Wing vs. tail-mounted engines?
Wolfgang Keller
as a passenger, I've always preferred aircraft with tail-mounted engines
(German BA's F100, for example) over those with wing-mounted engines like
the 737 - because they are usually less noisy, because I felt a lot more
safe with those burning chambers far behind me, away from the ground and
from the fuel tanks (not that flying makes me frightened) and...
[tomato-shield on]
...because they look so much more elegant. :-)
[tomato-shield off]
As an engineer who had also some classes about aerodynamics, structural
and engine design at university, I always thought that the relevant
tradeoff between those two options is basically aerodynamics vs.
structural weight. Wing-mounted engines should result in a lower bending
moment on the wing and the hull (but they need longer and heavier landing
gear) while tail-mounted engines should result in 'cleaner' wing
aerodynamics (and possibly a smaller wing for the same takeoff weight),
thus less drag. But the numbers I have seen, for example, for the Fokker
F100 and the Fairchild-Dornier 928 don't support this at all. In fact the
928 seems to be even a bit heavier (empty weight) for nearly the same
payload (but less range, maybe due to more efficient engines). Am I
missing something here or are my numbers simply incorrect?
928: empty weight 25202 kg
F100: empty weight 24375 kg
It seems that today all manufacturers prefer wing-mounted engines over
tail-mounted ones for new designs, in fact the Embraer 145 seems to be the
largest new design (in the 'west') for an airliner with tail-mounted
engines in the last decades - Boeing 717 and MD 90 are just 'evolutions'
of an existing design, the Fokker F70/100 as well, the CRJ 50 is a
derivative of a business jet, so all these don't really count. Some time
ago, the situation was quite different, with the 727, DC 9 and VC 10 etc.
So here's (finally :-) the question:
What are the actual design tradeoffs today and how did the situation
evolve over the past decades to favor wing-mounted engines that much? What
would be the impact of, for example, drastically increased fuel prices
and/or significantly more strict noise regulations (=> engines with bypass
ratio >>10)?
Regards,
--
Wolfgang Keller
Zu Risiken und Nebenwirkungen von Junkmail lesen Sie
de.admin.net-abuse.mail und fragen sie Ihren Postmaster oder Provider
PPowondra
>From: Wolfgang Keller
>I felt a lot more
>safe with those burning chambers far behind me, away from the ground and
>from the fuel tanks
Wolfgang,
Wouldn't there be some concern for how the fuel for those "burning chambers"
gets back there from the wing or centerbody tanks? I seem to recall that
those fuel lines approximate the size of an adult arm, running along the
length of the fuselage. Certainly a consideration (among others, I'm sure)
in new airliner designs. The rear-mounted engine layout apparently
has been relegated to the RJ's and bizjets.
I wonder how the CG concerns impact the tail vs. wing designs for large
a/c...
Best,
Paul
leo...@earthlink.net
> What are the actual design tradeoffs today and how did the situation
> evolve over the past decades to favor wing-mounted engines that much? What
> would be the impact of, for example, drastically increased fuel prices
> and/or significantly more strict noise regulations (=> engines with bypass
> ratio >>10)?
I haven't read any articles addressing the issues of weight, balance, and
aerodynamics.
However, safety might be a major reason for "below the wing" mounted
engines. When an engine is in serious trouble in the rear of the AC, it can
take out the other engines or critical hydraulic systems. (DC10 at Sioux
City, IO)
If an engine is mounted in a wing, then the engine can damage the wing or
fuel tanks.
When mounted below the wing, the engine parts (catastropic failures) have a
smaller probability of hitting critical wing parts or the fuel tanks. Also,
external mount allows the engine to actualy depart the AC leaving the AC a
chance to fly.
Although the following paragraph may not relate to modern jet design, you
may find it interesting.
Remember the old Boeing Stratocrusers (passenger form of the B29-B50). An
engine could (and did) fail in such a way that the entire engine would twist
out of the wing and depart the AC. This added so much drag to the AC that it
could not fly with three engines at full power. At least 4 accidents were
directly attributed to the complete physical loss of the engine and there
were several other near misses where the pilots could make a powered glide
to an airstrip.
Dennis
ME Incorporated
> What are the actual design tradeoffs today and how did the situation
> evolve over the past decades to favor wing-mounted engines that much? What
> would be the impact of, for example, drastically increased fuel prices
> and/or significantly more strict noise regulations (=> engines with bypass
> ratio >>10)?
I'm no expert, but the biggest concern is saleablity.
In other words, if all things are equal, what's the x factor in selling an
Airbus or a Boeing (or a McDonnell Douglas, before the merger...)
The DC10 was orignally thought of as a twin engine. Then a four engine.
The three engine was perfect, given it's requirement. But how to mount it?
And of course, Lockheed got into it too, and designed the Tristar. While
they both have tail mounted engines, they did their differently.
DC10 way up on a "banjo" fitting, the largest forged piece at the time, and
the L10 in the fuselage using a duct.
In BOTH cases, it was a harder sell than a twin or four engine, because of
the mainenance of the third, tail mounted engine.
In a 717/DC9/MD80 or a 727, this was a little be easier, because they
weren't as high off the ground. But in a DC10, well, this was a BIG deal.
And so, with more powerful engines, and the complaints/requests of big
clients such as American, United, etc, these tail mounted engines are
becoming a non desired design. Hence, the design of the 777. Very much an
airplane designed by and with the airlines, and not just the plane makers
designing around a spec. You can bet that Boeing wanted more engines, and
maybe in the tail, but because of that maintenance thing, two is cheaper,
and wing mounted cheaper still.
Anyway. It's economics. The airlines would rather have simpler maintenance
requirements than the DC10/747s require, which translates into less money
spent. Bean counting anyone? 8^) Two underwings are simpler than 4, and
way simpler than any tail buried DC10.
BTW, from an engineering standpoint, the wing is easier to build/design when
hanging engines off of it, instead of "clean" like the 727.
At least, that's what I've read/heard/figured out. I'm sure someone's gonna
boot me in the head now... Go on then! 8^)
Daniele Procida
> as a passenger, I've always preferred aircraft with tail-mounted engines
> (German BA's F100, for example) over those with wing-mounted engines like
> the 737 - because they are usually less noisy, because I felt a lot more
> safe with those burning chambers far behind me, away from the ground and
> from the fuel tanks (not that flying makes me frightened)
As someone who is scared out of his wits by aeroplanes, I much prefer
MD-80s and the like, because the sight of those enormous heavy engines
making those delicate wings bend up and down in that terrifying fashion
is something I can really do without. At least on a MD-80 I don't spend
the entire flight checking to see if the wings are going fall off.
Instead I worry about an uncontained blade failure ripping into the
fuselage or destroying the tailplane.
