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Tuesday, February 4, 2014

Flight School 5: Power Efficiency Hover vs. Forward Flight

RC aircraft, in particular RC helicopters and quadcopters have idiosyncrasies regarding their flight characteristics that should be considered to fully appreciate their capabilities. For instance, I like to keep some forward motion on my quadcopter at all times as it requires much less power than hovering.  This phenomenon is known as "helicopter translational lift" (Google it for detailed info). I know this seems counter-intuitive, but it's very true. The forward motion provides airflow over the blades, which enhances the lift on the quadcopter.  As such, forward flight is much better for motor life and to increase battery flight time. IMO keeping some forward motion also allows for more entertaining flight video than simple hover views.

However, there is a limit speed that if you exceed the aerodynamic drag on the quadcopter will start to kick in, requiring extra power to overcome. In effect, flying high speed runs are less efficient than hovering.  You need to determine a sweet spot of forward speed that will give you efficient flight. You can gauge an efficient speed by listening to the motors.  After takeoff hover, apply some forward pitch to increase speed.  You should notice that you'll soon need to reduce power to maintain the altitude.  

As example, please watch and more importantly listen to the following video. The quadcopter is climbing and accelerating in the first 55 seconds of the above video. Thus the sound of its motors are higher pitched.  But listen to the motor pitch reduce after it levels off in forward flight.  It also accelerates to high speed at various times.  In these instances, drag becomes more pronounced, requiring more power, and higher pitch sound of the motors.

4 comments:

  1. Yep, this article is true, if you fly straight forward the whole time, you'll save energy.

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  2. Is there any general formulation for energy consumption of helicopter/drone when it is flying or hovering? I found a formulation like "mgh" for energy consumption of drone at time t, that m stand for weight, g for gravity and h is height. I think it makes sense, but what about hovering? thanks

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  3. Can you supply some figures for this lift? How much more efficient is translational flight compared to hovering? How much will I throttle down to maintain altitude?

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    Replies
    1. I'd say around 10-20% saving, depending aircraft drag, etc... just based on anecdotal evidence, nothing scientific.

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