Smoking....

[Martin McIntosh]

There could lie your problem then. You have 0.75 sq in in and only 1 sq in out. The inlets sound OK but due to the expansion of the heated air you need three times the csa outlet to inlet. Same as an i/c. The outlet needs the front and rear to be chamfered to ensure a smooth airflow. Sometimes actually reducing the intake csa helps. Put some vents on the side of the model if you need to.

[leccyflyer]

Bottom line - as other have said - it;s not a good idea to run an electric set up at full throttle on the ground until the battery is almost spent. It doesn't give you any useful data anyway - as advised a short run of 10-15 seconds until the current draw is stabilised is much more useful,

If your maximum current was 25 amps and your measurement of the watts in was 268W you are going to struggle to fly a 5lb model with that set up, on a 10x7" prop. You should be looking at something more than 400w for a gentle floater and something over 500w for aerobatic performance- using the old school rule of thumb of 100w/lb. You are also not likely to have the energy density for reasonable duration on a small 3S1p 3000mah pack. That would be much more suitable for a 3lb model - aim more for 4s1p 4200mah or 5s1p 4200mah for the sweetspot on a 5lb aerobatic model, with a motor and prop combination to give you >500-600w..

[Dickw]

Posted by Roger Dyke on 05/05/2020 21:04:30:

Dick: Thanks for the info and the chart. I don't quite understand what the chart is trying to say but that's me probably being a bit thick. I understand that the RPM is controlled and maintained by the ESC which it tries to maintain regardless of load. Obviously, the larger the load, the larger the current.

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

Roger

Not quite right. At full throttle the motor tries to reach an rpm of ‘kv x volts’ but is held back by the load and the ESC has no impact on that but just provides the commutation (basically they are DC motors). At part throttle the ESC applies pulse width modulation to its output which reduces power and so rpm drops. Some ESCs monitor rpm and can provide a constant rpm (at part throttle) if power available allows. We usually select a battery and prop to run our motors (full throttle) at about 75 -80% ‘kv x volts’ which gives about 75 - 80% efficiency.

In very simple terms, what that Kontronik chart is showing is that you can’t run a motor at its max watts rating at lower than max rpm (i.e. max supply volts). Unlike most mains power motors which operate at a fixed voltage we use our motors over a wider range of supply e.g 2s to 4s. and most manufacturers will quote the max watts on max volts. We should be considering the current rating not the watts rating because obviously if you run the motor at higher volts (i.e. higher rpm) the same current will give you higher watts.

The ratings depend on temperature/cooling anyway, and I regularly run motors over their current/watt rating but just make sure they don't get hot.

Dick

[PatMc]

Posted by Roger Dyke on 05/05/2020 15:11:25:

Bob: The motor was only running at 2/3 of the maximum stated current so was well in hand. Also it was only dissipating 268 Watts which is almost half of it's stated 505 Watts. Even with a total lack of ventilation I would have expected it to do better than 2.5 minutes from cold. I am looking at the cowl though to provide better ventilation. Absolutely no sign of burning or charring whatsoever either on the motor windings or the outer housing. Absolutely no sign of overheating on the ESC either. Wiring is also good. As the motor now judders when passing through about 1000 rpm I feel it has to be something to do with the internal windings of the motor.

Roger

Roger, Bob is correct when he says that the max current is the limiting factor not the max power. The only real limit on the max power is the voltage handling capacity of the ESC.
The power dissipated as heat = the current squared x the motors internal resistance.
The internal resistance is constant therefore this wasted power is dependent on current drawn but independent of applied voltage so long as the current is also kept constant.

Also as Bob says, the specified max current given is often only a short burst rating, sometimes this is stated but often not.

Re the problem - each motor wind usually consist of several strands of insulated wire connected together at each end. If one or more of the strands were damaged they can go open circuit but leave the rest still conducting. This will create a (relatively) high resistance on one phase causing an imbalance in the normal running & upsetting the ESC's sensing sequence.
I suspect that this may have been the cause of the smoke. If I'm right it's possible that the damage will only be visible if the motor is disassembled & due to the normal very low resistance of the windings won't be detectable with a normal ohmmeter.

