non-symmetrical blades

[Gav500D]

I was wondering if anyone could put in laymens terms how non-smmetrical blades work? Also do non-symmetrical blades produce a rougher ride? Also do they produce more or less lift in autorotation compared to symmetrical blades as. I know this is not an exciting subject but some advice would be great!

[Disguise Delimit]

Asymmetric blades are more efficient than symmetric blades. But they cost more to make.

Symm blades have the advantage that the centre of pressure (and the pitch change moments) doesn't move much with changes of angle of attack, so the blade grips can be less strong, lighter, and therefore cheaper.

Asymm blades are now easier to make with carbon fibre etc, and the grips can be made stronger and lighter than in the 60s and 70s.

So, you should find that modern machines have asymm blades, older machines and those made to a price don't.

[CRAN]

I assume that you are referring to the blade aerofoil sections, not blade planform. If that is the case then here goes...

The aerodynamics of the helicopter main rotor are extremely complicated. On the advancing side the aerofoils must be able to operate efficiently under a transonic flow regime, that is, the combination of the rotational speed of the rotor and the forward flight speed can produce Mach numbers that are greater than 0.85. This produces pockets of supersonic flow that terminate with a relatively strong shock wave resulting in a large drag penalty amongst other things. On the retreating side the aerofoil must be able to operate at very high incidence at moderate Mach numbers - frequently close to the dynamic stall limit.

On top of this, helicopter aerofoils must be designed so that they do not produce large pitching moments. If they did, then huge control loads would be produced that would require a very strong rotor control system.

Clearly, there are lots of conflicting requirements here. The trusty old NACA0012 symmetrical aerofoil found wide application in early helicopters because it is a moderate thickness aerofoil that combines enough thickness to be able to operate up to about 12 degrees without separating (statically) while being thin enough to avoid developing strong shocks too early. And all of this, with virtually zero pitching moments until those nasty shocks start to appear.

The problem is that the early symmetrical aerofoils are rather dragy, they don't behave very well at higher mach numbers and they have some nasty characteristics at moderate Reynolds numbers. Therefore, in the quest for higher performance machines, we needed to develop aerofoils that delayed the development of shock waves until higher mach numbers and then produced weaker ones. In addition, we needed aerofoils that could operate at higher incidence without separating. Again in all of these regimes we wanted low drag and moments.

So in summary, cambered, or asymmetric aerofoils achieve greatly improved sectional aerodynamic performance, meaning lower drag and higher maximum lift in all of the flow regimes described. This means our modern aerofoils limit the strength of the shockwaves on the advancing blade and delay the onset of dynamic stall on the retreating blade. This makes our helicopters fly faster, further and higher, while vibrating less and being significantly quieter (Reduced transonic noise). However, very often this is at the expense of significant or even large pitching moments.

In most modern rotor designs this problem is countered by using a variety of aerofoils along the span, the BERP 3 rotor uses three different aerofoils for example. This allows you to balance the nose up pitching moments generated by one aerofoil with the nose down pitching moments of another. However, this isn't always enough and so it is sometimes necessary to resort to planform modifications.


I disagree with the earlier remark that 'asymmetric blades are more difficult and expensive to manufacture'. So long as it is a single aerofoil along the span, then the cost is identical to that of a symmetric aerofoil. Things get more complicated when you use more than one aerofoil, as you really need to use composite blades then, but many manufacturers use composite blades for other reasons anyway. Once you have gone composite then you can have quite alot of aerofoils with highly optimised shapes at little additional cost. If you got to complex tip shapes then some cost is added, but not nearly as much as some of the manufacturers would have you believe!

Hope this helps chaps,
CRAN
:ok:



Sorry Gav,

Didn't read the post properly! The one line answer is...

In general, modern asymmetric helicopter aerofoils produce more lift with less drag, than traditional aerofoils, which make is possible for helicopters to perform better.

...and the other bits...

They don't have a significant effect on the 'ride' on their own, as this has to be viewed from a rotor system point of view. Though they will improve autorational characteristics by virtue of their better lift to drag ratio and ability to delay separation to higher incidences.

Cheers
CRAN

[Lu Zuckerman]

I don’t claim to be an engineer nor am I an aerodynamicist however I have noted that when they switched from symmetrical to non symmetrical air foils the helicopters encountered a lot of vibratory forces mainly in my estimation from the pitching moments of the blades. A long time ago when I was in helicopter school the instructors and some of the engineers stated that the reason helicopters could only use symmetrical airfoils was that unsymmetrical airfoils generated climbing and diving moments. When the switch was made most if not all helicopters incorporated vibratory dampers to cancel some of the vibratory loads.

Ducking from incomming.

