Couple of blade construction questions.

[AdamFrisch]

Have a couple of questions for all you aerodynamical rotorheads out there that I've always wondered about, but haven't been able to find answers to.

1. One of the reasons heli's can't go very fast is because the retreating blade will have an unnaturally high angle of attack as to push air through the disc, causing it to stall. To increase stall resistance in FW, slats are often mounted to wings. Why couldn't you mount slats to the blade that pop out when the blade retreats?

2. Most modern rotors in helicopters seem to have constant chord blades where the tip has been twisted donwards towards the tip so as to produce less lift the further away from the hub you get. But the same thing could be achieved by having the chord change (and the twist remain unchanged) - by narrowing the blade the further you go out from the hub, the less lift it would produce (this was common in earlier helicopters). A twisted blade must be so much more expensive and complicated to make, so why was the changing chord design abandoned?

3. Also, older helicopters often had wooden blade rotors, and I think a few helicopters might still do (Brantly?). But I can't recall reading or hearing anywhere that wooden blades were more susceptible to failures than metal ones. What was the main reason wooden rotor blades were abandoned?

4. Rigid rotor question. As I understand it, in a rigid rotor system, lag and hinging is 'absorbed' by the blade itself, eliminating the need for dampers and such. Now, if I just took a normal, fully articulated rotor and removed the dampers for something solid and beefed up the blades, wouldn't this become a rigid rotor? If so, why aren't all manufacturers making rigid rotor systems, since they seem to have less moving parts that can start to misbehave?

Thank you.:8 :8

[Arm out the window]

For 1 and 4, I think there'd be structural reasons why they wouldn't be practical.

1. Slats would have to pop in and out 5 or more times a second as the blade travelled round and would be subject to all kinds of weird forces, so just making them work without breaking would have to be a massive challenge. Then I'd imagine vibration and forces imposed on the blades themselves would create more problems than the slats solved.

4. Again just my thoughts, but I'd reckon unless blades were specifically designed to do all the flexing required for feathering, flapping & dragging as well as handling normal flight loads and vibration, they wouldn't handle it.
Also it's not just the blades, but the head components, eg AS350 starflex, that come into play for that kind of system.
It wouldn't be a matter of just beefing up the blades, they'd have to be made right to handle all the fatigue caused by repetitive flexing.
My guess is that trying to cobble some kind of rigid head system from a fully articulated one would be like creating some kind of Frankenstein's monster - probably be easier and cheaper to design the whole thing from scratch as an integrated system.

[MightyGem]

As for number 3, compare a wire coathanger with a wooden one.

[NickLappos]

In general, just calculate the loads on a blade tip to see why fancy mechanisms don't stand a chance - the tip of a typical helo blade has between 600 and 1000 g's on it due to CF. That means a 1 oz flap is exerting about 60 pounds of whirling force on its tracks and hinges. Only for a few seconds, until it flys off, of course.

Wood is an excellent medium for low load work, but the inherent strength is very very low relative to metals. By inherent strength, I mean pounds per square inch. A wood beam must be much deeper to hold the load, so it becomes very hard to package as the loads go up, but the blade wants to be thin. Look again at that film "rotor.avi" to imagine what wood would do under the higher speed environment. Development of aluminum spar blades was one of the technological leaps for helos, allowing Vne to jump to 150 knots instead of the 100 or so prior. Blade motions and stresses at speed are not able to be handled by wood.

Here is a good discussion, note that aluminum is 10 times stronger than wood:
http://en.wikipedia.org/wiki/Tensile_strength
Some more complete strength data:
http://www.efunda.com/materials/common_matl/Common_Matl.cfm?MatlPhase=Solid&MatlProp=Mechanical#MechanicalStrength

Regarding swapping heads on a helo, it is not very simple. Imagine buying a set of racing tires and putting them on your Chevy Vega. Cut the wheel once and watch the front end come off. The entire helo is designed as a unit, with each connecting piece optimized for the loads it gets from its neighbor.
For a direct illustration of this, look at the mess the US Marines created when they decided to swap the 2 bladed Cobra head for a 4 bladed soft inplane rotor. They ended up redesigning the entire fuselage to chase the cracks they were getting from the higher loads of the new head. Actually, Bell was chasing these cracks, and paid handsomely for the effort. Nobody ever took out the Bell salesmen/engineers who sold this nightmare to the USMC and spanked them for their mistakes, I am sure. After all, they then turned around and sold the Yankee and Zulu, with similar development nightmares!

[i4iq]

So, whats going on with the blades here...

"Sikorsky Aircraft has submitted better than projected performance test results of its latest testing on the 4th generation rotor blade to the US Marine Corps.

The 4th Generation™ blade follows in a long chain of innovations in the area of rotor systems. It builds on the work done for Sikorsky’s state-of-the-art Growth Rotor Blade (GRB™) currently used on the UH-60M and S/H-92™ helicopter..."

... to make them so much more efficient?

[Rigga]

Good Questions!
1.
Cant add to the previous answers!

2.
Modern, metal sparred, rotor blades are/were cheaper to make with a constant chord as they only needed a twisting of the Spar to get the correct Wash-In/Out. In the case of Whirlwind/Wessex Blades, his was almost literally done in a big Vice-type machine that was manually twisted/tuned. The leading edges and pockets were then attached to the Spar and gaps caulked.

In the case of Carbon Fibre blades, I believe it is now easier to lay a constant chord pattern than a variable chord pattern, though I stand to be corrected. It is of course possible to lay whichever pattern you like, but time, money and difficulty all come in to play when laying large tracks of strands/mats/ribbons.

