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HomeAircraftHelicopterHelicopter Main Discussion › Torque vrs rpm
09-04-2017 08:11 PM  14 months agoPost 1
3dorbust

rrNovice

Hudson Florida usa

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Is high kV high torque or vice versa can anyone suggest a good high torque brushless motor for mild 3d in a 450 heli?

I know how Icarus felt

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09-04-2017 08:24 PM  14 months agoPost 2
Heli 770

rrProfessor

USA.

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09-05-2017 05:16 AM  14 months agoPost 3
Flyin for Jesus

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Troy, IL. 62294

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The only purpose of kv is to pick a motor within the range of the gearing to achieve the HS you want.
If that motor simply does not have enough power to do what you need it to, then a larger motor is the only option.
My own personal preference is to get higher kv and lower gearing thinking that if the motor RPM is higher, it will pull more air through it keeping it cooler. How much cooler? Probably no more than a few degrees but if it all costs the same, then why not. But if I already had a motor with a lower kv, I would use it.

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09-05-2017 05:28 AM  14 months agoPost 4
3dorbust

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Hudson Florida usa

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High kv
Great user name I'm with you on that. The only gain I have heard of with high head speed is better cyclic control and better tail lock on the other hand low head speed gives less heat longer run times I

I know how Icarus felt

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09-05-2017 05:36 AM  14 months agoPost 5
3dorbust

rrNovice

Hudson Florida usa

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High kv
Great user name I'm with you on that. The only gain I have heard of with high head speed is better cyclic control and better tail lock on the other hand low head speed gives less heat longer run times I wanted to find a happy medium im running 4000 kV 13 pinion and low pitch and it gets hot what's a good motor in the 2500 kV range with a 15 pinion I can increase my pitch a bit what do you think?

I know how Icarus felt

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09-05-2017 06:02 AM  14 months agoPost 6
3dorbust

rrNovice

Hudson Florida usa

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Pinions
Hey there flying for Jesus I also have a 3500kv with a 13 pinion it seems to bog down more than the 4000 so your saying I can change to an 11 tooth on my 3500 kV with less bogging I read that top rpm is better for motor efficiency

I know how Icarus felt

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09-05-2017 12:19 PM  14 months agoPost 7
Richardmid1

rrProfessor

Leeds, England

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We need to know more about your setup. Which heli, batteries, esc and esc settings i.e. governor or throttle curves?

There is mechanical torque which is increased by reducing pinion size and motor torque which is increased by higher voltage or simply a larger motor.

Running a motor and esc closest to 100% throttle is the most efficient however this means no governing which means some bogging will be observed during high pitch manoeuvres, therefore governing with the correct gearing for the rotor rpm is best for minimal bogging.

60% of the time, it works every time!

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09-05-2017 12:35 PM  14 months agoPost 8
jharkin

rrApprentice

Holliston, MA - USA

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Electrical engineers have explained kV to me as an "electronic gear ratio"

It doesn't change the theoretical max power output of the motor (watts), but it does change the speed range you generate that power.

If you have two motors that are otherwise identical (stator dia, height, pole count) the higher kV motor will want to spin faster than the lower kV motor, while capable of the same power.

-Jeremy
Whiplash-G
Helix 700G
T-Rex 450 fbl conversion
alot of planks

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09-05-2017 03:43 PM  14 months agoPost 9
AirWolfRC

rrProfessor

42½ N, 83½ W

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The "importance" of torque is an automotive advertising boondoggle.

What counts is power - - as in horse power.

Power is torque multiplied by rpm.
- - - as in 1 ft lb of torque x 1800 rpm = 0.3427 hp
- - - 1 x 2 x pi x 1800 / 33,000 = 0.3427 HP

You can have all the torque you want from things like a torque wrench.
- - - but that won't get your rotor turning.

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09-05-2017 03:43 PM  14 months agoPost 10
Flyin for Jesus

rrVeteran

Troy, IL. 62294

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I read that top rpm is better for motor efficiency
Top RPM for the motor is more efficient but just to clarify, a higher kv motor running more RPM is not more efficient than a lower kv motor running lower RPM but still running at the top RPM for its kv.

