Maybe not? Cut grasses, peel potatoes, pick apples, etc ... But seriously I would say the propellers are dangerous and only suitable for flying without close audience. An attacking bird would probably get very severe injuries ...
3-Blade propeller for MP1 ?
- Thread starter mnis
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FoxhallGH
Well-Known Member
Don't screw them up tight though!!!! The screws are into brass bushes that hold the screw quite nicely. The prop' blade should swing forward and back on the pivot point so that a) the prop' will fly correctly and b) you'll get some shock absorption if the prop hits something. Tighten the screw just enough to get rid of excessive up/down movement.
Yes - check them [as you should check stock DJI prop's] as part of the pre-flight, and carry a T6 screwdriver or equivalent hex-key in your flight bag. I've never found my [2-Blade] CF versions of this prop working loose though.
FoxhallGH
Well-Known Member
A question regarding rpm ... When I fly the 2-blade version of these CF prop's, I've recorded a marked drop in rpm over using the standard 8331 prop' on my Mavic Pro Platinum. Standard prop's around 515 rpmx10 in the hover, CF prop's show 475 rpmx10 in the hover (indicated on the RC controller). Have you noted the rpm that these prop's hover at??
FoxhallGH
Well-Known Member
If you do the calculations based on prop' diameter and motor rpm, you'll find that the tips of the prop's are travelling at around 560 metres/sec (speed of sound is 343 m/sec). So it doesn't really matter if it's CF or standard plastic, it's gonna hurt!
FoxhallGH
Well-Known Member
I'm waiting for a bit of good weather to do a test to compare flight duration between stock 'quiet' 8331 prop's, and CF 8331's - as I've seen that motor rev's are lower with CF, but I don't know if that means the motors are working harder. My theory is that if the flight duration is the same or less with the CF prop's, then the current drawn must be higher than with stock DJI prop's i.e. the motors are working harder. But if the flight duration is longer, then that will indicate that the motor is not having to draw as much current from the battery, and that will be a positive!Until I come to the test I stay with the statement: Seeexyyy
I'd be interested to see the same test done on the 3-blade prop's ....
FoxhallGH
Well-Known Member
Had a brief chance to fly in the back-yard today, as I wanted to test ideas for a decent method to compare two consecutive flights (with the different 8331 prop sets), and get meaningful & measurable results. There was a slight but gusty cross wind of 1 to 2 metre / sec - and I was really surprised to notice how the Mav' was so very much more stable with the CF 8331 prop's on it. But with the stock 'quiet' 8331 prop's my Mav' was noticeably unsteady in the hover, though the video was [vertically] steady as the gimbal was doing its job admirably, . Position keeping was OK, in that it didn't move far, but it [gently] jiggled & rocked back and forth continuously to keep position. I could see some sideways movement in the video taken while it was doing this ... But the video taken while in hover was rock steady - like the drone - when hovering with the carbon fibre prop's on. When I come to do the actual testing, I'll try and get some comparison video footage of that as well.
I think in the delivery state, the screws of the 3-sheet replicas are well adjusted. In direct comparison to the originals, it is only a minimal higher strength of the joints ... As already mentioned, it is with this product a surprisingly high overall quality recognizable... [emoji106]
No, I could not do any tests yet. I currently have a nasty male flu. The weather here is humid, cold and windy. The apartment is small and built with many furniture and things. Unfortunately there is no hall nearby. So it will take some time before I even come to tests with the 3-blade propellers ... I suspect the CF blades have a higher slope than the ordinary 8331F. And this results in a slightly lower speed in the hovering flight ... [emoji848]
YES Ouch, it will hurt a lot ... But I suspect these 3-blade propellers cut effortlessly through thin branches and more. The blades are sharp and very hard. The originals polycarbonate or nylon or whatever are harmless ... [emoji848]
You are free to try the 3-blade propellers, I mentioned the seller at the beginning and the product is certainly offered globally.
And hey, if these things do not bring any significant improvements, I'll take mine as a decoration. Because I want to hang my MP somewhere in the living room, at least for a while. And the price is OK for me.
Although I only skate on the surface, I dare to think about the energy consumption of the MP1:
For example, to move a certain amount of air for the hover you can use the "8331F" to make 5150rpm. For this, the motor is operated with a comparatively high voltage and low current. If you add an otherwise identical propeller blade, then theoretically a lower RPM will be required. This means a lower voltage and a correspondingly higher current for the motor.
And this could mean for the battery that it breaks down faster with increasing current load in the voltage. The battery and the ESC for the engine will probably heat up more, as if you had more charge on board and the standard propellers would have to compensate for it at higher engine speeds. Especially the ESC's probably need more energy if they need to down convert the battery voltage for the motors. With increasing voltage difference, the power loss will also increase.
