Prop advice please.....

[Peter Beeney]

Many thanks in turn for your reply Roger, much appreciated. And at the same time I’d also tend to think that your ability to be able to consistently fly a somewhat underpowered and at the same time rather portly model regularly and safely must be a honest and lasting tribute to your model flying skills too…

Being naturally a bit lazy I’ve always picked up the tacho first, at least I can very nearly understand this particular device! When tinkering with a new electric motor I alway check the unloaded revs first, then bolt on a likely looking prop and check again; then in a trice it’s possible to be able to predict to within a reasonable degree of accuracy as to how it’s going to perform and how close (or not!) I am to arriving at the ideal prop size. Of course an added look at the current flow with a clip on power meter and a temperature check for local hot spots with a contact thermometer is never a waste of time either and makes it all a little more interesting anyway; also it might help to find out how wrong, or perhaps not quite so wrong, my guesstimates were in the first instance. Although a slightly more difficult decision to make before you even begin this caper though, is maybe choosing what size of propeller to use for a particular model and also how fast it has to turn to get the result you want…

Anyway, all best wishes for a future long and unbroken life for your model…

PB

[perttime]

How much does the current 3S pack weigh? Do you need the weight to balance?

Perhaps there's a 3S pack that fits and is lower capacity, lighter weight, higher current rating. The lighter weight might improve your power to weight ratio slightly.

[Trevor Crook]

Roger, there is one more thing you could try. In my experience, bigger props at a lower rpm are more effecient than smaller props at a higher rpm, so if you installed your lower kV motor you could try an 11" or 12" prop, with the pitch chosen to give the appropriate current. This depends, of course, on the ground clearance available.

[Roger Dyke]

Hi Peter,

Thank you for your very kind words regarding my flying abilities. That was very generous of you, but I'm not that good really. Thank you also for your very detailed explanation of how you go about motor and prop selections for your own models which I found interesting. I intend to keep this particular model forever. Even it it has an unscheduled arrival which results in it being grounded as it holds a lot of sentimental value. I also have another one (also converted) which holds a lot of sentimental value which I built in 1973 (Mercury Galahad). It has been repaired so many times but still flies. All my other models don't really hold the same sentimental value, so if they went in big time, they would probably reluctantly be replaced.

With the one in question, my governing factor was the battery and had to work back from there. The biggest capacity battery I could get that would fit in the bay in all directions was a 3S 3000mA graphene type. Then in order to have any reasonable flying time (5 minutes) I had to calculate the current draw that I could afford (about 30A). This then set the scene for the motor and if I couldn't get one that was powerful enough to pull it around the sky the show was over. Anyway, I found a 3536 1050kv that at 30A would turn a 9x6 prop at 10900 rpm with a static torque of 2.9 lb (my own static tests). Not ideal but should work. I did test a few more motors too but found this suited my requirements rather better. So, in this shortened version of the conversion path, this is where I have ended up. I am now reasonably happy.

Roger

[Roger Dyke]

Perttime,

Thank for your reply. My battery pack weighs 214 grams. There is also a lot of lead ballast in the nose of the model as the motor/battery combo weighs a lot less than the old engine/exhaust/fuel tank (which also had some lead ballast).

I'm not sure what you mean by a battery with a lower capacity and a higher current rating. The power from a battery is the same no-matter how we derive it (volts x amps). Higher voltage with lower amps or lower voltage with higher amps. Size wise and power wise they're the same.

A lower capacity battery would give me less flight time, which at the moment is about 5 minutes (leaving about 20-30% of capacity left).

Roger

[Roger Dyke]

Hi Trevor,

I was considering doing that as I do have a 3542 800kv motor and which pulls about 30 amps with a 11x6 prop at 8500 rpm (my own tests). However when playing around with eCalc, it didn't show any improvement to performance to what I currently have. In fact it was a little worse so decided not to bother. maybe I might have another look on my static test set-up though to confirm my findings.

Many thanks.

Roger

Edited By Roger Dyke on 04/07/2020 10:39:45

[perttime]

Apparently, you need the weight of the battery for balancing too, so a lighter battery wouldn't help.

