent IC setup.

 

But I do imagine, that for all the stuff we typically fly at the local club, electric is fine.

 

Cheers,

 

Nigel

 

 

I run a wide range of electric aircraft and my feeling from building and specifying the equipment in them was that the battery weight and motor weight increased proportionately with the aircraft size, not exponentially as you state. Why would they increase exponentially, it is well known that a motor weight for good performance is typically 1g per 3W of power output, and for a standard type flight you want 100-125W/lb of model weight. This suggests that both power and by definition motor weight will increase linearly with model weight. As the battery needs to provide a certain power for a certain period of time, then battery size will also increase linearly (proportionately) with model weight.

 

I have always kept a spreadsheet of all my model and power set up details with Wattmeter and rpm readings, model weights, battery weights and flying weights. So I have looked at that record and plotted 2 graphs, one showing battery weight vs flying weight and one showing motor weight vs flying weight. For the purposes of this analysis I have removed all my EDF models, the ones shown are propeller driven only. It is known that EDFs require more power and larger batteries than the same weight model with propeller drive, but if plotted on their own, the EDF follow the same linear trend.

 

The data I have shows in the first graph that over a wide range of flying weights (0.3kg to 8.2kg) the battery weight is proportional to flying weight. There is some scatter as I only use 4 battery types for all these planes, plus there are a wide range of model types with different power to weight ratios (more later). The second graph shows that the motor weight is also on the whole proportionate to the flying weight, but the scatter is somewhat larger in this case. This is due to the different power: weight ratios required for the different flying styles. For example at the 7 to 8kg part of the charts are 3 planes all about 2m (80") wingspan. One is a 3D aerobatic plane, once is a Mosquito and one is a quarter scale Tiger Moth - pretty easy to decide which is which.

 

I do not have weights for the ESCs but these will be dwarfed by the battery and motor weight and will probably also be proportionate to the power.

 

Your other question around the flying weight of the different setups. A fair number of my models were glow powered and I converted these to electric, including the 3 large models described above. In every case I have found that the flying weight of the glow powered version and the IC version was very similar. In fact the mosquito was 17lbs 10oz glow and 17lbs 12oz electric. I do not know what it would be for larger petrol power as I have never flow petrol.

 

 

[leccyflyer]

Marvellous post and data Peter - thank you so much for that. 😎

[paul devereux]

13 hours ago, Ron Gray said:

Laser 200 in-line twin sounding nice

 

 

That does sound nice! Definite WW2 warbird vibes!

[Paul De Tourtoulon]

Ok Peter, but what about flight times ?.

[Jon H]

14 hours ago, David perry 1 said:

How is this done?  A gear box i understand, but what engine?  It does sound terrific

 

 

14 hours ago, Ron Gray said:

Moki radial?

 

 

Yea its a moki radial with a planetary gearbox on the front. Brings the rpm right down and allows the use of the very large prop. 

 

The snag is it aint cheap. I think the propeller alone is £700. 

 

Clearly that is excessive for most, but if you have a 15-20grand model, which that very well could be, is another 700 quid going to matter? probably not. It would be nice to scale it down in both complexity and cost but my first effort was unsuccessful and the boss gave up immediately when it didnt work instead of taking what we learnt and going again with a new design. sad times. 

[PeterF]

3 hours ago, Paul De Tourtoulon said:

Ok Peter, but what about flight times ?.

Typically most of my propeller drive planes are 8-9 minutes with aerobatics thrown in but some like my quarter scale Moth will do 15 minutes. The only ones with limited flight time are EDFs where I get 4 to 8 minutes depending on the model. 

[Ron Gray]

1 hour ago, Jon - Laser Engines said:

and the boss gave up immediately

That’s a shame as I was thinking about it for the DVII

[Jon H]

2 hours ago, PeterF said:

Typically most of my propeller drive planes are 8-9 minutes with aerobatics thrown in but some like my quarter scale Moth will do 15 minutes. The only ones with limited flight time are EDFs where I get 4 to 8 minutes depending on the model. 

 its not the flight time but the quick turn around that does it for me. I can refuel and go back up in just a few minutes if i choose rather than having to wait for a battery to charge. 

 

2 hours ago, Ron Gray said:

That’s a shame as I was thinking about it for the DVII

 

yep. Its unfortunate as i did learn a lot from the first experiment. Mostly that its harder than it looks, but also that gears wont cut it due to the noise and that my calculated performance predictions were accurate. This latter nugget of information is the most important to me and was very reassuring. Now all i need is to find a suitable belt drive and go again

[Nigel Heather]

16 hours ago, PeterF said:

I run a wide range of electric aircraft and my feeling from building and specifying the equipment in them was that the battery weight and motor weight increased proportionately with the aircraft size, not exponentially as you state. Why would they increase exponentially, it is well known that a motor weight for good performance is typically 1g per 3W of power output, and for a standard type flight you want 100-125W/lb of model weight. This suggests that both power and by definition motor weight will increase linearly with model weight. As the battery needs to provide a certain power for a certain period of time, then battery size will also increase linearly (proportionately) with model weight.

 

I have always kept a spreadsheet of all my model and power set up details with Wattmeter and rpm readings, model weights, battery weights and flying weights. So I have looked at that record and plotted 2 graphs, one showing battery weight vs flying weight and one showing motor weight vs flying weight. For the purposes of this analysis I have removed all my EDF models, the ones shown are propeller driven only. It is known that EDFs require more power and larger batteries than the same weight model with propeller drive, but if plotted on their own, the EDF follow the same linear trend.

