Help! Propeller size vs battery vs motor

[rgc213]

I'm going to be scratch building the Long EZ next week and wanted to use an engine I have laying around to power it. It's a crack series 2305 1500kv motor.
I was wondering what prop size I should use with it, using a 2s battery?

Also, in general, what is the relationship between the kv of a motor, the prop size and the battery cells? I've always just used what FT has suggested, but I would like to know why I'm using it, so that I may be able to scratch build my own stuff and size the engine/prop with it correctly.

thanks,
reub

 

[DamoRC]

I'm going to be scratch building the Long EZ next week and wanted to use an engine I have laying around to power it. It's a crack series 2305 1500kv motor.
I was wondering what prop size I should use with it, using a 2s battery?

Also, in general, what is the relationship between the kv of a motor, the prop size and the battery cells? I've always just used what FT has suggested, but I would like to know why I'm using it, so that I may be able to scratch build my own stuff and size the engine/prop with it correctly.

thanks,
reub

Specs from the twisted hobby website - is this your motor?
"Our Recommended setup:

• 2s w/ 8x4.3 or 9x4.7 Prop

Optional setup, not recommended unless watt tested and prop/hubs are true and balanced.

• 3s w/ 8x4.3 Prop * Please Note * 9x4.7 is not recommended when running with 3s lipo's
• 3s Lipo's this can damage motor or esc.

Specs:

• Constant Amps = 10 / 20 Burst
• 2s 60 watts /3s 120 watts"

They also say "can be used for any 6-12oz foamie model " so given that the FT Long EZ weight without battery is 5.5 oz, you should be good with this motor and their recommedation of 8x4.3 or 9x 4.7 when using a 2S battery.

There is no real general relationship between the Kv, cells, and propeller size because you can have "weaker" or "stronger" motors that have the same Kv. Typically checking the published specs for your motor is a good place to start.

There are also some rules of thumb around the watts per pound which can be informative but not everyone agrees with these. If you are putting 60watts into your Long EZ and say the battery brings it up to 8 ounces flying weight then you will be at approx 120 watts per pound. According to one version of this watts per pound rule - you will be in the sport flying to aerobatics flying region.

• Less than 50W/lb - very lightweight / low wing loading slow flyer.
• 50 to 80 W/lb - light powered gliders, basic park flyers and trainers, classic biplanes and vintage ('Old Timer') type planes.
• 80 to 120 W/lb - general sport flying and basic/intermediate aerobatics. Many scale (eg warbirds) subjects suit this power band.
• 120 to 180W/lb - more serious aerobatics, pattern flying, 3D and scale EDF jets.
• 180 to 200+W/lb - faster jets and anything that requires cloud-punching power!

DamoRC

 

[rgc213]

Thank you, this info is great!
I now see why my scratch built yak54 wouldn't work with 2s..... (it weighed in @ almost a lb, once I got it balanced)

Yup, that's my motor. 8043 will work for me.

.....but what would happen if I use, say, a 6x3 prop? Or a 5x3x3 prop? (With the same setup)

-reub

 

[ElectriSean]

Thank you, this info is great!
I now see why my scratch built yak54 wouldn't work with 2s..... (it weighed in @ almost a lb, once I got it balanced)

Yup, that's my motor. 8043 will work for me.

.....but what would happen if I use, say, a 6x3 prop? Or a 5x3x3 prop? (With the same setup)

-reub

Generally speaking, slower motors (low kv) spin larger props. Motors designed for smaller props are generally higher kv. That motor with a 5" prop wouldn't produce much thrust.

 

[DamoRC]

Thank you, this info is great!
I now see why my scratch built yak54 wouldn't work with 2s..... (it weighed in @ almost a lb, once I got it balanced)

Yup, that's my motor. 8043 will work for me.

