as OAT decreases what happens to a/c thrust
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BEEF SUPREME
Well-Known Member
It decreases. A high ISA really slows things down.
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BEEF SUPREME
Well-Known Member
At the end of the day: suck, squeeze, bang, blow still applies. And all the laws of physics that go with it.
Jets ain't magic. If there is a MX spot on the field where you fly ask if you can take a peek at a jet engine all taken apart. It's literally not rocket science.
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ppragman
FLIPY FLAPS!
You mean as OAT increases, no? Denser air should mean more thrust...
Cessnaflyer
Wooooooooooooooooooooooooooooooo
The naval aviator handbook is great for these questions.
Stone Cold
Well-Known Member
Colder air produces more thrust, all else being equal. The higher you go, and the hotter it is, the worse the engine performance...hence high and hot takeoffs being a special item always taught.
Flyinthrew
Well-Known Member
Thrust available equals Mass Flow multiplied by the change in velocity from the intake to the exhaust (usually written as Ta=Q(V2-V1))
Q=mass/time.
So Ta=mass(v2-v1) all divided by time.
Density=mass/volume
Density of a gas(air) also equals Pressure/(R x Temp). R is a constant (don't worry about that little guy). This formula is why we always say that as temperature decreases air density increases.
So setting them equal to each other and swapping some stuff around: mass = (Pressure x Volume)/(R x Temp). Sub that into the very first equation.
Ta= [(Pressure x Volume)(V2-V1)]/[(R x Temp)(Time)]......
A much easier way to say it for our purposes: Ta=C/Temperature.
OAT is in the denominator so when it gets smaller, thrust available gets bigger linearly.
This equation also answers what the effects of altitude and/or aircraft velocity have on thrust.
drunkenbeagle
Gang Member
Also, humidity gives you less thrust and lift as well.. Water vapor is less dense than air, and it doesn't burn so well either. Temperature is still the major component though.
BEEF SUPREME
Well-Known Member
Every jet aircraft I've flown measures air temperature in terms of ISA.
ISA is -35 deg C above FL310
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ppragman
FLIPY FLAPS!
Sure, not a jet guy here...but at a given altitude, if the temperature goes down, the air is more dense - thus you're getting more mass moved through the engine, thus more thrust I would think. All other things being equal, cold air should be more "giddyup."
BEEF SUPREME
Well-Known Member
It's basically the exact same principle as piston flying. As I mentioned on my second post. The laws of physics are the same for prop or jet.
I feel bad for the guys who haven't gotten a chance to fly a jet aircraft getting picked on by their more experienced peers at a job interview.
It seems like PFM but it ain't. The Naval Aviators book is good but after a few flights in a jet it's just the same as anything else. Its actually way easier.
Good luck to the OP on the interview if I can help in any way I will.
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Flyinthrew
Well-Known Member
Yes, this. If you want to talk about the effect of temperature, then you have to hold everything else (altitude, airspeed, compressor speed, etc) constant.
And of course the other factor that you should probably tack onto the end of your answer that is when it is so hot outside that you hit the temperature limit before you reach the fan speed limit.
Stone Cold
Well-Known Member
Right but your first post in this thread was backwards on the first sentence. That's where the confusion came from. Colder air, everything else being equal, produces more thrust. If you're at 310 at ISA, and it gets warmer, you lose thrust. if it gets colder than ISA then you'll produce more thrust.
BEEF SUPREME
Well-Known Member
Crap! I could have sworn that said increases not decreases. I can't edit my OP.
Sorry everyone! My bad!
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