Helicopter G limits/manoeuvering speed(R22) + Takeoff w/ Gs

[ifresh21]

Hello,

I found this video on youtube a while ago. The pilot accelerates to around 65 knots and pulls pretty hard for the takeoff.(takeoff at 0:33)

http://www.youtube.com/watch?v=9iJLtSQmSk8


Is that bad for the helicopter/dangerous in terms of G forces? He is clearly outside of the HV diagram, but I would like to know about the Gs.

Also, in the R22 POH, it doesn't talk about G limits at all. There is no maneuvering speed. Does that mean that no matter what speed you are at (below never exceed), you won't exceed any G limits? (basically, NE is below the maneuvering speed)

I am guessing that some other helicopters have maneuvering speeds?

Thank you!!!

[Dantruck]

I'm not a fixed-wing pilot and I'm guessing you are. For me, and from my limited knowledge of such things, I thought manoeuvring speeds were a fixed-wing preserve. My point being, you'll likely break other things on a helicopter long before you get to manoeuvre-induced airframe limits on a rotary bird.

Dan

[Shawn Coyle]

There is no maneuvering speed listed for helicopters as it's not possible to generate enough +ve G with the rotor to do any structural damage. Most +ve G that can be generated from the rotor (without doing some pretty amazing tricks) is about 3G, and the structure is designed for much more than that.
I know of no helicopter that lists a maneuvering speed in the flight manual. (and I've flown quite a few).

[rotorfan]

The point of maneuvering speed (Va) in a seized-wing is that, below that speed, a sudden increase in load-factor (for instance, hitting a sharp updraft) will result in the wing stalling, rather than overstressing the wing. The stall, of course, unloads the wing.

As noted above, helicopters don't have such a speed. The R22 does have in the limitations section of the POH a note concerning moderate or above turbulence. You are to fly between 60KIAS and 0.7Vne, but not lower than 57 KIAS.

[Ascend Charlie]

As I recall, the manouevring speed limit on a plank allowed for full-scale deflection of the ailerons (one each way) causing twisting forces on the wing. Above Vmo, only gentle aileron inputs are permitted.

Uptycopters don't have ailerons, the blades are continually on full opposite pitch at high speed, so it probably doesn't matter a rat's patootie.:ok:

[Graviman]

In gliders you are limited to 1/3 control inputs above Va, so are taught to accept g over speed in spin recovery.

Does that 3g rotor limit apply only to centred cyclic? I imagine there would be limits on mast bearings, if not mast itself, for say roll inputs. Not that i'm advocating trying any kind of hard manouvre in a teetering heli! :eek:

[rotorfossil]

A good hard pull at Vne, or sometimes even less at altitude will get you into retreating blade stall on some types particularly the older types.

[ifresh21]

Shawn cole - Why is it not possible to pull enough Gs for structural damage? I don't understand what keeps the Gs from going high

Graviman - It doesn't seems like a helicopter like an R22 is responsive enough to create a strong side force. Idk it just seems like that. And heli movements are dampened. Anyone heard of damaged bearings from turning

pulling hard is a very extreme case.



So it is ok to pull as hard as you want when flying an R22?

[FSXPilot]

Stick to what your instructor teaches you. R22s can fall out of the sky without too much encouragement as it is.

[topendtorque]

So it is ok to pull as hard as you want when flying an R22?


Gees, what a class act.:{

[MartinCh]

maybe you should quickly PUSH HARD (on cyclic/T bar) while MCP or max take off power and climbing at Vy or Vx.

OR, quickly PULLING HARD (back) on cyclic while somewhere between Vno or Vne. :ugh:

If you're still alive, answer and comparison with exemplary NTSB reports, on a postcard, please.

Who cares about V speeds in helicopters? Follow the 'avoid' curve/diagram, no abrupt movements, avoid turbulence, avoid unloading disk/creating low G on the bloody flimsy thing. Avoid high RoD, low RPM, too high engine and rotor RPM, keep safe speed when practicing autos. Etc etc.

ifresh, not sure you're aware, but Shawn Coyle wrote pretty thick book about helicopter training/flying and aerodynamics. It's got more stuff than FAA rotorcraft manual.

What keeps us from pulling hard on either control as not desired/recommended? Common sense and some sort of self-preservation left in us.
Sure, frames like Bolkow 105 with rigid rotor system and mil specs durability, they can do some +G maneuvres. None of that in R22 design.

FSXPilot is spot on in a very succint reply.

[ifresh21]

Oh I didn't know about shawn. What is the book - I might buy it.

