4 engines at the rear….

Well said Michelf and Vega ECM. Am I the only one who thinks the government of the day did BAC a great favour by refusing to support the rear-engined wide-bodied Three-Eleven in the early 70s?
Much wailing and gnashing of teeth at the time, but I’ve always felt the Three-Eleven was an overweight commercial disaster waiting to happen.
The same government also refused to back the A300, but at least HSA decided to stay in on a business basis.

Buddy,
53131 pounds are equal to 53131 times 454 grams or .454 kilograms, so in the end effectively 24121.5 kg, or 24.12 metric tons.

I didn’t know you were using Metric Tonnes as the distinguisher as the XR is 26.5 tons. I would be surprised if the airframe couldn’t be pushed that last 1900 pounds to exceed your magic 25t number.

Not true, the EMB-145XR has a MTOW of 53,131 lbs.

Buddy,
53131 pounds are equal to 53131 times 454 grams or .454 kilograms, so in the end effectively 24121.5 kg, or 24.12 metric tons.

Less than 25t MTOW.

Not true, the EMB-145XR has a MTOW of 53,131 lbs.

What about Embraer 145?

Less than 25t MTOW. The CRJ-1000 is close to 35t. E-Series goes up to 52t.

Indeed. NO business jet except Mcdonnell 119/220 has underwing engines. Even the biggest Gulfstreams and Bombardiers have tail engines.

I would think that for the top end business jets a underwing config would make sense, too, but design heritage overruled it. For a business jet manufacturer the investment into a totally different configuration doesn’t pay off given the overall small numbers he produces. And even if it sounds strange: fuel costs due to 5 or 10% more weight are no big concern for business jets.

The critical size seems to be somewhere close to the current Bombadier CRJ series. B717, ARJ21 and CRJ900 have rear-engine configuration rather due to heritage.

What about Embraer 145?

The business jets need lower landing gear. Additionally, they gain from the wing being positioned farer back (wing box does not interfere with cabin). But business jets are not really representative for jet design of all weights. Pain increases with weight.

Indeed. NO business jet except Mcdonnell 119/220 has underwing engines. Even the biggest Gulfstreams and Bombardiers have tail engines.

For these smaller and lower a/c the rear mounted engines offer the optimum solution as wing weight is less critical…as the scale increases the relief offered by mounting the engines along the wing become the driver, provided they are located equally and correctly….so the real 747 has it right.. the twin podded version seen on the ‘Dunsfold’ mod for the James Bond film are most definitely wrong…

The interesting fact is that aft mounted engines were the choice for
– DC-9 (kept until latest model)
– Tu-334
– Fokker 70&100
– B727
– BAC 1-11

While similar capacity aircraft of later designs have underwing configuration:
– Dornier 728/928
– Embraer E-Series
– Bombadier C-Series
– Mitsubishi MRJ
– Sukhoi Gayjet

The only exception is the ARJ-21, which cannot really be called an independent design.

Yes – the main reason that rear mounted engines are not done anymore on A/C over 50-70 Ton MTOW is the mass reduction resulting from the wing root bending relief offered by the mass of the engine on the wing. Its a lighter more efficient solution.

Also the on ground cg range is very limited. The IL62 had water tanks/a tail prop to alleviate this. ( I think the Super VC10 had something to do the same ?)

The other points on this thread which are unsound are;-
– the notion that all severely damaged engines are designed to break free in flight. Sure one of the major manf adopts this approach, but the other major manf does not (others, including Vickers, did not/do not) . Both approaches are certifiable with different means of compliance.
– Engine blade release protection is not done external amour. By design the blade should be contained within the engine casing and to certify the engine this is demonstrated by test. In the unlikely event that anything gets out of the engine it is considered to have an energy level which is unstoppable. Hence for airframe components the protection mechanisms includes fuel tank dry bays, system segregation and keeping critical system routing away from debris zones.

The primary reason for having under-slung engines is weight reduction. A wing structure that has no weight along its span (ie those with rear mounted engines) needs to be stronger than one which has the engine weights at the correct points along the span.

The reason for this is simple, in flight the main weight is at the wing root (the load being the fuselage) and the wing needs to cantilever out from there to the tip, with the lift acting along its entire length with no relief (counterweight). The further from the wing root the greater the bending moment. The entire wing structure has to be designed to accommodate this moment with acceptable deflection.

Imagine the engines under the wing, the bending moment of the wing is countered by the mass of the engine(s) along the length of the wing. The placing of the heavy engines along the span to reduce the bending moment (the distance to a relatively fixed point is reduced) which in trun means a less onerous structure required to accommodate the deflection. Less structure in turn leads to less mass.

