The Future of Propeller Technology
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maxwelltub
Member
Ya but you additionally have more drag. In traditional heli it means more responsiveness and more lift per disk radius. Sometimes you need that because at a certain speed you loss lift at the tips which are spinning faster. So you can get more lift with shorter blades and lower head speed. I could be wrong but I've heard this from full scale heli pilots, and this is what I learned when doing 3d flying. I don't understand what you mean about turbines? Are you talking about the first compression stage fans.
econfly
Member
KDE provides some nice technical data on their design. I'm not sure how up to date this is (or if I found the right material), but the results of 3 vs 2 blades seems to be more thrust with a slight efficiency loss when moving to 3 blades:
The linked file is not ideal. Efficiency (grams of thrust per watt) very commonly declines as throttle is increased for any given motor/ESC/prop combination. What we really want is the efficiency curve and not just a few data points so we can compare directly the efficiency of, in this case, two blades to three, for any given level of thrust. That ultimately (at least with respect to power consumption) is what matters: take a desired level of thrust and then find the design that delivers it with the least power consumption.
Of course, other things can matter too (convenience of folding design, low vibration, etc.).
The linked file is not ideal. Efficiency (grams of thrust per watt) very commonly declines as throttle is increased for any given motor/ESC/prop combination. What we really want is the efficiency curve and not just a few data points so we can compare directly the efficiency of, in this case, two blades to three, for any given level of thrust. That ultimately (at least with respect to power consumption) is what matters: take a desired level of thrust and then find the design that delivers it with the least power consumption.
Of course, other things can matter too (convenience of folding design, low vibration, etc.).
Pumpkinguy
Member
I've learned the hard way that manufacturer data doesn't mean squat in the real world. The only way I will accept that these are better is to have a completely independent person put 2 identical rigs up against each other. One with a conventional prop and one with the "new" style. Then and only then will I be a believer.
Patrick has built a very successful company in KDE. A large part of his success is his service and his ability to "pitch" a product. He is very likeable and intelligent but I'm not convinced his products are heads and tails above the rest.
Patrick has built a very successful company in KDE. A large part of his success is his service and his ability to "pitch" a product. He is very likeable and intelligent but I'm not convinced his products are heads and tails above the rest.
Old Man
Active Member
My personal experiences has always shown a 3 blade propeller to be less "efficient" than a two blade in modeling activities. That experience continued with the use of three blade propellers on small UAV's. In all cases the props were used on fixed pitch, fixed wing applications. Three blade props provide a higher RPM reading but less overall thrust in my experience. Perhaps that changes with controllable pitch props but that's not part of this discussion. OTOH, three blade props can make a little less noise when designed correctly, but if not designed well can end up at a hertz that is extremely annoying that also carries a long way. We might consider the original reason for three blade props was to come up with something that would provide a little ground clearance when radial engines became so large and powerful that a two blade that took advantage of that power was too long to use without extending the landing gear or designing a "bent" wing to put short gear closer to the ground, a la the F4U Corsair.
Three blades do look pretty sexy though
Three blades do look pretty sexy though
maxwelltub
Member
Ya, I think a lot of this stuff will come down to really slight differences in efficiency but I'm excited to test. I read an interesting article about wind turbines and why they have 3 blades, Check out the link.
http://www.theguardian.com/theguardian/2011/sep/06/why-wind-turbines-three-blades
http://www.theguardian.com/theguardian/2011/sep/06/why-wind-turbines-three-blades
SleepyC
www.AirHeadMedia.com
I can tell you first hand the 3 blade props made a HUGE difference for me. The smoothness of the rig, the transportability, just those factors make me a believer. Also, a 15" 3 blade is ruffly equivalent to a 16.5" 2 blade so for smaller frames it's awesome as well!
Pumpkinguy
Member
I'd like to see super x osd data and load test motor and esc temp data for 3 blade vs 2 blade.
Load her to the max. If electronics are cooler and vibrations are equal or less then I may be a believer.
Hexacrafter
Manufacturer
Sleepy...
Are the props your using the Tarot / Foxtech? Are they Plastic or Carbon Fiber? Are the Pitch of the propellers the same?
I have no doubt that the "flex" in the plastic props will give you a "smoother", "softer"... flight response.... as do all blades with less rigidity...but they are also less responsive..... The cheaper Carbon blades are also much more flexible than say a Foxtech Supreme or T-Motor.
