Why don't helicopters have published G limits?

[n5296s]

That's about it really. For some reason this came up in discussion with my instructor a while ago. He didn't know either. (Maybe some do, the R44 doesn't anyway). It occurred to me again watching the Brazil "accident waiting to happen video", since they seemed at one point to be in an 80 degree bank.

Of course normally you wouldn't get anywhere near the limit (I assume, although of course I don't know what it is)... but then that's true of my 182 too (+3.8/-2.0).

Anyway, answers greatly appreciated...

n5296s (/n9888s)

[Ascend Charlie]

The versatile machines like the BO105 and BK117 are stressed to +3.5 and -1.

Teetering machines are generally limited to operate between +1 and +2.7

The 2.7 comes from being able to sustain a 60 degree bank turn (2g) plus a fudge factor for turbulence and clumsy pilots.

[tecpilot]

As any instructor should know, load limits are generally specified in the construction rules and therefore there is no further need to explain the limits in the RFM - of course if the a/c is certified in accordance. And the Certification Specifications are valid for any type not only for BK and BO.

Limit manoeuvring load factor
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
(b) Any positive limit manoeuvring load factor
not less than 2.0 and any negative limit manoeuvring
load factor of not less than – 0.5 for which –
(1) The probability of being exceeded
is shown by analysis and flight tests to be
extremely remote; and
(2) The selected values are appropriate
to each weight condition between the design
maximum and design minimum weights.

[Phoinix]

This is all I could find in ICAO Annex 8:

3.4.1 Manoeuvring loads
Manoeuvring loads shall be computed on the basis of manoeuvring
load factors appropriate to the manoeuvres permitted
by the operating limitations. They shall not be less than values
that experience indicates will be adequate for the anticipated
operating conditions.


Where did you find your quoted factors?


EDIT: AAAAAAAh... just stumbled upon JAR27... stupid me.

JAR 27.337 Limit manoeuvring load factor
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
(b) Any positive limit manoeuvring load factor
not less than 2.0 and any negative limit manoeuvring
load factor of not less than – 0.5 for which –
(1) The probability of being exceeded
is shown by analysis and flight tests to be
extremely remote; and
(2) The selected values are appropriate
to each weight condition between the design
maximum and design minimum weights.



EDIT 2: So, according to you statement, where in this article do we find helicopters with teetering-semirigid rotor systems?

[n5296s]

load limits are generally specified in the construction rules and therefore there is no further need to explain the limits in the RFM
But you could make the same argument for fixed wing... Normal Category means 3.8, yet the limits are stated explicitly in the POH. It wouldn't seem THAT hard to include an explicit limit in the POH for a helicopter - especially since, from the rest of the thread, it would appear that in fact lower limits do exist.

As any instructor should know
Ah, but if they all did, it would deprive you of the opportunity of telling us all what a superior person you are.

[puntosaurus]

The EC130 has (2.5G), and a little light to tell you to cool it !

[Shawn Coyle]

I can only speak for the positive load limits, as there really is no justification for not having a G meter for the <1.0G stuff in teetering rotors.
The reason there is no G meter for the positive (>1.0G) stuff is that the helicopter has to be designed to meet pretty large G values (but these do not have to demonstrated in flight), and it's pretty much impossible to have a rotor system generate that amount of G for anything but the briefest possible instant.
The most G I've ever heard of a helicopter generating is 3.0 G, and that was after a most abrupt hydraulic actuator hardover in cruise flight. So, pretty much, you can't generate enough G load with manuevers in the pitch or roll plane to hurt a structure that's been properly designed. Yaw might be another matter, but we'd need sideslip instruments for that...

[tecpilot]

Ah, but if they all did, it would deprive you of the opportunity of telling us all what a superior person you are.

n5296s, seems to be easy to be superior with instructors never have looked into the basic construction rules of their a/c and never questioned such or other basics like load limits.

In fact we will found in all RFM i know the term "aerobatics are prohibited" and more any rotorcraft have more load limits than just on the structure. BTW, the FAA aerobatics definition is ”Any manoeuvre involving an abrubt change in an aircraft’s attitude, an abnormal attitude, or abnormal acceleration, not necessary for normal flight” whatever it means. :=

Generally we have also factual and not in the RFM described load limits in the fuel (feeding) system and on the hydraulics/controls and the (blade) stall speed depends also on the working load factor.

