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Vertical Autorotational Descent
Bryan, I agree with the conclusion you seem to have come to in that you can do vertical (no airspeed) autorotations all day long. As you mentioned, I would say that as long as you have sufficient inflow from the bottom of the disk (uplow if you will) you can autorotate and actually the more vertical the upflow the better... the only problem is that you are effectively in the autorotative state of settling with power. Yes, I said settling with power. The three factors that set up for settling with power are decent rate of at least 300 ft per minute with little or no forward AIRSPEED while having 20% to 100% power applied. In my mind the "power applied" aspect of this is an angle of incidence/angle of attack from the collective input as much as it is power demanded from the engine... furthermore, the 20% to 100% power applied doesn't ever define where the power is coming from (engine or autorotational forces). So, what I'm trying to say here is that if you do a vertical autorotation and try to stop the vertical decent with no forward airspeed you will just fall through probably with little if any decelerative effect what so ever. This is the final phase of settling with power known as the autorotative or windmill brake state. The distinctive difference of this phase is that there are no longer two vortices being produced. Settling with power starts when the upflow from the inner part of the disk moves outward to the center of the blade span while the rotor tip vortices moves inward. In the early phases there is still some downward thrust being produced between the two vortices. In the windmill brake state or autorotative phase there is only clean upflow and no vortices are being produced. This idea demonstrates perfectly what you already think and that is: vertical autorotation works and can be backwards explained and proven using the settling with power lesson. It's actually a very good exercise in correlative understanding of how each of the two subjects are really one in the same. Simply put, it's all just helicopter!
Thanks Mark.

I guess everyone has gotten their sugar mixed up with their salt at sometime. I had not studied for a rotary wing written or checkride in years and had not watched the Robinson SFAR lectures recently either.
My knowledge had gotten stale and just like witness' statements in a car crash, with time, inaccurate. In my mind I had mixed-up the "rotor-stall=fall" scenario with "aerodynamic forces in vertical autorotation."

I was prepared to argue that a helicopter cannot maintain RPM in a straight down descent without an engine. Luckily I got corrected BEFORE I made an idiot out of myself. (Some feel I did that long ago. LOL)

Now my only question is, why have I never had this demonstrated to me or done one? I'm sure I didn't DO it in the past 10 years because I thought I'd die. This makes me wonder what other concepts I use to live my daily life which are wrong right out of the starting blocks!
Bryan, I was not taught vertical or backwards autos in flight school either but I learned in the Huey not the TH-55 (a.k.a. Hughes 269). I don't think schools are teaching the backwards autos in the Robinsons very much either but I know they teach them in the Shweizer. I think this has a lot to do with the increased possibility of low g flight and therefore mast bumping and boom strikes associated with semi-rigid heads. As you know the Shweizer has an articulated head and is much safer for doing these maneuvers. I don't know any of this for sure but this is what I think.
I don't feel even close to qualified to even pipe in on this conversation. However, I will say that the Mosquito (turbine version anyway) is more than capable of performing safe 0 airspeed vertical autos. I know this because I have done quite a few of them in my XET, starting from as low as 700ft AGL and as high as 4000ft AGL. With a little wind the VSI on my XET's EFIS usually reads about 1,000ft/min or so vertical descent speed once the helicopter stabilizes in the vertical auto. I actually have a few Go Pro vids of me doing 0 airspeed autos from different altitudes that I just haven't uploaded. My rotor RPM usually stays around 600 or so once I slightly increase the blade pitch angles to keep the RPM constant. Just so some of you guys with the 2 strokes don't freak out, the XET flies at 590RPM, not 540. So I try to keep it around 600 or a little less in an auto. Once I get near the ground I push forward for airspeed and do a normal power recovery auto at the bottom. Like Mark said, the only thing keeping you from going into full blown settling with power in a vertical auto is your power/pitch angle. Once I have stabilized in a vertical auto I have never attempted to stop my descent or substantially slow my descent by lifting the collective and adding power, I always push forward for airspeed to terminate the vertical auto. Trying to stop your vertical descent with collective alone seems very dangerous, and the only way I see that MAYBE actually working is if you rapidly pull most of your collective to stop it as quickly as possible. However that would put tremendous G-load stress on the helicopter so I definitely never recommend doing that to terminate a vertical auto for any reason. I've looked at the time stamps in the Go Pro vid I took of my 4,000ft vertical auto and it took me 3 minutes and 50 seconds to get to the ground from 4,000ft AGL, descending vertically 90+% of the time. That corresponds well with my VSI reading of 1,000ft a min. Not bad if you ask me. I am very confident in the XET's ability to perform vertical autos. If anything I just said doesn't seem completely accurate in any way, please correct me Mark. I only wanted to share my experience with vertical autos in the Mosquito, not attempt to teach anyone anything.
Very good info Jesse! I should note here that the XET should autorotate better than an XE because of the extra weight. The extra weight results in higher rotor rpm that must be checked by increasing collective to keep the rpm in the green, which in turn decreases decent rate. I think the higher rotor rpm of the XET is to decrease coning that would be excessive at the normal 540 rpm of the XE. This also plays well into quickening the control response and improving overall rotor performance to include the autorotation. You are doing the right thing by pushing forward before pulling in any collective during the power recovery phase of the auto. The vertical autorotation is good experimentation especially for teaching yourself what to do if you need to make it to a forced landing area that is beneath you or very close. Thanks for posting your experiences Jesse.

That't pretty cool.

That's a long ride down, 4 minutes, wow.
I've never gone that high, about 1500 ft is the highest I've ever flown, even training in the Schweizer.
Like Jessie said, I've also done a ton of these in my XET, and like him always push out of the decent with cyclic instead of trying to pull collective, that to me just doesn't seem like a good idea. What it does do however is increase your decent rate and shorten you glide or intended point of landing if you were say in a established 50 mph normal auto and decided to slow up. What's handy about this little maneuver is allows you to change your intended point of landing like I just described. A good thing to practice if say mild s turns wouldn't work say due to some sort of obstruction such as terrain or power lines, and this has personally come in handy for me in real life experience. Just remember to plan to accelerate towards the bottom to allow for a safe touchdown

i did one at the robinson safety course in 06. i knew it could be done but still took some courage to do it. from 4,000' the descent felt strange.rpm management, very sensitive pedals, pitched over before 500' agl.  
During my training to solo in the R22, my instructor decided to concentrate on safety procedures so we did a lot of 180 Auto's with a couple of vertical and reverse Autos in the mix. The key to it all, was to ensure the forward speed was kicked in before getting to close to the ground - usually around the 400ft mark. That ensured the normal completion of an auto. The descent rate did not appear high, but as these were done from a starting altitude of around 1500ft it was hard to tell (I don't remember what the actual rate was), and the final push to gain airspeed burned up a fair amount of sky.
Great info guys! I'm enjoying reading everyone's account of their experience.

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