Daniele
--
Apple Juice Macintosh service, support & sales Cardiff 029 2041 0050
drink.d...@apple-juice.co.uk www.apple-juice.co.uk
JohnMcGrew
<w_ke...@gmx.de> writes:
>What are the actual design tradeoffs today and how did the situation
>evolve over the past decades to favor wing-mounted engines that much? What
>would be the impact of, for example, drastically increased fuel prices
>and/or significantly more strict noise regulations (=> engines with bypass
>ratio >>10)?
Weight: Putting engines in the back requires extra load carrying structure,
both for the thrust and the weight of the engine itself. On the other hand,
little extra structure is required for the wing mounted engine since there's
structure already there; the wing is mostly structure. Advantage: Wing Mount.
Noise: Clearly, advantage rear, except perhaps for those sitting in the back.
Advantage: Rear Mount.
Maintenance: Most maintenance on wing-mounted engines can be performed at
ground level without lifts or scaffolding. Significant appeal to maintenance
staffs. Advantage: Wing.
Engine-Out Performance: Single-engine control is much better with rear-mounted
engines over wing, simply because thrust is far less asymmetrical because the
engines are so much closer towards the center. Advantage: Rear
Aerodynamics: Rear-mount allows for a cleaner wing design, and less conflicts
with airflow and mechanics for slats and flaps. Advantage: Rear.
However, most rear engine configurations necessitate a T-Tail configuration for
the horizontal stabilizer, which requires extra weight for structure at the top
of the vertical stabilizer, as opposed to conventional horizontal stabilizers
attached to existing structure at the end of the fuselage.
CG Considerations: There are differences in handling and stall characteristics
that tend to favor wing with the engine weight near the CG vs the increased
moment as a result of having a longer nose and all that weight in the rear.
However, advanced control systems and pilot training tend to negate these
differences.
Ground Handling: Many rear-mounted designs allow the use of reverse thrust for
backing up the aircraft with limited danger of FOD ingestation, thus
eliminating the need for a push-back at the gate. This is appealing to
airlines as it means that less ground support required. Advantage: Rear.
That's all I can think of at the moment.
Personally, as a passenger, I prefer the MD-80 to the 737, mostly because of
less noise, the 2x3 seating, and the 737 is "bumpier". But the reality is that
what really determines what plane you will be flying is the economics.
John
JohnMcGrew
(Daniele Procida) writes:
>As someone who is scared out of his wits by aeroplanes, I much prefer
>MD-80s and the like, because the sight of those enormous heavy engines
>making those delicate wings bend up and down in that terrifying fashion
>is something I can really do without. At least on a MD-80 I don't spend
>the entire flight checking to see if the wings are going fall off.
Actually, having the weight of the engines carried elsewhere on the airframe
puts more of the load on the wing spar, which is what causes the bending you
seem concerned about. On the other hand, on wing mounted engines, the weight
is carried directly by the wing as opposed to the spar.
John
Nate Meier
news:airliner...@ditka.Chicago.COM...
> As someone who is scared out of his wits by aeroplanes, I much prefer
> MD-80s and the like, because the sight of those enormous heavy engines
> making those delicate wings bend up and down in that terrifying fashion
> is something I can really do without. At least on a MD-80 I don't spend
> the entire flight checking to see if the wings are going fall off.
Having the engines mounted under the wings actually reduces some of the load
on the wing (lift force on wing - engine weight on wing).
> Instead I worry about an uncontained blade failure ripping into the
> fuselage or destroying the tailplane.
Uncontained blade failures are fairly rare and engines (and casings) must be
designed to contain any blade failures. Uncontained rotor bursts, on the
other hand, are much more common than uncontained blade failures. I'd be
more worried about rotor bursts since they release wayyyy more energy and a
lot more flying bits.
Nathan Meier
Mad Monks
Daniele Procida <{$usenet$}@apple-juice.co.uk> wrote:
>As someone who is scared out of his wits by aeroplanes, I much prefer
>MD-80s and the like, because the sight of those enormous heavy engines
>making those delicate wings bend up and down in that terrifying fashion
>is something I can really do without. At least on a MD-80 I don't spend
>the entire flight checking to see if the wings are going fall off.
The wing mounted engines are there because it makes for easier
lengthening and shortening of the a/c. The chief engineer for the 747
pushed through the wing-mounted engine for the 737 so that the a/c
could be better balanced and also lengthened (so we now have a/c as
short as the 737-500 and as long as the -900). A tail-mounted engine
would have limited this flexiblity.
Don Stokes
>MD-80s and the like, because the sight of those enormous heavy engines
>making those delicate wings bend up and down in that terrifying fashion
>is something I can really do without. At least on a MD-80 I don't spend
>the entire flight checking to see if the wings are going fall off.
Actually, the wing stresses are lower with the engines on the wing
because the wing root is not carrying the weight of the engines as well
as the fuselage. Remember that the wings carry the plane, not the other
way around.
Of course, the tail mounted aircraft have correspondingly stronger wing
root and tail structures to deal with the higher stresses, so the chances
of the wings falling off are about the same: vanishingly small.
(I can't think of any case of failure of the wing box or root structure
causing an accident on a post-war commercial airliner -- can anyone?)
-- don
leo...@earthlink.net
Since the B47 design in the late 40s, wings have been built to flex .
Keep in mind the the wings are doing most of the lifting, even though as a
passenger, you might feel like the empenage is lifting the wings.
Thus, the weight of an engine on a wing is weight being directly lifted by
the wing on which it is attached and that weight is not affecting the wing
where it attaches to the empenage. If that engine were mounted at the rear
of the plane, the weight would still have to be lifted, but all of the
engine weight would be present at the wing root, thus increasing the amount
of bend.
Be of good cheer. Wings that bend work. B52 wings can flex 16 feet at their
ends. B52s have been flying since the early 50's and some are expected to be
in service into the 2040s. I stood near the end of a runway at Dyess AFB
(Abilene TX) and watched the entire takeoff roll. It was really interesting
the wing tips move from very low (near the runway) to very high as the AC
gained speed.
Dennis
Robin Johnson
>What are the actual design tradeoffs today and how did the situation
>evolve over the past decades to favor wing-mounted engines that much? What
>would be the impact of, for example, drastically increased fuel prices
>and/or significantly more strict noise regulations (=> engines with bypass
>ratio >>10)?
Engines these days are very reliable. It is not unknown for an engine
to remain in position for five years.
On the other hand, more and more ETOPS flights are being undertaken -
the majority of North Atlantic crossings for example. Despite this,
there have been very few ditchings.
Just this month a United 767 on the climb-out from Lihue and bound for
California, had a loss of power on both engines. No harm done,
but.....
