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More prop; less noise

Old 02-22-2010, 10:44 PM
  #1  
Radbuster
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Default More prop; less noise

I'm flying a scratch built pusher jet with a 2212-06 motor & 3S lipo. The APC 6x4 prop works great, but is pretty noisy. I switched to an APC 7x4 prop--the noise level went down, but the flight time and speed dropped.

I like the lower noise level, when I fly in parks and don't need complainers coming by. But I was surprised by the drop in performance. Should I be?
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Old 02-23-2010, 04:03 AM
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aramid
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Most likely, you didn't actually lose performance, you just changed it to a different situation. Going to a bigger prop means you're drawing more power (which is why your flight time went down) and getting more thrust (so you'll have better climb performance) but reducing the pitch speed (which is why your plane is now slower).

Be careful, though - you didn't give enough information for me to find your motor, but all the 2212-06s I could find were very high-Kv, which makes sense for a pusher. Motors like that will get very hot trying to turn a 7" prop - make sure you're running within your ratings; a power meter of some sort is indispensable for an electric pilot.

Pusher jets are often loud, because they tend to use small, fast props. You can still get plenty of speed from a slower motor running a bigger prop, but that kind of setup doesn't seem to work as well with jet designs, which may struggle with the extra torque. You could put together a warbird and use a steep-pitch prop on about 1500 Kv to get more speed than you ought to fly in a small park.
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Old 02-23-2010, 04:59 AM
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Kosh
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I've been dealing with some pusher Funjets that have shown up running 5 cell packs on 4 inch props. These planes exceed 100 DB and would not be allowed on any AMA field in this state that I know of so I banned them. Having a prop that close to the end of a plane in turbulent air is gonna makes noise. This can be greatly reduced by extending the motor shaft back into clean air. On one Funjet we got a 20DB reduction in noise by moving the prop back with a simple extension.

Because our field is electric only and we cant piss off the neighbors I set the DB limit at 86. Many AMA fields in the area use 89 to 90 for gas planes so were well under that. Even fast pushers can be made quiter with a little thought.
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