Jet engines run better in cold environments?

Not related: You guys have thrown out more acronyms in just 1 page [of this thread] than I've seen in a long time .. and what's weird, is that I understand all of them... LOL - OMG

I'll be graduating ERAU in may with a BS in aeronautical science... the pilots degree, not engineering... but all my roommates are engineers so I'm well aware of how much I'm lied to ;)

Yes, I know lift isnt real.

Yes, I know lift isnt real.

Lift as a resultant force is very real. It's just that the definition of lift that most people see/hear/read is either completely wrong or only tells part of the story.

So.... did everyone concur that cold air = more mass flow?

did we then move on to consider if this is equally off set by drag??

or is it that we ALSO have a lower take off speed???? (as I assume we all mean at sea level?)

but what any of this has to do with R/C warfare I don't know.

No, you do get better overall performance when it is cold. Significantly so.

@Topic. Did a test yesterday...

-50°C

Engines were running at 60% ENG RPM, as usual. But as soon as I touched the throttle they spooled up pretty quickly and started pushing the plane. When I returned the throttle to idle the engines would idle at about 80%, they never came back to 60. A bug?

However - the performance in cold envinonments is SICK! :D

@Topic. Did a test yesterday...

 

-50°C

Engines were running at 60% ENG RPM, as usual. But as soon as I touched the throttle they spooled up pretty quickly and started pushing the plane. When I returned the throttle to idle the engines would idle at about 80%, they never came back to 60. A bug?

 

However - the performance in cold envinonments is SICK! :D

Your referring to game A-10?

General question:

Is the temperature vs altitude implemented in the game?

If so, is it the temperature at MSL the one you set in ME?

Cold air is denser than hot air so it makes sense that a turbine would be more efficient on the ground due to the extra oxygen content. But do you get a higher service ceiling on a cold day?

And when looking this deep into it, does rain increase efficiency during flight as it vapourizes and expands in the turbine?

the extra oxygen content.

AFAIK the amount of oxygen is never a problem as turbines are always limited by the exhaust temperature way before they run anything resembling the optimum fuel to air mixture. What makes the big difference is that EGT is lower and massflow is bigger.

But do you get a higher service ceiling on a cold day?

Short answer, no.

While the temperature decreases with the increase of altitude, so does the density. The temperature on the ground may still be colder than on a "standard day" (+20C or 288K), but when it comes to altitudes of for example 20000ft and 30000ft (where most of us normally "can" fly the A-10C :)) the temperature is still roughly ~250K for 20000ft and ~230K for 30000ft. This may of course vary a little bit but this is the general rule.

Compare this to the density on a "standard day" which is 1.225 kg/m3 (which would be higher if it was colder outside).

At the altitudes of 20000ft and 30000ft the density is 0,66 kg/m3 and 0.47 kg/m3 respectively (standard day).

One can easily see that the service ceiling won't really be affected to any significance by the denser air at a colder day.

Uh...that's not right.

On days when it's ISA -10, the CRJ has adequate performance at MTOW to climb straight up to FL330. If it's ISA +10, you won't make it above FL270 with a full boat. I'm going off of memory here, but there are Altitude Capability charts right inside the back cover of the QRH with the actual numbers.

Temperature plays a huge role in the operational ceiling of aircraft that are thrust limited.

The lapse rate dictates that the temperature drop per foot of altitude remains about the same, hence the temp below ISA follows you up as you climb. In other words, you will be at a higher pressure altitude when you hit the ceiling.

What you read on your altimeter is pressure altitude. A given pressure altitude equals a given pressure.

What matters for performance is the density altitude. A given density altitude equals a given density.

The density at 20,000 feet pressure altitude on a standard day is 0.65 kg/m^3. At ISA -30 deg C (15 below on the ground), the air density is 0.74 kg/m^3 at this 20,000 ft pressure altitude, and you get better performance than you normally would.

To get down to 0.65 kg/m^3 and a density altitude of 20,000 feet again (and thus the same performance) you'll have to climb to around 24,000 feet pressure altitude. In other words, you gain 4,000 feet pressure altitude to the ceiling.

However, you will not be 4,000 feet further away from the ground, due to the fact that the colder air means more of a pressure drop for every foot climbed. Adding four percent of pressure altitude for every ten degrees below ISA to maintain the same height above ground is a rule of thumb used. Using this, your 24,000 feet pressure altitude (what's read on the altimeter) means about the same height above ground as where your altimeter would read a little above 21,000 feet pressure altitude on a standard day (112% of 21,400 is about 24,000). Probably stretching the rule of thumb a bit, but it's in the ball park.