The type of blade selected should fit the intended use....
If flying slow and smooth for aerial video... the flexible props can provide an added gracefulness to the flight...
If flying hard & heavy chasing a car or train.... the flexible props just cannot provide the needed stiffness for hard flight....
A properly balanced blade should have the same amount of vibration whether it is plastic, carbon fiber, two ear or three ear....
From my limited understanding, in full sized aircraft, three ear props are used to reduce noise, but usually are of less pitch......This is what really causes the reduction in noise....but usually produce less thrust.
Also, I believe if you do significant research, you will find that there is a 10% or so loss in thrust with three ear props.... this is the same thrust loss/ efficiency loss that occurs in a coaxial design as well... add the two together (3 prop & Coaxial) & there is a quite significant loss in efficiency.
I do agree that a folding prop design can really help with transport.
Folding Propellers for Multi's have been around a while....KDE did not invent them..... obviously a spin-off from the single rotors days...... but for different reasons..... Multi's do not have collective pitch or flapping dampners.....
Just my understanding & opinion....
Andrew
Giovanni59
Member
Very interesting debate and although I do understand some of the technical aspects there is still a lot I don't understand being fairly new to the industry. Thinking in practical terms the first thing that jumps into my head if the claim of less vibration is achieved with folding props, (I will leave the 3 blades aspect out of my theory) is the ability to descend at a good rate with no added vibration. It is not always possible to do a video shot by ascending and then reversing the video in post if there are people or other moving objects like cars or boats. Can anyone comment if this concept of lowering vibration because of folding props would effect rapid descending with a MR? I fly a HC1100 with 18" CF props.
SleepyC
www.AirHeadMedia.com
The best way to descend is while moving backwards or forwards, even if it's just a bit, coming straight down no matter what props will cause you to descend in your own prop wash. If you can even come back a little bit so the props get fresh air it smooths the descent completely.
Racerx1962
Member
So if a motor is spec'd to spin a max prop size of 15, can it handle 15 trips or should you step down in size?
SleepyC
www.AirHeadMedia.com
A 3 blade 15" prop is ruffly equivilent to a 16.5" 2 blade. So if 15" is max, you would need to go down.
learningrc
Member
Regarding wind turbines, I don't think you can draw any conclusions from them. One reason to use 3 versus 2 on a wind turbine is because wind speeds are much greater at higher altitudes than lower altitudes. If you have a 2-blade turbine that is located vertically at 12 o'clock and 6 o'clock, then the faster wind near the top blade is going to cause greater force than the slower wind near the bottom blade. This will cause uneven loading on the shaft even if the blades are perfectly balanced. If you have 3 blades, however, then the total vertical distance will always be less than with 2 blades (assuming the same length) because of the 120 degree angle between them.
Another thing to consider is that engineers may be less concerned with efficiency (measured in kWh produced per day or month or year) than they are with "cost efficiency" (kWh produced per $).
If I recall correctly, I also think there are issues with resonance/vibration on turbines with even number of blades due to when the blades pass by base that supports the turbine. I don't remember why that is though.
My understanding with helicopters and multirotors is that more blades means the props can spin slower but require more torque to produce the same power. More torque generally requires bigger motors so adding extra blades on a quad doesn't give you any advantages.
Another thing to consider is that engineers may be less concerned with efficiency (measured in kWh produced per day or month or year) than they are with "cost efficiency" (kWh produced per $).
If I recall correctly, I also think there are issues with resonance/vibration on turbines with even number of blades due to when the blades pass by base that supports the turbine. I don't remember why that is though.
My understanding with helicopters and multirotors is that more blades means the props can spin slower but require more torque to produce the same power. More torque generally requires bigger motors so adding extra blades on a quad doesn't give you any advantages.
Benjamin Rowland
Member
Been using KDE's three blade prop adapters for a while - they are awesome. We used them on Insurgent for flying a Red Scarlet around and they worked great. Glad to see that more sizes will be available directly from KDE. They build darn good motors too.
Mactadpole
Member
I am curious which 15" props y'all are using with these adapters? I am seriously considering going this route since all of the cheap to mid-range 15" props I keep getting are horribly out of balance.
maxwelltub
Member
I wonder what results you would get with a coax with 2 blade on top and 3 on bottom?