3.
They Failed! Wooden blades were fine for low weight and low rev rotors of the time but would not be able to withstand some of the more powerful strains of modern times. They were also susceptible to various warping problems and diseases in more exotic climates. Wooden rotors should not be confused with wooden airframes or wings. Wings don’t do all that twisting and bending per Rev.

4.
“For every action there is and equal and opposite reaction”, The same works for Rotors – If you make the Head rigid you must also make the surrounding structure hard enough to support the Gear (Actuators) to make those movements. and then the airframe needs to be strong enough to support the rest!

In other words; There is no sense in building a Tank if it is going to be used like a Pram!

Hope this helps

[Pan Pan Splash]

A further point to Q2.

IMHO.. The constant chord design lends itself better to the adjustment of trim tabs. Remember the whole idea is equal lift along the blade length (as much as possible, however it is almost impossible) but also equal lift generated from all of the blades. To do this each blade needs to be tuned as you are probably well aware, therefore in a blade with a twist, you are essentially fine tuning the twist, and in a blade which was designed to be twisted, this will create very little additional stress loading. I would think a blade designed to be flat would have problems... raising the point that if you had a variable chord, designed to be twisted.. whats the point??? I would suggest twisted blades give the designer an easier ballpark to play in to achieve equality of lift.

PPS

[AdamFrisch]

I have a couple of follow up questions.

1. Since load and high stress seems to be what killed off wooden blades - what if I had more blades and therefore spread the load between more of them? Then they could be thinner and more agile, no?

2. Isn't there a benefit in having more blades in that since they can share load, they can also create more lift per revolution. It must be easier to create X sq.ft of lifting area spread over 8 blades rather than over 2. And 8 thin-ish blades must make less noise than 2 huge ones.

3. A big rotor diameter means that the rotor can turn slower (since the tip speed limits the design of any rotor). Slow turning rotors create much less noise. Why isn't this done more on helicopters?

4. The constant chord design of blades seems easier to construct, as you guys pointed out. But surely, the twisting if them to create wash out must be a rather finicky thing to do. It must be very hard for two twisted blades to have the same twist at exactly the same point throughout the blade. This must create a vibrant and unruly rotor path that needs a lot of tracking. So my question is still - doesn't a variable chord rotor (that doesn't need to be twisted and matched up), create less of a headache in the end when all the work spent tracking the CC one has been factored in?

Thank you!

[IFMU]

Adam,

There is no free lunch. For one thing, as you add blades, you also add profile drag. You also add cost, because any one blade is about as hard to make as another, even if they are skinnier. So if your blade cost doesn't go down, but you have twice as many, you pay twice as much.

Large helicopters tend to have more blades, like the 53E. I suspect there is a tradeoff between disk area and solidity. If they made the disk area on a 53E such that the disk loading was like a Schweizer 300, you would need bigger aircraft carriers to land on. But, with the extra blades, you can turn more horsepower into thrust compared to if you had lesser blades.

Building blades with twist is about as hard as building them without, and keeping them true. It takes tooling and processes. So, once the tooling and processes are set up, building twisted blades is no big deal. And since we can track & balance them, that's not a big deal either.

I would also suspect there is a limit to how big you can grow a rotor, eventually the rotor would not be stiff enough to support itself when not turning. It would seem to me that getting the aeroelastics stable on a loooonger blade could be a challenge too.

I don't think it is just loads and high stress that killed wooden blades, though that is certaintly a factor. There was a high-profile FW accident before metal airliners where the wooden spar failed due to either dry rot or termites (if there are any historians out there maybe you can help me out on this statement). Metal doesn't dry rot (though it might corrode) and bugs don't eat it. And, good wood is harder to come by, it has gotten pretty expensive. If you ever build a wooden homebuilt, and have to buy spruce spars for the wing, you will pay a lot more than a comparable metal spar. The stuff may grow on trees, but slowly, and you won't be buying it at home depot.

-- IFMU

[NickLappos]

Adam, here is an attempt to discuss the excellent questions you pose:

1. Since load and high stress seems to be what killed off wooden blades - what if I had more blades and therefore spread the load between more of them? Then they could be thinner and more agile, no?
Wood as a structural material has great limitations - it cannot be strong enough without being made very thick - ok for buildings, pretty bad for blades. Much of the stress is not because of the number of blades, it is because of the need to have them aerodynamically thin while they have to be skyscraper-strong. The combination makes wooden blades a marginal idea.

2. Isn't there a benefit in having more blades in that since they can share load, they can also create more lift per revolution. It must be easier to create X sq.ft of lifting area spread over 8 blades rather than over 2. And 8 thin-ish blades must make less noise than 2 huge ones.

Two issues here - the real issue is the amount of blade area for the weight of the helo. More thinner blades are in the same hole as fewer thicker ones. If you made the greater number of blades wider in chord and thicker, the wood would work, but the blades would need much more engine power to swing them around and make the necessary lift. More power than the skinnier metal or composite blades.

3. A big rotor diameter means that the rotor can turn slower (since the tip speed limits the design of any rotor). Slow turning rotors create much less noise. Why isn't this done more on helicopters?
Very true, but slower rotors stall earlier, and have a lower Vne, and lower cruise speed. In fact, that is why older helos have Vne's at about 100 knots - they have slower rotors and less blade area. Good for noise, and hover efficiency, bad for high speed.

