Dynamic Rollover

Another dangerous condition for a helicopter pilot to experience is called dynamic rollover. It is again, where you have a series of conditions that combine to make a dangerous situation. Once again, 3 key elements make up this hazardous condition. They are: A pivot point, a rolling moment, and weight equal to thrust at some time during the manoeuvre. What actually happens is that the helicopter, which is still on the ground, will start to roll over on its side using one skid, or wheel, as the pivot point.



Once the aircraft starts to roll, a downward collective movement is the only thing that will stop the forces in action from flipping the aircraft on its side. By reducing the collective, the thrust to weight ratio decreases, which allows the aircraft to settle back down in a level attitude. If this is done on sideward sloping terrain, a collective reduction performed too quickly can cause the aircraft to roll over on the other side, down the slope. Care must be exercised when performing slope operations, but dynamic roll over can occur on the flattest of surfaces if the pilot becomes complacent.




It is normal practice to tackle a slope from the side and not from the front or back because most helicopters have skid type landing gear with no brakes. Skid gear will most likely slide down a hill if the toes or heels of the skids are pointed up hill once the power is taken away holding the aircraft in place.

Once that force is no longer applied, the weight of the aircraft will get it started sliding and, depending on the slope, could pick up so much speed that it crashes severely at the bottom of the hill. The ones that have wheels and brakes could slide also depending on the degree of slope and condition of the ground.




Other reasons not to attack a slope from the front or back is that the tail boom may strike the hill before the skids do (Again, depending on the degree of the slope) or the rotor system may impact the hill before the skids do. Usually, if the standard 8 degrees of slope are used as a maximum, then a sideward approach to the slope will have the skids touching before the rotor system. Care should be used when passengers depart the aircraft on a slope as they may walk into the rotor if they go up hill. Always brief the passengers to leave the aircraft on the down slope side of the aircraft.

Settling With Power

Settling with power can be a dangerous condition that any pilot may face, and if he or she is not on their toes, it may cause a serious uncontrollable situation. Settling with power is basically when the helicopter settles into the rotor wash produced by its own main rotor system. It requires 3 key elements to occur, and these conditions should be avoided in combination with one another.

These are: A near zero airspeed, up to 100% power applied, and a better than 300 foot per minute rate of descent. Once you have all of these situations in occurrence, the aircraft will settle in its own down wash from the rotor system. The only way to recover is to gain forward airspeed and allow the rotor system to fly into "Clean air". Once the rotor system is clear of the rotor-wash, it will become efficient again, and the settling with power conditions will cease to exist.

This can become a real problem at an out of ground effect hover (Above 10 feet from the ground), and during landings.

'Settling With Power' or 'Settling in your own downwash' is a dangerous situation that any rotary wing machine can experience. The term "Vortex Ring State" is used to describe the actual swirling of the air within the rotor system itself that causes "Settling With Power". Vortex Ring State can begin to occur when you have 300 Feet per minute (FPM) as a rate of descent. Pilots need to be aware of the situation and avoid it at all costs.

Counter-Rotation Vs Contra-Rotation

One thing that people often get confused with is the difference between "Contra-Rotation" and "Counter-Rotation". The terms are used incorrectly more than you could possibly imagine in books, manuals, and on web sites. I wanted to take this opportunity to clear up the difference between the two.


As you can see by the first diagram, "Counter-Rotation" is where there are two individual shafts driving two propellers or rotors in different directions. Although we have chosen to show this example on a CH-47 Chinook from a top view, it is exactly the same on a twin engine airplane that has one propeller turning one way, and one turning the opposite way (Like on a P-38 "Lightning"). Sometimes, as in the case of the CH-47, the rotors will mesh, so the synchronization of the systems is crucial.

On airplanes, where the propellers do not mesh it is not as critical that the systems are in synch. In an airplane, if the systems are out of synch, it can put undue stress on the airframe, and cause harmonic vibrations throughout the airframe. You can usually hear an airplane that has the engines out of synch, as it will make a varying strobe like sound.

Each propeller in an airplane counter rotating system has its own set of mechanical controls to vary the pitch of the blades. Often it is a hydraulic system, but in some cases (Like the P-38), other means can be employed such as electric power. In a helicopter, both rotors are manipulated by one set of controls for the pilot.


contra-rotation
"Contra-Rotation" is where the propellers or rotors are mounted "Co-Axially", meaning one in front of (or on top of) the other on the same axis. Usually, the drive mechanism is a single source, but the direction of rotation is spilt by a gearbox to drive the two systems in opposite directions. This is usually done to reduce the "P" factor or "torque" in a turn. While we have chosen to show this example in the form of a Royal Navy Fairey Gannet, it also applies to helicopters (Like on the Soviet "Hokum").

