Middle engine placement

[chornedsnorkack]

Where can a plane have a middle engine?

A plenty of aircraft have a S-duct from inlet down into the rear fuselage where the engine is. So on Trident the original trijet, Boeing 727, Yak-40, Tristar, Tu-154, Yak-42, Dassault Falcon 50, 900 and 7X.

Only DC-10 and MD-11 have the engine higher in tail, aligned with inlet.

Is there any technical reason why such tail engine placement has not been popular with small trijets?

[Rainboe]

1- on a smaller aeroplane, the trim/thrust effect will have more power than on a large long fuselage
2- the engine would have to be placed further back causing bigger trim problems. This rear positioning would be necessary to allow for easy removal and clearance from the rudders.
3- it would waste a large part of rear tailcone volume which is already there.
4- it would call for a higher tailfin as rudder space would be wasted. These jets live in tight hangars, so it would not be popular.

[chornedsnorkack]

Then it makes one wonder how the Tristar fared in comparison to DC-10.

Tristar was delayed because they bankrupted Rolls-Royce by developing a suitable engine.

No other trijet bankrupted their engine manufacturer... Boeing 727 did not, Dassault Falcon did not, DC-10 did not...

What was so difficult about the Tristar engine?

[Rainboe]

Now you are asking a different question! We are talking big fans. RR did not have the resources the other companies had and it went for a more technically complicated solution (3 spools). The Tristar did not sell as well as the DC10 as it did not already have a captive market of DC8 type aeroplane operators and it's range was inadequate. It seems to have been a better short/medium range high capacity aeroplane than the DC10, though not matching the 10 for long range. To get LHR-LAX, it had to be made in a shrunk version, and even then it was no good for that whilst the DC10 was happily flying the route.
The Rolls Royce RB211 went on to become a very efficient and reliable engine powering all Tristars and BA 747s, 767s and even 757s (I think it may even power half or more of all 757s). Derivatives are now powering the A380/777, so there was nothing 'difficult' about the Tristar engine, just a slightly more protracted development.

Now your turn! What are these questions about and why?

[seacue]

We had an engineer who had worked at Douglas when the DC-10 was being designed. He said that the reason the DC-10 didn't have an S-duct for the middle engine was that they couldn't afford to design / develop one. I don't know whether the shortage was entirely money or was also time. Thay may well have not had enough computing power. You'll note that the MD-11 got a pinched-waist housing for the middle engine to reduce drag. The DC-10 should have had that. It almost looks as though quick and dirty won the day with the '10.

[chornedsnorkack]

Well, the central engines are fascinating, and unfortunately there are limited numbers of new trijets in production - not sure how the Tu-154 and Yak-42 lines are doing, the only brand new trijet is Dassault 7X.

I think that Tristar was also limited by the fuselage and landing gear design - there was a massive keel beam which prevented adding a central landing gear and increasing take-off weight or something.

But otherwise, Tristar and DC-10 were pretty close... why could´t Tristar just use similar engines as DC-10?

And as for quick and dirty... wonder why then MD-11 got just pinched waist instead of a whole S-duct?

[Rainboe]

It needs a major design change switching engines on an aeroplane. For example, the RR engine with 3 spools is heavier than the GE and P&W equivalents. The mountings are different, the thrustlines are different, the whole interaction of the engines on the wing change, the ducted airflow supplies, hydraulics and electrics are different, and then you have to carry out all the development flying and manual and performance manual testing! Big Job. I don't think there was enough of a delay by RR to make swapping engines worthwhile.

As for the MD11 duct, I don't think Douglas thought that there was any effective gain in efficiency to pay for the major design change.

So, out of interest, why the questions, or are we just chewing the cud to pass the time?

[chornedsnorkack]

It needs a major design change switching engines on an aeroplane. For example, the RR engine with 3 spools is heavier than the GE and P&W equivalents. The mountings are different, the thrustlines are different, the whole interaction of the engines on the wing change, the ducted airflow supplies, hydraulics and electrics are different, and then you have to carry out all the development flying and manual and performance manual testing! Big Job. I don't think there was enough of a delay by RR to make swapping engines worthwhile.

But why was RR trusted with developing engines for Tristar to begin with - why not some other engine easier to develop and from a more reliable manufacturer?

[Rainboe]

You answer your question first!

[barit1]

From the dustbin of history -

The RB211 originally was to use a composite fan blade (called Hyfil, I think) that would have been wonderful - except that it couldn't pass the bird & icing tests. They had to redesign it in titanium. This delayed the 1011 project enough that Delta, for one, bought a Plan B (DC-10's, later resold to other operators). Delta was the first airline to operate 747, DC-10, AND L-1011.

Modern composites, had they been available to RR in 1969, might have made a huge difference in the Lockheed sales.

More L-1011 history (http://www.globalsecurity.org/military/systems/aircraft/l-1011-history.htm)

[CV880]

DC10's with General Electric CF6 engines did not have the waisted look to the No. 2 inlet duct. Northwest and JAL wanted Pratt and Whitney engines on their DC10's which had a larger fan diameter so Douglas fitted a larger inlet duct with the waisted (area ruled) look. The MD11 had larger engines than the DC10 so the larger waisted duct from the PW powered DC10 was used on all MD11's.
The 4 large banjo structural fittings that carried the tail loads were designed with this in mind and the GE powered DC10 had a smaller diameter duct lining passing through the banjo fittings. With the PW powered DC10 and all MD11's the duct lining was roughly the same diameter as the hole through the banjo's.