4. The constant chord design of blades seems easier to construct, as you guys pointed out. But surely, the twisting if them to create wash out must be a rather finicky thing to do. It must be very hard for two twisted blades to have the same twist at exactly the same point throughout the blade. This must create a vibrant and unruly rotor path that needs a lot of tracking. So my question is still - doesn't a variable chord rotor (that doesn't need to be twisted and matched up), create less of a headache in the end when all the work spent tracking the CC one has been factored in?

Lots of issues raised here: Twisting a blade isn't hard, and making two or 1,000 of them exactly alike also isn't that hard, really. Variable chord could work, but the chord would be screwy - thinner at the root, thicker in the middle, and then thin at the tips. It would be harder to make, but not terribly, but the aerodynamic advantage would be slight, I think.

[paco]

I believe Shawn did an article on just this subject (variable geometry blades) in the HAI edition of Rotor & Wing.

Phil

[ShyTorque]

Another problem with wooden blades - they were made by hand, by the old boys who had previously made wooden propellors, almost a generation before.

They had to be made as a set to have any chance of getting the track and balance correct. Ding one blade, change the set.

[slowrotor]

Adam asked: 3. A big rotor diameter means that the rotor can turn slower (since the tip speed limits the design of any rotor). Slow turning rotors create much less noise. Why isn't this done more on helicopters?

I am working on a large and slow rotor.
The Boeing Co. has a slow rotor design called Hummingbird A-160 (search google for info)
But mostly, the interest seems to be about increased speed.

If your design is slow, then wood might work OK. I think wood was abandoned because wood absorbs moisture from the air and the blade can go out of balance. Also, the blade should have chordwise balance at 25% from the Leading Edge. This requires a large steel bar in the nose of a wood blade. Why not just make the nose section out of metal? That way you have the balance and strength in one part.

The blade design and material would depend on the size and use of helicopter.

[Graviman]

Number of blades is an interesting problem, and i suspect there is much more to it than just aerodynamics...

In theory as long as the rotor has the same solidity ratio, it produces the same lift at the same Nr for the same induced power. Since more thinner blades have approx the same wetted area the total profile drag will be the same for both rotors. Since the thinner rotor has less overall depth there is an apparent weight saving, since each thinner rotors would each carry less load. However, with stress being inverse square of section depth then the need for thicker walls makes weight ends up the same

In practice each blade will have it's own free resonant frequency, affecting the effective hinge offset. There is a need to make sure that the rotating blade frequency is not overly excited by the aerodynamic loads. There is also a need to make sure that the blade modes do not occur in phase, to transmit vibration to the hub. In newer machines this must be the case over a wider range of Nr. Finally the cost of assembling many blades to a rotor dictate that the design team use as few blades as will work.

Mart

[Dave_Jackson]

The Reynolds number must also be considered when looking at blade count and chord width.

http://www.unicopter.com/1003.html

[AdamFrisch]

Thanks - this is all very interesting. I will persist with my questions, though:}

1. The more blades a rotor will have, the easier it must have to stabilize itself in the case of a malfuncioning blade, I assume. Lose a blade in a 2-blade rotor and you'll shake to pieces in an instant. Lose a blade in a 10-blade rotor and you'll probably survive. Therefore more blades must be safer, no?

2. Does a multiple blade rotor (3 or upwards) of the same diameter as a 2-blade rotor produce more lift? What I'm getting at - could a hypothetical 10-blade rotor with the same diameter as a 2-blade rotor create so much more lift that I could slow the rotor RPM down to make it less noisy?

Also - thanks Mr. Lappos for answering my questions. I think I read somewhere (being quite new to this board) that you helped develop and test fly the S-76 when it came out. It's such a looker that aircraft - must be one of the most graceful helicopters out there. I do however notice that whenever one flies over me here in London, they produce a lot of noise. One can instantly recognize a S-76 approaching on it's noise alone. Is this because Sikorsky chose to go with a pretty high RPM and narrow diameter rotor for this aircraft, or is it just some acoustic anomaly?:ok:

[IFMU]

Lose a blade in a 10-blade rotor and you'll probably survive

Maybe, but it's best if they all stay attached.

could a hypothetical 10-blade rotor with the same diameter as a 2-blade rotor create so much more lift that I could slow the rotor RPM down to make it less noisy?
Orlando Helicopters, which later became vertical aviation technologies (purveyors of the hummingbird helicopter based on the S-52) made a 5 or 6 blade S55 which was very quiet.

http://theamericas.org/DRSLOA2.jpg

IFMU

[Graviman]

Adam, i'll do my best to help..

1. The more blades a rotor will have, the easier it must have to stabilize itself in the case of a malfuncioning blade, I assume...

Actually, i was taught by a guy who worked at Rolls in the pioneering days of gas turbines (Bill Brownhill). One of his colleagues was debating that if you spring isolate ANY rotating assy below the rotational frequency, then no out of balance vibration is transmitted. To prove his point, and develop the optimum damping system (friction was considered), he built what to me was an incredible model. The model was spun up, a button was pressed and a mass representing a blade was shed. Sure enough if critical damping was set up the model sat there, easy as you please, running out of balance!

In principle a subframe mounting system allowing low pass isolation of lateral and longitudinal vibrations, but tranferring roll pitch torques, could be developed using ball joint links, springs and dampers. In practice there would be a mass penalty, with consequential reduction in payload. For the high number multiblade solution you are proposing, all mounting points would have to be strengthened to take out of balance loads for as long as it took pilot to safely land and shutdown. It is best to design the blades not to fail.

