Stupid questions about aviation

[aaron886]

Can't think of any airplanes in which the throttles are linked... LD, other line pilots/freight dawgs, any transport category aircraft you know of with linkable throttles?

[rextar]

Reheat has always been a thing of beauty,but how does it work?

[IvanK]

Can't think of any airplanes in which the throttles are linked... LD, other line pilots/freight dawgs, any transport category aircraft you know of with linkable throttles?

 

Yes I think the B17 had a means to just use Two levers that dragged the other throttle along with them. You still had the option to tweak each throttle individually as well.

[aaron886]

Reheat has always been a thing of beauty,but how does it work?

 

Literally, dumping additional gas into the hot exhaust through little sprayer heads aligned around the exhaust duct. It adds energy to that exhaust to augment thrust. If you look in the tailpipe of a jet with afterburner, you'll see a ring-shaped device in there aft of the turbine blades... this is called a "flame holder" and helps shape the afterburner flame via some kind of fancy schmancy fluid dynamics mumbo-jumbo that I couldn't describe to ya. ;)

 

 

Yes I think the B17 had a means to just use Two levers that dragged the other throttle along with them. You still had the option to tweak each throttle individually as well.

 

Nice... thanks for the example. Wouldn't be too surprised if that standard has held up in many-engined airplanes.

[Yurgon]

Thx for the input, guys!

[Yurgon]

Got a new one (which sort of came up in MH370 Theories).

 

Does pilot training for CPL include lessons on previous disasters?

 

Obviously, when NTSB and their counterparts outside of the US identify insufficient pilot training as the cause of a disaster, pilot training will in turn be changed so that such things (should) never happen again.

 

But does pilot training also include lessons on actual disasters and crashes? Obviously, AF447 would be a crash that newbies should be taught about for generations to come, but I guess there are many more examples that, if taught to new pilots, should make it much less probable that they'll make the same mistakes that have happened in the past.

[Bushmanni]

What I have been wondering is how the jet engines provide thrust in only one direction instead of both aft and forward as its just an open pipe with some fan blades inside. How the pressure inside pushes the gases in only one direction? Is it accomplished just by shaping the burn chambers in a clever way or is there some other things to it?

[sobek]

Is it accomplished just by shaping the burn chambers in a clever way or is there some other things to it?

 

Well, you're extracting a lot of energy from the exhaust stream to compress the air on the intake side. I suppose the burner can shape has a big influence as well.

[Griffin]

What I have been wondering is how the jet engines provide thrust in only one direction instead of both aft and forward as its just an open pipe with some fan blades inside. How the pressure inside pushes the gases in only one direction? Is it accomplished just by shaping the burn chambers in a clever way or is there some other things to it?

To any jet engine related questions I suggest you turn to AgentJayZ on Youtube. He does Q&A videos and just a couple of weeks ago his Questions 28 video has an answer to that around 15:30.

 

 

Basically the compressor of the engine is driven by the turbine and the shaft power on that setup is tens of thousands horsepower. All these horsepower spin the compressor which rams air backwards into the engine increasing the pressure at each stage. The turbine on the other hand is basically an open duct with relatively free passage for the air, thus it has nowhere to go but back. It has to do with the pressure differential. A fluid or gas will always try to go to lower pressure from high pressure. As you have an immense pressure in front of the combustion chamber, the air has no other way to go than back.

 

AFAIK burner shape doesn't have anything to do with it but it does transfer thrust forces.

Edited April 4, 2014 by Griffin

[sobek]

AFAIK burner shape doesn't have anything to do with it but it does transfer thrust forces.

 

We'd probably have to go into depth and do some math to know for sure. Unfortunately i'm an unknown quantity regarding thermodynamics.

[Griffin]

I don't claim to know everything about jet engines but in the industry or literature I've never come across anything telling that burner shape affects the direction of the air flow. It's the compressor that brutally shoves the air in the right direction and pressure difference between compressor outlet and rear of the engine. The burner is naturally carefully shaped to be most efficient, but no matter the shape, the air/gas won't change direction.

Edited April 4, 2014 by Griffin

[Bushmanni]

Griffin, thanks for the video link. So how I understood it is that compressor pushes air forcefully through the pipe and it's the main culprit behind air going from intake to exhaust. The pressure inside the engine is created by the compressor and the reason why the pressure keeps up is the rapid expansion and acceleration of the gas in the burner ie. expansion makes up for the "leak" through the turbine. As aerodynamic forces act on objects through surface pressure it's mainly the pressure on compressor blades that makes the engine and thus the aircraft move forward.

 

Now with this I think I also figured out the afterburner. I spent maybe three hours trying to explain how I think the pressures inside afterburning engine work but couldn't make it simple enough that other people would be able to follow my thought and understand what I wrote. The essential point of my idea is that afterburner doesn't create "pushing pressure" but instead reduces "pulling ones" by allowing larger exhaust nozzle area while still providing same backpressure for the engine as smaller nozzle without afterburner. Maybe this is enough that someone who knows can tell if I'm at the right track.

