Why use Firm Dampers?
A firm damper forces the helicopter to react faster to cyclic inputs, a.k.a. the fuselage will follow the rotor head faster, while a softer damper will react a little slower to the input. Please note that the firmness of the damper only affects the initial movement. For example, if two pilots started doing continuous flips with equal machines and setups with different dampers (remember, this is hypothetical as no two machines and pilots are alike) the one with the harder dampers would react faster, but both machines would be flipping at the same speed after the initial input. To keep it simple, firmer dampers will give quicker response to an input, but do not affect the total end response.

Why use Soft Dampers?
After reading all the benefits of a firm damper above, one might think "Why would I consider softer dampers?" There are numerous reasons why a pilot would want softer damping. When dampers and blades are too stiff, the heli will wobble and nod in the hover and slow maneuvers. Also, a heli with stiff dampening is very twitchy and reacts to even the slightest transmitter stick movement, such as a pilot's jitters. Softer dampers are very stable in the hover and in nice slow moves, such as figure eights and scale style flying. Softer dampers will give the heli flight characteristics closer to a full size helicopter--not too many full sizes do 3D! Beginners tend to prefer softer dampers as the heli is smoother and slower to react, giving the pilot time to adjust and think. The main concerns with going too soft include possible boom strikes due to the retreating blade being forced down to the boom by a large cyclic input/low head speed, and more flutter in flight due to the blades flapping to a stall angle.

Here is a real life example of a heli and its damper choices. The Hirobo Freya Evo helicopter has two types of dampers available from the manufacturer, the stock ones are a 60 durometer and they also offer a set of 3D ones that are rated at 80 durometer. For a heavy 3D pilot, the 80 durometer would be the best choice, while an FAI precision type pilot or beginner may want the 60 durometer units.

Damper Types
There are a lot of damper options out there for most helis, including a number of aftermarket sets. Most manufacturers offer hard and soft version of their dampers. These will usually look alike but be of a different color. Some companies use O-rings, while others use soft plastic. The aftermarket companies offer an array of options, including:

Hard plastic dampers with O-rings on the outside. These dampers are about as stiff as it gets. The O-rings provide the stability needed for hovering and flight, but in a hard maneuver they will contract and the plastic bottoms out in the head block giving instant response. These types of dampers run very well in 3D flight. Their main drawback would be the fact that they can be damaged more easily when a crash occurs.

O-ring upgrades. These dampers are usually of a harder durometer rating than what the manufacturer offers and they are the whole damper. They generally consist of two or more O-rings that the spindle rides inside the head block. Available in many durometer ratings that are suitable for all flight types.

O-rings in a sleeve. Some manufacturers offer sets that have a number of smaller O-rings inside a metal sleeve that goes into the head block. These work much like the ones above.

With so many options out there for dampers on most machines, there will be a set that will make your heli fly the way you want it to. As to which to choose, look at your flight style and what other pilots with the same heli are using, and that should be a great place to start.

How to Change and Install Dampers
Dampers are generally installed at each end of the head block with the spindle riding inside them. There are some helis that have the dampers in the head block at a teetering point, but the general installation and setup is the same. Most dampers should be lubricated with a type of grease that won't eat the rubber. I use dielectric tune up grease (found at an auto parts store) with great results.

1) Coat the inside and outside of the damper with a thin layer of the grease and install into the head block.

2) Next, lube the spindle shaft with the same grease and slide it into the dampers. I like to put a piece of paper towel or cotton swab into the spindle at the end to keep any grease from getting in the bolt threads--I'd hate to have a spindle bolt free itself in flight--very ugly. The grease allows the spindle to move in the head block so it can center in flight.

3) Install the blade grips and it is ready to go. There are some dampers made with a special plastic that should not be lubricated, and as always, see the instructions first.

Dampers need to be checked often for wear and damage. The main rotor takes some abuse, and the dampers carry the whole assembly! To check the dampers without removing anything, simply grab the blade grips and rock them in the head block. If there is significantly more movement than when the dampers were new, it may be time to replace them. The heli can also exhibit tracking and fluttering problems when dampers are worn out, so watch for those signs. As always, check them after a crash too.

Conclusion
Dampers play a very key role in the RC helicopter world. Good dampening systems contribute greatly to the stability and predictability of the modern heli. Adjusting the dampers on the helicopter can have a great affect on how it flies and gives the pilot another place to fine tune a machine to his/her liking. Enjoy!