MolaKule Q&A on Aircraft Structures VII Engines

High bypass engines prioritize efficiency and reduced noise, making them ideal for passenger aircraft.
Low bypass engines prioritize speed and performance, making them suitable for military applications

 
Bypass ratio is the ratio of the air entering the fan inlet but bypassing the engine core.
The advantage of a higher bypass ratio is that it reduces the average speed of the exhausted air to something closer to the speed of the aircraft, which is more efficient.
In a sense, the core is driving the fan which is driving the aircraft.
This also makes the aircraft quieter, since the large amount of lower velocity air surrounding the turbine exhaust masks a lot of the noise.
Anyone who airport spotted back in the seventies observed this in how quiet the new widebodies were versus things like the B727 or B707.
Current generation engines have bypass ratios as high as 10:1.
 
Last edited:
MolaKule said:
The turbofan was a developmental outgrowth of the turbojet engine. Most modern commercial aircraft are powered by two- to three-spool axial-flow turbofan engines with "High-bypass" ratios.

Q1: What is meant by the by-pass ratio?

Q2: What advantages do a large bypass ratio (> 2:1) turbofan engine possess?

Ref: https://www.century-of-flight.net/turbojet-vs-turbofan-explained/
Click to expand...
You wouldn't see cheap prices if airliners kept using the JT8D such an iconic engine but a fuel hog. Then the Kunetsov NK-8's have that iconic piercing whine that Russian aircraft are known for.
 
97prizm said:
You wouldn't see cheap prices if airliners kept using the JT8D such an iconic engine but a fuel hog. Then the Kunetsov NK-8's have that iconic piercing whine that Russian aircraft are known for.
Click to expand...
You haven't heard a whine until you've been around a Vickers Viscount.
Fortunately, you can still hear it in The Beatles Back in the USSR.
 
Last edited:
Engine discharge air temp is directly related to discharge speed. The hotter the discharge air, the faster we can move it before stagnation, choking or a limiting shock wave forms in the duct.

Although most pilots don't have the instrumentation necessary to see the relationship between duct air speed, temperature, pressure and airspeed, I promise, it is closely related.

In our Gulfstream jets, I've seen 250Kts TAS at low altitudes. with equal inlet and outlet pressures! This happens because the heat of combustion effectively allows the discharge velocity to be very high without stagnating in the duct. (remember F=MA)

Afterburner.webp

The Afterburner (or reheat) simply heats up the air in the duct, to 2200ºF or more. This raises the speed of sound to 2000+MPH (the stagnation point) and expands the air. Without changing the load or pressure on the engine itself, it can double the thrust and substantially improve top speed.

Another interesting way to go fast is to use a multi stage fan. This heats the air via compression, and therefore increases duct discharge velocity. The hotter we make the air, the faster we can go.

affinity_2018_hero_small.jpeg


This is why driving a turbofan with an electric motor is generally limited to 350Kts. The discharged air is cold.

On our faster corporate jets, we try hard to mix hot exhaust with cold fan air, well before the nozzle.

PW815.jpg
 
fdcg27 said:
Bypass ratio is the ratio of the air entering the fan inlet but bypassing the engine core.
The advantage of a higher bypass ratio is that it reduces the average speed of the exhausted air to something closer to the speed of the aircraft, which is more efficient.
In a sense, the core is driving the fan which is driving the aircraft.
This also makes the aircraft quieter, since the large amount of lower velocity air surrounding the turbine exhaust masks a lot of the noise.
Anyone who airport spotted back in the seventies observed this in how quiet the new widebodies were versus things like the B727 or B707.
Current generation engines have bypass ratios as high as 10:1.
Click to expand...
Here is an interesting slide show from P&W on the "Geared" turbofan project:

https://academieairespace.com/wp-content/uploads/2018/05/prattw.pdf
 
THRUST: The force that a jet engine develops and the one that drives the plane forward in normal flight, is called "THRUST." The engine thrust is transmitted to the pylon, then to the wing spars or empennage, and then to the fuselage.

Supply the missing word: According to Newton's Laws., any jet engine develops thrust by imparting -------- to the fluid passing through it.
 
MolaKule said:
THRUST: The force that a jet engine develops

Supply the missing word: According to Newton's Laws., any jet engine develops thrust by imparting -------- to the fluid passing through it.
Click to expand...
That's an easy one and begins with the letter M, M. er, ah, MONEY, lots and lots of it. That money converts to Horsepower, heat and noise, insane amounts of each. All of which is required to impart momentum or acceleration.

I know it's silly, but at takeoff thrust, one of our thirsty Gulfstream jets, when flying out of the expensive NorthEast, the fuel flow rate was $22,000 per hour.
 
Cujet said:
All of which is required to impart momentum or acceleration.
Click to expand...
According to Newton's Laws., any jet engine develops thrust by imparting Momentum to the fluid passing through it.

I.e., Momentum - Mass of air X Velocity of the air. The sum of the momentum of the bypass air + the momentum of the exhaust gas gives rise to the total thrust of the engine.
 
Yep, momentum - and the exhaust velocity has to be higher than the airspeed of the airplane - or no momentum is imparted to the air - and there is no net thrust.

