Propellers...for beginners - please keep it simple

[Mr.B.]

Total agreement. Apologies for grumpiness (I'm at work)

[Ed Anderson]

I will add a few thoughts.

We need to look at the motor specs (volts/amps/kV) the prop and the battery as a system.

As a general rule smaller props with deeper pitch will give you more speed than wider props with less pitch even though they might consume the same watts.

Think high gear vs. low gear. You go into high gear for steady speed cruising on the highway. You would go into low gear to get up to speed quickly. If you came away from the light in high gear the car would accelerate very slowly assuming you did not stall the car's engine, but would eventually get to that high speed for the highway.

So, are we gearing up for speed or low end power?

We need to look at goals. As I discuss this, think about cars or bicycles and gears. We can't change gears on planes so we set-up for the goal we are working toward.

A pylon racer is looking for top end speed on the flat more than low end acceleration for steep climb. We would probably go for a higher kV motor that spins a smaller deeper prop faster for high pitch speed.

A sailplane is looking for low end power to climb at a steep angle to get to height. We would normally select a lower kV motor that can spin a wider prop slower for climbing power even though the top speed may not be very high.

Both motors might be rated for the same wattage but they deliver the power in very different ways.

Lower kV motors usually receive manufacture recommendations for wide props and are optimized for low end power.

Higher kV motors usually receiver manufacturer recommendations for narrow props that are optimized for high end speed.

 

Edit:

I mentioned the battery as part of the system but left that out of the discussion.

If a given motor is rated for 300 watts we can achieve that power level in several ways based on battery voltage and prop size.

Often the motor specs will express a voltage range by listing 2-3S lipo and a prop range, for example 9X6 to 11X8 prop.

People miss read this as that they can put an 11X8 prop on this motor with a 3S lipo and that is good.   More likely what the specs tell us is that you should use an 11X8 prop on a 2S lipo and a 9X6 on a 3S lipo.   Both will result in a power draw within the motor specs but they achieve it differently and produce different results.

The 2S is spinning a larger prop more slowly.

The 3S is spinning a smaller prop faster.

Going back to the descriptions I provided above thin of 3S for the pylon racer and the 2S for the sailplane.

Edited By Ed Anderson on 29/09/2014 14:36:04

Edited By Ed Anderson on 29/09/2014 14:38:00

[Peter Beeney]

I think I’m inclined to take a slightly different slant on some of this. When I look at an electric motor I’d consider that the kV figure represents the unloaded speed that the motor will obtain for a given applied voltage. (Actually it means something slightly different, but for our purposes it easily transposes to revs per volt.) As soon as we put any propeller on the drive shaft this figure will decrease and the current will rise in a direct ratio. Therefore as we increase the load, by increasing the size of the prop, the current flow will proportionally increase.

The load, the revs, the amount of current flowing and thus the torque are all directly linked, if one is altered then the others will change in proportion.

I certainly agree that every situation needs some consideration of what type of motor is required, but I also like to consider the propeller as well. As an example, a small foam model with a black plastic flexible 9 x 6 prop on has a current flow of 19 amps. Changing this for an APC 9 x 6 i/c version reduced this flow to 13.5 amps. Now the motor turns faster, which straightaway makes for better flying, and the battery capacity lasts a bit longer, too. The power output from the battery, which we measure in watts, does not always translate directly to the performance at the propeller. The caveat, of course, is the belly landing, the flexible prop is the compromise for any less than perfect gentle touch downs.

Being even a little more pedantic, a joule is the energy expended when a force of one Newton is exerted through a distance of one metre; it’s also the energy expended when a current of one ampere passes though a resistance of one ohm for one second; a watt is the power when energy is expended at the rate of one joule per second. But I’m sure that this stuff is generally not of very much interest or importance anyway, and as has been said in pervious posts, Power is simply Energy divided by Time… …as far as modellers are concerned, watts are volts x amps…

Just another sideways view…

PB

[fly boy3]

Thanks all for all the info. Quite a bit to read up on. I do like Steve,s analogy of a ww1 model and a pylon racer. LoL I agree also the type of model and how it is being flown comes into the equasion. Can anyone give me some figs. on kv, as to what is a slow, sport model, pylon racer. ? IE 1000kv would be slow ? Thanks

 

Edited By fly boy3 on 30/09/2014 12:30:23

[Steve Hargreaves - Moderator]

Its not just the kv but the battery voltage you are going to use as well....1000kv on a 2S battery would be pretty slow but it would get a lot more exciting on say 6S...

At the risk of sticking my neck out I'd offer the following as a general start point for general sports models....

Battery Voltage	kv range	Suggested prop	Typical Power (Watts)
2S (7.4V)	1300 - 1600	6 - 8”	Up to 150
3S (11.1V)	900 - 1200	9 – 11”	Up to 450
4S (14.8V)	600 - 800	12 – 14”	400 to 800
5S (18.5V)	450 - 600	15 – 17”	800 to 1000
6S (22.2V)	300 - 450	16 – 19”	900 to 1500

 

 

Hope that helps.....

PS.....the higher suggested kv figure would tie in with the lower prop size....& vice versa of course....

Edited By Steve Hargreaves - Moderator on 30/09/2014 14:42:00

[Ed Anderson]

Good chart.

One note on kV. There is no direct correlation between kV and the watt capacity of a motor. You can have a 75 watt 1000 kV motor and you can have a 500 watt 1000 kV motor.

What would be the difference? The size of the wire that is used to wind the motor. Larger wire can carry more current and produce stronger fields and more powerful motors. But it also weighs more.

So, in the end a motor's power is measured in Watts, always watts.

kV is what you use to adjust for prop size and battery voltage.

While kV applies to inrunners in the same way as outrunners, inrunners typically reside in the higher kV ranges.

Inrunners, can drive a prop directly but are often teamed up with a gearbox which effectively lowers the net kV rating at the prop. This is common in helicopters, gliders but can apply to airplanes too. While you can team an outrunner with a gearbox typically gearboxes are teamed with inrunners.

[fly boy3]

Thanks Steve and Ed,as Ed says good chart, it does help. Funny after flying ic for over 25years I never thought of revs of motors. Looked at the model, thought this could use a 25, 40, 60, 90 and so on. Could never afford brand new stuff, but I would look up info of new motor and certain size prop would be offered for the motor. I would then change diam. or pitch for my style of flying. Still got a lot to learn with electric flight or "fright" as I once read. LoL Cheers and thanks.

Edited By fly boy3 on 30/09/2014 16:45:56

[Ed Anderson]

fly boy3,

 

We are all in the same boat. When you know something it seems easy, even obvious. When you don't it seems difficult and complicated. Remember learning to ride a bicycle?

If I had to buy a glow or gas motor for a plane I would be lost. sizes, sleves, tuned pipes, 5%, 10% 15%, oil? and all that glow plug stuff, sticks, starters, changing CG as the fuel goes down, a clink on the fuel line ..... black magic. And fuel proofing, vibration considerations ... really?

 

Tune before a flight?  Tune what?  I tuned it last time.  Rich?  Lean?   Choke?

 

Give me a nice simple electric set-up.

Watts per pound target

Target battery or target prop.

Use one of the calculators and it becomes trivial.

Once it is installed it works every time. 

 

Don't know how you fuel guys ever got them into the air.

 

Edited By Ed Anderson on 01/10/2014 14:17:07