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Old Sep 09, 2012, 10:04 PM
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Quote:
Originally Posted by psguardian View Post
HKs 'efficiency claims' are "max efficiency" which comes well before max Watts out... And looking at the heat Watts compared to drive Watts is crazy.
Yes, I think we now know why they don't actually show a graph of heat generated in the motor, right?

For the motor whose data I showed earlier, HK had one mathematically generated plot claiming 80% peak efficiency. The actual dyno data showed 75% efficiency. And the ad copy claimed the motor was good to 70 amps, at which point the dyno data showed the motor was more of an electric heater than an electric motor (less than 50% efficiency).

So, after looking at the dyno data for this supposedly high-quality Hobby King motor, I see no reason to change those rough estimates of 70% - 75% efficiency and about 3 watts of power handling per gram of motor weight that I've been using for some years now.

The good thing about these motors is they're cheap enough to just buy a bigger one if you really want more power. The bad thing is the bigger motor inevitably also comes with more weight, which your model has to carry around everywhere it goes.

As I've said, this is usually not a big problem with big slow models (they have lots of wing to carry the excess weight), or with warbirds (which are already heavy to start with, and often need excess nose-weight just to balance them). But if you want a model that's both light and very powerful, you discover that 75% peak efficiency and 3 watts per gram power handling is just not going to do the job for you. Then it's time to step up to a better quality motor, or give up the plan for light and powerful.

In IonSlo's case, the problem was that he wanted to carry a very heavy payload (2 lb floats). So he needed a light and powerful motor, and nothing from Hobby King looked like it was going to do the job.

-Flieslikeabeagle
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Old Sep 10, 2012, 08:57 AM
If I build it, it will fly
United States, NY, East Rochester
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Indeed! I see now that if you're going to run near 50% of the max amps listed then average/cheap outrunner (75%/70% eff) are likely to be close, but if you're going for max amps dirt cheap outrunner (60% eff) is likely to be generous in its est.

With this revised understanding of eff estimates, I've taken another look at the 1050kv motor I linked earlier. I really like that this motor comes with two prop mounts & a spare shaft. A reviewer listed a few sets of 'watts up v2.1' readings, (for my application 9x6TE is what it would be running) it showed to come in at 75-78% eff on that prop with a 3s... Even at 70% it would be enough to satisfy my bipe. The reduced motor weight & equipment I have in mind would produce an AUW of 23-24oz & the 1050kv @ 10.8v 13.8a (70% eff) yields 45.5mph | 26.1oz thrust & 16min flight time... Or 15.9a (75% eff) yields 48.8mph | 30.1oz thrust & 14min flight time.

I'm even thinking of scavenging my hk450 of it's servos & esc... It would save me a few more bucks, but then I'd like to just buy its crash parts.... Decisions decisions.

~psguardian
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Old Sep 10, 2012, 09:08 AM
If I build it, it will fly
United States, NY, East Rochester
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Side bar question:

Why does the Power To Weight Ratio use Watts In?
It would seem to me, due to the great variance of efficiency options out there, that this should be a reflection of Watts Out.

~psguardian
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Old Sep 10, 2012, 09:14 AM
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Quote:
Originally Posted by psguardian View Post
...Why does the Power To Weight Ratio use Watts In?...
Because that is how much power being used?
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Old Sep 10, 2012, 09:20 AM
If I build it, it will fly
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That's what I thought at first... But I hear a lot of people taking about needing 100w p/lb for their 3d stunt birds & it looks like they are talking about Watts at the prop, i.e. actual pulling power. Its like measuring horse power in a car, do you advertise HP at the crank or HP at the rear wheels where power meets pavement. Power used vs power applied to travel surface (be it air or pavement). Since the power absorbed by drivetrain inefficiencies doesn't ever reach the pavement it isn't counted amongst hot rodders, only RWHP (rear wheel horse power) is used for legitimate comparisons. Hence my thought that it should be power out.

~psguardian
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Old Sep 10, 2012, 11:07 AM
If I build it, it will fly
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I just checked drive calc & my 1050kv motor (turnigy 2217-16t) is in there!
EDIT: had wrong voltage entered in drive calc.....

