Hot and High
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Joined: Apr 2002
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From: Uranus
Hot and High
What is the difference between N1 & Thrust in an engine when taken from an airfield at sea level to an airfield that's hot and high?
I'm thinking N1 increases and thrust reduces right?
Cheers
I'm thinking N1 increases and thrust reduces right?
Cheers
Joined: Feb 2003
Posts: 1,078
Likes: 46
From: Brighton
Depends on how the engine is rated for max thrust. Most modern engines are "flat rated" to a designated temperature, often around 30 deg C, so to get the rated thrust you need more N1 as the temperature increases. Flat rating can also include an altitude range, so as altitude increases you need more N1 to achieve the rated thrust.
However, many airline take-offs are made with reduced thrust, called "derate" or "Flex" by different manufacturers, so you are not then using the full rated thrust -it's kinder to the engines and lengthens their life.
However, many airline take-offs are made with reduced thrust, called "derate" or "Flex" by different manufacturers, so you are not then using the full rated thrust -it's kinder to the engines and lengthens their life.
Joined: Dec 2006
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From: The No Transgression Zone
just to add that without a derate or a flat rate
that thrust is [if i remember right] is about proportional to about N1^3.5
so if thrust or[ drag] decreases with altitude as thrust at any altitude =T[msl]*sqrt [pi]
at any temperature T [msl]* sqrt[theta]
pi = the pressure ratio
theta =temp. ratio
and it can be seen that the required N1 would be mathematically increased if necessary to counter drag until either a limiting rpm [ if N1 vs EPR limited] or a limiting temp. is achieved for non-accelerated flight
example: T[msl]*sqrt [sigma] ~N1^3.5...
...but for the purpose of meeting performance, N1 would increase to maintain the thrust required to meet performance [although Drag would decrease too] but for acceleration you need thrust GREATER than drag, for performance it's referred to a 'thrust required'
so you are correct
PA
that thrust is [if i remember right] is about proportional to about N1^3.5
so if thrust or[ drag] decreases with altitude as thrust at any altitude =T[msl]*sqrt [pi]
at any temperature T [msl]* sqrt[theta]
pi = the pressure ratio
theta =temp. ratio
and it can be seen that the required N1 would be mathematically increased if necessary to counter drag until either a limiting rpm [ if N1 vs EPR limited] or a limiting temp. is achieved for non-accelerated flight
example: T[msl]*sqrt [sigma] ~N1^3.5...
...but for the purpose of meeting performance, N1 would increase to maintain the thrust required to meet performance [although Drag would decrease too] but for acceleration you need thrust GREATER than drag, for performance it's referred to a 'thrust required'
so you are correct
PA
Last edited by Pugilistic Animus; 15th May 2009 at 15:11. Reason: theta and pi are the correct designations, sigma is 'density altitude ratio
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Joined: Mar 2009
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From: USA
N1 V/S EPR hve been a conflict between engine manufacturers and operators for years.
None the less there is a small variance between the importance of the two in respect to rated thrust.
High and hot elevations limit performance to MAX EGT limitations in most cases...In simple terms the ambiant temperature and environmental air pressure being the limiting factor.
N1 is limited to a fixed speed (should not exceed) and thrust is the calculated variable based on EGT limitations.
More importantly "hot" reduces the wings lift capability (less dense lift producing air under the wings). Having said this Sea level and hot can affect MGW limitations for takeoff at sea level as well.
Runway length becomes the most prohibitive factor as well as terrain for alot of operators trying to get off the ground in hot thin air. The heavyweight freight operators are affected most.
None the less there is a small variance between the importance of the two in respect to rated thrust.
High and hot elevations limit performance to MAX EGT limitations in most cases...In simple terms the ambiant temperature and environmental air pressure being the limiting factor.
N1 is limited to a fixed speed (should not exceed) and thrust is the calculated variable based on EGT limitations.
More importantly "hot" reduces the wings lift capability (less dense lift producing air under the wings). Having said this Sea level and hot can affect MGW limitations for takeoff at sea level as well.
Runway length becomes the most prohibitive factor as well as terrain for alot of operators trying to get off the ground in hot thin air. The heavyweight freight operators are affected most.
