Graviman

Good for Sikorsky/Schweizer!

Must admit i was astonished at the original intention to get the thing flying by last year. To me it shows a responsible realisation within the organisation that the engineering for such a project is far less expensive when the time frame is given to trouble shoot problems.

I'd be interested to know more about the Moog active vibration control though. I could have a reasonable guess at how it might work, but will dig up some papers.

Mart

NickLappos

Grav,
The Moog system is the same as that in the S-92, and it works like a charm. I understand the EC225 has a similar system.

The Moog vibration absorber is a electronic motor is digitally controlled to have very precise rpm capability. The digitally controlled motor spins counter rotating weights which are driven eccentrically in and out by separate jackscrews to produce higher or lower vibration amplitudes. Because the rpm of the motor is precisely controlled, the phase of the counterrotating weights can be adjusted, where the phase is the precise direction of the maximum amplitude of the weights. As a result the vibration absorber is capable of being adjusted to precise RPMs and to precise vibration absorbtion direction (lateral, vertical, horizontal). It can also be adjusted automatically by a computer as the aircraft flies. A vibration pickup is located near the place where you want to quell a vibration the most. Using that pickup the computer determines the best way to tune the weights to drive the rpm to its lowest. The computer uses precise logic that learns as the flight conditions change and so the computer becomes specific for the aircraft it is installed in. You can use up to six absorbers with the computers so that it's quite possible to quell the vibration across a very large area. Each of the absorbers weighs around 40 pounds and the amplifier maybe 10 pounds.
Try this patent number: 6,869,375

Graviman

Thanks Nick, glad i asked.

Patents online
or
US Patent Office

Not sure if DOD prevents you giving details, but what sort of vibration control system was used on Comanche to allow the high roll rates? I guessed it might be a system to trim pitch around azimuth to tune out vibration, again requiring predictive software to deal with system lags. This may once again be the engineer seeking out the most complex solution to a simple problem.

Mart

NickLappos

With the five blades and small diameter (high frequency for n/rev) as well as the narrow, stiff fuselage with two strong keel beams, the Comanche had no vibe absorbers, and needed none!

This illustrates something that is counter to another of those helo pilot myths, that vibration is not normal, and it indicates "something wrong" with the rotor.

N/rev vibration is a part of every helicopter. It is created by the rotor is a natural function of the way each blade chops up the amount of lift that is needed by the aircraft. The amount of natural vibration at the rotor head is almost purely due to rotor head hinge offset, number of blades and their stiffness, and the speed range of the helicopter. How much vibration the pilots and passengers feel is almost purely due to the way the fuselage responds to the vibration the rotor is providing. A wide, long, soft cabin will respond worse to vibration and have hot spots and cold spots where people feel more and less vibration. A narrow, stiff fuselage will prevent the vibration from creating hot and cold spots and generally provides a more comfortable ride. In reality the amount of vibration the pilots feel is driven by many factors it's quite possible to change a helicopter in some way (by cutting a door or moving the seats around) and discover that the perceived vibration is made vastly better or vastly worse.

I should also point out that in fact the vibration felt at the seat is often due to the way the seat itself responds to the vibration. It is quite possible for some pilot seats to multiply the N Per Rev vibration of the rotor by a factor of two or three! Often an aircraft vibration "gets worse" and all you have to do is tighten down the seat mounting bolts.

The inventor of that patent, Bill Welsh, is a great vibration engineer at Sikorsky, I worked with him on two projects.

Graviman

Thanks again Nick. Certainly overturns another misunderstanding i had about blade 5P modes (or 5/4 wavelength eigenmodes) being significant. Then again the 15% hinge offset will push blade 5/4WL above rotor 5P, by between 1.1 to 2.7 times (say 1.7 or 8.6P). A combination of this and good internal damping lets 5 bladed system work.

Albeit not aero ( ) i can definately concur the importance of seat dynamics in perceptions of overall vehicle tranfer function. As part of a small team i am the dynamics and durability engineer (as well as designer, draughtsman, general dogsbody and teaboy ). I like to amaze folks with how well a BIG truck actually can ride&handle (and oversteer if nobody is watching ).

I would imagine that composites make life a little easier, because of the construction techniques allowing good joins between beam members, and the material having inherent damping properties. A larger machine with more panel area, and tough weight requirements, introduces all sorts of new eigenmodes which in the auto industry can be dealt with by swaged stiffeners. Composites allow thicker/stiffer sandwich construction, with some of the panels on EH101 proto being amazing (Heli museum UK).


Edit (24/01/07):

Using flexural waves (frequency dispersive bending wave) for blades in combination with rotating frequency gives:

Rotating_Blade_Freq = SQRT( (N^2 x Static_Nat_Freq )^2 + ( N x Rotor_Freq)^2 ) ; where N = blade N/4 wavelength mode.

For Comanche example this would gives "actual" flexural modes within ranges:
1/4 mode: 1.12P - this mode causes the swashplate lead angle
3/4 mode: 3.36P to 5.44P (1.12 to 1.81 x 3P) - say 4.28P
5/4 mode: 5.60P to 13.56P (1.12 to 2.71 x 5P) - say 8.71P
7/4 mode: 7.84P to 25.69P (1.12 to 3.67 x 7P) - say 14.19P

The range is because the calc does not consider construction, so that lower range is for stressed skin only and upper range is for solid section. This is not confirmed in FE, but would explain why high offset results in such a smooth helicopter. Nick, i imagine Bill Welsh is pretty knowledgeable...

