Espritmodel.com Telemetry Radio
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Old Sep 08, 2005, 12:22 AM
Got shenpa?
flieslikeabeagle's Avatar
Los Angeles
Joined May 2004
10,940 Posts
Quote:
Originally Posted by koko76
Weather there are 6kw's of power spinning a prop, a 2x4, a couple of rodents tied to sticks what have you, the belt still is undergoing 6kw of power transfer.
Certainly. The problem is that having the gear ratio wrong can change that 6 kW to 1 kW (which would be healthy for the belt, but bad for Pink Floyd's flying potential), or to 12 kW (which would be bad for both the belt and the health of the motors).

With the wild discrepancies between previous estimates power estimates, I was concerned. Now that Martyn's and my numbers have largely converged, I am a lot happier, and I now agree with you that, at a minimum, Ultra Pink should indeed fly once you get the gearbox to handle 6 kW. In fact you may only need the gearbox to handle 3.5 kW, if Martyn's calculations are spot-on...
Quote:
Originally Posted by koko76
I simply think our philosophies on how and what to measure differ a little here.
<snip>
To fix the problem at hand I feel quite confident that testing the box with power levels approximating the levels that the motors are rated for should be *good enough* for this app. And I don't belive that I need a prop to do it with.
I agree on the philosophical differences.

As I said earlier, if you get the gearbox to hold up under 6 kW, that will certainly ensure it holds together when installed in Ultra Pink. However that by itself does not ensure you will actually have anything close to 6 kW of power actually available, unless the gear ratio is optimised for those motors, that battery, and that prop.

I have a bit of a hard time seeing a 6 kW setup reduced to putting out "only" 3 kW due to improper gear ratio, due to my compulsion to optimise power trains in my own model planes, but I have to agree with you that 3 kW will almost certainly be enough to fly Han's biggest foamie. Yet.

Okay, I'll go look in a mirror and remind myself not to to take my own viewpoints on anything (power plants for RC included) too seriously.

-Flieslikeabeagle
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Old Sep 08, 2005, 12:53 AM
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koko76's Avatar
Joined Jul 2005
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No offense taken by any of that
A bunch of hard lessons from robots, even though the numbers for what you have look freakin awesome, it's always the one you didn't calculate that turns out to be your limiting factor.
You are of course correct that knowing the prop constant will help in determining the load I need to test for and it can of course be affected by the gear ratio. I guess I just figured that knowing what the motors could put out, if the prop were to draw that many times more than that I'd rather see the belt shred than the $750 in motors, not to mention the controllers and batteries.
I'm certainly not always correct, I just make some pretty good guesses
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Old Sep 08, 2005, 01:50 AM
Got shenpa?
flieslikeabeagle's Avatar
Los Angeles
Joined May 2004
10,940 Posts
Quote:
Originally Posted by koko76
I'm certainly not always correct, I just make some pretty good guesses
That's already a lot better than I usually do. Don't ask me about the time I guessed how much aileron throw I needed for the maiden flight of my World Models Rambler EP!

-Flieslikeabeagle
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Old Sep 08, 2005, 06:59 AM
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Neil Stainton's Avatar
United Kingdom, England, Stratford-upon-Avon
Joined Feb 2001
3,887 Posts
My previous post was with the wrong 5330/18 motor. Changing the MotorCalc setup to use the AXI 5330/24 motor it gives the following results for a 5:1 gear ratio:

Motor Temp Calculations Off:
279A, 34V, 9.5 Kw in, 72% efficiency, 144lb thrust at 38 mph

Motor Temp Calculations On:
169A, 35V, 5.9 Kw in, 58% efficiency, 86lb thrust at 30 mph

From these it still looks as though the prop is too big or the gear ratio too low. I agree with Flieslikeabeagle the benefit of testing the setup with the actual prop isn't just to ascertain the gearbox will hold up, it will also allow Hans to check the whole powertrain before the next flight attempt.

Regards,

Neil.
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Old Sep 08, 2005, 07:57 AM
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Neil Stainton's Avatar
United Kingdom, England, Stratford-upon-Avon
Joined Feb 2001
3,887 Posts
Quote:
Originally Posted by Martyn McKinney
If 3 motors are used with Motocalc, the Kv should remain the same, the Rm should be divided by 3 and Io should be multiplied by 3.

The 5330/24 motor, 5 to 1 gear ratio and a prop K of 1.0 gives the following: Approximately 40A per motor at full throttle.
MotoCalc has different motor constants to the one you used. Adjusting them as above, and feeding them into your spreadsheet with a 1mOhm (for 12P cells) battery resistance gives a current of 155A (52A per motor) and 89% efficiency. Still very different to the MotoCalc results.

Regards,

Neil.
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Old Sep 08, 2005, 10:29 AM
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Toronto Canada
Joined Dec 2002
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At the risk of asking a silly question, what motor constants are being used in Motocalc ?

Is Rm also being divided by 3 and Io multiplied by 3 ?
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Old Sep 08, 2005, 11:39 AM
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Neil Stainton's Avatar
United Kingdom, England, Stratford-upon-Avon
Joined Feb 2001
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Quote:
Originally Posted by Martyn McKinney
At the risk of asking a silly question, what motor constants are being used in Motocalc ?
The constants are:

Kv = 197 RPM/V
Io = 1.5 A
Rm = 0.057 R

But there is also this field:

Parallel Motors = 3

I presume it affects Io and Rm as you posted previously, but without input from the author we can't be sure.

