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Can't find reason for increase in RPM


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Ok, so I spent the weekend going through my books and can't seem to find a definitive answer on "Why does aft cyclic help increase RPM during forward flight." The question was posed by my CFI, acting as student for my own CFI training. I understand that aft cyclic increases RPM, I just can't find out why.

 

Now we know that during an auto, the air flow below the rotor up drives the disc (without getting too specific in regards to driven, driving, stalled). But during normal flight, when air is still being drawn in from above the disc, how does slight aft cyclic help increase RPM?

 

The only thing that makes sense to me is that you're changing the relative wind in a way that decreases drag on the disc, but that doesn't really cover it all and I can't pinpoint why.

 

So long story short, I need to be able to say something better than "it just does" ;)

 

Thanks in advance.

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It is only a temporary effect.

 

When the disc is tilted back, the airflow is more from below than from above, so there is less induced flow. Angle of attack goes up, more lift produced, blades cone up.

 

Conservation of angular momentum takes over, and because the blades have a shorter virtual radius (being coned up) their rotational speed must increase. Thinks of the spinning skater.

 

But it is only temporary, the aircraft will slow down (because you pulled the cyclic back, and attitude is airspeed) lift will go back to where it was, coning goes away and the revs will settle down again in a while.

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Thanks for the reply, Eric. This is exactly what I found out late last night after I had turned off the PC for the day. I posed the question to one of my old CFI's who couldn't explain the physics either, so she pulled out her copy of Principles of Helicopter Flight by W.J. Wagtendonk and looked it up. Great resource I plan on getting soon.

 

I knew it had to be something with angle of attack, I just didn't think about coning. Dang physics and it's multiple effects.. ;)

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  • 4 months later...
Conservation of angular momentum is part of it, but drag on the blades is also decreased because the vertical component of airflow through the rotor is reduced. Just like ground effect reduces the vertical induced flow.

 

 

This is incorrect. Reducing the download of air through the blades would increase AOA. Raising the nose and having more air pushing up would increase the vortices. Ground effect reduces the diameter of the vortices which increase AOA without a corresponding increase in drag.

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