Correct values @ 10.8v (10-12min flight on 2200mah)
1105kv
18.4a
34.9oz thrust
51.4mph
71.6%eff

I figure this power train will make me a happy camper over a few scratch builds. Maybe after I destroy the bipe (it is DTFB in the hands of a new flier after all) it can go in a sub 20oz edge or extra type bird.

~psguardian
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Old Sep 10, 2012, 02:26 PM
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Quote:
Originally Posted by psguardian View Post
Side bar question:

Why does the Power To Weight Ratio use Watts In?
It would seem to me, due to the great variance of efficiency options out there, that this should be a reflection of Watts Out.

~psguardian
Output watts is what WebOCalc uses internally in making its flight predictions. Of course that is what dictates the actual performance.

However, how many of us in this hobby have any way of actually measuring mechanical watts out? The only way to do it is to own or have access to a calibrated motor dyno. These are expensive, complex, and usually mostly DIY. Few of us have the money or engineering skills to put one together, or the time and knowledge to use one properly.

On the other hand, most RC pilots using electric power end up owning a wattmeter. It's relatively inexpensive and easy to use. It's also a usable proxy for the actual output power - good enough to get us in the rough ballpark, if we know something about the motor we're using. And it's enough to tell us that our battery, ESC, and motor are operating at safe power levels.

So when someone says their model has "120 watts per lb", they are virtually always talking about input power to the motor.

So WebOCalc displays the input watts, because it is something the user can easily check with a wattmeter. It also displays the (estimated) output watts, so the user can understand that in fact much of that input power never makes it to the propeller.

-Flieslikeabeagle
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Old Sep 10, 2012, 02:29 PM
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Quote:
Originally Posted by loNslo View Post
Because that is how much power being used?
Good point! We also need to know the input power in order to know how hard the batteries and ESC are working, and how long a flight might last.

That last item - how long a flight lasts on a charge - is the hardest one to get any kind of usably accurate handle on. There are far too many unpredictable variables involved, including the pilots mood!

-Flieslikeabeagle
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Old Sep 10, 2012, 02:45 PM
If I build it, it will fly
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I hadn't thought of it being a check for the Batt/esc... I am glad the rest of the results are based on the est Watts out though. I like this calc more every time you answer a question lol.

~psguardian
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Old Sep 11, 2012, 11:55 AM
Sure, I can fly after sunset!?
United States, MI, Novi
Joined Jan 2011
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Grades of motors

Sorry if this is obvious but, regarding the different grades of motors

70% Cheap outrunner
80% Good outrunner
90% Exceptional Inrunner

How do you tell which is which?
Where is the best (cheapest) place to find each?
How wide a selection at each level?
Is it worth it going better $$$?
What grade do most "experienced" fliers typically get?

I have a PS S.E.5a and a PZ Radian. As an example, where do their motors fit as far as grade?

Radian
http://www.parkzone.com/Products/Def...ProdID=PKZ4716

S.E.5a
http://www.parkzone.com/Products/Def...ProdID=PKZ4416
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Old Sep 11, 2012, 12:17 PM
That's a funny word
NE Ohio
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A couple of questions:

What would it take to get the GP SF props http://www.electrifly.com/miscproducts/gpmq6610.html put into the calc? Would you be willing to do it. I just got off the phone with Jack at Hobbico and he found out and told me that the max rpm is 165k/D. (same as masterairscrew) After we hung up I looked at some of my speed charts and thought perhaps He meant 65k , but went to windsors site and confirmed that 165k is the figure they use for the electrics.

I'm also wondering why the flyzone and ST model planes seem to use a kv motor that is much higher than the webcalc recommends? They in turn use a smaller prop and in turn seem to be at the top end of the rpm MAX. Maybe they just want more ground clearance and less torque for newbs?

Another question about the calc: Does the flight mission ONLY affect the speed and thrust recommendations?