I'm a little concerned about the dynamics of a ditching with
underslung engines, though. The Ethiopian 767 off the Comores,
although a hijack situation with armed men on the flightdeck, looked
good on the video until the last minute, when it seemed that the
engines dug in asymmetrically, causing the fuselage to break up.
Does anyone know of a successful ditching by a jet?
I would feel safer in a rear-engined model.
-Robin Johnson
(who once saw a recently-ditched Convair 240)
c...@panix.com
>> What are the actual design tradeoffs today and how did the situation
>> evolve over the past decades to favor wing-mounted engines that much? What
>> would be the impact of, for example, drastically increased fuel prices
>> and/or significantly more strict noise regulations (=> engines with bypass
>> ratio >>10)?
Long ago I remember reading a comparison of Boeing 707 vs DH Comet, which
claimed that the 707 design of engines slung under the wing had
aerodynamic advantages over the sleeker, engines-inside-the-wing approach
of the Comet. As I recall (from reading this years ago) the placement of
engines under the wing encourages airflow into the engines which enhances
performance; someone with greater knowledge, on this news group, could
make better sense of this.
Certainly the four-engines-at-the-tail configuration, as in the old VC10,
entailed the obvious danger that if one engine threw a turbine blade or
caught fire, its adjacent engine was almost certain to be damaged.
Plus, aircraft with engines mounted at the rear require the T-shaped
stabilizer configuration, which, as I recall, means that the stabilizer
ceases to function if the aircraft is in a nose-up position where the
stabilizer drops down into turbulence from the main wing. Correct?
Stephen H. Westin
<snip>
> What are the actual design tradeoffs today and how did the situation
> evolve over the past decades to favor wing-mounted engines that much? What
> would be the impact of, for example, drastically increased fuel prices
> and/or significantly more strict noise regulations (=> engines with bypass
> ratio >>10)?
The factors I can think of are:
1. Center of gravity. With the heavy engines at the back, but the
payload forward of that, the CG has to move drastically with different
loads and different distribution. That would make it more difficult to
maintain consistent flight characteristics under all situations.
Aren't there folk tales of unladen VC-10's and IL-62's tipping
nose-up while parked?
2. Structure. Instead of carrying the weight of the engines
through the wing roots and fuselage, they are attached to the
part that holds them up, the wing. Likewise, the wing presumably
generates most of the drag of the airplane, so you would like
the engines pulling it directly through the air, rather than
pushing the fuselage that then drags the wing along.
3. Clean air to the engines. I believe that MD80's and the like
are somewhat more susceptible to compressor stall, especially at high
angles of attack or strong crosswinds on takeoff. My wife and
daughter were aboard an AA MD80 that suffered a compressor stall
during the takeoff roll at DFW some years back; the pilot aborted
and went back to have things looked at before trying again.
4. Maintenance issues. Others have pointed out how engines below
the wing are just easier to get to.
5. Simpler mounting of four engines, though this only matters for the
747 and A340 these days. In the '60s, there were the VC-10 and IL-62.
I think the DC-9 and 727 got rear-mounted engines in part so they
could sit lower to the tarmac for easy access at primitive
airports. Both have rear integral stairs, at least in some
configurations. Boeing worked around that for the 737 by squishing the
engines in between a low wing and the ground, which created a
challenge in mounting higher-bypass engines. But that all is a moot
point except for regional jets these days, and they have tended to
stick with rear-mounted engines. I suspect that a minimum engine
distance from the ground is also desirable to avoid inhaling foreign
objects from the ground.
--
-Stephen H. Westin
Any information or opinions in this message are mine: they do not
represent the position of Cornell University or any of its sponsors.
Eric Hagerstrom
wrote:
>On 23 Mar 2001 17:40:08 , Wolfgang Keller <w_ke...@gmx.de> told us:
>
>Engines these days are very reliable. It is not unknown for an engine
>to remain in position for five years.
>On the other hand, more and more ETOPS flights are being undertaken -
>the majority of North Atlantic crossings for example. Despite this,
>there have been very few ditchings.
Well, actually no ETOPS flight has ever ditched. The Ethopian ditched
while struggling with a hijacker in the vcockpit, certaily not because
of ETOPS operations.
>Just this month a United 767 on the climb-out from Lihue and bound for
>California, had a loss of power on both engines. No harm done,
>but.....
>I'm a little concerned about the dynamics of a ditching with
>underslung engines, though. The Ethiopian 767 off the Comores,
>although a hijack situation with armed men on the flightdeck, looked
>good on the video until the last minute, when it seemed that the
>engines dug in asymmetrically, causing the fuselage to break up.
>Does anyone know of a successful ditching by a jet?
13 January 1969, An SAS DC8-62 (four-engines under the wing) ditched
in the ocean off Los Angeles (apparently after running out of fuel).
15 fatalities. I don't recall how many were aboard but the aircraft
stayed afloat for hours.
Don Stokes
>(I can't think of any case of failure of the wing box or root structure
>causing an accident on a post-war commercial airliner -- can anyone?)
To answer my own question:
The Aviation Safety Network has a list of airframe failure accidents at:
http://aviation-safety.net/events/afw.shtml
It seems that the L-188 Electra and Vickers Viscount had a number of
"wings falling off" accidents due to actual design flaws. Pre-war
designs like the DC-3 and C-46 seem to have fallen out of the sky in
seriously bad weather on a fairly regular basis.
There are few jet aircraft in this list. They are:
15.07.1964 Boeing 720 Aircraft overstressed and disintegrated
Ansbach, DE Lufthansa after loss of control by training crew
performing a (deliberate) roll.
03.06.1973 Tupolev Tu-144 Canard failed in demonstration flight
Paris, FR Aeroflot
09.05.1976 Boeing 747-131F Lightning strike to wing caused fuel
Madrid, ES Iran Air Force explosion that unravelled wing structure
14.05.1977 Boeing 707-321C Separation of horizontal stabilizer due
Nairobi, KE IAS Cargo to metal fatigue and inadequate inspection
06.10.1981 Fokker F28 Aircraft disintegrated in tornado
Moerdijk, NL NLM Cityhopper
12.08.1985 Boeing 747-SR46 Poorly repaired pressure bulkhead failed
Tokyo, JP JAL in flight, causing loss of hydraulic control
Of these, only the Lufthansa 720 and NLM F28 count as stress failures
of the main wing -- I'm not even sure about the 720 as to what failed
first -- and in both cases the overstresses were pretty extreme -- it's
not considered normal procedure in commercial ops to fly aerobatics or
into tornadoes...
-- don
JohnMcGrew
westin*nos...@graphics.cornell.edu (Stephen H. Westin) writes:
>I think the DC-9 and 727 got rear-mounted engines in part so they
>could sit lower to the tarmac for easy access at primitive
>airports.
