Technical Jet Question

[sydeseet]

Stump the pilot:


At a Mach .82 cruise speed, what is the velocity of the engine exhaust as it exits the engine??

What is the velocity of the exhaust during the takeoff roll, is it constant or does it change as the aircraft accelerates?

We thought of these yesterday while cruising at .82 and thought I'd get some input from the crowd.

 

[avbug]

Sydeseet,

Not enough information is provided. The speed of the exhaust is one thing (which is what I think you're asking), but the answer also applies to the amount of airmass being flowed (total mass). To get the same thrust, a lesser air mass must be accelerated to a higher speed, while a greater airmass need not.

On a hot day with a lesser EPR, you may require higher engine RPM to accomplish the same function, and consequently due to increased weight or temperature, you may be running the engine faster, contributing more fuel and therefore heat, and be experiencing a higher exhaust gas velocity.

It's not so much the exit velocity, as the airmass moved aft. Then again, I failed the last math class I attempted, so somebody here can probably give you the technical physics of it all.

 

[sydeseet]

Good answer. We came to basically the same conclusions. Total thrust is what moves the aircraft forward which is dependant on the mass of the aircraft vs the mass of the air being moved through the engines.

I'm curious as to how the velocity of the exhuast gas changes as aircraft speed changes.
ie: at a constant thrust setting (straight pipe jet engine so no by-pass air to worry about) is the exhaust velocity the same at .82 (or whatever speed you wish to use) vs a stationary or accelerating aircraft.

Again, just something to chew on.

 

[gsrcrsx68]

looks simple enough...

http://www.allstar.fiu.edu/aero/Propulsion2.htm

good luck

 

[checkessential]

There's a better way to think about jet propulsion. Think of a blown up balloon, and the equal amount of pressure on every side of the balloon, until you open it. The compressed air will act unevenly on the side with the opening and "push" the ballon away from that direction. It's a poor example but the velocity of the exhaust is not actually what propels the a/c. It's the unequal pressure on the inside of the engine, which of course, is attached to your 727. Think of the pressure change occuring inside the engine. That's the principle of jet propulsion. Sorry for the lecture...

 

[gsrcrsx68]

uuhhh???

I thought the question was about exhaust velocity?

So what is it exhaust velocity of that balloon?

Is there fuel burning and expanding air in that balloon?

Is there any relationship to the speed of the balloon and the velocity of exhaust?

I am open to learning, just not understanding your example...

 

[Groucho]

Velocity and Mass Airflow

Source: Aerodynamics for Naval Aviators, Pg. 104
Ta= Q (V2-V1)
Ta = total thrust
Q= Mass flow, slugs per sec
V1= inlet(or flight)velocity, ft. per sec
V2= Jet velocity, exhaust, ft.per sec.

It would appear from the formula that exhaust velocity may in fact be constant or even increasing but that thrust decrease is somewhat due to :
1. a reduction in the value of V2-V1, or a loss of velocity change
2. Mass airflow would go down with altitude increase.

In fact, although the text doesn't really address the issue of velocity vs. airspeed and altitude, it would appear by experience that velocity V2, exhaust, actually increases but primarily with altitude. Engine RPM increases, and the air is less dense and although velocity change in the engine is decreased exit velocity would increase.... I think.....

"Men we will fight for this women's honor, which is probably more than she ever did" Groucho Marx

 

[checkessential]

Yes, there is a formula for knowing exhaust velocity. The exhaust however, does not push the airplane forward. It's the uneven pressure inside the engine. The engine compresses air by burning and expanding the air inside it, the result is merely a function of design. Standing at the gate, there is obvious airflow, but the question of the exhaust velocity at cruise is purely trivia, if you know how a jet engine actually works.

 

[checkessential]

The correct measurement of jet thrust is the engine pressure ratio (EPR), there is no measurement of exhaust speed because it does not factor into the thrust equation. When you speak "thrust", you are talking pressure- inside the engine, not exhaust speed.....

 

[JetPilot500]

Stump the pilot:


At a Mach .82 cruise speed, what is the velocity of the engine exhaust as it exits the engine??

What is the velocity of the exhaust during the takeoff roll, is it constant or does it change as the aircraft accelerates?

We thought of these yesterday while cruising at .82 and thought I'd get some input from the crowd.

Oh my god! I can't believe you guys don't know this.

It's 872.492 Meters / Second.

You guys really need to get studying some more!

Geeeeez,
JetPilot500

 

[rumpletumbler]

That sounds like some of the questions I got on my first CFI checkride with the fed guy. He would have found a way to relate it to adverse yaw though.

RT

 

[avbug]

EPR is only a correct measurement for some engines. Not all. Ity is not always the primary power measurement, nor is it always a reflection of thrust. Alone, it does not speak directly to exhaust gas velocity, either.

