Various prop and battery size reccomendations

[mmeyer]

I was wondering what some good sizes for props would be which i can use on a range of different planes. I would like to be able to swap different props in and out depending on what i want and i was wondering what some common sizes would be to purchase. I would also like to have a range of batteries for different planes and was wondering what were some good sizes for them. I am using 18A speed controllers but obviously can upgrade. At the moment i have a 1400kv Brusless outrunner (http://hobbyking.com/hobbyking/store/__20627__2205C_1400Kv_Brushless_motor_AUS_Warehouse_.html) and a 1300kv 24g hexTronik (http://hobbyking.com/hobbyking/stor...rushless_Outrunner_1300kv_AUS_Warehouse_.html)
Thanks!

 

[earthsciteach]

Props, motors and speed controllers are matched, ideally. Using too large a prop on a motor will result in a burned out motor and/or esc. Using too small a prop will cause the plane to be underpowered. You really need to match the motor/esc/prop/battery to the size and desired performance of a given plane.

 

[earthsciteach]

Here is a resource for understanding how to select power systems. Making that selection can be quite overwhelming!

http://www.wattflyer.com/forums/showthread.php?t=18521

 

[mmeyer]

Thats not to say that i cant put different props on a motor though?

 

[earthsciteach]

No, but the range is limited. Both of those motors suggest 8x8.3 prop. What do you have in mind?

 

[FlyingMonkey]

You can always put a smaller prop than suggested on but it will be less and less efficient the smaller you go. The larger the prop is, the more work it puts on the motor. After a certain size, it will either burn out the motor, or the speed controller. Sometimes both.

The recommended prop has been tested to get make the most efficient use of the motor it's being recommended for.

 

[mmeyer]

At the moment i have 8x4 props. What then would be a good motor for say a 6x4 or similar? And do you have any good combos which i could put in more than one plane?
Btw thanks for all the replies If you havent already noticed i am new to the hobby so any help is greatly appreciated

 

[earthsciteach]

You really need to approach it from a "match the motor to the plane" standpoint. Coming from the other direction is WAAAYYYY too open-ended. If you can post a given plane or design, we can make recommendations.

 

[mmeyer]

Fair enough. This is the plane i intend to build next: http://www.rcgroups.com/forums/showthread.php?t=1174874
My current plane is much like the one built in this series: http://www.youtube.com/watch?v=hWIZVOmgxj0
The power combo i have for this plane i feel is a bit anemic as i always have to run at full throttle just to keep it in the air.
I would also like to build some of the planes in the swappable series at some point in the future.

 

[FlyingMonkey]

You really need to approach it from a "match the motor to the plane" standpoint. Coming from the other direction is WAAAYYYY too open-ended. If you can post a given plane or design, we can make recommendations.

Chad was telling me that he's a "pick the prop first, then find a motor to match" kind of guy.

Weird. I always picked the plane first, then went from there...

 

[FlyingMonkey]

When picking the motor for a project, I first try to determine how heavy my finished plane should be.

The "watts per pound rule of thumb" is roughly as follows...

Quote:

50-70 watts per pound; Minimum level of power for decent performance, good for lightly loaded slow flyer and park flyer models 70-90 watts per pound; Trainer and slow flying scale models 90-110 watts per pound; Sport aerobatic and fast flying scale models 110-130 watts per pound; Advanced aerobatic and high-speed models 130-150 watts per pound; Lightly loaded 3D models and ducted fans 150-200+ watts per pound; Unlimited performance 3D and aerobatic models

So, if I have a project that is going to weigh 24 oz and I wanted to fly aerobatic, I'd look for a motor and prop combo that would provide me with about 150 watts of power. Since I'm wanting more thrust than speed I'd go for a combo that lists a larger diameter propeller. I'd want a prop with a wide spread between the pitch and the diameter, for example, an 8x4 verses an 5x5.

 

[mmeyer]

Ok so here are my calculations.
My plane weighed in at 0.79 pounds. I want my plane to fly in the "70-90 watts per pound; Trainer and slow flying scale models" category. So lets take the average and say 80 watts per pound. That means that if my plane is 0.79 pounds i should multiply 80 watts by 0.79:
80 x 0.79= 63.2 watts per pound. So i would be wanting to get a power combo with 63.2 is watts correct? And as i am a learner i would probably want more thrust than speed so going with a larger prop with a shallower pitch would be better, ie 8x4?
As i said before i was always flying at full throttle. Do your numbers mean that it will be flying like a trainer or slow flying scale model only at full throttle? Could i upsize and just fly the plane on 1/2 throttle or something like that?

 

[aiidanwings]

Chad was telling me that he's a "pick the prop first, then find a motor to match" kind of guy.

Weird. I always picked the plane first, then went from there...

Me too. Plane, then prop, then motor, then ESC (not really, I use 80amp is almost everything) then battery.

 

[FlyingMonkey]

Yes, at the low end of the watt scale you're flying at full power. You can certainly put in a larger motor, and fly at half throttle, but have the rest of the throttle curve to get you out of trouble.

It looks like you have a good grasp on it. Treat the plane like it's a full pound, and that should give you plenty of reserve power.

 

[mmeyer]

If the motor doesn't say what it's "max power" is, what can i do to work it out?