Question about motors

[Bobmic]

Just a quick question to understand if I am not missing something.
Looking at three motors - Power52, Power60 and Power90.
Based on their spec the power difference is 1650W/590Kv/346gr, 1700W/Kv/380gr, 1800W/325Kv/450gr
Since actual power difference is so low and when looking at power to weight ratio the 52 is almost 20% higher than the 90.
is it realy "only" and exrecise Kv and number of cells when considering one of these?
- So is the consideration only the type of plane used - prop diameter vs pitch that should be considered?
At first look I was under the impression that a Power90 will have significantly higher power for a plane like the P51 blue nose but when looking at the numbers I am not so sure anymore.

Thank you
Bobby

[Ededge2002]

Tough question to answer but the higher kv motor "potentially" has thicker or more strands of wire in its winding and thus "might" be a factor in its power handling.

Although similar in watts the size of prop the
Motor will turn can make a huge difference in the power OUTPUT of the motor.

Pitch speed, thrust,  weight and efficiency are all a mess of values needed

[MarcV]

The main loss at high power is winding loss, I x I x R.  The way to increase the power of a motor is to use thicker shorter wire in the windings.  This increases the motors Kv.  For helis this isn't a big deal, as they can change the gearing to maintain the same head speed as a lower Kv motor.  But for planes, in general, you prefer a slower turning large prop for better efficiency.

That's why for a given size, as the Kv of the motor is increased the power handling of the motor increases.

Marc

[Bobmic]

That's why for a given size, as the Kv of the motor is increased the power handling of the motor increases.

..but higher Power motors usually have lower Kv (beside scorpion and others maybe)
Is there a prop efficiency calculator?
So for a P51 or similar that can handle both Power 60 400kv at 7s vs Power 90 8s 325kv will the actuall output power for a given prop be higher on the power 60?
Since the current ratings in the 60 are 10% higher it means that even at 6s it better. Now adding to the confusion why would someone use the power 90?
Can I assume the Pout in eCalc refers to to the power the prop will generate ? In that case can I use this data as expected power? In that case the numbers for the power 90 are better....only to mess up my thinking

[MarcV]

That's why for a given size, as the Kv of the motor is increased the power handling of the motor increases.

..but higher Power motors usually have lower Kv (beside scorpion and others maybe)

I'm not 100% sure, but higher power motors are usually powered by higher voltage packs and spin larger diameter props, hence the need for lower Kv.

Marc

[Tom]

I think the best way to look at it is to compare it to a car.  A car may come with 2 engine options - a 4 cylinder with 200HP and a V6 with maybe 220HP.  The HP difference is very little but they will have very different characteristics.  The larger engine will have more low-end grunt and typically a lower redline.  The 4 cylinder will need to spin faster to create that same HP because it will have lower torque (Power = Torque x RPM).

I am just learning about prop selection (actually i'm still learning how to glue balsa) but it seems to me that the prop would dictate which motor you choose.  For example, a 60 size motor may happily spin a 2-blade prop in a particular plane but you might want to switch to a 90 for a 3-blade prop in the same plane.

[Bobmic]

Hi Tom,

Thank you!! I like your comparison.

Bobby

[sihinch]

My understanding is that 3 blade props are less efficient than 2 blade props.  Not sure if this is correct for larger props too......

[Finnigan]

Hello,

A 2-blade prop needs to have longer blades to give the same thrust as a 3-blader, so its tips run faster through the air, making more noise. This also reduces ground clearance, making a prop strike more likely. However, it is simpler to make and lighter, which can be critical for nose heavy aircraft.

Its all about how much "lift" which acts as thrust the propeller can output. 1 blade would be more efficient, however 4 blades are quite inefficient as the airflow they encounter is quite turbulent. Thus reducing the "lift" the prop creates.

The angle of twist in the prop is probably the most important feature, not number of blades, im quite sure vari pitch props act different however im not sure to what extent.

[Ededge2002]

Hello,

A 2-blade prop needs to have longer blades to give the same thrust as a 3-blader, so its tips run faster through the air, making more noise. This also reduces ground clearance, making a prop strike more likely. However, it is simpler to make and lighter, which can be critical for nose heavy aircraft.

Its all about how much "lift" which acts as thrust the propeller can output. 1 blade would be more efficient, however 4 blades are quite inefficient as the airflow they encounter is quite turbulent. Thus reducing the "lift" the prop creates.

The angle of twist in the prop is probably the most important feature, not number of blades, im quite sure vari pitch props act different however im not sure to what extent.

Great post!

Funny thing is it has no input to the original question. The statement with the most assuring information is " however im not sure to what extent."

[Finnigan]

Hello,

A 2-blade prop needs to have longer blades to give the same thrust as a 3-blader, so its tips run faster through the air, making more noise. This also reduces ground clearance, making a prop strike more likely. However, it is simpler to make and lighter, which can be critical for nose heavy aircraft.

Its all about how much "lift" which acts as thrust the propeller can output. 1 blade would be more efficient, however 4 blades are quite inefficient as the airflow they encounter is quite turbulent. Thus reducing the "lift" the prop creates.

The angle of twist in the prop is probably the most important feature, not number of blades, im quite sure vari pitch props act different however im not sure to what extent.

Great post!

Funny thing is it has no input to the original question. The statement with the most assuring information is " however im not sure to what extent."

Thank-you for taking the time to post as well, although I'm not sure what value or purpose it served.

I was answering the previous poster's question regarding props, however you would be ill advised to have a discussion regarding motor performance without including prop configuration and dynamics as well.

[Michael]

It's great to have such enthusiastic discussion.

By the way Finnigan, you seem to have a good knowledge about the hobby.

Are you a member of TEMAC?