Engine/Electrical question unrelated to recent events
Posted by NeatPomegranate5273@reddit | aviation | View on Reddit | 5 comments
From my current knowledge of aircraft, the primary generators of electrical power are powered by the engines spinning. If in the case of the Gimli Glider, both engines fail, wouldn't the engine driven generators still produce power as the engines spooled down and the air hitting the engine inlet spins the turbines? I understand that there might not be enough power to run all the systems, but wouldn't there be a decent amount of electricity generated? If any of you guys have watched the F-22 pilot MIT lectures, he says that the air passing through the inlet spins the turbine enough to make the aircraft controls work in case of dual engine failure. *I am deliberately leaving out the contributions of the APU and RAT*
escape_your_destiny@reddit
No, because the engine would be spinning below the minimum control speed of the Constant Speed Drive (CSD), causing the frequency to drop, which would cause the Generator Control Unit (GCU) to disconnect the generator.
The generator needs to spin at a certain RPM to make 400hz AC power. Most generators I know of spin at 12,000 RPM. In order to provide a constant speed to the generator from the variable speed of the engine, the CSD uses a hydraulic piston pump with a wobble plate to drive the generator.
When the engine slows down, the wobble plate on the CSD will tilt to provide more fluid flow. At some point as the engine slows down, the wobble plate will be at full tilt to provide max flow. If the engine slows down further, the generator will start slowing down and drop frequency, at which point it will disconnect, usually at 395hz.
mafidufa@reddit
Aircraft run at 400hz AC? That seems very high. Why not 50 or 60hz like ground based generators?
What voltage are they running?
bbcgn@reddit
According tohttps://fcxinc.com/why-the-aviation-industry-operates-on-400-hz-power/ the higher frequency allowed for smaller and lighter generators leading to increased cargo capacity and reduced fuel burn. Over time the 400 Hz power became standard so now everyone uses it.
escape_your_destiny@reddit
115v 400hz. Allows for smaller generator and transformer windings, making them lighter.
Some planes like the 787 use higher voltage, 200+.
Apprehensive_Cost937@reddit
It depends on a lot of factors.
Airspeed is one, if you're flying at high speed, than the engine might be turning fast enough to still run the IDG (generator) at the sufficiantly high speed for it to produce electrical power.
Type of an engine failure would be another one - if you just have a flameout (which is quite rare with modern engines), then the above is possible. If something has happened to the shaft driving the accessory gearbox (usually N2/high spool), such as a bearing failure/seizure, or the accessory gearbox itself has failed, then the generator wouldn't work, regardless of the airspeed.
Generally with a dual engine failure, you want to fly fairly slowly (around 210-220kts for something like a B737 or an A320, depending on weight) to maximise gliding distance, and at that speed there won't be enough windmilling force for the engines to drive the generators.