Engine RPM fluctuating.

So when at a low RPM setting in situations like taxing and landing descent, I've noticed that placing N1 at around 75 -80 percent causes a weird RPM fluctuation that goes between 70 and 80 percent RPM and it just bounces back and forth at a moderate rate. Is this a correct modelling of the engine characteristics because I'm very sure it's unlikely to be engine surging as I have checked the manuals and no symptoms arise. Any L39 pilots care to take a jab at this question?

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According to the flight manual the LPC bleed air valve (A1-25TL engine, 3rd stage) closes at around 74-76%. Could be, that the bleed valve is constantly opening and closing. A throttle setting, that keeps you above or below that N1 should avoid the fluctuations.

EDIT: Reading further in the manual, a power setting of 80-85% is suggested for taxiing. And from the engine limitations section: the use of N1 between 74-77 and 86-89% is only permitted for a short duration (because of BOV - Blow off valve).

So that means don't set throttle in between 73 and 77 percent then.

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I will try it when I get home.and see.

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That should do the trick, yes.

Below 86-89% is the closing point for the 5th stage bleed valve. It's quiet possible that there are minor fluctuations in that range too.

Is this a correct modelling of the engine characteristics because I'm very sure it's unlikely to be engine surging as I have checked the manuals and no symptoms arise.

From T.O 1T-L39C-1, Aero-L-39-Flight-Manual

FINAL APPROACH

NOTE

When established on final, it is recommended to maintain a minimum of 70% RPM in order to assure optimum engine acceleration if required.

CAUTION

At engine RPM about 75 and 85%, the surge bleed valves open and close. When engine operation is not stable, increase or decrease power to leave this RPM range.

Well shit that explains a whole bunch. Bit the real question is this preventable and are you able to shut down some systems that work of the bleed air to stay in that range?

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Bit the real question is this preventable and are you able to shut down some systems that work of the bleed air to stay in that range?

AFAIK

The AI-25 bleed air values help to "tune" the engine for different operating speeds.

Bleed air used for air conditioning and anti-icing (preventive heating of the inlet guide vanes) come from a different set of compressor points/valves.

A general description of different "types" of bleed air.

Operation of the 4x AI-25 bleed valves (2 pairs of 2) is shown at 9 min 0 sec

The bleed air used for aircraft internal systems comes from stage 9 (customer bleed - usually continuous adjustment possible). The stage 3 and 5 bleed valves (handling/surge/overboard bleed - only open/close) are solely for stabilizing the engine in transient states, like accelerating from low rpm without surging. This air goes over board and isn't used otherwise.

Modern engines take additional use of their variable stator vanes to fine tune the rpm to avoid this constant valve opening/closing and the mass flow fluctuations.

Great image of a common type of those bleed valves from an AI-20. In the AI-25 they are slightly different as the video above shows.

So when at a low RPM setting in situations like taxing and landing descent,

Bye, Skull.

Can't view the video but if it bounces between rpms without throttle input then yes. OK so it seems then that that is an engine limitation for its generation then. So lesson is don't leave the rpm in that range in order to avoid the bleed valves from opening and closing. Thank you all for your assistance and guidance.

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