Re: [Paparazzi-devel] contra rotating propellers?

[Prof. Dr.-Ing. Heinrich Warmers]

Dear  Chris,
i think point 2 is wrong, because the resistive  los power  is  i*i*R. 
(Maximum  about 70% of lunch  power)
Same brushless controller have no free run  pwm  (about 10% lost  if the 
voltage  < 10V).
The gear box and the second propeller has a mass so the aircraft need 
more power.
Pleas read the web side:  
http://www.mh-aerotools.de/airfoils/pylonprops_2.htm.
The maximum of a propeller for model aircraft is in the range of  60..80%
The maximum diameter is about 0.3 span.
Regards
Heinrich

Chris schrieb:

> 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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