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Vertical Autorotational Descent
#21
Hi Bryan,

Like arrow123 I too was taught vertical auto's in the R22 and R44...you come down pretty darned quick but provided you recover into a minimum descent forward flight before the normal flair, all was well. Forward, sideways,backwards, the heli doesn't really seem to care, though it sure mixes up my brain!
It might be a Kiwi thing of course.
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#22
Some of you may have seen this video already but I think it is relevant to the original discussion of vertical autorotation proving that it does work and can be done.

http://www.youtube.com/watch?v=cpHMqv...
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#23
Mark: Thanks for posting the lastest video. I have watched the video some 15 times and re-read the thread in the forum another 15 times and I must admit I am still confused. (Maybe I'm not the only one!)

Before today, my perception was that while VERTICAL AUTOROTATION is fine at altitude, one must have adequate KINETIC ENERGY (i.e. forward airspeed) to effect successful flare. In the case of Mosquito, that would be 40-50 mph by about 200' agl (am I correct on the numbers?)  The video however shows a LOW SPEED AUTOROTATION at an indicated airspeed just above TRANSLATIONAL AIRSPEED (30-40 kts in the R22) and shows a successful autorotation, arguing that the final flare is just like a HOVERING AUTO.

A "NORMAL AUTO" uses forward airspeed only makes the landing easier by using CYCLIC FLARE to convert kinetic energy into altitude, and then using COLLECTIVE for the final settling to the ground.

In a Hovering Auto, one trades the stored energy in the rotor (at 100% rpm) into altitude cushioning the flare using COLLECTIVE PITCH alone. In way of deduction, in a VERTICAL AUTO, as long as the rotor is at 100%, a successful flare is possible using COLLECTIVE alone. (albeit at tremendous risk of misjudging the altitude to initiate the use of collective). Please explain why maintaining at above TRANSLATIONAL SPEED, and won't indicated airspeed be grossly inaccurate due to angle of the airflow on the pitot tube?

Being a novice, the thread still leaves me with many unanswered questions. While an R22 with its low inertia rotor can demonstrate a successful Vertical Auto, a similar maneuver in a Mosquito should surely be more successful. My conclusions so far is that if ALTITUDE is adequate, a 360 or Turning Auto would keep the LZ close below while maintaining adequate forward airspeed for the flare and landing. At a pinch, a Vertical Auto with a safe landing is theoretically possible while the possiblilty of misjudging the commencement of collective flare is high.

Further analysis and comments from everyone is most welcomed.

Andy 
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#24
For what it may be worth, I have had to perform a normal 180 deg auto, then a max distance auto and finally a near vertical Auto in my Mosquito (XE) all in the same autorotation and it was all possible and by the time I was a few feet above the ground the rotor speed was getting to the lower end of useable (not sure exactly what it was as my focus was outside the helicopter), but descent rate was low so if the landing spot available to me was a little flatter it all would have been very successful. What it did mean was that I did not get hurt and that an auto at low forward (nearly zero) speed to the ground was possible.
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#25
Andy, you have a very clear understanding of the conversion of kinetic energy (horizontal & vertical) for the purposes of maintaining RPM and controlling sink rate (either cyclic flare or collective pull or both). That being said, don't be mislead by the video. I think in this vertical autorotation video the 30-40 kts is nothing more than turbulent airflow acting on the static ports and the pitot tube. If you watch the airspeed indicator throughout the descent you will notice that it drops to zero right when the helicopter actually starts to move forward when the collective is pulled for the landing. The highest sink rate reached on the VSI is 1700 fpm and that converts to 16.7 Knots vertical speed. The vertical view looking down at the runway proves the helicopter is not moving in relation to the ground all the way down and then there is a little forward motion induced and that is exactly the point where the airspeed indicator drops to zero. I don't think there is a 30-40 kt wind blowing because if that were the case the collective cushion would have been much more effective. The point I'm trying to make is that the auto in the video is truly a zero airspeed and therefore below ETL. The helicopter might be close to ETL when it goes forward at the end but that is an efficiency that is hardly helping especially since there is no way to make an effective cyclic flare with the rotor disk at near horizontal. This is obviously not a hovering auto but a true vertical autorotation where RPM is maintained with aerodynamic autorotation. I believe that a quicker "pop" of the collective will provide a more effective cushion. A slower collective pull will allow drag enough time to act to slow RPM to the point where max collective is less effective. A quicker pull or pop of the collective will change the angle of attack completely and immediately to get the maximum cushion before the kinetic energy stored in the rotor has time to decay. Think of the difference in an airplane of a slow progressive flare versus a quick pull back on the elevator that results in ballooning.
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