XNV
Topic Author
Posts: 141
Joined: Mon Jan 31, 2000 1:45 am

Bypass Significance

Mon Sep 18, 2000 3:16 pm

I understand what bypass is, so you don't need to waste time discussing that concept again.

My question is WHY is it so significant? Your standard JT-8D has a ratio of about 1.3:1 or so, and a CFM56 has a ratio of 5:1.

Why does high bypass matter?

Thanks in advance.
 
dnalor
Posts: 346
Joined: Fri Mar 10, 2000 7:58 pm

RE: Bypass Significance

Mon Sep 18, 2000 3:36 pm

Basically (the same as my knowledge) the higher the ratio the more efficient/powerful the engine as the bypass fan is driven by the engine/gasses and for want of a better terminology is free power, it also acts as a muffler of sorts too. As well as being kinder to the enviroment.

Engines like on a 727 were pure jets, no bypass at all, hence no bypass to help quieten the exhaust down.

I'm sure some of the other guys here will have more detailed anwsers.

cheers
 
JETPILOT
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RE: Bypass Significance

Mon Sep 18, 2000 3:38 pm

Jets work on the principle that every action has an equal and opposite reaction.

There are two ways this action/reaction can be created.

One way is to move a small volume of air at very high velocities like a low bypass jet, or you can move large volumes of air at low velocities.

The high by pass jet is much quieter because of it's higher volume lower velocity cold steam bypass.

It also takes quite less energy to accelerate a large amount of air to a relatively low speed as opposed to moving a low volume at high speed adding to fuel efficiency.

JET
 
JETPILOT
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RE: Bypass Significance

Mon Sep 18, 2000 3:39 pm

The JT8D used on the 727 is a low bypass engine.
 
dnalor
Posts: 346
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RE: Bypass Significance

Mon Sep 18, 2000 3:54 pm

I stand corrected!
 
timz
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Joined: Fri Sep 17, 1999 7:43 am

RE: Bypass Significance

Tue Sep 19, 2000 2:31 am

Like JETPILOT said, the point is to shove more total mass rearward at a lower speed. I may be going off half-cocked here, but I think you can see the advantage if you consider the classic example of the man sitting with a load of bricks on a sled on a frozen lake. If he throws a brick rearward he and the sled start to move. He has expended a certain amount of energy; by far the largest part of that energy has ended up in the brick, doing him no good. But if he expends the same total amount of energy (not the same amount per brick) on throwing two bricks at once, or four, or twenty (so that the bricks end up going slower) then a larger fraction of the total energy expended will go into his velocity instead of the bricks. (Yeah, I know-- the other advantage is that he then has that many fewer bricks on the sled. But we'll pretend he has so many bricks that that difference is negligible.)
 
Greeneyes53787
Posts: 817
Joined: Tue Aug 08, 2000 10:34 am

RE: Bypass Significance

Tue Sep 19, 2000 6:04 am

Timz' explanation is practally perfect...for a rocket motor. A rocket operates on this principle of expedient weight transfer.

A jet engine pushes air backwards.

The bricks out the back theory relates not to relative wind created by the bricks but by the great transfer of weight aft. Thrust is created but by a different formula altogether than a jet.

GE

Ps-Aaron can give the formula. Consult him.
 
sabenapilot
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RE: Bypass Significance

Tue Sep 19, 2000 6:57 am

Hello guys,

Thrust is the product of the difference between the intake speed of the airflow (TAS) and the exhaust speed of the airflow (vjet) with the mass of air per second used by the engine (Dm);

F = Dm x (vjet - TAS)
Dm has to be in kg/sec.
TAS and Vjet in m/sec.
F is then given in Newtons.

