maneuvering load factor FAR 27

[slowrotor]

For the certification of rotorcraft in the USA, FAR 27.337 states: The rotorcraft must be designed for-
(a) A limit maneuvering load factor ranging from a positive limit of 3.5 to a negative limit of -1.0; or
(b) Any positive limit maneuvering load factor not less than 2.0 and any negative limit maneuvering 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 maximum weights. END

A positive limit of load factor of just 2.0 is very low in my opinion. Are small helicopters such as Hughes 269,Brantly B2b and R22 designed to just a limit load of 2.0 ?

Airplanes are usually 3.7 normal, 4.4 utility and 6.0 acrobatic


I would think a limit load of 2.0 could be exceeded in a steep turn.

Also, the gust load standard for rotorcraft is 30 feet per second, airplane standard is 50 feet per second.
Why lower for rotorcraft?

P.S. I bought Shawn Coyles, The Art and Science of Flying Helicopters (great Book!) and Ray Proutys, Helicopter Performance, Stability and Control.These books should help limit my strange questions to this forum hopefully.

[Vfrpilotpb]

Hi there SR,

Many books will fill your mind and hard disk with acres of information, asking a question on this site give you more because most of these guys have been there, and most of them got the Tee shirt, there is nothing to beat passed on expertise, keep asking!

[NickLappos]

slowrotor,
As Chief R&D test pilot and also as a member of the design team for Comanche, I had a chance to really probe the question you ask (a really good one, and fundamental, yet ignored by most agencies in their pilot training literature). There are two kinds of load factor the designer must worry about:

1) the limiting loadfactor of the static structure - the airframe (static means it doesn't rotate). This is the big number that defines when the tail falls off, or when the engines might get out of their alignment. It is usually 3.5 G for a part 27 or 29 helo, and has almost nothing to do with what you can actually pull in flight. The design is subtantiated for the g level, and must be shown to have a 1.5 safety margin (3.5 times 1.5 = 5.25, the acutal g at which failure will occur). Often, crash load factors or landing gear loads are more important to the structure.

2) the actual thrust capability of the rotor and the rotating components, expressed in g's. Usually about 1.5 to 2.5 for most helos at sea level, this is what the rotor can actually pull, so this is the number you will see on a g meter if you do an aerial maneuver. The limit is rotor retreating blade stall, where the rotor can't produce any more thrust, or where control is nearly lost as the thrust is increased. Unlike an airplane, rotor stall can't be easily felt by the pilot. Do not believe the training guides, which give all kinds of advice on stall detection, based on 1953 helicopters. They are of little help for machines with advanced airfoils, where stall causes sharp change in the blade pitching moment, and therefore increases in the loading of the pitch links, and therefore can actually drive the controls around the cockpit (jack Stall). As you climb above sea level, the g falls off, and at the service ceiling, the g capability might be 1.1 g's.

Most helos are designed as transports, for carrying things and people, just as most cars are. Don't maneuver your helo agressively for kicks, any more than you wouuld take your car to Indy for some trials. the awesome surprise some folks showed on this forum when a Robbie blade was fould cracked after the guy was doing nothing more than "mustering" is an example.

[Shawn Coyle]

Nick, as usual, is right on.
Most helicopters can only generate a steady load factor of 2.0 at most, and transiently not much more than that. The manuevers to get even that load level are pretty interesting, and not stuff that is normally done.
The interesting thing about the FAR requirements is that they are 'designed to' requirements, and no necessarily 'demonstrated to' requirements. The tail boom requirement is one that comes to mind.
The other issue is whether the fatigue spectrum the manufacturer designs to is what is finally used. Using a helicopter for logging is not the same as VIP transport, as we have found out in the industry to our cost.
But a great question.

[slowrotor]

Thanks Nick and Shawn,

The gust limit was 35 feet per second years back then they raised it to 50 fps (for fixed wing) if my memory is correct. This was based on a better assessment of what gusts a pilot may encounter I suppose.
But rotorcraft is still 35 feet per second.

So the point of this inquiry is this: should a small helo be cruised at a reduced speed if the pilot is concerned about hitting a strong gust.
The R22 has an airworthiness directive prohibiting flight into moderate turbulence. Sometimes owners and instructors are not aware of airworthiness directives,they may think that is for the maintenance people.
I have hit extreme bumps without warning (fixed wing).
Would it make sense to limit cruise in the R22 to say 70 or 80kt?

[NickLappos]

Slow,
The gust speed is not a significant contributer to the helicopter's load factor, I think, unlike starched wings, where the wing loading allows some real load factor to be generated by the gust.

Its a simple velocity match. If a common airplane with normal wing loading (maybe 20 lbs/ft2) and cruising at 120 knots (200 feet per second) experiences a 50 ft/sec gust, there is a very real shift in the angle of attack (the angle of the new speed vector has a tangent of 50/200, or a 14 degree angle of attack change!)

For a helicopter, the blade loading is in the order of 75 to 100 lbs/ft2, and the average blade speed is about 350 feet per second, so the resulting angle of attack change is smaller, and the net change in lift is also smaller.

The AD on gusts is really a limitation for new pilots only, and seems to control the possible overcontrol hich might lead to mast bumping. This has more to do with the pitch axis re-trim in the gust than it does with load factor, I'll wage:

http://www.airweb.faa.gov/Regulatory_and_Guidance_Library/rgAD.nsf/WebCurrentADFRMakeModel/91BE0874983FB92686256A4D0061449D?OpenDocument

[helislave]

Hi Guys, a little off topic, but do any of you know where i can get a copy of the JAR/FAR 27-29 for free? I need to read up on some stuff, potential job you see.

thanks in advance

[slowrotor]

Helislave,
For USA FAR's try this http://www.airweb.faa.gov/Regulatory_and_Guidance_Library/rgFAR.nsf/MainFrameNetscape4x?OpenFrameSet

or the link in Nicks post above for AD's