:E :E

[Gav500D]

ceers guys, for taking the time to reply your've made it heaps clearer

[delta3]

Vibrations

Lu, as you and Cran pointed out assymetrical foils do indeed have the disadvantages of producing moments along the blade cord. Even if these moments are more or less balanced out along the blade as they are in the case of the Lynx by carefully chosing different sections along the blade, these different moments still produce higher order harmonics which are not so well balanced. This is the cause of these vibrations

Delta3

[NickLappos]

The airfoil sections for all modern wings and rotors are not symmetrical, since they are optimized for lifting, which occurs one way. The shapes are selected for their ability to more efficiently develop lift wthout requiring more power.

Moment is a very secondary reason for selection of the airfoil, because even very poor moment characteristics can be solved with more structure (weight) in the swashplate and controls/servos. To double the weight of the controls might add weight equivilent to perhaps 1% of the rotor thrust, while a very optimal airfoil might add 3% to the net lift, so why not push for efficiency of lift?

We speak of figure of merit, which is the efficiency of the rotor as compared with a perfect one. Where all the engine power is converted to pure lift, we say the figure of merit is 1.0. Older symmetrical airfoils and simple tips produce rotors with figure of merit of perhaps .70 to .72. Modern assymetrical airfoils and swept tips produce figure of merit in the .78 to .80 range.

That means a 10,000 lb helo with an old rotor system could become an 11,400 lb helo with the same engines, at least as far as power efficiency is concerned.

There are some ways to see this in the flight manual, using the "cruise charts" and climb charts in the performance chapter. If ppruners are interested, I can try to post some examples.

In any case, the use of CFD (computational fluid dynamics) has produced a new set of extremely efficient rotors with carefully designed airfoils that move air and create lift very efficiently. These airfols are not symmetrical, by design.

It is my experience that vibration has no relation to symmetry in the airfoil. Why should it? What has symmetry about the chord axis to do with anything "balanced" about the way the rotor moves air? Most blade induced vibrations are due to the structural tuning of the blade itself (which produces harmonic forces at the hub) and the interaction of the blade with the shed vortex from the previous blade in the sequence. We often tune the harmonics with weights along the spar to try to make the blade spin more harmoniously.

[[email protected]]

Delta 3 - the reason for the firm ride on the Lynx is the semi-rigid rotor design where every bit of feedback from the blades is transmitted to the fuselage. I suspect that since most cambered aerofoils on helos are on ones with modern semi-rigid or rigid heads then this is why the blades may be associated with a harder ride.

[delta3]

Crab

I fully agree with your statement.

Nick

I would love to have some examples.
As far as vibrations is concerned, I am not going to argue with you, but it appears to me that the extra 'harmonics' so to speak produced by the pressure point changing over the span of the blade could be a cause of extra vibrations. These harmonics would be rather 'torsional' with respect to the blade as opposed to the classic harmonics caused by speed difference, blade twist etc which act lonigitudinal with respect to the blade (flapping harmonics).

Delta3

[NickLappos]

delta 3,

We always want to subscribe to differences as causes, so the belief that pressure points are some how changing develops into a theory of vibration. Let me try this mental model on you:

The assymetrical airfoil develops more thrust for the same power. Therefore the airflow around it is LESS disturbed, and less likely to cause any disruptions in force that might be measured as vibrations. The energy in the vibrations that you feel comes from your fuel tank, of course. It would also follow that modern airfoils would produce less noise, because that, too, is wasted energy.

It is interesting that our human view of perfection and symmetry are biases that sometmes produce misunderstandings about physical behavior! Why is the symmetrical shape of the blade, which is more pleasing to our eye, believed to be more pleasing to the air? Dave Jackson, where are you?

[delta3]

Nick

If my comments would argue against asymmetry, then that was certainly not the purpose, asymmetry has been the way forward. Yes this produce more lift, and so much more that there is a lot of margin to work with if needed for to compensate possible side-effects.

My statement is a pure 'engineers' reaction. When going from the symmetry to asymmetry some new 'disturbance modes' may be triggered which have to be taken care off if they come in the neighbourhood of some resonance frequencies. As an engineer, permit me to reason as a pilot: better to be safe then sorry. Apparently torsional resonances do not occur, or at least they are far less important then vortices, drag divergence etc..

delta3.

[NickLappos]

delta3,
Actually, your comments are very astute, the structural response of the blade is one important the reason for vibrations. Blade design forces us to make sure that torsionals are very well damped. That is because the blade has enormous aspect ratio so the torsional stability is derived from the mass balance and not from structural stiffening. A wing can have a deep spar box, but the blade is a noodle by comparison. For this reason, we place significant weight in the tip of the blade, forward of the spar and just under the nose skin. This creates a strong positive torsional stability, probably much stronger than the aerodynamic shifts you mention.

