Head speed calculation - confused!

[YOUNG GUN]

Hi all,

Got the motor installed...

Set the ESC to Govenor mode...

and without the main rotor blades on (just tail blades and paddles)

I have been trying to set the % throttle on the TX to give me a HS of 1,700rpm.

My calculations are as follows:
Motor RPM: 1100kv 60 amp
Main gear 111 teeth
Desired HS: 1700
LiPo: 6S 5000mah

So.
1100kv x 22.2v = 24,420 motor rpm (full power)
24,420/1700 = 14.36 required gear ratio
111 teeth main gear / 14.36 gear ratio = 7.7 = 8 tooth pinion at 100% power will give about 1700 rpm

90% power = 9 tooth pinion approx.


BUT -

I have it setup with a 9 tooth pinion, and yet at just 35% "throttle" i'm getting my 1700rpm measured using my tacho.


I was expecting to require nearer 85 - 90%

Is there something i'm missing or done wrong?

Would it matter if i only need 35% for 1700rpm?

Thanks
Matt

[solentlife]

(02-05-2011 16:42)YOUNG GUN Wrote:  Hi all,

Got the motor installed...

Set the ESC to Govenor mode...

and without the main rotor blades on (just tail blades and paddles)

I have been trying to set the % throttle on the TX to give me a HS of 1,700rpm.

My calculations are as follows:
Motor RPM: 1100kv 60 amp
Main gear 111 teeth
Desired HS: 1700
LiPo: 6S 5000mah

So.
1100kv x 22.2v = 24,420 motor rpm (full power)
24,420/1700 = 14.36 required gear ratio
111 teeth main gear / 14.36 gear ratio = 7.7 = 8 tooth pinion at 100% power will give about 1700 rpm

90% power = 9 tooth pinion approx.


BUT -

I have it setup with a 9 tooth pinion, and yet at just 35% "throttle" i'm getting my 1700rpm measured using my tacho.


I was expecting to require nearer 85 - 90%

Is there something i'm missing or done wrong?

Would it matter if i only need 35% for 1700rpm?

Thanks
Matt

Are you sure you are only tacho'ing 1 holder / blade ? and not both ?

I looked at maths and it seems sound ... you have a 9/111 ratio = 0.08108 ... therefore 24420*0.08108 = 1980rpm

Now question ... is the KV rating of a motor based on a loaded motor or an unloaded motor ?

[YOUNG GUN]

I am a bit confused Solent...

The maths is right...

The Motors KV is 1100 which i assume is unloaded rpms (i've not had a motor yet that quotes a loaded kv rating) as you could say - if the load was high enough you could stall the motor giving 0kv. manufactures always want to give the best spec possible.

All i can think, is theres something wrong with my Tacho?

It can be set for 1, 2, 3 or 4 blades Props. I was measuring off the flybar which has two paddles, so i set it to a 2 bladed prop - which would be correct.




To me, the Headspeed does not "Sound" high enough BUT i've not had a model this size, which unlike a trex 450, has a much lower headspeed anyway.

I will be using wooden blades to start with so i need to be sure my headspeed isnt too high - but high enough to be flyable.



So not sure IF to buy another Tacho - a laser one? (how much are they?)

[QuackingPlums]

(03-05-2011 18:19)solentlife Wrote:  Now question ... is the KV rating of a motor based on a loaded motor or an unloaded motor ?

Neither. It's the RPM at which the windings in the coils generate 1V of back EMF - at this RPM the motor would be generating zero torque, which is impossible as there is resistance in the air and the bearings.

I would work on an efficiency factor of 80-90% to give you actual RPM for a given input V.

Can't help with the tacho question though...

[solentlife]

(04-05-2011 10:12)QuackingPlums Wrote:  

(03-05-2011 18:19)solentlife Wrote:  Now question ... is the KV rating of a motor based on a loaded motor or an unloaded motor ?

Neither. It's the RPM at which the windings in the coils generate 1V of back EMF - at this RPM the motor would be generating zero torque, which is impossible as there is resistance in the air and the bearings.

I would work on an efficiency factor of 80-90% to give you actual RPM for a given input V.

Can't help with the tacho question though...

But that would make the results even more confusing .....

Thing to do is to check the tacho on something else ........ ie car ? On the main fan-belt wheel of the crank ... that should be at indicated rpm on the dash..... I assume that you got rid of all other models as you threatened before ? Cause I klnow you had your 450 all governed up ok ...

I wonder if putting a wattmeter inline so you see the volts going to the motor ? then try the sums again based on wattmeter numbers ? I just have this sneaking suspicion that running up the head without load has something to do with it.

[YOUNG GUN]

i don't quite understand what QuackingPlums is talking about - all i know is i put electricity in = motor spins.

I have bought a new laser tacho which i will use and compare with my other tacho (which relies on sunlight)

Either both will give the same reading and its a problem with the model - or its the tacho!

I did use it to setup my 450 a while ago - but I have sold everything so cannot cross check it.

I got a lot of jobs do today but i'll try do a quick tacho see what its doing!