Anyone not confused yet? :D

Hey guys; sorry for bumping up an old thread, but I was studying up jet-engines and their performances with varying ambient temperatures and this thread was one of the first results on google.

What I've read here concurs with what else I've seen online - that as temperature falls air density rises and as such more mass is pushed through the engine leading to better performances at the same power.

While the material I've been given to study from agrees with this, it also states that the Speed of Airflow through the engine is proportional to the square root of temperature.

So if we take Thrust as Mass * (Jet Air Velocity - Aircraft Velocity), we can get:

Thurst = Density of Air * Area of Entry for air at engine inlet * Velocity of Aircraft * (Jet Air Velocity - Aircraft Velocity)

These two mean the same thing.

Now, the step between here and the formula I show next has not been explained, but I believe it goes along these lines - given that mass flow increases as pressure does, but decreases as temperature does, we can say that Mass is proportional to (Pressure/Temperature)

Thus, Thrust can be taken as proportional to (Pressure/Temperature) * (Square root of Temperature) * (Square root of Temperature).

Thus, the temperature term cancels out, and we see that Thrust produced by the engine is actually independent of the Temperature of the ambient air.

So I don't understand this - if the Thrust produced by the Engine is independent of the Temperature of the ambient air, then how does it give better performance at lower temperatures?

Where do the two square root terms come from?

As the air temperature increases, the air density would decrease resulting in a

decrease in mass flow. However, the temperature increase will also result in an

increase in jet velocity (Vj) due to an increase in the rate of expansion in the

turbine.

 

Mathematically, the speed of airflow is proportional to the square root of

temperature change V ∝ √T

 

Thrust = m(Vj - Va)

= ρA Va(Vj - Va)

∝ (P/T) x √T x √T

∝ P

 

hence the thrust output will be independent of temperature variation and the

correction factor for temperature will be equal to 1

Quoting my notes there. No further explanation given.

Then what's your point, are you asking a question or trying to disprove the physics community at large, Mr. Newton?

Then what's your point, are you asking a question or trying to disprove the physics community at large, Mr. Newton?

So I don't understand this - if the Thrust produced by the Engine is independent of the Temperature of the ambient air, then how does it give better performance at lower temperatures?

No need to be snarky; just asking a question.

Did not read all of this website, but some info in it might help.

http://www.grc.nasa.gov/WWW/k-12/VirtualAero/BottleRocket/airplane/ngnsim.html

Did not read all of this website, but some info in it might help.

 

http://www.grc.nasa.gov/WWW/k-12/VirtualAero/BottleRocket/airplane/ngnsim.html

Yea, I found that link earlier; unfortunately the application is blocked on the computers I'm at now so will have to check that out later.

It'll be fun to play around with but doesn't supplement a theoretical reason.

Personally I feel there is something simple I'm overlooking here...but I can't seem to find what it is.

I don't feel like making hte calculation ,but your reasoning is based on Bernouilli's principle () and the ideal gas law (pV=nRt).

But there's something missing in your calculation. It's not taking into account that with colder air you have more air density at the turbine inlet and in the combustion chamber. Higher air density means more oxygen, which means a better combustion of fuel, which means more energy is released. And since more air mass is injected in the engine and heated to the engine exhaust temperature you get more volumetric air at the exhaust than with hot air at the inlet, resulting in more thrust.

Jet engines are limited by both compressor pressure and turbine temperature. The ambient temperature and pressure determine which limit you will encounter first.

 

At a given pressure altitude, a low compressor inlet temperature makes it possible to reach the compressor discharge pressure limit before reaching rated EGT/ITT/TIT. This temperature is the "flat rating" temperature, below which temperature has no effect on maximum allowable thrust.

 

At high compressor inlet temperatures, you generally reach the temperature limit first, so thrust must be reduced in order to prevent exceeding rated EGT/ITT/TIT.

 

Compressor discharge pressure can also be exceeded at high airspeed/mach.

 

Edit to add: This is a functional reason why there is more thrust available at lower temperature. I think that the theoretical answer you're looking for is that as air temperature decreases, air density increases. The increase in density is what actually causes the increase in thrust.

That makes sense, I get it now. Thanks! :)

I don't feel like making hte calculation ,but your reasoning is based on Bernouilli's principle () and the ideal gas law (pV=nRt).

But there's something missing in your calculation. It's not taking into account that with colder air you have more air density at the turbine inlet and in the combustion chamber. Higher air density means more oxygen, which means a better combustion of fuel, which means more energy is released. And since more air mass is injected in the engine and heated to the engine exhaust temperature you get more volumetric air at the exhaust than with hot air at the inlet, resulting in more thrust.

That's useful too.

Cheers for the help guys :)