Prop sizing

[Vimana89]

Like I said, I'm not the math guy. Others have covered that better than I could and I'm still learning. I have, however, taken the time to do some research to understand more about the inefficiency of three or more bladed props. @quorneng was correct that two bladed props are most efficient, and that any blades beyond that will have greater energy demands than the added thrust really compensates for, from a pure efficiency standpoint. So other than space or clearance concerns, or an absolute scale look for some planes, why even use a prop with more blades than two at all? Turns out there are some reasons.

A three bladed prop is a good option to train on some planes, because the inefficiency "tames" the plane a bit by letting it fly slower while still maintaining enough thrust for easy take offs and climbs(and I could see how it would make high alpha stuff easier too). You'll lose a bit of efficiency and your battery will drain a bit faster. In some cases, this actually sounds worth it to me. I'm a hands on kind of guy, so I will be trying this with my V Sliver. The three bladed 5x5.5 it has on now works very nicely with no complaints. This plane is a good testbed, as it does more than enough slow flying and high alpha. I'll definitely notice a difference if my climb ability and high alpha diminish at low speeds with a two blade prop. So I'll try a 5x5.45x2 and a 5x5x2 speed prop, and see what happens.

All that said, I can't see any real justification for that six blade prop. I have some of the exact ones in question, Raycorp 5x4x6, and have tried them on quite a few planes, and I'm pretty convinced they are just jank.

 

[Morgajus]

What about folding props? Do they sacrifice power? Are they forgiving in crashes?
I know they are meant for gliders to reduce drag when unpowered.

Edit: more thoughts

Also, for a dual prop, would counter rotating props be a good idea? My research mostly leads to people saying it doesn't matter but the balance of forces seems more stable with counter rotation...

 

[Arcfyre]

What about folding props? Do they sacrifice power? Are they forgiving in crashes?
I know they are meant for gliders to reduce drag when unpowered.

Edit: more thoughts

Also, for a dual prop, would counter rotating props be a good idea? My research mostly leads to people saying it doesn't matter but the balance of forces seems more stable with counter rotation...

For fun once I set up my cruiser with non-counter rotating props. It flew fine, once it was up to speed, but the takeoff was a nightmare with the torque of the motors. Counter rotating is the way to go.

 

[Morgajus]

For fun once I set up my cruiser with non-counter rotating props. It flew fine, once it was up to speed, but the takeoff was a nightmare with the torque of the motors. Counter rotating is the way to go.

Its good to hear. Take off has been my biggest worry; if something goes wrong, there's no time to correct it before a crash.

Now i need to source some cheap ccw/cw props from the same manufacturer... more searching...

Now for my other question: would folding props make crashes potentially less devistating?

 

[Wildthing]

Counter rotating props on dual setups will virtually eliminate any and if not all torque roll .

Folding props less thrust and less efficient, they have their purpose just like any other prop.

 

[Vimana89]

For a sec I though you were asking about contra-rotating, which is something different. Yes, counter-rotating for a normal twin engine👍. I don't know if worrying about folding props matters that much.

 

[Vimana89]

Counter rotating props on dual setups will virtually eliminate any and if not all torque roll .

Folding props less thrust and less efficient, they have their purpose just like any other prop.

🤔🤔🤔🤔I have toyed with the idea in my head of trying one of my high-mount pusher slender delta builds with a modified nacelle with a motor on both ends featuring inline counter rotating props. Would that lend to better high alpha and vertical stability?

 

[Wildthing]

For a sec I though you were asking about contra-rotating, which is something different. Yes, counter-rotating for a normal twin engine👍. I don't know if worrying about folding props matters that much.

Both are used for the same purpose ,

Aircraft equipped with contra-rotating propellers, also referred to as CRP, coaxial contra-rotating propellers, or high-speed propellers, apply the maximum power of usually a single piston or turboprop engine to drive two coaxial propellers in contra-rotation (rotation ... Contra-rotating propellers are also known as counter-rotating propellers, ...

 

[Wildthing]

🤔🤔🤔🤔I have toyed with the idea in my head of trying one of my high-mount pusher slender delta builds with a modified nacelle with a motor on both ends featuring inline counter rotating props. Would that lend to better high alpha and vertical stability?

It would definitely get rid of some torque roll but on you slender delta you would be adding a ton of weight so not sure if you would gain anything of what you want and just possibly make it more unstable. I am no expert though, somebody else might have something for you.

 

[Vimana89]

It would definitely get rid of some torque roll but on you slender delta you would be adding a ton of weight so not sure if you would gain anything of what you want and just possibly make it more unstable. I am no expert though, somebody else might have something for you.

Yeah it doesn't suffer too much from torque at the moment, so not sure what it would gain. Maybe a full 4ch 3D version would like it. I'd have to scale that up or use smaller motors. The wing loading is surprisingly low on the design due to the long wing chord.

 

[F4vet]

Still learning - flying my Old Fogey to pieces, it is heavily patched but still flying. My newest plane is an FT Explorer with the 3-channel wing. At first I flew it on a power pack A (1806 2280kv) the recommended 6x4.5 prop and a 3S 2200 mAh battery. It flew ok, nothing overheated, but it was clearly under powered. I built a new engine pod and changed to a 2208 1100kv engine with 8x4 prop. I just flew it this afternoon - it was still under powered - and the engine got HOT while the ESC stayed cool. 8 inch is the largest prop the pusher configuration will allow. The FT Store recommends power pack B or C for the Explorer. The B motor is 2212 1050kv so I thought the 2208 1100kv I am using would be close. Power Pack B also comes with 9x4.5 props (C is 10x4.5) but the max prop diameter for the Explorer is 8". Should I go with a higher pitch 8" prop to compensate? Other recommendations?

 

[quorneng]

F4vet
kV is not an indication of motor power but the RPM is will rotate at a particular voltage.
The recommended 2212 has a longer internals (12mm rather than 8mm) so it is heavier and more powerful motor better able to handle a bigger prop. The 8x4 was clearly overloading the 2208.
An 8x6 on a 2212 should absorb about the same power as a 9x4.5 but it will not generate quite the same static thrust. However the higher speed of the air leaving the prop will allow the plane to go faster.
If you need more static thrust you could try a 3 or even 4 blade 8x4 on the 2212.

 

[Merv]

...and the engine got HOT while the ESC stayed cool.... Other recommendations?

I agree with @quorneng, kv is not an indication of a motors power. For power comparisons, look at watts. With kv think transmition. Consider two motors that are the same size. A high kv motor, 2,000 kv, will swing a smaller diameter prop very fast. A lower kv motor will swing a larger diameter prop more slowly. The lower kv motor will always produce more thrust on the test stand, but it will have limited top end speed. Similar to low gear on a transmission. The higher kv motor has less thrust but more top end speed. Similar to high gear on a transmission.

A tractor and race car may have the same horsepower. Hook them to a plow, the tractor wins. Put them on a race track, the car wins.

The question is at what speed do you want your power?

If you want to swing a larger diameter prop, get a motor with more watts or a lower kv.