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....

[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]

Originally Posted by Gomer Pylot

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:



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 ) then why not let the machine respond quickly for you?

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

[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