Wooden Helicopter blades?

[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! .

[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..

[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.

[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

[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.

[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]

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

Nice! $13,500 for a rebuild sounds like a bargin for what parts cost now.Are they still making new blades?

[philbur]

On reflection I think you may be correct in that the stress due to the angular velocity has a tendency to reduce the bending stress due to the weight, but nevertheless the two stresses are additive and the sum is always greater that each individually on the lower side of the blade and always less than the sum on the upper side of the blade.

Phil

Im not so sure your correct on that

[The Artful Bodger]

Doesnt seem to be much bending stress here!

[Thruthefence]

Raising collective pitch before the rotor is below operational rpm induces extreme bending loads in the blades, (eg, "coning"), leading to failure of the blades. The centrifugal force adds considerably to the ability of the rotor disc to lift the aircraft.

And if you look at a bell 47 rotor head in profile, you will see that there is a bit of "pre coning" built into the assembly. On a bell 206, it's a little more obvious. It's analogous to dihedral in a fixed wing aircraft.

[Evan]

Same with trying to suck the last little bit of lift to avoid a hard landing on an autorotation.

[J Tiers]

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.

Still "guv'mint" interference...... it it was viable, it would stand on its own with no university nonsense.....

[A.K. Boomer]

Doesnt seem to be much bending stress here!

Right -- now try stopping the blades and supporting the entire copter by the stagnant blades ----- depends on the material - take your pick - snap - or bend...

remember -- the outer parts of the blade are where the majority of the lift work is being done (due to higher tip speeds) so you don't get to grab the props close to the hub and call it good -- u actually have to grab it more out at outer parameters and a little inboard as well - snap - crackle - pop

[lazlo]

There's not a better material out there then wood, for building airplanes. The latest-greatest "high tech" props are wood.

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

I'll preface this by saying I don't know d!ck about aeronautical design, but that doesn't seem to matter here

That link compares wood versus aluminum props, and the comparison table makes perfect sense to me: wood having superior moment of inertia and vibration damping. But isn't carbon fiber superior in almost all categories, except for fatigue life? All the ultralight human-powered aircraft are carbon fiber, and my model aircraft friends use carbon fiber props.

I'm pretty sure all modern military helicopters use carbon fiber/composite rotor blades -- I know the Apache does, and the rotors are rated to survive a direct hit from a 20mm shell and continue flying...
The Hind uses aluminum honeycomb, but that's a Vietnam-era design.

[dalesvp]

New Blades

Nice! $13,500 for a rebuild sounds like a bargin for what parts cost now.Are they still making new blades?

I do not believe anyone is making new blades. A rebuilt blade is certified to be "as new" and they do last for years of normal use. The big enemy of wood rotor blades is not use, rain or age; it's trees!