which is cheaper,IC or ELECTRIC?

[Geoff S]

Posted by John Stainforth on 05/12/2018 03:36:05:

Electrics can be quite expensive. I saw a drone plus camera at the field today that cost 85,000 pounds!

Everything can be expensive if taken to extremes and £85k for a drone is a prime example. I guess it's one intended for professional filming where it's replacing a full size helicopter and hence a cheaper alternative.

I'm not a quad/drone fan but the technology is fantastic and not really feasible with ic propulsion.

Geoff

[Dave Hess]

Posted by supertigrefan on 05/12/2018 11:50:29:

I have IC engines that I bought new over 20 years ago and still use, does that get factored in?

Edited By supertigrefan on 05/12/2018 11:51:49

That's interesting. I still have the first IC engine I bought for RC flying. I've never worn out or broken an IC engine, though I've had plenty of broken needle valves. On the othe other hand, I've never worn out or broken an electric motor either, and I still have some from 30 years ago.

So, yes we can factor it in, but what are the factors?

[Nigel R]

In my (fairly short) experience of electric flight:

Electric motor bearings wear out.

Shafts are somewhat easily bent with a belly lander.

Magnets become detached.

Some or all of which can, practically, write off a motor. Unless it is an expensive fancy one, of course.

My dad recently gave me a few motors which included the first motor I owned about 25 years ago. A mighty Magnum 25 GP. Might get it out and give it a bench run.

[extra slim]

Good on The Wright Stuff for having a stab with his excel skills, which was bang on topic. Now before I provide my feedback, I fly all manner of IC, Glow, Petrol, Diesel, and electric both fixed wing and occasional rotaries and I really dont think about the costs.

For me the example TWS exposed was useful, but missed a few elements:-

1) Under IC, you need to include plug and throttle servo, engine mount

2) I think your fuel pump and model box, could be £10 each (hand pump, and general tool box), plus an ASP 46 is £66

But... what was glaringly missing from my perspective, was the Lipo's, as you offset the cost of the lipo by a number of flights... but didnt do the same for the IC motor. For me the lipo cost should be above the line based on a flying session, and not just one flight, so is a fixed cost.

What I mean is... to get a good benchmark, take a kit built WOT 4 type plane, and a general flying session of say half a day with 5 flights.. on your electric side of the equation, there would need to be I guess 2 options.. either the cost of 5x 4s 3700 batteries (for a WOT 4).. 3s 2200 wouldnt cut it on a balsa WOT or would maybe get your airbourne for about 2 mins. OR.. just a couple of batteries and the cost of a leisure battery to take to the field to charge one, whilst the other is in use..

In doing this, the results are somewhat different.. Just my conclusion is that based on a flying session, not just one flight, smaller models with smaller batteries are cheaper when electric, but somewhere just short of a WOT4 is where there is a break even, and then above..(5s, 6s etc) more expensive.. obviously equipment varies wildly in costs, so this is broad brush..

Luckily, I dont care about the cost and love all forms

[Martin Harris - Moderator]

You could also factor in pilot experience and the likelihood of a significant accident to the original question. A reasonable impact into grass often damages an electric motor beyond economical repair - and a bent LiPo is a write-off even if it doesn't end in conflagration. Glow engines usually survive similar arrivals with little or no functional damage. Just avoid planting one into a concrete runway!

[Former Member]

[This posting has been removed]

[cymaz]

This debate will run and run. Just like my trusty SC 52 ( now sold).

[Peter Beeney]

Coming at this from yet another direction, to my mind the ‘comparison’ factor must also play at least a small part in this. I’ve always thought that the battery cost might be related directly to fuel cost; something I’ve poggered over a few times in the last 10 years at least.

So another fag packet calculation, (incidentally, empty ciggie packets are getting harder to find these days, I’ve not seen an empty Senior Service or du Maurier lying around for some time now) to decide between methanol and amperes.