[Griffin]

The main part of thrust is the acceleration of gases through the engine i.e. Newton's third law of motion. Pressure plays a minor role.

Thrust is created by burning the fuel and air thus expanding it and accelerating the mass. This acceleration of mass pushes the aircraft forward.

Compressor compresses the air to achieve conditions for efficient combustion.

Compressor in itself does produce thrust forces in forward direction but the turbine creates forces in the opposite direction which can completely counteract the compressor forces and even surpass them by a large margin. Thus these forces can be disregarded when thinking about the fundamentals of thrust creation.

 

Afterburning doesn't increase pressure in the jet pipe and it's actually undesired as it would affect the operation of turbine. It's again an expansion and acceleration of mass rearward.

 

EDIT: I had the wrong Newton's law but the second law applies to jet engines just as well.

Edited April 4, 2014 by Griffin

[Bushmanni]

I'm not trying to understand the big picture, ie. energy principles and conservation of momentum. etc. that are usually used to describe how the jet engine operates. What I want to know is what is pushing the engine forward, ie. how the exhaust and air molecules bounce off the surface of the engine so that the net force exerted makes the engine move forward instead of just sitting in there. As an analogy aircraft wing produces lift by pushing air downwards so that the impulse imparted on air is equal to gravity but the actual force pushing the wing upwards comes from air molecules hitting the bottom surface with more speed and/or more often than the upper surface ie. due to pressure difference. The impulse imparted on air and the pressure differential are two sides of the same coin as you can't have one without the other. What I'm trying to understand is the pressure distribution inside the engine that actually pushes the engine. It's obvious that this isn't a self evident problem to figure out.

 

The turbojet engine needs some pressure in the burn chamber and the compressor will create it but there still needs to be some way of keeping it there for the engine to work which means you need some constriction for the flow besides the turbine. Obviously the afterburner can provide this backpressure even if the nozzle is opened up and there's either more pressure on surfaces that push forward or less at surfaces that push aft (ie. "pull") because the engine is pushed forward more when afterburner is on.

 

In order to have more pressure pushing forward you would need to have some kind of mini nozzles in the afterburner where the ignited jet fuel is ejected. Otherwise the pressure would push air both aft and forward which would disrupt the operation of the engine. The nozzle theory isn't able to explain either how the required backpressure is generated as the nozzle is opened. As I'm pretty certain there ain't any nozzles that leaves only the option of less pushing backward.

 

When you open the nozzle more there's less area for the pressure inside the afterburner to push the engine aft. The opening of the nozzle would reduce the pressure differential at nozzle (the required backpressure for the engine) unless the flow speed is accelerated at the same time by expansion of the exhaust gases. Higher flow speed at the nozzle creates the same pressure differential with larger nozzle area.

 

As I'm thinking about this I can't but wonder how on earth the guy who invented jet engine was able to come about with such an idea. That was some out of the box thinking.

[Griffin]

In order to understand the internal operation of the engine, you must understand Bernoulli's law. You understand how it works with the wing but it can be counterintuitive when thinking about internal engine operation.

It states that when there is a convergent duct, the pressure drops and the velocity is increased. Thus afterburning doesn't increase pressure inside the jet pipe nor does it increase the engine pressure ratio (EPR). If it did, it would be working against the rest of the engine which is trying to accelerate air backwards to create thrust. It's also a common misconception that there is an increase of pressure in the combustion chamber, which is not true.

 

From Rolls Royce The Jet Engine book (1996 print):

"The nozzle is closed during non-afterburning operation, but when afterburning is selected the gas temperature increases and the nozzle opens to give an exit area suitable for the resultant increase in the volume of the gas stream. This prevents any increase in pressure occurring in the jet pipe which would affect the functioning of the engine and enables afterburning to be used over a wide range of engine speeds."

 

"The effect of afterburning is to increase the volume of the exhaust gases, thus producing a higher exit velocity at the propelling nozzle."

 

While thinking about thrust distribution and the conditions (P, V, T) in different parts of the engine, the big picture must also be remembered as total thrust equals the sum of internal forces at different stages of the engine.

And thrust results from acceleration of mass, not creation of high pressure behind the engine. Pressure plays a role but it's a small one.

Some engines may operate with the nozzle in choked condition which brings additional thrust from pressure. But since it doesn't apply to all jet engines, I won't go into that.

 

Here's a picture from the RR book. It shows the thrust distribution in all parts of the RR Avon engine. It's a bit misleading though as it doesn't show compressor and turbine stators which transmit thrust to the engine case. The rotors on the other hand act through the thrust bearing but it's not so simple as there is thrust balancing involved in the rotor system.

