High Inertia V Low Inertia

[bugdevheli]

Given That A Machine With Low Inertia Blades Will Recover Rpm More Quickly Than One With High Enertia Blades . What Is The Downside For A Rotor System That Has Extreemly High Inertia Blades Whilst Not Requireing A Heavy Hub To Restrain The Additional Centrifugal Forces.

[manfromuncle]

More weight, more complex, more maintenance, more power required, bigger engine required, more fuel, more cost...

[SimonCFI]

Low inertia rotor system:
Pro: fast RPM recovery, lightweight,...
Con: fast loss of RPM

High inertia rotor system:
Pro: keep RPMs longer, but if you loose them:
Con: it also takes longer to get the RPMs back,
+ the stuff mentioned above ;)

[georden]

is there less maneuverability/more stability with higher inertia blades?

[generalspecific]

nope dont think so... once its "up to speed" on rrpm is basically a rotor disc so the level of inertia wont affect the manovreablility or stability.

Of course the inertia will affect how quickly the rrpm decays and recovers during agressive manouvering and thus how much collective work is needed to maintain the green... 180 auto's in the R22 with two fat blokes on board springs to mind :{

[NickLappos]

The inertia in flapping is roughly controlled by the same factors as the rpm inertia, so a high inertia blade will take longer to flap, and thus is less agile than a lower inertia blade. Note how much you can wiggle a Huey or 206 stick with no corresponding motion of the disk.

High inertia blades also need more gamma (lead angle) because they flap later relative to the swashplate tilt.

[bugdevheli]

On the assumption that a rotor system can be manufactured that whilst having high inertia, is in total no heavier than say the rotor system on a 22, and yet gives four times the inertia, produces no more drag, does not lead and lag, and is competitve on price to manufacture. What is the downside?

[generalspecific]

I bow to Mr L's superior knowledge.. never thought of it like that but it sure makes sense when you think about bell v robbo stick wiggle...

Boy I could've used you all those years ago when i was tring to do my commercial knowledge test....:8

[rotornut]

High inertia versus low inertia? Just do an auto in a Robbie then try one in a 206.

[SASless]

Nick,

Perhaps a better example would be a Hiller 12E and a MD500E .

The lag one gets by flying a paddle compared to the five bladed articulated system of the 500 would seem to be a wider spread than the Huey with its stabilizer bar.

[Matthew Parsons]

Nick,

I agree that a high inertia blade would flap less for an equivalent change in blade lift, but I'm not convinced the inertia would change gamma too much, for a teetering blade. Whether high or low inertia, there is no hinge offset, so gamma should be pretty close to 90*. If it was a blade that bends, then an increase in the tip mass would decrease the effective offset, and that would increase gamma.

Remember that a teetering bladed acts very much like a simple pendulum. The period is dependent on the length, not the mass. When driven at resonance the phase difference (equivalent to gamma) is 90*.

As far as the stick wiggle in the 206 or Huey, I have only a couple hundred hours in the 206 and next to none in the Huey, but I thought the disk moved but the body of the helicopter didn't when you wiggled the cyclic.

Cheers,
Matthew.

[rick1128]

Requiring more maintenance? I don't believe so. Case in point is the Enstrom. Blade life is On Condition. A very small number of life limited parts.

[NickLappos]

Matt,

You're right, the generality that inertia changes gamma is not true for a teetering rotor, but the stresses that the rotor sees when "forced" to flap out of sequence by the rigid connection to the other blade is a factor.

Every pound of blade weight is probably a lost 2 pounds of payload, since the blade's inertia puts large stresses on the head, the mast and the transmission case and feet.

Some inertia is needed, too much is very costly to the rest of the design.

[Gomer Pylot]

You can move the cyclic of a 206 or UH1 a considerable distance, and take out that movement, sitting on the ground so the fuselage can't move, and get no movement of the disk. It takes time for the disk to move, and it's quite a bit of time. That's why flying one is a little different from flying a model with a semi-rigid system. With a Bell, you have to know where to put the cyclic for a desired result, put it there, and leave it for awhile, waiting for the reaction. It's not immediate, or even quick, and I think it's because of the high inertia. You just can't move that much inertia quickly. One positive side effect is stability - the UH1 is a capable instrument platform without any artificial stabilization at all. No SAS, no nothing, it just plows along. But it doesn't maneuver well. The 412, with the same fuselage, but different head and relatively light blades, flies very differently, and flying it without artificial augmentation will wear you out. For IMC, you have to have the full ATT mode. It's unstable, and far from the UH1 or 212.