Actually, primitive airports were a major consideration for both these planes,
as they were being sold as short and medium range "feeders" to replace places
that were still being served by DC-3s in the early '60s. The 727 was actually
certified for and used on gravel runways in Alaska; conditions that a
under-slung engine could not survive.
John
Steve
Daniele Procida wrote:
> )
>
> As someone who is scared out of his wits by aeroplanes, I much prefer
> MD-80s and the like, because the sight of those enormous heavy engines
> making those delicate wings bend up and down in that terrifying fashion
> is something I can really do without. At least on a MD-80 I don't spend
> the entire flight checking to see if the wings are going fall off.
Ahh, but looks are deceiving ;-)
the biggest *drag* force on an aircraft comes from the wing- whether the
engines are on the tail or on the wing. Also, its the wing that holds the
plane *up* in the first place. So, if the engines are on the wing, there's
actually less stress overall since the thrust of the engines has to be
transmitted through less structure to counter the drag on the wings. And
besides, thats all trivial compared to holding the whole thing up anyway.
> Instead I worry about an uncontained blade failure ripping into the
> fuselage or destroying the tailplane.
It happens... but so do car wrecks, strokes, heart attacks, and meteor
strikes.
No sense living in terror of everything....
Ian Barclay
>What are the actual design tradeoffs today and how did the situation
>evolve over the past decades to favor wing-mounted engines that much?
Others have covered most of the main issues (wing bending relief, clean
wing aerodynamics, ground clearance, FOD ingestion, etc) but a few other
considerations are :-
Wing mounted engines put mass in front of the flexural axis of the wing.
This damps out the aeroelastic forces & gives greater flutter margins or
allows lighter wing structures.
Rear mounting moves the cg aft reducing tail moment arms thus requiring
a larger tailplane and maybe even fin.
Rear mounted engines require heavier rear fuselage structure.
Rear mounted engines can be fitted with bucket reversers which tend to
be cheaper and more effective than cascade or 4 door reversers used on
wing mounted engines.
External noise levels are generally less for rear engine installations
as the fuselage partially shields one engine for sideline noise
measurements and the wings partially shield both engines for approach
measurements.
Wing mounted engines can easily be struck and damaged in a misjudged
crosswind landing.
The downwash from the wing varies with angle of attack and tends to
result in a relatively small variation in AoA at the engine inlet of
rear mounted engines making the inlet aerodynamics more closely
optimised across the flight envelope.
Rear mounted engines often require soft (rubber/fluid) engine mounts to
absorb vibration and blade off loads. For wing mounted engines the
flexible wings act as effective dampers thus allowing engines to use
cheaper hard mount arrangements.
Wing mounted engines are ideally located to supply bleed air for wing LE
anti-ice.
If the APU is tail mounted, bleed piping can be simplified with rear
mounted engines.
The length of fuel lines is minimised for wing mounted engines.
Ice shed from the wing can be ingested into rear mounted engines.
Wing mounting may limit the flap span to allow for the exhaust stream.
There is the possibility of high drag from the convergent/divergent
channel formed between the nacelle and the fuselage wall on rear mounted
engine installations.
More available fuel volume for rear mounted engines as no dry bays in
the wing fuel tanks to cater for disc bursts are required.
It may be possible for engine nacelles to be common port and starboard
for wing mounted engines.
It is easier to make accessories, piping & harnesses common on port and
starboard sides for wing mounted engines.
At the end of the day its pretty evenly balanced. Hence the
popularity of both configurations. I think the two factors that tend to
swing the decision are the manufacturers experience (e.g. Boeing &
Airbus go for wing mounted, McDonnell Douglas used to go for rear
mounted because that was what they had done before, they had the
engineers with that design knowledge, it was the low risk approach) and
ground clearance (e.g. regional jets want to be able to load baggage
holds from the ground and passengers without jetways so rear mounted is
better for this case).
--
Ian Barclay
Salwick, UK
RF-X
robinj...@bigfoot.com wrote:
> I'm a little concerned about the dynamics of a ditching with
> underslung engines, though. The Ethiopian 767 off the Comores,
> although a hijack situation with armed men on the flightdeck, looked
> good on the video until the last minute, when it seemed that the
> engines dug in asymmetrically, causing the fuselage to break up.
> Does anyone know of a successful ditching by a jet?
> I would feel safer in a rear-engined model.
I was under the impression that jets with underslung engines had a
method of jettisoning said engines in a ditch scenario, and this wasn't
employed in the Ethiopian 767 due to the hijackers' complete ignorance
of their situation - insufficient fuel for post-hijack voyage,
incomplete understanding of commercial flight realities, not to mention
rejection of the ditch inevitability, etc...
Don Stokes
Robin Johnson <robinj...@bigfoot.com> wrote:
>I'm a little concerned about the dynamics of a ditching with
>underslung engines, though. The Ethiopian 767 off the Comores,
>although a hijack situation with armed men on the flightdeck, looked
>good on the video until the last minute, when it seemed that the
>engines dug in asymmetrically, causing the fuselage to break up.
>Does anyone know of a successful ditching by a jet?
>I would feel safer in a rear-engined model.
If you watch the video again, you'll notice the wingtip went into the
water first, and the fuselage went in more or less sideways. Not good
for the structure, that.
Engines have "fuse pins" that will shear them off given the kind of
stresses involved in a water landing. This was demonstrated a year or
two ago by a 707 freighter crew that rather badly misjudged the approach
to Mwanza, Tanzania and put the bird into Lake Victoria. There's photos
somewhere around on the net (Aviation Week reprinted 'em) of the 707
floating in the lake, more or less intact but minus all four engines,
having been towed to shallow water and unloaded.
-- don
matt weber
----- Original Message -----
From: "Robin Johnson" <robinj...@bigfoot.com>
To: <airl...@chicago.com>
Sent: Saturday, March 31, 2001 4:43 PM
Subject: Re: Wing vs. tail-mounted engines?
> On the other hand, more and more ETOPS flights are being undertaken -
> the majority of North Atlantic crossings for example. Despite this,
> there have been very few ditchings.
> Just this month a United 767 on the climb-out from Lihue and bound for
> California, had a loss of power on both engines. No harm done,
> but.....
> I'm a little concerned about the dynamics of a ditching with
> underslung engines, though. The Ethiopian 767 off the Comores,
> although a hijack situation with armed men on the flightdeck, looked
> good on the video until the last minute, when it seemed that the
> engines dug in asymmetrically, causing the fuselage to break up.
You have to remember that at that point, both engines had failed (fuel
starvation), and I doubt there was time to deploy the RAT, and with no fuel,
the APU isn't going to be helpful either...
> Does anyone know of a successful ditching by a jet?