2. Does a multiple blade rotor (3 or upwards) of the same diameter as a 2-blade rotor produce more lift?...

No, assuming the same hover power. Lift for a given power requirement is a funtion of disk loading, hence for a given weight a function of disk radius. More blades or wider chord blades will increase the solidity ratio, which is fraction of rotor occupied by blade. An increased solidity ratio will reduce each blade angle of attack for same total lift. Angle of attack is best off as high as possible for the best "figure of merit" measure of rotor system efficiency.

In practice, solidity ratio is usually chosen to give machine a good manouvering margin and high speed performance. At speed retreating blade will already be running a high AOA, so compromising hover figure of merit will allow higher speed flight before retreating blade stall.

Also - thanks Mr. Lappos for answering my questions...

Must admit i also owe a great deal to Nick for helping my understanding of rotorcraft. Theory is one thing, but in the disagreement of theory and facts it is better to respect the facts... ;)

----

Dave, agreed Reynolds number is important when optimising the blade profile for a given size, Nr, and flight regime. Detailed aerodynamic interaction of tip votices acting on following blades is also an optimisation. The point is that at the design stage there is no real reason to consider any particular number of blades, other than for reasons of rotor dynamics and rotor system cost.

Mart

[AdamFrisch]

Thanks Graviman. But this is what I don't understand: Lift is a product of disc loading. Surely a wider blade (or one with more angle of attack) would create more downwash = lift. Hence, more blades, and the bigger they are, the more lift, no? Obviously, drag would also increase, but let's ignore that for now.

Also, more blades on a rotor system must mean that they can "slipstream" behind the other blade to some degree, and therefore have less air resistance per blade as compared to a rotor design where the blades can't "hide in the wake of the blade ahead of it", as in a 2, 3, 4, maybe even 5-blade system?

Basically, what I'm getting at - when you look at fanjet engine on an airliner, they have tremendous amounts of blades. There must be a reason for that - why don't they have just 2 fan blades if that's just as good?

[NickLappos]

The amount of blade area determines the total lift available from the rotor. A low-solidity rotor stalls earlier than a high solidity rotor, all other things being equal (the factor is called Aerodynamic blade loading. It is coefficient of thrust divided by solidity). Thus, more blades means more total lift available (more G's or more speed at 1 G).

The disk area determines the downwash, because the disk area is the size of the "pipe" through which the rotor accelerates the mass of air - the bigger the "pipe" the more mass of air acted upon, and thus the less velocity needed to be imparted on the air (momentum is mass times velocity - the less mass of air, the more the speed you need to induce to get an amount of lift). For this reason, bigger rotors make less downwash than smaller rotors - when they are both lifting the same load.

Tied to this is the fact that Power is used to turn the rotor, and the bigger the rotor, the less velocity it needs to induce on the air, so the very much less the power it needs to make the lift. Bigger rotors need less power to lift a load.

So, to lift a big load with a small engine, get a bigger rotor. To maneuver a helicopter, get more blade area.

[IFMU]

Also, more blades on a rotor system must mean that they can "slipstream" behind the other blade to some degree, and therefore have less air resistance per blade as compared to a rotor design where the blades can't "hide in the wake of the blade ahead of it", as in a 2, 3, 4, maybe even 5-blade system?
Adam,

It's not quite like drafting a truck with your geo metro. First, the induced airflow is from the top, the wake wants to depart downward, so you can't hide one blade behind another. Unless, of course, your solidity is 1.0, but then there is no place for the air to go because you just have a solid disk.

And, when one rotor does hit the wake of another rotor, usually there is vibration associated with it. And if there is vibration associted with it, you can bet it is causing drag and loss of performance.

-- IFMU

[NickLappos]

IFMU,
You are on the right track about blade interference - impact by the wake of the preceeding blade costs power, adds vibration and makes noise. Otherwise, it is nice....

[Dave_Jackson]

IFMU,

And what is wrong with a solidity ratio of 1.0?

http://youtube.com/watch?v=0ANt1lLDtQw :)


Dave

[Graviman]

Adam, you might like to get hold of a copy of Prouty "Helicopter Performance Stability and Control". It is in imperial units, but is well layed out and covers all the stuff you need. There are newer texts, but it is always Prouty i refer back to.

It took me a while to get the "Coefficient of Thrust", "Solidity Ratio" and "Figure of Merit", but Nick patiently guided me through it. :ok:


Basically, what I'm getting at - when you look at fanjet engine on an airliner, they have tremendous amounts of blades. There must be a reason for that - why don't they have just 2 fan blades if that's just as good?


That is actually a good question - and the only one not answered! A turbine stage is operating with bounded tips, albeit an idealistic simplification. This means that a pressure differential can exist across the stage without causing tip vortices. A helo rotor is in free space, so the tip vortices equalise the total pressure above and below (although blade surface will see pressure differential to produce lift). This means that while a tubine rotor must be designed to avoid leakage, a heli rotor is designed to optimise tip vortex induced flow. Thus the number of blades is greatly less in a heli rotor.

Before the introduction of Computational Fluid Dynamics aerodynamicists often produced suprisingly accurate results by modelling the flow field resulting from the tip vortices. Vortices have the characteristic of rotational velocity being inversely proportional to radius, outside of the vortex tube (so linear velocity is constant in the rotor plane). The field can be estimtated from an idealised tip vortex distribution (from observations usually) - you may be familar with the Biot-Savart equation from magnetic fields around coils from A-level phys. A tip vortex will stay in plane, until the following blade pushes it out of position. In fact the latest generation of rotor blades are optimised to position this vortex as far out from the rotor as possible, with no doubt further improvements to come.

