Building your own homebuilt aircraft means dedicating a huge amount of time, perseverance and resources into your personal aircraft project. Do not think that this can be done without too much trouble, it will not. It will take a lot of effort, time and dedication to finish and get flying.
Propellers create thrust from the power of the engine thereby moving the machine forward and in doing that the engine uses fuel from the fuel tanks. And this process is our subject here.
The power from an engine, be it an internal combustion engine or electric, must be transformed to useful thrust. Or else there isn't much flying going on. Propellers are used for that purpose. They come in two, three or more blades and can be made from metal, wood or composites. This depends (among other things) on the designer or on how much power, torque and RPM the engine has available.
Most small aircraft have two blades, sometimes three or more. The limitation for a propeller is blade length, the tip speed may not exceed the sound barrier at the maximum RPM. The resulting noise is deafening and thrust is lost. But with more engine power you will need more blades to absorb that and convert this power into useful thrust. Changing from a two bladed propeller to a three bladed with the same engine means the blade length will be shorter so the engine can reach it takeoff RPM / output power again. Vibrations are also less with a three bladed propeller.
My experience in a Tecnam P92 replacing the two blade fixed pitch propeller with a three bladed fixed pitch model was that takeoff RPM increased from 5000 to 5200, meaning more power from the engine at takeoff and at the same time cruise speeds stayed the same: 4800 RPM at 90 kts. The smaller blades also made less noise on takeoff! Win win win!
This lower noise effect is confirmed by Hartzell Propeller: "Will a 3-blade prop make my airplane quieter inside? Yes, in most installations increasing the number of blades helps to reduce noise. Cockpit noise comes from a variety of sources; engine, exhaust, air flow around the fuselage, and the propeller. Vibrations are also perceived as noise in the cockpit. In a single-engine airplane, the propeller blade wake will beat on the windshield producing noise. A 2-blade propeller produces two pressure pulses per revolution, where a 3-blade propeller will produce three smaller pulses per revolution (for the same amount of total thrust) which is inherently smoother and therefore quieter. The 3-blade propeller will generally have a smaller diameter than the 2-blade propeller that it replaces, which also reduces the tip speed and noise. In a twin-engine aircraft, the reduced diameter of the 3-blade propeller will result in less tip-generated noise and a greater clearance between the blade tip and the fuselage. Both of these characteristics will reduce cabin noise.". Source: https://hartzellprop.com/faq/technical-questions/
Propeller blades can be fixed (for climb or cruise purposes) or adjustable (on the ground or during flight). Adjustable propellers have an advantage over fixed types: they can be set for start, climb and cruise RPMs. Constant-speed models keep the preset RPM constant relieving the pilot from this task.
These propellers are controlled by either an electric motor in the spinner or a cylinder actuated by oil pressure from the engine. The electric types can be made rather sophisticated by a microprocessor controller with settings like: feather, takeoff, cruise, climb, manual control and hysteresis function. All values can be set by the pilot for the right conditions.
Choosing the right propeller for your aircraft is a difficult task as a number of variables, like engine RPM, torque, power, noise limitations (regulatory) type of aircraft (its ideal cruise and climb speeds, ground clearance) influence each other so that usually the chosen propeller design is a sum and compromise of all these variables.
Read more about this subject in our aircraft propeller basics section or click the next link for information about a number of propeller manufacturers.
Piston powered aircraft generally consume AVgas, aviation gasoline. It is like a high octane auto gas, only more expensive (in Europe about 10 USD/gallon), high lead (despite its LL, low lead, designation) and it is more controlled and more stable. Problem is that the future of AVgas is not certain, but I have been hearing that for 20 years now. No surprise that diesel engines are getting more and more popular. These engines can use JET-A(1), road diesel, agricultural (colored red, if you can find it) diesel and even biodiesel is possible.
Small airplanes like the Pipistrel, Pelican PL, Zenair types and the likes mostly have a Rotax 912, 912iS or the 914/915 turbo engine capable of running on Mogas. Much cheaper but not certified and the fuel is not controlled the way AVgas is. Some use an additive in the fuel when using AVgas, it helps with reducing the lead deposits in the engine. The composition of Mogas can vary daily but AVgas hasn't changed in the last decade or two. There are also some issues with bio-alcohol in Mogas.
That being said, I have been flying for years with Mogas and found no real issues apart from the alcohol in some type of aircraft.
Usually, high wing models like your average Cessna, Murphy etc. have a fuel tank in each wing. Aluminum fuel lines run then to a fuel selector in the cabin with Left, Right, Both (not always) and Off selections. The Cessna 150/2 has an On, Off type of selector and both tanks feed the engine at the same time. Fuel is gravity feed and sometimes a low pressure fuel pump is added to aid starting and used when switching tanks during flight and on takeoff and landing.
Low wing models normally would have a tank in each wing, fuel selector in the cabin and have an separate electrical fuel pump. Gravity feed is more difficult as the wing (and fuel) sits lower than the engine and the pump needs to push the fuel to the engine. There are some type of aircraft (Lancair has one) that have an extra header tank behind the instrument panel or other location in the aircraft.
Some aircraft have extra or auxiliary tanks in the wings or fuselage enabling them to fly extended ranges or for longer times. Fuel in these tanks is pumped to the main tanks via separate fuel pumps. More on this to be found in the fuel management section.
Written by EAI.
Experimental Aircraft Info