In the ideal idea (Mine, far from reality) both propeller variants will need the same amount of watts per minute. It is usually about 1, 615 watts per minute that the MP1 needs for hovering under ideal conditions. An improvement in flight duration can therefore occur only through the following changes: The battery has more energy (watt-hours per gram), the MP1 has a lower total weight, the drive (motor, ESC and propeller) receive an upgrade with better ratio (efficiency) of applied electrical energy, to converted mechanical energy.
Back on the ground of reality, I suppose that these 3-blade propellers will cause a lower total flight time. But how serious the difference will be, I can currently only guess ... minus one to two minutes maybe...
In return, if I get a generally more stable flight behavior of the MP1, it would be fine for me. Anyway ... I would be very happy if anyone could do tough tests ...
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That is and was exactly my assumption. The propellers made of carbon fiber bring the MP a generally more stable flight behavior. I consider this desirable, even if the gimbal sufficiently compensates for the unrest with the standard propellers. Anyway, I eagerly await your possibly following test results ...[emoji4]
FoxhallGH
Well-Known Member
I guess that by "higher slope" you are referring to the angle of the blades, or the 'pitch'.
Actually - that's not the case. The 8331 designation tells you this. That number is a 2-part code telling you that the prop' is 8.3 inches in diameter, and has a 3.1 inch pitch. Pitch measurement is all about 'how far the prop' will screw itself forward through the air, for one 360 degree rotation'. So your 3-blade prop' blade design [shape & pitch] is exactly the same as the original DJI 8331 'silent' prop set brought out as standard for the Mavic Pro Platinum. Having 3 blades doesn't alter what the pitch of the prop' does. What's different about the CF blades, is that they are stiffer and therefore 'maintain' their pitch while spinning at 5,000 rpm - whereas the standard plastic prop's tend to flex into the airflow - especially out near the tips - and therefore reduce the pitch and consequently, the lift they can produce. Having 3 blades gives the opportunity to move more air through the spinning disk, but as per your comments above, that must impact on the power required to spin the prop's on the Mavic. Voltage will stay constant from your battery, but current must increase to get the power required [Power (Watts) = current (Amps) x voltage (Volts)]. Therefore the battery runs out sooner decreasing the flight duration.
I too have loads of wind and rain outside - so no flying for me ...
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Oh, I hope to have already made it clear that I'm not an expert ...
I think I understood your comments ...
Your CF (8331CF) Props have the same profile as the original 8331F and my 3-blade things? Moving slightly more air comes from the better stability of CF / 3-blade propellers.
Regarding the engines, I think I have understood less...
Previously, my understanding of electric drives was that a higher voltage (volts) provides for a higher motor speed. And for a higher engine torque more power (ampere) is needed. In relation to the MP1 I thought so far, the ESC's will interrupt the battery voltage periodically (chop), just as in principle makes it a switching power supply, so that the output results in a variable voltage that is required for the desired speed. The respective required current (ampere) results from the endeavor to maintain a certain speed under changing conditions.
Please help me understand better if my assumptions are wrong ...
Oh, one more thing... Yesterday I read that a German professor in a study has proved the following: 3-blade propellers should have about 10 to 15 percent lower efficiency than 2-blade propeller. I do not know the exact source in this regard...
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FoxhallGH
Well-Known Member
My qualifications are in Telecom's and IT, so I'm no aeronautical engineer either.
The assumption about moving more air with the 3-blade prop', simply comes from the fact that you have 30% more propulsion blades on a 3-bladed prop, than you have on 2 ...
The study about 3 bladed prop's having less efficiency may be quite true. At the speed a Mavic's prop's rotate (4,500 rpm +), the tips of the propellers create an immense disturbance between the air that is being pushed back, and less than a mm away air that is not being 'pushed' by the prop at all! That disturbance is given a little time to clear when you have two blades, but with 3, you can have the situation where the blade tips are continuously running into the disturbance ('cavitation') caused by the previous blade. That's why sometimes you get worse results for multi-blade prop's - and it's also why you'll see a lot of strange curved prop' designs on aircraft that use propellers rather than jet engines.
In regard to the motors ... You have a fixed Voltage that is applied to the motor control . The Mavic motors are a design where an alternating current is applied to coils around the outside of the motor, to literally 'pull' the central 'stator' around on its shaft causing the attached prop' to spin. I think there may actually be a 3-phase control i.e. not a single wave going around the motor coils, but 3 - 120 degrees apart. My understanding of how the motor control works is that by varying the frequency of that alternating current, you drag the stator faster and get more thrust, or you 'brake' it to slow it down and get less. The balance of that between the 4 motors then gives you the directional control. I believe that the Mavic Pro uses a square-wave switching to control the motors, but the Mavic Pro Platinum, and the Pro 2 range use a more advanced sine-wave AC control making the control of the motors a lot smoother.
[Edit:] Correction to the above ... The Mavic 'brushless' motors actually have the coils wound on the central 'fixed' stator, with magnets on the moving drum that is attached to the prop'. Unlike conventional electric motors, its the outside drum that spins, while the central shaft is fixed.
