2 Blade aerobatics...

[andre127]

Hello everybody,

Could someone be so kind to explain me why a 2 bladed rotor can't perform negative g while I flown in the past several bi blades RC model that could also hovering upside down?!

Well I know why a R22 cannot do negative but why the design of the rotor head in pic for example cannot be upscaled to a real heli?

sorry but I have less than 8 post so I cannot upload pics or urls... please just search Align T-REX rotor head as an example of a 2 blade aerobatic!

thanks

Andrea

[Rotorbee]

That could become a long thread.
The rotorheads of model helicopters are rigid rotor heads. An R22 has a rotor that is semi rigid teetering rotorhead, which means it has hinges that let the blades flap up and down. This is needed, because otherwise you would not be able to fly very fast. The lift difference between advancing and retreating blade would make the helicopter roll and crash. You would have to compensate this manually, but that would make the controls more effective in one direction, than in the other. That is why hinges were added to rotorheads, because then the blades can correct that for themselves. The hinges and the rigging change the angle of attack on the blade constantly when it goes around.
In a model helicopter you don't need these hinges, because the speed difference between advancing and retreating blade is relatively low and easily compensated by the controls . You also don't care about vibrations, because nobody is sitting in there and a model helicopter is much stiffer and sturdier than a real one. And it will not fly for thousands of hours, therefore fatigue of components isn't really a thing. And if it crashes, because something breaks, you buy a new one. In a helicopter the rotor itself does create vibrations, because the center of rotation moves, due to the flapping of the blades and a few aerodynamic reasons. Therefore virtually all helicopter rotorheads have some sort of damping incorporated in their design.
By the way, no real helicopter can actually fly upside down, which means hold the altitude, because they don't have negative pitch. If you see the BO105 on its back, it is actually falling down.
And yes, some helicopter have rigid multi-bladed rotorheads, but in this case, the blades have virtual hinges. Which means, that the blade itself is meant to bend in a certain way to compensate.

[SplineDrive]

It's the stiff out of plane rotor head that is wholly impractical for a two bladed rotor on a passenger carrying aircraft. While a stiff out of plane rotor can be trimmed with cyclic for various flight conditions, it does require more complex control laws than a simple flapping rotor head. The vibrations are a particular killer. If Nb = number of blades, vibrations in the airframe are caused by Nb +1 and Nb-1 vibrations in the rotor. The 1/rev harmonic is the largest moment, 2/rev is less, 3/rev less than that, and so forth and so on (generally speaking). This is why the current Sikorsky X-2 aircraft have 4 blades, to help reduce the vibrations relative to the 3 bladed XH-59A. A two bladed rotor would be converting the largest and 3rd largest rotor oscillatory loads into airframe vibrations. It's a terrible idea. Comically bad, actually, and made worse because the human sensitivity to vibration increases as the frequency drops. So for a given diameter rotor, a rigid two bladed rotor will have both much higher vibration magnitudes at a worse frequency than the same diameter rigid rotor with, say, six more slender blades (holding solidity constant).

A two bladed rotor also has other 2/rev sources to deal with in the edgewise direction as well and an acrobatic stiff rotor mast capable of handling the high loads from the rigid rotor won't have the flexibility to reduce the transmission of those forces to the airframe.

There have been a few rigid two bladed human sized helicopters, though they were coaxial (to brute force the control moment terms). The Hiller X-2-235 was one and actually went through some wind tunnel testing by NACA to test the idea of this coaxial rigid rotor configuration in high speed flight, but testing was stopped due to fears of high vibrations and risk to the tunnel. Bendix made a line of coaxial helicopter prototypes and they started off as rigid out of plane rotors. The helicopter line moved to Gyrodyne and had some success, but only after adding a teeter hinge to each rotor.

Anyway, it's a terrible idea. One should notice that not even two bladed tail rotors are rigid on human sized helicopters. It's possible that rotor designers have for generations been given a stream of lucrative kickbacks by various bearing companies... or... flapping is a good thing and makes for a better aircraft, despite being a little more mechanically complex.