Are all watts created equal?

[Jon H]

Posted by David Mellor on 14/02/2019 17:07:33:

Jon, we are saying the same thing in different ways.

For electric system, it is clearly true that the wrong prop can be bad.

But to match the prop properly you have to take the kv into account as well. A 600 Watt 750 kv motor will require a very different prop to a 600 Watt 3000 kv motor And that is the whole point of the OP's question - knowledge of the power capability (Watts) alone is insufficient to select a propeller - you must have kv and voltage (cell number) as well. In that sense, "all Watts are not equal".

Edited By David Mellor on 14/02/2019 17:26:54

I agree completely that in your example watts are not equal. Its the same as a 2.5hp two stroke turning a 13x6 or something and a 2.5hp 4 stroke swinging a 17x8 or whatever. power is the same, performance and prop selection very much not. Im with you on that, its just that i was trying to point out that watts do not mean power from the point of view of how the model feels to fly. Chris's Spitfire example is exactly what i am talking about and i bet that he saw lower engine rpm (and thus calculated power) on the better prop and yet had better performance.

If you slap an inefficient prop on a motor and shove current draw into space, the big fat watts number on the watt meter looks great but your performance in the air would be horrible, to say nothing of your flight duration. Its the same for ic with engine temperature rising considerably when overloaded.

So anyway i think we are on the same page more or less, just looking at it from different angles

[Former Member]

[This posting has been removed]

[Nigel R]

In air we probably have a speed range that "feels right". Most of my sport models are typical "club sport", but, I like a bit of speed (not masses), but I have noticed my prop and motor choices tend to be a touch smaller props and a touch higher revs than most suggestions. Works for me. I have tried larger and lower pitch but it always feels to me like driving in second gear!

Our job is to optimise the prop for that range, given the motor. Really we are trying to maximise efficiency across that speed range, so we get best use of our Watts in terms of aircraft acceleration per unit power.

I think we're all saying the same stuff as mentioned from different angles.

 

"what happens to the engine/motor power output as the prop speeds up when in flight"

That's really worth repeating.

Power from electric drops rapidly as the revs rise, which ultimately means it won't "wind up" as much as an IC model would, Flying IC just as a different feel as you can build up speed with a big dive, and the nature of the motor means you can keep it moving (if you fly smoothly) once you are going faster. It can be a lot of fun to keep things lively like this. Electrics feel like they bleed off that speed quicker as the motor just doesn't produce as much until it is loaded up when the speed drops back again. It could even conceivable work against you if you built up too much speed. On the flip side, if you pull into a slow climb, the electric just keeps on trucking.

Horses for courses!

Edited By Nigel R on 15/02/2019 09:48:58

[Former Member]

[This posting has been removed]

[Former Member]

[This posting has been removed]

[Nigel R]

"rule of thumb is that each of these motors will therefore supply about 3 Watts to turn the prop for every gram of weight in the motor"

I thought that rule of thumb was more to do with guessing the maximum throughput of the motor, as determined by it's ability to shed heat, heat produced by current flowing through the resistance of its windings.

Which makes no account of the resistance itself, nor the cooling of the motor, nor the exact loading, nor your own use of the throttle.

Or in other words, it probably isn't a very good rule of thumb.

[Jon H]

Nigel

Im a fan of big props i have to say. I love the fact that when i pull the nose up a bit it just keeps going as that big prop takes large bites out of the air. That said, i fly all 4 strokes and many are long stroke meaning they wouldnt rev that well even if i wanted them too. I also like the feeling of instant power i get from a big prop as even a small increase in RPM makes a huge difference to thrust. Downline braking is another factor as it helps keep the speed of my warbirds under control when coming out of a loop or whatever.

Having said all that i flew a model that belonged to a friend and if it were mine i would have dropped an inch on dia to get the revs up. It was a quick ish sport/aerobatic model and i would have liked a bit more sparkle from it. IT didnt fly badly, it was actually very nice, but i would have tried the smaller prop if it were mine.

