RunRyder - Kora 15-12 on 5s

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e-Hirobo SRB Quark - Lepton - XRB Coaxial > Kora 15-12 on 5s

demosa
Veteran
Location: Tigard, Oregon, U.S.A.

I've decided to make my second Lepton a 5s ship, since 4s does not seem to keep me happy anymore and I already have a nice 4s setup (Hyperion 3025-10/23t pinion).

I've been flying the Kora 15-12/21t pinion on 4s. With the Jazz at 90%, I'm getting ~2700rpm. But not much power with +/- 9 degrees pitch.

I'm not sure which Kora is better on 5s. Some say the 15-12, some say the 15-14, some the 15-16... With the KV rating of the 15-12 being 920, it seems like 5s is a little too high for the Kora 15-12 especially given the max RPM for the Kora 15-XX series is 15,000.
So I'm leaning towards the 15-14 with 21t pinion on 5s.

But I already have the 15-12/21t pinion in there.

Should I just try it with 5s and simply turn down the Jazz to say 75% so that I stay below the 15,000 rpm limit or is it completely out of the question ? (the Jazz is the only ESC I know that can be run at lower % and still be perfectly happy).

Any comments please ?

05-04-2008 04:44 AM

Jafa
Elite Veteran
Location: Sydney, Australia

I run a Lepton with Kora 15-12/21t pinion on 4s
The difference is I use +/- 11 degrees pitch
it's been the best configuration I have tried

I'm also working on a second Lepton
5S on a Kora 15-12 with a 18T pinion
It will only only be slightly over the 15,000 rpm limit
for 1 minute until the pack voltage goes down a bit
I reckon it will be fine

The 15-14 on 5S will perform like the 15-12 on 4S
Power goes up with RPM - you've got get to 15,000

Lepton | TRex600Nitro | Sceadu | Freya | Avant | Predator Carbon Max

05-04-2008 09:28 AM

demosa
Veteran
Location: Tigard, Oregon, U.S.A.

I'm surprised that you can pull 11 degrees out of the 15-12/21t/4s combo. What ESC ?

Are you saying that you wouldn't even try the 21t on 5s ?
I figured I can keep the motor rpm below 15000 by adjusting the
ESC setting to whatever is necessary to keep my rotor rpm between 2700 and 3000.

The 15-14 is 790KV so at 18.5V it would get to 14,615 or mid 12,000 for 85% efficiency. How close to 15,000 do I need to get for max power ?

My thought for the 15-12 was :
920KV * 18.5 * .85 * 21 / 94 ~ 3200 which is perfect for me.
I can then have the Jazz at 90% for a rotor rpm of ~2900

What do you think ?
Thanks !

05-05-2008 03:26 AM

Jafa
Elite Veteran
Location: Sydney, Australia

Yup - 11 degrees works great
I'm using the RotorTech yellow carbon blades
If you are using the MAH blades that may be your problem
as they are known for bogging

I'm using a Jazz 55 ESC

With my 9C the Jazz red light comes on at 78 on the throttle curve
That is full power - any higher makes no difference at all

Do not run a low throttle curve - you will heat up the ESC
(The Jazz is one of the best out there but same principal)
(but 90 is not a low throttle curve)

I doubt that you will have the power required
to spin a 21T pinion for 3,200 on the head
It's not all simple maths
it's also what the equipment can do
(especially as your 90% is likely to be the same as 100% on the Jazz)

These reasons are why I am putting a 18T on
and running a high throttle curve to start with
Lower gearing makes life easier for the motor too
Less bogging and less heat

Using 85% efficiency
the 15-14 on 5S will spin
at a similar RPM as the 15-12 on 4S
The resulting power/performance will be very similar

The Kora manual shows power increasing at a linear rate with RPM
so run the Kora close to 15,000 using 85% efficiency

5S on the 15-12
will be above 15K at the start of the flight
and below at the end of the flight
I'm going to live with that

Your maths is correct
You just need to try a few different configurations
and then you'll see the real world practicalities kick in

Cheers...

Lepton | TRex600Nitro | Sceadu | Freya | Avant | Predator Carbon Max

05-05-2008 08:19 AM

demosa
Veteran
Location: Tigard, Oregon, U.S.A.

Yes, I'm aware of the Jazz "full power point". On my radio, the led comes on at 91%. So when I say I'll set the Jazz at 90% I really mean 90 * 91 = 81%

I understand that in reality things are different than on paper.
I'm already baffled at the head speed I'm getting out of my 4s setup. It doesn't match the math ... in fact rpm is higher than I expected.

I know that 21T is pretty big. I'm gonna give it a whirl anyway just to see what happens.

With the 18T do you have to modify the magnesium frame to push the motor forward or is it still ok ?

Thanks for your input !

05-06-2008 03:34 PM

demosa
Veteran
Location: Tigard, Oregon, U.S.A.

I forgot to mention... I'm using SAB blades with my 4s setup.
I'll have to try those rotortech blades !

thanks

05-06-2008 03:37 PM

Tday
Key Veteran
Location: Needham, MA

Hey demosa, I'm using the kora 15-12 with an 18T and it's great. I did happen to fry one doing continuous rolls for a battery pack...just didn't know better I guess.

