Holding speeds with A320 IAE V2500 engine
 

Hi,

can anyone be so kind and explain the following?

PER-HLD-GEN and PER-HLD-HLD

Holding tables established for clean config at 210kt and green dot with CFM56 engines.

For IAE V2500 engines it says clean config at 210 kt and green dot +20 kt.

Why this 20 kt increment?

Still can't explain it, here is my original thread about it from years ago!

Thx Cough,

it seems that in this forum there is nothing that has not been discussed. Next time I'll try the search function :)

Anyway, maybe they want to prevent to fly the engine in a specific range of N1 or N2 for a longer period of time, eg. holding patterns?

This is indeed confusing.....why would they do that i wonder.....i have checked the tables and its GD vs. GD+20 alright....
and what adds to it, i have also checked the holdong tables for all engine variations on other airbus A/C.
P&W and G.E. and Rolls Royce RB211 TRENT on A330
CFM56-5C on A340-300
Rolls Royce TRENT 500 on A340-600
and they all have 210kts and S spd. and GD!!!!!:ugh:
My only guess now is that its something related to the V2500 itself, because as far as i know, the CFM tables are like every other engine i looked at....
maybe what you guessed about the N1 range that needs to be avoided for long periods is the real reason behind it, afterall i know that the V2500 fan blades design is sensitive to flutter between 60% and 74% N1 at low speeds (unlike the other engines i mentioned above) thats why they have established a KEEP-OUT-ZONE for it during take-off, so, maybe this is valid in the air as well....

Another explanation may be the variation of engine thrust-specific fuel consumption TSFC at low power. See Boeing's Jet Transport Performance Methods example for the B757:

http://i.imgur.com/TC2ufng.png

I have thought about that issue, and it crossed my mind that maybe they want the aircraft to be flying at a speed away from the backside of the speed curve in order to achieve speed stability, but then again that should also apply to all other engines i mentioned.(which doesn't)
In addition, what really blew this assumption for me was this statement from the FCOM.

"Green dot speed in clean configuration and S in CONF 1 are speeds between the minimum fuel speed and the minimum drag speed.
These charts are established with air conditioning in normal mode and the center of gravity at 33 %.".

and yet the tables use GD for CFM and GD+20 for V2500!!!

Hold speeds
 

Yet HazelNuts39 has tackled an issue that might open the way to explaining this mystery.....
i have looked again at the all the engines that i mentioned above in my previous posts....
the only difference that i found between all these engines and the V2500, is that they all have bigger fan diameters / higher blade tip speeds and higher bypass ratios of a range 5.6 to 6.2 while V2500 has only bypass ratio of 4.9 which indeed affects the specific fuel consumption for each engine type (V2500 has specific fuel consumption of 0.560, compared to 0.591 of the CFM56), and might require the aircraft to be flown at speeds different from G-dot for each engine type, in order to maintain the speed for best fuel consumption for each specific engine.
Now if we consider: 1- that the G-dot speed is related to the wing and airframe rather than the engine itself (close to the best lift/drag speed).
2- that the A320 initial design and characteristic speed calculations were based on aircraft fitted with CFM56 engines rather than the V2500 which was not introduced until a later time.
then it would only make sense that this difference would impact all aspects of performance, and would show on the Perf. tables.

To add to the confusion, both the A319 and A321 with the V2500 hold at Green Dot no addition