As part of the PW1200G LPC redesign I worked on, I had to do a “Cold to Hot” assessment.
When things that fly are designed, aerodynamics are important. That is typically where the design starts. So for compressor blisks, the aerodynamic shape of each blade is defined. Now, the shape of that blade is designed to be that shape under normal operating conditions (aka: jet engine turned on). These normal operating conditions are considered “hot” conditions, and this blade shape is called the “hot shape.” So the “hot” blade is a couple hundred degrees F and attached to a disk spinning at several thousand RPM.
Obviously, it’s designed to move and not stay still, duh. When you make the part though, you cut it out of a block of metal. It would be really hard to do that if the block was a couple hundred degrees and spinning at several thousand RPM. No, you hold the block of metal still, and cut the part out. Well now, it’s 70ºF and spinning at 0 RPM. Here’s the problem. When the blade is 70ºF and still, it has a different shape than when it’s “hot.” This is called the “cold shape.”
So, we need to make sure that the manufactured cold shape will deform into the desired hot shape when the engine is turned on. This way, it meets the aerodynamic requirements and makes the plane fly.
Two big factors come into play here:
- Thermal Expansion
- Centripetal Force
A third factor does come into play, although it has a less significant impact. And that is the pressure on the surface of these blades. It is accounted for, but as I said, has less of an impact than the 2 big factors above.
When materials heat up, they expand. The titanium we make these parts out of is no exception. So when the metal goes from “cold” to “hot”, it expands. When it expands, the blade changes it’s shape.
Pretty much all materials act like the water in the picture above when heated. Including the metal we make these compressor blisks out of. This is a big part of the reason for a cold to hot assessment. Another is centripetal loading on the blade.
A good place to go for an example of centripetal force is the carnival. A good example of what’s happening to our blisk can be seen when we look at the carnival swings. When the ride isn’t moving, all the swings are hanging straight down. When the carny fires it up and it starts to spin, all the swings get pulled outward by centripetal loading.
The same thing happens to our blisk when the engine spools up. The cold blades get stretched out, and the metal actually deforms. To the laymen’s perspective, they don’t deform by much at all. To our analysis team though, the deflection is enough to significantly impact the performance of the engine. That’s why we do this exercise.
How do we know?
We can make a good and accurate simulation of the deflection using Finite Element Analysis. Using FEA we can calculate the deformation of the cold shape when it spins up to hot conditions. Then, an engine test is run. The aerodynamics team measures its performance. If the engine performs as planned, then we know our cold blade is deflecting to the hot shape it needs to be!