David

Funny you mention current draw. I fly mostly WWII scale and yet my prop selection and flying style grant me pretty low fuel consumption. Most are surprised when i tell them that my Sea Fury has a 60cc glow engine fitted. They comment that i must 'only get 3 flights a gallon' when the real figure is probably 6 or 8.

Flying style is super important when it comes to fuel consumption, irrespective of whether you fuel is measured in ml or Mah

[Former Member]

[This posting has been removed]

[Nigel R]

Jon, yes "bigger" props have more braking effect and are inherently more likely to be a lower pitch for a given IC, which suggests more pull at lower speed vs 'getting going' at speed. As you say, suits the model and the flying style. I wouldn't say my sport model choices would be the best fit for scale or slow fly. And a step further away from 3d.

[Jon H]

Posted by Nigel R on 15/02/2019 11:41:34:

Jon, yes "bigger" props have more braking effect and are inherently more likely to be a lower pitch for a given IC, which suggests more pull at lower speed vs 'getting going' at speed. As you say, suits the model and the flying style. I wouldn't say my sport model choices would be the best fit for scale or slow fly. And a step further away from 3d.

Your pitch comment is interesting as i usually fly with about 8 inches on most of my models. My slower biplanes have 5 or 6 inches, sport 7-8 inch and warbirds are running 8-10. Although this conversation is a bit off topic im just curious what you define as a lower pitch

[Dickw]

Posted by David Mellor on 15/02/2019 10:56:43:

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

A commonly encountered (and in my view altogether useless) rule of thumb is that each of these motors will therefore supply about 3 Watts to turn the prop for every gram of weight in the motor - about 306 Watts in this example.

But..... will they????

I think you have miss-understood that “rule of thumb”. I always understood it to mean a motor could safely run at a power level of 3 watts per gram continuously – not that it would produce that power. It just gives you a rough starting point for the size of motor you need for your application. After that you start looking at kv etc.

In your examples of how much max power those motors can produce it doesn’t say for how long.

If you run the “D3536/5 - 1450 kv: max current 45A, max power 655 Watts, ” on 5s, as you suggest is possible, that equates to ~ 830 watts at max amps – will it be OK at that? I know it will for short periods, but how long is a short period?

Dick

[Former Member]

[This posting has been removed]

[Dickw]

Posted by David Mellor on 15/02/2019 12:45:36:

Hi Dick

I haven't misunderstood the rule of thumb. Sorry if I gave that impression.

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

..... There are some truly astonishing accounts of prodigiously huge amounts of battery power being sucked into motors with huge square (pitch+diameter) props in short bursts to achieve phenomenal acceleration in F5B models, but after a very few tens of seconds the motor is shut off - presumably before it melts the insulation on the windings!

And you are right again - I wouldn't want to run a D3536/5 at 900 Watts for very long, though in fairness to me I haven't burned one out yet......

Dave

Edited By David Mellor on 15/02/2019 12:56:55

Fair comment David, and I do agree that a lot of people do miss-use that rule of thumb, so your warning was reasonable.

Regarding F5B - I do fly those and the motor runs are generally a max of about 3 seconds. We run them at a specific point on the motor performance curve utilising the power while the model is still accelerating and so the motor is under max load. If we left the motor on longer and allow the model to reach higher speeds than we need the curent would drop considerably as the motor unwound (accidently tried it!). We are after max acceleration not max speed.

Dick

[Former Member]

[This posting has been removed]

[Nigel R]

Jon a low pitch to me is something where pitch is under half the diameter. Say a 14x6. For a sport model I'd probably look at 13x8 or 12x10 to go with. Although 12x10 is getting quite high in pitch. And it also depends on revs as to what the pitch speed is.

bigger - for an ic - is simply something that would keep revs down. Which four strokes are quite good at working with. I'm guessing most of your airframes have a particular brand fitted..?

Edited By Nigel R on 16/02/2019 07:07:31

Edited By Nigel R on 16/02/2019 07:07:55