Engine locations 3

[sughew]

Hi

Many new large aircraft (Boeing, Airbus...) have the engines located under the wings.

I don't know if they are around anymore, but I remember the planes with two or three engines located at the tail of the plane.

What are the main benefits of installing the engines at the back? Is the balance and flight characteristics affected much?

 

[dalcazar]

For one thing, it reduces cabin noise to a certain extent, of course this wasn't true for the DC-9, especially if you were sitting in the back... god I remember that trip... never again if I can help it.

Other reasons include a lower ground stance for the smaller business size jets, and being higher they are less likely to injest rocks or anything else that might come off the runway. (like concord tires)

The main reason for putting the engines under and forward of the wing is because they generate the least possible amount of drag there, and they also help by being near the center of gravity. There is also a drag penalty caused by the air flowing between the engines and the fuselage on rear-mounted aircraft like the ones you mention.

aerodynamics and weight considerations aside, it's also much easier to work on an engine that is hanging right in front of your face instead of having to use a lift to obtain access to the engines

 

[Kwan]

Another benefit of wing mounted engines is to provide inertia relief. This will reduce wing bending.

 

[rb1957]

another complexity of hanging engines on wings to aero-elastic effects (the large mass of the engines has a significant effect on the mass distribution of the wing),

and then there are dynamic effects from landing

both of these are quite a bit less significant for fuselage mounted engines

 

[tbuelna]

How about this:

 

[rb1957]

us engineers are always thinking of new ways to "skin the cat" ...

i know this is the new honda-jet, a similar configuration was tried in the 70s/80s, the VFW614 i think; i guess it's a way to improve the ground clearance issues (of an under wing installation) i wonder ('cause i'm not smart enough to know) about the aerodynamic effects (is there a shock on the wing? (probably not) how does the the higher velocity airflow affect things ? (airflow above the wing is faster (locally) than below the wing)

there is also the option of submerging the engines in the wing root (a la comet, nimrod). this style has gone out of fashion, i guess because of the structural weight, manufacturing complexity, servicing/maintenance access issues.

 

[plasgears]

There may be some merit to mounting on top of the wing thru the wing spars and wing box. Reduced FOD for one thing, but it could be a problem climbing above the wing to service the engine. Also, I'd be haunted by an engine mounted just outside my window. Time will tell on this design.

 

[rb1957]

the offset pylon looks funny too ...

 

[rb1957]

and rotor burst on the rear spar ?

 

[debodine]

Excellent photo, tbuelna! Do you work at GSO? I do, and I occasionally get to watch this little beauty takeoff and land. A friend of mine is a mechanic on the project and has been working oxygen system modifications recently. He has explained that the Honda security is so tight over there, if I tell you any more I will have to kill myself!

debodine

 

[dalcazar]

I was reading a book on turbine design the other day and it mentioned the Fokker 614 in there, according to the book, another problem with mounting the engines above the wing was that the engine could accelerate the airflow over the wings to temporarily supersonic speeds resulting in drag penalties, I suppose this could be offset with a laminar flow wing, but that was part of the arguement

 

[ferdo]

Hi,

http://adg.stanford.edu/aa241/propulsion/engineplacement.html

http://naca.larc.nasa.gov/reports/1934/naca-report-462/

http://www.compositesworld.com/hpc/issues/2004/May/465

http://www.lissys.demon.co.uk/pug/c03.html#s08

http://naca.larc.nasa.gov/digidoc/report/tr/50/NACA-TR-750.PDF

Regards
Fernando

 

[CESSNA1]

SUGHEW: I think it is the whim of the designers. Boeing supposedly beat out the DC-9 (rear engines) by saying that the 737 (underwing engines)could hold 6 more passengers. I do not know if that is hanger talk or not. But wing mounted engines do move the center of gravity of the wing further forward to improve flutter charactersitics. Wing mounted engines require more structure and hardware in the wings. Rear mounted engines are noisier in the rear cabin. The bottom line is its probabaly nothing more than preference.

Regards
Dave

 

[FastMouse]

When the USAF and Boeing were discussing the 'plane that eventually became the XB-47, one of the requirements was to mitigate the effect of an engine fire on the airframe. Boeing's answer was unusual. Mount the engine externally on the aircraft, not internally as had been the norm to that point, and so the under-wing, pylon mounted engine installation was born.

Today jet engines are still frequently mounted below the wing as it is a structurally convenient location to do so. In addition to all the info in the above messages, the spar and skin of the wings are already "beefy" to take the shears and bending moments of flight, so not much additional thickening is required to react the loads of a wing-mounted engine. Aft fuselages without an aft-mounted engine tend to be fairly light, but placing engines in this area introduces significant additional structure (and thus weight) in order to take the loads. This is offset, to some extent, by the opportunity of using a shorter, lighter landing gear as in this case there is no protuberance beneath the wing to worry about.

This is made less straight-forward by rotation angles and runway clearances. When taking-off with a long fuselage, it is easier to get a tail strike on rotation, so to counteract that, longer 'planes tend to have longer gear. So if you have a long aircraft, you may as well use a wing-mounted, under-slung engine as you are stuck with using a tall, heavy landing gear anyway, and the wing is a good place to mount the engine. If you have a short aircraft and you can get away with a shorter, lighter landing gear and still achieve adequate runway clearance on rotation, then go for an aft-mounted engine and retain the short, light gear. Just beware of adding too much additional structure in the aft fuse to carry the engine.

