N1 takeoff thrust depending on outside air temperature

[737Andi 6]

Hello,

with the PMDG 747-400 I'm observing a strange thing. Raising the value for the OAT increases the N1 thrust on takeoff and vice versa.
The behaviour of the PMDG 737 NGX is different and to my knowledge correct.

Thanks!

Best regards Andreas

[MartinRex007 390]

In general hot days (increase OAT) require more thrust on takeoff, there is an upper limit where commercial jet aircraft will not be able to take off because the OAT is to high, effects both engine performance and aerodynamic lift on the wings. It has happened at KPHX in the past.

Edited January 29, 2019 by MartinRex007

[downscc 1,327]

7 hours ago, 737Andi said:

Raising the value for the OAT increases the N1 thrust on takeoff and

Do you mean increase OAT increases N1 RPM?  Increase air temperature decreases air density and it is the weight of air, not the volume, that determines thrust.  The EEC will adjust N1 by managing the fuel flow to provide the thrust setting set for takeoff.  If it is warmer then a higher volume of air is required for a given thrust.  As far as I know, the CFM engines on the 737 work the same in this regard.

[737Andi 6]

No...what I meant is the OAT setting in the derated takeoff performance page of the CDU. In the 737 I can see the N1 value going down with higher OAT settings. In the 747 its the other way?!

[PinkPony 171]

On 1/29/2019 at 2:44 AM, 737Andi said:

Hello,

with the PMDG 747-400 I'm observing a strange thing. Raising the value for the OAT increases the N1 thrust on takeoff and vice versa.
The behaviour of the PMDG 737 NGX is different and to my knowledge correct.

Thanks!

Best regards Andreas

The vast majority of commercial aircraft engines are flat rated up to a OAT of ISA + 15 C (i.e. 30 C).

Meaning: up to a OAT of 30 C, these engines will be able to achieve maximum rated thrust (i.e. flat rating)

All available 747-400 engine options have a flat rating of ISA + 15 C, as does the CFM56-7B on the B737NG family. 

Why are you seeing a N1 increase with OAT increase? When OAT increases, air density decreases. Meaning that in order for the engine to still achieve its maximum thrust rating, it must increase N1 and EGT. This concept applies up to the flat rating limit (ISA + 15 C).

When the OAT continues to increase and it eventually exceeds the flat rating limit (ISA + 15 C), N1 and EGT will decrease, and thus the maximum thrust begins to decrease for every degree of OAT increase. 

For both the -400 and 737NG mentioned above, maximum N1 will increase up to 30 C. When OAT > 30 C, maximum N1 will begin to decrease.

 

Edited February 1, 2019 by calzonister

[737Andi 6]

13 hours ago, calzonister said:

 

When the OAT continues to increase and it eventually exceeds the flat rating limit (ISA + 15 C), N1 and EGT will decrease, and thus the maximum thrust begins to decrease for every degree of OAT increase.

 

Thx for the good explanation!

So N1 decreases above ISA + 15C in order to protect the engines at too high temperatures?

[PinkPony 171]

6 hours ago, 737Andi said:

Thx for the good explanation!

So N1 decreases above ISA + 15C in order to protect the engines at too high temperatures?

As N1 increases so does EGT. Eventually (at the flat rating limit) the EGT must be reduced, and this is achieved by reducing N1 again. 

The optional Thrust Bump on the GE90-110/-115B engine for example provides increased takeoff thrust for OAT 32 C to 53 C. The FADEC logic achieves this by reducing the rate at which N1 drops, and by increasing the EGT limits for when OAT is > ISA + 15 C. This comes at the expense of reduced EGT margins and shorter/more costly engine overhaul periods.