 

[SuperD]

Checkessential is abosolutely correct with regard to how thrust is actually produced. There are only 2 ways that a fluid can transmit a force to a body, and that is through pressure and shear stress. We represent thrust as a function of exit velocity, mass flow, and entry and exit static pressures and areas because measuring and adding up all the internal pressures of an engine is nearly impossible. Basically, we encompass the engine in a "control volume". We don't care what's going on inside the engine, just about the air entering and exiting. This allows us to represent the thrust in a convenient manner.

Getting back to the original question, it is difficult to determine what the actual exit exhaust velocity is at Mach .82. But, ignoring the increase of mass flow due to fuel addition, you can bet that the exit velocity will be greater than the freestream velocity entering the engine. For a given power setting, jet engine thrust is generally considered to be constant with speed.

SuperD

 

[~~~^~~~]

The speed of the air coming out of the turbine is relatively constant, since the pressures and temps are relatively constant at a given thrust differential. There are parts of the engine where the air slows to less than 200 knots in the combustion liner, but there it is at very high temps and pressures. The air accelerates as it moves through the turbines and the diffuser. The speed will vary per engine, depending on the design of the diffuser. A straight pipe like those used on military jets can be 1.7 Mach before the afterburner accelerates it further.

Fuel hits the burner can in the Shuttle's main engines at about 250 miles per hour. All the rest is temp and pressure and it accelerates to Mach 25+ - so the flow coming out of that engine is really full tilt boogie!

The larger the opening of the diffuser, the larger speed reduction you will get. The speeds vary wildly depending on where exactly you are taking the measurements.

I asked a similar question relating to speeds at the compressor to NASA engineers at OSH this year and was told they think of it more as pressure than speed. Speed is all dependent on the shape of the inlet and diffuser.

If you have a book on the SR-72, you can use it as an extreeme example that illustrates the point. The engine is just an air pump in the middle of an inlet and a diffuser. The sexy part may actually be the inside of the cowling!

 

[ASApuppy]

Well, you guys are dodging the question. The answer is near Mach 1. That is true for takeoff as well as 41,000 ft at .82Mach. The secret is in the design of the nozzle, which in engineering terms, is where the exhaust leaves the engine. The diffuser that Fins is talking about is commonly called the inlet. It slows the air down and compresses it before it gets to the fan or first compressor. On the SR-71, the airflow speed at the fan is sub-sonic even when the aircraft is going 3.5Mach. Actually that is true for all supersonic aircraft. Back to the question. Air molecules communicate air pressure changes by bumping in to each other. This is why sound travels and that the speed of sound is defined as Mach 1. The fastest that you can push air through a hole is mach 1. That happens at a pressure ratio of around 0.5. That is not the EPR reading, which measures the pressure differential across the fan. So, if you have a compressed air tank and pressurized it to twice the ambient and opened the valve, then right at the exit of the valve, nozzle, the speed would be mach 1. If you pressurized the tank to 100 times that, the speed at the nozzle would be the same. The nozzle is said to be choked.

Now then, the smart ones out there are saying "If I can only get Mach 1 out of my straight nozzle regardless of how much I push, how can I get a plane to go supersonic?" The answer to that is something that proved Bernoulli to be dead wrong. In slow speed , he was right, but in the supersonic world, his equations are actually backwards. That is why it took so long to break the friggen "sound barrier". Anyway, back to the new question. The answer is in a special nozzle called a "converging-diverging nozzle". What that is is a nozzle that contracts the diameter to a choking point, throat, and then expands the area until the actual exit piont. If done right, the velocity will be mach one at the throat and then continue to expand and accelerate to the exit point. Velocities can be enormous. This nozzle can be seen on all supersonic fighters. Some call them turkey feathers. They can be adjusted to vary the exit diameter and therefore the velocity. Transport aircraft, except concorde, don't have them and therefore are limited to engine exit velocities of mach 1.

There will be a quiz next week!

The pup

Aeronautical Engineer in my past life

 

[Rush Limbaugh]

My brain hurts!!!

 

[sydeseet]

Too cool pup. I'll pass the info along to my associate. Nice to have an intellectual discussion on here every once and a while. Thanks all!!!!!!!!!

BTW: Is the quiz open book?????

 

[Captain X]

Dorks.

 

[KlingonLRDRVR]

Nice post ASApuppy, my head still hurts but it kind of makes sence.

I think I will just set the temps at 795 and take what I get on the airspeed. Cheers

KlingonLRDRVR

 

[Torqued]

One additional note: The exhaust velocity may be mach 1.0 at the throat of the nozzle, but the high temperature of the exhaust means that the actual velocity is much higher than mach 1.0 outside the engine at ambient temperatures. ( Because the speed of sound varies with temperature ) So even a subsonic jet engine has exhaust velocities well above whatever the local mach 1.0 may be.