[Robin Colbourne]

In-cowl airflow and cooling is something like 25% science and 75% art. The science is:to have rounded edges to any air inlets (look at the inlet to a ducted fan), duct the air so it can only pass through the motor, to have an air outlet 4 times the area of the inlet, and lastly to experiment to ensure this outlet is not in a high pressure zone on the airframe, otherwise the cooling air cannot escape. An angled 'gill' in front of the outlet may be necessary to make a low pressure zone behind it.

The art comes from appreciating that every installation is different. It is not uncommon for the air to actually travel forward through the prop nearest the spinner when ground running, this can be demonstrated with smoke tests (not the sort the OP did...) .

Full-size aircraft designers go to a lot of effort with baffles to make sure that the air if forced past the cooling fins and out of the cowl to ensure the heat the engine generates is reliably transferred into the airflow. Aircraft such as the Vought Corsair had very noticable adjustable gills on the back of the cowling to allow greater airflow out of the cowling at low speeds.

Its also worth bearing in mind that if you switch off your motor in the air, the speed of the cooling airflow is going to be the gliding speed of the model, do the same on the ground and its zero. This can be demonstrated by the fact that on a full-size installation, if the throttle is suddenly closed in the air, the cylinder head temperature will drop quickly (shock cooling), whereas on the ground it will rise for a while.

Going back to the model, unless it is very draggy, the power to maintain straight and level flight is minimal and the prop is only having to accelerate the air just enough to maintain level flight, whereas on the ground to achieve that same RPM, the propeller is accelerating the air from zero to that same speed.

[Roger Dyke]

Goodness. What a lot of replies. Brilliant, and I thank you for that.

Martin: Thank you for that useful cowl info.

leccyflyer: Regarding ground running. I have found an electric motor manufacturer that also states to do short ground runs only for it's motors. I have emailed them for an explanation. I fully realise that with this setup, the model is under powered, but due to battery space etc., it's the very best I can achieve. In the conversion I also had to add lead to the front end as the electrical setup was far lighter than the IC hence the overall weight. Of the two models that I ground ran, this one has had many flights on this setup, only with a 10x6 prop (not 10x7). The difference in current draw was 3 amps more with the 10x7. I agree that the model was a challenge to fly but fly it did. I'm not too sure that I will pursue this as it's 50:50 at the moment. A bigger motor and battery are out of the question I'm afraid, but if I can get it back flying I'll be happy.

Dick: Thank you for that really detailed information and explanation. Great stuff. It sort of says that all the manufacturers advertising information is not really worth a light if it's only fact providing certain caveats are met which are not stated in their advert. I can see that.

Pat: I thank you for your explanation. I do understand the electrical jargon as it has been my life and career. As you say, I think that the evidence will show once I open the motor up.

Robin: Thank you for your very detailed explanation of in-cowl cooling. All good stuff. I'm only too aware of shock cooling on full sized aircraft being an ex private pilot myself. Thanks again.

Roger

Edited By Roger Dyke on 06/05/2020 07:48:19

[Andy48]

Posted by Roger Dyke on 05/05/2020 21:04:30:You are so right about IC being much simpler. I am on a knife edge whether to convert it back or not.

Roger

I don't believe IC is any simpler, its just that you need different design requirements from the outset. When I build an electric plane, often from a traditional IC plan, I have to make significant changes to the fuselage. Weight distribution is very different, with a light motor in front and a heave battery further back. Also having room for a suitably sized battery is a key issue, as well as being able to easily remove it. Also as you've discovered airflow requirements are somewhat different.

Converting a plane from IC to electric has to consider all the above requirements and with a narrow fuselage may simply not be possible, as I think is the case of your model which will be very underpowered with the setup you've stated.

