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Make your bets please Ladies and Gentleman

or where are my amps going?

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Bob Cotsford19/09/2018 16:53:38
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Here is the background - I bought a Ripmax Bolero off e-bay which came fitted with E-Flite Power 52 motor, 85A esc, DS821 servos - all looked to be good value and well put together. I did wonder about the 14*7 prop which seemed an odd size on a 590kv motor but forgot to ask what size pack had been used.

Receiver installed and servos set up I ran the setup through e-calc based on a 6S pack and came up with a 13*6.5 prop as being just the job. Said prop installed and meter plugged in to check the telemetry I enabled and opened the throttle. At 1540W and 70+ amps I chickened out. I checked my entries in e-calc again, as to date I've found it gives accurate results, it still said 13*6.5 = 1200W. OK, try again with 12*8, again the current goes daft.

Next step, no prop, two white patches on the can, tacho and current to check the kv which an online calculator gave as 508. ?? Try again, this time I'm getting 9000 rpm on the optical tacho and 0 rpm on telemetry??????

Any bets on whether it's the esc or motor I'll be subsituting these later on as I have an Propdrive 5050 580 and a couple of unmarked but chunky escs to try.

Peter Beeney19/09/2018 18:42:37
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Bob,

I’m not sure those kV figures quite add up for starters. Assuming you used a fully charged 6 cell pack for the test then I’d expect to see around 14,800 rpm at no load. Even at 3.7 volts per cell it should still surely be about 13k. What was the current flow that you measured?

I’ve checked a number of motors for their kV figure. Mostly they are about as per spec but they can be suspect. One I did recently was an completely unmarked can but the model’s write up said it was an 850. A test figure proved to be 600! The model appeared to be flying a bit slowly! But, of course, that doesn’t mean it will necessarily use excessive current.

I use a 12V car battery for a nice stable voltage that I can easily read with a voltmeter and if the tacho is accurate, which it is according to the mains frequency, then I reckon you can get fairly close to what it actually is.

I always check the unloaded kV first then the tacho / prop figures always gives me an instant indication of what is going on.

I wouldn’t bet on anything but I think I’d be looking again at the kV figures in the first instance…

The motor does appear to be a 1,650W jobby? Around 65 amps constant?

Good luck…

PB

PS   Bob, reading the motor reviews you may not be that far off the iron anyway. Stuff like 12 x 12 props and 1700 plus watts I ask you...

 

Edited By Peter Beeney on 19/09/2018 19:00:37

Edited By Peter Beeney on 19/09/2018 19:02:21

Bob Cotsford19/09/2018 19:23:30
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Peter, it came out as 2.8A no load, I've swapped it out for a Propdrive motor which has a higher current rating.- 90A. What gave me concern was that I was getting odd results with the Power 52, I think I'll resolder the connectors before I try it again. I'd be happy with just 1kW, the model only weighs 3kg, what does that come to in the real world, 6 1/2lb? 1500W+ would probably disassemble the model!

Peter Beeney19/09/2018 22:44:02
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Bob, 2.8A sounds a trifle on the high side, maybe ok for a 25 volt input; so I guess the question maybe be with the motor’s kV figure. If your 9,000 rpm result on 6S is correct I don’t think it can be a 590 unit. However, as you’ve changed it for a similar unit it’s perhaps not that pressing now anyway. Still well worth having a little fiddle about around with to find out exactly what is going on. though, on the face of it this one does seem to like a touch of loud pedal too, one gentleman runs it regularly at 1750 watts static, that’s a 6S pack, 12 x 12 prop and a 75 amp whack of current. I’d estimate that at 22.5 volts and 75 amps the revs are in the 12,000 + bracket; that’s a theoretical forward speed of real hurry up stuff. Perhaps more than 130 mph! No wonder the owner says that when his Sundowner 50 goes by it turns heads. Almost overpowered then, even for an example of a Formula 1 racing lookalike model!

Sounds like fun!

PB

Edited By Peter Beeney on 19/09/2018 22:48:40

Bob Cotsford19/09/2018 23:39:33
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Too much for me Peter, these days I like something that doesn't send my eyes into a spin. The Bolero is supposed to fly slowly, on that basis I've got a 13*4 on order to try out.

I actually use a 12*12 APC-E on my Curare, that's got a 500kv motor on 6S and is about as quick as I want now. I aught to check the logs too see how much current it's using in flight. I suspect around 50A for 1100W.

There's more to these electrics than meets the eye, you really do need a good selection of props to tune the system. With ic I knew what prop would work for any airframe and engine, I haven't got there yet with electrics.

