In engine failure, how much time to lower the collective in R22 ?

In the R22, RHC state that the event of a power loss at maximum continues power, the rotor system will stall in 1.1 seconds if the collective is not lowered. A lot of pilots interpret this as they have 1.1 seconds to lower the collecctive. It is incorrect because once the rotor has stalled, recovery is impossible. In reality, the collective has to be lowered within approx 0.5 sec in order to be able to maintain RRPM within recoverable limits.

Ref: http://www.pprune.org/archive/index.php/t-502459.html

Will turbulence result in low-G condition for R22 ?

The low-G is unlikely to be too much of a problem in turbulence, because the low-G condition produced by turbulence is not usually sustained long enough for a roll to develop.

If a pilot’s reaction on encountering turbulence is to lower the collective and slow down, which will reduce tail rotor thrust and therefore remove most of the cause of the rolling tendency.

Ref: http://www.pprune.org/archive/index.php/t-502459.html

Why RHC advises pilots to slow to 60 Kts (IAS) – 0.7Vne (about 70 Kts) when encountering turbulence.

The reason is the power setting for this speed in the R22 is typically 17-18″ MAP (even lower in R44). This setting is considered low and produces low tail rotor thrust, and reducing roll tendency in the event of low-G situation encountered during turbulence. more ...

Then, why in R22 POH Limitations Section ?
“Adjust forward airspeed to between 60 knots indicated airspeed (KIAS) and 0.7 Vne, but no lower than 57 KIAS, upon inadvertently encountering moderate, severe, or extreme turbulence.”

The airspeed you fly in turbulance is a range. 60 is the low point 71.4 (0.7 of 102) is the high point. If 0.7 of any other Vne (remember it goes down with altitude and tempurature) is less than 60, you must go 60. However, at 10,000 feet and 30 degrees, you cannot go 60, because your Vne is 57! Hence the one exception, “no lower than 57″.

Sidetrack

If you fly at 10,000′, Vne is 57 kts (the lowest Vne on the chart) so although we may be flying along at 57 kts, we should not go any faster as we would be exceeding limitations and likely soon experience retreating blade stall. Lets say we flying along at 53 kts (best climb speed), trying to get max performance to be able to stay up at 10,000….Now we hit moderate turbulence again and we do the math – between 60 kts and 40 kts (57 Vne x 0.7). 40 kts is lower than the minimum 57 kts mentioned and we shouldn’t be going 60kts as that is above Vne, so according to the limitations we should speed up to 57 kts.

I think the reason RHC advises not below 57Kts is because 60Kts is the safe speed due to deadman curves, so they wants you to fly closer to 60Kts.  But if you are more than 500′ AGL, may be 53 Kts is better, as it produces minimum tail rotor thrust and will be less roll roll tendency in the event of low-G situation encountered during turbulence.

Ref: http://www.pprune.org/archive/index.php/t-502459.html

Why the approach in helicopter is so different from fixed-wing aircraft ?

When I learnt helicopter flying, I already had the FW license.  I had this question in mind.  The helicopter approach is normally through a glide path where you gradually reduce the air-speed and altitude together.  For FW approach, they keep flying at low air-speed and flare at the end which is like the flare in parachuting or quick stop in helicopter.

Helicopter can do the same thing like the FW, to fly at low air-speed and terminate by a quick stop.  But, in quick stop, you will need a high nose altitude.  That will involve a lot of coordinations in short time which demands very high skill.

Trying to land with a nose high attitude along with any other unfavourable condition (i.e., high gross weight, tail wind, gusty wind) is most likely to end in disaster.