I have made made advancements since my last postings. I will list them:
- Fully articulated rotor.
- Flapping induced washout.
- Heading hold stabilization.
- Semi-rigid rigidizing.
**Fully articulated rotor: **A fully articulated rotor allows the blades of a helicopter flap (move lengthwise) independently. Unlike a semi-rigid rotor where they flap in unison. Successful initial tests yielded mediocre performance though, ether poor general performance or massive vibrations.
*Initial testing: *
This early design directly limited the flapping of each blade, this caused vibration as the blades would not track. When blade flapping was directly limited, the angle of attack between the blades and shaft were not equal, causing uneven lift across rotor and vibrations.
It used a advanced ballsocket securing the angle between lengwise axis of the blades and shaft. This would not stay the same for both, and one blade would lag behind the rest slighty (the blades would not stay paralel to each other). This would shift the center of mass of the whole rotor, thus creating massive vibration).
This is a later design, It uses two levers attached to the rotor blades and shaft with axis joints. Lever movement is limited via advanced ballsocket.
The levers were installed to:
Limit rotor blade flapping.
Remove rotor blade tracking issues.
Rigidly secure blade lead/lag angle.
Although this worked fine, it presented instability during lateral flight, The blades would reach the flapping limit with collective. Allowing only downwards flapping, thus not completly canceling out dissymmetry of lift.
It’s building was also tedious as the levers required multiple constrains in the same position.
*More testing: *
Further tweaking simplified the rotor design. Blade flapping is directly limited via av. ballsocket and the limiting levers were removed. To coax with lead/lag issues, an expression 2 (is invisible in the video) which turns the rotor is placed in-line with the root of the rotor blades . The blades are av. ballsocketed to it. This removes major vibrations (lead/lag issues, tracking) and simplifies the rotor.
In order to increase stability and cancel out dissymmetry of lift. The blades can flap independently from each other under a certain angle which is less then the maximum flapping angle of each. This greatly increased stability and performance.
They also present flapping induced washout.
A fully articulated system offers three advantages from a semi-rigid:
Increased cyclic authority (for the same swashplate inclination, more torque is developed).
Increased lateral flight stability (During lateral flight, the craft will be less prone to roll without pilot input).
Increased hover stability (due to independent flapping, the blades will turn the rotor disk into a cone. This acts as dihedral on a airplane. If the helicopter is not level with the horizon, one blade will push more against gravity. The drawback of this is lateral movement, as the blades will also push sideways)
Coning is visible in this picture:
In order to have stable lateral flight, I had to weld a plate on the top of the helicopter to create drag. Fully articulated rotors are affected by the net drag and center of mass of the helicopter like semi-rigids. If the net drag on the helicopter is bellow it’s center of mass/gravity. When moving forwards/sideways the helicopter will pitch in to that direction the faster it moves. To cancel this out, I added some drag on top of the craft.
*Flight trials: *
Seeing as the prior helicopter performed very well, I decided to try out it’s flight performance.
General performance is similar to that of a semi-rigid but is slightly better and smoother.
Flapping induced washout: **Washout is the tendency of a wing to decrease it’s angle of attack (AoA) . Shifting the positing of the swashplate linkages moves the hinge line of each blade.
Moving the linkage farther away from the blade tips will induce washout (more flapping, less angle of attack - vice versa). If closer washin is induced (more flapping, more angle of attack - vice versa).
Induced washout provides:
- Improved lateral flight stability: Induced washout helps cancel out dissymmetry of lift. When the helicopter is moving laterally, the advancing blade (moving into the direction of motion) will flap upwards and decrease it’s AoA. The retreating blade (moving away from the direction of motion) will flap downwards and increase it’s AoA. Keeping the total lift across the rotor constant even though the blades don’t have equal speed.
Red arrows represent blade movement, blue arrows body movement.
Vibration decrease: The tendency of the blades to remain perpendicular to the rotor shaft helps dampen vibrations. Increasing/decreasing AoA in order to keep the forces acting on them equal.
Slight increase in maximum airspeed (2-7 km/h, 1-4 mph) . Since dissymmetry of lift is mostly cancelled out, more cyclic control can go into tilting the helicopter, thus pushing forward more.
**Heading hold stabilization: **Heading hold stabilization “holds” an object in a specified orientation. It works via accumulating the angular velocity of a object. This applied to helicopter control yields excellent results, it relieves pilot stress by tenfolds. The pilot only sets the orientation the craft should retain, letting the stabilization system hold said orientation.
In the video I compare simple angular damping (the wood helicopter/RH-3) with heading hold (the green helicopter/RH-4)
This is especially for beneficial for lateral flight, as it compensates for dissymmetry of lift by offsetting the cyclic. This allows the helicopter to reach critical airspeed to the point of retreating blade stall. Where one blade increases angle of attack to the point which it no longer produces lift or has very little airspeed. Something which would be very hard to achieve manually.
Semi-rigid rigidizing: **In a simple semi-rigid rotor, the blades are allowed to move freely lengthwise. This decreases cyclic authority (making it less effective, as some of it goes into tilting the rotor disk, instead of rolling the helicopter).
The wooden helicopter/RH-3 has a semi-rigid rotor.
To increase it’s effectiveness, blade flapping can be dampened via hydraulics or elastics. Putting some resistance to blade flap, tilting the rotor disk less. It’s effect can be varied by changing the strength of the dampers (hydraulics/elastics). The second helicopter has a dampened semi-rigid rotor, cyclic is more effective as the tail boom can be seen moving with changes in cyclic.
Another way to increase cyclic response is to limit the flapping. Letting the blades flap freely enough to cancel out dissymmetry of lift, yet allowing greater cyclic authority as the rotor disk cannot tilt as much. This greatly improves cyclic control but increases vibrations drastically when the blades cannot flap. The third helicopter has a limited semi-rigid rotor.
Limiting is done the say way with a lever as used in early fully articulated rotors.