Martin(and everyone): I am not saying(nor do I want to)actually try it and am trying to find out if it is safe. I am simply talking theoretically to build knowledge. As someone with a fixed wing backround who can look at the 172 POH and see limits like 4.4 positive G with a maneuvering speed, I am curious about the limits of helicopters and what they are capable of. If it is true that it is impossible to exceed 3Gs without exceeding Vne for example, that would be really impressive. I am just asking.

Is it wrong to ask?


Id love an little explanation from shawn as to why 3Gs can't be exceeded/why you can't due structural damage due to G forces (or is it in your book at least so I can read there). Thank you very much in advance

[Gemini Twin]

Design requirement fron FAR's

§ 27.337 Limit maneuvering load factor.

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 minimum weights.
[Amdt. 27–26, 55 FR 7999, Mar. 6, 1990]

Due to Vne limit and control response it it difficult to exceed even +3 g in your average helicopter. Negative G especially in a twp blade system can result in a much more critical condition and should be avoided like the plague

[Flying Lawyer]

ifresh21
Oh I didn't know about Shawn. What is the book - I might buy it.

Cyclic & Collective
More Art & Science of Flying Helicopters
by Shawn Coyle


Superb book. :ok:

[ifresh21]

Book is expensive lol

[Rotorwashed]

Book is expensive lol

book is worth every penny

[Heliport]

Worth every penny to a helicopter pilot, or aspiring helicopter pilot. :ok:

ifresh21 is a fixed-wing student pilot.

[NickLappos]

Heliport is right, Shawn is the consummate expert!

Having lurked here for a bit, let me weigh in on this pithy subject:

Unlike an airplane, the G's a helo is pulling are only half the story. That is because every helicopter is really two entirely different systems flying in formation - the fuselage, which behaves like an airplane and which we call the "static structure", and the main rotor, which behaves quite strangely under load, and is called the "rotating system".

In turn:

The Static Structure - The G's that the FAR refers to is the load factor experienced by the fuselage - the engine mounts, the tail cone, the feet of the main transmission, and the G meter on the pilot's panel. When the FAR refers to 3.5 g, they refer to the static structure, since the rotor would long prior give up the ghost as the pilot maneuvered that severely.

Frankly, regarding load factor, the FARs were derived from airplane requirements and so they shadow the way airplane behaves. And airplane's structure experiences load factor across the entire aircraft. The wings produce lift and delivered to the whole structure, until the wing stalls or the structure fails. The most extreme maneuver an airplane can successfully perform occurs at the speed where the maximum load factor at stall matches the structural strength of the wings. We call this maneuvering speed and suggest that the pilot slow below maneuvering speed under severe turbulent conditions. Maneuvering speed protects the airplane from structural harm by assuring that the wing will stall before harmful forces are produced. There is no rotorcraft equivalent to maneuvering speed.

The rotating structure – the rotor of a helicopter produces the load factor but virtually no helicopter can produce the load factor required by FAR since virtually no helicopter has a rotor designed for 3 1/2 Gs, except perhaps a very light H-60 or H-64. A rotor designed for 3.5 g at normal weight would consume far too much power in a hover, and require far too much blade chord, again at a cost of considerable payload. The limits to a rotor during high load factor maneuvers are related to the blade stall and the ensuing pitching moment, and the stresses on the pitch links, swashplate and servos. This topic has been much discussed when we've talked about servo transparency, where the blade forces during high load factor maneuvers result in high stresses on the aircraft's controls.

It is safe to say that most helicopters, under limiting rotating system maneuvering, will produce very high blade and control stresses under surprisingly low static system load factors. Because rotor stall is strongly affected by altitude, airspeed, RPM, and collective pitch, it is very hard to predict precisely what load factor would produce excessive rotor stresses. This is why several manufacturers have attempted to employ servo warning limits systems as a means to directly warn the pilot of excess rotating system stresses during maneuvering. The servo limit lights on some EC models and the cruise guide on some Sikorsky models comes to mind.

To give a sense for the kinds of forces the rotor blade can place on the controls and servos here is a thought experiment. Imagine that your helicopter was placed in a hangar and the rotor blade was passed through a hole in the concrete wall and that hole was then bricked up tightly around the blade. Now connect a hydraulic mule to the hydraulic system, bring the system to full pressure and start to move the cyclic around. You can imagine the enormous forces the hydraulic cylinders would impart on the swashplate, pitch links, blade horn, and rotor blades while the rotor blade is trapped in the concrete wall. This condition is precisely what occurs in Jack stall, also called servo transparency except it is the blades that produce the force to drive the hydraulic servos backwards against their maximum force capability.

