The Relevance of Propellers

[Rui Manuel Cravo Marques]

Hi again. Latelly I've been testing some different propellers on my quad and checking the main differences between them. For my emax gt 2218 930kv , my 3S batt and the 1500 / 1600 weight, I was using those cheap 12x4.5 props. I even bought 12 pairs for $25 or so, and one thing I can tell you about them... FORGET ABOUT IT. After 2 crashes from wich I could not tell if it was a motor or prop problem, I decided to test other props. I started thinking that too much flexibility would have a negative impact, both on material resistance after a couple of flights and prop deformation on higher RPMs giving it less efficiency.

Looking close to one of those cheap props with about 3 flights, I noticed the plastic was becoming fragile near the joint to the hub (becoming white as if it was heavily bended). At APC page you can read about the forces involved and afecting a prop, so I decided to start using higher quality and stiffer props, and I'm not regreted.

There are some models wich mix carbon with nylon giving it a good stiffness. I'm using composite APC E series and APC slowfly and also those carbon nylon ones and all seem to behave very well.

One thing you will be noticing is the much quieter noise and less vibration you get from the motors. Balancing is almost unnecessary, and with 12x6 apc e, I managed to get about 2 min. more flying time. So, forget about cheap propellers. I believe they are like tires in a car. If they're not good, you'll crash for shure if you push them.

If you just hover, and don't make sudden alt changes or high pitched maneuvers, the cheap ones will stand, otherwise, get high quality props.

[Jonathan M]

Good observations. I have experienced the same.  When you look at the 12x4.5 EPP props vs. the 12x3.8 SF props you will notice the planform area is much greater on the APC.  Also the airfoil on the APC is much "cleaner" (very blunt leading edge on the EPP).  As you observed, the material for the EPP props seem to be lower quality and softer. 

Carbon fiber props with a foam core would be nice but very expensive.  A well designed prop could potentially give 5-10% more efficiency as well. 

[Rui Manuel Cravo Marques]

There are lots of small things that can affect our multicopters performance. I have read (and experienced) lots of big problems that were no more than the amplification of small problems. A vibration or a bad motor alignment or uncalibrated ESC, tends to spread it's effect along the entire system, leading to bigger problems. So if we start to solve problems on peripheral base components, the software will have much more efectivness. Sometimes, programmers try to correct system malfunctions via software when the problem is at hardware level, so when you repair the hardware, the software will in turn generate malfunctions, trying to compensate for problems that no longer exist, therefore, it's important to create here some type of hierarchy and identifying what components can lead to what problems. I never thought that a simple piece of plastic would cause so much impact on the quad performance untill I started to think in terms of automobiles and their base components.

[Martin Szymanski]

Planning of good architecture comes first.

The programing works with hardware but does not make the real world.

However it can adjust hardware only so much.

At a certain expectation software will give up on a build.

I've always used composite propellers.

Welcome aboard.

[James Roney]

Anyone know where I can get cw and ccw APC 12.2.8sf props in South Africa?

[Graham Dyer]

12x2.8? never heard of that size, 12x3.8 are the smallest pitch I've seen. I haven't found many CW (pusher) props here in SA, and especially not in that size, I've had to order everything online/overseas. You can try www.netram.co.za though, they have some CW and CCW props.

[James Roney]

Yeah, sorry, I meant 12 x 3.8.  Netram do the same composite's that you can get off ebay.  I am hoping to find APC's specifically.

[Oliver]

I found a good paper on the relevance of good props and prop sizes on the efficiency of VSTOL aircrafts. Sadly it is in German, but main outcome was that the prop usually has the efficiency from 40-70% which may impact the flight time a lot. Additionally larger props usually also increase the efficiency of the multicopter.
http://www.rc-network.de/magazin/artikel_02/art_02-0037/Standschub.pdf

[Rui Manuel Cravo Marques]

Yes, this is all a question of balance (when it comes to sizes and pitches). Today with my 12x6 apc e composite, I got 15m fly time, most of it in loiter. I was getting 10m / 12m with the cheap ones, so I can see here some better efficiency. The size of the prop is also limited by the motor's torque and power. If you get too large props, and have a low torque, the motor will take too much time to achieve desired rotation, also, bigger props = + stress to the motor, requiring more power (wich is not always available). Smaller props will require more RPM but will give the motor much quicker accelaration and less power need. I'm happy with the 12x6 and 12x3.8 slowfly for my 1.5kg. If I had anything less than 1kg, I would be using 10" props.

 

[John Arne Birkeland]

Did you balance the propellers before usage? Cheap ones sometimes are so unbalanced from the mold that they will tear themselves apart in flight (and ruin the motor ball bearings in the process). Even brand names like APC may need some balancing before usage.

[Rui Manuel Cravo Marques]

Yes, I usually balance them all before usage. However, high quality props need lot less weight to balance properlly, and offer lot less vibration and a "smoother" motor noise when running.