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P3D/FSX effect of ambient temperature

Roy Holmes

Roy Holmes

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Several people raised the issue of the effects on performance of ambient temperature in the simulator.
We all know that in the real world hot temps reduce jet thrust and cold temps increase it, mainly due to the effects on density.

I did a test with my SimSkunkWorks Harrier II.
Full throttle, nozzles fully down and fuel weight well above that for VTOL, so I could get full static thrust.

Ambient temp 15°C. set through advanced weather at the airfield. Pressure 29.89 density 0.002375 slugs/cu ft. Thrust 20450 lbs at 110% CN1, 110% N1 EGT 441.2

Ambient temp 35.06°C. set through advanced weather at the airfield. Pressure 29.89 density 0.002219 slugs/cu ft. Thrust 20450 lbs at 110% CN1 113.8% N1 EGT 490.4

Ambient temp -34.99°C. set through advanced weather at the airfield. Pressure 29.88 density 0.002873slugs/cu ft. Thrust 20450 lbs at 110% CN1, 100% N1 EGT 316.9

At the three temps CN1 was the same because it is corrected N1 for standard sea level atmosphere. Because thrust is driven by CN1, the output from 1506 would also be the same, which it was.
(A:Turb Eng Jet Thrust:1, pounds) did not change with temperature change. Table 1506 says it is corrected thrust/static thrust, but that result is immediately adjusted for temperature and air pressure ratio, so the output is actual net thrust or, in my case actual gross thrust because there was no ram drag.
Note also that the density changes were representative.

So it would appear that jet thrust is not affected by temperature changes from standard in the simulator. Since the turbine tables do not have a direct input for temperature and there was no altitude change involved, the result, though counter intuitive, is theoretically correct.

Roy
 
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So going by what you say, if say my engine is a fadec engine and I have the rated N1 tables, should I use N1 instead of CN1? or have I understood it wrongly?
 
So going by what you say, if say my engine is a fadec engine and I have the rated N1 tables, should I use N1 instead of CN1? or have I understood it wrongly?
Click to expand...

For display you should always use N1, N2 and any other uncorrected values. This is because the corrected values, CN1, CN2 etc are intermediate mathematical terms used to generate actual uncorrected values. Actual thrust is used for the simulation of aerodynamic parameters.

Roy
 
Roy Holmes said:
We all know that in the real world hot temps reduce jet thrust and cold temps increase it, mainly due to the effects on density.
Click to expand...

Roy, you're missing something here. Density effect on thrust is not applicable to the statement "temperature affect on thrust."

For your comparison against a real engine to be accurate, you need to set constant density and look at what would be observed in the real airplane:

thrust at 100% N1 at temp X
vs
thrust at 100% N1 at temp Y


Assuming constant density, thrust will be lower for the higher temperature. Losses observed in the real airplane at constant density are due to lower CN1 speed as constant throttle position (N1) is maintained with an increasing temperature and vice versa.

CN1 and static thrust are always locked together based on massflow * velocity.

Maintaining a constant throttle setting in FS on a cold day will boost thrust vs a hot day at the same density.

That being said, the real question should be why no change in thrust was observed with the change in density? I know FS models density affect on thrust, but it seems odd that it didn't change in your test.
 
Joe,
I varied the temperature with constant pressure so that FS would have no altitude change when calculating thrust. Therefore the density changed.
If I had (and I'm not sure how you would do this) kept the density constant the pressure would have changed, therefore IAP would change and thrust would change.
My aim was to simply change temperature, keep altitude constant and show what happened.

I know FS models density affect on thrust,
Click to expand...

I do not think this is true for the sim and there are those who would argue that it is not true in real life as well!
Sim thrust is primarily affected by pressure ratio. There are no density effects on a jet, though there are density effects on torque with a turboprop or helo turbine.
Since pressure and therefore IAP did not change the pressure scalar was unchanged.
Note also that N1 changed but CN1 did not. Since CN1 drives thrust in the sim, not N1, then thrust remained constant.
Note also that I only considered thrust. There probably were other effects due to density that could have affected take-off performance, but I did not consider them.
Roy
 
Yes, agree with all that. That wasn't my point. Ignore my statement about density, that obviously distracted you from what I'm saying. That's just an 'all things being equal' statement.

