The airfoil equation of Lift = (Ct * V^2 * Area) also does apply to props blade count and scales linearly, where area is proportional to blade count * blade cord length. So a small number of skinny blades produces a small amount of thrust, and a large number of fat blades a huge amount of thrust proportional to relative areas.
This is a little nasty to be exact with, because thrust is directly computed from the sum of stations from blade root to tip where the circumference/velocity of each station is: radius * 2 * pi * RPM/60 is squared for the velocity term V in Lift = (Ct * V^2 * Area).
From that it’s just easier to know that it’s linearly proportional to cord length and blade count.
It’s easy to get this wrong because the popular prop thrust equation hides cord length and blade count inside the coefficient of thrust and drag for each prop, to make the rest of the equation more manageable for scaling wind tunnel data to general aviation real world scale.
It’s really easy to get this wrong, and I have to admit I produced early test results that failed to use the proper control variables, that resulted in misleading graphs that suggested the common wisdom of each blade was less efficient. When I later retested holding either rpm or thrust constant, then it was clear that prop count * cord length scales thrust and power linearly, and efficiency is constant.
I’ll post some of that flawed data in the next post, and explain again how it’s easy to make the wrong conclusions from it. I did it too … took a while to learn from it, which caused additional field testing to get things right. So I’ll share that path, to help others avoid it.
Back before I added load cells to the test stand, I printed a set of props to explore blade count. These are the 9 props that are all red, white, or blue at the back right of the table in a previous post a couple days ago. I used the hand held tach with the RC watt meter to collect this data:
The 0 column is the motor without a prop, to “easily” subtract the basic motor/esc power requirements, along with adjusting the data for the known motor/esc R*I^2 loses under load to estimate shaft power. That yielded this data:
While efficiency was pretty flat as expected, watts/blade wasn’t. After scratching my head a lot for several weeks, it became clear the mistake was not capturing the data at the same RPM. The simple answer is power (watts) is Torque (current) * RPM … so to compare power per blade requires a constant RPM … a simple mistake.
The other mistake is that the higher blade count props were pushing the motor and esc to their rated limits which was the hard testing limit, and graphs are arching over at this limit. Otherwise the graphs should have been close to constant slope. 25A esc at 10V is 250W. The low blade count props were voltage limited by the motor’s Kv. The larger props were current limited.
I should have not allowed a test at more than 80% of either voltage or current limit. This I believe is why the efficiency reflected in this data isn’t almost completely constant, as it should have been. Just a heads up if you want to repeat this for your own enlightenment.
When I got the load cells I switched to a larger capacity 22V battery, motor and esc so that I could keep the testing range to less than 60% of the battery/motor/esc limits, so the results would be more linear.
So those were the first 9 props that started this testing campaign. And over the last 9 years probably something close to another 90 or so have been carefully chosen to explore the real numbers behind the actual physics, and the equations that express this.
So I hope this OLD real data puts the N-Blade poor efficiency myth to rest. If there is interest I can explain more about why I design with high blade count props and the advantages they provide.
Actually an excellent point where the loading is spread across 30 shorter blades, at a higher RPM, to shift the noise spectrum and lower the blade loading. Two slow, highly loaded blades creates a lot more noise. The six motors will also have slightly different RPM, spreading the noise spectrum even wider, for even lower perceived levels.
The result would have been much higher noise with six 2 bladed props.
A couple days ago I said I would share some insights about why N-Bladed props are highly efficient, and often optimal over 2 or 3 bladed props. All mechanical systems have near zero efficiency at very slow speeds and gain efficiency as they gain velocity. Same is true for props too. This is the efficiency graph for the 9 N-bladed props I previously shared raw test data for.
Notice that the high bladed props all become much more efficient at slower speeds, since RPM * Pitch is velocity, and all the props have the same 7" pitch.
This strongly implies that a high blade count prop will most likely be a LOT more efficient at cruise, where a two or three blade prop will be very inefficient, which is for the majority of the battery capacity. In this test the 2 and 3 bladed props were RPM limited by battery voltage and unable to reach their best case efficiency.
For reference these are the raw power and estimated shaft power graphs.
Sorry Rick, it’s just High School AP Physics and Algebra, where following the science with accurate test data and math has a very different outcome than the “we have always done it that way” that snake oil sales men, hyped up on mushrooms, selling one bladed unicorns, are scamming with. I didn’t test for the one bladed case, but it’s pretty clear what it’s outcome would be.
Over 1,000 teams, mostly lead by University Prof’s and PhD’s failed the GoFly challenge for exactly this reason. Building scaled up quad, hex, and octal copters with two and three bladed props, just doesn’t work. They failed to do the math, and test properly. And they failed to win the $1M prize because of battery range at a low efficiency.
Of course I’m sure that you and your echo chamber will still have insults to beat your chests with.
The adjusted shaft power numbers were estimated by subtracting:
0 blade power at each PWM frequency. This is mostly PWM high frequency switching losses in the ESC, wiring, and motor.
R*I^2 losses at measured power. This is mostly resistive losses in the ESC mosfets, caps, wiring, and motor windings.
8% of the remaining power. This is mostly iron eddy current losses in the motor.
These three in practice net about 30% for most systems, which it what results in the 70% efficiency cap in the graph. If highly optimized it can be reduced to about 20%, but that will not change the shape of the N-Blade efficiency curves, or N-Blade ranking between 2-N blades.
This is why I proposed a high efficiency air-core motor combined with a high count N-Blade prop for the PPG market to lower costs and increase performance at a lower weight and size. The data is pretty clear that is likely to produce a much better product.
If anyone is interested, drop me a note, and I’ll save you an early adopter spot when I start taking pre-production orders.
A couple other notes. The E Prop guys have great props, and have got designing a prop to a specific engine down to an art. If you go to their web site, you will find that they also blow away the N-blade myths too. At the end of this page, they also clearly state their props can not fly in their own wake. They also clearly say " for the same diameter, more blades = more efficiency". They offer props with up to 6 blades, which for high torque engines is important for low speed operations like PPG. Wimpy engines don’t perform as well.
At the end of the day they are just providing the best prop they can for the motor their customer has chosen. When the customers motor can only deliver 60-80% efficiency, their nearly 100% efficient props still only deliver close to that 60-80% system level efficiency.
I come from a different perspective where I design with 95-99% highly efficient air-core motors, and can choose matching 95-99% highly efficient props that will deliver 90-99% system level efficiency for the best performance, across a wide range of applications. And design for safety and redundancy not found in similar products.
The proof of concept air-core motor with integrated prop I printed last week did it’s initial spin up today. It had 3 out of 18 stator phases hand wound, and target performance appears right on the mark using a 3-phase RC ESC. So I can start on the custom 9/18 phase trapezoid controller board design and hopefully have it back in a few weeks.
So things are still on track for high efficiency motor/prop testing before end of March.
I am going to have this topic unlisted as it has gotten quite long and one-sided so bit less of a “productive conversation”. Once you can demonstrate the stated efficiency number flying then i will relist the topic. Hope to see it and best of luck.