TANSTAAFL.

[bugdevheli]

Lets assume by design it is possible to design a rotor system that gives immense inertia. with no overall weight penalty. Is it better to have ten seconds at the beginning of an emergency to make a decision, or would the judgement of when to flare be a greater problem due to the slower rpm recovery.

[Matthew Parsons]

If you're worried about losing all engines, then go with high inertia.

[Graviman]

With a Bell, you have to know where to put the cyclic for a desired result, put it there, and leave it for awhile, waiting for the reaction. It's not immediate, or even quick, and I think it's because of the high inertia. You just can't move that much inertia quickly.


Gomer, i though that was because of the effect of the Bell bars on the pitch link? The Bars provide a gyroscopic input to "resist" the cyclic movement. SCAS (evolved from Lockheed mechanical system) lets the pilot directly control the gyro, rather than fight it, resulting in a fast but stable system.

[Gomer Pylot]

The bars have something to do with it on the UH1, but the 206 is rather similar, and has no bars. They do provide stability, but the inertia provides even more, I think. It's all inter-related, of course, and you have to consider the entire system, not just one variable.

[Graviman]

Thanks Gomer. This makes sense if you think about cyclic pitch generating aerofoil forces to cause gyroscopic precession. Basically a higher inertia gyroscope will have a lower pitch/roll rate for a given input torque:

http://upload.wikimedia.org/math/a/f/7/af758eeace825d72a30f51a264351e67.png

From site:

http://en.wikipedia.org/wiki/Gyroscope

So a lower inertia rotor makes for a snappy response. Also since low mass blades allow the rotor higher frequency bending modes, the effective hinge offset becomes more. So the time delay from cyclic input to required pitch/roll rate becomes less. In general low rotor inertia is good for handling (like Nick already said).

But high inertia is good for autos. My own take is if the Nr drop off response is too fast non-TP reflexes (like mine :uhoh:) then why not let the machine respond quickly for you?

That said stick to your ideas Bug, and you'll find an application. :ok:

[relyon]

Nick,

Every pound of blade weight is probably a lost 2 pounds of payload, since the blade's inertia puts large stresses on the head, the mast and the transmission case and feet.

Some inertia is needed, too much is very costly to the rest of the design.
I assume you're referring to the torsional stress produced while bringing a rotor up to rated RPM. To some degree that should be able to be migated by the limiting the rotational acceleration in engines/drivetrains where that's possible. Don't those components already need to handle the additional torsional stress created, for example, when an abrupt full or significant yaw input is made? I understand the forces acting on the head and [non-static] mast, and by inference the transmission gears. How are the transmission case and feet affected by the blade inertia?

Bob

[NickLappos]

Bob,
Your assumption is not correct, the stresses are a part of everything the rotor does in flight and on ground, not just during run-up. The difference in the structure that supports a blade that has a CF of 30,000 lbs as compared to a blade that has a CF of 40,000 lbs because of its extra inertia is not obvious when we talk about blades, because most pilots are unable to think of the whole helicopter when they debate the parts, separately. (See the silly thread "How fast would you like to cruise" which should be labeled "How much payload would you lose for 50 extra knots?")

The extra 10,000 lbs of CF makes the blade spar thicker, the head thicker and heavier, the mast also thicker, and (it may not seem obvious) the transmission housing much thicker, since it carries the forces from the mast to the mounts.

The discussions that pilots have about the merits of a thing are very nice, but designers chuckle to themselves. It is NEVER that inertia, power, speed, engine size, extra fuel are bad things. We should like infinite quantities of them. It is ALWAYS "what do you want left off when you increase the (insert one) - inertia, power, speed, engine size, fuel?"

[RVDT]

Cheaper, Faster, Better - pick any two! :rolleyes:

[georden]

has the idea of using a geared flywheel that spins much faster than the rotors ever been tried? would seem that you could increase inertia without adding stress to the rotor system, might offset the weight of the flywheel setup?