My point was "loss of thrust due to temperature" in a real aircraft is perceived and not actual. Corrected N1 performance is by definition equal to "engine performance at ISA temp." All engines will always produce ISA performance (what you observed in the sim) for a given CN1 at a given pressure altitude. In a real plane, pilots perceive the loss of thrust because the RPM reference for takeoff power 'looks' the same, but the thrust is noticeably reduced. In actuality, thrust isn't reduced, but the EGT margin is reduced due to the additional heat energy in the exhaust and therefore the engine can't run hotter to regain the lost CN1 RPM.

Simply put, if the engine could run hotter on a hot day, thrust would be the same as ISA at the same CN1 setting. This would naturally mean N1 RPM would appear higher.

Had you done your test using 110%N1 instead, it would have been a valid test to compare against a real aircraft. No pilot will ever use CN1 to set thrust on a hot day...if they did, no one would claim temperature reduces thrust because it wouldn't. CN1 would be constant and thrust too.
 
Joe,
Good points
Just to take the ISA and ISA +15°C examples I quoted.
To get 20450 lbs thrust at ISA required 110% N1. To get the same thrust at ISA + 15°C required 113.8% N1. If I was N1 limited at 110%, thrust at ISA +15°C would have been reduced.
So higher temps do reduce thrust in the sim as long as you set max limiting N1%, or have some code that does it for you.

Roy
 
great! and it works even better on EPR aircraft where you can compare actual thrust output. The thrust (EPR) is constant with N1 increasing until reaching the engine's break-point temperature, where thrust begins to decrease and N1 becomes constant due to EGT limit.
 
I repeated the test, but at ISA plus 20°C which is generally considered a "hot day"
With N1 set at 110% thrust was 18570 compared to 20450 with 113.8% N1.
The added 20°C was present at 30,000 when another check was made. This is in line with the general representations of non-standard atmospheres.
Roy
 
Hi Roy Holmes, I created a gauge EPR in XML, the dependence of the EPR dall'N1. But the EPR is an indicator of thrust, then from what was said by you above, at equal EPR, in the hottest days N1 I will have a different (higher).
So I want to depend on the EPR from the thrust factor.
I tried to do depend on this variable: (A: TURB ENG1 JET THRUST, pounds); unfortunately as the speed and height of the EPR it decreased very quickly. I was advised to add ram drag, but unfortunately it came out equations too complicated for a gauge. Can you give me some advice?
 
On the VC10 N2 is the principal measurement. And that is very much affected by temperature changes and therefore affects the resulting thrust.
Take-off from a sea level airport at ISA results in n2 of 1005 Whereas take-off from Kuwait will raise it to 105%. So avoid engine limitatations take -off thrust is lowered on a hot day. so in order to maintain a certain speed on approach for example higher n2 is required and therefore the thrust is maintained.
 
OK, but it is that N1 N2 is dependent on the temperature I want to find a way to calculate the EPR and it is not affected by temperature as in reality.
In fact in my case (747-100) on the table it says that the same EPR, every 10 degrees higher to 'ISA, I will have a 2% increase of N1.
 
Ignore those numbers...they are actually an attempt to reclaim thrust at the expense of maintenance costs. You are still losing thrust.

Example:
At ISA, 100% N1 = 10000 lbs thrust
At ISA+10, 100% N1 = 9664
At ISA+20, 100% = 9350

At ISA, 102% N1 = 10200 lbs thrust
At ISA+10, 102% N1 = 9857
At ISA+20, 102% = 9537

You gain some back, but never back to ISA. That engine is pressure limited to that EPR. If it wasn't, EPR would go up to regain full thrust.
 
It Would be interesting to compare the FSX variables for AMBIENT DENSITY, AMBIENT TEMPERATURE, and AMBIENT PRESSURE for your cases.
 
It Would be interesting to compare the FSX variables for AMBIENT DENSITY, AMBIENT TEMPERATURE, and AMBIENT PRESSURE for your cases.
Click to expand...
When you say "your" which post are you referring to?. In the original post I included that detail.
Roy
 
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