So, actually both of you are correct, you have the relative wind component, i.e. vjet - TAS.
You also have the weight component, i.e. Dm.
If you'd ignore eighter one of them you multiply by a zero in the formula, thus ending up with no thrust....
 
sabenapilot
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RE: Bypass Significance

Tue Sep 19, 2000 7:14 am

BTW,
some brainexercise for those of you who like it:

You can apply the same above formula from a jet engine on a rocket, even though both mechanisms are completely different!
Everything is relative, remember? (Einstein)

On a rocket going to the moon, a certain mass of fuel is burned per second, leaving the rocket at a speed vjet.
Since there is no intake on a rocket there in no need to talk about relative wind, the exhaust speed simply equals the relative wind if you want.

the above formula thus becomes:
F = Dm x vjet.

Sometimes some algebra can be so good to explain very diffenent things in a logical and similar method.

BTW, before someone asks, yes, the same formula can also be used for small balloons, jetfoils, hovercrafts, yachts etc. etc.
Maybe some of you will find all this strange, but that's what physics is all about....

 
Greeneyes53787
Posts: 817
Joined: Tue Aug 08, 2000 10:34 am

RE: Bypass Significance

Thu Sep 21, 2000 1:19 pm

This is wonderful. I am glad for this response. Remember though that with an unknown power source a fan could theoretically propell an aircraft. The jet engine would not need to exist. This fan simply does two things. It blows air in one direction and it propells the machine in the opposite direction. The machine never equals the exhaust speed and the exhaust is never the same as the surrounding air.

There is no weight distribution at all. This is not a matter of weight distribution but of leverage. Gasses contact other gasses--and push those. This causes an equal reaction in an opposite direction.

But in rocket motors, theoretically a large object could be dropped while tethered inside a tube (in a vacuum) causing the tube to rise (before the object impaled the bottom. The tube would not need to be open at either end.

These two ideas are related but only barely. The top depicts a fan engine. The larger the fan the less the power source of the fan in germane. The difference between the intake air and the exhaust air of the turbine engine only produces the power for the fan.

Think about it.

GE
 
UAL747
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RE: Bypass Significance

Fri Sep 22, 2000 8:36 am

So does the 777 have the engines with the largest bypass ratio? Would that make them the quietest as well?
"Bangkok Tower, United 890 Heavy. Bangkok Tower, United 890 Heavy.....Okay, fine, we'll just turn 190 and Visual Our Way
 
Greeneyes53787
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RE: UAL747

Fri Sep 22, 2000 9:29 am

No. They might be. They might not be. The higher the bypass, generally the lower the exhaust speed. Therefore a quieter engine is expected. However, the exhaust is not the only contributor to engine noise.

An independent concern has been working on a turbojet concept that is very quiet, clean burning and powerful. This concept is also more efficient than fanned engines.

My personal view is that the fan engine has only been a stop gap between the first generation turbojets and the next (for commercial use).

GE
 
timz
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Joined: Fri Sep 17, 1999 7:43 am

RE: Bypass Significance

Fri Sep 22, 2000 10:21 am

CMIIW, but as I recall the TF39 on the C-5 has an 8:1 ratio, and is not that quiet. (Does any in-service engine beat that ratio?)
 
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fanoftristars
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RE: Bypass Significance

Fri Sep 22, 2000 11:30 am

The 777 has the highest bypass ration on its family of engines. from 6:1 as high as 9:1. I think this is the GE 90 with the 9:1 ratio, since it has the biggest fan (123") I remember reading that somewhere.
"FLY DELTA JETS"
 
aaron atp
Posts: 517
Joined: Sun Mar 19, 2000 1:17 pm

Technical...

Sat Sep 23, 2000 4:34 pm

Let's say that we have two engines mounted on test stands. We feed both 2000 pounds of fuel per hour. Engine 1 is a turbojet, and produces 2000 pounds of thrust. Engine 2 is a turbofan (e.g. 6:1 BPR), with engine 1 as the core, and produces 4000 pounds of thrust.

Engine 1 has a TSFC (thrust specific fuel consumption) of 1.0, while the turbofan will have a .5 TSFC. A lower TSFC means higher efficiency.

