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Rotorcraft Limit manoeuvring load factor

Rotorcraft Limit manoeuvring load factor

(OP)
Hi community,

currently I am doing some structural analysis with loads resulting from required Limit manoeuvring load factor.

In CS VLR.337 following requirement is listed:

The rotorcraft must be designed for-
(a) A limit manoeuvring load factor ranging from a
positive limit of 3.5 to a negative limit of -1.0
; or

How should I interpret this requirement? Up with 3.5g and down with -1g? What about the factors for the side, forward and backward loads?? I really have some difficulties to understand this requirement. Requirements for aircraft are more exact since there are values for up,dwn,side,fwd,bwd load cases but for rotorcraft, it is very difficult to understand.


Thank you in advance for your inputs.

Regards

RE: Rotorcraft Limit manoeuvring load factor

DerGeraet...

This is a complex question... symmetrical and unsymmetrical loads are analyzed/tested separately... by meeting each requirement, the aircraft has structural assurance. And don't forget the rearward, sideward and yawed flight conditions. Also IF the power train cannot sustain these regimes... that can become a limiting load factor.

A fairly good discussion of flight parameters and design factors is in ... Army ADS-29 AERONAUTICAL DESIGN STANDARD - STRUCTURAL DESIGN CRITERIA FOR ROTARY WING AIRCRAFT

NOTE. Your negative load factor of -1G seems a bit high for a conventional Helo... Army uses -0.5G.

Regards, Wil Taylor

o Trust - But Verify!
o We believe to be true what we prefer to be true. [Unknown]
o For those who believe, no proof is required; for those who cannot believe, no proof is possible. [variation,Stuart Chase]
o Unfortunately, in science what You 'believe' is irrelevant. ["Orion", Homebuiltairplanes.com forum]

RE: Rotorcraft Limit manoeuvring load factor

Hello DerGeraet,
The manoeuvring load factor is for vertical loads only. Yes, 3.5g is "pull back" on the cyclic, causing your body weight (and the weight of any massive object in the rotorcraft) to experience in increase in perceived weight. The sensation is like pushing down on your seat. The other direction, -1.0g is "push forward" on the cyclic, causing your body and other objects to experience the reverse of the force of their weight. The sensation is rising into the straps of your harness as the nose of the helicopter pitches down rapidly.

All of the side-to-side and forward-aft loads are defined elsewhere. If you have CS VLR then I believe you are looking at the regulations for very light rotorcraft published by EASA, and while I do not have a copy of this code, it is largely based on the code developed by the USA in the 1950's and adopted by the rest of the world since then. Even the same chapter numbers are the same, hence, "337" is the chapter for maneuvering load factor for every civilian aircraft and rotorcraft since the 1960's regardless of the code book they were printed in. I would wager if you flipped to "VLR.561" you would find emergency load factors in forward, down, up, and sideward directions (and these are ultimate load factors, not limit load factors like 337, by the way). And if you perused the rest of the "VLR.3**" chapters you may find a sideward or yawing load factor to use.

Please let me know if I guessed right about the VLR code chapters!

Wil,
Guessing again, but the simplicity of VLR certification may not offer any freedom to reduce the load factor in light of limited lift capability of the main rotor or cyclic control afforded to the pilot.

STF

RE: Rotorcraft Limit manoeuvring load factor

(OP)

Quote (WKTaylor )

A fairly good discussion of flight parameters and design factors is in ... Army ADS-29 AERONAUTICAL DESIGN STANDARD - STRUCTURAL DESIGN CRITERIA FOR ROTARY WING AIRCRAFT

NOTE. Your negative load factor of -1G seems a bit high for a conventional Helo... Army uses -0.5G.

Tnx for the input WKTaylor! Do you know where to find this document? I couldn't find it via google :/

Well you are right, flight data also shows that the value of -0.5g is never exceeded but since the flight vehicle has a composite-primary structure, I am doing the analysis more conservative, I have selected the value of -1g :) Let's see what the fea-analysis results are. If the allowables are below the stresses for the UP-load case, I will reduce the value to -0.5g

Quote (SparWeb)


Hello DerGeraet,
The manoeuvring load factor is for vertical loads only. Yes, 3.5g is "pull back" on the cyclic, causing your body weight (and the weight of any massive object in the rotorcraft) to experience in increase in perceived weight. The sensation is like pushing down on your seat. The other direction, -1.0g is "push forward" on the cyclic, causing your body and other objects to experience the reverse of the force of their weight. The sensation is rising into the straps of your harness as the nose of the helicopter pitches down rapidly.

All of the side-to-side and forward-aft loads are defined elsewhere. If you have CS VLR then I believe you are looking at the regulations for very light rotorcraft published by EASA, and while I do not have a copy of this code, it is largely based on the code developed by the USA in the 1950's and adopted by the rest of the world since then. Even the same chapter numbers are the same, hence, "337" is the chapter for maneuvering load factor for every civilian aircraft and rotorcraft since the 1960's regardless of the code book they were printed in. I would wager if you flipped to "VLR.561" you would find emergency load factors in forward, down, up, and sideward directions (and these are ultimate load factors, not limit load factors like 337, by the way). And if you perused the rest of the "VLR.3**" chapters you may find a sideward or yawing load factor to use.

Please let me know if I guessed right about the VLR code chapters!

Wil,
Guessing again, but the simplicity of VLR certification may not offer any freedom to reduce the load factor in light of limited lift capability of the main rotor or cyclic control afforded to the pilot.

Hi SparWeb,
tnx for your reply.

So, the +3,5g acts "downwards" and -1g "upwards" with the z-axis acting downwards.

Well I do calculations for a rotary wing UAV which can be compared to the VLR-class. Within this class of rotorcraft, it is allowed to neglect the emergency landing conditions (as described in .561 :) ) since there is no one in the AV. Therefor I have considered the manoeuvring loads from .337 only (limit load) but I was very confused regarding the fwd/aft/side loading factors. But I will assume 2g for fwd/aft/side...


Therefor I will take +3,5g/-1.0g for down and up and 2g for side/forward/backward....

Thank you very much for your inputs.

RE: Rotorcraft Limit manoeuvring load factor

It was the second hit on Google: http://standards.globalspec.com/std/1703308/army-a...

Note that some helicopters, like OH-58D, are designed for air transport within fixed wing aircraft, so their transport requirements might exceed operational.

TTFN (ta ta for now)
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RE: Rotorcraft Limit manoeuvring load factor

I think I've published this before... but for what its worth...

Army Engineering Design Handbook series... fairly old, but useful for intro/overview purposes... and free.

Helicopter Engineering. Part One. Preliminary Design http://www.dtic.mil/docs/citations/ADA002007

Helicopter Engineering. Part Two. Detail Design http://www.dtic.mil/docs/citations/ADA033216

Helicopter Engineering. Part Three. Qualification Assurance http://www.dtic.mil/docs/citations/AD0901657

Helicopter Performance Testing http://www.dtic.mil/docs/citations/AD0785000

OK OK OK OK... all my experience is military oriented...

Regards, Wil Taylor

o Trust - But Verify!
o We believe to be true what we prefer to be true. [Unknown]
o For those who believe, no proof is required; for those who cannot believe, no proof is possible. [variation,Stuart Chase]
o Unfortunately, in science what You 'believe' is irrelevant. ["Orion", Homebuiltairplanes.com forum]

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