 

The data I have shows in the first graph that over a wide range of flying weights (0.3kg to 8.2kg) the battery weight is proportional to flying weight. There is some scatter as I only use 4 battery types for all these planes, plus there are a wide range of model types with different power to weight ratios (more later). The second graph shows that the motor weight is also on the whole proportionate to the flying weight, but the scatter is somewhat larger in this case. This is due to the different power: weight ratios required for the different flying styles. For example at the 7 to 8kg part of the charts are 3 planes all about 2m (80") wingspan. One is a 3D aerobatic plane, once is a Mosquito and one is a quarter scale Tiger Moth - pretty easy to decide which is which.

 

I do not have weights for the ESCs but these will be dwarfed by the battery and motor weight and will probably also be proportionate to the power.

 

Your other question around the flying weight of the different setups. A fair number of my models were glow powered and I converted these to electric, including the 3 large models described above. In every case I have found that the flying weight of the glow powered version and the IC version was very similar. In fact the mosquito was 17lbs 10oz glow and 17lbs 12oz electric. I do not know what it would be for larger petrol power as I have never flow petrol.

 

 

 

Many thanks for that.  I haven't done anything like the research you have done.  It was more from gut feeling and things like this

 

Acrowot Foam-E - flying weight 1200g, battery (3S 2200) = 170g, without battery 1030g

Acrowot EP - flying weight 3000g, battery (5S 4500) = 600g, without battery 2400g

 

Airframe weight ratio = 2.33x

Battery weight ratio = 3.53x

That's why I said exponential

 

Also my observation not none of the large planes being flown at Wings and Wheels were electric.

 

Also- would your graph keep scaling - if so why don't we have real electric plane - full size Cessna.  Why is a EP VW Golf significantly heavier than a petrol VW Golf.  The weight of EV cars is an emerging concern and the reason always cited is the battery.

 

I'm not saying you are wrong at at, just asking if there is a point where the weight of the battery becomes a problem

[leccyflyer]

13 minutes ago, Nigel Heather said:

 

Many thanks for that.  I haven't done anything like the research you have done.  It was more from gut feeling and things like this

 

Acrowot Foam-E - flying weight 1200g, battery (3S 2200) = 170g, without battery 1030g

Acrowot EP - flying weight 3000g, battery (5S 4500) = 600g, without battery 2400g

 

Airframe weight ratio = 2.33x

Battery weight ratio = 3.53x

That's why I said exponential

 

Also my observation not none of the large planes being flown at Wings and Wheels were electric.

 

Also- would your graph keep scaling - if so why don't we have real electric plane - full size Cessna.  Why is a EP VW Golf significantly heavier than a petrol VW Golf.  The weight of EV cars is an emerging concern and the reason always cited is the battery.

 

I'm not saying you are wrong at at, just asking if there is a point where the weight of the battery becomes a problem

 

[John Lee]

48 minutes ago, Nigel Heather said:

 

 

Also- would your graph keep scaling - if so why don't we have real electric plane - full size Cessna. 

 

A Cessna Caravan first flew under electric power back in 2020. Wikipedia lists dozens of full size electric aircraft under development.

 

[Ron Gray]

2 hours ago, Jon - Laser Engines said:

find a suitable belt drive and go again

This one warrants closely scrutiny, maybe somewhat over engineered.

 

https://www.facebook.com/groups/3214227645472741/permalink/3567895766772592/

Edited July 28, 2023 by Ron Gray

[J D 8]

         Tooth belt drive works well in many microlight installations.

[Zflyer]

The liquid fuel weight diminshes with it use  the electric powered ones do not. Full size aircraft using electric will have the same weight on landing as takeoff. In addition payload capacity i assume would lessen as fuel weight (battery) is taken into consideration. 

It will no doubt come to pass but i suggest with smaller passenger capacity. Sustainable jet fuel is another proposal which has many pros and cons.

[PeterF]

3 hours ago, Nigel Heather said:

 

Many thanks for that.  I haven't done anything like the research you have done.  It was more from gut feeling and things like this

 

Acrowot Foam-E - flying weight 1200g, battery (3S 2200) = 170g, without battery 1030g

Acrowot EP - flying weight 3000g, battery (5S 4500) = 600g, without battery 2400g

 

Airframe weight ratio = 2.33x

Battery weight ratio = 3.53x

That's why I said exponential

 

Also my observation not none of the large planes being flown at Wings and Wheels were electric.

 

Also- would your graph keep scaling - if so why don't we have real electric plane - full size Cessna.  Why is a EP VW Golf significantly heavier than a petrol VW Golf.  The weight of EV cars is an emerging concern and the reason always cited is the battery.

 

I'm not saying you are wrong at at, just asking if there is a point where the weight of the battery becomes a problem

For RC with typical flight times the battery will continue to scale linearly with model weight, the power required is linear with weight, so the energy stored will be linear, so the battery capacity and weight will be the same. For full scale they want longer duration. There is also the cost element of higher upfront costs, as Jon says, turn around time is very different, you either have to buy loads of batteries or charge them at the field. On my largest EDF plane I have to accept 40 minute recharge because I will not buy another set of batteries at several hundred pounds. I am happy to compromise like this. For my smaller models I can afford multiple packs and not be restricted. This is one of the reasons for not commonly seeing larger models with electric power. 

 

Your battery and flying weights fall within the scatter on my charts, so they add to my data confirming the proportionality, but you need more than 2 examples to see the average trend.

[John Lee]

Aerovolt have just announced that their first of a network of airfield chargers for electric aircraft will be launched next week.

[MattyB]

On 27/07/2023 at 18:18, Ron Gray said:

Moki radial?

Yes, coupled with a planetary gearbox. Full details here from the YT description…

 

https://seidel-props.de/en/sp-250-x-beschreibung/

 

Couldn’t find a price, but given you need a Moki 250 to start with and their props are in the stratosphere, I the total powertrain cost is going to be well into the divorce-worthy category… 😂