Think about converting your car to electric. It would need a very significant design change in much the same way.

[Nigel R]

"An angled 'gill' in front of the outlet may be necessary to make a low pressure zone behind it."

This would be my first port of call to sort the cooling, make sure the hot air is being 'sucked' out. Easy to fit something that will achieve this. Keeping the motor and the ESC both nicely cooled really is key to happy electric flight.

 

"Since I have come back into this hobby I have found that there is a lot of folklore"

Yes indeed.

I am in agreement with the assessment of 5lb and 3s3000 being a bit anemic, you might find this all a bit of a lost cause when it goes up in the air. If it was a 0.40 powered job you would want something of around 700 or 800W to recreate the same performance.

 

"I've worked with motors all my life and had them working flat our all day for months. Why are these any different?"

They're not. Keep them cooled, stay within in current limits, they're happy to run and run.

In fact, as Dick's setups will show, going way over current / power limits is perfectly possible with appropriate cooling and run time limiting, the key limiting factor being the final temperature at the end of that short sharp run. But that's specialist territory.

For a sport setup, the power dissipated by the motor needs to be extracted by cooling with motor temperature staying constant and under some maximum - just like any other electronic device.

Edited By Nigel R on 06/05/2020 09:17:19

[Roger Dyke]

Andy: I don't agree. I think that IC is much simpler to get to grips with. With the power plant there is all sorts of experimenting we can do which hardly ever results in the power plant being defunct. The plane may not fly well or might even have a few unscheduled arrivals if we get it wrong, but in my experience, I still have all my power plants from over 30 years and have never scrapped any except one that went in at a high rate of knots. I don't think the same could be said of most electric setups. We will have to beg to differ on that one. I totally agree with you that the design concept of an electric plane is considerably different to an IC one and conversion can be painful as I've found out to my cost. It was a good exercise to go through though as I've learnt so much. With hindsight I should have never started to convert this one but here I am. It was always going to be underpowered but thought with some concentrated effort I could get it to work. With an 8x6 prop on I have had many successful flights with it although maybe a little challenging at times. For purely nostalgic reasons (memory of a friend) from back into the early 90's I'd like to keep it going, although it will never be my main flyer.

Nigel: Thanks for you info regarding cowl cooling. I will look into that. Keeping the ESC cool is almost an impossibility due to the tightness of room. But immediately following the burn-out, I felt the ESC and it was slightly luke warm so didn't indicate a problem. The ESC has a large heatsink fitted to it. I am very aware about the guidelines of how many watts per pound etc., and I know what I have is very marginal, but it works. Not brilliantly, but it works. I would just like to get it back in the air again following this hiccup.

Roger

[Roger Dyke]

Hi All,

I have just stripped the motor and it looks like one of the 12 segments has burned (top). Strange really as I would have expected to see all of the segments look the same if it had started to overheat. Please see below.

Edited By Roger Dyke on 06/05/2020 10:41:19

[FlyinBrian]

Strange stuff lectric, it looks for weaknesses so it can let the magic smoke out, once the smoke is out its usually happy!

[Martin McIntosh]

I said it was probably that. From the pic. several of the windings look dodgy. This can happen even on expensive motors like AXI`s. At least with i/c you can simply replace bearings or a broken rod. Anything on a leccy which blows up is scrap.

[Roger Dyke]

Martin: Close inspection of the windings seem to indicate that they were wound that way (quick and cheap) as the lacquer and windings look undisturbed. Also it shows no signs at all of the other windings overheating. I am wondering if it was running nicely at full chat then suddenly one of the segments failed for some reason then took excessive current. And due to it having the full voltage still on it heated very quickly as it ran down to a standstill. If it had been gradually heating up I would have thought that all of the windings would have showed a uniform colour until one of the segments failed. Just a thought.

In the air, this model with this setup and a 10x6 prop has been doing 5 minute flights with the motor barely warm following the flight. This time it burned out at 2.5 minutes on the ground from cold. That seems rather quick to me. I realise that they are entirely different scenarios.

