Solved The right prop for my scratch built Rc plane

[Dawson]

I just finished building a huge 7.5 wingspan Rc plane and put the ft monster motor with the 80 amp Esc that comes with it on it. I’m thinking about getting a 6s battery to go with it but I don’t know which propellor size to use or find one that fits the motor shaft. I’m almost finished with it and I’m guessing I need to have the weight of the plane to find out what I need but I’m not finished and I didn’t order the battery yet.

 

[Merv]

First you need to determine if you have the correct size motor. Then the supplier of the motor should be able to give you prop recommendations. The size of the motor depends on how you want to fly. Here are some guidelines that may help. It’s the total flying weight of the plane that you need, including motor and battery.

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.

Once you have the size of motor. If you want to fly fast, pick a higher Kv motor with a lower diameter higher pitch prop. If you want to fly slower pick a lower KV motor with a larger diameter, lower pitch prop.

 

[Dawson]

So I have a 510 kv motor for this plane. I don’t need too much speed but need enough power. I’ve been wondering what pitch I would need but I just need a normal pitched prop to have power and some speed. Is this motor good for a higher pitched prop? If so what pitch? I’ll finished up the plane and order the battery then record the weight.

Thanks.

 

[FlyingTyger]

Running a very basic setup on eCalc, it looks like a 14x8 prop will pull about 65amps, produce about 11 pounds of thrust, and have a pitch speed of about 75mph.

 

[Dawson]

Running a very basic setup on eCalc, it looks like a 14x8 prop will pull about 65amps, produce about 11 pounds of thrust, and have a pitch speed of about 75mph.

If I have a higher pitch will it give the motor more thrust or will it give it more thrust and overload the motor?
Thanks

 

[FlyingTyger]

If I have a higher pitch will it give the motor more thrust or will it give it more thrust and overload the motor?
Thanks

Yes and no.
Increasing the pitch and holding the diameter will increase the load on the motor. But you will gain thrust and pitch speed.
If you increase pitch and decrease diameter, you can maintain the load on the motor. You will gain pitch speed but may lose some thrust.
Increasing the diameter and lowering the pitch will again maintain the load, but will increase thrust and lose pitch speed.

 

[Dawson]

Yes and no.
Increasing the pitch and holding the diameter will increase the load on the motor. But you will gain thrust and pitch speed. Here’s the propeller I thought might work-
If you increase pitch and decrease diameter, you can maintain the load on the motor. You will gain pitch speed but may lose some thrust.
Increasing the diameter and lowering the pitch will again maintain the load, but will increase thrust and lose pitch speed.

So if I use a 16 inch propeller something like a 16x7 propeller would work well for thrust for this motor? Here is a propeller I thought might work -https://www.amazon.com/gp/product/B07S1Z2T7B/?tag=lstir-20
I just measured the current weight of the plane without battery and landing gear and it weighs about 7 lbs. I think the battery is about 2.5 lbs so with landing gear I estimate it might be about 10-11-12 lbs when finished? Motor and Esc are in this measured weight. Would this propeller pull the plane well or do I need something else?
Also, what is pitch speed?
Thanks

 

[FlyingTyger]

So if I use a 16 inch propeller something like a 16x7 propeller would work well for thrust for this motor? Here is a propeller I thought might work -https://www.amazon.com/gp/product/B07S1Z2T7B/?tag=lstir-20
I just measured the current weight of the plane without battery and landing gear and it weighs about 7 lbs. I think the battery is about 2.5 lbs so with landing gear I estimate it might be about 10-11-12 lbs when finished? Motor and Esc are in this measured weight. Would this propeller pull the plane well or do I need something else?
Also, what is pitch speed?
Thanks

Based on what eCalc showed, you will exceed the limit of the ESC with that prop.
Pitch speed is the airspeed if the prop is at 100% efficiency.
eCalc can be a great tool for planning your power setups and is inexpensive. Here is a link to the site: Linky

 

[Dawson]

Based on what eCalc showed, you will exceed the limit of the ESC with that prop.
Pitch speed is the airspeed if the prop is at 100% efficiency.
eCalc can be a great tool for planning your power setups and is inexpensive. Here is a link to the site: Linky

Do you know what propeller would work for my motor and Esc while getting enough thrust? I’ve tried using that calculator before but it doesn’t let me choose the options I need to put

 

[FlyingTyger]

Do you know what propeller would work for my motor and Esc while getting enough thrust? I’ve tried using that calculator before but it doesn’t let me choose the options I need to put

A 14x8 will produce about 11 pounds of thrust.
The approximate weight of your plane and the desired performance will dictate how much you need.

 

[Dawson]

A 14x8 will produce about 11 pounds of thrust.
The approximate weight of your plane and the desired performance will dictate how much you need.

And is that good to work for my motor and Esc? I have to wait until January 12 for my battery to come in the mail and then I’ll record the weight.
Thanks

 

[Dawson]

A 14x8 will produce about 11 pounds of thrust.
The approximate weight of your plane and the desired performance will dictate how much you need.

Also, the shaft diameter of my motor says it’s 8mm and it can fit virtually any propeller with 8 mm hole. Do I need one with a 8mm hole?

 

[FlyingTyger]

The 14x8 should be good for your motor and esc.
The prop doesn't need to have an 8mm hole, most come with shims to adapt them to the size shaft you have. But if the props hole is smaller than 8mm, you will need a prop reamer to open it up.

 

[Dawson]

How do I know if it has those adapters? Do you know of a prop that will work on Amazon?
Thank you

 

[FlyingTyger]

Typically the plastic electric props will come with the shims. However, most of the props I am finding in this size have a 6mm shaft hole. You would need a reamer to make it bigger. I know some have used a drill, but it is really easy to make the hole angled and/or off-center doing it that way.
It seems to me that the ESC is the limiting factor, maybe someone with firsthand experience with this motor can share what they used.

 

[Dawson]

Typically the plastic electric props will come with the shims. However, most of the props I am finding in this size have a 6mm shaft hole. You would need a reamer to make it bigger. I know some have used a drill, but it is really easy to make the hole angled and/or off-center doing it that way.
It seems to me that the ESC is the limiting factor, maybe someone with firsthand experience with this motor can share what they used.

So how do I get a reamer? Is it like a huge machine or is it just something you can put on an electric drill?

 

[FlyingTyger]

There may be others out there, but here's the first one that popped up on amazon.
Linky

 

[Dawson]

Ok so I’d have to buy a 14x8 propeller with a 6mm shaft diameter with that reamer you found. I’ll be searching here too but are there any that do just 8mm? And if my motor is 8mm shaft does that mean the propeller needs to be 8mm?

There may be others out there, but here's the first one that popped up on amazon.
Linky

Ok so I’d have to buy a 14x8 propeller with a 6mm shaft diameter with that reamer you found. I’ll be searching here too but are there any that do just 8mm? And if my motor is 8mm shaft does that mean the propeller needs to be 8mm?

 

[FlyingTyger]

Yes, you can get a reamer that will just do 8mm. The stepped prop reamers are just handy because they will do multiple sizes.
Yes, the prop will need to be 8mm to fit on the motor shaft.

 

[Dawson]

Yes, you can get a reamer that will just do 8mm. The stepped prop reamers are just handy because they will do multiple sizes.
Yes, the prop will need to be 8mm to fit on the motor shaft.

Alright thanks I’ll keep looking for an 8mm reamer. One more thing, can the reamer cut wood and plastic/carbon fiber? There’s a lot of wood propellers on Amazon and I think they look really cool.