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QUESTION about prop(s) ??

#1
Hello to ALL...
I have built several Flite Test planes, and even flown a few, some into the air - and some into the ground!!
My question is - if I have a simple plane, lets say a Piper Cub and an average / proper sized electric motor for such, and the plans call for a 8 inch prop.
If a 6 inch prop is used, what will the most likely results be? AND also what about if a 10 inch prop is used?
I think I understand power / speed VS / torque - but can someone help me out? I like to build and attempt to fly my own plane designs, and must guess on prop sizes, etc...

Please keep it simple laymen's terms, as I am a simple minded 62 y/o that's medically retired - time on my hands, but get bored pretty quickly... LOL

Thanks in advance to ALL / any responses!!

Jim = stainless45
 

FDS

Well-known member
#2
Prop size is determined by motor choice and required thrust plus efficiency considerations.
The way to roughly work it out is to determine the all up weight of the finished item, then look at how much thrust per lb or thrust/kg you want, this determines if it will be a gentle slow flyer with a big slower prop or a faster,ore acrobatic plane with a smaller, faster prop. There’s an approximate table of thrust to weight that shows how it might handle.
The top and bottom prop size will be determined by how physically big the plane is.
You then choose a prop and motor combo to achieve desired level of thrust and handling. In general terms a larger, steeper pitch prop will want to turn slower and will produce less overall speed and a smaller, finer pitch prop will use a higher KV motor to spin faster which will make the model fly quicker.
There are several calculators that model builders can use to select motors, props etc.
Efficiency I don’t know much about, someone clever from here will help with that.
 

jaredstrees

Well-known member
#3
What FDS said. Basically you need to size the motor to the plane first. A small 1804 motor is not going to fly a simple cub, and a 450 size motor would turn a mini sized plane into a brick. Suppliers generally will post motor data, which will give you how much thrust a particular motor will provide with different props. Once you know how much your plane weighs, you can select a motor that provides enough thrust to fly. At that point you select a prop within that motors specs. If you go too far outside a motors specs on amps, etc, you'll risk frying it. Putting too large a prop on will kill a motor, too small and you probably just won't have enough thrust to fly.

Good luck!
 

Merv

Well-known member
#4
I agree with @FDS and @jaredstrees, the general rule of thumb. If you want to fly fast, turn a small prop very fast (high KV & high volts). If you want to fly slow or 3D, turn a big prop slowly (low KV).

the plans call for a 8 inch prop...a 6 inch prop is used, what will the most likely results be? AND also what about if a 10 inch prop is used?
If you turned the 6" prop very fast (3,000kv), the plane would be very fast. If you turned the 10x3 prop slowly (1,000kv) you would have much better vertical.

I have an FT3D with a 12x3.8 slow fly prop, a 3730 - 1,000kv motor on 3S. Had to make the gear a bit longer so the prop would clear. It has unlimited vertical and will hang from the prop as long as I can hold it there.
 

Merv

Well-known member
#5
Please keep it simple laymen's terms,
Look at the data sheet form the seller. They should give you thrust data with various props. If you want to 3D, look for a combination that has 50% to 100% more thrust than your plane will weigh. Also look at a similar size ARF, see what they are using and copy it.
 
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Merv

Well-known member
#7
I agree with @Bricks, a watt meter is an invaluable tool for finding the right prop. Don't want too much prop and burn out your motor or ESC. Always give you self a little head room.
 
#9
Thank you to everyone that chimed in, and overwhelmed me even further!!! LoL

I built the FT Bushmaster and used the power pack "C" (EMax GT 2215/10 @ 1100kv) with a 30A ESC using a 10"X4.5 slow fly prop.
I recently upgraded to a Turnigy D2826-6 @ 2200 kv with a 40A ESC, with the same 10"X4.5 prop. Both power systems are run with a 3 cell 1500 to 2200.

While the weather got bad in the N.E. corner of Kansas, so I didn't get a chance to try the new setup. Hopefully in the spring?

It looks like I am going to have to stick with buying / building known tried & true airplanes, and step away from the "experimental" unknown - "do you think it will fly?" types that I have been building lately...

