The Future of Propeller Technology

[KDE Direct]

Hey Guys,

KDE Direct was not able to attend NAB this year, as we were relocating our core office to a new shipping warehouse, but that's not to say we don't have new products to show off that are coming in the near future.

Through all our testing, we've never been a fan of "fixed" propellers, not only from the hassle of storage - but also in flight-performance. Fixed-propellers are useful in airplanes, where the requirement for the propeller to lead-lag to maintain dynamic balance is not essential. In a multi-rotor, the complete opposite is true - these fly in more relation to a helicopter and as such, separating the propeller "blades" is critical for dynamic balancing during flight and performance. So, it's been nearly 1+ years in development, but we are soon to launch our new propeller series that is going to change the thinking on propellers and what "smoothness" truly is in-flight. Let alone storage - just fold-up the blades, tuck away the multi-rotor, and you're done...how simple is that? No more unscrewing blades, hassling with propeller adapters...etc. Once the blades are installed, they stay there and ready for the next flight, after flight, after flight....

A full series of sizes and dual- and triple-propeller editions will be available very soon, full-production is in the works and we'll keep everyone posted. Here's a sneak-peak of the upcoming triple-propeller 27.5" edition, and these are all constructed of true carbon-fiber 3K materials - same as we supply to our military-customers at a price competitive to the current market. All propellers are dynamically-balanced and matched, so simply install the blades and go fly, no messing around with low-quality "hobby-grade" components anymore...

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[Carapau]

Edited to remove harshness!


Sent from my iPhone using Tapatalk

 

[stevemaller]

Look, I love KDE stuff but lead and lag is only applicable to variable pitch and in particular cyclic pitch not in fixed pitch props so this really is pure marketing BS. I'm sure these props will be exceptional but please don't take us for fools.

That's pretty harsh. It seems that KDE are in good company with moving to this kind of prop. First DJI, then Freefly, and now KDE...are you prepared to present data that can challenge this clear industry trend?

 

[Old Man]

He should not need to. The reference material is all over the web for those that choose to do their own due diligence.


Sent from my iPhone using Tapatalk

 

[Bartman]

i'm familiar with lead/lag concepts in single rotor helicopter heads but not in fixed pitch applications. the convenience aspects are obvious so for that I'm interested but can any0one give me the nickel tour as to what KDE is talking about in the first post?

does the prop blade lag as it turns into the wind and then accelerate as it comes across the front and picks up the tailwind component from the relative wind? is this the lag and lead that makes the blade produce more equal lift from the advancing and retarding sides?

 

[econfly]

I'm lost. Hopefully KDE can educate us.

For my interests, convenience of a folding prop combined with excellent thrust/watt and low vibes are more than enough to sell me on a prop system.

 

[Bartman]

@KDE Direct
hi patrick, would love to hear a more complete explanation regarding the benefits of a non-rigid prop assembly
thanks!
bart

 

[KDE Direct]

Look, I love KDE stuff but lead and lag is only applicable to variable pitch and in particular cyclic pitch not in fixed pitch props so this really is pure marketing BS. I'm sure these props will be exceptional but please don't take us for fools.

It is a bit harsh response, but that's okay - sometimes it's good to be direct and to the point...ha.

The physics of flight is nothing that I'm inventing, just applying it correctly to the method we are using multi-rotors. Your argument that lead/lag is only applicable to certain conditions is true - but ONLY in a perfect hover scenario, and one where there is no wind or other environmental effects. Once you introduce forward flight, wind, or any other motion, then absolutely the physics of lead/lag come into play from the changing attack-angles to the blade. By methods of lead/lag, the effective attack-angles are changed (even with fixed-pitch propellers), and the propeller assembly is able to dynamically balance the CG locations to the attack-angle of the blades and prevent vibrations. It's the reason ALL helicopters (even semi-rigid rotor head systems, such as used in aerobatic military designs, such as the Apache) have to allow for this motion, otherwise the helicopter will literally vibrate itself to death and/or critical failure of various components. In semi-rigid head designs, smoothness is sacrificed for maneuverability, which makes perfect sense for attack-helicopters. No different than the physics applied even in R/C Helicopters - try flying a rigid-rotor system in any form of forward flight, and immediately it becomes unstable and will vibrate itself to destruction. But, crank up the head-speed to help dampen the vibrations, and now you have a seriously maneuverable helicopter ideal for 3D flight popular in the hobby - something rarely needed with multi-rotor applications.

