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Jay Bunning

Why no lower than 57 kts in modertate + turbulence?

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Hi all,

 

I understand in moderate+ turbulence we want to reduce airspeed to reduce structural loads, but why do you think in the R22 POH it states the number 57 kts as the absolute minimum?

 

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."

 

I guess it is probably a performance issue - to be able to maintain best climb maybe, but that number is 53 kts...

 

Any ideas?

 

Without understanding more I think I would just aim for 60kts and try to keep the ship level.

 

Many thanks for your thoughts,

 

Jay

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Always just figured it gives us a margine of error to not go below that L/D max. I wouldn't want't to be at either extreme end in a 22 in moderate turbulence, just because you don't want to be making too many inputs at that point, but just have to ride it out while you vacate the area. Well, that's what I tell my students anyway, that it gives you a light "bufer" zone.

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Moderate, or greater, turbulance can lead to excessivly high angles of attack and/or excessive blade flapping which can lead to mast bumping, main rotor stall, or main rotor blade/fuselage contact! (R22 Normal Proceedures Section-Note)

 

At some of the higher altitudes 0.7 of the Vne (or 0.9 depending on which section you're looking at) will equal speeds that are lower than 60, thus the "no lower than 57".

Edited by pilot#476398

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Thank you all for your input. The definitive answer was provided Answer provided by Raven on another forum:

 

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".

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I have a related question;

 

Is this only for pilots with less than 200hrs in helicopters and 50hrs in the R22?

 

Yeah I wondered the first time I saw that page POH 2-15 - http://www.robinsonheli.com/manuals/R22%20POH/R22%20Pilot's%20Operating%20Handbook_2.pdf) but then re-read the first sentence: "The following limitations (1-3) are to be observed unless 200, 50 etc..." Number 3 does say "continued flight in moderate+ turbulence is prohibited", but the 0.7 Vne and 57 kts stuff - that comes after number 3.

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...but then I thought, is that a second paragraph to #3?,...I mean, if it were seperate, wouldn't it be #4, or another "note:"?

 

I guess I can wait until the next time I'm down at Robinson for that one. Seems like a good "rule of thumb" no matter how many hours in the R22 you have, anyway.

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Moderate, or greater, turbulance can lead to excessivly high angles of attack and/or excessive blade flapping which can lead to mast bumping, main rotor stall, or main rotor blade/fuselage contact! (R22 Normal Proceedures Section-Note)

 

At some of the higher altitudes 0.7 of the Vne (or 0.9 depending on which section you're looking at) will equal speeds that are lower than 60, thus the "no lower than 57".

Short answer;

 

Between 60 kias and 0.7 of the Vne.

The lowest Vne is 57kts!

 

That’s correct

 

When you get too hung-up on the 57 knots number, you loose track of the main reason, staying on the forward side of the power curve. On the backside of the power curve the unstable conditions with respect to the power required vs. airspeed lead to excessive angles of attack and flapping, these conditions can be amplified in turbulence leading to M/R stall or mast bumping.

 

That 57 knot number may have only been chosen because, like you wrote, the lowest VNE on the chart. Don’t let that 57 knots tie you in knots. That revision in the limitation section is based on LINK: AD 95-26-04. The AD answers the why question:

 

To prevent main rotor (M/R) stall or mast bumping, which could result in the M/R blades contacting the fuselage causing failure of the M/R system, and subsequent loss of control of the helicopter, accomplish the following:

 

a) Insert the following information into the Model R22 Rotorcraft Flight Manual. Compliance with the Limitations section is mandatory. The Normal Procedures and Emergency Procedures sections are informational.

 

LIMITATIONS SECTION

 

The following limitations (1-3) are to be observed unless the pilot manipulating the controls has logged 200 or more flight hours in helicopters, at least 50 of which must be in the RHC Model R22 helicopter, and has completed the awareness training specified in Special Federal Aviation Regulation (SFAR) No. 73, issued February 27, 1995.

 

1) Flight when surface winds exceed 25 knots, including gusts, is prohibited.

 

2) Flight when surface wind gust spreads exceed 15 knots is prohibited.

 

3) Continued flight in moderate, severe, or extreme turbulence is prohibited.

 

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.

 

Note: Moderate turbulence is turbulence that causes: (1) changes in altitude or attitude; (2) variations in indicated airspeed; and (3) aircraft occupants to feel definite strains against seat belts.

 

Main Rotor Stall: Many factors may contribute to main rotor stall and pilots should be familiar with them. Any flight condition that creates excessive angle of attack on the main rotor blades can produce a stall. Low main rotor RPM, aggressive maneuvering, high collective angle (often the result of high-density altitude, over-pitching [exceeding power available] during climb, or high forward airspeed) and slow response to the low main rotor RPM warning horn and light may result in main rotor stall. The effect of these conditions can be amplified in turbulence. Main rotor stall can ultimately result in contact between the main rotor and airframe.

 

Mast Bumping: Mast bumping may occur with a teetering rotor system when excessive main rotor flapping results from low "G" (load factor below 1.0) or abrupt control input. A low "G" flight condition can result from an abrupt cyclic pushover in forward flight. High forward airspeed, turbulence, and excessive sideslip can accentuate the adverse effects of these control movements. The excessive flapping results in the main rotor hub assembly striking the main rotor mast with subsequent main rotor system separation from the helicopter.

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I have a related question;

 

Is this only for pilots with less than 200hrs in helicopters and 50hrs in the R22?

 

To get the whole story read the AD....

 

 

ACTION: Final rule: AD 95-26-04

 

 

SUMMARY: This amendment supersedes an existing airworthiness directive (AD), applicable to Robinson Helicopter Company (RHC) Model R22 helicopters, that currently requires revisions to the Limitations section, the Normal Procedures section, and the Emergency Procedures section of the R22 Rotorcraft Flight Manual, revised February 4, 1993. These revisions limit operations in high winds and turbulence; provide information about main rotor (M/R) stall and mast bumping; and provide recommendations for avoiding these situations. Additionally, emergency procedures are provided for use should certain conditions be encountered. This action would require similar revisions to the Limitations, Normal Procedures and Emergency Procedures sections required by the existing AD, but the revision to the Limitations section would prohibit only pilots without a certain level of experience and training from operating in the flight conditions specified. This action is prompted by data that indicates pilots who possess a certain level of experience and training are more able to recognize and react to the adverse meteorological conditions specified in the AD. The actions specified by this AD are intended to prevent M/R stall or mast bumping, which could result in the M/R blades contacting the fuselage causing failure of the M/R system and subsequent loss of control of the helicopter.

Edited by iChris
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Good topic

 

Good stuff, but if you have ever been in severe turbulence, trying to maintain a specific airspeed and altitude is impossible. So you aim for 60 as the airspeed indicator is bouncing between 30 and 75! and you forget all about maintaining a specific altitude. I was on final just last week in the R44 and the airspeed went from 75 knots to zero a couple times! Don't try to react too fast, smooth is key. The airport was reporting 18/G25 but my GPS was telling me I had 50 knots up at 500 feet. Remember too that the pitching up from a wind gust can make your airspeed indicator less than accurate.

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