In a nutshell, load factor is not useful to warn pilots of the damaging maneuvers that the helicopter's rotating system can experience, and in fact a G meter might fool a pilot into thinking his maneuvers were acceptable while the rotor was screaming in protest. Meanwhile, it is very doubtful that the rotor would produce the load factors needed to bother the static system.

As pilots who've experienced Jack stall will tell you, under some conditions, it takes little load factor to create these enormous control system stresses, given some airspeed, altitude and gross weight.

[MartinCh]

ifresh21 - Don't get me wrong. I never said you're aspiring test pilot in R22. There WERE plenty guys in the past who taught us how to get killed in some helicopters through NTSB reports, serving us to avoid these.

heliport: ifresh21 is a fixed-wing student pilot.

Well, some of his/her past posts earlier on talk about C172 stuff. He/she obviously did some fixed wing flying in the past.
Considering ifresh21 user posted some stuff about own R22 lesson with some overspeed issues, I'm safe to say he/she just likes flying and interest in rotary aerodynamics has reason behind it.

I'm fixed wing PPL, glider pilot and can fly rotary - nearly PPL (with more to come this year, finally). Does that make me more into fixed wing than helicopters?

Back to Shawn Coyle's book.
Well, I prefer having good deal, value for money. Probably the best bet is Amazon marketplace with brand new or used books with considerable discounts. I didn't pay RRP for C&C.

[ifresh21]

Thanks for the super detailed response Nick.

So if I understand correctly, you are saying that something else would break before 3Gs is achieved? You also talked about rotor stall. Is it possible to stall the rotor if you remain inside the RPM limits in a helicopter like the R22? Or is it that you would run out of power(within MAP limits) to maintain RPM with before reaching a high G load?

But if the FARs say that the helicopter has to be able to withstand a certain number of Gs, why would that not count for the entire helicopter? That means that the helicopter would not be able to handle the Gs required by FARs - right?(cause the rotor system would break or jack stall etc.)

The R22 doesn't use hydraulics, so is there some sort of equivalent problem since jack stall is impossible?

Is it that the rotor can't physically produce the load factor to bother static system because of rotor stall?


Im pretty confused about this


What about helicopter that can do a loop like the apache ah64

http://www.youtube.com/watch?v=us18o7qOXjI

how many Gs would that be

[NickLappos]

ifresh21,
In general, yes, the rotor would stall, and produce a load factor much less than the static system is designed for, so the maneuver would not produce damaging G, but it would produce damaging fatigue forces in the rotating system controls.
That does not mean the rotor would break or any such thing, because generally, the components from the blade to the servo are designed to take everything the blade stall forces can hand out, at least a few times. They would fatigue, and most likely, the bearings would wear out very quickly. You would find pitch link bearings, swashplate trunnions and scissors wearing very prematurely as the big loads were being routinely passed through them. I would bet helos that are maneuvered a lot see lots of bearings being replaced, as well as minor cracks on the blade root areas.
Let's answer each piece:

So if I understand correctly, you are saying that something else would break before 3Gs is achieved?
no, just that the rotor will reach a stall condition, and produce no more thrust, and so no more G. It wouldn't likely break, but it would get worn out quickly.

You also talked about rotor stall. Is it possible to stall the rotor if you remain inside the RPM limits in a helicopter like the R22? Or is it that you would run out of power(within MAP limits) to maintain RPM with before reaching a high G load?

Great question. The engine power is only a small piece of the puzzle, because big maneuvers must be fueled by decelerating and using the aircraft's kinetic energy to produce the G, temporarily, as the speed is coming way down. I have experienced 20 knots per second speed loss while in big maneuvers in helos, while the engine power was only moderate.

But if the FARs say that the helicopter has to be able to withstand a certain number of Gs, why would that not count for the entire helicopter? That means that the helicopter would not be able to handle the Gs required by FARs - right?(cause the rotor system would break or jack stall etc.)

No, because the FAR is a static system requirement, later in the FAR it even say the forces assumed to produce the G are assumed to act on the mast or rotor center, because these limits are static system, theoretical limits. Once again, the static machine cannot come apart under load factor, but that design G strength limit is no guarantee that the rotor can produce that much thrust. Most helos cannot produce 2 G's under engine power, let alone 3.5!

The R22 doesn't use hydraulics, so is there some sort of equivalent problem since jack stall is impossible?