[Dave_Jackson]

Bob,

I think that the Robinson R-22 might be used as an example to support Nick's remarks.

It appears that Frank Robinson incorporated the additional two out-of-plane (coning) hinges in his rotorhead so as to reduce it's weight. It has also been said that at the beginning of the R-22, Martin Hollmann mentioned to Frank Robinson that the rotational inertia was too little. At which point the tip weights in the R-22 were increased slightly.


georden,

The following link is radical (futuristic) concept of utilizing a flywheel to provide rotor inertia.

The primary objective of this high-speed flywheel is to provide additional controllability to a 2-bladed rotor. This is done by providing a moment to the top of the mast that is 90-degrees out of phase to the hub-spring (or offset teetering-hinge) on the rotorhead. In addition, the 'flywheel' is the electric motor that drives the rotor.

Electrotor ~ Plus (http://www.unicopter.com/ElectrotorPlus.html)


Dave

[slowrotor]

I think a hybrid of electric and internal combustion would work. A small electric motor could produce two or three times rated power for about two minutes in the case of an emergency main engine failure. It would be almost like twin engines.

Also the main engine could be smaller for better fuel efficiency in cruise. The electric motor would provide the peak power needed only for hover.

The hybrid would be lighter than high inertia blades, I think.

[relyon]

Nick,The extra 10,000 lbs of CF makes the blade spar thicker, the head thicker and heavier, the mast also thicker, and (it may not seem obvious) the transmission housing much thicker, since it carries the forces from the mast to the mounts.I understand the additional CF necessitates a heavier rotor system structure (blades and head) and was alluding to that when I stated I understood the forces acting on the head. For a given structure, there is an optimum spanwise distribution where both minimum additional structure and maximum additional inertia results from the same minimum additional mass. Not having done the math, I imagine that distribution is biased predominantly or in some cases entirely at the blade tip (i.e. R22 tip weights).I also understand the additional weight of the rotor system must be supported while the helicopter is on the surface and that is borne by the mast, transmission, transmission mounts, etc. That shouldn't be too difficult since these same components support the entire weight of the helicopter when airborne, though admittedly the former stress is primarily compression and the latter primarily tension.I fail to understand how these components support the additional stresses in flight at rated Nr, since the lift provided by the rotor system itself counters it's own weight directly. How can that stress be transmitted to the mast? Seriously, what am I missing?

Dave,Interesting you should mention the R22 as I feel it could benefit significantly from more main rotor rotational inertia than it currently has, even with the tip weights. I suspect (only an geuss) the tip weight position may have had as much to do with minimal or no redesign of the structures that Nick mentions (blade spar and head) as it did with being the optimum spanwise position for maximum inertia at minimum additional mass. In any case, I seriously doubt any component below the teetering hinge was redesigned for the addition of several ounces of tip weights, but I could be wrong.

Graviman,One question regarding rotor system control implications discussed earlier in this thread:... Also since low mass blades allow the rotor higher frequency bending modes, the effective hinge offset becomes more. ...How is that possible in a semi-rigid head?Bob

[Graviman]

Bob,

For an articulated, or a teetering, head you are quite correct. The design trend seems to be towards hingeless heads, with the semi-rigid head a half-way house to reduce blade root bending stresses. My comments were general, but aimed at rotors designed to improve manouvreability by using blade root bending moments. I believe this is also where Nick is coming from when he comments about the need to beef up mast and associated structure from using a stiffer/heavier blade (ie keeping the same basic bending frequency).


(See the silly thread "How fast would you like to cruise" which should be labeled "How much payload would you lose for 50 extra knots?")


Nick,

Point taken about the practical loss of payload for the extra rotor, which must be stiff enough for ABC. That thread was intended to see what reaction could be provoked from the posibility of higher cruise speeds. I accept that the conclusions might not concur with immediate engineering practicalities, but i felt that better than reaching none at all. The other way to see it is that for a given payload you need a slightly heavier helicopter, which is more expensive to operate - that was why i was trying to guage how much folk are willing to pay for the performance increase.

[relyon]

Graviman,

Ok, that explains things a lot and clears up some confusion on my part. Though I may miss things at times, I really try to strive to understand and consider all aspects of things, and have no preconceived ideas as to what is good or not so good. To me, questioning is a key ingredient of a good engineer and pilot. I don't flatter myself with the things I'm on top of; it's the things I may be missing that concern me.