Consider this: To be efficient at low speeds, we want to move a lot of air; but that air cannot be forced through the combustors. The air which passes through the fan is accelerated above the free stream velocity, without requiring a large increase in fuel consumption. Thrust has increased without increasing fuel flow, but the Conservation of Energy still applies. Where did the increase in thrust come from? Better system efficiency.

As long as we are operating in the sub/transonic realm, that exhaust velocity is irrelevant, because it will increases proportionally to the free stream air velocity anyway. A turbojet's efficiency increases as the free stream velocity increases, but the turbofan will be more efficient at any stage; especially low speeds. Sabenapilot uses a derivation of the more commonly used thrust equation:

Thrust = m.eVe - m.0V0 + [(pe - p0)Ae]
where m. = rVa = density * velocity * area
m. is read "m dot" and is referred to as 'mass flow rate'

In english words, the exit mass flow rate multiplied by exit velocity, minus the free stream mass flow rate times free stream velocity {since you brought it up, a rocket has no free stream mass airflow, the equation is shortened to Thrust = m.eVe }

the section in brackets [] is omitted when the free stream pressure is equal to the exit pressure, as is usually the case because the nozzle most often equalizes the pressures.

Now I’m sure (quite positive actually) that you are asking “why does an increase in mass airflow rate result in a more substantial increase in thrust and efficiency than simply scaling the size of the turbojet?" Everyone wants to blame a trade in exhaust velocity for mass flow, which is an incorrect assumption. The exhaust velocity of the engine 2 core (see above example) will not be substantially decreased, relative to engine 1, by the addition of the fan. The average velocity of the exhaust may be reduced, but that doesn't mean much for this example.

that means: m.eVe - m.0V0 will remain the same for the core engine, but our final equation will look more like this:

m.eVe - m.0V0 + m.fΔV
where ΔV is the positive velocity increase for the fan output.

Read that as: thrust equals the thrust of the core engine, plus the mass flow rate of the fan multiplied by the change in velocity across the fan is the total output. Think of it as free thrust added to the core engine!


My question is WHY is it so significant? It is more efficient.



aaron


 
Greeneyes53787
Posts: 817
Joined: Tue Aug 08, 2000 10:34 am

RE: Bypass Significance

Sun Sep 24, 2000 11:00 am

But also be advised that the fan takes energy to run. We like to think of the fan's operation as free power but it really is not. However, at subsonic speeds the fan engine of today is most efficient.

GE
 
Guest

RE: Bypass Significance

Mon Sep 25, 2000 2:18 am

wouldn't a turboprop be the most efficient engine at subsonic speeds? the core engine produces only about 2% of the complete thrust, the bypass is in fact the only thing that matters, a turboprop moves great masses of air with little increase of speed for this air.
of course, it could be that the missing mantle around the propeller means a massive decrease in thrust.
 
Greeneyes53787
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RE: Jsbothe

Mon Sep 25, 2000 3:37 am

I need a little help from Aaron on this one. I, personally, like to call the high bypass engines ducted turboprops. They technically aren't "props," but the turbojet's exhaust thrust decreases as the greater drag on a fan increases--to the point of practically no thrust from the actual turbojet.

I think this is really a ducted turbo prop--but with a fan design instead of a propellor. But again I could use a little correcting from the great technical writers.

GE
 
aaron atp
Posts: 517
Joined: Sun Mar 19, 2000 1:17 pm

Turboprops

Tue Sep 26, 2000 11:49 am

>>>wouldn't a turboprop be the most efficient engine at subsonic speeds

Yes, but it's limited to low subsonic speeds as you've noted. Without the nacelle, we can't slow the air down to keep tip velocities under their critical mach number. Such a penalty prevents the turboprop from operating at the higher speeds being demanded today.

Regionals are switching to all jet fleets, because many pax are become apprehensive when they see the propeller, although the existing turboprops may be more efficient. In many regional routes, the speed isn't even an issue because of airspace restrictions.

