Wooden Helicopter blades?

[The Artful Bodger]

Nonetheless, metal rotor blades have been an option since the 47G2.

http://www.bell47helicopterassociation.org/Bell_47G%20article_1.pdf

Initially, the 47G-2 came equipped with wood -50 main rotor blades, but later production introduced the first metal blades on the 47. These blades are known as “-13s” and had a serviceable life of 3200 hours. ................. These - 13 blades are no longer Today, a G2 with original metal -13 blades with 1000 hours or more remaining is considered a prize.

[wierdscience]

Wood has one problem, eventually it degrades and develops cracks crosswise to the grain.... it becomes "brash".... and the breaking strength gos down by a LOT.

But, that takes a while, and the better the wood to begin with the longer it takes.

State Forest Products lab? Sounds like an expensive tax-sucking boondoggle type *government takeover* of commercial materials research... THAT should be closed down *immediately* to allow the free market to work.......

Nope,paid for by industry,staffed by university,establishes data points for products already in developement.

None of that slopping tax money to companies who don't have a worthwhile product like is all the rage these days.

[dalesvp]

Tulsa Rotor Blades

Years ago I worked on '47 and Hiller blades, they are wood with a metal spline down the middle, metal leading edge, laminated spruce spar, trailing foil made of balsa. The older blades were finished with varnish which was recently (1990s) upgraded to West Systems marine finish. Here is the company still in business:

http://www.tulsarotorblades.com/

Photos of the blade repair processes and tooling:

http://www.tulsarotorblades.com/pics/pics.html

[Evan]

You are both correct. The heavier the load the greater the "coning" angle of the blades. Instead of just bending the blades run at a larger angle from horizontal resulting in an increased cone angle. Rotors all have a maximum coning angle specification on every helicopter beyond which the rotor head will fail. Edit: The coning angle is both load and rotor rpm dependent.

[A.K. Boomer]

Im not so sure your correct on that - would not the bending of the blade be less due to the centrifugal force -- the outer part of the blade has mass and therefore tries to remain true to the horizontal plane the faster is it spun, therefore the blade will go through less deflection for the same load as one would in a static mode...

In a related topic --- a car traveling with a low tire still has the same load at a high rate of speed (forget about getting fancy with aerodynamics and such)

yet the tire is able to support more weight due to the outer casing having mass and being forced out against the road at incredible G-forces - some of these forces are applied to the casing which are of course linked to the rim...

anyone in doubt of this needs to take it to the drag racing circuit --- where cars show up all hunched in the back with low pressure wrinkle walls and then they "grow" in height upon take off...

Black moons description of "wooden tight ropes" was a good way to describe this effect IMO..

[philbur]

When in flight the blades have to support the vertical load due to the weight of the helicopter, it's payload and any additional vertically applied performance loads. These loads generate a bending stress in the blades that is not influenced by the "horizontal" stress resulting from the angular velocity of the blades. However the stresses are additive.

Phil

Note how much sag there is to the blades of a helicopter that is not spining, those blades can hardly support there own weight under static forces, its the g forces of rotating around a center axis that pulls them out, basicly suspending the helicopter from multiple wooden tight-ropes! .

[J Tiers]

Wood has one problem, eventually it degrades and develops cracks crosswise to the grain.... it becomes "brash".... and the breaking strength gos down by a LOT.

But, that takes a while, and the better the wood to begin with the longer it takes.

State Forest Products lab? Sounds like an expensive tax-sucking boondoggle type *government takeover* of commercial materials research... THAT should be closed down *immediately* to allow the free market to work.......

[wierdscience]

Years ago I toured the state forest products lab here.At the time late 80's they were experimenting with laminated lay ups of veneer from various species.One of the possible applications for the materials mentioned were aircraft spars and propeller blades.The stuff they were making was built up from veneer ranging from .062 to .015" in thickness,coated with epoxy and cured under pressure.The end result was a product that retained the properties of wood,but was consistent in structure as metal.

