Helicopter problems (design etc)
Created | Updated Jan 28, 2002
Helicopter mechanics and other stuff
****************************************************************
Helicopters by their very nature don’t really want to fly. They have to be forced into the air, and once airborne are subject to strange forces that their fixed wing relatives can safely ignore.
Phase lag.
Helicopter blades are designed to flap up. The amount of flap up depends on the angle of the blade to the airflow. The blade will flap to its maximum 90 degrees after it has experienced the maximum blade angle. Now, every student nods wisely at this point and matters something about gyroscopes.
It has NOTHING to do with gyroscopes, and is purely down to aerodynamics. Think about it - the blade will continue rising as long as the blade angle is above its starting point. It will be at its maximum blade angle, and still flapping upwards, 90 degrees after the point at which the blade angle changed. However, for the next 90 it is still above its starting angle and will still rise.
The effect of this is that the controls that change blade angle must be offset in various mechanical ways.
Obviously it is not a good idea for the blade angle to increase as it flaps upwards, otherwise it wouldn't want to come down again. Methods of controlling this include linking the jacks or rods that change blade angle so that they ahead of the blade; as the blade rises, providing the geametry has been well calculated, there will then be a tendency for the blade angle to reduce. Another way is by offsetting the hinge mechanism that allows the blade to fly up.
Coriolis.
The same force that allows ice-skaters to increase their spin by bringing their arms close to the body affects helicopters. As the blades flap up, the distance to the centre of the rotor decreases, so the blade wants to speed up. If it is restrained from doing so, there will be vibration, and possible damage to the blades or attachments.
Smaller helicopters less of a problem, but larger ones need some sort of mechanism to absorb and control this movement. One solution is to allow the blades to pivot in the horizontal plane, and use a hydraulic damper (or even a simple friction damper) to absorb and limit the resultant movement. Another solution is to make the roots of the blades flexible.
Hookes Joint Effect.
Any time that some blades are flapping up higher than others, the centre of gravity of the rotor head will not align with its true physical axis.
This will cause vibration as forces acting on the rotor mass try to find a more acceptable axis point.
Blade balancing.
Rotor blades need to be finely balanced both for weight and for aerodynamics. Main rotor blades have the weight marked on them so that maintainers can select a set of blades within laid down parameters.
Aerodynamic balance is trickier; although the blades are made to close tolerances, they will each have slightly different flight characteristics.
Sooooo, how do we resolve this? Well, we used to use a flag on a stick.
Basically, you put different coloured chalk marks on the tip of each blade, then gently introduced a taut canvas flag into the rotating disk until the blade tips just touched it. (the flag was stretched between the horizontals of a large “F” shaped metal frame) Examination of the coloured hit marks on the flag will show which blades need to be adjusted. If this sounds hairy and scary, its because it was. A more modern, and safer technique, is to have coloured reflectors on the tip of each blade and shine a strong beam of light on them.
Sideways drift.
Tail rotors counteract the torque of the main rotor, and stop the helicopter spinning on the spot. However, it adds an extra sideways component, which results in the helicopter wanting to drift sideways.
Compensation in early helos was by the pilot using the cyclic controls to move the helo in the other direction, and many early helos (noticeable Dragonfly) would always fly a bit lopsided. More attention is paid to design in modern choppers so that the effect, although still present to some degree, is less noticeable.
Ground resonance or Padding.
Under certain conditions, the harmonics set up in a helicopter that has its rotors turning while on the ground can cause it to start rocking from side to side. The terms “Ground Resonance” and “Padding” are generally used to describe this, although pedantically the first causes the second.
This can be worsened by the following factors:
Blades out of balance.
Blade damping mechanism not working properly.
Undercarriage shock absorbers not correctly maintained.
The ground moving. (this normal only happens on ships!)
Pilot error in not recognizing the symptoms.
Helicopters have been known to flip over as a result of padding. Fortunately it is easy to stop once the symptoms have been recognized. All the pilot needs to do is to lift the collective stick to change the pressure on the undercarriage; in practice, though, most pilots will lift the aircraft fully off the ground. Many years ago, the author had an unscheduled flight while lying on the main gearbox-servicing platform of a large helicopter during a between-flight check. The helicopter started bouncing and the pilot lifted off for a few moments. Perfectly safe, but scary!
Dunker
When a helicopter crashes into the sea a number of things may happen depending on the helicopter and luck:
If it has flotation equipment fitted (and it works as advertised) it stays in an upright position, the pilot shuts down the rotors, and exits sharply right.
If however it has no flotation equipment (or it fails) the situation quickly becomes life threatening. The helicopter will tip to one side due to torque from the rotors. As the rotors hit the water they will start to break off. This is NOT a good time to leave the helicopter. Rather, the safest technique is to take a deep breath, wait until the rotors/engine have stopped, try to remember which way was up (because by now the helicopter is almost certainly upside down,) then calmly(!) leave by the nearest exit.
As this survival technique of doing nothing until the danger has passed
is not natural to human beings, responsible operators of sea-going helicopters (Navy, Oil rig suppliers) insist that regular/all fliers practice the technique beforehand. The device used for doing this is known as the “Dunker”, and consists of a mockup of a helicopter cabin suspended over a pool of water. A crash can be simulated by hydraulic jacks, which would typically lower the cabin to the water, then rotate it while still lowering it. This is considered to be rather jolly fun by the operators of the Dunker; less so by those who have to escape from it. An added refinement is to black out the whole pool or blindfold the crew so that it simulates a crash at night. Although a bit traumatic, it has saved many lives – the author has spoken to aircrew after a crash and their common feeling was “It was just like the dunker, but easier”
*******************************************************
This article was prompted by SAM’s Helicopter article. Feel free to add comments – I may update the article to reflect any I feel are relevant. In particular a better title would be handy!