Sure there is an penalty to pay in order to hold the engines on in the first place but the equation works out in favour of the underslung engines every time on larger a/c; hence its now universal use for these larger a/c.

The VC-10 is a classic example, where the wing structure was far heavier than that of the 707 (admittedly it was a far more ‘technical wing’ with all the high lift aspects…)

This in turn makes everything else heavier…a vicious circle…

The prime reason the VC-10 went with the rear engines was to create a clean wing with high lift devices which Vickers thought was the best way to meet the BOAC requirements for hot and high operations. This worked very well and was the correct technical solution….but as ever at the time Boeing made the correct commercial call… betting that airports were going to extend runways to attract the larger aircraft, provided those a/c provided low enough seat mile costs to the operators to offer more wide spread services.

In smaller a/c the effects of scale make the rear engines more attractive… the wing span is reduced and so the bending moment reduction benefits of the underslung engines are reduced… there are often runway requirements that require high lift devices; often more economically achieved by a clean wing, the rear mounted engines are more protected from FOD…although the 737 in all versions seems to manage quite well..

For these smaller and lower a/c the rear mounted engines offer the optimum solution as wing weight is less critical…as the scale increases the relief offered by mounting the engines along the wing become the driver, provided they are located equally and correctly….so the real 747 has it right.. the twin podded version seen on the ‘Dunsfold’ mod for the James Bond film are most definitely wrong…

Interesting is that today the aft mounted engines were rather common in the 60ies, while nobody does it today (except for the copy-paste design ARJ-21).

The reason is as far as I know primarily a weight issue. The only advantage of aft mounted engines is lower landing gear and less yawing moment in case of engine failure. A reason why it made sense for the B727 to switch off an engine when in cruise. Obviously the “clean” wing was more famous in the 60ies. The wing mounted engine increases the pain for the designer of the wing, not only structurally but also aerodynamically.

I think, because most small jets find 2 engines enough and big jets put their engines elsewhere.

ALL the disadvantages you mention about tail mounted engines are equally applicabl to 3 and 2 tailmounted engines. 2 engines in the tail continues to be the standard layout for regional and private jets, for very good reasons. 3 engines in tail continues to be the configuration of Dassault Falcon 7X. And yes, you can have high-bypass turbofans in the rear, like the 2 engine MD-80 and B-717, and 3 engine Yak-42 and Dassault Falcons. However, BAC 3-11 was not built…

The critical size seems to be somewhere close to the current Bombadier CRJ series. B717, ARJ21 and CRJ900 have rear-engine configuration rather due to heritage.
The business jets need lower landing gear. Additionally, they gain from the wing being positioned farer back (wing box does not interfere with cabin). But business jets are not really representative for jet design of all weights. Pain increases with weight.

I seem to remember A VC10 being fitted with an RB211 on one side, and 2 conways on the other side of the fuslage, think it was G-AXLR, I dont recall any problems with engine mounts / bending of pylons etc.

Alas this was not the case, Nordjet

re 4 engines

I seem to remember A VC10 being fitted with an RB211 on one side, and 2 conways on the other side of the fuslage, think it was G-AXLR, I dont recall any problems with engine mounts / bending of pylons etc.

I think a VC10 would be fine with 2 x RB211 535E4s rated at around
44,ooolbs thrust, or a derated Tristar RB211 22B engine. what a shame this was not thought of back when the RB211 was first produced.

Regards
Nordjet415

IIRC 4 engines in such close proximity required take off performance to consider 2 engines inop due to the increased chance of an errant fan blade or two taking out the adjacant engine whereas conventional quads only require one engine inop for take off. Consequently they need to be grossly overpowered which then tends to make them inefficient.

Having those fat ******s hanging at the back

I wasn’t anywhere near the back!

Paul

Also keep in mind the fact that at the time of building of the VC10, jetstar etc, the certification requirements for isolation of engines in case of fire and disintergration were a lot less limiting than today.
The VC10 did have armour plating between the engines to help stop an errant fan blade or turbine blade and to delay the onset of fire to the other engine. This would cost a lot of armour plating today with today´s big fanengines, and would probably never achive certification by the authorities.

I think, because most small jets find 2 engines enough and big jets put their engines elsewhere.