Some info here ... How A Quadcopter Works With Propellers And Motors Explained
The assumption about moving more air with the 3-blade prop', simply comes from the fact that you have 30% more propulsion blades on a 3-bladed prop, than you have on 2 ...
The study about 3 bladed prop's having less efficiency may be quite true. At the speed a Mavic's prop's rotate (4,500 rpm +), the tips of the propellers create an immense disturbance between the air that is being pushed back, and less than a mm away air that is not being 'pushed' by the prop at all! That disturbance is given a little time to clear when you have two blades, but with 3, you can have the situation where the blade tips are continuously running into the disturbance ('cavitation') caused by the previous blade. That's why sometimes you get worse results for multi-blade prop's - and it's also why you'll see a lot of strange curved prop' designs on aircraft that use propellers rather than jet engines.
In regard to the motors ... You have a fixed Voltage that is applied to the motor control . The Mavic motors are a design where an alternating current is applied to coils around the outside of the motor, to literally 'pull' the central 'stator' around on its shaft causing the attached prop' to spin. I think there may actually be a 3-phase control i.e. not a single wave going around the motor coils, but 3 - 120 degrees apart. My understanding of how the motor control works is that by varying the frequency of that alternating current, you drag the stator faster and get more thrust, or you 'brake' it to slow it down and get less. The balance of that between the 4 motors then gives you the directional control. I believe that the Mavic Pro uses a square-wave switching to control the motors, but the Mavic Pro Platinum, and the Pro 2 range use a more advanced sine-wave AC control making the control of the motors a lot smoother.
[Edit:] Correction to the above ... The Mavic 'brushless' motors actually have the coils wound on the central 'fixed' stator, with magnets on the moving drum that is attached to the prop'. Unlike conventional electric motors, its the outside drum that spins, while the central shaft is fixed.
Some info here ... How A Quadcopter Works With Propellers And Motors Explained
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Check that arithmetic again. 30% more blades would be 2.6 blades.
Perhaps you mean 50% more ??
FoxhallGH
Well-Known Member
Yeah ... Thought about that afterwards ... You got me! Time for that second coffee!!!
jimlips
Well-Known Member
I tried to "hand catch" my Mavic Pro once, and got my fingers up too far. The original blade hit my finger, and it was VERY painful. It felt like it got hit with a whip. I thought for sure that it cut the tip of my finger off - but was relieved to see it wasn't even bleeding, or even scuffed! I attribute this to the fact that the original Mavic blades ARE flexible. If I was using a stiffer blade (carbon fiber), I may not have my fingertip any more. This is all theory, IMHO. I'm not about to do a study, although it sounds like a good project for Mythbusters - wish that show was still on...
CAG
Well-Known Member
For example, this plane, equipped with an APC 9x6 thin electric, 9x6 sport or the 4-blade 9x6. The simulation shows that the 4-blade can't even reach the performance of the 2-blade propellers because the high power consumption reduce considerably the rpm !
I know this thread has been going awhile and I did not read every post but generally, in RC aircraft the rule is down in diameter, up in pitch when you start adding blades and while multi blades are "sexy" two is better than three for performance. The F4U you see in my posting is an 84"" wingspan aircraft running a two blade APC because trying to make it look scale and going 3 or even 4 was great for static displays but would cause such a reduction in power from the OS 200 4 stroke on it that it might get off the runway, but then again, it might not! More is not better with RC props.
The 98" P-61 was running 2 OS 91s and would have been scale with 3 or 4 blade props but would not develop the necessary RPM to fly without danger of a stall.
I know this thread has been going awhile and I did not read every post but generally, in RC aircraft the rule is down in diameter, up in pitch when you start adding blades and while multi blades are "sexy" two is better than three for performance. The F4U you see in my posting is an 84"" wingspan aircraft running a two blade APC because trying to make it look scale and going 3 or even 4 was great for static displays but would cause such a reduction in power from the OS 200 4 stroke on it that it might get off the runway, but then again, it might not! More is not better with RC props.
The 98" P-61 was running 2 OS 91s and would have been scale with 3 or 4 blade props but would not develop the necessary RPM to fly without danger of a stall.
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FoxhallGH
Well-Known Member
That does illustrate the limited and 'niche' nature of fixed pitch prop's. The real Nothrop P-61 ran 4-blade constant-speed prop's which of course allowed it to run with a fine-pitch for take-off, and then allowed the prop pitch to be optimised for cruise (or even feathered for engine fail). Fixed pitch prop's don't come close performance-wise, because they need to be matched to the motor, and are usually optimised for the cruise part of the flight. They suit one part of the flight envelope, but make-do to varying degrees across the rest.
[Edit:] Nice model P-61 by the way!!!
CAG
Well-Known Member
Thanks. The P-61 engine was the same that was adopted for the F4U in it's later years. The R2800 double wasp.
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