Mart

[AdamFrisch]

Thanks so much everyone.

But, haha, I don't give up easily. I might have found a fatal error in my earlier blade design question - I quote myself:

"Most modern rotors in helicopters seem to have constant chord blades where the tip has been twisted donwards towards the tip so as to produce less lift the further away from the hub you get. But the same thing could be achieved by having the chord change (and the twist remain unchanged) - by narrowing the blade the further you go out from the hub, the less lift it would produce (this was common in earlier helicopters). A twisted blade must be so much more expensive and complicated to make, so why was the changing chord design abandoned?"

Now, in an autorotational state, the outer part of the rotor drives the inner part (roughly 1/3 of the blade is driving, as I've been told) by having less angle of attack (or even negative). This can only be achieved by having a twisted blade. Now, let's say we had a constant chord blade design instead (where the angle of attack stays the same over the entire blade), would it be able to autorotate?

Why I'm asking is because I'm almost 100% sure early heli's had CC blades - and they did autorotate. So how did they achieve this?

[slowrotor]

A twisted blade (that looks like an airplane prop) will lift more in hover but not as good in autototation, but still will auto well enough usually.

A flat blade (no twist) is better for autorotation and almost all gyrocoters have no twist because they are always in autorotation.

[Graviman]

Adam, that is a good point (if i understand correctly): Why not design a tapered blade instead of twisted blade?

To be correct the blade would need have it's chord length inversely proportional to radius from hub centre. This blade would then be equally efficient in hover, climb and autorotation - each element would always present optimum AOA to inflow. In fact many performance props are based on this design, with a cutout or inverse taper to meet the maximum chord radius (generally chosen to match spinner diameter).

The problem comes when you consider cruise, the speed of which has being pushed up in modern turbine helis. Here the taper becomes a disadvantage because the advancing side sees air with a large component of uniform flow. The reatreating side is the real limitation though, since the tip has to pull high AOA to keep the lift in balance. Wide chord tips are the order of the day.

In practice OEMs spend a great deal of resource getting just the right twist and taper to suit the average mission profile, and variations of it. All things considered, the benefits of taper are lessened so the additional cost pushes the designer towards constant chord. In modern machines the benefits of anhedral tips are more easilly exploited with a constant chord design too.

Hope this helps. Like i say Prouty is a very enjoyable read, but even better if you can gain insight from this forum to see how the theory ties up with practice. I have learned a lot on this forum.

Mart

[AdamFrisch]

Thanks. Yes I do see that the retreating blade will suffer in this regard with a narrow tip airfoil in a tapered blade.

I was also reading up on the Boeing Hummingbird unmanned helicopter that you mentioned. Not much info has leaked, but it seems terribly interesting. For instance, they reduce blade RPM at different altitudes and different weights. And the endurance and performance figures for the 6-cylinder heli are nothing short of astounding! 2500nm range, ceiling of 30.000ft and 24h endurance - all very quiet, too. Amazing.

Has anyone experimented with reducing rotor RPM at altitude? I assume this could safely be done in any helicopter given enough height. It seems quite logical to do so, yet that's not part of the way heli's are operated - they run at constant RPM's all the time throughout.

I think Boeing are def on to something here - helicopters stress out components and are expensive to use simply because they're flown at the limit all the time. If there was a procedure to safely reduce rotor RPM and engine RPM at altitude, many benefits might be had, no? FW aircraft lean out and throttle back for long flights - who says heli's can't do the same?

[Dave_Jackson]

AdamFrisch,

This earlier thread started by slowrotor should be of interest to you.

http://www.pprune.org/forums/showthread.php?p=3037184&highlight=Hummingbird#post3037184

[Graviman]

Adam, there was a good thread on this which i can't find right now. S76, among others, does vary Nr with speed, load factor, and height. It is becoming the norm now.

The headache is that the rotor blade eigenmode frequencies are a function of Nr. Since many orders of P may be aerodynamically excited, and could work there way back into the structure and systems, the machine must be carefully evaluated at different Nr. Worst case is that blades vibrate in phase to compromise fatigue life. The other problem is that the azimuth lead angle is a function of Nr, so you need additional complexity in the control system.

The golden panacea is the ability to vary rotor Nr from 100% all the way down to near 0%, in an advancing blade concept helicopter. This means that while efficient hover can be achieved, the machine can have a cruise factor unlimited by advancing blade compressibility. The limitation is the need for a heavier rotor system to transfer the root bending moments.

I am a very keen fan of the Sikorsky X2 project, although do not know it's current status (scrounging for resource probably).

Mart

[IFMU]

The headache is that the rotor blade eigenmode frequencies are a function of Nr.

Mart,

Got to keep things in order. I think you are due to drag out the eigenmode thingy on the "You want me to test fly what...?" thread before this one.

:)

-- IFMU

[JimL]

Mart,

I am about to go swimming out of my depth here so I will try to stick to what I know.
Adam, there was a good thread on this which i can't find right now. S76, among others, does vary Nr with speed, load factor, and height. It is becoming the norm now.You probably need to check your facts on this statement; apart from experimental craft, it is not clear that variable NR is being used (at the moment) in other than two cases:

Reducing Nr - to lower the speed of the tail rotor in the cruise to reduce noise.
Increasing Nr - to improve the (stored) energy in the blades thus providing a short term benefit should an engine fail early in the take-off or late in the landing manoeuvre (mainly used for Category A procedures).Some manufacturers use an automatic system of raising Nr that is related to airspeed; others a manual system of 'beeping'.

