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.
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Hi Brad, I've been reading the document and it's a great work. I can only apologize for not beeing an aerodynamics specialist and finding it a little difficult to interpret, but the graphics can really help a lot, specially identifying the most efficient type of prop. Thank you.
Propellers are extremely relevant, and most model airplane designs are terrible for static thrust, mainly because they're not designed to stand still. There are so many potential compromises of airfoil section, rotational inertia, and pitch angle. For our purposes, the lighter the propeller, the better. But we also want an airfoil section that is resistant to stalling at inflow rates of nearly zero, yet still manages decent thrust at practical speeds. This means the airfoil chosen must be thicker, which unfortunately adds mass and also makes the Cd (drag coefficient) higher. Lower pitch is not always better, but even the venerable Clark-Y section starts to stall at any pitch beyond 14º. Model airplane propellers are designed with thicker sections at the hub and thinner sections toward the tip, and these can stall at 8º or less. However, you do want the pitch as high as you can get it without stalling for the maximum Cl (coefficient of lift).
Airplane propeller pitch is calculated at 75% of the blade length, so a 12 X 4 (using basic trig) would have a pitch of 6.1º. The actual angle of attack is considerably less, however, because the minute the propeller starts turning, it produces inflow which lowers the angle, which in turn, lowers the Cl. This is why many high-performance (read high-speed) propellers have huge pitch angles and perform poorly in static thrust (to Rui's point, the blade is stalled).
To the point, there are so many variables involved that trying to find an off-the-shelf propeller that works well with a specific model weight and motor is a trial-and-error project for most people. Here's a link to a definitive work on model airplane propellers that I've posted before, but it is a good read if you're interested in practical results of aerodynamic theory:
http://www.ae.illinois.edu/m-selig/pubs/BrandtSelig-2011-AIAA-2011-...
Well, as to battery performance, there's not a lot of difference between both. Motors seem to have a smoother noise with SF 3.8, and I believe the motors are not needing more high RPMs to hover, but the quad gets a bit less responsive (sluggish). The E 6 give a quicker response but seem to work better at higher RPMs (as you said). The motors are 930kv so they handle both well, but in windy conditions I go for the 6 pitch because of it's faster answer compensating.
How do the 12x6's perform compared to 12x3.8 SF? Generally, lower pitch is better for quads because of the mostly static conditions. The more pitch, the more likely a good portion of the blade is stalled. It will still create thrust, just not as efficiently. The 12x6 Es have a skinnier profile and probably spin at a faster RPM. So, they probably match better with higher kV motors.
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.
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.
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.
http://www.rc-network.de/magazin/artikel_02/art_02-0037/Standschub.pdf
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.
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.
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