science

Power transfer unit

Power transfer unit is a science topic covered in the lgStudy science library. This page brings together a partial reference excerpt, illustrations, worked examples, real-world applications and a short study plan, so you can understand Power transfer unit rather than just read about it. In short: In aviation, a power transfer unit (PTU) is a device that transfers hydraulic power from one of an aircraft's hydraulic systems to another in the event that the other system has failed or been turned off. The PTU is used when, for example, there is right hydraulic system pressure but no left hydraulic system pressure.

Key takeaways

Power transfer unit belongs to science; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect Power transfer unit to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of Power transfer unit from memory before moving on to harder problems.

Reference excerpt

In aviation, a power transfer unit (PTU) is a device that transfers hydraulic power from one of an aircraft's hydraulic systems to another in the event that the other system has failed or been turned off. The PTU is used when, for example, there is right hydraulic system pressure but no left hydraulic system pressure. In this example, the PTU transfers hydraulic power from the right hydraulic system to the left hydraulic system. A PTU consists of a hydraulic motor paired with a hydraulic pump via a shaft. As the connection is purely mechanical, there is no intermixing of hydraulic fluid between the left and right hydraulic systems during PTU operation.

Design philosophy Large transport category aircraft with hydraulically powered flight controls and utilities typically have multiple, independent hydraulic systems powered by a combination of engine-driven and electrically driven hydraulic pumps. Multiple hydraulic systems are typically needed for redundancy, where for instance if one system fails or loses hydraulic fluid, a surviving system may still provide sufficient power for critical systems to continue safe flight and landing. On airliners or business jets with powered flight controls, it is typical to have at least two hydraulic power control units (actuators) for each critical flight control surface — these are the elevators, rudder and ailerons. Only two sources might be used if some form of mechanical reversion is present (i.e. the pilot can still fly the aeroplane manually, but with some difficulty, via mechanical linkages and cables if hydraulic power is lost). On fly-by-wire aircraft, at least three independent power sources are needed. Spoilers and flaps meanwhile are considered secondary flight controls, and may only have a single hydraulic power source, providing the flight control can be deployed symmetrically. Likewise, landing gear, brakes and nosewheel steering are systems which are not considered critical for flight, and are subsequently typically only powered by a single hydraulic system on an airliner or business jet. Where an aircraft utility is powered by a single hydraulic system, PTUs become beneficial in allowing a single source of power, e.g. a pump powered by one surviving engine, to power more than one hydraulic system if the source of power in that system has failed. PTUs only work on the proviso that the system has not punctured and lost its fluid, because they do not permit fluid transfer, only the transfer of mechanical work. For example, on the original design of the Airbus A320, the landing gear hydraulics (extension/retraction, brakes and steering) were solely powered from the green (left hand) system, powered by the left-hand engine driven pump. In the event of a port engine failure during take-off, the landing gear would not be able to retract as there is no auxiliary motorpump in the green hydraulic system on an A320. (Modern A320s have the nosewheel steering powered by the yellow system.) The PTU solves this problem by allowing a rotary mechanical coupling between both systems, so the engine driven pump for the yellow (right hand) system on the starboard engine, which is oversized for normal hydraulic demand, can dump the excess power into the green system via the PTU, and allow powered landing gear retraction to continue, while maintaining hydraulic pressure to the green system flight controls as well. Assuring landing gear retraction in a failure case is one potential assurance provided by a PTU. Alternatively, the designer may elect to have a second electric motorpump perform this role if a PTU is not desired. An additional motorpump may be heavier than a PTU however, and complex trade studies may favor one option or the other, depending on which failure cases are considered and how important weight is in the trade-off.

Uni-directional & bi-directional PTUs On the Airbus A320, the yellow system may power the green system, but because it is also bi-directional, if the starboard engine fails, the green system can help to power the yellow system by dumping excess power into it via the same mechanism. This is also known as a 'reversible' PTU. On some other aircraft, the direction of rotation of the PTU, and thereby the fluid flow through it, may be designed to work in only one direction. The Citation X business jet is one such aircraft with a uni-directional PTU, protected by check-valves and a back-pressure stall line, designed to allow the right hand hydraulic system to assist the left hand hydraulic system and the left hand auxiliary motorpump to retract the landing gear during a port engine failure only. On yet other aircraft, the function of a bi-directional reversible PTU can be accomplished with two uni-directional PTUs installed side-by-side arranged in opposite orientations to each other. The hydraulic system of the CH-47 Chinook helicopter uses twin uni-directional PTUs in this fashion.

In-line & bent-axis PTUs Hydraulic power transfer units are essentially nothing more than a hydraulic motor coupled to a hydraulic pump via a shaft; as such, they can be any kind of motor or pump such as a vane, gear, impeller or an in-line piston, or a variable displacement in-line piston pump. Commonly though, PTUs are paired in-line piston motors/pumps, in either bent or straight axis arrangements. A straight-axis in-line piston pump/motor relies on a canted internal swashplate to drive the piston shoes up and down around the internal piston slipway of the pump, lubricated by the fluid itself — this kind of PTU may appear to resemble two cylinders bolted together, with an inlet and outlet port at either end. An example of a straight axis in-line PTU can be found in the Cessna Citation X hydraulic system. A bent-axis in-line piston pump works the same way, but forgoes the canted swashplate; instead the whole rotating group is tilted to achieve the piston displacement. An example of a bent-axis in-line PTU can be found on the Hawker 4000 hydraulic system. In yet further representations, a bent-axis fixed-displacement motor/pump can be mated with a straight-axis variable displacement motorpump, as in the case of the Airbus A320 PTU.

… excerpt ends here. Continue reading the full article.

Worked examples

Example 1 — a first encounter with Power transfer unit

Start with the simplest possible case. Write down what Power transfer unit claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In science, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to Power transfer unit before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about Power transfer unit ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of Power transfer unit

In research: Power transfer unit appears in science research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses Power transfer unit in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: Power transfer unit is common in secondary-school and first-year university syllabi. It links to neighbouring topics Hydraulics, so understanding it makes those chapters shorter.
In everyday life: Look for Power transfer unit outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

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How to study Power transfer unit in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what Power transfer unit means in your own words.
Compare your version with the excerpt and mark what you missed.
Work through the three examples above with pen and paper.
Explain Power transfer unit out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Power transfer unit in simple terms?

Why does Power transfer unit matter?

Because it connects several science ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study Power transfer unit?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on Power transfer unit.