Help! Picking an appropriate motor, propeller, and ESC

[TheRealSeal]

This may not exactly be the right place for this, but since you guys hopefully know a lot more about electric motors than me maybe I can get some answers. This is kind of a loaded question, but if I were to go about scratch building a plane (not an FT plane), how can I know what kind of power train to put into it? I’ve looked at several RTF planes with similar size and power that have varying KV rating for the motors yet similar ESCs and propellers but completely different battery recommendations. Can someone lay down some general calculations I should use because I am totally lost.

 

[Merv]

The amount of thrust needed from the motor & prop depends on the weight of the plane and how you want to fly. A slow flying trainer, the thrust will need to be 60-80% of the weight of the plane.
A typical plane, 80-100%. A war bird or one with a little zip will be 100-150%. A 3D plane will need 150-200% thrust to plane weight.

Here is a FT thread with more information.

Thrust vs Power to Weight Ratios for Prop Planes

I'm scratch building the FT Cruiser and would appreciate all the help that anyone can provide in designing the power system from the ground up. I know that I can use the parts specified by Flite Test but I really want to learn the fundamentals for future builds. I'm not into 3D flying yet so I...

forum.flitetest.com

Watts per pound is another useful guideline. You'll need to weigh the plane and estimate the weight of the motor and battery.

• 50 to 70 watts per pound is the minimum level of power, good for park flyers and lightly loaded slow flyers.
•70 to 90 watts per pound is perfect for trainers and slow-flying aircraft.
•90 to 110 watts per pound is good for fast-flying scale models and some sport aerobatic aircraft.
•110 to 130 watts per pound is what you want for advanced aerobatics and high-speed aircraft.
•130 to 150 watts per pound is needed for lightly loaded 3D models and ducted fans.
•150 to 210+ watts per pound gives unlimited performance for any 3D model.

When it comes to Kv, think transmission. High Kv generally use smaller diameter higher pitch props. Resulting in higher top end speed and lower thrust at low air speed. Just like a high gear in a transmission. Lower Kv generally use larger diameter lower pitch props. Resulting in lower top end speed but greater thrust at low air speed. Just like a low gear in a transmission.

Two motors can output the same power (watts) but at different air speeds. A race car and a tractor may both have the same horsepower but at different speeds.

 

[TheRealSeal]

The amount of thrust needed depends on how you want to fly.
A slow flying trainer, the thrust will need to be 60-80% of the weight of the plane.
A typical plane, 80-100%. A war bird or one with a little zip will be 100-150%. A 3D plane will need 150-200% thrust to plane weight.

Here is a FT thread with more information.

Thrust vs Power to Weight Ratios for Prop Planes

I'm scratch building the FT Cruiser and would appreciate all the help that anyone can provide in designing the power system from the ground up. I know that I can use the parts specified by Flite Test but I really want to learn the fundamentals for future builds. I'm not into 3D flying yet so I...

forum.flitetest.com

Watts per pound is another useful guideline. You'll need to weigh the plane and estimate the weight of the motor and battery.

• 50 to 70 watts per pound is the minimum level of power, good for park flyers and lightly loaded slow flyers.
•70 to 90 watts per pound is perfect for trainers and slow-flying aircraft.
•90 to 110 watts per pound is good for fast-flying scale models and some sport aerobatic aircraft.
•110 to 130 watts per pound is what you want for advanced aerobatics and high-speed aircraft.
•130 to 150 watts per pound is needed for lightly loaded 3D models and ducted fans.
•150 to 210+ watts per pound gives unlimited performance for any 3D model.

When it comes to Kv, think transmission. High Kv generally use smaller diameter higher pitch props. Resulting in higher top end speed and lower thrust at low air speed. Just like a high gear in a transmission. Lower Kv generally use larger diameter lower pitch props. Resulting in lower top end speed but greater thrust at low air speed. Just like a low gear in a transmission.

Two motors can output the same power (watts) but at different air speeds. A race car and a tractor may both have the same horsepower but at different speeds.

How or where can the motor wattage be found? All I ever see on the electric motors are the barrel dimensions and kv rating. Do I have to calculate that based on the battery and ESC as they’re supplying the power?

 

[Merv]

How or where can the motor wattage be found?...

It’s in the description or specs of the motor, the exact location will vary from vendor to vendor. Sometimes it in the text, sometimes in a table.

Here is one example from FT

 

[TheRealSeal]

It’s in the description or specs of the motor, the exact location will vary from vendor to vendor. Sometimes it in the text, sometimes in a table.

Here is one example from FT

Okay, so let’s use this as an example. This is the motor that comes stock in the Dynam Waco, which weighs roughly 3.5lbs. This states it has an output of up to 630 watts (not sure if that electrical or mechanics though). So would that mean this would have a ratio of 180? How does the battery affect this with different cell number, mAhs, and discharge ratings?

 

[Merv]

...So would that mean this would have a ratio of 180?...

Yes, this setup would be about 180 the watts per pound.

...How does the battery affect this with different cell number, mAhs, and discharge ratings?

Im guessing the mfg of this motor would recommend a 2-4 cell battery. On the 2S use the 12 inch prop. On the 4S use the 9 inch prop. The 4S setup will be the fastest.

The battery capacity (mAh) will determine how long you can fly. The more mAh the longer the flight, up to a point. At some point the flight time will max out due to the additional weight of the battery. Example: If a 1000 mAh battery will give you 6 minutes of flight then a 2000 battery may give you 10 minutes and a 3000 maybe 12 minutes.

The amps a battery can supply are calculated, the formula is mAh/1000 x C = Amps. Example a 2,000 mAh 20C battery can supply 40 amps (2000/1000 x 20 = 40). The problem, in the past few years C ratings have become highly exaggerated.

Here is some more information