Prop Pitch Question

[nilsen]

Hi Ladies and Gents (are there any ladies on here?)

Anyway, I'm struggling with the prop pitch question, I have 12inch props on the hex and am switching between 12x6 and 12x4.5 but am not sure which are better.
The craft is an Aerial photography and videography build so stability is important.

As far as I understand the theory is in an ideal world 6 inch props would have more thrust as they move 6 inches up (or forward) for each revolution whereas the 4.5's move 4.5 inches forward for the same revolution.

The motors can handle the 1260 props no problem so should I just use them as I would get better lift? Is there anything else I should consider with regards to prop pitch?

Thanks.

 

[3DMad5]

I'm no expert, but from what I understand the lower pitch props would probably be better for your application. The motors will be able to accelerate/decelerate them faster leading to a more stable craft. The higher pitch props will give you more thrust, but you lose the fine control aspect of the lower pitch ones.

 

[jayz 84]

The high the pitch the more speed you will get but less thrust. Also higher amps and higher watts. Lower pitch more thrust but less speed. Lower amps and lower watts. Now may every depending how efficient the motor runs on a given prop. A pitch of 6 means you with travel 6inch per revolution as the same goes for the 4.5. But on a prop of that size youll most likly run better on the 12x6 prop

 

[SnowRocker88]

http://personal.osi.hu/fuzesisz/strc_eng/

^^This will give you an idea for thrust, speed and load (watts). As for the application, that's for someone else to chime in on as I would have no clue.

 

[FinalGlideAus]

Have a little think about your pitch versus thrust theory Jayz 84. I'm not sure you're on the ball with that one. So long as the motor can handle it a 5x4 prop will have both a higher forward speed and higher thrust than a 5x3 prop but at a cost of higher current draw.

Nilsen, I would suggest that so long as 12x4.5 can lift the copter fine and do what you want well enough then that would be the best prop to use (but mostly with regards to flight times) but really the proof is in the pudding. Tune and fly both and see for yourself which is best. There are many variables that can change the answer.

 

[Craftydan]

As far as I understand the theory is in an ideal world 6 inch props would have more thrust as they move 6 inches up (or forward) for each revolution whereas the 4.5's move 4.5 inches forward for the same revolution.

Almost, but not quite. They aren't implying forward momentum, for a given loading, but instead efficency for the prop.

A 12x6 prop will work most efficiently when they move 6" forward for each revolution, due to the steepeness of their pitch. They'll need to move that far forward to get fresh, non turbulant air to grab on to. The converse, however, is true: If they can't move 6" forward through the air, they can't produce the thrust efficiently, so it will suffer in static performance like hovering because it can't get as much clean air as it would like.

This is a bit non-linear and can depend a lot on the motor so it's hard to say difenitively, but too steep of a pitch on a fixed pitch rotor craft *could* produce less thrust than a much shallower pitch at the same throttle setting, but more often it will produce a little more thrust , but at a significantly higher cost in current (read: flight time). Sometimes it's worth the extra cost, sometimes it's not. It's possible to go the other way with a too shallow pitch, but unless it's a variable pitch prop, I'd expect the prop manufacturers won't sell you something so shallow.

All that being said, if you're not hurting for thrust to lift your airframe, more often than not, shallower will be better for multirotor performance.

 

[jayz 84]

Have a little think about your pitch versus thrust theory Jayz 84. I'm not sure you're on the ball with that one. So long as the motor can handle it a 5x4 prop will have both a higher forward speed and higher thrust than a 5x3 prop but at a cost of higher current draw.

Nilsen, I would suggest that so long as 12x4.5 can lift the copter fine and do what you want well enough then that would be the best prop to use (but mostly with regards to flight times) but really the proof is in the pudding. Tune and fly both and see for yourself which is best. There are many variables that can change the answer.

Depend on how efficient that prop is on the motor amps and watts will very. But the lower the pitch the less fwd. speed you will achieve but you will gain better thrust and will take off faster by achieving its top speed faster The 5x3 will have more thrust over the 5x4 amps may very but will draw less watts. I calculated thev12x6 over the 12x4.5 and dident find any thrust diff but pulled more watts now this depends on your setup

 

[joshuabardwell]

All that being said, if you're not hurting for thrust to lift your airframe, more often than not, shallower will be better for multirotor performance.

Interesting insight, Dan. Thanks.

 

[FinalGlideAus]

Depend on how efficient that prop is on the motor amps and watts will very. But the lower the pitch the less fwd. speed you will achieve but you will gain better thrust and will take off faster by achieving its top speed faster The 5x3 will have more thrust over the 5x4 amps may very but will draw less watts. I calculated thev12x6 over the 12x4.5 and dident find any thrust diff but pulled more watts now this depends on your setup

Let's look at a real world test for a moment
http://www.rcgroups.com/forums/showthread.php?t=2112708
I see a thrust trend. Do you?

 

[jayz 84]

Let's look at a real world test for a moment
http://www.rcgroups.com/forums/showthread.php?t=2112708
I see a thrust trend. Do you?