Leaving out all the hardware costs for a moment let’s look at a 5 lb 12 oz, 61 inch span, low wing sports model with a lump of Irvine 53 Mk II up front. This turns an 11 x 8 APC at 11K and for arguments sake soaks up 1kW in so doing. This runs flat out for 20 minutes on 9oz of fuel. (I have timed it, from 9oz start to an empty dead stick.) The model is quite lively too, with a theoretical speed of around 80mph. My fuel is straight synthetic with an extra pint of methanol added to each gallon to thin it a bit more. Let’s say it’s 20 quid a gallon. A gallon is 160 fluid ounces, so I’d get 160 / 9 = 17.8 full flights from a gallon. That’s a total run time of 20 x 17.8 = 356 minutes or around 6 hours.

To get the same performance from electric I’d again need a kilowatt of power; actually I’d consider that’s a bit conservative, the i/c power measurement is at the prop shaft whereas the electrical power as measured by a watt meter is that which is flowing from the battery; and they are not quite the same thing; due to losses changing power from electrical to mechanical in good conditions I’d need to see probably 1,200 watts, 20% more, on the watt meter. In some circumstances the disparity between these two measurements can be very pronounced indeed.

Anyway, let’s be generous and just call it a kW. In a random but practical way let’s select say a 5S 5,000mAh pack. To deliver a kW it would have to discharge at 1000 / 18.5 = 54A. Discharging at 54A would give me 5Ah / 54 = 0.0926 hours; or 0.0926 x 60 = 5.5 minutes flying. That’s a discharge rate of around 11C, nothing too drastic there.

The cost of a 5S 5Ah pack is around £60. I get three gallons of fuel for that; this give me 18 hours of flying time. If I get 5.5 mins per flight electric, then to be able to fly for 18 hrs would take 18 x 60 / 5.5 = 196 charges and discharges. How would the battery be feeling after that? In my experience, a little bit jaded to say the very least. Also another point, the 9 oz of fuel gradually finds it’s way though the engine and disappears; the engine’s performance stays fairly consistent throughout. The model gets a bit lighter, thus perhaps improving it’s flight characteristics. The CoG also moves back a little bit, (I like back a little bit CoG; it helps to to give the elevator a tad more authority!) The battery weighs around 30 ounces, so the model now weighs 7 lb 10 oz, this stays constant and the motor’s performance can only ever (slightly) diminish throughout the flight.

I fully appreciate this is all very speculative and perhaps my arithmetic is a bit wobbly anyway; and that there are very many successful electric models. Indeed, they’ve been winning World class competitions for quite a while now. It’s just that I’ve occasionally wondered what size of battery I’d need to get the same time flying time and performance from the model. Also I’m not sure why the Irvine is so economical; just the gently tickling of the needle valve perhaps; maybe an OS 55 AX would be twice as thirsty; very much the same spec. though, 1.25 kW at 17k. So I reckon a kilowatt at 11,000 rpm might just about fit again. Even if it did drink twice as much fuel the battery would still have to deliver the very think end of 100 flights to equal it’s duration.

I have to admit there’s not much flat out flying for 20 minutes at the time for me these days. Even 5.5 minutes might require some written notification and preparation. And like many other folk I don’t count the cost very much anyway, I might find it slightly disconcerting to be constantly counting the coins. I can cope with single tasking, just… as for multi tasking, no chance…

There is another option here too, I could go back to slope soaring, a little something I’ve fancied for a while now, as it happens. Now the motive power will be cheap enough, but the car fuel gauge might start to complain it’s a bit thirsty on a regular basis…

It seems I can’t win…

Please consider this as only another dusty and rambling chronicle, just my view…

PB

 

Edited By Peter Beeney on 06/12/2018 13:02:00

[Bob Cotsford]

The only point that I'll pick you up on Peter is that the conversions that I've done have shown far less of a weight increase. These have all been where I've replaced IC motors with 5055 or so sized electrics with 6S 4000mAh packs and 80A or so ESCs. So there's a Laser 80, Laser 100, ASP91FS and an OS61SF spring straight to mind. Maybe you need to allow for the much lighter motors, no silencer, no fuel and tank, no servo or linkage. I would say that electric usually ends up just a couple of ounces heavier at take off.