In the 1960's, a JAL DC8 freighter ended up well short of the runway at SFO,
and in the bay.
It was fished out, overhauled, and flew until the mid 1980's when it was
finally retired....
matt weber
failure of the engine mounts caused the wingbox loading to exeed the design
limits, and the actual failure was the wings came off the aircraft. The
fix was to stiffen the wing, and make the engine mounts stronger.
Matt Weber
Gord Beaman
--cut--
>As someone who is scared out of his wits by aeroplanes, I much prefer
>MD-80s and the like, because the sight of those enormous heavy engines
>making those delicate wings bend up and down in that terrifying fashion
>is something I can really do without. At least on a MD-80 I don't spend
>the entire flight checking to see if the wings are going fall off.
>
>Instead I worry about an uncontained blade failure ripping into the
>fuselage or destroying the tailplane.
>
>Daniele
Daniele, it might ease your mind to think that the engines are
lifted -by- the wing and because they're mounted -out- on it will
contribute much less stress on it than if they were mounted on
the fuselage and their weight had to be carried by the wing root.
You shouldn't worry about safety, after all, you did survive the
trip to the airport didn't you?...you're much safer in flight
than on the highway.
Jonathan Thornburg
<w_ke...@gmx.de> writes:
>What are the actual design tradeoffs today and how did the situation
>evolve over the past decades to favor wing-mounted engines that much?
In article <airliner...@ditka.Chicago.COM>,
JohnMcGrew <johnm...@aol.com> wrote:
>Weight: Putting engines in the back requires extra load carrying structure,
>both for the thrust and the weight of the engine itself. On the other hand,
>little extra structure is required for the wing mounted engine since there's
>structure already there; the wing is mostly structure. Advantage: Wing Mount.
[[...]]
>However, most rear engine configurations necessitate a T-Tail configuration for
>the horizontal stabilizer, which requires extra weight for structure at the top
>of the vertical stabilizer, as opposed to conventional horizontal stabilizers
>attached to existing structure at the end of the fuselage.
[[...]]
>Maintenance: Most maintenance on wing-mounted engines can be performed at
>ground level without lifts or scaffolding. Significant appeal to maintenance
>staffs. Advantage: Wing.
These are the big reasons in practice, especially the lower weight
(--> lower drag at the same lift/drag ratio --> lower fuel burn).
>Aerodynamics: Rear-mount allows for a cleaner wing design, and less conflicts
>with airflow and mechanics for slats and flaps. Advantage: Rear.
With modern design techniques (computer + wind-tunnel) the aerodynamic
difficulties of wing-mount (podded) engines can be dealt with without
serious difficulty.
The industry doesn't care much about it, but safety is another big
advantage of wing-mounted engines: What kills people in a jet crash
is often not the crash itself, but the ensuing fire. Tail-mounted
engines mean fuel pipes running from the wing (where the fuel tanks are)
aft through the fuselage to the engines, i.e. you've got fuel right
in the fuselage with the passengers. In contrast, wing-mounted engines
allow all the fuel to be kept in the wings, some distance away from the
passengers. (Some designs do put fuel tanks in the wing center sections
anyway.)
--
-- Jonathan Thornburg <jth...@thp.univie.ac.at>
http://www.thp.univie.ac.at/~jthorn/home.html
Universitaet Wien (Vienna, Austria) / Institut fuer Theoretische Physik
Q: Only 6 countries have the death penalty for children. Which are they?
A: Congo, Iran, Nigeria, (Pakistan[*]), Saudi Arabia, United States, Yemen
[*] Pakistan reportedly ended it in July 2000. -- Amnesty International
http://www.web.amnesty.org/ai.nsf/index/AMR511392000
Robert Briggs
> I'm a little concerned about the dynamics of a ditching with
> underslung engines, though.
> Does anyone know of a successful ditching by a jet?
Yes. In May of 1995, the Royal Air Force did quite a neat job with
Comet - correction, Nimrod - XW666 of 51 Squadron near Lossiemouth.
I've not found a picture on the Web (perhaps no big surprise for an
event that long ago), but the incident is mentioned at:
http://www.morayairnet.co.uk/News/Crashes.htm
Of course, that answers your actual question about a successful
ditching by a jet, but doesn't address your concern about underslung
engines in such circumstances. :-)
Filip De Vos
: Engines these days are very reliable. It is not unknown for an engine
: to remain in position for five years.
: On the other hand, more and more ETOPS flights are being undertaken -
: the majority of North Atlantic crossings for example. Despite this,
: there have been very few ditchings.
I am aware of exactly no ditchings caused by ETOPS. In fact, I am aware of
no mid-ocean ditchings in the jet age. All jets lost at sea came apart in
mid-air (TW800, AI182, KL007) or shattered on impact (SR111)
: Just this month a United 767 on the climb-out from Lihue and bound for
: California, had a loss of power on both engines. No harm done,
Engines require a continuous flow of fuel :)
: but.....
: I'm a little concerned about the dynamics of a ditching with
: underslung engines, though. The Ethiopian 767 off the Comores,
: although a hijack situation with armed men on the flightdeck, looked
: good on the video until the last minute, when it seemed that the
: engines dug in asymmetrically, causing the fuselage to break up.
: Does anyone know of a successful ditching by a jet?
A SAS DC-8 at San Fransisco or LA (pilots were preoccupied with a faulty
landing gear light)
A JAL DC-8 Super sixty pulled reverse thrust on approach to Haneda, pilot
heard voices in his head. Not really a ditching. 24 killed.
More recently, a 707 cargo in Lake Victoria. Engines and landing gear was
sheared off, but the aircraft kept on floating, crew safe.
: I would feel safer in a rear-engined model.
In a ditching, I do not know. The engines do not hang down, and therefore
cannot 'dig in', but aircraft with tail-mounted engines tend to have wings
that are straight instead of pointing slightly upward, so instead of
digging in an engine, they can dig in the wingtip, with potentially the
same results.
--
"Who needs credibility around | Filip De Vos
here?" -- T. L. Elifritz | FilipP...@rug.ac.be
Michael Butler
> (I can't think of any case of failure of the wing box or root structure
> causing an accident on a post-war commercial airliner -- can anyone?)
I think it was either a Viscount or a Convair with batteries in the wing
to make it self starting and the heat from the battery caused the
wing failure - but even then it was a mod to the original design.
--
Mike Butler Wellington NZ
Ben BC X New Zealand Dog Agility on the Web
Ella Kelpie http://homepages.ihug.co.nz/~mbutler/nala/
Don Stokes
>Noise: Clearly, advantage rear, except perhaps for those sitting in the back.
>Advantage: Rear Mount.
