Propeller inspection.

[FoxhallGH]

Thank you for the link, technical definition and thorough explanation. I guess my using the word pitch was wrong. What I actually meant to describe was the angle of incidence of the blade. Do you think the angle of incidence of the MA blade is the same as either the 8330 or 8331?

@FoxhallGH, can you please explain why those folks that chose to try the low noise props on their Mavic Pro were experiencing unwanted vibration? Also, aside from the obvious difference in length, why it might be a bad idea to use Mavic 2 props on a Mavic Pro?

By the way, thank you for being a gentleman, something which I am often not.

Thanks for the feedback @Thunderdrones - I'm happy that the explanation was understandable! Sometimes this subject can get too technical!
To look at the imperical difference between the prop's - you'd need to choose a point about 75% of the way along a blade, and then with it resting on a flat surface (with the locking keys up), measure the difference in height above the surface on both sides of the blade at that 75% point. That will give you a comparitive pitch measurement (in inches or mm), that you can then use to compare the different prop's against. It's also interesting that the width of the e.g. MA [Master Airscrew] prop's at their widest point, are about 6mm (1/4") wider than the DJI 'quiet' props.

There is a big difference in the prop' spec's between the Mavic Pro and the 2 Pro (8330 v's 8743) - However, the Mav' Pro flies quite happily on 8344 Master Airscrew prop's - So it would be interesting to see how a Mavic Pro flew the 8743 prop's - IF - you could get past the fact that they'd impact the rear body of the drone as there is not enough clearance for an 8.7" diameter prop. That additional 0.6" diameter may not sound like much, but in the real-world of aircraft design, it does not seem to take much to make a difference in prop' performance. Usually, if an aircraft manufacturer increases a prop' diameter, they do that along with an increase in engine horse-power.

The problem that 'sometimes' happens with low-noise prop's on the Mavic Pro, is due to feedback gain in the stability electronics of the Mav' Pro. As an aircraft, the MP is inherently unstable, and it uses feedback to the motors to adjust them to position-keep until a control movement pushes it outside of that stable envelope. e.g. If the feedback circuits 'feel' that the left hand side of the MP is dropping, then it will boost the rpm on the left motors (& drop rpm on the right) to bring it back level. Just watch the rpm indication on your controller display and you can see this continuous change in rpm in the hover as the Mavic compensates ... Of course, the prop's are part of this feedback system ... If you put prop's on the MP that are more efficient, you can push the boundary on this deleicate setting for the feedback. What in effect happens (with 'quiet' prop's) is that as the left side of the MP drops, the left motors compensate with increased rpm to bring it level, but because you now have better prop's, it overcompensates - so the motors throttle back in rpm and the left side drops (assisted by the right side throttling up in rpm - which over-compensates and has to drop again - and on it goes. The stability process - even with the correct prop's - is a continual correction process with micro-movements being input all the time. The difference is that with the 'quiet' prop's, the MP 'may' actually physically move further in the compensation cycles, and although the movement of the drone's body is still pretty much imperceptable, it creates a vibration frequency that the camera gimbal assembly is sypathetic with. That makes the camera vibrate at a lower frequency (a harmonic), and the video goes crazy! You don't usually see this effect happening all the time during a flight. It may be during the hover phase, or sometimes you'll hit that sympathetic vibration as you move the Mavic out off the stable hover (e.g. a slow transition forward, or back). It's a mystery as to why some Mavic Pro's do it and others don't ...

This is a very technical issue and the best description of what is happening and how to cure it is in this video. It's long, because the subject is complex, but it's clear and accurate in the description - and how to compensate/fix it:

 

[Former Member]

Thanks for the feedback @Thunderdrones - I'm happy that the explanation was understandable! Sometimes this subject can get too technical!
To look at the imperical difference between the prop's - you'd need to choose a point about 75% of the way along a blade, and then with it resting on a flat surface (with the locking keys up), measure the difference in height above the surface on both sides of the blade at that 75% point. That will give you a comparitive pitch measurement (in inches or mm), that you can then use to compare the different prop's against. It's also interesting that the width of the e.g. MA [Master Airscrew] prop's at their widest point, are about 6mm (1/4") wider than the DJI 'quiet' props.

There is a big difference in the prop' spec's between the Mavic Pro and the 2 Pro (8330 v's 8743) - However, the Mav' Pro flies quite happily on 8344 Master Airscrew prop's - So it would be interesting to see how a Mavic Pro flew the 8743 prop's - IF - you could get past the fact that they'd impact the rear body of the drone as there is not enough clearance for an 8.7" diameter prop. That additional 0.6" diameter may not sound like much, but in the real-world of aircraft design, it does not seem to take much to make a difference in prop' performance. Usually, if an aircraft manufacturer increases a prop' diameter, they do that along with an increase in engine horse-power.