You say that you are taking about 33 A from the battery. You can get the same 33 A, and more, from a smaller 3S pack. 30C packs are everywhere, now, and I've seen people mention 100C. Clearly, flight time becomes a limiting factor with a lower capacity pack, too.

[Roger Dyke]

Yes, you are correct. The weight of my battery is included in the amount of weight I need in the nose to balance the plane.

Yes, I can certainly draw more current form my battery or even from smaller capacity batteries, but drawing around 30 amps or so from my 3000 mA battery is my maximum limit that I can draw without impeding my flying time of around 5 minutes.

[Peter Beeney]

Yes indeed, you do have a little predicament, Roger, not really many easy answers here I guess. Changing from i/c to electric is not always that simple by the look of it.

Just a fag packet type of speculation but if you were starting with a bit more leeway here 2 x 2Ah 4S packs in parallel one behind the other in the fuz might have been an option. That in turn might now make it possible to find the right CoG without any additional weight so it might not be that much heaver anyway. Selecting a prop, to me something like a 10 x 7 looks like a likely first candidate for a fictitious 50in span speedster weighing in at something well under 5lb,(hopefully), that should be turning at least in the order of 10,00rpm. Such that a 40 i/c might do. But 11k would be a bit better, though. Such that a OS MAX-35AX might do. The extra watt minutes we have will allow us to bump up the current to around 35 amps and still have just about 5 mins duration down to 30% capacity. Using nominal figures, an 850 kV motor will spin unloaded at 12580rpm, ideally our prop needs to revolve at about 11000rpm to be within our preset limits, slightly faster would be better. So an 850kV 800 watt motor might be choice. 800 watts still gives us a bit of headroom on the max current flow with fully charged batteries.

First setback here maybe a bit of a lack of 850kV’s but a 900 pops up here straightaway which looks to be just about perfect all round for our fag packet…sorry, model. Job done…

But if it were only that simple…….

With the greatest possible respect all round Roger, I have to say I’m a bit suspicious of the 11/6 at 8500rpm though. As usual I may be completely wrong but that seems to me to be now lurking marginally beyond the borderline of a rather marginal place to be in the first instance!

Take care.

PB

[Trevor Crook]

Roger, I must admit I've never used ecalc, I tend to use a combination of experience, advice and trial and error!

A few years ago I wanted a bit more urge from a 3s model that was pulling about 40A, so fitted a 4s battery and came down in prop size until I was back to the 40A level. This involved going down from a 12x6 to a 10x6. The model (a Galaxy Magician) made more noise but didn't fly any better, despite absorbing 30% more power. I reverted to 3s and 12x6 and tried different makes of prop, and got an improvement from a different brand.

[Roger Dyke]

Hi Peter,

It is what it is I'm afraid. The battery compartment is maxed out in width, height and length and I have trawled the websites for any shape and combination of battery that might fit. So that was my starting point. The fuselage is quite narrow and the distance from the firewall to the wing bay former is what it is. I fully understand what you suggest, but if I could get a bigger battery in the bay in any combination at all it would open up all sorts of new possibilities. But I can't I'm afraid. So there we are.

Just for interest. Regarding the 11x6 prop. On my static rig and with the 3542 800kv motor fitted, I tried a few different prop sizes with a 3S battery.

The results were:-

10x7 - 8300, 11x6 - 8500, 11x7 - 8300, 11x8 - 8100. The currents recorded between 28 and 33 amps.

Roger

[Roger Dyke]

Hi Trevor,

Thanks for the useful info. and the benefit of your experience. I too tend to do my own experimenting, but as I have the iCalc tool I thought that I would give it a go. I must say that very often it doesn't come up with the same answers that I get. Still it's useful to do comparisons.

Regarding changing props. Unfortunately, I have already been through the prop experiment with all the thereabout sizes and makes. I've even tried the half sizes, but now think that I've found the optimum in the 9x6 thin APC. Some of the others come close. But none of them any better.

Roger

Edited By Roger Dyke on 04/07/2020 16:45:15