High MP low RPM Continental TSIO520 engine (or Lycoming engines)

Hi John. I enjoy and learn a lot reading your monthly mechanic`s tool. Thank you very much.

I am a pilot and would like to your point of view from a discussion some of us been having for quite a time: it is there any problem on a turbocharge engine to operate on low rpm and high manifold press. i.e.: cessna 421 at 40" and 1900 rpm for a 5 to 10 minutes period of time?.

I would appreciate your comments. 

Your joking right? Your not aware of Cape Air/Hyannis Air Services Inc.'s engines slinging pendulum absorbers through the crankcase? 

http://www.avweb.com/avwebflash/news/Cape_Air_Grounds_Cessna_402_Fleet_195402-1.html

or, Continental Service Bulletin SS107-5,

Or this from Continental:


SUBJECT: MINIMUM CRUISE RPM LIMITS
PURPOSE: To inform operators of the possible long term effects of low engine RPM in cruise conditions. To establish limitation of minimum engine RPM in cruise.
COMPLIANCE Upon issuance of this bulletin
MODELSAFFECTED: O-470-G; IO-470-N; IO-520-BB, CB, MB, P; IO-550-A, B, C, D, E, F, G, L, M, N, P, R; IOF-550-B, C, D, E, F, L, N, P, R;
TSIO-520-AE, BB, BE, CE, DB, EB, JB, KB, LB, NB, UB, VB, WB; LTSIO-520AE; TSIO-550-A, B, C, E, K; TSIOF-550- J;
TSIOL-550-A, B, C
Teledyne Continental Motors (TCM) has examined recent occurrences of crankshaft counterweight release and subsequent engine stoppage in two high time IO-520 and two high time TSIO-520 engine models. Investigation and reported service history lead us to believe that these occurrences are associated with engine operation at sustained cruise engine RPM of less than 2300 RPM. Power settings of less than 2300 RPM have been within the recommended cruise range allowed by TCM’s Model Specifications. It is TCM’s belief that the population of aircraft equipped with the affected engine models that operate using an RPM less than 2300 RPM for extended cruise operation is limited. TCM will continue to evaluate these reported counterweight releases in an attempt to establish a root cause, including any possible connection with power settings. TCM has not been made aware of any additional confirmed occurrences of this type beyond those mentioned above. Effective immediately, TCM strongly recommends the following limitation be observed on all the models affected above:
Engine cruise RPM settings should be no lower than 2300 RPM.
NOTE … This limitation applies only to cruise operation and is not meant to
supersede the aircraft manufacturers’ recommendations for other  operational modes such as emergency or holding procedures.  Any engine listed in the models affected that has been consistently operated outside the recommended limitation in this bulletin should contact TCM Technical Customer Service at 1-888-826-5465 Option 1 or 1-251-438-3411 x8299 for further information and instructions

Pendulum Absorbers mounted on crankshaft

-----------------------Here is the problem -----------------------

Counterweights are in fact pendulum absorbers that have a fixed capacity to absorb torsional crankshaft energy. If you feed in more energy than they can absorb they "detune" or "jump" . By detuning, the absorber, which is free to swing like a pendulum, no longer swings but bounces around violently. This violent bouncing will break or knock out the retaining rings and plates and detach the absorber. How do you feed in more energy? - by increasing the torsional twisting of the crankshaft. You do this when you:

a. increase engine torque by increasing MP

b. operating at an rpm that coincides with the crankshaft's resonant frequency. The resonant frequency is around 2000 rpm for the 520 crankshaft.

So by operating at low rpm/high mp you are close to peak energy input into the 6th order absorbers. But there is more to this story. When the absorber is far from peak energy it kind of sits there and wears a depression into the bushing (frets). This changes the pendulum length and the absorber's natural frequency. This means that its energy absorbtion capability is reduced. So as you're engine gets to "high-time" or close to tbo in hours, the 6th order counterweights are more sensitive to detuning.

Now about operating for a short period of time? Keep in mind that once the absorber detunes it jumps to a different curve and doesn't come back into tune unless you bring the power back to close to idle. Think of a child on a swing - your absorber is suspended in exactly the same manner (bifler suspension - at two points). The child swings smoothly, but if you disturb the swing it stops swinging and you must grab the child and completely stop the swing and start over.

Crankshaft with pendulum absorber removed.

Circlip, Plates and internal pin provide a bifler attachment

If I design a device and include a manual of operation, it is "safe" if operated within those boundaries. I have communicated clearly what those boundaries are.


Further reading on this subject