Thanks, ps. Remeber the 1000 kv motor we talked about and overspeeding SF GP props? I not only find out they are 165k/D , but now the motor is listed as 800kv and not 1000 kv.
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Old Sep 11, 2012, 01:33 PM
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Antony (France)
Joined Sep 2003
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Hi
A lot of people is using Webocalc
Me too
However I am surprised that there are very few feedbacks regarding Scorpion Calc Wizard (post #368)

If you try it, don't forget to parameter a high ground clearance concerning the prop.
SC wizard is computing and limiting itself the right prop diameter to the optimum value.
Also you could setup in the preferences "no warning" to avoid numerous warnings, if you are editing fields (before all fields are correct and coherent)

SC wizard is giving you generic parameters, automatically
a) Motor Kv and weight (could be any brand, not mandatory Scorpion ref)
b) Suggested battery
c) Suggested prop

Regards
Louis
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Old Sep 11, 2012, 01:38 PM
That's a funny word
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Flylite

I decided it would be interest to do the Flylite and while the webcalc comes up with a <1000kv motor the actual plane uses a 1700kv motor , 10+ amps of current and one of those "famous" SF 8x6 props.
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Old Sep 11, 2012, 05:04 PM
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Quote:
Originally Posted by mybad View Post
regarding the different grades of motors

70% Cheap outrunner
80% Good outrunner
90% Exceptional Inrunner

How do you tell which is which?
Put it this way: MP-Jet, Hacker, Hyperion, Torque, and Scorpion motors will get you into the 80% class. If you find a motor of the right size and Kv and price from any of those brands, you can buy with confidence that you're getting something good.

Pretty much all the budget-priced China-sourced outrunners that are popular these days - and sized to fit your model - are in the 70% category (sometimes worse).

One of the big simplifications I made in WebOCalc was to use a crude efficiency estimate based on price, rather than requiring complex technical knowledge on the part of the user. It works well enough. So just check the 70% or 75% box and use a budget Hobby King motor, and it will get your trainer flying just fine!

To answer your question more technically, the easy way to know motor efficiency is to have the manufacturer provide the three motor constants, which you then plug into motor modelling software. Hacker, Scorpion, MP-Jet, and a few other good manufacturers do provide you with these motor constants.

Unfortunately, most manufacturers of budget-priced motors either don't provide these constants, or simply lie about them. With these motors, the only way to really tell for yourself is to depend on 3rd party measurements.

You (or someone you trust) must measure motor rpm, current draw, and voltage with a variety of different loads on the motor, and mathematically extract the motor constants from the measured data.

There is a surprisingly large amount of this sort of data out there, if you have the time or inclination to look for it. For example Drive Calculator (I provided a link a few posts ago) includes measured data for lots of numbers.
Quote:
Originally Posted by mybad View Post
Where is the best (cheapest) place to find each?
Hobby King (in Hong Kong) is the mecca for cheap motors.

For the rest, there is no definite answer. There are hundreds of vendors, sales and discounts come and go, and you never know if there will be a better price tomorrow. Life as usual as a consumer in America, in other words.
Quote:
Originally Posted by mybad View Post
How wide a selection at each level?
I'm trying to keep it simple for you: Hacker, Hyperion, Scorpion if you want quality. Everything else (hundreds of different brands) if you want lower quality but more affordable.
Quote:
Originally Posted by mybad View Post
Is it worth it going better $$$?
For a trainer that you're not particularly passionate about? In my opinion, no, it's not worth it. You can get it flying nicely with a budget-priced Hobby King motor.
Quote:
Originally Posted by mybad View Post
What grade do most "experienced" fliers typically get?
It really, really depends. I have a friend who only flies hotliners. These are like the Formula One race cars of the RC flying hobby. He only uses the most expensive motors on the market - Neumotors, Kontronik, Plettenberg, etc. His models go from 0 mph to 150 mph in maybe a second or so - and I'm not kidding!

Another group of experienced senior flyers in Arizona still use ancient, incredibly heavy, incredibly inefficient brushed motors. They like to fly huge, light, floaty "old-timer" models for hours at a time. The motor is used only to get up to altitude, after that they sit around in deck chairs and hunt for thermals.

