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Old Dec 12, 2012, 07:51 PM
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murdnunoc's Avatar
United States, TX, Lubbock
Joined Oct 2007
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Picking your motor:

Quote:
Originally Posted by BpPilot View Post
Soooo, what MEGA for 6s-8s?

I'm pretty good with throttle management so it can be pretty high powered. Im planning on doing the build with the angled battery setup so probably no 7s configuration. I would like to be able to run 6s 5000mah and 8s 4000mah if possible. I think that is about what Peter M did on his GT40 build (correct me if im wrong...?)
You could still do the build to fit 8S, but run a 3S pack in place of one of the 4S to get 7S.
Gives you more options that way

For a setup that will be running with good throttle management, we can shoot for a couple different prop setups.

I've seen the highest top speed (171mph level, no-wind) on my Demons with a 6x6 prop at 37,500 RPM.
It's a tough hand launch, though, and takes a lot of room to get up to speed. But once it does, it squeezes the most speed out of the 16/35 and 16/40 motors that I ran it on.

For larger motors, you could shoot for turning that 6x6 prop at higher RPM and see what speed you get. It may be on the verge of becoming inefficient. Its tips will be transsonic over 38K.

My goal would be to run the higher pitched 6.5" pylon props at around 33K RPM.
They'll ensure you're not dealing with transsonic inefficiency. A 16/40 or 16/45 motor can safely turn these props at 33K.

So... start figurin'...
You need 33K RPM
Motor runs at about 85% of no-load KV. To get unloaded KV use the inverse (1/X) of 85% which is 117.6%
So 33K target x 117.6% = 38,808 no-load RPM.

You can then use this to figure out your target KV on any cell count you want. I'm using 3.5V/cell. If you have in-flight data, you can plug in your actual loaded voltage for more accurate predictions.

6S: 38,808 / 21V = 1848kv
7S: 38,808 / 24.5V = 1584kv
8S: 38,808 / 28V = 1386kv

Those are your ideal KVs for 6.5x5.5 prop at 33,000 RPM in-flight.

Then it's just down to looking at the 16/40 or 16/45 motor options to see which fits your cell count and target KV the closest.

Look! 16/45/1.5SCE is 1594kv!
That's stupid close to your ideal 7S motor.

16/45/2C is 1404kv. That's pretty close to ideal KV for 8S.
16/45/F5DE is 1360kv. Little lower than ideal 8S KV, but could turn a little larger prop, or account for batteries that hold better than 3.5V/cell under load.

If you
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Old Dec 12, 2012, 07:54 PM
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Peter M's Avatar
Newcastle, New South Wales, Australia
Joined Sep 2007
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Quote:
Originally Posted by BpPilot View Post
...... I would like to be able to run 6s 5000mah and 8s 4000mah if possible. I think that is about what Peter M did on his GT40 build (correct me if im wrong...?)
You are correct.
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Old Dec 12, 2012, 08:56 PM
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GryphonRCU's Avatar
United States, CA, SF
Joined Mar 2008
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Murdo,
Nice write up.

You know my favorite!!!!

-----

Also when same Watts is made using more cells, the AMPs are lower and same ESC like ICE 100A will have more head room.

-------

More number of smaller cells can have same weight as fewer larger cells.....and when/if at same power level: there will be same run time.


Gryphon
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Old Dec 13, 2012, 02:45 AM
Molon Labe
BpPilot's Avatar
United States, CA, San Luis Obispo
Joined Feb 2008
644 Posts
Wow, thanks guys. Exactly what I was looking for. Now I just have to decide...

I do want to retain the ability to somewhat easily hand launch the wing.

Also, who do I contact for prices on mega's these days?
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Old Dec 13, 2012, 08:30 AM
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murdnunoc's Avatar
United States, TX, Lubbock
Joined Oct 2007
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You can order either direct from Mega at megamotor.cz, or from Efflux RC in California. It'll be about the same 6-8 weeks from either. Efflux RC is more convenient for payment.
For the custom motor order from Efflux, you'll need to email them using the Contact on the website, as all the options for Megas are not on the website.
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Old Dec 13, 2012, 09:58 AM
Flirtin With Disaster !!
D.B. Hooper's Avatar
United States, OR, Mt Angel
Joined Jan 2008
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Quote:
Originally Posted by murdnunoc View Post
Make sure you specify 16/45/1.5RC

There are two different 1.5 turns, and C gets you a 5mm Combat shaft. Don't want to wind up with a bitty 3.17mm shaft. Can't get anything done with that little shaft!!

