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Old May 20, 2003, 04:28 AM
Turbines suck ;-)
Australia, QLD, Brisbane
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3 blade vs 2 blade props

Anybody have any information on the pros and cons of a 3 blade prop in place of a 2 blade prop. This is for an IC engined plane and I am trying to get the noise down. I was thinking of using a 3 balde in place of a two blade to get the diameter down and reduce tip noise which, is my main problem right now.

Please don't abuse me for flying IC as well as electric - every now and then I need those nitro fumes, a sticky transmitter and a mess to clean up after flying - makes me appreciate electric more .
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Old May 20, 2003, 09:52 AM
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Dick Huang's Avatar
Dallas,TX, USA
Joined Mar 1999
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rorywquin
Do a search on multi blade props on this website.
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Old May 20, 2003, 10:13 AM
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United States, NY, Spencerport
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A 3-blade prop is less efficient than a 2-blade. The blades are closer together, so the air has less time to "settle down" before the next blade hits it. The cleaner, less disturbed the air is, the better the "bite" the prop gets.

The difference is very minimal. What little you lose by going to a 3-bladed prop is not really worth worrying about, IMHO.
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Old May 20, 2003, 10:27 AM
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Yes, a 3-blade can help reduce noise. I ran one on my ST 75 powered Panic bipe for a while. The rule of thumb at .40-.60 size is to go down an inch in diameter (so at about the same rpm you automatically get lower tip speed), think I used an 11x7 on the Panic and a 9x6 (?) on an EXTREMELY quiet .35 FP powered semi-scale model that passed the 76 dB noise limit for my club's glider & electric field

BUT ... many 3-bladers have blunt-tipped 'paddle' blades (thinking of the Graupner grey ones I used), which are inherently noisier than the more modern APC and Graupner CAM designs.

So there are other ways to cut prop noise, with the Panic I eventually went to a 14x6 2-blader with a de-tuned (over-length) quiet pipe. That was pretty unobtrusive and performed a lot better.

As already mentioned, 3-bladers are often less efficient than 2-bladers (not such an issue if you have loads of power in the first place).

IC noise reduction is a funny thing, reduce the exhaust and blade tip noise is often dominant. Reduce that and (believe it or not) induction noise becomes an issue!
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Old May 20, 2003, 11:21 AM
Turbines suck ;-)
Australia, QLD, Brisbane
Joined Mar 2000
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Thanks for the input guys - It is actually for a Laser 150 (XtraWot).

It was 85 db (16x8 prop) so I made a silencer which really reduced the exhaust noise but the measurement was still 83 db. Used a 18x8 and just scraped through on 80db (our club limit) but the engine seems over propped now and the rpm is 15 to 20% down so I am looking for something where I can get the rpm up a and stay inside 80db - thought a 3 blade would do the job.

Does a 15" three blade prop have three blades each 5" long, or is the diameter still 15" ?

Perhaps I will try rounding the tips of the 16*8 Graupner prop !
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Old May 20, 2003, 01:08 PM
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A 15" 3-blader should be 15" diameter (i.e. 7.5" from centre of propshaft hole to blade tip).

Getting a big 4-stroke down to 80 dB is pretty tough. Most clubs have some leniency with low-revving 4-strokes as the sound in the air is far from unpleasant. At the other end of the scale I was asked not to fly a Cox TD 010 model (r/c) which sounded like a dentist's drill but was way under the limit in terms of dBs!

A 2-blader with less diameter and more pitch is also worth trying, go for an APC if possible. The Graupners are defintely noisy (but work well, I use a 12x5 on a piped Webra 40 ABC in a Moronic).

I can't advise on specific sizes as the biggest 4-stroke I have is an .80. Mind you I run that on a 14x6, so your 16x8 on a 150 sounds pretty small! Perhaps the Laser likes to rev, no wonder you have a problem with prop noise! What prop sizes do Laser recommend?

You might also consider rubber mounting the motor, that would cut back on resonance from the fus.

Oops this is getting a bit off-topic, erm ... how about an electric conversion instead?
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Old May 21, 2003, 02:00 AM
Turbines suck ;-)
Australia, QLD, Brisbane
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Bill the recommended prop is 16x8 and the 150 revs at 8000-9000 rpm with that prop however, it is incredibly quiet in the air - quieter than than many of the much smaller two strokes flying.

I will have to persevere and experiment with different props. Neil Tidy (Laser Engines) recommended a 15x12, but as we have a short patch, landing speed could be a problem so I will try that as a last resort!

Once again thanks for the input!


PS - I think it was Peter Cook who did an electric XtraWot and he was really happy with the performance !
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Last edited by rorywquin; May 21, 2003 at 02:03 AM.
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