Asking about bogging, now we're getting into a few different variables. The next main on is the battery and its condition.

Lets get back to that one in a second. Start with what HS you want to run. Most heli manuals have a HS range so look there to get started. Running a higher HS does give crisper cyclic and a stronger tail but at the expense of flight time. Unfortunately this is a trial and error kind of thing. Unless you are capable of calculating disc loading, heli weight and other sorts of things, you just pick a headspeed and go try it out. If it seems too low, then raise it and try again. Or just try several and pick the one that worked best for your desired flight time and performance.

Okay, so once you pick your HS, check available pinions and note your range of gear ratios. Go to this website ( http://calc.kfmconsulting.se ) and plug in the middle gear ratio and just ( guess and check ) the motor kv until the result comes out to the HS you want to run. Now that's the motor kv you're looking for but you may not get exact, but get close. Once you pick a motor, run the lowest and higest gear ratio through the HS Calculator just to make sure it will give you your desired HS and some room below and above so you can adjust the HS to your liking.

Now, talking about bogging. Sometimes it is the motor, sometimes its the batteries and sometimes its you demanding more than the heli is designed to do.

You demanding too much means running a really high HS and/or too much pitch. This is where putting in info about your setup like was posted above helps. People with experience with your heli/motor combo may already know the limits and if you're going above those. Next is motor, pretty simple... bigger motor, more power. But will a larger motor fit? Does the weight make your heli near impossible to set your CG? Most helis are designed with a more than powerful enough motor so this isn't one that needs to be changed unless you are just wanting more.
The battery is a vital part and gets overlooked often so it causes a lot of problems. I'll spare you a full LiPo lesson but give some basics. If the battery is going bad, it starts having too much voltage drop when under a heavy load. Here's a scenario... You do a high power maneuver and the heli bogs. You pull out the battery and check its voltage. Checker says battery is fully charged so you plug it back in and does the same thing still. The problem is that when the battery is needing to output a lot of current, it cannot and the voltage drops giving less watts to your motor resulting in less power. This is where either telemetry or an ESC with logging comes in handy. You can easily check and see how much the voltage drop is.

So, which pinion to run with your 3500kv motor, use the calculator and pick the pinion that puts your HS closest to the target RPM. If its still bogging, check your max pitch and see if its with the manual's suggested pitch range. If still bogging, time to check out batteries. I have a handy device if you don't have telemetry or a logging ESC so no need to spend a lot of $$$.
Anyway, got to get back to work, let us know how it goes.

Oh, I like my user name too. Everything I do, I do for the glory of God.

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09-05-2017 05:21 PM  14 months agoPost 11
ICUR1-2

rrElite Veteran

Ottawa, Ontario

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To shorten the long read ^

KV of a motor is how many rpm it will spin per volt.
The higher the KV the faster it will spin at a cost of torque.

For a model helicopter the less things spin the less energy it consumes.

What to consider when choosing a motor...
Battery size
The gear ratio you have available
The desired head speed
Weight of the model RTF

On my 500's I get 6-7 min /flight stock
Simply by lowering the head speed and raising the gear ratio
If I were shopping for a new motor I would find a lower KV than stock and lower the gear ratio. Then IMO my model would be more efficient.

spending time, paying attention

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09-05-2017 05:32 PM  14 months agoPost 12
EEngineer

rrProfessor

TX

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KV of a motor is how many rpm it will spin per volt.
The higher the KV the faster it will spin at a cost of torque.
Much more concise....

In other words, torque is inversely proportional to KV....

Logo 600SXs, 800XX, TDR IIs

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09-05-2017 06:10 PM  14 months agoPost 13
Flyin for Jesus

rrVeteran

Troy, IL. 62294

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Much more concise....
In other words, torque is inversely proportional to KV....
However both produce the same power ( you know, same size motor, just different kv )

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09-05-2017 06:45 PM  14 months agoPost 14
AirWolfRC

rrProfessor

42½ N, 83½ W

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If I were shopping for a new motor I would find a lower KV than stock and lower the gear ratio. Then IMO my model would be more efficient.
By doing both of those things you will not accomplish much of anything.