[Martin McIntosh]

Long ago I bought four `Donkey` motors from HK and right asses they turned out to be. The windings would hit the front of the motor casing resulting in exactly what you have.

[Nigel R]

"I think that IC is much simpler to get to grips with"

My take;

Electric is dead simple, providing, you use a pre-rolled powertrain, and don't change it. All it comes down to then is, charge battery, go fly. IC needs something of a skill set to be learned before you can fuel up your Wot4, start the brand new engine by yourself, tune it and then go fly.

TBH for any of us who want to roll our own, I don't know that there is much in it. IC takes more operational knowledge to run. Electric is biased toward understanding and using some maths to design and spec the right parts. Swings and roundabouts.

[Andy48]

Posted by Roger Dyke on 06/05/2020 10:34:31:

Andy: I don't agree. I think that IC is much simpler to get to grips with. With the power plant there is all sorts of experimenting we can do which hardly ever results in the power plant being defunct. The plane may not fly well or might even have a few unscheduled arrivals if we get it wrong, but in my experience, I still have all my power plants from over 30 years and have never scrapped any except one that went in at a high rate of knots. I don't think the same could be said of most electric setups.

Roger

OK, you've had a motor burn out. I never have. I've had one cheapo ESC give out the blue smoke, but otherwise all my kit is ultra reliable. There are more variables with electric motors, they are very much more forgiving on prop size provided you remain within the current limits. Once the electric combination is set up you can forget it and go fly.

As for the practical realities down at the field, I don't think I've ever been down to the field and not seen at least one IC flyer who has struggled to get their motor to fire up. Even the most experienced of them. As for the number of times I watch people walk out onto the strip and the engine cuts out. As an electric flyer, I have telemetry on my systems to tell me how much of the battery capacity I have left, plus if any battery cell is failing in flight. As a result, I have never had to do a dead stick landing since I took my "A" test. Perhaps I ought to practice that sometimes just in case!

[Roger Dyke]

Andy: I can well understand your bias towards electric flight. But for many of us old 'engineering' types, the stripping and refurbishing of IC engines, the smell, the sound, the manufacturing of various exhaust systems all added to the hobby. As well of course as the visit to the model shop for a load of balsa and a plan to make our next creation.

I left the hobby in 2004 as I took up flying full size flying to obtain my Private Pilots Licence. I returned to modelling in 2018 and managed to rejoin my old club again. I was gobsmacked as to how everything had changed. Most of the members of my club now buy off the shelf electric foamies and fly those. I must say, that I'm well impressed as they just walk out onto the strip, place their plane on the ground, throttle up, and away. The planes fly brilliantly too. But for me, there is a big lacking on the engineering and building side that I miss. As you said, messing with engines, crowding around a plane that won't start with everyone giving their opinion. It's what we did. I think nowadays, there are probably a lot more flyers than builders. Just my observation.

Everybody to their own I suppose.

[Andy48]

Roger. Exactly, its each to their own, it would be a horribly boring world if we all liked the same. - Imagine trying to find your car in a large car park full of identical cars! You are right though, I'm in a large club and one of the very few builders these days. It is really sad. Electric building is quite a learning curve if you are not used to it, and even today there are few plans or kits that are solely designed for electric, and it takes a bit of experience to know how to adapt the plan, especially weight wise for electric, but none the less a great sense of achievement when the maiden flight comes and little or no trim is required.

However, if you think electric is difficult, try getting to grips with a FrSky transmitter and OpenTX and all it can do. I'm toying with the idea of building a twin engine plane, and using the rpm telemetry from each motor to feed back to the transmitter so that the speed of both motors is always balanced.

[Martin McIntosh]

At least four of my models have been tried on i/c and electric. Similar power but the electric versions just do not seem to `groove` as well and have a mind of their own for some reason. They appear to wander off course, maybe due to the differing gyroscopic effects caused by a heavy item rotating at speed as opposed to a reciprocating motion.