In an airplane, it's a non-issue, as the propeller is spinning perpendicular to the oncoming airflow, so the effect of approaching/retreating blades is minimized - it's not worth the added mechanical complexity to limit this effect, and rather, it can be done in proper usage of materials and propeller design. In a helicopter, with the propellers now spinning parallel and/or at an attack angle to the incoming airflow during flight, this all changes - the blade spinning into the airflow GAINS lift, while the blade spinning away from the airflow (retreating) LOSES lift at a frequency equivalent to the RPM of the rotor, and as such, if there's no lead-lag allowance for the propellers to change their effective angle-of-attack, then the CG and effective lift is immediately put into a state of imbalance, and vibration occurs equivalent to this offset and the RPM.

In a multi-rotor, this can be cheated by using soft-blades (such as plastic composites) with the smaller-size ships (such as a Phantom class), The blades simply flex to balance their angles of attack, and the added cost of having "folding" propellers is negated. When going to stiff-carbon fiber propellers, to gain both efficiency and thrust for system 12" and larger, this forgiveness is lost and now, vibrations start to generate when being used in a "fixed" setup without lead-lag physics. With the ability to lead-lag in the folding-design, now the benefits of the carbon-fiber and higher-quality propellers can now be realized - and it can be both seen and felt in-flight.

I also was not a huge believer in this until we went into full-development on both designs (a fixed version and folding-version), and upon testing on the bench and in-flight performance, it become immediately evident that physics of flight were true and the smoothness of the folding-version was clearly easy to be recognized. Even with perfect balancing of the fixed-version, we could never reach the smoothness of the folding-versions and manufacturing was near-impossible to reach the level of quality we would be comfortable releasing to the market.

 

[stevemaller]

Patrick, thank you for your thoughtful explanation.

 

[Carapau]

Oh dear I have to have a laugh (at myself). Long day yesterday and re-reading this I have indeed been very harsh so apologies. The original post was a bit of fire and forget or should that now be fire and regret?! On the up side, one of the reasons I like KDE is for responses as above. However, here is my understanding of lead / lag and this ends in more of a question rather than a statement based on what was said above.

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?

Another question I have is how does the rapid deceleration / acceleration of our props effect pivoted blades, especially if active breaking is used. In these instances, you will get simultaneous lead or simultaneous lag on both blades in theory. A bit like above, I think the RPM probably cancels this out due to the centrifugal forces and it would seem that in the real world there isnt much of an effect at least not noticed on the 15inch blades in common use on things like the S1000 / S900s.

With all that said, the idea of folding blades is great in my mind. Clearly they are much easier to deal with in terms of storage and traveling with a multirotor and that really is a noticeable effect!

Apologies once again for my earlier harsh response Im now just fascinated to get to the bottom of the effects of pivoting blades in a fixed pitch prop.

 

[binlagin]

This community proves quality over quantity. Thx B-man

Ahhhh! I just ordered Tarot triple blade props for my x8... I want to try these!!

 

[Bartman]

This community proves quality over quantity. Thx B-man

Thanks for taking the time to post that, thanks to @KDE Direct for the explanation, and thanks to @Carapau for being moderator enough to apologize for his cantankerous outburst!

I've got a box full of Xoar beechwood props but I've also got another build forming up using the KDE 4012 motors I bought last year so maybe I'll have to try some of these very convenient non-rigid props.

 

[sledge57]

What sizes does @KDE Direct plan to make these available in? I like the whole concept but the day I build a rig that uses 27" props will be the day I have to find a new place to live

 

[Motopreserve]

Great thread with some really interesting information. Thanks for raising the bar of my knowledge when it comes to the effect of physics and props - or is that just confusion I'm feeling....

Seriously though. Great info and look forward to checking out another great KDE product.

 

[Timtim]

How tight or loose do the blades need to be to get the greatest benefit?

 

[maxwelltub]

Good read

 

[SleepyC]

How tight or loose do the blades need to be to get the greatest benefit?

I have several rigs with folding props on them, and I find that snugging the blades enough that you have to apply a little bit of pressure to move them into place works best. The force of the spinning motor aligns them quite quickly and I have to admit my rigs have never been smoother no matter how much balancing I have done to standard 2 blade ridged props.

 

[maxwelltub]

I'd be interested to learn the best way to find out the amp draw and thrust by adding one more blade to the mix. Multiply by .33?

 

[eskil23]

I'd be interested to learn the best way to find out the amp draw and thrust by adding one more blade to the mix. Multiply by .33?

Going upp one prop size adds more lift than current. Spinning the same prop faster adds more current than lift. I'm not sure that adding one blade will affect lift and current equally.

 

[maxwelltub]

Oh I see. I assumed more load from increased drag. 3 blades is less efficient but has more lift? Is this correct. That's my understanding from CP heli setups.