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Motor Sizing

alan0043

Active member
#1
Hi Everyone,

I have some questions about knowing what size motor to use on a FT design that has been enlarged. The plane that I am talking about is to enlarge the Tiny Trainer to use a full size power pod not the mini pod. When you enlarge a plane design how do you know what size motor to use ? Is it based off the wing size ? Or is it something else that determine motor size ? I would like to use the 'B' pack motor. I am open to all comments. I have never done this before. I am not sure what percentage to enlarge the Tiny Trainer plans. Open for thoughts here also. I would like to have a plane that has a wing that is over the fuselage and is held in place with rubber bands.

Open to all input,
Al
 

Grifflyer

WWII fanatic
#2
Hi Everyone,

I have some questions about knowing what size motor to use on a FT design that has been enlarged. The plane that I am talking about is to enlarge the Tiny Trainer to use a full size power pod not the mini pod. When you enlarge a plane design how do you know what size motor to use ? Is it based off the wing size ? Or is it something else that determine motor size ? I would like to use the 'B' pack motor. I am open to all comments. I have never done this before. I am not sure what percentage to enlarge the Tiny Trainer plans. Open for thoughts here also. I would like to have a plane that has a wing that is over the fuselage and is held in place with rubber bands.

Open to all input,
Al
Your motor size will be determined by the weight of the plane, and how many watts you want per pound.

If you enlarge the tiny trainer to around a 35-40 inch wingspan, the B pack motor should suit it well.
 

Merv

Well-known member
#3
I agree with @Grifflyer, look at the total weight of the plane in the air and how do you want to fly.

• 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.

If you want to go fast, select a high Kv motor, 1,800+. If you want to fly slow or 3D select a low Kv motor, 1,000 +/-. After I select the power plant, I look at the mfg recommend props at the cell count I intend to fly.
 

alan0043

Active member
#4
I agree with @Grifflyer, look at the total weight of the plane in the air and how do you want to fly.

• 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.

If you want to go fast, select a high Kv motor, 1,800+. If you want to fly slow or 3D select a low Kv motor, 1,000 +/-. After I select the power plant, I look at the mfg recommend props at the cell count I intend to fly.
Hi Merv,

Thank you for the chart. That is going to be a be help.

Al
 
#5
I agree with @Grifflyer, look at the total weight of the plane in the air and how do you want to fly.

• 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.

If you want to go fast, select a high Kv motor, 1,800+. If you want to fly slow or 3D select a low Kv motor, 1,000 +/-. After I select the power plant, I look at the mfg recommend props at the cell count I intend to fly.
Merv
How do I figure out what watts my F pack set up ( Radial 2205 2300Kv motor with 3S 850 mAh battery and 6x3 prop) will be? I know what my plane weighs but without a watt meter I don't know the watts.
 

Grifflyer

WWII fanatic
#6
Merv
How do I figure out what watts my F pack set up ( Radial 2205 2300Kv motor with 3S 850 mAh battery and 6x3 prop) will be? I know what my plane weighs but without a watt meter I don't know the watts.
That setup will make around 210 watts at full throttle.
 

Merv

Well-known member
#7
Merv
How do I figure out what watts my F pack set up ( Radial 2205 2300Kv motor with 3S 850 mAh battery and 6x3 prop) will be? I know what my plane weighs but without a watt meter I don't know the watts.
It will say on the box somewhere, watts = amps x volts. It may also say on the website, sometimes the actual motor in the F pack will change. Your F pack motor may not be the exact same as mine that I got 2 years ago.

Getting the exact watts is not important, just get close, plus or minus 15% or so.
 

Merv

Well-known member
#9
Thanks Grifflyer. That puts it at the top of the scale based on Merv list so if I want to use it in the trainer I need to run it at half throttle or less.
You could also use a smaller prop. Smaller props produce less thrust and pull fewer watts.
 

evranch

Well-known member
#10
Once it comes down to prop sizes and thrust numbers the best resource is eCalc. It costs several dollars, which are recovered the first time you don't buy the wrong prop or motor.

You can spend the whole evening looking at sizes and pitches depending how fast you want to fly or the target TWR. Sometimes a large prop at half throttle is better than a small prop at full throttle. Sometimes it's the other way around. Sometimes what sounds like a great cruise prop isn't capable of takeoff thrust levels!

The free trial is also useful for ballpark numbers but the guy is constantly researching new motors and props, throw him the dollar for his effort.