I don't think it's quite that clear-cut. Vibration from rear-mounted
engines is transmitted directly into the rear of the cabin, rather than
into the wing and wing root, and the wing itself protects the cabin from
exhaust noise (at least on most designs -- the 737-100/200 have exhausts
behind the wing, but modern designs are well underneath). Intake noise
is also closer to the cabin in tail-mounted designs.
The wing probably directs more noise groundward from the exhaust.
>Aerodynamics: Rear-mount allows for a cleaner wing design, and less conflicts
>with airflow and mechanics for slats and flaps. Advantage: Rear.
But at the expense of putting the engines in a messy place. Wing
mounted designs put the intakes in more or less clear air, whereas tail
mounted have aerodynamic effects from the wing and fuselage to contend
with. I believe this is why you don't see high bypass engines mounted
on the sides of the tail on any aircraft -- the big fans tend to be more
touchy about screwball intake flow than their lower bypass brethren.
(This doesn't seem to have stopped the DC-10 and L-1011 from having top
mounted intakes.)
-- don
Tim J Lee
news:airliner...@ditka.Chicago.COM...
> "Wolfgang Keller" <w_ke...@gmx.de> wrote in message
>
> > What are the actual design tradeoffs today and how did the situation
> > evolve over the past decades to favor wing-mounted engines that much?
> > would be the impact of, for example, drastically increased fuel prices
> > and/or significantly more strict noise regulations (=> engines with
bypass
> > ratio >>10)?
> And so, with more powerful engines, and the complaints/requests of big
> clients such as American, United, etc, these tail mounted engines are
> becoming a non desired design. Hence, the design of the 777. Very much
an
> airplane designed by and with the airlines, and not just the plane makers
> designing around a spec. You can bet that Boeing wanted more engines, and
> maybe in the tail, but because of that maintenance thing, two is cheaper,
> and wing mounted cheaper still.
An engineer writes.....
Structurally, underwing engines are a better bet. Reason? If you put the
weight of the engines on the fuselage (a la DC9), this adds to the weight
that the wing root has to transmit - leading to a more complex wing/fuselage
interface and carry-through structure. The object is to make the wing as
heavy as possible (so it carries it's own weight) and the fuse as light as
possible. It's the same reason that on aircraft with centre fuel tanks, they
use this first (usually for take off) - get the fuse light, keep the weight
on the wings. As an aside, it also makes maintenance easier, but this is a
secondary consideration against airframe efficiency.
There are good reasons for tail mounted engines - the VC10 had the best - it
was designed with the hot and high airports that BOAC were serving in East
Africa in mind, and so an efficient and highly flapped wing was created.
Fouling the trailing edge up with engine exhaust was not considered a good
bet, due to the loss of flap section.
As for containment - the bit of the engine most likely to go bang is the
large fan at the front - which in any case is mounted ahead of the wing and
fuel.
T
R J Carpenter
>
> 2. Structure. Instead of carrying the weight of the engines
> through the wing roots and fuselage, they are attached to the
> part that holds them up, the wing.
Am I naive in thinking that a "flying" tail could support the rear
engines without any of their weight being supported by the wings???
The first rear-engined jet, the Caravelle, didn't have a true T-tail.
The horizontal tail surface was still fairly low on the fin.
Bob C.
Gord Beaman
--cut--
>>
>>(I can't think of any case of failure of the wing box or root structure
>>causing an accident on a post-war commercial airliner -- can anyone?)
>That what caused the Two Electra accidents indirectly. The whirl mode
>failure of the engine mounts caused the wingbox loading to exeed the design
>limits, and the actual failure was the wings came off the aircraft. The
>fix was to stiffen the wing, and make the engine mounts stronger.
Bit more than that Matt,,,they discovered that the 'whirl mode'
problem was causing sympathetic vibration of certain stiffeners
and support structures in the wings and that the fix was to add
syncronization and syncrophasing of the propellers. This system
is now used on all twin and four engined a/c using Allison T-56
type engines. C-130, P-3, and Convair 580's are the types that I
know about, there's likely others. This system has the added
benefit of reducing crew fatigue due to less engine induced
vibration.
Don Stokes
>Actually, primitive airports were a major consideration for both these planes,
>as they were being sold as short and medium range "feeders" to replace places
>that were still being served by DC-3s in the early '60s. The 727 was actually
>certified for and used on gravel runways in Alaska; conditions that a
>under-slung engine could not survive.
Actually, they can with a bit of imaginative engineering -- 737-200s
were (are?) used for the same runways, with the aid of a "gravel kit"
consisting of a deflector on the nose gear and small bleed air pipes
pointing forward to disrupt any vortexes forming in front of the intakes
that might suck stuff into the engines.
-- don
Gord Beaman
--cut--
>
>Am I naive in thinking that a "flying" tail could support the rear
>engines without any of their weight being supported by the wings???
Yes, I'm afraid so, -all- of any a/c's weight is supported by the
wings (or canards), none is supported by the tail surfaces at
all. They're there -only- to control (along with the ailerons)
the a/c in flight. As a matter of fact, in normal flight, the
horizontal tail surfaces actually have a net downward force which
is used in the 'vertical stability department'.
>The first rear-engined jet, the Caravelle, didn't have a true T-tail.
>The horizontal tail surface was still fairly low on the fin.
The reason for the T tail config is to get the horizontal stab.
up out of the turbulance of the rear mounted engines. When in a
very nose high position the a/c can get into what's known as a
'deep stall' where the elevators are in such turbulent air from
the engines that you cannot get the nose down...this apparently
happened to a Trident (?) in the UK many years ago.
Doug Holik
"RF-X" <n...@spam.com> wrote in message
news:airliners...@ditka.Chicago.COM...
> In article <airliner...@ditka.Chicago.COM>,
> robinj...@bigfoot.com wrote:
>
> > I'm a little concerned about the dynamics of a ditching with
> > underslung engines, though. The Ethiopian 767 off the Comores,
> > although a hijack situation with armed men on the flightdeck, looked
> > good on the video until the last minute, when it seemed that the
> > engines dug in asymmetrically, causing the fuselage to break up.
> > Does anyone know of a successful ditching by a jet?
> > I would feel safer in a rear-engined model.
>
> I was under the impression that jets with underslung engines had a
> method of jettisoning said engines in a ditch scenario, and this wasn't
Not sure where you heard that but there is no way to jettison the engines.
Some commercial aircraft, notably the MD-11 feature a "ditching" switch.
All this switch does is close up the holes on the skin of the aircraft, APU
Ports, Outflow Valve, etc...
A means to jettison the engines does not sound like a good idea anyway, what
if someone accidentally pressed the wrong button?
Doug Holik
Geno Rice
> Am I naive in thinking that a "flying" tail could support the rear
> engines without any of their weight being supported by the wings???