There has been an interesting discussion on the Bell 412 thread which appears to warn against reducing the Nr in the cruise unless it has been authorised by the manufacturer. The thread discourages experimental excursions outside the RFM as it could take the helicopter outside of its design envelope. The net result may not affect the offender but could impact on later users.

Jim

[minibirdpilot]

There is a short bit in the new ROTOR & WING about varying RPM.

Hey Nick what about the S76 question he asked about it's very noticable louder noise?

Thanks
Clayton

[NickLappos]

minibirdpilot,
I haven't read the article to know what it said, but I do know that nothing has changed on the S-76 models to make the noise different. The S76A had variable RPM, and it worked well, but when the MGW rose as power was added to later models, the rpm was kept at the upper boundary to keep the speed up and reduce the blade stall factor.


This thread is interesting, but shows the flaws of the socratic method, where a question and answer session leads to debates that try to find out if witches are made of wood...(apologies to Monty Python!)

Some funny things Adam has said (this is not criticizing you, Adamfrish, just pointing out how much strange belief exists in the land of the regular people):

Has anyone experimented with reducing rotor RPM at altitude? I assume this could safely be done in any helicopter given enough height. It seems quite logical to do so, yet that's not part of the way heli's are operated
Yes, and it really makes the helo stall and go out of control, just like we knew it would. However, this didn't seem to be what the customer wanted, so we didn't deliver it that way.

Lift is a product of disc loading.

This proved to be a mystery, since we thought the blades lifted the helicopter, and when we removed the blades, the disk performed badly.

helicopters stress out components and are expensive to use simply because they're flown at the limit all the time

We had thought they were expensive because of all those pesky machined parts that are flight critical, so we were working on that, but now we find that if we just gave the parts valium everything would be easier.

Adam, it seems that most of the beliefs in your worldview about helicopters comes from a hodge-podge of press releases and TV shows, and a strong belief that a few quick phrases explain how a rotor works. I strongly suggest that you drop out of the Hummingbird press release school of engineering and just buckle down with a good Prouty or Padfield or Stepneiwski for a while, because most of the starting places for your concepts are not only wrong, they are dead wrong.

Here are some rotor basics:

The blades do the lifting, and they want speed. This means that you must mix blade area (number times width times length) and RPM (tip speed) properly so that there is enough lift to make the helo go.

The best blade design is not one design point, because hover and high speed cruise conflict. The hover where the blade should work a bunch, at maybe 3/4 of its maximum lift so that the power it eats is smaller for the lift it produces. High speed where the blade should be working less, where you would like the balde to be at 1/3 of its max lift. This paradox makes a variable rpm attractive, where you slow down the rotor RPM at hover, and speed RPM up at high speed cruise flight.

The rotor disk determines the size of the air package the blades move, and the bigger the disk, the less power the rotor needs in a hover. This means big, slow rotors are better for hover efficiency, which is why Harriers and tiltrotors eat so much power. It is also why older piston helos had big rotors (the 8300 lb piston H-19 had the same rotor diameter as the 21,000 lb turbine Black Hawk!) Turbines throw power away, because the weight of a somewhat bigger turbine engine isn't that much, realtive to the weight of a bigger rotor.

Areodynamics excites us, but weight, structures and dynamics is what designs a flying machine. The "best" rotor and blade is seldom chosen, but the lightest, strongest one almost always wins. It is like going to the movies, we watch Scarlett Johanssen, but we bring the girl next door. Mundane aspects like weight, cost to build, vibration suppression end up dominating the designer's life. Variable rpm sounds cool, until you try to match the vast band of rotor frequencies (which tickle the whole aircraft) to the structure, and find that you can't permit a 40% rotor rpm range without beefing up the tailcone so much the advantage is lost.

Helo designers aren't stupid, their "mistakes" that you notice are testaments to what you don't know about designing helicopters. The matchbook school of engineering says there are two or three simple rules, the real world says there are hundreds of comprimises that make a helicopter.

Here is a crash course on helo design with thanks to Lakshmi Sankar of Georgia Tech:

www.ae.gatech.edu/people/lsankar/AE6070.Spring2004/Part1.ppt (http://www.aerospaceweb.org/design/helicopter/)www.ae.gatech.edu/people/lsankar/AE6070.Spring2004/Part2.ppt (http://www.aerospaceweb.org/design/helicopter/)

www.ae.gatech.edu/people/lsankar/AE6070.Spring2004/Part3.ppt (http://www.aerospaceweb.org/design/helicopter/)


also a touch of the same from a micro-lite site:

http://halfdome.arc.nasa.gov/publications/files/Young_AIAA02.pdf

[AdamFrisch]

No, I don't know anything about helicopter aerodynamics except from what I learned in my PPL (H) training and have gathered around the net - that's why I'm asking all these (apparently) stupid questions. Never did allude or pretend to be eductaed in the same either - I just like to educate myself and am naturally curious.

Now that we know I'm an uneducated fool and that's out of the way, let's get back to business and leave haughtiness behind.:}

"Reducing RPM in flight"

If you were to reduce RRPM in a, say, H269 just for the hell of it, it will of course stall at some point. But not immeditaley at lower red line - there must be an area of lesser performance before it stalls, no? Also, if I recall correctly from my aurorotation days, one had to keep a vigilant eye on the RPM so as not to over-rev the rotor on the H269. Therefore, if you were to reduce RPM to just when it stalls, can not the RPM be regained by a combination of/or autorotation and throttling up again?