So thrust acts throughout the engine imposing forces on the structures.

I won't claim to understand it completely either as I'm not an engineer and there are a couple of counterintuitive things that I haven't yet figured out.

 

[Griffin]

Do we have someone here good with physics or maths? I have a question regarding jet engine thrust.

 

Rolls Royce The Jet Engine book (1996 print) gives the following thrust equation:

 

This is easy to work with especially since there are great examples of calculating thrust for the whole jet engine. This equation is also the same given in a Boeing presentation.

 

However the latest Rolls Royce book gives the following equation:

F = W (Vjet - Vflight) + A (Pexit - Pinlet)

 

And NASA gives (essentially the same):

F = (m dot * V)e - (m dot * V)0 + (pe - p0) * Ae

 

EDIT: The two last equations are basically the same as the first one without the gravitational constant. At least so I believe. Might be something I'm missing.

 

So the old RR book and Boeing give the equation with a gravitational constant but the latest RR book and NASA give the equation without it. NASA gives a good explanation of the equation breaking it down before combining it to the above form but nowhere there is a gravitational constant. The latest RR book doesn't explain it at all.

It goes without saying that without gravitational constant the result of the thrust equation is not the same so I can't use the NASA and latest RR equation to calculate thrust.

 

So what am I not understanding about the NASA method? Why is there no gravitational constant and why is it needed in the first place?

Edited April 6, 2014 by Griffin

[sobek]

What's the M in the picture?

 

Edit: A good tip how to figure such things out is always to cancel down the units (in this case for the fraction on the right) and see what you are left with.

 

PS: A heartfelt :doh: to RR for using imperial units. ;)

Edited April 6, 2014 by sobek

[SFJackBauer]

The g is there in the first equation since it is used to find the mass flow rate from the weight flow rate.

 

Since weight W is mass m * g (at least on Earth), then the mass m is equal to the weight W / g.

 

Note that in the first equation legend, it seems there is a typo since W is the weight flow, and it should've appeared in the equation in place of M.

 

See also http://www.grc.nasa.gov/WWW/k-12/airplane/mflchk.html and http://www.grc.nasa.gov/WWW/k-12/airplane/wcora.html.

Edited April 6, 2014 by SFJackBauer

[Griffin]

Ah, thanks guys! Makes a lot more sense. See, the last time I did this was in school so while it's simple and very basic, I just don't remember. :doh:

PS: A heartfelt :doh: to RR for using imperial units. ;)

I don't mind as I'm pretty fluent in imperial units and use pounds and horsepower instead of Newtons. Should propably get used to the SI system like the rest of the world. The book edition is from 1986.

Note that in the first equation legend, it seems there is a typo since W is the weight flow, and it should've appeared in the equation in place of M.

 

See also http://www.grc.nasa.gov/WWW/k-12/airplane/mflchk.html and http://www.grc.nasa.gov/WWW/k-12/airplane/wcora.html.

Yeah the typo appears on two equations on the first page of calculations and is corrected on the rest of four pages.

[Griffin]

The essential point of my idea is that afterburner doesn't create "pushing pressure" but instead reduces "pulling ones"...

You are really on the right track in a way as afterburning reduces the rearward thrust forces acting on the nozzle. It's really confusing to me as how the hell can the nozzle that produces the final thrust of the engine actually create rearward forces on the engine. Really counterintuitive despite understanding the theory behind it. The rearward thrust forces come from the pressure differential in the jet pipe vs free air as far as I understand. However afterburning doesn't reduce the rearward thrust forces by reducing or increasing the pressure, but with the acceleration of the gases. From RR book again:

 

 

Sorry, I didn't think all the way through the jet engine all the way back to the afterburner nozzle. I've been so narrow sighted trying to understand individual parts of the jet engine that I forget the rest parts and their variations.

 

Of course talking about afterburners involves choked and convergent-divergent nozzles (about which I refused to talk about).

So far the best book explaining the operation of the jet engine has been Jeppesen's Aircraft Gas Turbine Powerplants, which unfortunately I don't have physically, yet.

 

 

This isn't so much about lecturing other people as much as it's useful to me to write things down as it forces me to put it out in an understandable form, thus clearing the picture to myself.

Please correct me if you see something wrong.

 

EDIT: If the attachements were deleted on purpose then please tell me so I won't do the same mistake again.

Edited April 8, 2014 by Griffin

[Bushmanni]

After reading some theory from the Jeppesen's book I think I figured out what I was after. First of all compressor forces the exhaust out from the back of the engine instead of both ends. Pressure inside the engine is highest just after the last compressor stage before combustion chamber. The ducting and other flow constrictors are shaped so that the pressure (total pressure = static + ram) after the combustor declines as the flow moves towards exhaust. As the pressure difference always creates an acceleration towards low pressure the flow keeps going from forward to aft. If the compressor would fail to provide enough pressure or there was enough extra constriction added after combustor the exhaust would start going out from both ends and the engine would stop functioning.