****************************************************************
Helicopters by their very nature don’t really want to fly. They have to be forced into the air, and once airborne are subject to strange forces that their fixed wing relatives can safely ignore.
Phase lag.
Helicopter blades are designed to flap up. The amount of flap up depends on the angle of the blade to the airflow. The blade will flap to its maximum 90 degrees after it has experienced the maximum blade angle. Now, every student nods wisely at this point and matters something about gyroscopes.
It has NOTHING to do with gyroscopes, and is purely down to aerodynamics. Think about it - the blade will continue rising as long as the blade angle is above its starting point. It will be at its maximum blade angle, and still flapping upwards, 90 degrees after the point at which the blade angle changed. However, for the next 90 it is still above its starting angle and will still rise.
The effect of this is that the controls that change blade angle must be offset in various mechanical ways.
Obviously it is not a good idea for the blade angle to increase as it flaps upwards, otherwise it wouldn't want to come down again. Methods of controlling this include linking the jacks or rods that change blade angle so that they ahead of the blade; as the blade rises, providing the geametry has been well calculated, there will then be a tendency for the blade angle to reduce. Another way is by offsetting the hinge mechanism that allows the blade to fly up.
Coriolis.
The same force that allows ice-skaters to increase their spin by bringing their arms close to the body affects helicopters. As the blades flap up, the distance to the centre of the rotor decreases, so the blade wants to speed up. If it is restrained from doing so, there will be vibration, and possible damage to the blades or attachments.
Smaller helicopters less of a problem, but larger ones need some sort of mechanism to absorb and control this movement. One solution is to allow the blades to pivot in the horizontal plane, and use a hydraulic damper (or even a simple friction damper) to absorb and limit the resultant movement. Another solution is to make the roots of the blades flexible.
Hookes Joint Effect.
Any time that some blades are flapping up higher than others, the centre of gravity of the rotor head will not align with its true physical axis.
This will cause vibration as forces acting on the rotor mass try to find a more acceptable axis point.
Blade balancing.
Rotor blades need to be finely balanced both for weight and for aerodynamics. Main rotor blades have the weight marked on them so that maintainers can select a set of blades within laid down parameters.
Aerodynamic balance is trickier; although the blades are made to close tolerances, they will each have slightly different flight characteristics.
Sooooo, how do we resolve this? Well, we used to use a flag on a stick.
Basically, you put different coloured chalk marks on the tip of each blade, then gently introduced a taut canvas flag into the rotating disk until the blade tips just touched it. (the flag was stretched between the horizontals of a large “F” shaped metal frame) Examination of the coloured hit marks on the flag will show which blades need to be adjusted. If this sounds hairy and scary, its because it was. A more modern, and safer technique, is to have coloured reflectors on the tip of each blade and shine a strong beam of light on them.
Sideways drift.
Tail rotors counteract the torque of the main rotor, and stop the helicopter spinning on the spot. However, it adds an extra sideways component, which results in the helicopter wanting to drift sideways.
Compensation in early helos was by the pilot using the cyclic controls to move the helo in the other direction, and many early helos (noticeable Dragonfly) would always fly a bit lopsided. More attention is paid to design in modern choppers so that the effect, although still present to some degree, is less noticeable.
Ground resonance or Padding.
Under certain conditions, the harmonics set up in a helicopter that has its rotors turning while on the ground can cause it to start rocking from side to side. The terms “Ground Resonance” and “Padding” are generally used to describe this, although pedantically the first causes the second.
This can be worsened by the following factors:
Blades out of balance.
Blade damping mechanism not working properly.
Undercarriage shock absorbers not correctly maintained.
The ground moving. (this normal only happens on ships!)
Pilot error in not recognizing the symptoms.
Helicopters have been known to flip over as a result of padding. Fortunately it is easy to stop once the symptoms have been recognized. All the pilot needs to do is to lift the collective stick to change the pressure on the undercarriage; in practice, though, most pilots will lift the aircraft fully off the ground. Many years ago, the author had an unscheduled flight while lying on the main gearbox-servicing platform of a large helicopter during a between-flight check. The helicopter started bouncing and the pilot lifted off for a few moments. Perfectly safe, but scary!
Dunker
When a helicopter crashes into the sea a number of things may happen depending on the helicopter and luck:
If it has flotation equipment fitted (and it works as advertised) it stays in an upright position, the pilot shuts down the rotors, and exits sharply right.
If however it has no flotation equipment (or it fails) the situation quickly becomes life threatening. The helicopter will tip to one side due to torque from the rotors. As the rotors hit the water they will start to break off. This is NOT a good time to leave the helicopter. Rather, the safest technique is to take a deep breath, wait until the rotors/engine have stopped, try to remember which way was up (because by now the helicopter is almost certainly upside down,) then calmly(!) leave by the nearest exit.
As this survival technique of doing nothing until the danger has passed
is not natural to human beings, responsible operators of sea-going helicopters (Navy, Oil rig suppliers) insist that regular/all fliers practice the technique beforehand. The device used for doing this is known as the “Dunker”, and consists of a mockup of a helicopter cabin suspended over a pool of water. A crash can be simulated by hydraulic jacks, which would typically lower the cabin to the water, then rotate it while still lowering it. This is considered to be rather jolly fun by the operators of the Dunker; less so by those who have to escape from it. An added refinement is to black out the whole pool or blindfold the crew so that it simulates a crash at night. Although a bit traumatic, it has saved many lives – the author has spoken to aircrew after a crash and their common feeling was “It was just like the dunker, but easier”
*******************************************************
This article was prompted by SAM’s Helicopter article. Feel free to add comments – I may update the article to reflect any I feel are relevant. In particular a better title would be handy!