The main use for this on a helicopter is that it negates the need for a tailrotor (Anti-torque rotor) to maintain directional control at a hover. It also tends to relieve some of the effects of retreating blade stall as both sides of the aircraft have advancing rotor blades.

In an airplane, one set of controls will adjust the pitch of both propellers at the same time. Usually, it is done by varying hydraulic pressure in the propeller hubs. In a helicopter, both rotors are manipulated by a single set of pilot controls as well, but two sets of control tubes working off of two alternately rotating swashplates are needed to adjust the rotors at the individual hub.

Dissymmetry of lift

One cannot begin to talk about the mechanics of helicopters until the problems associated with rotary wing aerodynamics are understood. When the first rotary wing pioneers started trying to make a helicopter fly, they noticed a strange problem.

The helicopters rotor system would generally work just fine until one of two things happened: Either the aircraft began to move in any given direction, or it experienced any sort of wind introduced into the main rotor system. Upon either of these events, the rotor system would become unstable, and the resultant crash would usually take the life of the brave soul at the controls. The question then was; Why does this happen? The answer is what we refer to today as "Dissymmetry of lift".

What "Dis-Symmetry of lift" means is, when the rotor system is experiencing the same conditions all around the perimeter of the rotors arc, all things are equal, and the system is in balance. Once the system experiences a differential in wind speed from any angle, it becomes unbalanced, and begins to rotate. Take for instance forward flight. Imagine a two bladed rotor system spinning at 100 MPH.

The blade moving toward the forward end of the aircraft is going 100 MPH forward, and the blade moving toward the back of the aircraft is travelling at 100 MPH in the other direction. This is just fine when the aircraft is not moving or is in a no wind condition. It is experiencing 100 MPH of wind in all directions, so it is totally in balance. Once the aircraft moves forward, it begins to change this balance. If we travel 10 MPH forward, then the forward moving, or advancing rotor blade, is experiencing 110 MPH of wind speed, and the rearward, or retreating blade, is experiencing only 90 MPH of wind speed.

When this happens, we get an unbalanced condition, and the advancing blade experiencing more lift wants to climb, while the retreating blade experiences less lift and wants to drop. This is where we get the term "Dis-Symmetry of lift". The lift is not symmetrical around the entire rotor system.

How do we compensate for this situation? We compensate by allowing the rotor to flap. By allowing the advancing blade to flap upward, and the retreating blade to flap downward, it changes the angle of incidence on both rotor blades which balances out the entire rotor system. As you can see in this simple graphic, there are a few ways to allow for blade flapping.

One is to allow the blades to flap on hinges (Articulated rotor system). Another way is to have the whole hub swing up and down around an internal bearing called a trunion (Semi-rigid rotor system). Unfortunately, we can not compensate completely for dis-symmetry of lift by using blade flapping. Once the aircraft gets to a certain airspeed, and the rotor had flapped as much as it possibly can, then "Retreating blade stall" may be experienced. In retreating blade stall, the retreating blade can no longer compensate for dis-symmetry of lift, and the outer portions of the blade will "Stall".

This situation, when not immediately recognized can cause a severe loss of aircraft controllability. This is a major airspeed limiting factor for helicopters. For many years, aeronautical engineers have tried to figure ways to eliminate this problem and increase the forward airspeed for single rotor helicopters. Although many breakthroughs have been made, the manufacturers of single rotor helicopters are usually not willing to change the entire design on their products because of the extra costs involved for little airspeed payoff. Most have resigned themselves to slower airspeeds for their aircraft, at a lower cost and less maintenance.

Controls in a RC Helicopter

Collective: The up and down control. It puts a collective control input into the rotor system, meaning that it puts either "all up", or "all down" control inputs in at one time through the swash plate. It is operated by the stick on the left side of the seat, called the collective pitch control. It is operated by the pilots left hand.

The collective lets you change the angle of attack of the main rotor simultaneously on both blades.

Cyclic: The left and right, forward and aft control. It puts in one control input into the rotor system at a time through the swash plate. It is also known as the "Stick". It comes out of the centre of the floor of the cockpit, and sits between the pilots legs. It is operated by the pilots right hand.

The cyclic changes the angle of attack of the main rotor's wings unevenly by tilting the swash plate assembly. On one side of the helicopter, the angle of attack (and therefore the lift) is greater.

Pedals: These are not rudder pedals, although they are in the same place as rudder pedals on an airplane. A single rotor helicopter has no real rudder. It has instead, an anti-torque rotor (Also known as a tail rotor), which is responsible for directional control at a hover, and aircraft trim in forward flight. The pedals are operated by the pilots feet, just like airplane rudder pedals are. Tandem rotor helicopters also have these pedals, but they operate both main rotor systems for directional control at a hover.

