There are many forces at work when a helicopter flies, and many are specific to helicopter flight. We will touch on some of these briefly. We all know about lift, drag, gravity, and thrust, so discussion of these would not really be necessary. I would rather talk about specific conditions experienced exclusively in rotary wing flight. Here are some examples.
Translating tendency is defined by the textbooks as: The tendency for a single rotor helicopter to drift laterally, due to tail rotor thrust. One may not think about how much thrust is produced by the tail rotor, but we must remember that the tail rotor has a 6 to 1 rotational ratio to the main rotor system. It actually spins 6 times faster than the main rotor, so it can compensate for the torque of the main rotor without the need for a massive tail rotor span. The thrust it produces tends to push the aircraft sideways at a hover. We compensate for this by adding left cyclic control inputs (On American Helicopters, the opposite in foreign manufactured aircraft, because their rotor systems turn the opposite way from ours). This makes the helicopter hang left skid, or wheel, low at a hover. If you ever see an American helicopter hovering, you may notice this left side low condition. If you ask a helicopter pilot how he is doing, and he answers, " Left skid low", that means everything is normal.
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.
I have personally experienced it during night operations while taking off from a pinnacle. We started to descend once we left the ground effect of the hill top we were departing from. Because it was at night and we were not under night vision goggles, it was hard to recognize the situation. Corrective action was applied, and we flew out of the situation without incident. A good lesson was learned that night without a major accident taking place.
If you go to the section of this web site that discusses the V-22, you will read a lot about "Vortex Ring State", "Settling With Power" (Army), or "Power Settling" (Navy). Unfortunatly, the different branches of service have different meanings for the latter two terms. The Navy uses the term "Settling With Power" as a means to explain how high temperatures and high humidity will reduce the available power a helicopter can generate and use. The Army uses the term "Power Settling" to describe the same phenomena.
In addition, the Army uses the term "Settling With Power" to describe the "Settling in your own downwash" phenomena where the Navy calls the same exact thing "Power Settling".
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" (Army) or "Power Settling" (Navy). Not only that, but the Army manuals say that Vortex Ring State can begin to occur when you have 300 Feet per minute (FPM) as a rate of descent. The Navy says 800 FPM is a more accurate figure. Of course, I would always side with the lower number as being the more safe. As you can see, this gets very confusing when the branches interact with one another or manuals are made by a company that normally deals with one specific branch of the military. Are you confused yet?
How ever you wish to describe it, and which ever terms you wish to use, it is a dangerous situation that any rotary wing machine can experience. Pilots need to be aware of the situation and avoid it at all cost. For more information, see the V-22 section. There is a copy of the Congressional hearings which discuss the phenomena covered here.
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 maneuver. 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.
Someone asked me once why we attack a slope from the side and not from the front or back. The first thing one must remember is that 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. (IE: Wet, muddy...).
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.
There are many famous photos of Chinooks with the back wheels on some building or hill top. That is a standard slope landing for a tandem rotor helicopter.
I had a volunteer from the local aviation museum contact me asking why the rotor system on a display helicopter they had was not set up properly. It was a home built helicopter and they were not sure that the person who built it knew what they were doing. It seemed that the control inputs were 90 degrees off from where he thought they should be. I informed him that what he was seeing was normal. It was a phenomenon called "Gyroscopic Precession" and the helicopters rotor system was actually built properly.
A rotating body acts like a gyroscope and the forces that act upon the gyroscope require some adjustment to allow for the rotation itself. A spinning body will take inputs placed at one part of the cycle of rotation and react later in the cycle of rotation. Now without getting too technical, the main thing to remember here is that with the rotation comes some extra planning. If you want a control input to take effect, you just have to be a little ahead of where you want it to happen. In this case, 90 degrees before the spot where the action you desire is to take effect is where you have to plan to put it into the system. Input is placed in one location and as the blade swings 90 degrees more in the direction of rotation, the desired effect will be realized.
An interesting side note: When I went to the museum to look at the helicopter on display, they had a podium there with the specifications of the display helicopter. On the podium was one of the graphics from this site that I had created with the breakdown of the parts of the helicopter (Each individual part labeled). It was a nice surprise to walk into the local museum and find my graphics being used to teach their visitors.