.....but what would happen if I use, say, a 6x3 prop? Or a 5x3x3 prop? (With the same setup)

-reub

If you used the 6x3 prop you will just get less thrust because you are not taking advantage of the power capabilities of the motor. You could take this to the extreme and think about having no prop (motor will run fast and cool but you ain't goin' anywhere). The 5x3x3 is perhaps a little more complicated. I recently had to do this calculation for a Tigercat build. Here is quote from the thread:

"One approach for dealing with blade number conversions was described as calculating the prop load as D^3 x P x Sqrt (N-1) where D is the diameter, P is the pitch, and N is the number of blades (apologies for the math). So for the 12 x 4 2-bladed prop this works out as approx. 6900 (not sure what the units are supposed to be). I had a rough idea that a 9 inch 3-bladed prop would produce much the same load. Then I found 9 x 6.5 B-25 3-bladed replacement props at Motion RC. If you run the numbers on this prop, the load is 6700. Pretty close. To check the numbers I measured the static thrust, Amps, and RPM for the two props at full throttle.

12 x 4, 2-blade prop generated 4.4 lbs of thrust at 10400 RPM pulling 31 amps
9 x 6.5, 3-blade prop generated 3.4 lbs of thrust at 9700 RPM pulling 38 amps"

In your case - the 5x3x3 prop generates a load number that is about 1/4 that for the 8043 prop so the load on the motor will be a lot less and you are likely to get significantly less thrust. Additionally, increasing the numbers of blades on a prop makes it less efficient.

DamoRC

 

[rgc213]

Interesting.
I knew it would produce less thrust, but did not realize that the lower kv motors work better with bigger props, and vise versa.

With the motor, and recommended prop (8030), and the low weight of the long ez, it put me into the sport flyer/intermediate aerobatics category, so I might still try the smaller props (because I have them, and will still have to buy the 8030), and see how well it flys. I was hoping to get a slow flyer out of it anyway.

Thank you both, you have been a wealth of information.

-reub

 

[DamoRC]

Interesting.
I knew it would produce less thrust, but did not realize that the lower kv motors work better with bigger props, and vise versa.

With the motor, and recommended prop (8030), and the low weight of the long ez, it put me into the sport flyer/intermediate aerobatics category, so I might still try the smaller props (because I have them, and will still have to buy the 8030), and see how well it flys. I was hoping to get a slow flyer out of it anyway.

Thank you both, you have been a wealth of information.

-reub

Sounds like a plan - always worth experimenting with this stuff. The trick to a slow flyer is a light wing loading, so the smaller and lighter you can build, including the selection of battery and motor, the slower you can go. Let us know how the other props work out.

DamoRC

 

[Arcfyre]

In my experience, the best way to experiment with prop sizes and motors is to find a common ground, and that common ground is watts. Watts measure power, and to measure watts, you need a watt meter. They are extremely cheap at around $15, and take a lot of the guess work out of building your setups.

I got my nerd out a little while ago and bench tested some of my motors with different props. I then made an excel spreadsheet with the watt meters, Lipo cell count, and throttle position all recorded. This gave me an easy "cheat sheet" to reference for when I was putting a new plane together. I can then apply the "watts per pound" rule, as DamoRC mentioned, and have a pretty good idea of how my plane will fly before I even maiden it.

 

[rgc213]

Here's my update to the prop saga:

build went well, plane turned out looking pretty sharp:

I was origianlly going to use the 5x5x3 for the maiden, but forgot the motor was of the prop saver variety and I couldn't figure out how to use a 3 blade prop. I used an 8045 for the maiden instead.

It went ok.... the 8045 was way too overpowered for this plane, and it was fairly windy, so the plane was 200 yards away within 20 seconds of takeoff. Once it got almost out of sight, I "landed" it and went looking for it (flying site is 27 acres of scrub land. took about 15 mins to find).

Second flight did not go well at all...............

.....oh and, upon takeoff:

A trip to the urgent care and 3 stitches later, I'm back at my desk thinking about fixing and which prop I'll be trying next

Think I'll go with the recommended 6030, which I know is way underpowered, but may get the flight characteristcs I'm looking for

-reub

 

[DamoRC]

You are now initiated to the prop cut cult - welcome!

The 6030 is worth a try and you could also try the 8045 at significantly less throttle. It's always nicer to fly knowing you have the power of you need it.

DamoRC