A320 V2500 questions

[Eurotraveller]

Hi,

I note that on the V2500-A1 there is a 50 second 'crank' before ignition on start and I understand that this is due to possible rotor bow which the V2500 is apparently prone to.

Is somebody able to explain exactly what rotor bow is and why it happens, why the V2500 is prone to it and how the crank during start helps?

Also, I understand the concept of 'bump' in that it provides approximately 10% extra thrust on top of TOGA, but is anyone able to explain the theory behind how it actually works?

Any other comment on the V2500 vs CFM56 is also welcome!

Many thanks,
Eurotraveller

[idg]

ET,

My understanding is that with a 'fully shrouded' engine such as the -A1, heat build up after shut down is particularly bad if there is no head or tailwind to move the hot air through the engine. In addition with such high pressure ratios (achieved with densely packed fan blades) the engine wasn't as good as others at clearing away that heat. With a long two-spool engine the heat build up was felt in the rotor shafts (obviously worse for the outer, high pressure, shaft) which caused the rotor (shaft) bowing with excess heat.

Cranking the engine for extended periods before putting fuel and ignition on allowed a stabilisation of the thermal load and allowed the shaft to straighten thereby minimising the blades rubbing on the case. This rubbing had caused blade failures in the engines early on which led to the discovery of the fault.

The fix on later engines was to pump up the oil pressure to the bearings in the middle of the shaft which 'squeezed' it straight while the heat was evening out across the engine and shaft.

Bump Thrust is possible because when setting thrust and beginning the TO roll there is a strong depression in the intake. This lower pressure (clouding on a damp day) temporarily increases the EGT due to lower gas flow. As the ac accelerates the pressure increases and the EGT reduces. If we were now to inject a little extra fuel we would 'restore' the engine to the limit EGT but in the process achieve a higher thrust rating.

If you watch the EPR closely during the roll you will see the EPR increase as the ac accelerates with bump selected. The A5 engine does the same thing automatically with TOGA selected.

This is why we should wait for 80 knots before checking to see that TO EPR has been achieved and more importantly check that the EGT is within the limits at this point.

Well that what I think anyway....standing by for other inputs!:ok:

[Eurotraveller]

Thanks idg, much appreciated.

[lomapaseo]

I thought the cranking was to get oil into the bearing compartment under pressure to activate the oil damped bearings to keep the shaft from sagging the bearing outer race against it's support.

Could be that the manufacturer has issued a SB describing why something is done and how it works

anybody got one??

[Metro man]

The crank is 30 seconds on our engines, is the figure different for various series of the V2500 or is it a typo ?

[idg]

I think that Lomeo has it right on the technicalities wrt what the higher oil pressure achieves.

The cranking time is (from memory) different between the mod states of the engines. Once the initial fix (higher pressure pumps?, modified FADEC software etc) was introduced the cranking time was reduced from 50secs down to 30secs.

Could be wrong tho.

[Dani]

yes, I agree with most of it. But you don't have to wait until 80kts but latest at 80kts? It's a while for me...