3-blade propellor for an 0-320?

[seagull]

Thank you Bill R & Bill G,

I hear what you both say. My basic understanding is that 3-blades tend to damp down some of the vibrations slightly, give less noise and as you say, a smaller diameter arc so less (sorry that should read 'more') ground clearance. And I agree that for a small aircraft like this with only 160hp the diffences may be negligible at best. But some fliers seem to swear by them and my personal taste is the 3-blader looks better. 35 years ago when I joined EAA we didn't have much of a product choice (at least in my country) so now I have a lot more options to evaluate with this, my retirement project. The choice is exciting! - So I am keeping an open mind and checking things out as best I can. I must be getting quite ancient 'cos I just dislike the modern square glass panel screens on principle. Yuck! - much prefer the old analog instrument displays I was brought up on. Who cares if they are proven to be more efficient - they're just not right. Back to the project - I'm trying for good take-off performance out of some short strips in the hills. Hopefully not too overweight with my bag full of NZ Rainbow Trout.

Cheers

Barry

PS - Here's a baby we had to throw back - undersized!

[Bob H]

I fly from a relatively high altitude airport at 6800' with density altitudes reaching close to 10K' during summer. To accomodate operations, I have a wooden GSC ground adjustable 2-blade prop, which allows custom tweaking of pitch for the conditions. Works well and is inexpensive. Plane is a Pulsar and engine is Rotax 912 with high comp pistons at around 90 hp max.
Props have to absorb all the power the engine can produce without lugging the engine down with too much pitch or having insufficient pitch and over speeding and not developing full power. In general terms, you want the largest dia blade to move a high volume of air but also need proper pitch for power absorbtion and maintaining engine rpm within power band. The Prince Prop folks have a good formula for computing prop dia at cruise;

Dia (inches) = 125[ [HP/ (RPM^3 x mph)]^.2] for 2-blade. Multiply the 2-blade dia by .933 for 3-blade, and .891 for 4-blade.

[seagull]

Thank you for the formula Bob, I hadn't seen that anywhere else. As for the 'hills' I intend flying it to the highest I would mostly go to are around 4,000 feet density altitude even on a hot day, and that would be for a venison trip where my companions and I would likely hire a chopper to get us in and out. Most of my intended excursions flying myself in would be in the lower 'rough country' where the best trout streams are, not the real high stuff. It's more important to me to get the power development for a short take-off than a very fast cruise speed, although exactly what that prop compromise might be is possibly too early to tell.
Thanks again for your interest

Cheers
Barry

[Dave Prizio]

There seems to be a glitch in the prop diameter formula. I keep getting answers in the range of 1". Someone else try it and see if it is me or the fomula. I used a GlaStar with a 180 hp engine, 2700 rpm, and 160 mph speed. I got an answer of 1.1.

Dave Prizio

[spungey]

Agreed. Something seems amiss. Here's some results from dumping it into a spreadsheet. ... Transcription error?

rpm	Rpm^3	times mph	180 / rpmstuff	fifth root	Times 125
2700	19683000000	3149280000000	5.71559213534522E-011	0.0089415384	1.1177
2550	16581375000	2653020000000	6.78472080873872E-011	0.0092535078	1.1567
2400	13824000000	2211840000000	8.13802083333333E-011	0.0095962987	1.1995

[Bob H]

I got 66".

Take the quantity 180/(27 X 160)=.04167
Raise that to the 0.2 power which equals .529
Multipy that .529 by 125 and result = 66.2"

[Snowflake]

Take the quantity 180/(27 X 160)=.04167
Raise that to the 0.2 power which equals .529
Multipy that .529 by 125 and result = 66.2"

Yeah, but that's a different formula. The formula in Post #13 says RPM^3, not RPM/100.

[Max Torque]

seagull - recommend you get together with Jan at JC Propellers www.jcpropellerdesign.com . He offers an excellent prop design program that is well worth the price and he is extremely knowledgable about propellers. 2-blade or 3-blade depends upon many things. I'm designing a prop for my direct-drive Corvair and the rpm dictates a 3-blade prop for what I want it to do for the plane it's going on.

[seagull]

Thank you for the contact Max - I'll go and check them out.

Cheers

Barry

[Max Torque]

My bad - Jan's website is actually www.jcpropellerdesign.com

I also highly recommend getting a copy of Jack Norris' book: Propellers - the first and final explanation http://www.propellersexplained.com/ Different writing style, but an informative book.