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Information required about props (2 vs 3 blade)

#1
Hi guys,
I am about to start building a HK FPV250 and I have seen people with 2 blade and 3 blade props on this multirotor.
I would rather just stick some 2 blade props on it as they are generally easier to come by in the event of a crash. However, would there be any benefit to having 3 blade props as opposed to 2 blade? Unfortunately I'm not that wise when it comes to choice in props. If flight time was to be influenced positively by using 3 blade props I'm all for it as I don't want to be in and out charging batteries. This being said I seem to remember on one of the Q&A episodes (i think) David mentioned that 3 blade props were inefficient as the blades hit the disturbed air quicker or something along those lines. Anyway as I said I'm not to clever when it comes to props.

Any information regarding amount of thrust, efficiency, flight time, etc... would be much appreciated.
also if you have the multirotor in question (HK FPV250) please share any useful information regarding any aspect of the multirotor you may have.

Thanks.
 

Craftydan

Hostage Taker of Quads
Moderator
Mentor
#2
dual bladed props require longer length to get the same thrust, but will burn far less energy doing so. The effect gets worse with increasing KV.

Sometimes it's not an option, though -- The tri-blade will fit better on a smaller frame (which is why they're popular on heavy micro and mini quads). What you gain from the smaller/closer thrust columns, however, you'll loose in flight time.
 
#3
dual bladed props require longer length to get the same thrust, but will burn far less energy doing so. The effect gets worse with increasing KV.

Sometimes it's not an option, though -- The tri-blade will fit better on a smaller frame (which is why they're popular on heavy micro and mini quads). What you gain from the smaller/closer thrust columns, however, you'll loose in flight time.
so, what gather from that is:
Flight Time: 2 blade > 3 blade
thrust: 2 blade < 3 blade

shame if so as i cant wait for the day it take me 1 min to charge a battery and it lasts 60 mins worth of flight. (i assume every rc pilots dream)

Thanks for the information thus far. any more regarding either prop choice or tips on 250 size quads would be much appreciated.
 

Montiey

Master Tinkerer
#4
I was watching an episode, can't find it now, but David mentioned that three bladed props are less efficient, but more powerful.
 

FinalGlideAus

terrorizing squirrels
#5
Correct. They have more surface area than the same sized 2 blade so they move more air and therefore more thrust. BUT... Because the blades are closer together they hit the turbulence of the blade in front of them and because they are using less "clean" air they become less efficient. You will nearly always get the same or better thrust and efficiency by going up a size with a two blade prop.

Simply put, performance wise, the only reason you would use a 3 blade prop is if lack of space stopped you from going up a size in a 2 blade prop.
 
#6
Correct. They have more surface area than the same sized 2 blade so they move more air and therefore more thrust. BUT... Because the blades are closer together they hit the turbulence of the blade in front of them and because they are using less "clean" air they become less efficient. You will nearly always get the same or better thrust and efficiency by going up a size with a two blade prop.

Simply put, performance wise, the only reason you would use a 3 blade prop is if lack of space stopped you from going up a size in a 2 blade prop.
so having said that, in theory these:
http://www.hobbyking.com/hobbyking/...tating_5Pcs_Bag_.html?strSearch=propellers 6x

will perform just as well as if not better than:
http://www.hobbyking.com/hobbyking/store/__23460__5030_black_Three_Blade_Propeller_ABS_3pc_.html

for example?
so possibly longer flight times with the first ones?
 

FinalGlideAus

terrorizing squirrels
#7
In theory yes but you need to prove it in real life. It's not just the size, pitch and number of blades that count. The brand of prop and what the prop is made of also play a part. The speed of the tip has an effect also as props have an speed range which when they go above they loose efficiency. Plus are you looking for your efficiency at half or or full throttle? It gets complicated doesn't it :)

The good news is if you search hard enough there are plenty of people out there willing to do the leg work for you. Take this example of a thrust test on the SunnySky2204 2300kv motor that Soma did in order to find the best prop for it.
http://www.rcgroups.com/forums/showthread.php?t=2112708
I don't have the time or patience to do this so iñstead I look for wonderful people like Soma to do it for me.
 

Balu

Moderator
Staff member
Admin
Moderator
#8
Another reason for three blades is if something is blocking your air flow - a prop mounted in front of a big wing for example. Everytime a two bladed prop is parallel to the wing it can not push the air, while a three blade prop still has two blades that do.
 

anmol77

Junior Member
#9
The brand of prop and what the prop is made of also play a part. The speed of the tip has an effect also as props have an speed range which when they go above they loose efficiency. Plus are you looking for your efficiency at half or or full throttle?????
 

jipp

Senior Member
#10
welcome to the forum, where did you find this thread from? its quite old. things have seem to change, in favor of the multi blade props for speed for the mini quad racers. im cheap so i will be using 2 blade and i have a drawer full of them.. so why not. :D

chris.
 

Craftydan

Hostage Taker of Quads
Moderator
Mentor
#11
Things may change, but the physics behind them hasn't.

What FGA mentioned in both his posts remains valid -- add more blades, get more thrust but at the cost of shorter flight times.

It has come to the point where getting that extra bit of brute-force speed, a 6" prop just isn't enough for some 250 frames, even at higher voltages. As FGA mentions, if the frame can't hold the prop that big, trade flight time for blades.

For example . . . In a race with three laps averaging 1.5 min (or less) means the race won't last 5 minutes -- putting a power system onboard that lasts 8 minutes is fine for practice, but in a race leaves unused fuel in the tank. This, and the flash of "I'm swinging 16 blade props now" is what's driving that trend. On the high end of racing, it's an edge. On the low end . . . DNF is more often than not the deciding factor.

For the points about brand . . . A propeller is just a wing that spins really fast. Just like the optimized airfoil for a slow thermal duration glider will be dramatically different than one for a high-speed pylon racer, a putting on a airfoil better suited to the RPM you're spinning can give you dramatic improvements. The stiffness and durability also come into play -- the stiffer the prop is the better it holds that airfoil as it spins to higher speeds . . . but generally the easier it is to get dinged or broken. The "brand" comes into play because each manufacturer builds their own molds and picks their own materials. Better than even chance a few are merely copying other manufacture's designs (a few may be building to TLAR, but that usually comes out in the poor performance). The better brands take the time to make good estimates of the RPM range of the props and try to optimize the airfoil to that speed envelope . . . and it shows.
 

jipp

Senior Member
#12
morning dan. thanks for the feed back.. so yeah the 180 frame with its 4" bullnose would probably handle better with a 3 blade.

and at that size probably even steven on fuel, or would a 2 blade still get better flight time at 4"

again, i will stick to 2 blades until the multi blades become the same price i guess which i don ot see happening.

and i agree i use blades that are durable.. as id rather take a hit on performance for durability.
i have to be frugal, so if i can save a buck here and there yay me. but i do appreciate people pushing the edge, and it sure looks fun to race.. but i doubt ill ever be in a race.
chris.