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High Bypass Engines In Low Atmos Pressure?  
User currently offlineKaitak From Ireland, joined Aug 1999, 12565 posts, RR: 35
Posted (9 years 10 months 2 days 21 hours ago) and read 3858 times:

Hi,

I was chatting with an EI pilot a while back and he was telling me that there are issues about the operation of high bypass engines in low atmospheric pressure. This is obviously something which can (no, really!) happen in Dublin.

Low atmospheric pressure brings rain (again, a great rarity in Dublin) and that brings about performance limitations, because of the runway length. This part I understand, but I'm not clear on whether there is specific issues relating to high bypass engines; i.e. why wouldn't this apply to any engine?

Hope I've made myself reasonably clear!

Thanks.

6 replies: All unread, jump to last
 
User currently offlineIFIXCF6 From United States of America, joined Sep 2004, 108 posts, RR: 0
Reply 1, posted (9 years 10 months 2 days 14 hours ago) and read 3782 times:

I'm not certain from your post as to whether the pilot was referring to low pressure on the ground, or in the air. But here I go...

High bypass engines loose efficiency in the thin air up high quicker than low bypass or turbojet engines. The reason is that the large diameter fan has to spin faster to maintain its (approx) 80% thrust total. That 80% bypass is contributing nothing to engine operation. The air mass is considerably lower due to the pressure, so what does the engine do as torque requirement lessens on the low pressure turbine? Speeds up (N1). N1 speed (the fan) becomes the limiting factor.

To quote you, "Hope I've made myself reasonably clear!"

Regards,

Mike


User currently offlineSlamClick From United States of America, joined Nov 2003, 10062 posts, RR: 68
Reply 2, posted (9 years 10 months 2 days 14 hours ago) and read 3778 times:

I am trying to recall something about this subject, but it has been ten or fifteen years since I even thought about it.

If I'm not mistaken there was some issue about low barometric pressure for ground operations, say startup. I associate it with the CFM-56 but again, it's been a long time.

Wish you had a more specific reference. I've thumbed through an old 737 manual without finding what I'm talking about. Maybe someone will pop up with a better memory.




Happiness is not seeing another trite Ste. Maarten photo all week long.
User currently offlinePhilsquares From , joined Dec 1969, posts, RR:
Reply 3, posted (9 years 10 months 2 days 14 hours ago) and read 3779 times:

I guess I'm not clear on the original post, but here goes.

Speaking about the PW 4056/58 it's not a problem at all. I pass through ANC quite often and it's not unusual in the winter to see a baro of 28.XX. With the FADEC/EEC there isn't a problem at all. Even the older JT-9/RB211 did just fine.

Can't add much more than that...


User currently offlineWIDEBODYPHOTOG From United States of America, joined Jun 1999, 917 posts, RR: 67
Reply 4, posted (9 years 10 months 2 days 3 hours ago) and read 3730 times:

All turbofan or turbojet engines/airframe combinations are effected by lowered barometric pressure or QNH (higher density altitude) to some degree. Lower air pressure also lowers lifting surface performance and increases V1 for a given TOW and in order to maintain specific field performance you must reduce allowable TOW when the QNH goes down.

But what were talking about here is the thrust lapse rate (difference between the highest thrust at sea level and normal power at altitude) of an engine. In terms of HBPR turbofans, generally the higher the bypass ratio the greater the thrust lapse rate as the operating altitude increases. All engines lose power as altitude increases and this can be compensated for by increasing the turbine temperatures (amount of fuel) and/or the rotational speed of the engine spools (pressure ratio and mass of airflow). Of course there is a limit to the degree of which you can increase either of these to maintain thrust and after that the thrust lapse takes over and engine output falls down that slope whatever it is. Turbofan bypass air does not undergo significant thermodynamic cycle and that low temp cycle is not adjustable so the thrust produced by bypass air is really just along for the ride as far as atmospheric conditions are concerned. Core airflow air has "fuel and fire" added to it after being highly compressed so the output is somewhat less dependent on atmospheric conditions in terms of output. You put the two together in some proportion and you get a HBPR turbofan engine. You sacrifice ultimate thrust at increased altitudes for greater overall fuel and propulsive efficiency and because the greater proportion of the thrust is produced by "cold" bypass air it is more subject to unfavorable atmospheric conditions on the ground or in the air.

I'm sure someone can correct me if I'm wrong, but I believe that the A330 loses about 1000lb of allowable TOW for every 100ft in decreased density altitude above a given outside temperature. This holds true for every commercial aircraft in some proportion. So your pilot friend should blame physics or the weather for his payload/performance woes and not HBPR turbofans in general...


-widebodyphotog



If you know what's really going on then you'll know what to do
User currently offlineDarkblue From United States of America, joined Sep 2003, 233 posts, RR: 10
Reply 5, posted (9 years 10 months 2 days 1 hour ago) and read 3711 times:

Does low barometric pressure decrease thrust? As stated by several others above, yes. Lower pressure mean lower thrust.

However, the effect of weather on barometric pressure in Dublin is small when compared to effects of altitude on barometric pressure. I'm not familiar with Dublin, but I assume that it is less than a couple hundred feet above sea level.

Dublin on a low pressure day would be no different than a runway at 1000ft on a normal day.

DB


User currently offlineWIDEBODYPHOTOG From United States of America, joined Jun 1999, 917 posts, RR: 67
Reply 6, posted (9 years 10 months 1 day 23 hours ago) and read 3697 times:

Does low barometric pressure decrease thrust? As stated by several others above, yes. Lower pressure mean lower thrust.

Lower barometric pressure means higher density altitude, lower air density, and without a commensurate decrease in OAT (outside air temperature) will result in lower thrust from a given engine.

However, the effect of weather on barometric pressure in Dublin is small when compared to effects of altitude on barometric pressure. I'm not familiar with Dublin, but I assume that it is less than a couple hundred feet above sea level.

A .11 inch of mercury decrease in barometric pressure is about a 100ft increase in density altitude and that may result in a 1,000lb or more reduction in allowable takeoff weight above a specific temperature for a particular aircraft on a particular runway.


-widebodyphotog



If you know what's really going on then you'll know what to do
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