There are many exceptions. The originally short DC-9 with aft-mounted engines was eventually stretched into the long MD-90 (although they tried hard to do most of the stretching forward of the landing gear to reduce the tail strike issue, I think), and the A320 with its under-slung engines and long gear was shrunk to the short A318. However, that these derivatives both are a long-way removed from their original concepts and they inherited their configurations. And there is also the case of high-wing aircraft, like the C-141, but they have to have different requirements compared the usual Airbuses & Boeings.

I recall that Dan Raymer's "Aircraft Conceptual Design" has a nice summary of engine-mounting options.

FastMouse

 

[Beggar]

Well-written and thorough.

Thanks, FastMouse

--------------------
Bring back the HP-15

http://www.hp15c.org

--------------------

 

[Sparweb]

FastMouse's excellent post makes me wonder where Bombardier is going with its ever-growing fleet of regional and business aircraft. All models continue to have the aft-mounted pair of engines - a configuration that hearkens back to the design of the CL-600. Now that the Global Express is longer than the Boeing 737 (>30 ft long), and proposed models promise to be even bigger still, will a change be economical a few years down the road? Or does the MD-90 (46 ft) prove that there is room on this point either way?


Steven Fahey, CET

 

[FastMouse]

Regret the typos in my previous post! Think it still makes sense, though.

Bombardier's C-series, currently existing on paper only, departs from their traditional aft fuse-mounted engines in that it has wing-mounted engines. (As an aside, it will be interesting to see how it performs in the market compared to Embraer's 170-195 family. Does Bombardier have the investment grunt required to make the C-series significantly better than the Embraer products and overcome their first-mover advantage and established product? Time will tell... Ah, it'll all come down to a subsidy shouting match anyway!)

Just for fun, I listed Embraer's & Bombardier's current commercial jets and sorted them by length. The data came from the companies' websites. I was vaguely hoping to see a definite fuselage length, on one side of which all the aircraft had fuselage-mounted engines, and on the other side of which all the aircraft had wing-mounted engines. However, this is what I got:

Manufacturer Aircraft Length (m) Engine Mounting
======== ===== ======= ==========
Embraer 135 26.33 fuselage
Bombardier CRJ 200 26.77 fuselage
Embraer 140 28.45 fuselage
Embraer 145 & 145 XR 29.87 fuselage
Embraer 170 29.90 wing
Embraer 175 31.68 wing
Bombardier CRJ 700 32.51 fuselage
Bombardier C110 35.56 wing
Embraer 190 36.24 wing
Bombardier CRJ 900 36.40 fuselage
Embraer 195 38.65 wing
Bombardier C130 38.76 wing

While the general trend is in-line with the previous post (short 'planes have aft fuse-mounted engines, and long 'planes have wing-mounted engines), the cross-over point is not distinct. This analysis is courageously simplistic, but it just goes to show that while you can forecast the trends, the constraints of design & tooling cost, time to market, etc, mean that it is hard to predict exactly what configuration will be chosen be the design team. I mean, what were the criteria that lead each manufacturer to acknowledge that its existing aircraft configuration had been fully exploited and a new configuration was needed?

It would be nice to include additional data from other manufacturers in the above list, and do some sort of correlation with range, manufacturer and product family. One could maybe even do a rough wing loading and high-lift analysis and guess how much growth potential Bombardier & Embraer have built into their initial designs for the C-Series & 170-195 family. Hmmm, well off topic now.

FastMouse

 

[tbuelna]

FastMouse,

Concorde had underwing engines and tall landing gear, yet still needed a "wheelie bar" to prevent tail strikes:

 

[Onemorechance]

As can be seen in previous responses, putting an engine at the aft fuselage or tail, or putting it below or above the wing all have their advantages and disadvantages and is a compromising decision.

1. Engine below wing:
Some Advantages
Less engine noise
Inertial relief (lighter wing structure weight)
Easy alternative to solve wing flutter problems by shifting the engine mass enough forward
Easy access for engine repair and overhaul
Undisturbed inlet flow for the engine inlet
Might be safer in case of engine fire

Some Disadvantages
May require longer landing gear
More chances of ingestion of debris from the runway during Take Off.


2. Engine above wing:
Some Advantages
Less engine noise
Inertial relief (lighter wing structure weight)
Easy alternative to solve wing flutter problems by shifting the engine mass enough forward
Less long landing gear required
Can have a positive influence on maximum lift capability in short take off runs (shorter TO distances)

Some Disadvantages
Less chances of ingestion of debris from the runway during Take Off
Disturbed air flow (shock waves possible and also possible wing boundary layer ingestion) to the inlet possible especially at high angles of attack
Less accessible for engine repair and overhaul

3. Engine at aft fuselage or tail:
Some Advantages
Less long landing gear required
Less chances of ingestion of runway debris during Take Off
Looks more esthetical

Some Disadvantages
Disturbed air flow to the inlet possible especially at high angles of attack
More engine noise
Maybe smaller aircraft W-CG limits constraints
No structure weight saving due to inertial relief
Usually requires a T-tail which is heavier in construction and has the potential for dangerous stall characteristics (deep stall)


OneMoreChance

 

[Onemorechance]

Sorry,

Less chances of ingestion of debris from the runway during Take Off

Should be in the Advantages-list.

OneMoreChance