Here is our trusty rotor blade flpping itself. Note how little torsional movement there is:
http://www.s-92heliport.com/ROTOR.AVI

[Flytest]

Nick, Delta 3

Do either of you subscribe to the idea of adjusting tip weights (Note: not addition or subtraction, just moving fwd or aft along the chord) as a method of controlling pitching moments on asymmetrical blades?

[delta3]

Flytest

The mass balance is (see previous remark by Nick) apparently very important in mastering the torsional disturbances. Adjusting the tip weights (in all directions) is part of the total mass balance. So it should help but will not be capable of providing for all the control the weight distribution and concentration provides for over the full blade (both in span and along chord). The blade end does however not enjoy the same 'preferred' position with respect to torsional disturbances as it does for flapping disturbances, because the centrifugal forces don't play here.
This is similar to the multiple bearings of the tail-rotor shaft, where several bearings are needed to provide for stability (of torsional nodes)

Delta3

[NickLappos]

That longitudinal CG of the blade (the chordwise CG) is a very important term in its torsional stability - in other words, its flutter stability. It would be a really bad thing to tamper with the tip weights, since much aft movement might make the tip area les stable, and perhaps more likely to flutter and perhaps fail.

Again, I think the whole idea of this thread, that assymetrical blades are more likely to produce vibration, is suspect.

[imabell]

way way back in another time rotor blades were made symmetrical for the sole reason of limiting the hunting tendencies of blades due to centre of pressure changes.

this reduced the fatigue of the rotor head and the grips.

they didn't know about this other stuff.

[NickLappos]

imabell said, "way way back in another time rotor blades were made symmetrical for the sole reason of limiting...."

Not true, the old days used aluminum spars, and making an assymetrical airfoil was too hard, so symmetry was imposed by manufacturing.

Modern blade spar materials and modern adhesives allow us the shape them the way we want (and twist them oddly, too.) That is why we have those airfoil sections and tip shapes.

The myth that somehow the pressure distribution and moment wander on a modern airfoil will not die will it?

OK, does anyone out there have some real data to show the "center of pressure changes" that are now part of pprune legend?

[Lu Zuckerman]

I may be wrong but the hunting (lead / lag) is a function of introducing cyclic pitch with the resulting imposition of the laws of conservation of angular momentum.

And for the true believers there are gyroscopic precession and centrifugal force.


:O :E :E :O

[Wunper]

Ref using the tip weights for track control.

Nearly all of the Bell model helicopters employ what they term Product Balance using front and rear pockets on the upper surface of the blade near the tip

Product balance on the M412 is used to control the track pattern change between the ground and hover test conditions and thereby prevent lateral vibration readings on opposing sides of the polar plot for the two conditions.

With around 1000 G acting on the mass at this span location any chordwise changes of mass distribution have a profound influence on controlling the “noodle” (nice term Nick!) torsionally when the disk is coned. For this reason it is most important that the aircraft is flown at the correct AUM for the RTB flights as the response sensitivities for product balance are only good for a narrow range of AUM.

The RTB in flight test conditions use climb and let-down (hi power low power) to further expose Chord-wise tip CofG mismatches in the pack that would point to a need for a product balance correction.

With regard to the procedure for using it, it is well documented in the MM and most effective, I personally wish more manufacturers would use it

Wunper

P.S.

By the by the M412 has a heavily reflexed asymmetric aerofoil section, and just how relevant that is to using Product Balance vs a symmetrical section I’ll leave to the egg heads…

[imabell]

show me an aluminium spar in a wooden d1 blade.

whenever a wing accellerates the c of p moves forward, whenever a wing descellerates the c of p moves aft. thus the term "hunting" for the centre of pressure. or so it said in the theory that was taught when i was student. i know i'm old.

"underslinging the head, to reduce the hunting stresses on the rotor head as the blade moves in the direction of the centre of pressure" was the wording i think.

that is the correct answer in the aerodynamics exam in two countries. ?????????

i know things have changed, (slightly), since but.

all that vibration stuff is beyond me. it's not that deep.

:confused: :ok: :uhoh:

[Flytest]

Phew, Wunper thanks for that mate..

Was always taught that where the procedure exists on type, then you should always make use of whatever adjustables are available.. chordwise weight adjustment included..

Glad I'm not the only one :ok:

[NickLappos]

The notion that the Center of Pressure shifts (as induced by hunting or anything else) is simply wrong, where does that idea come from? Does anyone have any such data?

Up until stall or Mach 1.0, the CP of an airfoil is a constant, unless you are flying airfoils taken from barn doors.