A couple engines come to mind, the GE J79/LM1500 family and the RR Avon. These engines, or more specifically, these engine's compressor rotors share a common first design critical, in the 5700-6400 rpm range.
It seems that no matter how well you balance these rotors, it is still a crap shoot as to how severe the FDC will be. Some engines won't even cause the vibe meter to wiggle going through the FDC, others will shake your teeth fillings out. Rolls' "fix" is to numb down the vibration metering in this RPM range, and/or get creative as to where they place the vibration probe on the engine. No rhyme or reason, every installation is slightly different, often within the same facility that has multiple Avon units operating side-by-side. GE toughs it out by sticking to it's alarm and shutdown values through the whole RPM range, which leads to some perhaps unecessary shutdowns. During balance of these engines' compressor rotors we try to reduce the residual imbalance to minimal levels, but sometimes this is not effective. Over the years we have found that of more importance is balancing the rotors "in the middle", by removing approximately 60% of the imbalance in the middle of the rotors, and then performing a final trim balance on the rotor ends. With the J79/LM1500's, this is easy, as there are specific balance locations to accomodate this. With the Avon, this is a bit more tough. Also, we pay particular attention as to where any residual imbalances lie, as they cannot be in couple.
To top it all off, we leave a significant imbalance in the turbine rotor, and specifically do not 0-balance this assembly. If upon testing of the engines on the cell, midframe vibration is high through the FDC, by re-indexing the turbine rotor 180 degrees, we have a good chance of reducing the FDC.
I have a copy of a report GE made up in the mid 1970's, of a study done over a period of 11 years of J79 production. The report is a good inch thick, and covers a lot of ground. It can all however be boiled down to the last paragraph, on the last page. Basically, GE concluded that they really cannot answer what is going on with the compressor rotors, and no matter what, all Airforce engines had a 1.9 percent failure rate on the test cell due to high vibration, and the Navy engines 1.8 percent.
Other recommendations? If you have a GE engine with a significant FDC, and upon accelerating through it, vibration levels are acceptable, do what Rolls does, and dampen down the vibe monitoring through the FDC. It works for Rolls, it will work on a GE engine. Avons?, well they all shake through the FDC, some more than others. One thing I did run into is in high ambient temperature conditions, the FDC shifts up the RPM range. I was in Iran last August, commissioning a Mk-1533 Avon, and it was +46 degrees C. outside. The FDC was pronounced in the 6200-6400 rpm range, and their vibration monitoring returned to full sensitivity at 6400. It was touch and go as to whether the engine would come on-line or not, and at exactly 6400 the vibes would drop off. Too close to call, and I recommended they raise their vibration dampning to 6450 RPM.
Robin.