Aircraft Engine Maintenance 3-Spool-Engine Concepts
What is triple spool design on the RB211? what are the advantages/disadvantages?
A 3 spool engine is one that has three sets of compressors before the combustor and three sets of turbines behind it.
A spool is made up of a compressor and a corresponding turbine used to extract the power from the exhaust gasses to turn the compressor.
Each spool is given a name. N1, N2, and N3. N1 is the large fan section in front of the engine. N2 is the low pressure compressor section. And N3 is the high pressure compressor section. Some engines incorporate N2 And N3 into one rotating mass and call it N2. Hence the double spool engine.
Each section of the compressor wants to rotate at it’s own speed, and if allowed to do so as in a triple spool engine, it is able to operate more efficiently. It can turn at it’s optimum speed, and not have to compromise between the optimum speed for the N2 and N3 sections when attached in the double spool engine.
All modern engine have 2 sets of compressors (HP and LP) and a fan section providing a vast majority of the thrust.
In a 2 spool motor the HP section and the LP section are joined. The number of spools in an engine tells how many sets of compressor blades and corresponding sets of turbine blades the engine has. A single spool engine has one set of each, a double spool engine has two sets of each, and a triple spool engine has three sets of each.
3 Spool Engine advantages and disadvantages:
- More sets of blades results in a greater engine weight, but the corresponding increase in thrust possible more than offsets the weight increase.
- Major advantage of triple spool engine design is it’s ability to minimise engine surges
The drawbacks of a 3 spool engine are increased weight, complexity, and cost to purchase and overhaul, but they are the most efficient engine flying.