Edited By Bob Cotsford on 19/09/2018 23:40:19

Nigel R20/09/2018 09:11:10
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Bob is that 6.5lbs including the lipo?

1500W sounds about right, either way, it is a 3d model - 150W/lb is low end power, 200W/lb is more like it. Most of its time will be spent at under or around half throttle.

13x4 also seems good, pitch needs to be kept low.

I'm probably not telling you anything new here!

BTW, Ripmax recommend a 5S lipo for that motor.

Bob Cotsford20/09/2018 10:45:59
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Nigel, the 5S motor that Ripmax recommend is iirc 700kv hence the need for a lower kv on 6S to get similar results. Running the figures through e-calc the Ripmax setup gives 1094W electrical which equates to 956W of mechanical ponies. What's that, a 90FS on real world props? My 5050 580kv on 6S and a 13*6.5 comes up as 1246/1123W, while in the real world on new Gensace 4000mAh 6S it's drawing 1400ish watts for 66A which is as high as I'd want to go on XT60 connectors.

The Bolero is a bit of a chunky monkey for a 3D model but that does mean it's not as fragile as the true 3D airframes. I see it as a big funfly as that is about the measure of my ability.

Biggles' Elder Brother - Moderator20/09/2018 17:15:31
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Firstly this isn't meant as a criticism of anyone, its just a thought this thread triggered and a view that has been forming in my mind for some time - so I'm going to get it off my chest!

6.5lb with 1500W! That's 230W/lb - bonkers territory!!

The model may be a 3D model but:

a) I don't think, from what I have read, that Bob is looking for that.

b) 150-170W/lb is plenty for 3D. Approaching 200W/lb well maybe, but 230 is far too much in my view!

I have been thinking about this for a while now. As electric-power flyers we came from the "NiMh, just about enough power to stay airborne for 4 mins" tradition. Now we can have more power we almost seem to have gone power mad!! I actually believe we are overpowering most models now - just because we can. And yes, I'm as guilty as the next man!

Today people view 100W/lb as "puny - very low power" The reality is for most aircraft 100W/lb gives a good general all round performance. OK verticals might not be unlimited, but they will be as big as most folks need them to be! !20-150W/lb and again a lot of us are sniffing at that now! But the truth is that's very lively performance. Scale warbird for sure!

Danny Fenton I believe is of the view that lower power to weight ratios often lead to much smoother and scale-like flying in scale models. And just to show what might be possible, at the extreme, look at David Mellor's experiments flying on 10W/lb!

Perhaps we could save ourselves some money, weight and space (as well as gaining in endurance) if we weren't so uncompromisingly power hungry like we have "something to prove"?

BEB

Edited By Biggles' Elder Brother - Moderator on 20/09/2018 17:16:56

Nigel R20/09/2018 17:53:10
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BEB, for what its worth:

My 3d foamie has about 230W fresh and about 200W at end of flight. Weighs 1lb. Power level is perfect. Not bonkers territory for 3d at all.

My hack speed model has around 550W and comes in at 2 3/4lb. Just about perfect again. Enough beans to get it moving. I should very think so. Vertical? Yes, lots. In a ballistic style.

A smooth pattern aerobat, ~5lb, 800W - but just about enough to motor through a constant speed (well, within my best abilities, anyway) vertical eight. With a lower power level it would need speed to be built up before going for tall maneuvers and I wouldn't be able to try and fly a constant speed. Incidentally, flight times is 12 minutes on this - I am able most of the time to fly with a low throttle.

On the flip side, I also fly several lightly loaded trainers and an aerobatic bipe, all with ~100W/lb. Vertical is not endless. It is good enough to pull a decent stall turn, but giant loops they will not. Spritely, yes. Overpowered, very far from. The drag is too high on both of them.

Desired speed and other flight performance is a big factor in motor choice.

YMMV.

PS - they're my models and if I want to stuff a big motor in, I will cheeky

 

Edited By Nigel R on 20/09/2018 17:58:39

Bob Cotsford20/09/2018 18:45:00
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We have a couple of members in the club that I belong to who own 'nifty' models. By this I mean models that I have trouble following visually. They tend to be fairly small 3-4S models. The owners also tend to have bigger, more civilised models. I don't think any of us worry too much about W/lb as long as the models do what we want. Generally speaking, I'm with BEB on power levels aiming with my models to get 60-150W depending on the model. My ancient 1 1/2 Strutter is probably around the 40W/lb level. My Curare is more the 140-150 sort of level. Horses for courses. I would worry about structural integrity above these sort of power levels, I still remember playing with motors on my MiniPanic and the nose completely disintegrating.