Which is why airliners have a top speed about 0.85 Mach - you simply can’t move the bypass air through the engine at supersonic speeds by using a big fan section.

On a fighter, at supersonic speeds, the exhaust velocity is considerably higher than the airspeed of the airplane - it has to be, or no net momentum is imparted.
 
Take for example the P&W4000 as used on the B767 and Airbus models:

https://en.wikipedia.org/wiki/Pratt_&_Whitney_PW4000

Bypass ratio = 4.8

Two-Spool Design (two drive shafts) with high and low pressure turbines

Inlet (fan tip) diameter = 93.4 inches

Thrust range is 50,000 to 65,000 pounds depending on model

Overall Pressure Ratio = 30.8

Total airflow rate = 1750lbs/sec

High Pressure Turbine inlet temperature = 2450F

Materials: Nickel Steel, Titanium, carbon fiber.
 
On 9/12/2001, walked out of my house in Boston to hear two low bypass aircraft overhead. Knew the sound from military service.

Two F-15’s were flying CAP over Boston. Nothing else in the sky until a few planes came in Sunday 9/16.
 
Cujet said:
That's an easy one and begins with the letter M, M. er, ah, MONEY, lots and lots of it. That money converts to Horsepower, heat and noise, insane amounts of each. All of which is required to impart momentum or acceleration.

I know it's silly, but at takeoff thrust, one of our thirsty Gulfstream jets, when flying out of the expensive NorthEast, the fuel flow rate was $22,000 per hour.
Click to expand...
Yup some jet engines can eat more fuel than all the rows of toilets in the rest rooms at a fair. :ROFLMAO:

I think some of the larger engines are in the $100,000,000. range for new cost, and some of the blades or buckets are likely in the $20,000 range. I think they are machined and also 3D printed. Then some parts are now sintered as well, and have caused failures if memory is correct.
 
Last edited:
We know from previous BITOG discussions that a specially formulated POE oil of ~5 cSt is used to lubricate the engine's bearing cells.

Bearings support the drive shafts: "Bearings are lubricated inside a bearing sump [cell], which is sealed. Often labyrinth type seals are used together with air, which is also holding back the oil. Air and oil have to be separated and the air is eventually vented over board. Some oil is lost along various paths causing oil consumption. Only oil lost through bearing seals upstream of the bleed ports can contaminate the bleed air."

According to the white paper below, what is the average volume of oil loss in terms of liters/hour during operation?

https://www.fzt.haw-hamburg.de/pers...tEngineBearingsAndOilConsumption_18-04-02.pdf
 
Skippy722 said:
I haven’t yet had the (dis)pleasure of cutting titanium… but inconel fights back. Super tough stuff. Amazing finishes though without polishing!
Click to expand...
Both those materials are very easy to cut nowadays, but back when the only choice was highspeed/ cobalt cutters both were not fun at all.
I'd have to look for old notes but I think when running some drilling tests on inconel I was pushing a 1/8 carbide through spindle coolant drill at over 8 inches a minute, it may have been faster its been awhile.
 
We know from previous BITOG discussions that a specially formulated POE oil of ~5 cSt is used to lubricate the engine's bearing cells.

Bearings support the drive shafts: "Bearings are lubricated inside a bearing sump [cell], which is sealed. Often labyrinth type seals are used together with air, which is also holding back the oil. Air and oil have to be separated and the air is eventually vented over board. Some oil is lost along various paths causing oil consumption. Only oil lost through bearing seals upstream of the bleed ports can contaminate the bleed air."

Question still stands: According to the white paper below, what is the average volume of oil loss in terms of liters/hour during operation?

https://www.fzt.haw-hamburg.de/pers...tEngineBearingsAndOilConsumption_18-04-02.pdf
 
MolaKule said:
We know from previous BITOG discussions that a specially formulated POE oil of ~5 cSt is used to lubricate the engine's bearing cells.

Bearings support the drive shafts: "Bearings are lubricated inside a bearing sump [cell], which is sealed. Often labyrinth type seals are used together with air, which is also holding back the oil. Air and oil have to be separated and the air is eventually vented over board. Some oil is lost along various paths causing oil consumption. Only oil lost through bearing seals upstream of the bleed ports can contaminate the bleed air."

Question still stands: According to the white paper below, what is the average volume of oil loss in terms of liters/hour during operation?

https://www.fzt.haw-hamburg.de/pers...tEngineBearingsAndOilConsumption_18-04-02.pdf
Click to expand...
0.18L/h~0.43L/H on average
 
You must log in or register to reply here.

Similar threads

MolaKule
MolaKule Q&A on Aircraft Structures III
2
Replies
38
Views
571
97prizm
C
  • Locked
The internal combustion engines--history
Replies
9
Views
7K
HerrStig
Falcon_LS
  • Locked
Engineer's "opinion" on synthetics
3 4 5
Replies
97
Views
67K
SubieHo
D
  • Locked
Q&A with Chief R&D Chemist for Fuchs/Silkolene
Replies
9
Views
8K
Oilgal
Back
Top Bottom