Adriaan,
Can you clarify exactly which helicopter that was?
the airfoil is filled with massive iron rods and aluminium or iron pipes.
That's not even remotely similar to the helicopter blades I have worked on. Can you retrace your steps to that video you saw?
When helicopter blades are retired from service (they have a fixed life) they are destroyed by the maintenance facility by sawing them to pieces. I can't overstate the criticality of this action on the part of the maintenance facility - they can lose their permit to practice if they fail to disable or destroy parts that they retire from service. At least, that's normal practice in North America and I don't know if it's exactly like that elsewhere in the world. The objective is to prevent re-selling helicopters parts on the black market since they are extremely valuable (to some who think fatigue does not apply to them).
My point being (sorry for taking so long to explain) that I've seen many types of helicopters blades after being cut apart. They all have very similar cross-sections. The following is from the UH-1 "Huey". It's a screen capture from the manufacturer's overhaul manual.
To be clear there is no pipe, because such products have nowhere near the accurate dimensional tolerance to be acceptable in a helicopter. The hollow sections are extrusions, and I expect these are machined/lapped after the extrusion process to ensure extremely tight tolerances before being assembled into the rotor blade. The nose block is a typical balancing mass that prevents oscillation. I've seen copper used since it's dense, chemically inert against aluminum, and has similar stiffness to aluminum. Mixing radically different materials (e.g. steel and aluminum) doesn't work so well on a helicopter rotor blade because they will deform and disbond under the combined change in length from both elastic stress and thermal expansion. So I would be surprised to see different types of metal in a helicopter rotor blade, except at the root end attachment where high-strength stainless steel is essential.
As helicopter, one needs a symmetrical airfoil because the rotor must be able to work as a wind turbine rotor if the motor fails. If the angle of attack is positive for helicopter use, it will be negative for wind turbine use.
Not true. Symmetrical airfoil is much easier to manufacture. The successor model to the blade pictured above (Bell 212) has blades with airfoil profile that varies from root to tip, and the tip is not symmetrical. The need for reverse flow is entirely true, as you say, but that doesn't commit helicopter designs to have symmetrical airfoils. The real limitation on the types of airfoils that should be used is the high variation of Reynold's Numbers where they must operate. The tip traveling forward is nearly at the speed of sound when it is flying forward, and on the rear-ward stroke it has nearly zero velocity. Each blade tip experiences this cycle on every revolution or the rotor.
I've been told many times that I should use old helicopter blades in my wind turbines. It's an interesting place to start, and I do have access to them, but they are designed to be so structurally strong that they would make an extremely heavy wind turbine rotor. And, it would be discouraging to repair blades that have been sawed apart by the shop before they can give them to me.