The Bolero instructions explicitly warn against overpowering/overspeeding the airframe. I'd have compared it to certain brick built outhouses but there you go. I'll be satisfied with around 1kW but I think the thick wing will shrug off a moderate excess such as the extra 300W or so I'm seeing, especially in view of the solid fuselage construction.

Now, if the weather would just play ball I could find out. Instead I'm spending days putting batteries back to storage voltage sad

Edited By Bob Cotsford on 20/09/2018 18:47:03

Biggles' Elder Brother - Moderator20/09/2018 20:32:38
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Whoa Nigel, I meant no slight to you or anyone. Of course they are your models mate and you're free to do what you want with them! I did more or less say that right at the opening.

All I really wanted to do was see if we could have a sensible debate about the current situation (no pun intended!) regarding power levels. I feel there two thing we could think on:

1. I feel, just my opinion, that "standard expectations" for power are creeping up under the influence of us all. So a few years ago 100W/lb was seen as respectable, now some folks are openly saying things like "an average model needs 150W/lb" - and I'm not sure it does! And more,...how long before 180W/lb becomes "the norm" and then 200W/lb and so on.

2. Now you might think "so what? if more power is the norm what difference does that make?" Well I think it's potentially missing a trick - in reality we all know that electric power is balancing act, we are trading: power, weight and endurance. But if we become too wrapped up in one of those three maybe we miss the chance to advance in other areas? So we go all out to squeeze 1.5kW out of the model, but it's duration is 6 mins? If we backed off just a little - maybe to 1.2kW we might get 8 mins from the same set up? All I'm saying is surely this is worth discussing? 1.2kW would still be lots of power for that model - so why not have the power and the endurance. Also, once the verticals are unlimited - can they really be even more "unlimited"!?

In short - if you want to to be "ballistic" and enjoy that - that's fine. But we should be careful I think of drifting into making that the normal expectation.

As I say, no challenge intended - just an invitation to discuss. But maybe its better on another thread of its own while we ponder Bob's missing amps!

BEB

PS I have no idea what YMMV means!

 

Edited By Biggles' Elder Brother - Moderator on 20/09/2018 20:33:09

Mike Blandford20/09/2018 20:44:37
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YMMV - Your Mileage Might Vary! Almost what you are saying anyway!

Mike

Nigel R20/09/2018 23:15:25
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BEB - No offence taken I can assure you!
Nigel R21/09/2018 08:57:54
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Bob

re: the Bolero instructions, and not overpowering the model, I reckon the concern there is the enormous control surfaces, excess speed, and flutter. Rather than general structural strength.

BEB, bit more time to type now...

Re, point 1, I don't disagree, absolutely the opposite. In times of yore, we'd have fitted a 40 into a 5lb (dry) aerobatic sport model and that would be "about right". For a hot model, it would be a piped 45. In my (approximate) idea of equivalence, an average 40 tuned for average sort of revs would put out about 600W at the prop, making it the equivalent of a 700W or 750W electric system. Bearing in mind the 1/2lb of fuel load going up with it, that's (700/5.5) around the 120W/lb mark.

If I was flying an average kind of 40 trainer or more laid back sport model, that would more likely be a 40FP or something, slightly lower power, and ending up about 100W/lb.

So yes, do we need more than 100W/lb? Depends on the model, but, certainly we never had much above that. A lightly build aerobatic model could push that a bit higher (I built a fun fly model at 3 1/2 lb including a piped Irvine 46).

re point 2, yes, it is a balancing act, like everything involved with engineering a flying machine. I would say the answer here generally lies more in the pilot's use of the loud pedal than the maximum output. Going electric allows us to have our cake and eat it - if you get it right and you fly it right - you can have a light power train that has the capability of a high output (even if constant use of that high output burns the lipo down in 5 minutes). If the model has enough power at part throttle, then with discipline on the throttle you can fly it at part throttle and have the flight times, too. Perhaps we're not using our power wisely if we end up with short flights.

But there's a flip side to that - my opinion is that most electric power systems I see, particularly "manufacturer recommended" types seem to be woefully underspecced in terms of lipo size. I assume this is done on the basis of weight - selling the benefits of a nice light electric model ("lighter flies better" - which is true of course). But a different answer (and this is what I shoot for) is to simply accept some extra lipo weight to get the duration up.

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