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[Paparazzi-devel] contra rotating propellers?


From: Chris
Subject: [Paparazzi-devel] contra rotating propellers?
Date: Sat, 18 Jun 2011 20:59:41 +0300
User-agent: Mozilla/5.0 (X11; U; Linux i686; en-US; rv:1.9.2.17) Gecko/20110424 Thunderbird/3.1.10

I have posted this in rcgroups also so apologize in advance if posting it here was not the proper thing to do, i just felt that some people here might know more than i do or have already
experimented on the subject.
I am planing on experimenting with contra rotating propellers in my quest for increased cruising efficiency.
Here are my thoughts:

1) It looks like contra rotating propellers have higher efficiency
by 5-15% ( http://en.wikipedia.org/wiki/Contra-rotating_propellers )
than single propeller minus mechanical losses.

2) One motor can be stopped and the other set at 100% throttle during flight
for increased efficiency during cruise. The front propeller can be used
only when needed, typically at low speeds.
Brushless motors at partial throttle below 90% are very inefficient.

3) The back propeller typically has a higher pitch to diameter ratio which
improves efficiency and on top of this it can't stall since the front
propeller will provide airflow while the plane does not move (static thrust)
thus aiding take off.

Any opinion, added information or correction is welcomed.
Chris




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