The APU Gen.?

Question

I understand that the APU runs to power the Aircraft when the engines are off. But what allows the APU Generator to spin? Fuel? or What is it?

Answer 1

The others have explained what the APU is.But your question is about the APU Generator. the APU Generator is powered by the rotation of the APU/ Gas turbine through the accessory gear box mounted to the APU. Of course the the speed is geared slower than the Turbine RPM . They can be a DC Stater/Generator like the Sun Strand C14 used on the EMB 145 or a AC Generator used on the CRJ 700.

Answer 2

As a couple of the others mentioned, a starter/generator unit is responsible for the initial starting of the APU. Once started and is ready to load, the flight crew can put the APU online and supply the aircraft with electrical power and pneumatics. The APU can supply electrical power up to 41,000ft but only pneunatics up to about 17,000ft.

There are a couple of other things when the APU is started, there is a start power unit which takes the 115 vac (ground power) or 28 vdc (cold start from battery) and converts it to 270 vdc. This 270 vdc is then sent to the start power converter which turns the 270 vdc into 270 vac and sends it to the APU starter generator. There is also a APU ECU (electronic unit) which controls fuel flow and the like to the APU.

Answer 3

The APU is basically a small Jet Engine. It Generates the Power and Air needed to Start the Mains
The APU runs off of the Fuel that the Main Engines run off of.

Answer 4

An aircraft APU is a relatively small, self-contained generator used to start the jet engines, usually with compressed air, and to provide electricity, hydraulic pressure and air conditioning while the aircraft is on the ground. In many aircraft, the APU can also provide electrical power in the air.
Although APUs have been installed in many locations on various military and commercial aircraft, they are usually mounted at the rear of modern jet airliners. The APU exhaust can be seen on most modern airliners as a small pipe exiting at the aircraft tail.

In most cases the APU is powered by a small gas turbine engine that provides compressed air from within or drives an air compressor (load compressor). Recent designs have started to explore the use of the Wankel engine in this role. The Wankel offers power-to-weight ratios better than normal piston engines and better fuel economy than a turbine.

APUs fitted to ETOPS (Extended-range Twin-engine Operations) are a critical safety device, as they supply backup electricity and compressed air in place of the dead engine or failed main engine generator. While some APUs may not be startable while the aircraft is in flight, ETOPS compliant APUs must be flight-startable at up to the aircraft service ceiling. Recent applications have specified starting up to 43,000 ft (≈ 13,000 m) from a complete cold-soak condition. If the APU or its electrical generator is not available, the airplane cannot be released for ETOPS flight and is forced to take a longer route.

A typical gas turbine APU for commercial transport aircraft comprises three main sections:
• Power section
• Load compressor
• Gearbox

The power section is the gas generator portion of the engine and produces all the shaft power for the APU. The load compressor is generally a shaft‐mounted compressor that provides pneumatic power for the aircraft, though some APUs extract bleed air from the power section compressor. There are two actuated devices, the inlet guide vanes that regulate airflow to the load compressor and the surge control valve that maintains stable or surge‐free operation of the turbo machine. The third section of the engine is the gearbox. The gearbox transfers power from the main shaft of the engine to an oil‐cooled generator for electrical power. Within the gearbox, power is also transferred to engine accessories such as the fuel control unit, the lube module, and cooling fan. In addition, there is also a starter motor connected through the gear train to perform the starting function of the APU. Some APU designs use combination starter/generator for APU starting and electrical power generation to reduce compelexity.
With the Boeing 787 being an all electric airplane, the APU delivers only electricity to the aircraft. The absence of pneumatic system simplifies the design, but the demand for hundreds of kilowatts (kW) of electricity requires heavier generators and unique system requirements.

Hope this helps

Answer 5

apu is basically a small jet engine and runs off the same fuel tank as the engines.. Which once running, it can supply electrical power , cabin conditioned air , It has the ability to start the aircraft engines.

Answer 6

On the ground, maintenance and operation of aircraft systems are powered by ground power units (GPU). During preflight, power requirements are provided by the on board APU (Auxiliary Power Unit) a small jet turbine started by the GPU. With this source of power all flight systems can be tested.

Most commercial and large aircraft have another source of power that is deployed from the fuselage and powered by the passing air stream. It's called 'Ram Air Turbine and is for Emergency supply of Electric and/or Hydraulic power. Unlike the APU, 'RAT's' only supply power to essential equipment.

You can probably get good information from Wikipedia for either one.

Answer 7

it is in fact a small jet engine. some of them like AI-9 of russian helicopters may switch its function from generator to the starter - then it provides compressed air for starting the rotation of compressor of the common engine.