Welcome back to Helicopter Lessons in 10 Minutes or Less!
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This video covers breaks down Autorotations, more specifically the basics. This subject has quite a lot of information when considering pilot techniques, environmental factors, etc. But what I'd like to do with this video is set the ground work for general knowledge at the most basic level. From there, I'd like to work on a follow-up video expanding more in-depth.
To set the stage, we'll start by defining an Autorotation. This is a condition where the rotor system is being driven by the upwards flow of air rather than by engine power. As the name implies, in an autorotation the blades "rotate" "automatically." This is important because it gives helicopters the ability to have a controlled glide to the ground in the event of an engine(s) failure.
To expand on this, we talk about the stages of an autorotation.
1. Level Flight: let's set the stage by saying we have a helicopter flying straight and level at 500' above ground level (AGL). At this point, the engines turn the blades enough to overcome drag produced by the pitch of the blades. This produces enough lift to overcome weight and the helicopter can maintain level flight. It's important to note that in this stage the airflow is downwards through the rotor disk.
2. Entry: at the onset of engine failure the pilot instinctively lowers the collective to maintain rotor RPM. If the pilot did not do this, drag on the rotor system would cause it to slow down out of limits and make the rotor system useless. In order to maintain aircraft control, the rotor must be maintained. So as the collective is reduced, the rotor decouples and spins freely, a rate of descent is established, the airflow now moves upwards through the disk, and the helicopter enters an autorotational profile.
3. Descent: while in the descent the helicopter has somewhat of a glide instead of fall. The pilot is looking for a suitable area to maneuver the helicopter while cross checking instruments to maintain rotor RPM, airspeed/attitude, and trim as appropriate. A good cross check is key.
4. Flare: at some point near the ground (approximately 100' but dependent upon conditions like aircraft design, density altitude, weight, etc.) the pilot trades this airspeed for a reduction in rate of descent. This is accomplished by "flaring." This is accomplished by an aft cyclic input and results in the flight path trajectory leveling out and becoming more parallel to the ground. In the event the pilot is landing to an unimproved area (e.g. not a runway), the pilot would use this flare also as a means to reduce all forward airspeed to prevent rollover on landing.
5. Touchdown: at the end of the flare as the helicopter begins to settle the pilot cushions the landing with collective application.
When performed correctly, this whole maneuver allows the pilot to turn an engine failure into a safe touchdown instead of a crash.
Looking back at the actual autorotation, there are changes in the aerodynamics of the rotor system. The rotor system is broken down into 3 distinct regions:
A. Stall: this takes place in the first 25% of the blade radius area closest to the hub. Here exists a stall region where the airflow exceeds the critical angle of attack and stalls. This is turbulent, non-lift producing airflow.
B. Driving: here the rotor extracts energy from the upwards flow of air. This region drives or increases rotor RPM and exists roughly 25%-70% of the blade radius area.
C. Driven: lastly, this area of the rotor system is being driven by the driving section. This is the rotor-absorbing, lift-producing area of the rotor which is located in the last 30% of the blade radius area furthest from the hub.
In forward flight these regions shift towards the retreating blade side due to its lower Angle of Attack. However, this is compensated for by blade flapping and cyclic feathering. For more information showing why that takes place, check out my video:
Compensation for Dissymmetry of Lift (https://youtu.be/culBbvszRpk)
That wraps up basic autorotations. Much of this material that I pulled into this video and many others is outlined in depth detail by detail in Shawn Coyle's book Cyclic and Collective. It's a great book for expanding your professional reading library and I'd recommend checking it out. Here's a link to the book (http://amzn.to/2ifQGLx)
Like I said before, there is a lot more information out there addressing tips, techniques, environmental factors, autorotations with turns, etc. I'm interested to see what you guys and gals have to share on the subject. Leave your comments and questions below.
Thanks for watching and Safe Flying!