You would still be operating at the same % of motor power and same head speed.

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09-05-2017 09:00 PM  14 months agoPost 15
Andy from Sandy

rrElite Veteran

Bedfordshire, UK

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What ESC are you using. Is it for 3S or 6S?

For a 450 I would just get the Align 460MX in either 1800kv for 6S or 3200 for 3S.

No point in reinventing. Just use the parts Align supply for pinion and main gear.

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09-05-2017 09:38 PM  14 months agoPost 16
ICUR1-2

rrElite Veteran

Ottawa, Ontario

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Airwolf
For myself I find the stock motors has too high a KV and having to use smaller pinion than necessary.Plus having to run at reduced throttle makes them inefficient. The power needed to make it fly might remain the same but the efficiency can certainly be improved.

spending time, paying attention

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09-05-2017 11:15 PM  14 months agoPost 17
AirWolfRC

rrProfessor

42½ N, 83½ W

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Motors typically run more efficient when run somewhere between 1/3 to 2/3 load. That goes for most all motors, electric, gas, etc. The world rarely runs motors at best efficiency unless fuel efficiency is the objective.

Keep in mind that manufacturers will advertise their motors using maximum output which is always beyond best efficiency. A full spec on motors is usually not available.

Also realize that "full throttle" does not specify if constant rpm setting in the ESC is being used or not.

You do want a KV rating that is above the rpm you intend to run with a given battery. The question is how much above. Think of the KV rating as describing the back EMF created by the motor at a given rpm. KV is the no load rpm when a certain voltage is applied.

Something like servo ratings.
Servo speed is a no load speed.
Servo torque is a stalled motor output torque.

One could always setup a make-shift dyno and find the answers for the specific motor they have.

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09-06-2017 12:41 AM  14 months agoPost 18
EEngineer

rrProfessor

TX

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Think of the KV rating as describing the back EMF created by the motor at a given rpm. KV is the no load rpm when a certain voltage is applied.
KV rating is the back EMF created by the motor at the RPM that causes 1V....at no load....

Logo 600SXs, 800XX, TDR IIs

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09-06-2017 01:39 AM  14 months agoPost 19
learnedthehrdwy

rrNovice

Saginaw, MI

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Are you using a governor? That in itself can greatly reduce bogging.

Also what size 450 is it? 325-360 or 380mm blades

Beyond kv there's ​also motor size like 2618, 2520, 2820
325^, 360^, 380^

Those are 6s motors but just to give the gist. Too small of a motor won't have enough torque and will bog at any rpm.

Something else to consider is the C rating and condition of your batteries. If they're getting hot during the flight they're probably not quite up to the task. C rating usually isn't that important on old 3s 450's but still worth mentioning.

Another thing is the max headspeed that the old 325mm Trex 450's were rated for is 3500rpm just fyi. I would gear it to get your desired headspeed at 80% flat throttle and you should be about as good as you're gonna get as far as bogging.

If you're looking at buying a new motor check out the Lynx EOX motors. Same price or cheaper than a Scorpion but better specs but make sure your esc is up to the task. One of those should eliminate your bogging issues no matter what rpm you want to run

MSH P380/Lynx'd Mini Protos×3/Chase 360/Trex 450/180cfx/230s/SK540's,YGE,MKS,KST, Xnova,Scorpion,Switch,RJX/UMX Pitts/Valiant 1.3/RochobbyMX

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09-06-2017 01:50 AM  14 months agoPost 20
dkshema

rrMaster

Cedar Rapids, IA

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Torque is a function of motor current and the motor torque constant.

RPM is a function of voltage.

-----
Dave

* Making the World Better -- One Helicopter at a time! *

Team Heliproz

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HomeAircraftHelicopterHelicopter Main Discussion › Torque vrs rpm
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