> The first rear-engined jet, the Caravelle, didn't have a true T-tail.
> The horizontal tail surface was still fairly low on the fin.
I thought that for stability the tail is required to provide negative
lift.
Geno Rice
John Liebson
>The industry doesn't care much about it, but safety is another big
>advantage of wing-mounted engines: What kills people in a jet crash
>is often not the crash itself, but the ensuing fire. Tail-mounted
>engines mean fuel pipes running from the wing (where the fuel tanks are)
>aft through the fuselage to the engines, i.e. you've got fuel right
>in the fuselage with the passengers. In contrast, wing-mounted engines
>allow all the fuel to be kept in the wings, some distance away from the
>passengers. (Some designs do put fuel tanks in the wing center sections
>anyway.)
What about wing-mounted-engine aircraft that _also_ have tail-mounted
fuel tanks?
John Liebson
>Actually, primitive airports were a major consideration for both these planes,
>as they were being sold as short and medium range "feeders" to replace places
>that were still being served by DC-3s in the early '60s. The 727 was actually
>certified for and used on gravel runways in Alaska; conditions that a
>under-slung engine could not survive.
B737s could have, as a factory-option, gravel kits installed, and
aircraft with this option operate from gravel runways. I suspect that
these 737s have under-slung engines and that they survive such
operations....
JWizardC
complexity of the wing main spar/load box. Also, the engines take up a bunch of
room that could have been used by fuel.
As far as the T-tail goes, a 'conventional' empannage can also stall under
certain circumstances. On the other hand, the t-tail allows a slightly longer
moment arm from the center of gravity.
TheFNG
Robin Johnson
I didn't get much response to the above question in another ng (about
successful ditchings). But I do remember that some of the early jets
with underslung engines, specifically the DC-8, had a pylon
arrangement that disconnected easily in the event of increased
resistance. The example I saw was a DC-8-50 that overran into a soft
ploughed field on attempted takeoff at Heathrow sometime in the
mid-60s. One engine (or more) had completely severed connection. I
was told at the time it was arranged so to avoid smashing into the
wing fuel tanks: it presumably did not happen to the ET 767. Which
aircraft had/have that as a feature, and why was it dropped?
--Robin Johnson
>
>Doug Holik
JP Caputa
Mad Monks wrote:
> Daniele Procida <{$usenet$}@apple-juice.co.uk> wrote:
> >As someone who is scared out of his wits by aeroplanes, I much prefer
> >MD-80s and the like, because the sight of those enormous heavy engines
> >making those delicate wings bend up and down in that terrifying fashion
> >is something I can really do without. At least on a MD-80 I don't spend
> >the entire flight checking to see if the wings are going fall off.
>
> The wing mounted engines are there because it makes for easier
> lengthening and shortening of the a/c. The chief engineer for the 747
> pushed through the wing-mounted engine for the 737 so that the a/c
> could be better balanced and also lengthened (so we now have a/c as
> short as the 737-500 and as long as the -900). A tail-mounted engine
> would have limited this flexiblity.
What about the DC9/MD 80-95/717 series, those came in lengths between 46.5 m
for an MD-90 to 37.2 m for an MD-95, seems pritty strechable to me!
Jonathan Thornburg
John Liebson <jlie...@cybermesa.com> wrote:
>B737s could have, as a factory-option, gravel kits installed, and
>aircraft with this option operate from gravel runways. I suspect that
>these 737s have under-slung engines and that they survive such
>operations....
Indeed, ISTR DC-9 engines being rather prone to damage from ingesting
gravel or dirt flung up by the main wheels. Wing-mount engines may well
be *preferable* in this regard.
--
-- Jonathan Thornburg <jth...@thp.univie.ac.at>
http://www.thp.univie.ac.at/~jthorn/home.html
Universitaet Wien (Vienna, Austria) / Institut fuer Theoretische Physik
Moore's Law: computer hardware speed doubles every 18 months
Gates' Law: computer software speed halves every 18 months
Gord Beaman
--cut--
>
>What about wing-mounted-engine aircraft that _also_ have tail-mounted
>fuel tanks?
But those tanks are comparatively very small John, their purpose
is for trimming out the aerodynamic drag of the aircraft's
'stability system' when it's not essential. It's only used in
stabilized cruise conditions, so for takeoff and landings these
tanks are always empty. The engines are not fed from them.
Gord Beaman
matt weber
Synching up the props is a lot older then the Electra. The L1049
(Constellation) has it.
However had the wing been adequately stiff in the first place, it would
have damped out the sympathetic vibrations in the first place, and raised
the frequency at which the wing would resonate to a frequency well beyond
what could be generated by precession caused by partial engine mount failure..
Matt Weber
Jonathan Thornburg
| The industry doesn't care much about it, but safety is another big
| advantage of wing-mounted engines: What kills people in a jet crash
| is often not the crash itself, but the ensuing fire. Tail-mounted
| engines mean fuel pipes running from the wing (where the fuel tanks are)
| aft through the fuselage to the engines, i.e. you've got fuel right
| in the fuselage with the passengers. In contrast, wing-mounted engines
| allow all the fuel to be kept in the wings, some distance away from the
| passengers. (Some designs do put fuel tanks in the wing center sections
| anyway.)
In article <airliners...@ditka.Chicago.COM>,
John Liebson <jlie...@cybermesa.com> asked
>What about wing-mounted-engine aircraft that _also_ have tail-mounted
>fuel tanks?
Not nice:
* fuel pipes in the aft fuselage (--> big fire risk in crashes)
* fuel tanks far from center of gravity --> big trim change as you burn
that fuel --> hurts aerodynamic efficiency (fighting that trim change
will cost extra drag)
JohnMcGrew
jth...@galileo.thp.univie.ac.at (Jonathan Thornburg) writes:
>The industry doesn't care much about it, but safety is another big
>advantage of wing-mounted engines: What kills people in a jet crash
>is often not the crash itself, but the ensuing fire. Tail-mounted
>engines mean fuel pipes running from the wing (where the fuel tanks are)
>aft through the fuselage to the engines, i.e. you've got fuel right
>in the fuselage with the passengers. In contrast, wing-mounted engines
>allow all the fuel to be kept in the wings, some distance away from the
>passengers. (Some designs do put fuel tanks in the wing center sections
>anyway.)
I doubt there is much in the way of actual crash data to justify that opinion.
Any accident serious enough to compromise the fuel system almost certainly will
compromise the fuel tanks as well. And with many longer-range planes today,
that argument is moot since fuel is now being stored in tanks other than in the
wings or at the spar, and even the horizontal stabilizer, resulting in fuel
lines, pumps, etc all over the airframe.
John
Stephen H. Westin
> The biggest disadvantage to the wing-mounted engines in the Comet
> was the complexity of the wing main spar/load box. Also, the engines
> take up a bunch of room that could have been used by fuel.