Here are some of the component parts that make up a helicopter. While this is an example of one specific helicopter (UH-1C), not all helicopters will have all of the parts listed here. Some of this may be a bit more of the same old stuff we have just discussed, but it will show everything as it relates to everything else on the aircraft and the location of each component. Just mouse over the grey spots to see the explanation of the parts of the helicopter below.

The Tail Rotor

The tail rotor is very important. If you spin a rotor using an engine, the rotor will rotate, but the engine and the helicopter will try to rotate in the opposite direction. This is called TORQUE REACTION

The tail rotor is used like a small propeller, to pull against torque reaction and hold the helicopter straight.

By applying more or less pitch (angle) to the tail rotor blades it can be used to make the helicopter turn left or right, becoming a rudder. The tail rotor is connected to the main rotor through a gearbox. When using the tail rotor trying to compensate the torque, the result is an excess of force in the direction for which the tail rotor is meant to compensate, which will tend to make the helicopter drift sideways. Pilots tend to compensate by applying a little cyclic pitch, but designers also help the situation by setting up the control rigging to compensate. The result is that many helicopters tend to lean to one side in the hover and often touch down consistently on one wheel first. On the other hand if you observe a hovering helicopter head-on you will often note that the rotor is slightly tilted. All this is a manifestation of the drift phenomenon.

Main Rotor System

Root: The inner end of the blade where the rotors connect to the blade grips.

Blade Grips: Large attaching points where the rotor blade connects to the hub.

Hub: Sits atop the mast, and connects the rotor blades to the control tubes.

Mast: Rotating shaft from the transmission, which connects the rotor blades to the
helicopter.

Control Tubes: Push \ Pull tubes that change the pitch of the rotor blades.

Pitch Change Horn: The armature that converts control tube movement to blade pitch.

Pitch: Increased or decreased angle of the rotor blades to raise, lower, or change the
direction of the rotors thrust force.

Jesus Nut: Is the singular nut that holds the hub onto the mast. (If it fails, the next
person you see will be Jesus).

RC Heli Rotor

A RC helicopter main rotor or rotor system is a type of fan that is used to generate both the aerodynamic lift force that supports the weight of the helicopter, and thrust which counteracts aerodynamic drag in forward flight. Each main rotor is mounted on a vertical mast over the top of the helicopter, as opposed to a helicopter tail rotor, which is mounted on a tail boom. A helicopter's rotor is generally made up of two or more rotor blades. The blade pitch is typically controlled by a swashplate connected to the helicopter flight controls.

Blade Structure

1. A helicopter blade comprising:

(a) an elongated metallic nose spar having a closure member therefor to form a nose spar assembly of material having a high modulus of elasticity and having upper and lower trailing flanges closed by said member with a coupling bushing directly connected to said assembly for direct transfer of primary loads from said spar to said bushing,

(b) a trailing skin structure attached to and supported from said nose spar assembly,

(c) bands of unidirectional fiberglass material having a lower modulus of elasticity than said assembly and extending the length of the blade and bonded to the upper and lower interior surfaces of said nose spar assembly with a loop formed at the root of said blade by said bands and attached to said bushing for secondary transfer of load from said blade, and

(d) grip means engaging said bushing to couple said blade to a helicopter rotor mast whereby said nose spar assembly normally will operate at high strain levels and said bands operate at low strain levels until failure of said nose spar assembly and thereafter maintain the load.

2. In combination, a helicopter blade having an elongated nose spar extending the length of the blade forming the primary tensile member, and a trailing blade structure attached to and supported by said nose spar, the improvement which comprises a closure channel for said nose spar secured to the trailing edges of said nose spar and the leading edges of said trailing blade structure to form a nose spar assembly, unidirectional fiberglass bands secured to the inside of said nose spar assembly and extending the length of said blade to form a secondary tensile structure to carry the centrifugal load of said blade should said nose spar fail and means to receive centrifugal loads directly from both said assembly and said bands.

3. The combination of claim 2 wherein a plurality of external doublers extend the width of said blade near the inboard end to transfer the loads at said inboard end.

4. The combination of claim 2 wherein the modulus of elasticity of said band is substantially less than the modulus of elasticity of said nose spar.

5. The combination of claim 2 additionally comprising a connecting sleeve on the inboard end of said blade, whereby said blade may be connected to a helicopter, means attaching said sleeve to said nose spar, and means attaching said band to said sleeve.

6. The combination of claim 5 wherein said means connecting said band to said sleeve comprises a loop formed in the end of said band and positioned around said sleeve.

7. A redundant helicopter blade in which an elongated nose spar extends the length of said blade and forms the primary tensile member, the combination comprising:

(a) a spar closure channel having upper and lower flanges which overlap the inside surface of the trailing edges of said nose spar;

(b) a trailing blade structure supported from said spar through said closure channel; and

(c) unidirectional fiberglass band secured to said upper and lower flanges and extending the length of said blade to form a secondary tensile member.

