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 This page is intended for college or high school students. For younger students, a simpler explanation of the information on this page is available on the Kids Page.

Most modern passenger and military aircraft are powered by gas turbine engines, which are also called jet engines. Jet engines come in a variety of shapes and sizes. To evaluate the performance of a turbine engine, we have to determine the thrust generated by the engine, the fuel consumed to produce the thrust, and the weight of the engine itself. The weight of the engine is important because it contributes to the overall weight of the airplane. Aircraft range, rate of climb, and maneuvering capability depend on the thrust to weight ratio of the aircraft.

On this page we present a simple model for estimating the weight of a gas turbine engine. The model is used in the EngineSim computer program. The modeling method is the same for each type of gas turbine engine, although the number of components and the values of some parameters are different for each type of engine. On this page we use a turbofan engine as an example. In general, the weight W of any object with a uniform distribution of material of density r is equal to the density times the volume of the object:

W = r * V

For our model, we represent each major component of the engine by a cylinder. Each component has a distinct length L and diameter d. The volume of a cylinder is:

V = pi * L * d^2 / 4

where pi is the ratio of the circumference to the diameter of a circle and equal to 3.1415...

If we know the volume and the density of each major component, we can calculate the weight wn of each component. The weight W of the engine is the the sum of the weights of the components:

W = sum (wn)

On the figure we use the Greek letter "sigma" to denote the sum over all the components.

For our model, we must determine a density rn for each component. Using the compressor as an example, it is obvious that in the real component the material is not evenly distributed. There are rows of blades separated by air spaces and connected to a central shaft. The blades and the shaft are made of different materials. The external cowling is made of a third material. For our model, we are going to represent all of the different materials by just one component average material. We determine the density of the component average material by calibrating our model using the weight and volume of the component for an existing engine. If we are using EngineSim, and decide to change the material of the compressor from titanium blades to aluminum blades, we change the compressor average material density by multiplying by the ratio of the density ofaluminum to the density of titanium, and then re-compute the weight. Here is a table of the density of materials which are used in jet engines:

 Material T Lim K Density kg/m^3 Aluminum 500 2726 Titanium 833 4693 Stainless Steel 1111 7633 Nickel Alloy 1388 8252 Nickel Crystal 1666 8252 Ceramic 1666 2630

We have included the temperature limits on the table because weight is only one consideration in picking the materials used in an engine. EngineSim checks the computed temperature in each part of the engine cycle against the temperature limits of the component and signals the user if a temperature limit is exceeded.

The model which is given here accounts for the change in weight for both geometric changes (change in length or diameter of the component) and for a change in material for the component. Notice that the procedure outlined here is just a way to estimate the weight of a new engine. It is just a model, but it gives the engineer an indication of how the weight changes based on design decisions. Engineers often use estimates and models when making design decisions. The estimate has to be checked during testing. In the case of aircraft engine weight, this value is closely monitored during design refinement by actually measuring the weight of the components.

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