The previous discussion of the UDF has a little more discussion on the topic.




aaron

 
Greeneyes53787
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Joined: Tue Aug 08, 2000 10:34 am

RE: Turboprops

Wed Sep 27, 2000 6:24 am

Aaron's info is good here. I only want to point out that there is a current duscussion on the UDF. I haven't read it since the earlier one was thorough. Check the General Aviation section. That might be where the newer one is. Then find the older one.

GE
 
Fly-by-pilot
Posts: 180
Joined: Mon Sep 04, 2000 10:45 am

RE: Bypass Significance

Wed Sep 27, 2000 1:11 pm

The most efficient is a helicopter rotor. I think. What do you guys make of that PW-8000 geared turbofan.
 
Greeneyes53787
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Joined: Tue Aug 08, 2000 10:34 am

RE: Fly-By-Pilot

Sat Sep 30, 2000 12:10 pm

This puzzles me, if it is true. A helocopter loses efficiency as it begins travel forward. The rotors must adjust their angle of attact constantly during forward flight. For every revolution each rotor must adjust. This is costly both for maintenance and fuel economy.

Because I am not in the loop on this one, I'd like to hear from an aerodynamics expert.

GE
 
Buff
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Joined: Fri Mar 16, 2007 1:29 pm

RE: Bypass Significance

Sat Sep 30, 2000 1:14 pm

Embedded in aaron's most excellent posting above is the single reason high bypass turbofans have become so popular amongst pilots and operators:

A turbojet's efficiency increases as the free stream velocity increases, but the turbofan will be more efficient at any stage; especially low speeds.

In two words or less - "Shorter Runways". The modern jet aircraft uses significantly less runway for takeoff than older turbojets.

With regards fan vs core thrust, please excuse my taking the liberty of re-posting an explanation of mine from some months ago:

As promised, I am posting my findings on a topic from last week discussing reverse thrust on a high bypass turbofan engine. The discussion surrounding propulsive efficiency of high bypass turbofan engines seemed to end up with two opposing schools of thought. My opinion was that at high forward speeds, a high bypass turbofan engine produced most of its thrust from core exhaust with little thrust from the fan. An opposing viewpoint, held by most contributors was that at all speeds, core exhaust was little or no factor in overall thrust output of the engine.

To resolve my thinking on the issue, I’ve spent the last day digging through several text books on turbine engine theory. Without offering a lot of mathematics, I will put forth one of the examples used to determine propulsive efficiency of a high bypass turbofan engine. Propulsive efficiency is only one of two types of efficiency, the other being thermal efficiency, used in determining the overall efficiency of the engine, i.e. its ability to do work.) Part of this formula gives a thrust calculation in which the resultant (kinetic) energy of the total exhaust (bypass & core exhausts combined) is one-half of the mass of gas times the square of its velocity. The forward speed of the engine through the air is also factored.

The example goes like this: “A high bypass turbofan is at cruise altitude with a flight speed of 532 mph or 780 fps (feet per second). Its fan exhaust velocity is 995 fps with a mass of 550 lbs/sec; its core engine exhaust velocity is 1,450 fps with a mass of 110 lbs/sec.” These numbers are plugged into a complicated formula to determine that propulsive efficiency of this example is 80%.

From the text, the thrust calculation part of the overall propulsive efficiency formula gives the following values (I don’t know what the unit is, but I think it is ft/lbs/sec): The fan produces 92,235,000 units or 61.6% of total thrust; the core produces 57,486,000 units or 38.4% of total thrust. More simply, the core forms only about 38% of the total thrust at speed/at altitude. If the formula is calculated with the aircraft at a standstill, as in at takeoff, the resultant numbers are: fan – 426,855,000 units or 77.4%; core – 124,410,000 units or 22.6%.

So my assumption that the fan merely idles along with the engine core thrust providing the predominant propulsive power while at speed and altitude is incorrect. So too is the statement that all engine power is completely absorbed by the turbines and that residual exhaust from the core provides little or no thrust. This last statement is much closer to the truth though when discussing turboprops.

