The Future of Propeller Technology

[KDE Direct]

Hey Everyone -

Just posted the new KDE Direct Design Engineering - Episode XI: Carbon Fiber Propeller Blades video to YouTube. Rather than try to explain all the Engineering logic and design/manufacturing decisions via e-mail, the best manner is to simply show the technology via video and verbal discussion. One of the key discussions is why we chose to drop all research and development on a fixed-propeller series - shifting gears to the folding-propeller category for the final KDE Direct editions. This became a much more difficult and involved project, to bring this level of quality and performance to the market, but we are confident the effort was well worth the final results. I hope you all enjoy and please e-mail me with any further questions.

 

[R_Lefebvre]

Lead and lag is only required if a blade moves up and down, something which happens in a rotor head with cyclic inputs. The reason being that as the blade moves away from the perpendicular (caused by changes in pitch on the cyclic) the change in angle of the blade relative to the axis of rotation moves the blades CoG nearer to the axis and angular momentum causes it to spin faster- think of an ice skater pirouetting with their arms out and the speed they gain when they bring their arms to their chest. Of course as the blade then returns to the perpendicular it slows down and hence why lead and lag is required. In a full scale helicopter the amount of flex is very notable and the pitch is constantly changing by a fair amount. However on a multirotor both blades are not changing pitch relative to their spinning axis or if they are it is in very very small amounts. I fully get the change in lift due to the very slight changes in angle of attack as the prop blade goes from advance to retreat (relative to the airflow). So here is the question. Is the change in lift really that significant that it causes the blade to flex sufficiently such that its CoG shifts up or down? I'm not sure it does based on the distance of the blades CoG from the hub and the stiffness of a carbon blade and also the speed at which the prop is spinning at. The speed seems to me important on two factors, the faster a fixed pitch blade spins the less effect wind over it has in terms of changing angle of attack. Centrifugal/centripetal forces are stronger and therefore help considerably to keep the blade flat (aided by the stiffness of carbon blades) let alone near to the CoG and also there is less time for the blade to accelerate, decelerate thus reducing the lead lag. Is is therefore really an issue ie are the effects actually noticeable?

Your statements here are all correct.

I have to say I'm a bit disappointed here in KDE. They make really nice stuff but... I can't get behind the technical explanation they are giving here.

Lead lag is needed to allow for changing angular moment as the blades flap up and down due to either cyclic pitch or dissymmetry of lift due to forward flight. Multirotor propellers do not have cyclic pitch. And they don't flap to equality (to eliminate dissymmetry of lift) because they are too rigid. Also in typical usage, the dissymmetry of lift is not nearly as great as helicopters due to the rotor speed and low airspeeds.

3 blades do help reduce vibration (4 and even 5 blades would be even better) because 2 blades do suffer from vibration due to oscillating torque loads due to what little dissymmetry there is.

But I don't really see any benefit that lead/lag would give you on a fixed teeter rotor.

I would love to see somebody make a free teetering rotor head (with lead-lag) for multirotors, that should give even less vibration.

 

[KDE Direct]

You are correct in that the blade "flapping" is an aspect of changing characteristics of lift, but take into consideration wind effects, forward flight, and also tolerances of the propellers. If the propeller is indeed fixed (single-body), then there is no freedom for the propellers to naturally balance out their out tolerances in the pitch range (alternate sides), and there is no method to balance out their angular momentum (resulting in cyclic oscillation of the propeller disc). Even with the carbon-fiber propellers being extremely rigid and strong on the bench, they still flap more than one would expect - the forces on the propellers are much higher out towards the end of the propellers, which is where tracking is key to reduce noise and vibration. You can see this especially on the heavier-lift systems, where the propellers tend to "cone" away from the motor in the opposing direction of the thrust force, and the more weight added - the more benefit the lead-lag will impose to the system to balance out the angular momentum (especially in non-ideal weather conditions and turbulent air).

 

[Carapau]

Well I am now flying the folding blades and very nice they are too- very convenient which is why I bought them. I still dont buy the benefits of lead lag at all in this. I get that one blade might have a slightly different pitch to it that is paired other blade but by lead lag does nothing to counteract this does it? (genuine question here) For lead lag to occur the blade needs to have a force on it causing it to move forward and back which given the large centrifical forces this wont happen for pitch differnece. If the pitch were changing during a rotation then yes got it but without any pitch changes there will be no lead lag.

The only area that I can see the hinge producing better performance is in the overall balance of the blades. With the fixed blades, balancing for differences of weight across the chord of the blade is not easy and at least with swinging blades this naturally gets cancelled out as the blade to will swing out such that the CoG is in direct line with the axis. But that's about it from what I can tell. Patrick- do you have any specific quotable science that backs up your claims because I would love to be proven wrong on this / learn more about the aerodynamics?

 

[eskil23]

If I understand it right, it works like this; If one blade have larger pitch it will also have larger drag and larger lift. The larger drag will give it larger lag which compensates for both the extra drag and lift.