ALL the disadvantages you mention about tail mounted engines are equally applicabl to 3 and 2 tailmounted engines. 2 engines in the tail continues to be the standard layout for regional and private jets, for very good reasons. 3 engines in tail continues to be the configuration of Dassault Falcon 7X. And yes, you can have high-bypass turbofans in the rear, like the 2 engine MD-80 and B-717, and 3 engine Yak-42 and Dassault Falcons. However, BAC 3-11 was not built…

Having those fat ******s hanging at the back is not exactly optimal for area ruling either. Manageable with a turbojet, a problem with a turbofan.

Btw, the commercial BWB designs will also put all three/four in the back, but as artists impressions show, in a horizontally stacked configuration.

Actually, weight has very much to do with it! Engines are very heavy, and having to add them to the rear fuselage presents four mayor weight issues:

• The actual support there will weigh a lot. The rear fuselage is not particularly strong as, with the exception of the tail, there are not that many forces pulling on it. Not nearly as much as on the wing anyway.
• The tailwing will still need to go somewhere. By having the engines at the rear you have to put the tailwing on top (F28, DC9 etc) or somewhere mid of the tail (Caravelle). This requires further reinforcing of the tail, and the rear section it is attached to… thus a double penalty!
• Engines are designed to shear off in case of critical damage. It happens only very rarely, but the engines are designed with that in mind anyway.

  The shear is probably the most important issue here, I forgot about that.

  However, doing so on a tail engined plane is tricky due to center of gravity issues. If the engine where to fall off you will have a large center of gravity issue longitudually, though a small bonus diagonally (hope I put this right).
  Engines that fall off are due to maintenance problems. They shear off if there is a catastrophic failure or a siezure.
  The CoG is a good point, harder to correct by putting more thrust on the other side (Harder than if it was out on the wing)

  A diagonnal CoG failure can be dealed with, to some extent, with steering and power inputs. With a longitudial failure this becomes much harder.

• You would need considerably more fuel lines and stronger fuel pumps.

You mentioned that the additional strenghtening of the tail versus the wing negates the weight issues. As mentioned above the center of gravity is a potential issue, even if only in emergencies. There is another thing though. At the end of the day the wing will need reinforcing anyway to resist the impact from the landing gear.
No, the gear loads are taken up in the Bermuda Triangle, (The Rear of the rear spar to fuselage triangle, where the pintles and gear beams etc. are) that area will always have to be beefed up…..more so if there is a lump of engine outboard of it.
The wingbox, the section where landing gear and fuselage attach, is the strongest part of the plane. (Torsion box actually)
The wingspars go out of that box and the engines are attached to the spars. Beyond the engines the spars will thin out, or go down in number.

The Front Spar carries most of the Engine load, but obviously the rear is used, or a mid stubby spar. The Spars, and stringers/stiffeners all taper out right from Rib 1 or Rib 0, without a lump of engine to cope with the whole structure outboard of the Bermuda triangle could be optimised, it would also be less complicated to manufacture
As you mentioned ease of access is vastly superior for wing mounted engines then it is for tail mounted engines. That’s another reason that is very important. A fast turnaround time is not possilbe if you need half an hour just to set up a ramp to touch the engine!
Indeed!

This is also the reason why tail mounted engines are only used for regional jets and business jets. The landing gear is not tall enough to make an engine fit under the wing. Another way around that is to mount your engines on top of the wing like the Honda Jet and the VFW-614.

And, I’ve just thought of another….The Leading Edge anti ice system takes bleed air from the lump, and if that lump is handy..more the better!

Electric anti ice is just coming to the fore now, however.

Answers in the text in bold

Why did this concept die with the VC10 IL62 and OK the Jetstar?

Actually, weight has very much to do with it! Engines are very heavy, and having to add them to the rear fuselage presents four mayor weight issues:

You mentioned that the additional strenghtening of the tail versus the wing negates the weight issues. As mentioned above the center of gravity is a potential issue, even if only in emergencies. There is another thing though. At the end of the day the wing will need reinforcing anyway to resist the impact from the landing gear.

The wingbox, the section where landing gear and fuselage attach, is the strongest part of the plane. The wingspars go out of that box and the engines are attached to the spars. Beyond the engines the spars will thin out, or go down in number.

As you mentioned ease of access is vastly superior for wing mounted engines then it is for tail mounted engines. That’s another reason that is very important. A fast turnaround time is not possilbe if you need half an hour just to set up a ramp to touch the engine!

This is also the reason why tail mounted engines are only used for regional jets and business jets. The landing gear is not tall enough to make an engine fit under the wing. Another way around that is to mount your engines on top of the wing like the Honda Jet and the VFW-614.