What I found to support my assumption at least in part is that fan thrust is much more a part of the total thrust on the ground at low speed than at altitude and high forward speed.

I hope that explanation clears the air on this very interesting topic. I most enthusiastically welcome more postings/discussion of this issue and by no means purport to be an expert on the subject. My thanks to Jetpilot for making me look at this topic in greater detail.

For copyright purposes, the text from which the above example was taken is Jeppesen Sanderson Training Products: Aircraft Gas Turbine Powerplants © 1996. Other texts I used to find information were Rolls Royce’s The Jet Engine © 1973 and Pratt & Whitney Canada’s The Aircraft Gas Turbine Engine © 1984.


Best Regards,

Buff
 
Fly-by-pilot
Posts: 180
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RE: Bypass Significance

Sun Oct 01, 2000 2:11 pm

Not sure greeneyes. So what do you guys think of that PW8000 geared tubofan. Do you think all turbofans will be geared in the future.
 
AFa340-300E
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Reliability?

Tue Oct 03, 2000 3:41 am

Hello,

What makes bypass engines more reliable than non-bypass ones please?

Thank you,

Best regards,
Alain Mengus
 
AC183
Posts: 1496
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RE: Reliability?

Tue Oct 03, 2000 11:13 am

I see lots of posts, so if this has been said already then I apologize, I looked at them all, but I just scanned them quickly, and I didn't see this info said.

I think the question at hand (why higher bypass engines are more efficient) can be answered using a thermodynamic approach. Basically in a turbine engine we have a simple cycle taking place. First there is a compression of air that causes the air to heat up as well as increase in pressure (as per the ideal gas law). This is followed by a constant pressure addition of heat (in the combustion chambers), then an expansion of the hot air generates power in the turbine, and the resulting warm air is sent out the back of the engine (I say warm, as it's cooled significantly from the hot section by the air expanding in the turbine). This is the Brayton cycle. I think most people here probably understand this process quite well. Now to bring the question of efficiency into this problem. There are ways to calculate efficiency in theory, but I won't go into figures. Basically the theoretical efficiency of the cycle is increased if the temperature in the hot section is increased. Now when you have a turbine operating like this there is a significant amount of excess energy. One approach is to simply allow it to go out the back of the engine (ie- turbojet) and provide forward thrust. However, if you're looking at a turbofan engine, if you increase the hot section temperature, you have more excess power available to drive a larger bypass ratio fan. So basically I think this answers the question: the combustion section temperature dictates efficiency of the turbine to a good extent, and the hotter the temperature the larger the fan it can power. In actuality it is the heat resistance of the metal used in the hot sections of the engine that determine how high the temperature can go, and thusly the efficiency.

Just an anecdote: turbine inlet temperatures were at about 1000F (540C) during the 1940's, and have increased to roughly 2600F (1425C) today, obviously this is a good part of the factor in improving engine efficiency in the last 60 years since Sir Frank Whittle.

It is my understanding that this also explains the poor efficiency of many Russian built engines. I have been told, anyways, (not by a prof, but by someone who is in the aviation profession) that the Russians didn't have the high-temp alloys like in the western world. This kept them down to lower temperatures, and thus their relatively fuel inefficient engines, at least in the past.

If anyone is particularly interested in the calculations of efficiency, look up the Brayton cycle in a thermodynamics textbook. I believe that most thermodynamics books will essentially evaluate the numbers are for the core, and don't give thrust numbers, rather they examine the shaft output power, but it's interesting to see, anyways, and it give an idea of the power output that goes to the bypass fan. BTW, the thermo book I have is "Thermodynamics: An Engineering Approach," by Cengel and Boles. You should be able to find it, or a similar book in a university library somewhere, if you're interested.