Edit - This is for conversions, if you build for electric from the start then the whole structure can be much lighter with a less rigid area around the engine mount as it doesn't have the vibration to cope with.

You also have the freedom to move stuff around more to balance the model so no lead needed most of the time! Admittedly it doesn't get lighter throughout the flight but neither does the trim change. I would expect to get realistic safe flight times of around 8 minutes, then again my Wot4 on 4S 4500mAh can give mixed style flight times up to 20 minutes. I don't recall any of these models getting beyond 8-10 minutes safely and the OS61SF in a Tornado generally needed it's wheels back on the ground after 7 minutes.

It's interesting to while away the winter days with these speculative comparisons but really it's down to what is practical for each of us. I'd like to still be flying IC but since having to go electric I'm coming at appreciate that it does have some benefits.

Edited By Bob Cotsford on 06/12/2018 13:31:38

[Andy48]

I've just finished and tested a new Wot4 running on electric with a 13x8 prop. It uses 5S 4000 batteries. With static thrust I get about 3.5kg thrust at full throttle. While I had some batteries, I bought a couple extra, and far from the £60 quoted above, these new 40C batteries cost £30.88 each from HK with free delivery.

In flight, the thing goes like the clappers, as might be expected on 5S. I have full telemetry on board so I can measure the current actually used. At half throttle (plenty fast enough for most of the time) it uses just 15 amps. After an 8 minute flight there is easily enough for a similar length flight again and a 20 minute flight is, as Bob suggests, well within the battery limits provided one is sensible with the throttle. Again at half throttle, there is little stress on the battery and its life will be extended.

There is one thing to bear in mind for those who move over to electric, whilst the relationship between the thrust and the joystick position is linear, the current consumption rises significantly above half throttle. At full throttle my Wot 4 runs at 65amps. Thus if you fly flat out all the time on electric you will get very short flights. Bear in mind that with drag being the square of the speed, you will likely not see a great increase in overall speed flying like this.

As for robustness of motors, I now stick to Turnigy SK3 motors and these have proved very reliable, but cheap. No bent shafts, no bearings worn out and no magnets come loose in the last several years.

[eflightray]

To throw in another odd-ball,. What IC engine would be needed to turn a 17" x 8" prop ?

I fly a scratch built Douglas SBD Dauntless using a 17" x 8" prop, on 3s at 340 watts. It's 67.5" span and flying weight is 4 Lbs - 12.5 oz, and that includes retracts.

It flies quite scale like, obviously not a pylon racer.

For scale model, an electric motor can swing a much bigger prop for its weight, plus if you want to you can build quite light for electric.

I built the TN 70" Spitfire, (free plan in RCME Autom Special 2010, mine actually ended up at 72" span). But I used Depron and a small amount of balsa. Still used the TN molded exhaust stacks, lower cowl and cockpit. The plans quote a expected flying weight of 15 Lbs for the original build and IC engine power. Mine is 7 Lb 2 oz flying, on 6s, turning a 15" x 8", (originally a 16" x 10" but still flies as well on less amps).

I'm not arguing whether IC or electric is cheaper or better, but that it's just different, if you want it to be.

Ray.

[The Wright Stuff]

Peter makes a good point, which refers to a question I asked earlier in the thread. Whether to calculate cost per flight, cost per minute of flight, or cost per flying session?

Personally, I find that although my I.C. flights tend to last maybe 2 to 3 times longer than my electric flights, I don't fly 2 to 3 times more flights in the session. Therefore for me, cost per flight is the best metric. Others might differ.

[Nigel R]

Pete, I don't think you're a million miles out. But some numbers need adjusting.

In my experience, I'd expect a Irv53 to spin that 11x8 APC at 11k just like you did, and run an 8oz tank for about 15 minutes until dry. In fact my dad flew a Chilli Wind with this exact setup for a while, until a runway inconveniently ended the fun for that particular engine.

