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Low G Entering Autorotation


smoltz
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I'm a low hour student training in an R44, I try and read as much as I can, from FAA books, the POH, here, etc.


Today was my first day of training on auto entries. While entering the auto, there is a clear feeling of less than 1G. A couple years ago when I did an intro flight the instructor I had did an auto and I experienced the same feeling.


A bit confused about Low G when entering autorotation. I have read what seems to be conflicting information. Seems like some say that it's normal. "The low-G which occurs during a rapid autoration entry is not a problem because lowering collective reduces both rotor lift and rotor torque at the same time." That's from Robinson SN-11. This makes perfect sense to me.


At the same time, I read something like this "In entry, if the pilot slams down the collective, there can be a bit less than 1 G for a fraction of a second, but even that is not zero g. In short, unless the pilot slams down the collective, mast bumping is very unlikely in an auto." This makes it seem as if low G during auto entry is a technique issue that can be avoided. This was posted by Nick Lappos on PPrune, who from what I can tell is both an expert level pilot and engineer.


What am I missing? What is the best technique here?


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It would take a pretty specific situation, in my opinion, to put yourself in a low g condition that would result in mast bumping during an auto entry. I feel like you would seriously have to TRY and make it happen. Be in a climb, BURY the collective, pushing the cyclic forward, and not apply required pedal to maintain in-trim flight. Then again.... even though you're entering an auto here, all of those things are what lead to a perfect storm regarding mast bumping.

 

Regarding entering an autorotation and best technique, lower the collective immediately, apply aft cyclic at the same time to regain any lost RPM (assuming it's an actual power failure), apply pedal as required to maintain trim.

 

Aft cyclic is a huge thing though. We all know what happens when Rotor RPM drops too low, and depending on the situation, lowering the collective might not be enough to allow RRPM to start increasing. It might require you to do a cyclic flare at altitude to regain RRPM before doing anything else. In the R22, I'd wager that it will be REQUIRED before doing anything else considering how fast the RRPM decays following a power failure before the pilot recognizes and reacts.

 

 

 

For practice autos, where you KNOW you are doing an auto and are refining techniques for aircraft control to hit a predetermined spot, I teach students to enter a bit on the slow side so they can get used to coordinating all the controls. Simulated engine failures, however, I teach them to enter as fast as they can. If they don't have the collective down by the time I finish saying "simulated engine failure" it was too slow.

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"Hold the attitude, stop the yaw,

Get the lever to the floor."

 

What Nick was saying was that an auto entry isn't anything to be scared of regarding less-than-1-g.

 

You will never get the true zero g (dust lifting off the floor, maps rising out of map cases, etc) unless you made a massive mistake, in which case you would probably die anyway in a Robinson. Just play it easy but positively.

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I'm a low hour student training in an R44, I try and read as much as I can, from FAA books, the POH, here, etc.
Today was my first day of training on auto entries. While entering the auto, there is a clear feeling of less than 1G. A couple years ago when I did an intro flight the instructor I had did an auto and I experienced the same feeling.
A bit confused about Low G when entering autorotation. I have read what seems to be conflicting information. Seems like some say that it's normal. "The low-G which occurs during a rapid autoration entry is not a problem because lowering collective reduces both rotor lift and rotor torque at the same time." That's from Robinson SN-11. This makes perfect sense to me.
At the same time, I read something like this "In entry, if the pilot slams down the collective, there can be a bit less than 1 G for a fraction of a second, but even that is not zero g. In short, unless the pilot slams down the collective, mast bumping is very unlikely in an auto." This makes it seem as if low G during auto entry is a technique issue that can be avoided. This was posted by Nick Lappos on PPrune, who from what I can tell is both an expert level pilot and engineer.
What am I missing? What is the best technique here?

 

First, real world- you're cruising along at speed, power applied (collective pitch) and forward cyclic for attitude and speed. And the engine quits- BANG- you lose the collective pitch lift component and thrust component, now it's cyclic pushing the nose down and you're going to be at reduced G as the aircraft drops and pitches down...

If you enter the auto with a power chop first, then collective and cyclic, you'll get the same reduced G, but I'd call it poor technique (although it's more like a real failure). Being light in the seat makes over-control more likely, and when were you encouraged not to be smooth and coordinated on the controls? A smooth entry: throttle, collective, pedals and cyclic disrupting the aircraft as little as possible in the transition makes it look like you know what you're doing.

