A constant speed propeller is required to achieve the maximum performance of any high performance aircraft design.
For example, how well do you think your car would perform if it was stuck in only one gear? Let's say you only had the equivalent of 1st gear. Driving at 55 mph on the highway would not be possible or practical. Conversely, how about starting from a stop in 5th gear? If possible, the acceleration would be anything but ideal. This is exactly what takes place with any airplane operating with a fixed pitch propeller.
The typical aircraft engine will develop full power around 2700 rpm, but it is not uncommon for a fixed pitch cruise propeller to only turn up 2100-2200 rpm's on take off. This means the airplane is only able to use 65% of its maximum horsepower available on take-off, and this is when you need horsepower the most.
With the same fixed pitch propeller, a 260 hp engine will only develop slightly over 170 hp, radically and adversely affecting T/O performance. It doesn't matter how fast your new speed machine is if it can't clear the trees at the end of the runway!
Now, can you imagine how your airplane will accelerate and take-off with an extra 90 to 100 HP? Take-off distance can be reduced by as much as 40% with a constant speed propeller! This is what a constant speed propeller can do for your airplane.
Rate of climb is a function of excess power. Required horsepower over and above that of Vmins (minimum sink airspeed) for level flight translates to power used for climb. A constant speed propeller allows the engine to develop full power at any climb speed (Vx, Vy) thus maximizing climb performance. Rate of climb can be increased by as much as 35% with a constant speed propeller!
By adjusting the manifold pressure and rpm, this will allow the pilot to choose a wide range of power settings whether for maximum endurance, maximum range, or maximum speed. Only a constant speed propeller gives the pilot these options.
Constant speed propellers make approaches and landings easier and safer. In flight, an engine at idle will create drag due to the slow turning propeller. The less pitch the propeller has the more drag created and conversely, the more pitch the propeller has the less drag created.
On the approach to landing sequence, the constant speed propeller is in low pitch (more drag), thus allowing the pilot the option of a steeper more accurate approach than that of a fixed pitch propeller. The landing roll is reduced with the added propeller disc drag.
Not only does a constant speed propeller increase the total aircraft performance, but it also decreases the pilot engine management workload.