Twin Otter w/four-blade props?

[rmsmith 1]

Are the Twin Otters with four-blade propellers really that much quieter than the three-blade setup?

[Hooknswoop 19]

I think the 4-bladed props on the -34 Super Otters are more for climb power than to reduce noise.

Derek

[FrogNog 1]

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I think the 4-bladed props on the -34 Super Otters are more for climb power than to reduce noise.

Derek

I don't mean to quote this as an authoritative source, but clearly someone else things so:

"Another modification is the mounting of four-blade propellers. These propellers, designed in the USA and installed for the first time in France by Mike, bring an answer to the problem of noise pollution. They were tested during the RW worldcup in 1992 on one of Gap's Porters with a huge crowd invading the field. The change was obviously very positive even for the fiercest defenders of the alpine skies. Since then, other DZs have changed for four-blade propellers in order to reduce noise pollution.
"
(http://home.nordnet.fr/~paramag/archives/n142/article/article-us.html)

-=-=-=-=-
Pull.

[quade 4]

THE largest producer of noise on prop aircraft (jet or piston) are, the props.

At take-off RPM, the prop tips reach the speed of sound and the majority of the really loud noises you hear are actually little sonic booms.

4 blade props can have a smaller diameter than 3 blade props so the tips don't travel as fast for the same RPM setting so there is less of a sonic boom so less noise.

They are, however, a bit more expensive to install and maintain, so that's the trade off.

quade -
The World's Most Boring Skydiver

[alain 0]

I duno about climb power but I can tell you about noise:
We can hear the difference very often down here with some porters flying with 3 and many other flying with 4 (talking of porters running PT6/A27 turbines here)

edit: we have barely no more conflict with neybourhood since our porter has been upgraded to A34 4 blades...

regarding the A34 mod on the porter: as far as I know it requires a 4 blade prop
http://www.icarius-aerotechnics.com/anglaisstcpt6a34.html
but i could not tell if that's true for the otter or king air too... (Mike Mullin's one has 4 blades thou)

alain

[airgord 1]

All of the SD AZ Otters have -34's and 4 bladed props and they are quieter inside than the lower powered Otters with 3 bladed props. The 4 bladed propellors are an STC for most aircraft, expensive and somewhat heavier, but most operators will tell you worth it.

[ypelchat 0]

Our King Air A100 has PT-6 -28, and has 4 bladed props.



Yves.

[kmcguffee 0]

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Are the Twin Otters with four-blade propellers really that much quieter than the three-blade setup?

I've never tested it myself but, according to what I learned in my college engineering classes, the more blades you add to a prop the quieter the blade chop noise is. Compare a UH-60 Blackhawk to a UH-1H "Huey". The UH-60 has four blades as compared to the Huey which has two blades. The Uh-60 is much quieter. You get very little of the blade slap that is so characteristic of the Huey in all of the Vietnam movies. Also, the more blades you add the less efficient each blade. This is due to each blade having less clean air to use due to the wake of the preceding blade. It has something to do with laminar airflow vs turbulent airflow. Turbulent is easier to move through but gives an airfoil less lift. The total lift of the prop might increase with an added prop but each blades efficiency decreases. It is a case of diminishing returns.

So, my guess would be that it provides more power and is quieter.

Now this is all from memory and from classes that I took over 10 years ago. So I could be completely wrong.

"Any fool can criticize, condemn and complain and most fools do." Ben Franklin