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What am I missing? What is the best technique here?

 

Learn the basic entry techniques first. That is, simultaneously, lower the collective with aft cyclic - peddle to control yaw…. It’s all about control coordination. As you gain experience, you’ll be able to speed up this process to the point of it being an unconscious reaction, albeit the same coordinated inputs…..

 

I’m not sure why low-G is even in this discussion. Don’t worry about it. Smooth application is the key. Over-controlling is a fail….

 

Once while attending a Bell 407 refresher, the instructor had me go vertical while pulling to the max 2000 foot rate-of-climb and just when I hit the red mark on the VSI, he snapped the throttle off. During our pre-maneuver brief he told me to get the collective down as fast as possible, which I did. We both were lifted completely out of our seats, strained against the lap belts, while hitting the top of my headset against the headliner….. Low-G bigtime.. No pounding… And yeah, the 407 is not a Robinson but the important part of this is; technique needs to be there before anything else. Again, fail to do this and your autos will suck and, god help you if the real-deal occurs……

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If its forced landing practice, i.e. a throttle chop, then slamming the collective down is what you want, although you don't need to be quite as quick with it in the 44 as in the 22. I did it all through training and that short hop out of your seat negative G feeling is nothing to worry about.

 

If its auto practice then just slowly and smoothly lower the collective and you won't get that negative G hop.

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So I believe the question was about Low G on an auto entry, not how to do an auto.

 

Bottom line, you feel that falling feeling because you are falling.....and if you are falling, then air is coming up from under the rotor disk, and you cannot get into a low G in an auto, it's basically impossible.

 

Try it sometime. Go up to 2000 agl and do a vertical zero airspeed auto...straight down. Get to 1000 feet and push that cyclic till the nose is pointing at the ground....no low G.

 

Just to be clear.....since you are a bit early in your career to do this by yourself, go do it at the Robinson safety course with one of their high time guys (Simon & Doug are awesome).....but bottom line....an entry to an auto is not the time to worry about Low G conditions....high speeds with turbulence and a light load is.

Edited by Goldy
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Most of my flight time is in bell products, the 206 and 205 although the military versions. There is a caution in the operators manual that specifically warns about simultaneous left cyclic input and collective reduction during autorotation entry. This typically was brought up before attempting 180 autos although common sense would say that you get the aircraft under control before you slam it into a turn. None of the senior guys I spoke with ever had any low g issues entering an auto, even when done poorly but I'd always wondered if some idiot actually had balled one up and so they decided to add a caution. Anyone here ever heard of, or been involved in an incident like that, or is the consensus that this incredibly unlikely to ever occur?

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To clarify what low G is. It is when the weight of the air craft which is normally hanging from the rotors is suddenly pushing up into the rotors. Such as a climb with a cyclic push over at the top. Think roller coasters and the top of a hill on the tracks.

The rotors dont like to be pushed. They act like a brat when they start loosing in the push pull game.

 

In flight the lift/ thrust vector of the rotor is constantly pulling the aircaft along and we call this a loaded disk. Where the weight of the aircraft is pulling down on the rotors.

 

In an auto even though we are going down rapidy and suddenly, the air craft is still pulling down on the rotors, and we still have a loaded disk to work with.

 

The differences are in how the weight or momentum of the air craft is vectored.

Edited by WolftalonID
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Guest pokey

the procedure is: straight and level, full back elevator followed by full left rudder. once wings level? break the stall with brisk forward stick while simultaneously applying opposite rudder. Snap to the right? a bit of aileron will counteract the torque effect upon exit-------i never tried this in a helicopter tho, any feedback or hints is appreciated

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From the R22 POH, Safety Notice SN-11:

 

"...(The low-G which occurs during a rapid autorotation entry is not a problem because lowering collective reduces both rotor lift and rotor torque at the same time.)"

 

This means that the rolling moment (to the right) that normally occurs during a low-G event is not present, hence mast bumping is no longer in play.

 

Bottom line, just get the collective down ASAP.

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The sideways drift you mentioned is called translating tendency. It is a byproduct of the anti-torque rotor, in the Robinson this tendency is compensated for by pilot input.

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