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Re: [Paparazzi-devel] cannot reach 100% throttle on takeoff

From: Tobias
Subject: Re: [Paparazzi-devel] cannot reach 100% throttle on takeoff
Date: Thu, 6 Aug 2015 17:05:29 +0200

Hi Joseph,

I ll try to explain in more depth - if you have defined
   <define name="POWER_CTL_BAT_NOMINAL" value="14.8" unit="volt"/>
in your airframe file and your battery (fully) charged has 16.8V   PPZ will 
automatically adapt the throttle setting to get the same thrust you would get 
with a 14.8V battery.
Seem wired, I know but here is the explanation:

PPZ is using mostly linear control, at least for throttle. 
So if were are looking at normal cruise power (lets say 50% throttle at 14.8V 
to hold the desired altitude and speed. And now your battery drops below 14.8V 
du to discharge. 50% throttle is not enough anymore to hold the speed and/or 
altitude. this results (in most cases) in an integrator to fill in the cap so 
you can reach your desired values. But this is slow and not the best way...
So one nice way would be that 50% throttle would alway result in the same 
thrust setting. 
Thats exactly what the above stated define does! If you have a input voltage 
that is a little less that the defined one it increases the throttle setting to 
get the same thrust or
if the input voltage is a little higher than the defined voltage PPZ will 
decrease it a little.

You could also argument its a kind of "derated takeoff" like in the big (real) 
planes. If 14.8V is enough for takeoff  there is no need to go beyond if you 
have more available. 
just kidding :-)

hope that helps - maybe you could add that to the wiki?


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