[QuackingPlums]

(06-05-2011 11:03)YOUNG GUN Wrote:  i don't quite understand what QuackingPlums is talking about - all i know is i put electricity in = motor spins.

Hehe. I get that a lot.

I know Raj and Sam call me Intel Inside, but unfortunately I suffer from the Pentium floating point bug so my mental math sucks. Here's an example using 'easy' numbers:

Say I have a motor rated at 1000kv and a battery pushing 10V. At full power (no load) the motor would theoretically spin at 10,000rpm.
In practice, there are losses in the bearings, coils etc that reduce the efficiency so the actual rpm may be 80-90% of the theoretical maximum, giving 8000-9000rpm.

Forget the back emf for now - that only comes into play when you start looking at watts under load...

[solentlife]

(06-05-2011 16:33)QuackingPlums Wrote:  

(06-05-2011 11:03)YOUNG GUN Wrote:  i don't quite understand what QuackingPlums is talking about - all i know is i put electricity in = motor spins.

Hehe. I get that a lot.

I know Raj and Sam call me Intel Inside, but unfortunately I suffer from the Pentium floating point bug so my mental math sucks. Here's an example using 'easy' numbers:

Say I have a motor rated at 1000kv and a battery pushing 10V. At full power (no load) the motor would theoretically spin at 10,000rpm.
In practice, there are losses in the bearings, coils etc that reduce the efficiency so the actual rpm may be 80-90% of the theoretical maximum, giving 8000-9000rpm.

Forget the back emf for now - that only comes into play when you start looking at watts under load...

I think everyone expects 10% or more losses in anything thats powered as a minimum .... I think YG's problem is the Back EMF bit ... now luckily I did Electrical theory as part of my training back many years ago and remember a little about it ...but of course in those days BL motors were not around retail.

My previous about no-load and loaded .... lets take an example :

Model railway engine. Lift it of track .. apply power to the wheels by touching leads ... you need very little on the speed control to get full rpm out of it. But load the wheels as in put it on track and make it propel itself and speed control has to increase significantly to mantain it.

Now lets look at governed head speed. I know why YG uses it and I will be as well later ... but YG knows that as he increases pitch on the blades - the governor function increases power to the motor to compensate for higher load to maintain rpm. Now my thoughts are it works in reverse as well ... remove rotor blades and head is now so light without any load literally ... bit like the railway engine with leads touching wheels and not on track.

I think the governor should be set as YG found ... and maybe when blades are fitted and run up - better figures are obtained ? Maybe !!

[YOUNG GUN]

Thanks both - I understand whats been said above...

But getting to the nitty-gritty...

I have been doing some testing with a new laser tacho, and it seems to be a problem with the ESC's Govenor.

I have had a real good play about with it, set it to govenor mode, set it to 30% power - which gives about 1,600rpm HS (again, i dont know why the maths doesnt work out for this BUT at just 30% - 35% power i'm getting my required headspeed.

Now keep in mind - the Govenor is supposed to 1st) detect the motor slow down when load is applied and then 2nd) increase power to compensate.

It does NOT seem to be doing this.

I spooled up, tail gyro set to rate mode so the tail blades are at 0 degrees. HS = 1,600. I then started to apply left rudder (so increasing load) - straight away you could hear the motor bogging down and the HS started dropping to around 1,200 rpm. thats a 400 rpm drop of the ROTOR head with just the tail pitch changing a little.

At no point did the ESC attempt to compensate for the load.

So all in all... i think the ESCs Governor isnt working or is very crap!


The ESC on my mini titan was very good - and from tests held the rotors rpm very close to the desired HS even through big fluxuations in pitch!


So... What to do... just refit the original motor & esc maybe!!!!

[solentlife]

(09-05-2011 19:24)YOUNG GUN Wrote:  Thanks both - I understand whats been said above...

But getting to the nitty-gritty...

I have been doing some testing with a new laser tacho, and it seems to be a problem with the ESC's Govenor.

I have had a real good play about with it, set it to govenor mode, set it to 30% power - which gives about 1,600rpm HS (again, i dont know why the maths doesnt work out for this BUT at just 30% - 35% power i'm getting my required headspeed.

Now keep in mind - the Govenor is supposed to 1st) detect the motor slow down when load is applied and then 2nd) increase power to compensate.

It does NOT seem to be doing this.

I spooled up, tail gyro set to rate mode so the tail blades are at 0 degrees. HS = 1,600. I then started to apply left rudder (so increasing load) - straight away you could hear the motor bogging down and the HS started dropping to around 1,200 rpm. thats a 400 rpm drop of the ROTOR head with just the tail pitch changing a little.

At no point did the ESC attempt to compensate for the load.

So all in all... i think the ESCs Governor isnt working or is very crap!


The ESC on my mini titan was very good - and from tests held the rotors rpm very close to the desired HS even through big fluxuations in pitch!


So... What to do... just refit the original motor & esc maybe!!!!

Sorry to sound a twat YG ... but why change what was working ? If the replacement is failing due to ESC ... then if you really want to change gear - replace ESC and get it working ?