Thanks again, my fellow feathered flyers.

Jim=stainless45
 

FDS

Well-known member
#11
You won’t have to step away from experimenting, you will just need to weigh your creations when done then look for a motor with the thrust needed to lift it, then pick the optimum prop for that motor.
If you have enough thrust to overcome the all up weight, then it will go up/forward, the rest will depend on how big the control surfaces and wing are!
If you are buying the cheaper motors that don’t always have prop specs on them you can compare them to a more expensive one with identical or very similar specs that does and go somewhere in the middle.
Also higher KV, as noted above, with all things equal, wants a smaller prop. A larger prop will be less efficient and increase the motor load, without much gain in thrust.
 
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Bricks

Well-known member
#12
If you are going to try that Turnigy 2200 KV motor with a 10X4.5 prop good luck, might get the smoke of death. With out a Watt meter to see what the motor is pulling for amps.
 
#13
OK - One more question about props...

Let's say I go to a 8"X4.5 two blade prop. What would happen if I tried a 8" three OR four blade(s) props??
Should there be a difference in thrust, climb rates, etc??

BTW: I have used this motor & ESC combo upgrade in my Millennium Master with a 9"X4.5 prop. The same prop that came with the plane, and no problems - (so far!)

Once again, many thanks!!!
 

FDS

Well-known member
#14
Usually multi blade propellers are selected with reduced size VS the two blade equivalent, for the same reasons in terms of efficiency and motor loading. Pushing more prop at the same speed range isn’t always a guarantee of more thrust. The maths supports a reduction in blade size for more blades.
There’s a lot of different forces at play in props. The biggest thing that seems to happen with multi vs two blade is that multi blade props are much less efficient. This impacts battery duration and performance. Good article here, with real world data.
Personally I am sticking to two blade props for my builds, despite them not being scale for some things. I build stuff to fly and it seems easier to achieve that with a simple solution at this point. When I start building my own designs that will be my starting point also. It has been with all my other technical hobby builds and it has served me well over the years.
You can always tune something once you have it doing its primary purpose reliably, as in running it a bit you get to understand more about what your changes are doing, vs simply making them at the design stage.
A counter point to that is that “failure” is an essential part of the design process for anything and should not be considered as grounds to stop or abandon a project. If you really want to create something unusual then go at it, if it fails on its maiden then go back and work out what was the biggest issue, fix that, then go again, rinse and repeat!
 
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Merv

Well-known member
#15
What would happen if I tried a 8" three OR four blade(s) props??
Multi blade props are less efficient, the most efficient prop, in theory, is a 1 blade. Multi blade props are used to get more thrust without needing more ground clearance.

Here are the rules
More pitch requires more power
Larger diameter requires more power
More blades requires more power
More RPM requires more power

Your motor is going to try to spin as fast as it was made to spin at a given voltage. If you prop, spinning that fast, requires more power than you motor, ESC or battery can supply you are going to burn something out. You need to reduce one or more of the above. Likewise if you under prop, you will not hurt anything, but the larger capacity it just dead weight. Your battery needs to supply more amps than the ESC needs. The ESC needs to supply more amps that the motor AND servos need. The motor needs to supply more power than the prop needs, as measured in amps or watts.
 

Vimana89

Well-known member
#16
Great answers with the fine details. These were things I was still unsure of also. What I've been doing is just using the default/recommended prop for any given motor, although I have seen some motors that give you a bit of wiggle room as to the exact specs, its usually in a fairly tight range, like half and inch to an inch in diameter. You might find a motor that's rated for various 5-6'' props for example, or various 9-10'' slow fly props, but you won't find one that will do like a whole multitude of sizes and pitches.
 

Merv

Well-known member
#17
You might find a motor that's rated for various 5-6'' props for example, or various 9-10'' slow fly props,
When you see a range like that. It’s the pitch that changes, a high pitch 9” prop OR shallow pitch 10” prop. (example 9x7 or 10x4). Sometimes they will give a range, like on 2s 10x6 on 4s 7x5. You can infer on 3s a 8-9 inch prop.
 
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