In other words, it's not so much how experienced you are as a pilot, it has more to do with your preferences. What type of model? What type of flying?
Quote:
Originally Posted by mybad View Post
I have a PS S.E.5a and a PZ Radian. As an example, where do their motors fit as far as grade?

Radian
http://www.parkzone.com/Products/Def...ProdID=PKZ4716

S.E.5a
http://www.parkzone.com/Products/Def...ProdID=PKZ4416
Probably in the 60% - 70% category. Remember, those motors probably were bought in bulk in China for under $10 each, then re-sold to you with a 350% price mark-up.

You can find similar motors for maybe $25 - only a 150% mark-up - at Hobby King.

The Radian has tons of wing and is an easy to fly powered glider. It does not really need a light and efficient motor. Cheap fun at 70% efficiency is just fine for a Radian.

An S.E.5a was light, slow, and underpowered by today's standards, like all WW I aircraft. It would fly more scale if designed around a very light and efficient motor. However, as this model stands, you need the nose weight to get the CG, so a cheap and heavy motor is fine.

-Flieslikeabeagle
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Old Sep 11, 2012, 05:28 PM
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Quote:
Originally Posted by gulio View Post
What would it take to get the GP SF props http://www.electrifly.com/miscproducts/gpmq6610.html put into the calc?
The hard part is finding propeller constants for each prop in the line. Someone has to measure thrust and motor torque for each propeller at a variety of rpm levels, tabulate that data, and extract motor constants from it. It's a ton of work and requires expensive and specialised equipment.

And if you or I or someone else actually did do all that work, we'd still be faced with all the other prop brands out there: Graupner, Aeronaut, nameless Hobby King props, etc, etc.

Perhaps it is simpler to use WebOCalc to find a similar prop from the APC line, then swap on the same-size Great Planes prop and use a wattmeter to verify that you're within safe limits for the motor, battery, and ESC. Similar sized props of different brands and models usually have current draw within +/- 20% or so of each other, all else being the same.
Quote:
Originally Posted by gulio View Post
I'm also wondering why the flyzone and ST model planes seem to use a kv motor that is much higher than the webcalc recommends?
We can only speculate. Cost? Crudely made motors with big air-gaps and weak magnets end up with high Kv, forcing the small prop. Ignorance? Very few people in the glow and electric sides of this hobby seem to ever consider propeller efficiency.

But the people into rubber power and endurance flying (and solar-powered electric flight) know all about it - you'll never find a tiny inefficient prop on those models!

In the mid 1800's the first approximate propeller theories were developed (for ships, of course). Using that approach, it turns out that if you halve the diameter of a propeller, you have to turn it four times as fast, and reduce the propeller pitch by a factor of four, in order to put the same amount of power into it. And if you do all this, you end up with only 60% of the thrust you got from the twice-as-big propeller!

In other words, if you do everything right, you still lose 40% of the thrust-per-watt by halving the propeller diameter.

Usually it isn't done right; usually pitch speed inevitably becomes higher as well, making the situation even worse, and costing you even more thrust.

Nowadays there are more sophisticated (and much more complex) mathematical techniques for studying propellers, most of them developed in the 1980's for human-powered flight and refined very little since then. But the old 1865 theory got the essentials right, and it's probably "good enough" for our purposes - sport hobby flying using off-the-shelf propellers.
Quote:
Originally Posted by gulio View Post
Does the flight mission ONLY affect the speed and thrust recommendations?
As far as I remember, yes. "Flight Mission" is a much easier concept for most people to understand, and the wizard translates that into the necessary thrust and speed to achieve that particular level of performance.
Quote:
Originally Posted by gulio View Post
Remember the 1000 kv motor we talked about and overspeeding SF GP props? I not only find out they are 165k/D , but now the motor is listed as 800kv and not 1000 kv.
Check this out: http://www.apcprop.com/v/html/rpm_limits.html

APC's (fibreglass reinforced) Thin Electric props - which are really beefy - are only rated to 145,000/D. It makes me a little leery of apparently much more fragile propellers that supposedly have a higher rpm limit!

-Flieslikeabeagle
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Last edited by flieslikeabeagle; Sep 11, 2012 at 09:47 PM. Reason: Clean up the writing and add some new information
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