With a replacement fuselage, I'm flying the model, sporting electric doohickeys, which spin (wait for it) an APC 11x8 at (wait for it) 11k. No, it's not the exact same prop, the first one left bits of itself in the runway. The lipo being used is a 4S 5000mAh. Now, a normal flight time, 12 minutes, with careful throttle, BUT the kicker of course is that at full bore that is drained in 6 minutes.

Let's say I've bough 4 gallons of Weston Prosynth @ £15/gallon. At around 20 goes per gallon, 20 x 4 x 15 = 1200 minutes for £60.

The lipos cost £30 each (Turnigy jobs). 6 minutes x 100 charges (before expecting end of life), or 600 minutes for £30. Rather astonishingly - after adjusting to match the IC runtime - that's £60 for 1200 minutes.

I didn't massage anything. I might have a couple of details a bit hazy. Irvine 53 consumption needs to be checked! The lipo setup is a definite.

Edit: If you shop at Overlander or 4-max or one of the fast vanishing Ye Olde Local Model Shoppe, electric starts looking less cheap.

 

Edited By Nigel R on 06/12/2018 15:39:21

[Dave Hess]

Posted by eflightray on 06/12/2018 15:05:30:

To throw in another odd-ball,. What IC engine would be needed to turn a 17" x 8" prop ?

I fly a scratch built Douglas SBD Dauntless using a 17" x 8" prop, on 3s at 340 watts. It's 67.5" span and flying weight is 4 Lbs - 12.5 oz, and that includes retracts.

It flies quite scale like, obviously not a pylon racer.

For scale model, an electric motor can swing a much bigger prop for its weight, plus if you want to you can build quite light for electric.

I built the TN 70" Spitfire, (free plan in RCME Autom Special 2010, mine actually ended up at 72" span). But I used Depron and a small amount of balsa. Still used the TN molded exhaust stacks, lower cowl and cockpit. The plans quote a expected flying weight of 15 Lbs for the original build and IC engine power. Mine is 7 Lb 2 oz flying, on 6s, turning a 15" x 8", (originally a 16" x 10" but still flies as well on less amps).

I'm not arguing whether IC or electric is cheaper or better, but that it's just different, if you want it to be.

Ray.

Can you tell us how much the motors, ESCs, motor mounts and batteries in those two planes cost so that we can compare?

[Geoff S]

I've just done the same Andy except my Wot4 is running on 4S 4AH LiPos mainly because I have a lot of them I use in other models. I'm using a fairly expensive motor I happened to have fitted to a model I very rarely fly (a Foxy). It's being very understressed with a 12x6 prop which delivers 700 watts at 10k rpm. As the model only weighs about 5lbs ready to go it has adequate performance and I get an easy 10 minutes without taking the battery anywhere near a critical discharge state.

It suits me perfectly, especially for winter flying when what's needed is a reliable model and no faffing in the cold. I don't think it really matters what you fly or how you choose to power it as long as you enjoy it and it doesn't frighten the horses

Geoff

[eflightray]

..........................................

Can you tell us how much the motors, ESCs, motor mounts and batteries in those two planes cost so that we can compare?

Not easily as the Dauntless was built 10 years ago, the Spitfire 6 years ago, (and both are still flying).

Dauntless - Emax 4020 - 520kv motor, Zippy 4000mAh 40C 3s battery x1 (current battery, have used others)

Spitfire - Emax 4030 - 385kv motor, Zippy, as above, 3s x2 (6s power).

Most electric motor come complete with a mount, ('X' mount), and prop driver.

The Zippy batteries, I have 6 3s packs, (plus some older ones still usable).

I would add that cost is probably not that easily compared, I have no clue how much the electricity cost charging them.

Ray.

Edited By eflightray on 06/12/2018 15:37:46

[Andy48]

To give you a guide as to what my Wot 4 setup cost:

ESC Emax BLHeli series 80a UBEC £26.00 (Found these heli ESCs work just as well)

Turnigy SK3 4250 500kv 1350watts motor £30.61.

Motor Mount, included with motor

Battery 5S 4000 40C battery £31 (heavier battery balances the Wot 4 without extra weight.)

Charging cost per battery (2 flights at least) less than 1p.

[Former Member]

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