Are all watts created equal?

[Chris Freeman 3]

I asked this question on another forum a few years ago and never got a answer that made sense to me. In the early days of Giant scale a Quadra 32cc was rated as 1.2 hp which was equal to a good 40 size glow motor but it would use a 18 to 20 inch prop and fly aircraft up to 20 pounds in weight. My Rossi 105 was over 6hp which is more than a DLE 55 is rated today.

Now the actual question is if my Spitfire with a 5055 motor produces 1300 watts on 6 cells with a 14x6 prop and the other with a 5060 motor also produces 1300 watts on a 18x10 prop can I expect the same performance?

[Dickw]

The bigger prop will be turning a lot slower than the small prop. The performance will depend on how well a large slow turning prop or a smaller fast turning prop suits your model.

it's not so much how many watts, it is more a case of what you do with them.

Dick

Edited By Dickw on 14/02/2019 10:02:58

[Nigel R]

Yes, all Watts are equal. That is kind of the definition of a unit of measurement.

For our game, maximising performance on an airframe usually means obtaining the best thrust over a particular speed range (usually concentrating on the typical speed you want you plane to fly at) and that depends on picking the right prop for the job.

Will your two setups perform the same? Close, I would say, but not exactly the same.

[Dave Hess]

You need to make sure you're dealing with output power and not input power. When you say that the motor produces 1300w, where does that come from? Efficiency ranges from about 50% to 80% depending on the motor's speed, Kv, voltage, propeller and things like that, so if 1300w is consumed power (amps x volts), output power could be anything from 650w to 1040w. That's motor output power. Actual motive power will be less due to propeeler efficiency, which depends on things like type, quality, size, pitch, and rotation speed.

I would say that the best way to evaluate an electric motor is to measure its static thrust while measuring the voltage and current with a wattmeter. All you need is cheap luggage scales that cost about £2 and a wattmeter for £8. Those measurements will tell you the thrust per watt, which is a good measure of efficiency and power, which will tell you more about what your motor can do than any spec sheet or size chart.

[Don Fry]

Dick is right. Watt is a measure of work potential. Physics don't care if you use the work as noise, heat, or thust. Mick Reeves produces a gearbox for a Zenoah 38. Same motor, see the different aircraft it flys in naked and gearbox forms. Same watts in either case

[Mowerman]

The difference that matters is the rated KV ( rpm/volt) of the motors so for the same power the higher KV will produce higher rpm. with a finer pitch prop.

Edited By Mowerman on 14/02/2019 10:48:06

Edited By Mowerman on 14/02/2019 10:48:31

[Nigel R]

"I would say that the best way to evaluate an electric motor is to measure its static thrust "

Yes.

And no.

That sort of doesn't quite work for really high pitch or high speed props that are stalled on the ground at zero forward speed. Like EDF perhaps, or a pylon style power setup. I guess after that you're into ASI telemetry territory, or performance against a stopwatch. I'm digressing a bit, it's probably into specialist stuff at this stage.

[Chris Freeman 3]

Interesting comments, I have used a watt meter to see which prop gives the best reading as I am finding it hard to get the watts from the 5060 motor as it needs a bigger prop. I am not a fan of static thrust as a finer pitch prop will give you higher readings but limits the speed of the aircraft. More pitch can result in less static thrust like a motor boat with a high pitch can cavitate. I tend to be a try and see type of modeller so I look forward to see in practice what difference it makes. To compound this you also get some props of the same size but different makes that also suit an aircraft better than others.

[Peter Beeney]

My take on this would be that the output power of a motor is that created by the deflecting action of two magnetic fields, an electromagnet and a permanent magnet. This is mechanical power and to be accurately measured would have to be checked on a dynamometer; the amount of turning action in a given time by the prop shaft. The wattmeter measuring the electrical power from the battery is measuring the voltage and the current flow also in a given time. However, in so doing the current flow creates heat and in the case of model aeroplanes heat is something we wish to avoid. The wattmeter is measuring the heat generated; and that’s not always the power at the prop shaft.

In extremis these two measurements can be diametrically opposite.

I’m always a little cautious when comparing input and output power; so I invariably finish up by changing props until I find the best size to suite any particular model.

I’d consider that all watts are equal in the sense that they can easily be converted from one unit name to another, for example, HP to watts; and talking of horse power, when I’ve played around with this it soon seems to be a little bit of a remarkable power unit…

PB

[Dickw]

Posted by Chris Freeman 3 on 14/02/2019 11:32:25:

Interesting comments, I have used a watt meter to see which prop gives the best reading as I am finding it hard to get the watts from the 5060 motor as it needs a bigger prop..............

That sort of suggests you need a higher kv version, or more volts on the one you have.

Dick

[Former Member]

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[Nigel R]

Units of power makes no distinction as to torque and rpm, or current and volts, or work done and time.

Of course, and you're absolutely correct in saying so, the answer when talking about propellers, is more complete than "yes they are", to whit, rpm and torque are very important.

Also worth noting that lots of tractors do not actually have (or need) that much power. They do their work slowly, by appropriately gearing the engine to wheels + accessory drive. The work can be done quite slowly. Whereas a Galliardo needs enormous power to accelerate quickly, the mechanical energy must be imparted extremely fast to do this acceleration.