"We had thought they were expensive because of all those pesky machined parts that are flight critical, so we were working on that, but now we find that if we just gave the parts valium everything would be easier"

Of course, but there is a reason that Robinson de-rated his Lycoming engine so it would last longer. No engine likes to run at top RPM for long periods. That's why Rotaxes don't last. The H269 is apparently quite known for having a highly strung engine that fails quite a lot, which I myself experienced.

I will definitely look into reading Prouty and other aerodynamic books, but forums like these are invaluable for empirical data.

[Dave_Jackson]

Adam,

Reducing the RPM of the rotor will require that the pitch of the blades be increased to maintain the same thrust. This is an advantage in that the efficiency (power/thrust) is increased.

Unfortunately, this also increases the risk that a perturbation, malfunction, etc. will cause the rotor to loose all lift.

The Boeing Hummingbird is an Unmanned Aerial Vehicle therefore its crash would be unfortunate but not tragic. The loss of life in a helicopter crash is much more serious. With all regards for the relatives and associates of the deceased, helicopter fatalities have also resulted in the demise of a number of aspiring helicopter manufacturers.

IMHO, and subject to elaboration or correction by others.

Dave

[slowrotor]

The engineering books agree with Nick and state "Optimum hovering performance requires a lower tip speed than is desired for forward flight".

Adam,
I think Nick gets frustrated with some of the wild claims of promoters such as the A-160 Hummingbird and unleashed his anger toward you.

The A-160 claims a fivefold increase in endurance. This is a rather extraordinary claim. "Extraordinary claims require extraordinary proof" (quote from Carl Sagan, I think)

[IFMU]

The A-160 claims a fivefold increase in endurance. This is a rather extraordinary claim. "Extraordinary claims require extraordinary proof" (quote from Carl Sagan, I think)
I wonder how much of this claim is based upon an engine who's performance and efficiency claims hasn't been proven in the real world yet.
-- IFMU

[minibirdpilot]

Thanks you guys. I enjoy learning more about these things. I didn't think Nick was harsh at all...

"Lighten up Francise or I'll have to kill you" seems like a good, famous quote to apply. Remember it must be said through clenched teeth.

[Graviman]

Now that is the sort of knowledge that you just can't get from the books! Practical experience outweighs theory anyday...

Mart

[AdamFrisch]

Sorry to revive this old thread again, but I have a new couple of questions:

1. In most fast helicopters it seems to me that the rotor disc area seems to be erring on the smaller size. Now I know Mr. Lappos mentioned that a bigger rotor will stall quicker since it turns slower - is this a factor on the retreating blade that limits top speed? What I'm getting at - does a fast helicopter HAVE to have a smaller rotor disc, or is it just so that they happen to be mounted on choppers with strong engines (S-76, A109, MD500 etc) that can really pull well?

2. A high wing loading on an airplane normally means that the ride will be smoother and the initial turn quicker (but sustained turns slower). Does the same apply to rotor discs and their loading? High disc loading = smoother ride?

Thank you. Your answers are always very good and informative.

[KrisRamJ]

1. Retreating blade stall begins at the tip of the retreating blade due to two factors - downflap and reduced blade velocity. If you shorten the blade, you shorten downflap and also reduce blade velocity causing a corresponding large reduction in lift so you must increase rotational velocity of the blade to compensate. Of course, you will eventually get to a point where you've increased rotational velocity so much that you run into compressibility (supersonic flow) on the advancing blade. The record breaking Westland Lynx walked a fine line between retreating blade stall and advancing blade compressibility, and that's why it's the realm of test pilots and engineers. Screw up your temperature or pressure calculations just a little bit and you've lost your rotor system. :ouch:

2. I'd say definitely, yes. High wing (or disc) loading = smoother ride. I actually had some first time helo passengers mention this just today. They were expecting the helicopter to be bumpy and the smoothness surprised them. The exception, of course, is at high speed because a heavy helo gets into retreating blade stall earlier, and therefore the vibration levels for a heavy helo are higher at speeds close to Vne than those for a light helo.

[AdamFrisch]

Thanks.

Let's say I was mad enough to want to design my own helicopter. Let's also assume I'm otherwise sane and responsive and need more answers:

1. As a tail rotor flies at speed, it presumably encounters the same effects as a main rotor - that the retreating blade has less bite. This gets continually worse the faster you go. But, at speed, a tail rotor also becomes less important as the weather-waning fin and structure of the helicopter now provides most of the anti-torque. My question is: do I need to construct the tail rotor with the same complex lead/lag and flapping hinges as the main rotor, or can I just ignore the fact that the retreating blade will stall and design it rigidly?

2. Is good autorotational qualities mainly achieved by having a low disc loading, or a heavy rotor? The two are not the same.

3. I'm a little bit confused about the autorotational aerodynamics over a blade. I've always assumed that the angle of attack of the inner part of the blade that drives the autorotation had to be negative to sustain blade rpm. And since the blade is twisted more at the root, that part could have negative AoA, whilst the rest of the blade had positive AoA. I've been told that's not the case - a rotor will autorotate even if this is not the case or all parts of the blade have positive AoA (like in a Autogyro). Why I'm asking is simple - I want to know if a variable chord blade (a blade that's wider at the root than the tip), but with NO blade twist will autorotate.