There is nothing special about putting the motor in place (other than it's difficult with any motor). I used a ball-head hex for the nut under the main gear and pulled the right side pulley for the other screw.

Tom

2 + 2 = 5 for extremely large values of 2.

05-06-2008 04:10 PM

HOMEPAGE

Jafa
Elite Veteran
Location: Sydney, Australia

The variation in headspeed will be the battery voltage
Your calculations are on the nominal voltage
which is 3.7v * number of cells

But at the start of the flight
the voltage is more like ~4.3v per cell
and at the end it's more like ~3.4v

If you tached the heli at 2.5 minutes you would get a different result

Lepton | TRex600Nitro | Sceadu | Freya | Avant | Predator Carbon Max

05-06-2008 05:22 PM

demosa
Veteran
Location: Tigard, Oregon, U.S.A.

I didn't tach it, I used the on-board eLogger.

Hovering rpm was actually the same both at the beginning and the end of the 5:30 min flight. This just tells me that at the end of the flight my batteries still have enough juice to maintain the rpm that the Jazz governor is asking for.
(Battery voltage is going down which only means current is going up.
But power consumption and rpm stays the same as long as the battery has juice to handle the job).

Anyway, I suppose I'll have to try a few pinions and 18t will definitely be one. Thanks for the info on the motor Tday.

05-06-2008 08:10 PM

demosa
Veteran
Location: Tigard, Oregon, U.S.A.

I finally tried the Kora 15-12 on 5s with 18t pinion.
Because my 5S batteries are new I only hovered in the back yard.

The facts:
The average voltage reported by eLogger is 18.78Volts.
The Kora 15-12 is 920 KV
The efficiency of the Kora motors is reported to be 83%.
The maximum rotation speed of the Kora 15-XX motors is reported to be 15000rpm.
The Kora 15-12 is a 14 pole motor.

Based on the above, I was expecting a main rotor rpm of :
920 * 18.78 * 0.83 * 18t / 94t = 2746rpm with wide open ESC.

Based on my eLogger measurements, I'm getting 2970rpm with 80% open ESC. This results in 2970 * 94/18 = 15509 motor rpm (over the limit !)

The motor, ESC, and batteries are not more than 104F with 78F ambient temp. at the end of a 6min hover. I'm putting back about 2000mAh into my 3300mAh batteries.

So either the KV of the Kora is higher, or it's a lot more efficient or I'm smoking dope...

Can someone explain this to me ?

In any case, this setup looks great to me but I'm a little concerned about the motor rpm going over 15000.
On the other hand I think it's exactly that fact that makes this a very powerfull combination. I wish I could put in a 17t pinion but I would have to modify the magnesium frame to do that (elongate the motor mounting holes).

Maybe the Kora 15-14 would be better for a 5s setup ?
Any thoughts or comments ?
Thanks !

07-08-2008 09:14 PM

husafreak
Senior Heliman
Location: NorCal

I just test ran my Kora 15-14 with 19t and 6 cells. I ran it up to full 100% and put a tach on the thing. 3300+ rpm holy cow! I had her lightly tethered with 0 deg pitch. As I walked back to pick up my tx POW! a Readyheli CF tail blade let go. I hit throttle hold before any real damage was done, just a bent tail rotor shaft and ruined bearings. I'll post my results with 18t pinion in another thread.

07-08-2008 09:37 PM

demosa
Veteran
Location: Tigard, Oregon, U.S.A.

Oops, sorry to hear that. Good thing that you didn't get too much damage.

However, your post supports mine ! In your case:
790 KV * 22.2 * 0.83 * 19/94 = 2942rpm which is a lot lower than the 3300 you got !

So again, either the Kora efficiency is much higher or the KV ratings that they report are low. Another possibility is that the motor efficiency doesn't come in the equation the way I think, i.e. is not directly multiplied in.... I don't know.

It seems to me that in your case even 18t is going to be too much.
I don't think you should be going anywhere over 3100rpm. Too much stress on everything.

07-08-2008 10:36 PM

husafreak
Senior Heliman
Location: NorCal

Hopefully DocJoe has something to add as he is running 6s with a 19t pinion but different esc.

07-09-2008 12:30 AM

Qooo
Senior Heliman
Location: Hong Kong

if you are still using Lepton's 14T, 3300rpm headspeed means you are running the tail with 13200rpm ...

... I tried 12700rpm tail and it is still very smooth and solid. I don't know if this 500rpm tail speed will kill the tail. I am also using 70mm CF tail blade from Readyheli.

Qooo - MARZ

07-09-2008 09:02 AM

HOMEPAGE

demosa
Veteran
Location: Tigard, Oregon, U.S.A.

Sorry, this is off topic

The question here is how can the Kora 15-12 on 5s with 18t pinion give a headspeed of around 2973 with 80% ESC setting when the motor KV is 920 and the motor efficiency is 83% !!!

07-09-2008 07:57 PM

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e-Hirobo SRB Quark - Lepton - XRB Coaxial > Kora 15-12 on 5s

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