 

The engine is pushed forward by air molecules mostly from combustion chamber, secondly from compressor blades and a little bit by turbine exhaust cone. The thrust created by compressor is similar to any fan or propeller, ie. airfoils moving fast through air. The combustion chamber is basically a pressure vessel that has larger outlet than inlet. Compressor forces air through the inlet creating pressure inside the combustion chamber and the expansion of the gas inside the chamber makes up for the greater flow through the outlet so that the pressure stays up. Turbine exhaust cone is bacwards facing cone which is pushed forward by the pressure inside engine exhaust (or afterburner) section. Turbine blades push the engine backwards due to drag of the turbine blades and exhaust nozzle due to pressure pushing the inside surface of the nozzle backwards.

 

When the afterburner is in operation the nozzle can be opened so that the pressure pushing the nozzle from inside is directed more to the side and less backwards than during mil power. You need to speed up the gas to produce the same backpressure in the open nozzle than when the nozzle is closed. The pressure inside afterburner stays the same but the force pushing the nozzle is directed more to the side so the force component facing rearward is smaller.

 

I was puzzled by the thrust equation where thrust created by pressure and that of flow acceleration is combined in the same equation. It contains the other half of my confusion that I was having with jet engines. Basic physics dictate that there are four basic forces ie. electric force, gravity and two nuclear forces that are relevant mostly to nuclear physics. Electrical force is responsible for atoms sticking together, colliding and bouncing off from each other. There's no other mechanism for fluid and solid to affect each others movement but by molecule collisions at the surface of the solid. The expanding and accelerating gas and the jet engine don't just agree to go separate ways and magically move in different directions. They have to interact through collisions which is to say through pressure. How this exchange of momentum is done in practice is what I am interested to know as by looking at the jet engine it's quite hard to see. Also having force from pressure and from change of momentum in the same equation for thrust is puzzling at first.

 

Simple answer is that pressure gradient accelerates flow and applies a force to the walls that contain the flow. If you integrate the force applied to the wall and the force applied to the flow by the pressure gradient (for which you can then calculate acceleration or change in speed) you get the same force (but they are to the opposite directions) for both as should. If you know how much the flow accelerated inside the engine you know how much force the pressure gradient applied to the gas and hence how much it applied to the engine. You don't need to know where and how hard the gas molecules are pounding the engine to calculate the thrust. All you need is the pressure and area at the exhaust nozzle of the engine and the force that is applied inside the engine to the flow through velocity change of the flow inside the engine. The part for change of momentum has the "hard to calculate with"-pressure gradient hidden inside of it.

 

Talking and writing about this stuff with others is educational and reduces the chances of making some stupid mistakes or assumptions. And it's of course more fun too.

[Griffin]

That sounds quite correct although I'm still learning here too. It's nice that people comprehend things differently which forces me to think stuff from different perspectives. I had to really process that text!

The thrust created by compressor is similar to any fan or propeller, ie. airfoils moving fast through air.

This is true for the rotor part. The compressor rotor blades add kinetic energy to the air and shove it backwards. The compressor stators do the opposite by converting the kinetic energy into pressure, thus slowing the air down. The total velocity of the air through compressor doesn't increase, so the quoted part doesn't apply to the compressor section as a whole.

Also the (high pressure) compressor rotor doesn't really apply a significant force to the engine structure because it may be cancelled by the turbine (depends I guess). However the compressor stators do apply significant thrust forces to the casing.

 

[Bushmanni]

How are the stator wanes arranged in relation to the compressor blades ie. in what kind of angle they are put in relation to the flow and the compressor blades? The airduct in compressor is converging which means the duct would speed up and drop the pressure of the flow. It seems a bit odd that airfoils increase the pressure of the flow and slow it down (or prevent acceleration in this case) so there's definitely something interesting going on.

[Griffin]

If no stator vanes would be present the velocity would indeed increase in a converging duct. But compression reduces volume. If the duct wasn't convergent, the air would slow down considerably. Converging duct keeps the total velocity constant by keeping the space/volume ratio constant for the ever more reducing volumes of air.

I think I could explain it a bit better but I have to leave work in just a moment.

 

Maybe the Jeppesen book has a nice picture of the angle?

Maybe this video would clear it up a little? Also the first few stator stages have a variable angle to keep the airflow optimal. They are called variable inlet guide vanes and variable stator vanes.

 

[Bushmanni]

Ok, found the secret of the stators from the Jeppesen book. They actually form a diverging duct as they are much thicker at the front. The airfoil nature has nothing to with the diffusion but only changes the flow direction to be optimal for the next compressor stage.