Bob

[Graviman]

Bob, best way to be - any system is only as good as its worst component. ;)

[bugdevheli]

Consider a rotor system that has eight blades. The blades are surrounded by a ring that is fixed to the end of each blade. The ring is weighted and provides whatever inertia one requires. The blades no longer lead or lag. Any thoughts on this concept. ? Bug.

[Heli-phile]

If you can achieve a 4 blade system that weighs same as a R22 sytem (and works - not fails) you will be a very wealthy man and Robinson would be very pleased too.

[Heli-phile]

Like all things there are trade offs

Low inertia means faster rpm recovery = Good thing, if you can?

Low inertia means faster rpm loss = Bad thing, could kill you?

High inertia means slower rpm recovery = not ideal but, you will have longer time to recognise and react to it.

High inertia means slower rpm loss = great, time to react and recover.

Only one of these scenarios has a high chance of a fatal outcome. Low inertia blades are well known to be beyond rpm recovery in as little as 3 seconds (depending on AOA etc at time of loss) Thats why when you are in your trusty R22 you should be permanently psyched to get that collective down sooner rather than later.

Also low inertia blades make the aircraft very maouverable, but the trade off for this is they are also much more susceptible to upset and track divergence in low g and or turbulence. Overall medium to high inertia blades seem to be the best overall way to go and the fatalities/type/hr (weighted for numbers produced) support this.

[Graviman]

Consider a rotor system that has eight blades. The blades are surrounded by a ring that is fixed to the end of each blade. The ring is weighted and provides whatever inertia one requires. The blades no longer lead or lag. Any thoughts on this concept?


Bug,

From a 1D analysis point of view (ie hand calcs or Excel) your concept makes sense. From a performance point of view, particularly in hover, the tip ring will force the blade tip vortices to remain along the rotor circumference. This will increase the effective rotor area, in the same way as a fenestron uses the cowling. This will probably also work in forward flight.

I know you've had some CFD work done, but the full performance envelope must be examined. So forward flight, climbing, descending, pure climb, autorotation, VRS, and turns. I have limited the cases to static conditions, as the only way you will get transient affects (like fast manouvre) is from test flying. VRS might concern me, since those bounded vortices might well set up the conditions for sudden entry into a very strong vortex ring state - not a nice characteristic. Also the design will likely need some optimisation, since a circular cross section ring may not work well in all flight conditions.

Have you done any finite element modal simulations? I would be concerned that the inertia at the tips would also allow blade flexure in new ways (Rolls have much experience with blisks - in both blade torsion and bending). You will find that coning and wee-wa lead angles will be affected by the inertia too - this should all be checked. Also, are you considering non-teetering applications? It will be more difficult to determine the blade effective hinge offset from blade natural bending frequency. You may need to use the 1st rotor pitch/roll frequency in place of blade 1st flapping frequency to get the delta3 right.

Finally, regardless of FEA results, test test then test some more. Your worst enemy will be fatigue cracks. What is the expected duty cycle? Are there any dynamic loads? How much can you accelerate the durability testing? How can you validate that the hours predicted to say B10 failure tie up with practice? These are all things that must be considered.

Remember: Any new project is a collection of faults waiting to reveal themselves. The job of an engineer is to find them before the customer does...

Mart

[bugdevheli]

Graviman Your comments are all valid. I have had FEA carried out on a one fifth scale model and the results were to say the least, exciting. As you predict the design of the ring is critical, and i have done a considerabe ammount of work on various shapes, including winglets and ducts etc. I am in the process of building my Mk4 machine which will be the vehicle on which i will test the full size ring rotor. Thanks Bug

[Dave_Jackson]

Bugdevheli,

The patent US 6,845,941 may be of interest, if you do not already know of it.

Rotary/fixed wing aircraft (http://patft.uspto.gov/netacgi/nph-Parser?Sect1=PTO1&Sect2=HITOFF&d=PALL&p=1&u=%2Fnetahtml%2FPTO%2Fsrchnum.htm&r=1&f=G&l=50&s1=6,845,941.PN.&OS=PN/6,845,941&RS=PN/6,845,941)

Dave