I still have a sample somewhere of some Faux Rosewood they made that was produced by pressure laminating 65 layers of Dyed Birch veneer .015" thick and compressing them to just 3/4" thick.It was dense,heavy and felt like the real deal,but cost about as much too

In the materials used by man has there been anything more useful to us than Wood besides possibly stone?It's amazing stuff.

[Evan]

A lot of wooden rotors don't look like wood as they have a metal leading edge cap and are fiberglassed. The spar is still wood with balsa for the trailing edge filler. I had a look and found this sketch of a typical wooden blade. This is the one from the Bell 47, I believe.

[Thruthefence]

Let me assure you Kaman knows exactly what to do with Rotor Blades. They make a nice Guitar, as well.

There's not a better material out there then wood, for building airplanes. The devil is in the upkeep. It doesn't do well without constant attention.

The latest-greatest "high tech" props are wood.

http://www.steenaero.com/Products/propellers.cfm

And not just for little aircraft; there's a guy Locally operating a Piper Cheyenne 400LS, 1000 flatrated HP per side, running 4 blade M T propellers.

It's extremely important to maintain the integrity of the composite exoskeleton; a rock nicking the blade will let moisture into the wood.

[A.K. Boomer]

This explains allot ---- it's really not some wooden airplane blade with a coating of varnish ---- these are exoskeleton structures that use the most strongest materials on the very outside (where it counts) --- both carbon and glass fiber are bonded to the pre-shaped wood.

I tried to find a pic like I seen on TV - I don't think the wood part of the blades are anywhere near the hub area due to stress - it's not to say they couldn't be built that way but the design would have to be much larger and reinforced in this area ------- just the same - hardly the "norm" but im impressed - I have a friend that has an airplane with a spruce wing (main spar) and i'll have to mention this to him, actually maybe I shouldn't cuz that will just make him push it even further and he's already on the edge

I don't get one thing - Kaman states that they use the outer wrap to strengthen the blades - OK - but there's a fatigue life on composites like that is there not? so what's the advantage of using wood? does not the outer reinforcement have to be shelved after so many running hours?

or is it because it's semi-transparent or "visible" that makes all the difference - it would (in general) after all be the first point of failure if it's a stiffer material and on the outside.

At the waldo canyon fire one of the smaller K-maxes out-lifted a massive "sky crane" --- not all at once - but over the course of an hour it dumped more water on the fire...

http://www.compositesworld.com/artic...il-helicopters

Quote:

Although Kaman's commercial helicopter, the K-MAX, was certified in 1994, before the latest FAA rules were drafted, the company selected metals, primarily aluminum, for the fuselage's structural components, using composites only in the rotor blades and some nonstructural parts. "You always have to think about market size and the investment in tooling for composites," explains George Schafer, K-MAX program manager. Called an "aerial truck" by the company, K-MAX was designed for external lift operations required in oil rig and pipeline construction, utility power pole erection, fire-fighting and timber harvesting. "When we started K-MAX development, the market size was unknown," Schafer says, but notes that, to date, the company has produced 38 K-MAX helicopters.

Unable to justify the investment in tooling required for composites, Kaman engineers also had a practical motivation for selecting metals. "The environment our customers work in is like the environment for bulldozers," Schafer contends. Field repairability was, therefore, a critical objective. As a result, the only fuselage components made from composites were those with critical contoured surfaces, the nose cone (fiberglass/epoxy) and the tip caps on the vertical stabilizers. These solid-laminate parts are fabricated by wet layup and cured at room temperature.