Mega's standard bearings are tough. They'll survive any crash that doesn't bend the shaft (and it's TOUGH to bend their 5mm shaft)
Ceramic really only becomes a significant benefit over 50K RPM. Won't hurt to have them if you want just that extra tiny edge, or a tiny bit more insurance, but not necessary in this application IMO.

16/40/1.5SCE would be sweet on 7S.
Decided on the Mega 16/40/1.5 SCE it's in stock and don't mind going 7s.
Thanks for your advice and number crunching

Now it's back to my 200+mph 70mm ME 163 Komet EDF build
Might just need the 100,000 rpm hybrid bearings!
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Old Dec 13, 2012, 10:37 AM
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DB, I think that 16/40/1.5SCE will put you in a great spot on 7S with 6x5 prop.
At least as much power as you had on the Rokett, with headroom for no concern about motor damage.
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Old Dec 13, 2012, 11:03 AM
Flirtin With Disaster !!
D.B. Hooper's Avatar
United States, OR, Mt Angel
Joined Jan 2008
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Quote:
Originally Posted by murdnunoc View Post
DB, I think that 16/40/1.5SCE will put you in a great spot on 7S with.

" with headroom for no concern about motor damage"

Imagine..... carefree flying at 160mph! The 100,000 Mink's over at the mink farm aren't going to like this! Apparently evey pass makes them poop simultaneously.

Charging batteries, wating for the ice fog to lift 31 degrees. TL50 and firewood loaded up. Fly all day.
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Old Dec 18, 2012, 09:19 PM
Flirtin With Disaster !!
D.B. Hooper's Avatar
United States, OR, Mt Angel
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Quote:
Originally Posted by murdnunoc View Post
DB, I think that 16/40/1.5SCE will put you in a great spot on 7S with 6x5 prop.
At least as much power as you had on the Rokett, with headroom for no concern about motor damage.
Got the Mega installed in the Demon
3 day shipping from Gary at EffluxRC.com
WOT for 5sec.bench test.
7s 4000 6x5 carbon prop
120amps
2675watts
35,820RPM
Hope to fly this weekend.
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Old Dec 18, 2012, 10:57 PM
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murdnunoc's Avatar
United States, TX, Lubbock
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Your packs are pukey!
3.18 volts/cell.

That's low even if the test was done with a battery that wasn't fresh off the charger. If it was fresh off the charger, you'll puff em for sure in flight.
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Old Dec 18, 2012, 11:35 PM
Flirtin With Disaster !!
D.B. Hooper's Avatar
United States, OR, Mt Angel
Joined Jan 2008
1,435 Posts
Quote:
Originally Posted by murdnunoc View Post
Your packs are pukey!
3.18 volts/cell.

That's low even if the test was done with a battery that wasn't fresh off the charger. If it was fresh off the charger, you'll puff em for sure in flight.
Have some new batteries on the way but don't think they will be here by the weekend. I'll try not to catch them on fire
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Old Dec 18, 2012, 11:50 PM
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murdnunoc's Avatar
United States, TX, Lubbock
Joined Oct 2007
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Might be a good idea to run the first flight with 6x4 prop. New motor, just to be safe?!
I always like to start low just so I can see when the motor begins to heat up, rather than risk a surprise in case I missed something in calculation.
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Old Dec 19, 2012, 09:59 AM
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DoctorAudio's Avatar
Milwaukie , Oregon
Joined Feb 2003
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Yeah , what he said. the bigger the motor the longer the smoke trail will be.

Muahahahahaaaaaaaa!
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Old Dec 20, 2012, 08:37 AM
Why the dirt on the wings?
moparmyway's Avatar
New York
Joined Jan 2009
4,997 Posts
Quote:
Originally Posted by D.B. Hooper View Post
Might just need the 100,000 rpm hybrid bearings!
Just remember, you will need to add oil to those ceramic bearings should you choose them.

The MEGA 100k bearings wont need you to keep them wet.
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Old Dec 21, 2012, 10:33 AM
Flirtin With Disaster !!
D.B. Hooper's Avatar
United States, OR, Mt Angel
Joined Jan 2008
1,435 Posts
Quote:
Originally Posted by murdnunoc View Post
Might be a good idea to run the first flight with 6x4 prop. New motor, just to be safe?!
I always like to start low just so I can see when the motor begins to heat up, rather than risk a surprise in case I missed something in calculation.
Good idea! I'll do that for the maiden flight then check the flight log.

Dr Audio . I see your not far from me. Nice flying weather we are having! Hard to believe they are having a dust storm in Texas. Almost makes me want to move back there.

Thanks again to Murd and Mopar for the number crunching and advise.
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