4. As a blade spins its centrifugal force will want to pull the blade outwards. At the same time the weight of the helicopter + the g's you pull will want to bend the blade upwards. These are forces one has to bear in mind. But, common sense also suggests that the faster the blade spins and the more centrifugal force it encounters, the less it will want to bend. Or to put it another way, you trade bend forces for centrifugal forces at speed. At what point or speed does this start to affect the calculations and is there some kind of rule of thumb here? I'm assuming that a certain speed, one could all but ignore the bending forces and just construct a blade to take the (very high) centrifugal forces, no?

Sincerely,
your local noodle sharpener.

[Graviman]

Adam,

1. Allowing TR to teeter gives the lightest design. There is no design reason for rigid tail rotors - fenestrons normally use cyclic trim to keep TR in plane.

2. Low disk loading allows low auto rates. High disk inertia supplies energy to gain forward speed from hover and/or flare. Both are desirable features.

3. Any blade will autorotate as long as the pitch is set low enough to allow no net torque across span. A tapered blade will be more efficient in hover, but will suffer in forward flight.

4. Rotor coning is carefully considered in any helicopter design. A compliant root is desirable to allow a range of operational g-loadings.

[HELOFAN]

I have been looking for a good explanation on solidity ratio.

HF

[Graviman]

Solidity Ratio (Sigma) is the actual blade planform area divided by the disk area. It is a means of non-dimentionalising various rotor performance coefficients. The assumption is that the hub area is trivial.

[Dave_Jackson]

HELOFAN

This might provide what you are looking for. Aerodynamics - General - Solidity [σ] (http://www.unicopter.com/0948.html)

Dave

[AdamFrisch]

But blade solidity can be achieved in two ways, no?

Either have a few fat chord blades, or many narrower ones. Surely there must be differences in how they perform and fly even if they have exactly the same blade area?

[Dave_Jackson]

Adam,

Prouty has a chapter entitled 'Tail Rotor Design' in his book 'Helicopter Aerodynamics'. Some considerations are; noise, power consumption, and aspect ratio.


Perhaps, some of the answers to the tail-rotor blade count are the same as those for the main-rotor blade count currently being discussed on the thread Number of rotor blades? (http://www.pprune.org/forums/rotorheads/320555-number-rotor-blades.html)


Dave

[AdamFrisch]

Thanks, I've ordered Helicopter Performance, Stability and Control from Prouty now. Hopefully that incorporates most of the info from the older discontinued books.

[AdamFrisch]

OK, one last question.

On an articulated head, the blade is allowed to hinge in both lead/lag and up/down. Lead/lag is all fine to me, but the up down thing I have some trouble understanding.

Isn't it correct to assume that as the helicopter gains lift and the blades cone upwards, the blade must reach its up hinge stop. If not, the blade would just continue to fold upwards producing no lift and leaving the heli on the ground.

Now if this is the case - then what possibility does the blade have to flap upwards in the hinge if it's already at the up stop? None, I would assume. And if that's the case, that means that the blade itself absorbs that up-flap. Which makes it a rigid rotor blade in flight, by all definitions. So what's the point of having that hinge if it has no effect in flight and isn't allowed to hinge?

This ancient rotor blade in flight footage which you've all seen, seems to describe the above. From the age of the film, I'm assuming this is articulated head (rigid rotors are after all a pretty new invention). But look at thhe blade closest to the hub - it doesn't move at all. The rest of the blade and the tip is doing all the bending.

YouTube - Slow motion video of a helicopter rotor blade (http://www.youtube.com/watch?v=Ug6W7_tafnc)

Don't get it. Please explain.

[Dave_Jackson]

Centripetal Force [Centrifical Force] (http://www.unicopter.com/B264.html#Centripetal_Force) and Thrust (http://www.unicopter.com/B263.html#Thrust) set the Coning Angle (http://www.unicopter.com/0735.html)

[IFMU]

Adam,

CF pulls the blades out. Even with no hinge stop they would not go straight up. If you consider lift and CF as roughly perpendicular, the coning angle will be defined by adding the CF and lift vectors together. At flat pitch there is no coning angle, at high power there is significant coning angle. CF is much bigger than lift so the angle never gets crazy, i.e. straight up.

Juan de la Cierva's contribution to the helicopter was the flapping hinge. On the advancing side in forward flight, there is greater airspeed which would result in greater lift, and on the retreating side the opposite is true. As the advancing blade flaps up, the relative wind changes such that there is less AOA on the blade, and less lift. On the retreating side the blade is flapping down, which due to the change in relative wind produces more AOA and more lift. This tends to equalize the dis-symmetry of lift. However, as the blades flap up and down, they are trying to conserve their rotational momentum. Spin yourself in your office chair, pull you legs & arms in, you speed up. The lead/lag hinges allow the blade to accelerate and decelerate without causing huge bending moments at the root.

-- IFMU

[perfrej]

Folks!

This is fantastic reading. When I started my PPL(H) 1993 one of the first things I did was to obtain a subscription to Rotor & Wing, and from reading some re-runs of Prouty articles I decided that I just HAD to get "Practical Helicopter Aerodynamics". That was great reading and a really good starting point for some basic understanding of what actually transpires above your head during flight.

I have always had a need to understand things and would instantly repspond to , to take a fictive axample, a driving teachers statement "You need to press the clutch before putting this thing in gear" with a solid "WHY?" Prouty gave me that, and I must add that Nick Lappos´ answers keep my gray cells in motion.

Thank you all! (and keep this thread going so that everyone with a stick in each hand gets a better understaning of "things on the roof")

All the best,

PF