Yes, there are questions about the negative g especially on the teetering rotors. But if you read the rules carefully it is not necessary to reach the -1,0g limit for certification, -0,5 is enough (and there is no specified time holding that working force), cause "The probability of being exceeded is shown by analysis and flight tests to be extremely remote" :oh: means nothing more than "if the pilots fly the normal way - than our a/c comes never in -g"

And to add some mathematics for our instructor friends (just irony!), load limits are easy to calculate at least on steady turns.

n (load limit) = 1/cos bank angle

Means 60° = +2,0G but on 80° the actual load factor reaches +5,88g and that's much more than the certified.

BTW, as i asked the famous Red Bull helicopter arobatics team, they never exceed +3,0g and -1,0g with their BO105 cause impressive aerobatics are not (only) a question of g-loads. And they have installed a load limit indicator inside, but only to make sure to stay within the real important limits.

[lelebebbel]

Honest question: If helicopters really have to be able to survive -0.5G, how is this tested on two-bladed machines? Is it actually possible to achieve a negative load factor with a teetering rotor?

I always thought the requirement for negative G-loads doesn't apply if the flight manual prohibits low/negative G manoeuvres, and the minimum G-load was +0.5G for most machines.

[AMDEC]

Increasing the airspeed on a fixed-wing aircraft allows to reach higher load factors, sometimes beyond the structural strength. This is why some fixed-wing aircrafts have g-limitations.
There is nothing similar on a helicopter. Beyond 100 Kts the load factor capability only decreases. The load factor you are able to reach in 100 Kts conditions cannot be exceeded in the whole flight envelope. The structural strength substantiation takes it in account.

[Darkhorse30]

Hueys and Cobras (AH-1G's through AH-1S's) are limited on the low side to +0.5g's because of main rotor flapping as well as the dreaded so-called "dihedral roll". If you push over too hard and get negative g's you get an un-commanded right roll due to tail rotor thrust. If one adds left cyclic to correct the right roll you enter mast bumping which is usually fatal. The correct procedure is aft cyclic, keep it in trim, and ride/fly it out. Nothing to do with structure.
I was in it once in RVN and I paid attention after that.

[n5296s]

Increasing the airspeed on a fixed-wing aircraft allows to reach higher load factors, sometimes beyond the structural strength. This is why some fixed-wing aircrafts have g-limitations.
Interesting. So are you saying that effectively a helicopter is always operating below Va, the speed where the wings (fixed wing) will stall before you can break anything?

Does that mean that if you try to pull cyclic hard enough to break things, the rotor blades will stall? That sounds fairly ugly since if they do, they will stall asymmetrically.

Re the Red Bull team... you can do just about any aerobatic manouver, except tumbles and outside loops, within an envelope of -1 to + 3. From that pov any Normal category fixed wing aircraft can be safely looped and rolled, you just have to be very good to not accidentally pull a bit harder. Bob Hoover used to demonstrate this. Given a G-meter and a parachute, I would be willing to loop or roll a 182, though not mine thank you very much. (And to be legal of course it would have to classified experimental). If an R44 is good for +3.5 then I guess you COULD loop it (not that I'm suggesting anyone should), though it's not something I'd care to try even WITH a G-meter and a parachute. You'd have to be super-careful on top, where normally you're pulling around +0.5 but it's VERY easy to float. Would be a lot of fun to try in a sim, if there was one.

[OBX Lifeguard]

Remember the 60 degrees of bank equals two Gs is in level flight at constant airspeed. You can easily roll to 80 degrees pulling very little G by giving up altitude or attitude. At one time we were allowed to do 90 degree banks in the UH-60 Blackhawk, and inverted flight happened (unintentional of course :) though lovely wing-overs resulted). You cannot maintain altitude with 90 degrees of bank (well..., actually... you can in some aerobatic airplanes by using the fuselage as an airfoil...but I digress ) which apparently some Army Pilots couldn't grasp...besides it was driving Safety Officers crazy to see pilots having fun flying...

An aileron roll is about a 1.2 G maneuver because you start with 30 degrees pitch up and 1 G and end with zero degrees pitch up going thru 360 degrees of bank.... A slow roll experiences negative Gs as it is a constant pitch attitude/altitude maneuver...it is significantly harder to do well than an aileron roll.