8. The combination of claim 7 in which the glass fiber in said unidirectional fiberglass band is in the form of a flat strap and is secured to the inside upper and lower flanges of said closure channel.

9. The combination set forth in claim 7 in which the modulus of elasticity of said fiberglass band is substantially less than the modulus of elasticity of said nose spar.

10. The combination set forth in claim 7 additionally comprising a connecting sleeve on the inboard end of said blade, means attaching said sleeve to said nose spar, and means attaching said band to said sleeve.

11. A helicopter blade comprising:

(a) an elongated metallic nose spar having a closure member therefor having upper and lower inner flanges to form a nose spar assembly of material having a high modulus of elasticity and having upper and lower trailing flanges closed by said member with a coupling bushing directly connected to said assembly for direct transfer of primary loads from said spar to said bushing,

(b) a trailing skin structure attached to and supported from said nose spar assembly,

(c) bands of unidirectional fiberglass material having a lower modulus of elasticity than said assembly and extending the length of the blade and bonded to the upper and lower interior surfaces of said upper and lower inner flanges with a loop formed at the root of said blade by said bands and attached to said bushing for secondary transfer of load from said blade, and

(d) grip means engaging said bushing to couple said blade to a helicopter rotor mast whereby said nose spar assembly normally will operate at high strain levels and said bands operate at low strain levels until failure of said nose spar assembly and thereafter maintain the load.

Flying Hobby RC Helicopters

With hobby-grade RC helicopters there are many more actions that the pilot can do and needs to perform to keep the helicopter aloft. Variable pitch rotors and other design features allow the helicopters to do more diving, climbing, rolls, and loops in addition to going up and down and hovering. These actions along with adjustable speed make hobby helicopters extremely challenging to fly but also more exciting.

Transmitters for hobby RC helicopters may come with many channels to control basic helicopter functions, provide more precise control of mixed actions, and change settings on the helicopter from a distance; but, for basic flight four or five channels is normal.

All four or five channels are activated with just the two sticks on the transmitter. The movements typically controlled by a 5-channel transmitter are:
Throttle
More throttle equals more power and speed. Less throttle slows down the helicopter.

Main rotor up and down movement
The collective keeps the pitch of the main rotor blades level with the fuselage and allows for the ascent and descent of the helicopter.

Tail rotor side to side movements
The tail controls yaw -- keeps the helicopter from spinning around and around. The tail rotor also acts like a rudder for turning.

Main rotor forward or backward tilt
The elevator or cyclic pitch controls forward and backward movement and altitude (diving and climbing) when in flight.

Main rotor left and right tilt
The aileron or cyclic roll causes the helicopter to bank left or right or roll to the left or right.

Controlling RC Helicopters

What you can do with an RC helicopter (such as going up and down) are actions initiated by radio signals from the transmitter. The number of channels on a transmitter tells you the number of actions that you can control on the RC.

These actions usually involve things like changing the pitch (tilt) of the rotor blades or making the blades spin faster. A hobby-grade RC helicopter normally requires at least four or five channels for normal flight that closely mimics the controls and flight of full-size helicopters. Toy-grade helicopters may have only 2 or 3 channels and much more limited actions.

Flying Toy RC Helicopters

The typical toy heli is a 2- or 3-channel model that can fly up and down, maybe forward and sometimes backward, and go left and right. It may run at a constant speed. It can hover in place but it's probably not going to be able to do high speed chases, loops and rolls, or inverted flight.

In order to provide more stable flight, the tail may not have the familiar tail rotor and blades of real helicopters that are set perpendicular to the main rotor. Instead they often have fixed pitch, counter-rotating dual main rotors (ringed for safety). These rotors eliminate the need for the operator to use tail rotor controls to counteract a natural phenomenum of helicopter flight that makes the body of the helicopter want to spin around and around.

Because the main rotors are fixed pitch (blades don't tilt independently), there are no cyclic controls -- tilting of the main rotor -- for climbing and diving or doing banking turns. Instead, the dual main rotors provide level turning. Some models have a small rotor on the tail (parallel to the main rotors) or vertical rotors in other locations that control forward flight and provides further stability.

These design changes sacrifice some of the maneuverability found in hobby-grade helicopters but it also means that the pilot needs to perform fewer actions to keep the helicopter in flight. Simpler controls, slower speed, and less aerobatics ability makes these toy helicopters easier to fly and provide children and novice pilots with more entertainment value. It doesn't mean that you can master RC helicopter flight right out of the package though. Even with the toy helis it takes patience and practice to hover, fly around the room, and land upright.

For a step up from toy helicopters but with the stability features that make for easier flight, consider a hobby-grade Blade CX. It provides easier hovering and control but has the advanced features of hobby helicopters.