Prop differences...
 

Hello!

There's somebody who can help me to understand the difference between differnce of some propellers. I have a 26cc gas engine on a 12 pounds airplane, I got 18 x 8, 18 x 6 and a 16 x 8 wood prop, what should I wait from these 3 kind of prop?

Thanks a lot.

RE: Prop differences...
 

The rule of thumb is bigger diameter equals more torque and more pitch equals more speed. It depends on what kind of plane you have and how you want to fly it.

RE: Prop differences...
 

I'm getting 7150rpm with a Turnigy 16x8 with my 26cc, more props to be tested next Thursday.

RE: Prop differences...
 

with what "soarrich" said I would think an 18 inch prop will be to big,, I would think you will want a diameter of 16 or 17 inches and a pitch of 6 or 8 inches, "17x6" or "16x8"

keep in mind a Zinger 16x8 should rev higher than a 3W 16x8, the 3W prop will load the engine more than the Zinger

as "flatspinJim said a rule of thumb,,
smaller diameter more pitch = more speed, and usually at higher throttle settings, so more throttle will be needed at lower speeds because of the smaller diameter,,

larger diameter less pitch = more pull at lower throttle settings, less top speed because of less pitch,,

once you have found a prop and RPM that is close to what others guys are getting on the same engine you go out and fly,

once in the air notice how much throttle you need to keep the plane at a normal flying speed,

if you find you need a lot of throttle (say 70% or more) you may want to go up 1 inch in diameter and down 1 or 2 inches in pitch, that will make your engine pull better at 50% throttle,,

now, if you find your flying at 30% throttle and-or you do not go much faster after 50% throttle and you want more top speed you will want to go down 1 inch in diameter and up 1 or 2 inches in pitch..

do a search, find what prop others are using on the same engine, then go from there..

Jim

RE: Prop differences...
 

Thanks guys for your answer, at this moment I tried only an 18x8 on my sv26, the plane is a high wing of 84 inches and a lenght of 63 inches, the max rev I got is 6200 0r 6400 rpm at the full throttle, but nothing change from 60% throttle to 100%

In my hand now I get a 18x6 and 16x8, so I should try the 18x6 first?

RE: Prop differences...
 

You might want to check this out. I have a SV26, I haven't opened it up, but it looks to use the carb block from their SV50, I gained 300rpm with this simple mod. Use a dremel sanding drum and it takes about 30 seconds to do. It lets the air/gas mix get a straight shot at the reeds.

RE: Prop differences...
 

There is a big difference in props going from a 16x8 to an 18x8, in the the two brands I've looked at anyway. I have the same engine as you and was recommended to use the 16x8 first and then the 18x8, both Bambula props. I broke the 16x8 and just put the 18x8 on and test ran it. Didn't tach it, but after readjusting the needles, I could tell the rpm was down. The engine also was getting hot. The 18x8 is not only longer, but wider. It's even thicker at the hub. I'm not even going to fly the plane with it. The 16x8 was perferct for the plane which is a Goldberg Ultimate. The differences between the 16" and 18" props were apparent in the Zinger brand also. The 18" props had bigger hubs, wider blades and therefore, weigh more. Use the 16x8 and you'll be happy.

Rich, that looks like a good mod. I might have to try it out.

RE: Prop differences...
 

Thank you, I'll try it next time, the wurst can happen, it's I don't get enough speed or torque!

RE: Prop differences...
 

""the max rev I got is 6200 0r 6400 rpm at the full throttle""

tuned for flight I am running my MVVS 35cc at 7200, and my friend is running his DA 50cc at 6500, so I would think you would want to rev a 26cc somewhere around 7400 to 7800 tuned for flight,,


""but nothing change from 60% throttle to 100%""

if an engine's Throttle Linkage is setup correctly you should not feel much of a difference in power from somewhere around 85% to 100%,, so make sure your Throttle Linkage is linear..

Jim

RE: Prop differences...
 

For that size engine - prop for at least 8000rpm
the best usable powerband is in the higher rpm ranges 8000-10000

RE: Prop differences...
 

....yup, when I was running a 26, I saw significant increases in performance, propping for RPM 9000 up to 9600 (aerobatic flying) with standard in-cowl exhausts. Below 9000 and the engine would never get into its power, and above 9600 with a prop that would spin this fast, just wasn't suitable for that style of flying on a standard exhaust. Running a larger prop, on a tuned pipe at 9600-10000 would have been just fine though. :)