Regards,

Neil.
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Old Sep 08, 2005, 11:54 AM
HyperFlight Support
Neil Stainton's Avatar
United Kingdom, England, Stratford-upon-Avon
Joined Feb 2001
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My Error

Oops, I have just seen there is a number of props field, and if left blank (as it is by default) it seems to assume 3 props if there are 3 motors. Explicitly changing it to one prop I now get these results.

Motor Temp Calculations Off:
127A, 36V, 4.5 Kw in, 85% efficiency, 65 lb thrust at 45 mph

Motor Temp Calculations On:
122A, 36V, 4.4 Kw, 83% efficiency, 62 lb thrust at 44 mph

MotoCalc now agrees quite closely with your results. Hurrar!

Regards,

Neil.
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Old Sep 08, 2005, 12:04 PM
It's been a while.
Daniel G's Avatar
Irvine, CA
Joined Jan 2005
2,690 Posts
Quote:
Originally Posted by Neil Stainton

MotoCalc now agrees quite closely with your results. Hurrar!
Great discovery Neil!
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Old Sep 08, 2005, 01:00 PM
Flying motor mount master
fly_boy99's Avatar
San Jose, California, United States
Joined Oct 2004
8,745 Posts
Had the pitch speed almost dead on in post #446 but it hard to figure out a t constant for a 60inch prop!!! So to give you an idea my original calcs came out with 42lbs of thrust so the tconst has definitely gone way up!!!

More then enough to fly her if you can get her to stop eating her belts.


Quote:
Originally Posted by Neil Stainton
Oops, I have just seen there is a number of props field, and if left blank (as it is by default) it seems to assume 3 props if there are 3 motors. Explicitly changing it to one prop I now get these results.

Motor Temp Calculations Off:
127A, 36V, 4.5 Kw in, 85% efficiency, 65 lb thrust at 45 mph

Motor Temp Calculations On:
122A, 36V, 4.4 Kw, 83% efficiency, 62 lb thrust at 44 mph

MotoCalc now agrees quite closely with your results. Hurrar!

Regards,

Neil.
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Old Sep 08, 2005, 01:18 PM
Registered User
Perth, Australia
Joined Mar 2004
872 Posts
If I've read the previous posts correctly, It seems that if a 15mm belt is used instead of 5mm and the mainshaft is modified to not overtension the bearings that the problem should be fixed.

Ron...
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Old Sep 08, 2005, 01:30 PM
Just one more plane!
JWarren's Avatar
Port Saint Lucie, Florida, United States
Joined Aug 2004
2,537 Posts
Was there any thought to using bicycle chain and sprockets from a 10 speed bike? I am sure the machine shop at your school could adapt them to the motors and prop shaft. Just a thought.

John
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Old Sep 08, 2005, 03:05 PM
MotoCalc Developer
stefanv's Avatar
Moorefield, Ontario, Canada
Joined Mar 2001
315 Posts
Quote:
Originally Posted by Hovertime
LOL

My latest discovery of motocalc "surprises" is that it calculates aircraft performance using input watts.....
Actually, it does no such thing. It doesn't calculate performance on Watts at all. Using Watts to calculate performance is just a rule of thumb, not a sure fire method. MotoCalc calculates power reaching the propeller, determines RPM, and then calculates performance based on the suitability of the propeller, its RPM, and its thrust at various speeds, to the characteristics of the airplane.

Can you post or e-mail me an example that makes you think it's calculating performance based on input Watts?

Thanks,
Stefan Vorkoetter
Capable Computing, Inc.
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Old Sep 08, 2005, 03:34 PM
Always right
Hovertime's Avatar
Chicago
Joined Feb 2003
5,924 Posts
Wow, impressive, words directly from the maker, always gets me exited!

How I found it?
Calculated small airplane data with brushed motor.
Then calculated same airplane data with brushless motor, tiny bit lighter.

Results more prop rpm, at less input watts, slightly less weight with BL motor. Everything else is identical.

Motocalc shows faster climbs with brushed motor. (Less prop rpm, but more input watts)

Thats how. Explanation?

PS- where is promised helicopter calc? I bought motocalc what - 2-3 years ago? It said that heli calculation will be added "real soon now"
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Old Sep 08, 2005, 03:51 PM
MotoCalc Developer
stefanv's Avatar
Moorefield, Ontario, Canada
Joined Mar 2001
315 Posts
Quote:
Originally Posted by Hovertime
Calculated small airplane data with brushed motor.
Then calculated same airplane data with brushless motor, tiny bit lighter.

Results more prop rpm, at less input watts, slightly less weight with BL motor. Everything else is identical.

Motocalc shows faster climbs with brushed motor. (Less prop rpm, but more input watts)

Thats how. Explanation?
Without seeing a specific example, it's hard to comment. One possibility is that the lower rpm was more suited to the flying speed of the plane. Just because a propeller is spinning faster doesn't make it better. But I'd really have to see a specific example to comment on it.

Quote:
Originally Posted by Hovertime
PS- where is promised helicopter calc? I bought motocalc what - 2-3 years ago? It said that heli calculation will be added "real soon now"
Hey, we never promised it. It was just an idea for the future. But of all the things we get asked for, that's probably the least frequent.
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