[Former Member]

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[Jon H]

As Dick points out right at the beginning the biggest factor to impact model performance is the propeller.

No matter what powerplant you use and how suited it is to your application, if the big fan at the front cannot turn the output power of that device into some sort of useful thrust then all bets are off and the actual theoretical power output is completely irrelevant. Many many props are extremely good at turning power into noise. Unfortunately that noise is your engine/motor power disappearing into the sky and not actually moving you along.

It is for this reason that Laser dont quote HP figures as they are not an accurate representation of the power you feel when flying the model. you can also create fantasy power figures by revving the snot out of it but this is not really representative of real world use.

[Former Member]

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[eflightray]

Cheat, use what others have used for the same model, or same type of model and motor.

Different people like to fly their planes differently. Some like high speed, (high pitch speed), some like slower slogging power, (higher thrust from a slower but bigger diameter prop).

My Spit - 72" span - 4030 385kv - 6s Lipo - has flown with 14x7, 15x8, 16x10 APCe, I prefer the 15x8 for general flying.

Ray.

Addition: That's from 600 watts to 850 watts, not a lot of difference in how it flies, just the way it 'feels'./

 

 

Edited By eflightray on 14/02/2019 15:01:24

[Jon H]

Posted by David Mellor on 14/02/2019 14:25:54:

Posted by Jon - Laser Engines on 14/02/2019 14:22:05:

As Dick points out right at the beginning the biggest factor to impact model performance is the propeller.

Perhaps true for IC, not so much for electric where the motor's kv and propeller are the determinants.

Nope, the problem is worse for electric as i crummy prop can put current draw through the roof. A friend is an almost exclusively electric flyer and we have discussed many times how the propeller is the governing factor. Even if you have everything else right the wrong prop will still kill the deal as it is the only thing that actually makes the model move.

[Former Member]

[This posting has been removed]

[Peter Beeney]

I have to say in general terms that I’ve never really found props to be a big issue. I’ve found that when propping electric motors the secret is to find the right size, too big and the motor is running too slowly, the current flow is increasing, as is the torque, but is unable to maintain the necessary revs. My short answer to me would be to find a more powerful motor; if I needed to actually use to that particular prop, that is.

I’ve also found that checking the revs in the first instance gives a general guide as to the performance; as with i/c. Comparing the prop revs with the unloaded revs will usually give me some idea as to what the current flow is going to be.

I would also agree that the kV is also one very important governing factor. The prop is alway going to turn slower than the unloaded revs per minute. So there is always going to a speed at which the prop, and consequently the aeroplane, can never exceed. Of course this can be altered by changing the applied voltage, more cells, but this will change everything else, too.

PB

[Former Member]

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[Peter Beeney]

Exactly so David, but my more regular rule of thumb is when I’m getting hot performance combined with expected battery capacity flight times I know I’m on the right track.

PB

[Simon Chaddock]

Another aspect that is often missed when comparing the performance of IC and electric is what happens to the engine/motor power output as the prop speeds up when in flight.

Brushless motors have a kV value which represents the theoretical maximum revs. At that speed the motor would draw no current and or deliver any power. Full power is achieve at an rpm below the kV figure (about 10%) but if the prop speeds up above that figure the power the motor produces fall away very rapidly.

Put on a bigger prop that pulls the full power static rpm down by 10% to allow for an in flight increase and there is a risk that the extra current drawn could overload the motor and/or the ESC.

IC on the other hand does not suffer in the same way, unless it is a very highly tuned unit. A 10% rpm rise will have only a modest effect on the power output, Indeed as an IC motor is not harmed if the prop pulls the full power static rpm to below the peak power then in this case as the plane's speed rises so does the power delivered!

Hence for the "best" performance with electric the prop choice is rather important, as is the use of a Watt meter!

[Former Member]

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[eflightray]

For an IC engine there is a limited range of props, for a brushless motor, there is a much greater range of props by changing the voltage.

e.g. my Dauntless - 4020-08 - 520kv - this motor quotes 5s to 7s Lipo 13x8 to 14x8 props.

But I use a 17x8 and use a 3s, the plane flies fine, on 350 watts.

You never know what may work until you try.

Ray.

[Chris Freeman 3]

Lots of good views and information here. Prop selection is vital, in the old days most of our aircraft were marginal on power so propellers were very important. You even used to see articles in the mags on how to modify the propellor shape to get those few extra rpms. The brushless motors and lipo's have resulted in our aircraft being very overpowered so prop selection is less important than what it was. I had a 1/5 scale Yellow aircraft Spitfire with a 30cc ST that would not loop from level flight with a Master airscrew glass 18x8 yet would easily loop from level flight with a Puntillo 18x8 wood prop.

My view is that a larger prop turning at lower RPM is more efficient than a smaller ones, that is why people such as Mick Reeves have made gearboxes. A watt meter is important to ensure that you are using the motor correctly as many of the motor specs are not a true indication of the actual requirement. The thinner air in Johannesburg also normally require a little more pitch on the propeller to get the same figures as that at sea level.