The problem that 'sometimes' happens with low-noise prop's on the Mavic Pro, is due to feedback gain in the stability electronics of the Mav' Pro. As an aircraft, the MP is inherently unstable, and it uses feedback to the motors to adjust them to position-keep until a control movement pushes it outside of that stable envelope. e.g. If the feedback circuits 'feel' that the left hand side of the MP is dropping, then it will boost the rpm on the left motors (& drop rpm on the right) to bring it back level. Just watch the rpm indication on your controller display and you can see this continuous change in rpm in the hover as the Mavic compensates ... Of course, the prop's are part of this feedback system ... If you put prop's on the MP that are more efficient, you can push the boundary on this deleicate setting for the feedback. What in effect happens (with 'quiet' prop's) is that as the left side of the MP drops, the left motors compensate with increased rpm to bring it level, but because you now have better prop's, it overcompensates - so the motors throttle back in rpm and the left side drops (assisted by the right side throttling up in rpm - which over-compensates and has to drop again - and on it goes. The stability process - even with the correct prop's - is a continual correction process with micro-movements being input all the time. The difference is that with the 'quiet' prop's, the MP 'may' actually physically move further in the compensation cycles, and although the movement of the drone's body is still pretty much imperceptable, it creates a vibration frequency that the camera gimbal assembly is sypathetic with. That makes the camera vibrate at a lower frequency (a harmonic), and the video goes crazy! You don't usually see this effect happening all the time during a flight. It may be during the hover phase, or sometimes you'll hit that sympathetic vibration as you move the Mavic out off the stable hover (e.g. a slow transition forward, or back). It's a mystery as to why some Mavic Pro's do it and others don't ...

This is a very technical issue and the best description of what is happening and how to cure it is in this video. It's long, because the subject is complex, but it's clear and accurate in the description - and how to compensate/fix it:

This explains so many things.Thank you again for the clarification.

Further along this line of differences in props... If the MA props are wider at the widest part by 6mm, will this give the prop additional lift vs being narrower by 6mm? What effect if any would it have on the "bite" of the prop, and also on the drag? The reason why I ask, is if it gives the prop additional lift, then top speed might be greater, but acceleration might be slower? Also, if the 6mm increase in width does give the prop more lift, will it also create more drag?

 

[FoxhallGH]

This explains so many things.Thank you again for the clarification.

Further along this line of differences in props... If the MA props are wider at the widest part by 6mm, will this give the prop additional lift vs being narrower by 6mm? What effect if any would it have on the "bite" of the prop, and also on the drag? The reason why I ask, is if it gives the prop additional lift, then top speed might be greater, but acceleration might be slower? Also, if the 6mm increase in width does give the prop more lift, will it also create more drag?

Hmm - lots of good questions ...

Lift is one component of how the prop' works, but don't forget that's where the 'pitch' or 'angle of attack' comes into play. While a wing is using airfoil effect to get it's lift by being pulled through the air, a propellor gets a lot of it's push just by virtue of being an angled 'oar' against the air its being pushed through (plus lift from airfoil effect). The width helps of course, because the wider the blade is, the more air is pushed back as it rotates ... However, as you rightly point out, it takes more effort to push that wider blade through the air than it does a thinner one. Aircraft that want to make the most out of flying straight and level have long thin wings (e.g. gliders) While aircraft that need to make rapid course changes have big flat wing platforms (e.g. military fighter-jets). The prop' is more like the fighter jet in that it's looking to 'push' air rather than 'slide' through it.



Bearing that in mind, then yes - it will usually take more effort to push a courser pitch prop through the air than a finer one.

A major difference between the DJI Stock prop's and MA prop's is actually their stiffness. DJI have designed their prop's to act almost like a shock-absorber on a car. The stock DJI blades flex quite considerably in all planes - but one that's important, is that they will twist into the airflow i.e. the blades will tend to decrease their pitch as rpm increases due to air pressure (known as 'feathering'). So an easy way to get your prop's to work harder, is to make them less flexible - which is what you get with MA and carbon-fibre prop's. The immediate effect you see is a drop in rpm needed to keep the Mavic in hover. However, it means that in turbulence, the Mavic may not be as stable, because the prop's are not going to absorb some of that movement due to the turbulence. Conversely, because the MA prop's don't feather to anything like the degree that the DJI prop's do, they give the Mavic more accelleration - and actually, it's reported that the Mavic is more stable with MA prop's, because the feedback changes that are being fed to the motors to keep the Mavic steady, aren't being absorbed by blade-flex.

There is a 'negative' effect on top speed however ... Now, if you think about it this way, it's quite logical ... When the Mavic is flying forward, its prop' disks are angled forward toward the direction of travel - and the faster the Mavic goes, the more it/they angle forward. Therefore, they are presenting a cross-sectional 'block' to the horizontal airflow around the aircraft. The faster that you can get your prop's to spin, the less of a barrier those prop's will be to that airflow (or the higher the forward velocity is, at which they become a barrier). It's for that reason, that the best choice of prop' for high speed on a Mavic Pro, is the stock standard 8330 square-tipped prop's, because they spin with the highest rpm. Again using real-world aircraft comparisons, cruising mode for a variable-pitch (constant-speed) prop, is having the pitch set to 'fine'. I think you'll find that all the prop' tests that compare top speed of a Mavic, will back that one up, showing C-F and Master Airscrew to be the 'slowest' ...