Also, to comment on another thread on this tech-ops forum, I wanted to say that although I don't frequent this forum quite as much (due to lower activity here), and although I haven't bothered to fill in my list of respected users (maybe I'll bother to do it later), I have to say I have great respect for the people here, and in the main forum, who are seriously interested in discussing information in a good, constructive manner. Thanks.
 
aaron atp
Posts: 517
Joined: Sun Mar 19, 2000 1:17 pm

RE: AC183

Tue Oct 03, 2000 2:56 pm

I think you've missed a key point in this discussion. Combustion efficiency has nothing to do with explaining the effects of bypass ratio; as was the point of the original post (and all subsequent replies). In this previous discussion of thermodynamics, we discussed that topic. Case cooling would be the only (albeit minor) exclusion to these divisions. I've replied to this, in part, to ensure that readers keep these two parts of total system efficiency separate. Combustion chamber efficiency shouldn't not be discussed in conjunction with more mechanical (fluid flow) efficiency.

>>>I think the question at hand (why higher bypass engines are more efficient) can be answered using a thermodynamic approach

Combustion efficiency, as it is discussed in your reply, has nothing to do with the effect of BPR on engine efficiency. The two are entirely exclusive of each other.

>>>One approach is to simply allow it to go out the back of the engine (ie- turbojet) and provide forward thrust. However, if you're looking at a turbofan engine, if you increase the hot section temperature, you have more excess power available to drive a larger bypass ratio fan

That's an oversimplification of what engine designers are doing in modern engines. My first post explains why. You could essentially turn any turbojet into a core engine for a turbofan, and the efficiency would increase without significantly affecting the thermodynamics of the combustion process. Combustor efficiency can then be further increased, but the effect of decreased fuel consumption would be small in comparison to the effect of adding the fan.

__________________________________________

>>>What makes bypass engines more reliable than non-bypass ones please? Alain Mengus

I don't think it is wise to group engine reliability by BPR. Of course modern technology devotes itself to creating more efficient engines for propulsion purposes, so the bypass engine has received more modern refinement of material and processes than the turbojets of yesteryear. I can't think of any of any modern subsonic jetliners propelled by turbojets (for reasons other than reliability, e.g. efficiency), but I would guess that such a turbojet designed to propel an aircraft today would be quite reliable by any standard.


aaron
 
AC183
Posts: 1496
Joined: Tue Jul 06, 1999 10:52 am

RE: AC183

Wed Oct 04, 2000 12:45 am

Aaron, I think we're looking at very different questions. Thus far the discussion has been answering strictly about bypass. (by the way, could it be as simple, perhaps, as conservation of momentum / conservation of energy???)

The question I am seeing when I read the first post, is not strictly bypass, but rather I am seeing "why have bypass and efficiency increased together in the time since the RR Conway?"

For that question, what I am trying to say (although I seem to be muddling it again...) is simply that when you look at the whole engine (as I am looking at the question as being why the whole engine is more efficient on newer, higher bypass engines) that the increased turbine inlet temperature has both increased core thermal efficiency, as well as having made it possible for smaller cores to power larger bypass fans. I am not trying to explain why larger fans are better, rather I am saying they are possible because of improved cores. Sorry about my muddling with that...
 
aaron atp
Posts: 517
Joined: Sun Mar 19, 2000 1:17 pm

RE: Bypass Significance

Wed Oct 04, 2000 4:03 am

I replied because the initial question was Why does high bypass matter? If it is your prerogative to delve in other gas turbine principles, no problem. I just want everyone to know that combustion efficiency is a entirely different topic.


>>>but rather I am seeing "why have bypass and efficiency increased together in the time since the RR Conway?"

I won't try to argue you away from that, but I think XNV was wondering 'why a fan added to an existing core produces more thrust than simply scaling the size of the core turbojet.'

>>>(by the way, could it be as simple, perhaps, as conservation of momentum / conservation of energy???)

If you'll take it on faith, yes. That's why we call it the First Law of Thermodynamics

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