Yes, the stick forces would rise, the cyclic would feel throbby and start to jump around in the pilot's hands. In effect, the pilot is the "jack" and he/she would find it hard to control the aircraft.

Is it that the rotor can't physically produce the load factor to bother static system because of rotor stall?

Yes, now you've got it.


Im pretty confused about this.

You should be, it is hard, nobody ever teaches or discusses it, and frankly, few folks know the rotor theory and the pilot lingo, so the two worlds don't often meet.

What about helicopter that can do a loop like the apache ah64 how many Gs would that be

A loop takes about 2.5 G when done at helo speeds, few of us who fly helo aerobatics pull much more, because we can't - the rotor will not produce that much thrust. At 80 knots, 2.5 G is a tiny maneuver radius, perhaps 300 feet, a very wonderful loop!

One macho comment, I used to demo 3+ g's in light Black Hawks using a wind-up turn, and trading airspeed for load factor at 75 degrees of bank. John Dixson taught me how, it is ok on a light machine at perhaps 13,500 lbs (and bad bad on a heavy one). Enter at 125 knots, roll out at 75 or 80, and have hit 3+G for a 2 second count. At that weight, I used to trim at 80 knots, 60 degrees of bank and 2 g's and make 720 degree slowly climbing turns while trimmed at 2 G's. A light Black Hawk is a screamer!

[ifresh21]

AWESOME post Nick. Thank you!

I will have to read through that a few more times :). But my preliminary question is - I don't think you answered whether or not a rotor stall is possible if RPM is maintained.

That has actually been a question since day 1 for me. I asked it to my instructor on my first intro flight. Of course, I didn't get a good answer (instructors generally keep things very simple when dealing with aerodynamics).


I just really would love to know if a stall is possible if RPM is maintained where it is supposed to be(top of green in R22/104%).


Thank you and I will probably have more to ask after a read your post again a few times


Also, you served in the military? - Thank you for your service, Sir.

[Vee-r]

ifresh21,

Since each individual blade can be considered a wing, as long as it is operating at it's normal operating RPM (blade tip speed is around 400 knots), stalling the blade will be unlikely. My (fuzzy) memory on aerodynamics leads me to believe that rotor stall is only going to happen as RPM decays, I can't think of a scenario where it could happen at 100% (vortex ring state?)
Retreating blade stall, on the other hand, can occur due to excessive forward speed.
Buy and study the books, this stuff is hard! I know I need to get my head back in them!

[ifresh21]

Any books in mind that deal with this?

I did some math 80% RPM is 366mph(318kts) - 104% is 476mph(413kts). This is tip speed in the R22.

Just a lil info since it was brought up.

[Vee-r]

Rotorcraft Flying Handbook is a start, and free as a PDF on the FAA website.
Principles of Helicopter Flight -W.J. Wagtendonk, is a bit more advanced.
Cyclic and Collective comes highly recommended too, but I don't own that yet ;)

[ifresh21]

I read the first 2 books and they definitely don't answer this question, unfortunately.

[[email protected]]

ifresh - if you pull hard enough on a R22, you won't stall the jacks (because there aren't any) but you can experience retreating blade stall (RBS) - this can be achieved at normal RRPM but only at high speed or density altitude or in a harsh pull-up or turn (or any combination thereof).

The highest aerodynamic backloads (highest AoA) are on the retreating side of the disc so that is where the stall occurs when you load the rotor whilst manoeuvring. The symptoms are a pitch nose up and a roll to the retreating side (left in an R22) and the recovery (although it will come out itself as the speed washes off due to the pitch up) is to lower the collective and reduce the severity of the manoeuvre (unload the cyclic).

So in answer to your question - yes you can get rotor stall at normal RRPM and within engine limits but only by mishandling the aircraft. Note that unlike a FW, you have only stalled a part of your lifting area, not all of it.

Aerospatiale (now EC) always maintained that jackstall (servo transparency) was designed in to provide early warning of RBS - the symptoms and recovery are identical.

Sustained G in a helo is down to engine power - a 2G manoeuvre (60 degree level banked turn) is usually started at a higher speed (120 kts for example) and will settle at about 80kts (in Gazelle and Lynx) with almost max continuous Tq.

As Nick said, loops and the like are achieved within 2.5G and it is only on the pull up (high speed) and recovery (accelerating) that you have to be careful as the higher speed means you are much closer to RBS when you load the disc.

[ifresh21]

Well thats just partial rotor stall though (RBS). During almost all flight regimes, a portion of the disc is stalled. So thats not really what I meant.

I meant stall to the point where there is no recovery.


Good reply though