I have a question regarding propeller selection. Currently I am using Master Airscrew props.
My quad is 24" x 24" aluminum tube with Turnigy 2217 16turn 1050kV 23A outboards. 2 motors are powered by a Zippy 5800mAh 30C battery, the other 2 motors by an identical battery. A third Zippy 4000mAh 20C battery powers the IMU.
I've noticed that when I was initially running everything on the single 4000mAh battery, I had about 8 - 10 minutes of flight time. Now that I am running more batteries, I get about 3 minutes of flight time and the quad requires 3/4 to full throttle to climb.
I am starting to question if the props I selected are not providing the same amount of lift the APC props would. By that I mean a propeller pitch issue. If I can get more lift out of lower RPM's that would increase battery life and require fewer batteries on-board. Thus dumping weight and improving performance.
I haven't weighed my quad lately, but I would guess I'm right around 7lbs UAW.
I'm curious as to suggestions to improve flight time. Thanks in advance.
Mo
Replies
Mo, thank you for providing me an opportunity to pontificate about one of my favorite aerodynamic issues: the Reynold's Number paradox. Conventional wisdom says that because lift increases with the square of the velocity and drag increases with the cube, turning a propeller more slowly will always be more efficient. For larger scale propellers that is true. However, for those props of less than around 24" in diameter (in other words, just about ALL model-scale props), the exact opposite is the case. This is due to the fact that the actual coefficient of drag for an airfoil section increases with low Re - higher RPMs actually cause the Cd to go lower, eliminating the usual cubed-with-velocity relationship with total drag.
Here is the data for your Master Airscrew 10 X 6 propeller taken from the University of Illinois wind-tunnel testing (as real-world and scientific as it gets):
RPM Ct Cp Calculated FM
Notice that the figure of merit or FM, which is the ideal power/actual power ratio, has a positive correlation with increasing RPM. This is true up to around 6,000 RPM, where with FM starts to fall off.
If you're not turning this prop to at least 4K RPM in hover, you're losing - not gaining - efficiency.
Also, think of pitch as analogous to aerodynamic "gearing" of sorts. Higher pitch is equal to a higher gear. This is why low-pitched props are preferred for static thrust; it takes less torque to twist them faster. Even so, higher pitch with most propeller airfoils also risks having part (if not all) of the blade in a stall condition at lower RPM.
But efficiency and pitch aren't your problem.
To solve for actual thrust using Ct:
T = (Ct) * (0.00238) * (RPM/60)^2 * (Diameter/12)^4 = lbf (pound force)
So the MA 10 X 6 prop turning at 4220 RPM should be giving you (0.102) * (0.00238) * (70.3)^2 * (0.822)^4 or .548 pounds of thrust. At 6505, it's (0.1028) * (0.00238) * (108.4)^2 *(0.822)^4 or 1.31 pounds, which should be considered maximum thrust. Dude, you are seriously underpropped for a 7 pound quad.
Just for comparison sake, the APC 10 X 4.7 can produce 1.728 pounds at 6520 RPM. If that's what you were using before, it's no wonder your quad seems rather sluggish. That said, 6500 RPM is usually around the recommended limit for a 10" prop, so you should check to see if you're tempting fate as it is.
"A third Zippy 4000mAh 20C battery powers the IMU." That's a bit much too - it's just extra weight to drag around for no real benefit. If your quad flight time is 15 minutes, there's no reason to have battery capacity for >> 4 hours for the IMU.
Battery life is all about current drain. A 4000 mAh battery supplying 4 Amps will last 1 hour. A battery supplying 40 Amps will last 1/10 as long or about 6 minutes. The motor selection, prop selection, weight of the copter (hence the average throttle position in flight), and number of cells in series (ie Voltage) will all impact the current drain. My guess in your particular situation is that you went from a 3S to a 4S (ie higher Voltage so higher current drain) and that may be the big issue. But if not, keep reading.
There is not an easy answer without some testing. I recommend you get a current meter and measure the current drain on your motor / prop combination. Try the same test on your previous combination. The prop selection can have a very large impact as well on current drain, all other variables being equal. Try other combinations of props with this same test. You can also experiment with the number of cells and lower the weight of the copter as well but start with different props.
Eddie