[chornedsnorkack]

More L-1011 history (http://www.globalsecurity.org/military/systems/aircraft/l-1011-history.htm)
Ah, thanks!

The explanation offered there is that the GE and PW engines suitable for DC-10 (mounted outside) were too big to be mounted inside the Tristar tailcone.

So, for some reason, smaller trijets can mount their tail engines inside the tailcone without trouble...

[barit1]

The early CF6's were probably too long (although I don't fully understand that...), but about 1983 some L-1011 operators were shall we say fed up with the maintenance cost of the RB211.

Delta and All Nippon had large TriStar fleets and had bought early 767's with CF6-80A's, which were a foot shorter than earlier CF6's due to new combustor design.

So these two carriers were very interested in retrofitting the 1011 with CF6-80A's - but they couldn't achieve a critical mass to convince TWA or others to join them. :ugh:

[Rainboe]

I find your first statement questionable for 2 reasons. It is well known the RB211 is shorter than the competing big fans because of its 3 spool design allowing more efficient pressure increase across the compressor section and better matching of the combustion chamber. As for the remarks about maintenance costs, strange then that so many operators selected the RB211 in preference to GE and P&W for rather a large number of 747s and 757 and 767s?

I also very much doubt anybody seriously considered re-engining the Tristar with all the work and expense involved redesigning the wing and the S-duct and all the ancillaries- electrics, hydraulics, air compressors and all the other connections....now seriously- all that to save a small percentage (allegedly) maintenance cost? I suspect this is hearsay passed around the GE or P&W factory, at least one of which is located in Ohio? The problem with accepting the RB211 as the success it has been is the old NIH syndrome!

[barit1]

Then you may not have heard of the RB211-22C. :)

[Rainboe]

So tell us all about it!

[barit1]

Since you asked --
Initial RR sales of the RB211 were the -22B rating. However, they could not produce engines that actually met that thrust rating, so they delivered a derated -22C model with attendant aircraft penalties (and financial penalties to RR & Lockheed, I'm sure).

But development refinement continued, until the machine met its original guarantees. Existing -22C's were upgraded at last to -22B, and finally the -22C was stricken from the type certificate. (Swept under the carpet, we might say)

But as late as 1986 I witnessed a dozen or more RR technicians at a medium-sized Middle East engine shop, scurrying to keep a fleet of RB's in dispatchable condition--average time on wing was a small fraction of the competitors. RR provided the labor force free of charge - which helped contain the airline's costs somewhat.

[Rainboe]

Well the early days of any engine type are usually filled with problems. I seem to remember P&Ws grounding rather a lot of 747s for rather a long time- I bet their engineers were scurrying all over the place trying to repair an engine that should never have been let loose on the public in the first place. But they get fixed and burble away merrily for years. You seem to have a thing about RR engines- I like them. After shutting down many P&Ws on the 747-100, the RB211 was like a breath of fresh air. If you can only dredge up 20 year old bad stories, wake up and look around.....they are selling very successfully on the latest equipment.

[barit1]

No quarrell - all the engine mfgrs. have greatly improved their statistics in 35 or so years.

But this whole thread is historical in nature, isn't it?

[old,not bold]

[quote=chornedsnorkack]Where can a plane have a middle engine?

Let's get down to earth here and back to the question! Jets aren't everything.

The Ford trimotor had one on the nose, as did the main German communication aircraft of about the WWII era....was it a Heinkel?

Now, in the 21st century, to get ourselves right up to date, we have the Trislander in service with one mounted a little way up the fin leading edge.

I vaguely recall a turboprop on the nose of an experimental DC3 conversion, along with replacing its radial pistons with turboprops (PT6-??, I think).

Surely these, with the L1011/DC10 etc installations mentioned so far, just about cover the available options? Or did anyone ever put one on a pylon on the centre-line above the fuselage (as per the Lake amphibian, although that was the only engine on that lovely airplane)?

[barit1]

The Lake was a single; a classmate of mine used to ferry green ships from the factory to a paint shop. He'd stop by the campus to catch a class or two enroute.

The German Junkers JU-52-3m was built in large numbers. Read Gann's "Band of Brothers" for a wonderfully funny tale about one. :ok:

And when the first AA 727 was delivered, they parked it nose-to-nose with their restored Ford trimotor for a photo op. You can probably find it online. The Ford is now hanging in the NASM in Washington.

[barit1]

The Italians built a couple of lovely "middle engine" types too:


http://img.photobucket.com/albums/v293/boacphotos/FiatG212CP.jpg
First the Fiat G212CP


http://img.photobucket.com/albums/v293/boacphotos/S73.jpg

Savoia Marchetti S.73


I remember seeing the Fiat at Vigna di Valle (http://www.aeronautica.difesa.it/SitoAM/Default.asp?idsez=717&idente=1404)

(note to Pprune mod - I resized these to 300x800 or less, but they still show bigger here. I dunno...)

Barit - since you tried, as a favour I have done this for you - NB I do not intend to do this all the time - my time is too precious! Full instructions are in the notes at forum top, and your images were respectively 1179x399 and 1109x306. I suspect you reposted the originals? Doh!