Propellers

[asra]

G'day guys,A question concerning propeller pitch, I have a 4 blade bolly prop at 72inches with a gearbox ratio of 2.64:1. At 5200rpm there is a harmonic that is hard on the ear drums, we have found that de - pitching 2 opposing blades 2deg this harmonic is lessened. (These opposing blades are perpendicular to the remaining opposed blades)However what ramifications are there to the hub/prop with 2 blades @ 14deg & 2 blades @16deg?We have done the same to a warp drive 4 blade at 76inches, same ratio and has considerably quietened the prop with no performance loss, and a gain of 250rpm.Regards,ChristopherLockwood Sth. VICSpeaking without thinking; is like shooting without taking aim - Spanish Proverb

[asra]

You will probably find if you change to a 3 blade prop, the harmonics will disappear. I found a similar situation with the 3 cylinder 3701 Hirth and using a 3 blade Powerfin 74 inch prop. After changing to a two blade prop, I found the harmonics had gone but didn't bother seeing if it was the same with a 4 blade prop. It's worth a go.

[asra]

Thanks Barry, but have just changed from a 3 blade warp drive at 72inches that needed 17 deg pitch to get similar performance.Regards,ChristopherLockwood Sth. VICSpeaking without thinking; is like shooting without taking aim - Spanish Proverb

[Tim]

Chris, how is your Subaru exhaust set up? Are the outlet/s "firing" through the prop?Tim McClureBroken Hill

[asra]

G'day Tim,EJ22 exhaust into one centrally under the sump then an outlet approx 10inches long into right angle and 6inch extension to the side, see pic attached.Image Insert: 14.38

[Tim]

Christopher, I found that I could only stop the "harmonics" from an open exhaust/prop by dumping the exhause gas outside the prop circle, and just after the prop. Sometimes you can get away with a good exhaust pulse "diffuser" like a good muffler (not straight through) or a turbo and angle it sideways instead of straight at the prop, but even then it is not guarenteed. Many Rotaxes will still harmonise, especially with lighter mor flexable props like the Ultra Prop and the Brolga.Tim McClureBroken Hill

[asra]

Something else you can do to help performance is to cut the exhaust tip back at 45 degree's to the oncomming airflow. This causes a vaccume at the pipe tip helping expell spent gases.Image Insert: 20.7 KBAs for your problem with harmonic prop sounds, I actualy like the sound, and it is usually heard more during sudden wind gusts blowing through your prop. Remember it will all go away as soon as you open up the tap.Regards Sam.

[asra]

Thnaks for the input guys, but am more concerned about the effect on the hub with different pitches for each pair of blades.Sam - it already has the tip cut, and the tap is open mate only another 400rpm left.Regards,ChristopherLockwood Sth. VICSpeaking without thinking; is like shooting without taking aim - Spanish Proverb

[asra]

This set up would be beter off with a 76" prop with the reduction you have or if you are stuck with 72" then a 2.2 reduction would have pitch back to 15deg & max revs of 5200 aproxButch S.

[asra]

Thanks Butch,Looks like I am stuck with a 72inch prop, however it's not my ears that are having the problem.Regards,ChristopherLockwood Sth. VICSpeaking without thinking; is like shooting without taking aim - Spanish Proverb

[asra]

Tim, I think you are right with the exhaust behind the prop.That's what I have on my other 2 seater with the same prop, although the box ratio is 2.25:1Regards,ChristopherLockwood Sth. VICSpeaking without thinking; is like shooting without taking aim - Spanish Proverb