And something that I'm sure wasn't considered in the '50s: the
complexity of upgrading to other engines. Though the RAF Nimrods are
slated to be fitted with BR700's soon; I wonder how they managed that.
<snip>
--
-Stephen H. Westin
Any information or opinions in this message are mine: they do not
represent the position of Cornell University or any of its sponsors.
Stephen H. Westin
<snip>
> In article <airliners...@ditka.Chicago.COM>,
> John Liebson <jlie...@cybermesa.com> asked
> >What about wing-mounted-engine aircraft that _also_ have tail-mounted
> >fuel tanks?
>
> Not nice:
> * fuel pipes in the aft fuselage (--> big fire risk in crashes)
> * fuel tanks far from center of gravity --> big trim change as you burn
> that fuel --> hurts aerodynamic efficiency (fighting that trim change
> will cost extra drag)
I believe the 747-400 offers optional fuel storage in the horizontal
stabilizer. I suspect that this could be used to trim CG during
cruise; it might even wind up as a net advantage, as with Concorde.
ME Incorporated
"Jonathan Thornburg" <jth...@galileo.thp.univie.ac.at> wrote in message
news:airliners...@ditka.Chicago.COM...
<snip>
> The industry doesn't care much about it, but safety is another big
> advantage of wing-mounted engines: What kills people in a jet crash
> is often not the crash itself, but the ensuing fire. Tail-mounted
> engines mean fuel pipes running from the wing (where the fuel tanks are)
> aft through the fuselage to the engines, i.e. you've got fuel right
> in the fuselage with the passengers. In contrast, wing-mounted engines
> allow all the fuel to be kept in the wings, some distance away from the
> passengers. (Some designs do put fuel tanks in the wing center sections
> anyway.)
That's all well and good, but ya still have center wing tanks on most
airliners. That's potentially gonna get into the cabin as well. While it's
not piping, true, I wonder how much of the fuel in a CWT can get into the
cabin. Or outright just explode due to vapors...
John Liebson
>But those tanks are comparatively very small John, their purpose
>is for trimming out the aerodynamic drag of the aircraft's
>'stability system' when it's not essential. It's only used in
>stabilized cruise conditions, so for takeoff and landings these
>tanks are always empty. The engines are not fed from them.
True, but that does not detract from my original comment, which is
that fuel can be present and sometimes _is_ present, a fact which
contradicts the first statement that wing-mounted aircraft don't have
fuel where tail-mounted ones do. The tail tanks are never completely
empty, nor is the piping from them to the main tanks.
A reminder (from someone, namely me, who happens to be a retired fire
chief), empty tanks may be considerably more dangerous than full ones!
Adding to that, there are also, on some aircraft, auxiliary fuel tanks
that are mounted in the cargo areas, and those tanks are used on
aircraft with wing-mounted engines.
So, to conclude, the theory of the message that started this, namely
that wing-mounted-engine airliners are inherently safer in regards to
fuel than rear-mounted-engine airlines is, I submit, somewhat of a
canard. (And, yes, the pun IS intentional ...).
Pete Mellor
> John Liebson <jlie...@cybermesa.com> asked
> >What about wing-mounted-engine aircraft that _also_ have tail-mounted
> >fuel tanks?
>
> Not nice:
> * fuel pipes in the aft fuselage (--> big fire risk in crashes)
> * fuel tanks far from center of gravity --> big trim change as you burn
> that fuel --> hurts aerodynamic efficiency (fighting that trim change
> will cost extra drag)
Exactly the opposite, in fact. This misrepresents the purpose of
the tail tank, which is not to store masses of fuel, but precisely
to effect a trim change. (In fact, the tail tank is often referred
to as the "trim tank".)
This is used on long-haul aircraft for which efficient cruising at
high altitude is desirable. Examples are A300, A310, A330, A340
and (I think) most Boeings in the medium to long haul range.
(Note that it does *not* apply to the A320, A319, A321, and similar,
which are short haul.)
As an example of the size of the tail tank, that on the A330 holds
about 7000 units compared to around 25000 in the wing and centre.
(I am going by memory from the last time I rode in the jump seat
of an A330 several years ago. I think the units the crew quoted
were gallons. If I'm wrong, please correct me.)
During cruise, fuel is pumped between the tail and wing/centre
tanks in order to move the CoG as close to the Centre of Lift as
possible while still retaining marginal static stability.
The result is that elevator trim and hence drag is reduced to the
minimum compatible with safe cruising. Therefore fuel burn is
reduced with a significant cost saving.
On modern aircraft, this transfer is (surprise! :-) controlled by
computer. A similar feature exists on Concorde, and as it goes
supersonic, the centre of lift changes, hence the fraction of the
fuel load required in the tail changes also, but the transfer is
performed under the manual control of the flight engineer.
The disadvantage of the system is that the aircraft *must* be
controlled by the autopilot while in this trimmed-back
configuration. The reason is fairly obvious. The aircraft is
up in coffin corner where the stall speed approaches the cruising
speed *and* it is only marginally stable. The suddent disconnection
of AP can therefore lead to a spectacular upset. This is thought
to have been a factor in the crash in Russia a few years back
(the A300 or 310 in which it was suspected that the captain's
teenage son was at the controls). I think there is now some
doubt about the "kid at the controls" theory, but even an
experienced pilot has great difficulty retaining control if the
AP is unexpectedly disconnected. (A Lufthansa pilot was quoted
to that effect in Der Spiegel at the time of the Russian crash.)
I would be very interested to hear any other thoughts/information
on this topic.
Pete
----------------------------------------------------------------
Peter Mellor, Centre for Software Reliability, City University,
Northampton Square, London EC1V 0HB
Tel.: +44 (0)20 7040 8422 ) NOTE: Code recently changed from
Fax.: +44 (0)20 7040 8585 ) 7477 to 7040
e-mail: Pete Mellor <p.me...@csr.city.ac.uk>
----------------------------------------------------------------
matt weber
>The biggest disadvantage to the wing-mounted engines in the Comet was the
>complexity of the wing main spar/load box. Also, the engines take up a
>bunch of
>room that could have been used by fuel.
It is also impossible to fit a high bypass (read fuel efficient) engine
inside the wing.....
Matt Weber
matt weber
separate if loaded enough in the correct direction. If you examine the ET
accident, the aircraft was not under control when it went in, and frankly
whether the engines were still attached when it went in or not, would
probably have made very little difference.
On the other hand, in the early 1960's JAL put a DC8 freighter down in San
Franciso Bay. Engines remained attached, in fact the aircraft was fished
out, rebuilt, and flew for 20+ more years with JAL before being retired in
the 1980's
Matt Weber