Craftydan explained it best for you. Ive done all mine in real life of my own testing. Besides i work on planes for a living so i know all about how it really works
And that test you posted would be more accurate if they were useing the same type of prop and brand and the thrust will go up when you not running it at its full potential. Of couse youll get more thrust when your increasing prop pitch and size to reach the motors potential

 

[Balu]

Didn't David mention in the last(?) video that he was amazed by the small racing quads with their high pitched props that are great for speed, but shouldn't work for a stable hover? He found it confusing that hovering works mainly because they are so inefficient in grabbing enough air while not racing that it still works for a hover.

 

[FinalGlideAus]

You do realise you just contradicted yourself in the above statement?

Let's just agree to disagree as there are many factors niether of us are taking into account.

 

[FinalGlideAus]

Didn't David mention in the last(?) video that he was amazed by the small racing quads with their high pitched props that are great for speed, but shouldn't work for a stable hover? He found it confusing that hovering works mainly because they are so inefficient in grabbing enough air while not racing that it still works for a hover.

I had a chuckle when I heard that. Most people run a 5x3 prop on a mini quad with some (like me) running a 4.5" pitch. What does David run on his tricopter.... An 8x5!?! So he's running a far steeper pitch prop than the mini quads but claiming their "high pitch props" are an issue when he's running a higher pitch himself. Hmmmm...

 

[Cyberdactyl]

I'm lovin' the HQ0645s on my 2600kv brutes. Wish I had the skill to open them up on 4S.

 

[FinalGlideAus]

I'm lovin' the HQ0645s on my 2600kv brutes. Wish I had the skill to open them up on 4S.

Yeah that setup would go nicely Cyber! Are you getting hot motors during extended WOT (10-20 second bursts) or have you not pushed it that hard? I would be worried about letting the smoke out in that situation but I haven't tried that setup myself. I think you'd start puffing batteries anyway with the current draw at Extended WOT. What's the amp draw per motor at WOT?

 

[Cyberdactyl]

What's the amp draw per motor at WOT?

I would imagine the 20g wires on the Afroslims are choking it down to no more than the upper teens. I initially was going to replace the wires with 16g, but never did. I was too concerned seeing how it would fly and put it off.

As to WOT, there's no way possible I can do WOT LOS for 10 seconds with 3S, let alone 4S. And FPV, I would have to have some room, but haven't tried on my occasional visit to the flying field when I have. But I do notice the Afroslims can get very warm pushing hard off and on continuously for around 2 minutes with 4S.

One of my upcoming projects is to build a test stand with a scale, tach and watt & amp meter. I have had all the components I need for a couple months, but other projects keep pushing it back. I would like to see exactly what's going on.

I had a chuckle when I heard that. Most people run a 5x3 prop on a mini quad with some (like me) running a 4.5" pitch. What does David run on his tricopter.... An 8x5!?! So he's running a far steeper pitch prop than the mini quads but claiming their "high pitch props" are an issue when he's running a higher pitch himself. Hmmmm...

One thing to keep in mind is bigger props can have a higher pitch but actually 'dig' air less aggressively than a smaller prop with a shallower pitch in that the larger prop generally spins slower. Take 0645's. The pitch/prop ratio is 0.75. For an 0850 it's actually shallower at 0.625

You can check this by going to a pitch speed calculator and plugging in a realistic prop rpm for an appropriately sized prop.

That's why I love the 2600kv 28x22's, they are cheap and a bit heavy, but they will poop some air if I can keep them fed without smoking.

 

[FinalGlideAus]

But see it's the size and speed of the prop, not just the pitch that effects things. David's tri is wider and heavier which helps stability. Mind you the slower larger props end up being worse in wind. I thought the mini quad gave better footage in the wind than Davids tri. Funny that the FT crew don't use tri's or kk2 boards anymore. At the end of the day one airframe type can't have all the best features and you need to choose the design that best suits your intended purpose. Mine is power, speed and agility so mini quad all the way for me.

 

[Cyberdactyl]

Another thing David still uses are the HK F20As in his latest tri build.

They are older tech and a bit heavy, but I would have to agree, in that they are workhorses in that they are dirt cheap and just plain work darn well and never fail.

 

[FinalGlideAus]

Can't argue there. They just keep ticking along. Mind you they're not being pushed hard.

 

[nilsen]

Well this was a great response, thank you.

Crafy, thanks for the explanation, I think I understand and what I will take away from this is if you want a significant change in lift then try and increase the prop size, the pitch is more related to flying speed and less with lifting ability but at the cost of higher wattage.

What I will do tomorrow is charge a battery to full, fly for exactly 5 minutes at a hover using the APM to do the takeoff and landing thus removing human error. I'll put on the 4.5pitch and the 6pitch, both 12 inch and once I recharge the battery I can see which one was more efficient on the the hex.

If I were to fly at 6 inches up a per revolution then they would be most efficient but as a video / image craft I mostly simply hover so now I think that perhaps some 1238 would maybe be something to look at for ultimate stability vs filght times and low motor stress.

You learn something new every day which this magnificent hobby.