Mart

IFMU

Cool picture there:

They must have really been burning the midnight oil!! Either that or it is really an NHRA picture. Maybe it is so top secret they only test at night.
-- IFMU

Dave_Jackson

Graviman

Good looking machine, however you view it.

Mart

Collective Bias

I always thought lateral and horizontal vibration was the same. What would be the diffrence, or is lateral another word for roll vibration?



CB

Dave_Jackson

From Market Watch article on the delay of the X2 project.

What a strange remark.

Is he is saying that technological advancements do not create markets?

If this is the attitude of the industry then it's no wonder that the American Helicopter Society didn't publish my provocative letter. Perhaps they believe that rotorcraft is a 'sunset industry'.


Is this an example of technologically driven capitalism being replaced by monetaristically driven capitalism?

Graviman

CB,

I have come to appreciate on this forum that a helicopter is a collection of vibrations that come together to form a flying machine. The best any engineer can do is to make sure they don't coincide. The local aerodynamics are best off not affecting the various blade flexural modes, the blade flexural modes are best off not affecting the cabin modes, the cabin modes are best off not affecting the seat modes and acoustic modes. The pilot is best off being vibration tolerant.

Lateral generally means side-to-side, but can be taken loosely as horizontal. As a blade flexes it's mass to hub radius alters so you get coriolis vibration, normally solved with lead-lag dampers. Depending on the general layout of the machine all of these can cause either lateral or roll vibrations. I understand the theory, but that is not the same as a lifetimes experience solving helicopter specific problems.

Dave,

I imagine Sikorsky are internally justifying the engineering budget on a conservative market. If helicopters had not been developed there would be no helicopter market. I imagine that once the machine practically proves it's capabilities, market growth will be exponential...

Mart

slowrotor

Dave,
Sikorsky builds large helicopters for the government. The risk of lawsuit is low compared to the profit per unit. For Sikorsky or any large company to build a small helicopter would be foolish.
New technology must come from new entreprenuers that have limited assets.

If your interest is the private market, then look at Robinson for ideas about capitalism in the helo market.

Dave_Jackson

slowrotor,

I agree with some of your remarks and politely disagree with others.

Even the US military has expressed its frustration about the inability of rotorcraft to meet its requirements. The X2 is a prototype for a suite of coaxial-ABC helicopters; from small to very large.

I understand that Sikorsky acquired Schweizer, in part, so that it would have the ability to do fast prototyping and development. Unfortunately, the cost of today's technology and regulations, combined with the limited assets of a 'start-up', makes it difficult for an individual to bring something 'new' onto the playing field.



The X2 delay has to do with the transmission. A secret document says that Sikorsky has realized the obvious and is going from the 'I' configuration to the 'V' configuration.


Dave

NickLappos

slowrotor,
The X2 is small because it is a technology demonstrator, a full scale X2 could be the size of a house.

I know Richard Aboulafia well, he is a smart fellow, but he has no crystal ball.

Jack Carson

I disagree with Nick. Scale up is always an issue. Just look at the growing pains associated the XV-15 evolution to the V-22. A smarter individual than me once said, “The bicycle was designed and built at turn of the last century, yet today you see very few two wheeled Greyhound buses.”

IFMU

I'd have to disagree with this analogy. At the turn of the 1900's we had powered flight, now we have 747's which fly quite well. In 1939 we had the VS300, and the helicopter grew up quite nicely from this humble start. But, there are some concepts that work very well for small things, like bicycles and model airplanes that hover, and don't scale up well. It seems to me that this latest Sikorsky machine is still a helicopter, and I would expect it would scale up like most helicopters.

-- IFMU

Jack Carson

IFMU
I agree in principle. Igor Sikorsky flew the Grand, a 4 engine, 5 ton machine in 1913. Sixty years later that vehicle evolved into the 747. Similarly, he flew the VS-300 in 1939. 35 years later Sikorsky flew the 35 ton CH-53E. However, some subsets of rotorcraft technology may approach physical limits due to power requirements or material limitations. To draw a slightly different analogy, I believe that the VTOL Harrier and F-35 are approaching practical limits for turbo-fan thrust born flight. Co-axial rotor systems may have just such a boundary.
Jack

Dave_Jackson

IFMU,

"In 1939 we had the VS300" AND it had 3 tail rotors.

Two of these tail rotors were removed by abortion. Or; - were they abortions, which were removed?

The third tail-rotor only grew up because it was force fed.


Dave

IFMU

The two were aborted because they promoted lateral symmetry!

-- IFMU

Graviman

Jack, your comment is spot on. The boundary you are refering to is compressibility. By opting for a counterrotating main rotor system you avoid the retreating blade stall, but the limit you now hit is the advancing blade shock wave formation. Improved modelling and damping of blade modes will allow Nr to reduce at higher speeds without fear of structural divergence. Eventually the machine will operate at speeds approaching turboprops, so Sikorsky are being sensible with a 250kias start - remember it has to work.

One day in the distant future speed might get into the turbofan arena. Much work on blade dynamics will be required - likely with active systems. This is similar to commercial fixed wing struggling to justify projects over M1...

Mart