For the helicopter's 7.3m/24-ft long composite rotor blades, Kaman uses wood laminate spars, based on technology dating back to the 1950s. Pickett explains that the company selected wood for its damage tolerance and fatigue resistance and to take advantage of field experience and qualification data amassed from a similar spar on its HH-43 helicopter, built for the U.S. Air Force in the '50s and '60s. Layers of wood are glued together with penacolite adhesive and carved to aerodynamic contour on a 5-axis NC machine. To enhance the spar's stiffness, unidirectional IM7/8552 carbon/epoxy prepreg tape from Hexcel (Dublin, Calif.) is bonded to the spar with Henkel Corp. (Bay Point, Calif.) Hysol EA 9394 two-part epoxy. Finally, the spar is wrapped in a dry glass cloth, which is wet out with EPON 828 epoxy, from Resolution Performance Products LLC (Houston, Texas).

The blade "afterbody," Kaman's term for the portion of the blade aft of the spar, is composed of woven 7781 and 120 glass/epoxy prepreg, supplied by J.D. Lincoln Inc. (Costa Mesa, Calif.), over Hexcel Nomex honeycomb core. Unidirectional carbon/epoxy also is added to the trailing edge spline to provide edgewise bending stiffness. The afterbody skins are fabricated via hand layup from prepreg ply patterns cut on automated equipment from Gerber Technology Inc. (Tolland, Conn.). The skins are bonded to the core with Sovereign Specialty Chemicals (Buffalo, N.Y.) Plastilock 717 adhesive and the assembly is autoclave-cured at 121?C/250?F and 103 kPa/15 psi. The afterbody and spar are assembled and bonded with Hysol EA 9394, which is cured at 66?C/150?F.

Each rotor blade has an aerodynamic control surface called a servo flap, a feature unique to Kaman. This 91-cm/36-inch long flap is attached along the blade's trailing edge at two locations and is centered at about 70 percent of the blade span. "That FAR aft of the blade pitch axis, it has to be light," Schafer says. The servo flap is hand layed up from Hexcel woven carbon/epoxy prepreg and cocured in a single operation using a heated matched mold tool at 177?C/350?F. [/quote]

[Hobbyman]

In the 1960's one of my instructors who maintained Bell 47's told us of balancing the blades with masking tape wrapped on the tips. During the day as temperatures went up and the blades dried out they would re-balance the rotor assembly by unwrapping or adding tape.

[hojpoj]

The K-Max is an interesting aircraft, someone I work with is actually working on the unmanned resupply stuff pointed out in that wiki article. The thing that struck me most about it when I saw it during some flight tests was that it was so danged quiet. And you're getting a whole lotta lift capacity out of what amounts to a Huey engine.

It's not surprising that rotors are still made from wood, on a rotorcraft every pound matters, and mother nature does an exceptional job of creating lightweight, strong materials. While fatigue life is a definite issue for rotor blades, the predominant forces are the centrifugal force, and to some extent the bending moment over the length of the blade. I'm not a rotors guy, nor am I familiar with the synchrorotor configuration's dynamics.

For the military, at least, I imagine that wooden rotors have fallen out of favor due to erosion issues. It's easily justifiable for civilian aircraft to either stay grounded in inclement weather, or only land on nice prepared surfaces. For the green-suiters, they fly in some amazingly harsh environments that would eat a wood rotor for breakfast. Another reason the military likes the composite/metal rotors is that the blade mass properties can be tuned. If a pilot has to execute an autorotation (due to engine failure), he'd like for more of the blade mass to be further from the rotor hub so as to keep as much rotational inertia stored in the rotor system as possible. Does it add mass? Yeah, but requirements in the military don't always work together.

[gwilson]

I have a bunch of sitka spruce that came from NASA. Apparently years ago,they bought a large bunch of Sitka spruce to use on their wind tunnel fans,and changed their minds about what to make them out of,or else had a good surplus of wood. Anyway,there was a bunch of it available for workers to have. I can't recall how I got mine. Must have been from an ex employee.

[The Artful Bodger]

Welll.... not all Bell 47's have wooden blades and I vaguely remember hearing that the wooden blades suffered from rain erosion more than the metal blades.

One of the more remarkable wooden aircraft ever built was the DH98 Mosquito. http://www.dhmosquito.com/