Besides the engineering interest in i/c there is the extra adrenaline factor due to the chance of a motor cut if you are not good enough to tune the thing correctly and pay attention to cleanliness with fuel. I have many electric models but only because i/c would not be practical in most.

One of my most expensive crashes building time wise was with my Mosquito which suffered an esc failure on take off. Both motors were running but due to lack of sound I did not realise that one was way down on revs.

I also find that landing an electric is more difficult because it is all too easy to reduce the power too much, again due to lack of sound.

[Roger Dyke]

Andy: It was really strange when I re-joined the club back in 2018. The first couple of visits were during the week, and both times there was an older (different) guy already there when I arrived. Both of these guys had IC models so I never thought anything about it and felt quite at home. Then the next couple of times were at a weekend and there were rather more members there. The age group at the weekend were rather younger (some a lot younger) and nearly all were flying electric foamies apart from a couple of guys who were flying electric quads. No IC models in sight. They were also talking in jargon that I didn't understand. I suddenly realised that I was someone from a bygone era. I stuck around for a while and watched as they buzzed around the skies. Some of the flyers were really good but I got a definite impression that they were all flyers and no builders. It was not long after when I decided to convert two of my four IC models to electric so as I had a foot in each camp. Probably not the right thing to do with hindsight, but I have learnt so much along the journey and it has been good experience. I don't regret a thing. I think the biggest problem with the conversions was the C of G. The weight of the battery and motor against the weight of the engine and exhaust system were vastly different. Also the fuel tank compartment was nowhere near big enough to hold a suitable battery. They were very big problems, but I got there. They are not ideal and they don't fly to well, but they fly and the experience has been invaluable.

I don't find electrics difficult as I've spent most of my life submerged in it. It's model electric motors I find puzzling as no-one seems to specify the correct parameters for their motors by leaving out the caveats. I think that I will leave the over-complex transmitters alone as my DX6 does me very nicely. I do like the idea of a twin though with telemetry feedback to try to control the motor balance. As I spent a lifetime designing computerised test equipment it sounds just up my street. That's quite an interesting project. I wish you well.

[Roger Dyke]

Martin: I do agree about the adrenaline factor with IC models. When I flew my first electric plane the first thing that struck me was how to operate the throttle. I had always been used to hearing it and must have psychologically used the feedback from my ears to regulate the throttle. Now it was silent and frequently found myself either at full throttle or hardly any which led to a few precarious landings. Just something that we get used to I suppose.

I really like the engineering side of IC models when as you said, we never know if the motor is going to quit or there is power available when we need it. It all adds to the excitement. Then suddenly a deadstick. Oh dear. All good stuff. Happy days.

[Andy48]

Posted by Roger Dyke on 06/05/2020 22:43:45:

As I spent a lifetime designing computerised test equipment it sounds just up my street. That's quite an interesting project. I wish you well.

My first job was as a production engineer in an electronics factory making power supplies and test equipment, long before computers were anything but slow terminals linked to a distant mainframe. However, I was involved in computer control from the late 70s.

[Roger Dyke]

Andy: I remember the transition to computer and microprocessor control very well. I started off in Radio and Television as a t/v engineer then moved into electrical/electronic test in the mid 60's. It was quite a journey up until 2007 when I retired. Networks, mainframes, and minis, back in the early days were a complete nightmare, taking forever to get any information, then only to find out the information was probably corrupt. Of course, that's if the link or system hadn't crashed first. It's odd, as we accepted it back then as being the norm. History in the making I think.

I used to have a mate who was a design engineer for a power supply company. I think they used to make mainly switch-mode ones as I remember.

[Andy48]

I worked on the first commercially available switch mode power supply back in the 70s. Farnell.

[Roger Dyke]

Well I never.... I probably used lots of your power supplies as we had an account with Farnell. Used to very occasionally go to lunch with your rep too. Happy days.