stdun

I have a Funtana with a Saito 91, currently using an APC 14-4 prop. I plan on going to the 15-4 soon. BUT I found a Graupner 3 blade prop I got for this motor to use on a dfferent plane. The problem is I don't understand the size and pitch numbers. There are two sets of numbers. 32-18 is one set and the other is 12,5-7''. What is the size and pitch of this prop? How would it work with the Saito on the Funtana? I haven't flown this plane alot but it seems the 91 is to much power, it hovers at 1/3 or less throttle.

3Dn4life

your prop is a 12.5 inch diameter and has a 7 inch pitch, the other set of numbers is cm i think. A 12 inch prop is to small for the 91 and you could end up over-reving it and ruin your engine, i have a funtana with a 91 also and have a 15x4w prop on it

halfandhalf825

there is no such thing as too much power

3Dn4life

True.....True

seanychen

My Feedback: (4)

3-blade 12.5 x 7 has the same load as 2-blade 13.75 x 7. I bet it will run the same rpm as the APC 15x4W, which my Saito 91 peaks @ 9800 rpm.

Graupner indicated that this 3-blade 12.5 x 7 as well as their 3-blade 12 x 8 is for 4-stroke .80 ~ .91 or 2-stroke piped .61.

[link=http://www.hobby-lobby.com/grprop.htm]Hobby Lobby Link[/link]

stdun

Does the 7 pitch compare in the same way on a 3 blade as a 2 blade? In other words, if my plane is fast now with a 14-4, it would seem logical that it would fly faster yet with the 7 pitch, or not? Or would the extra drag calm the motor down a little?

seanychen

My Feedback: (4)

Pitch for 2-blade functions the same as 3-blade, if rpm is identical. So a 6" pitch 3-blade, compared to 4" pitch 2-blade @ the same rpm, will theoretically go 50% faster, ignoring plane drag & prop slip. The difference between 2-blade & 30-blade lies in the efficiency to produce thrust: more blade = less efficient (only generally speaking for R/C use in this rpm range)

Your plane seems fast because your rpm is pretty high. I imagine the Saito 91 must swing the 14x4W @ close to 11000 rpm, which is a little fast for a 4-stroke this size. Try to keep the rpm to 10k @ ground.

Although, 4" pitch @ 11000 rpm is roughly 45 mph level flight, which is not that fast. You can always throttle down for level & down leg, and reserve full throttle for vertical.

Higher pitch means you'll be able to maintain speed better @ up leg. That's why pattern planes use high pitch props: they need to keep going. Lower pitch means you'll accelerate out of zero-speed better. So depending what you want, 3-D: low pitch; pattern: high pitch.

I think Saito 91 swinging the 3-blade 12.5x7 is a good trade-off between speed & thrust, because you'll have about 8 lb. of static thrust, and be able to go 70 mph theoretical top level speed. I think a 2-blade 14x6 is perfect though: 9 lb. of static thrust, 60 mph top level speed. For all-out 3-D, either the 14x4W or 15x4W will get you like 10 lb. of static thrust, w/ a top level speed of 40~45 mph.

The only advantage of a 3-blade for this application would be ground clearance, and cool looks.