• Hi Guest! Forum rules have been updated. All users please read here.

Bréguet Transport & Airliner Projects and Prototypes

iverson

ACCESS: Secret
Joined
Sep 24, 2009
Messages
411
Reaction score
248
Breguet Br. 851
192 m wingspan and 1.000 tons

How practical would a 15-engine aircraft be? With so many separate components, I'd expect multiple engine failures per flight, just as a matter of statistics. So the remaining engines would have to be powerful enough to maintain altitude with X number of props feathered and the consequent extra dead
weight and drag. Thoughts?
 

gabrielorosco

I really should change my personal text
Joined
Dec 10, 2016
Messages
126
Reaction score
88
Breguet Br. 851
192 m wingspan and 1.000 tons

How practical would a 15-engine aircraft be? With so many separate components, I'd expect multiple engine failures per flight, just as a matter of statistics. So the remaining engines would have to be powerful enough to maintain altitude with X number of props feathered and the consequent extra dead
weight and drag. Thoughts?
It was to be powered by 120 Hispano-Suiza 12Z motors, placed in fifteen octuple mounts along the wing. A nightmare!
 

dan_inbox

ACCESS: Top Secret
Senior Member
Joined
Sep 3, 2006
Messages
872
Reaction score
419
I'd expect multiple engine failures per flight
Yes, complexity is bad. Now what other options if the tech of time did not offer powerful enough engines?

Note that most likely, engineers would have access to the engines, and would be able to fix and repare them during the flight.
So unless something unusual happens, you'd have only one of the fifteen out at the same time.
 

avion ancien

The accidental peasant!
Joined
Mar 6, 2013
Messages
292
Reaction score
106
How practical would a 15-engine aircraft be? With so many separate components, I'd expect multiple engine failures per flight, just as a matter of statistics.

I'm not sure that the logic of that theory is sound but, if it is, then presumably one would have to aver that a four piston engine aircraft should expect more than one engine failure every four flights. I'd be surprised to find that the statistics bear that out.
 

Arjen

It's turtles all the way down
Senior Member
Joined
Nov 6, 2010
Messages
2,806
Reaction score
677
On the question of how many engines are practical - the Dornier Do X had twelve engines. It struggled to get out of ground effect.
 

iverson

ACCESS: Secret
Joined
Sep 24, 2009
Messages
411
Reaction score
248
How practical would a 15-engine aircraft be? With so many separate components, I'd expect multiple engine failures per flight, just as a matter of statistics.

I'm not sure that the logic of that theory is sound but, if it is, then presumably one would have to aver that a four piston engine aircraft should expect more than one engine failure every four flights. I'd be surprised to find that the statistics bear that out.

No, I wouldn't expect a failure every four flights. We aren't talking about the reliability of the ENGINE, which depends on a mean time between failure (MTBF) per engine determined during testing and operation. I was talking about the reliability of the AIRPLANE--that is, its ability to avoid falling out of the air, all else being equal. The question is, what kind of safety margin do additional engines provide compared to the extra complexity and weight?

My reasoning goes as follows.

Assume that a given engine has an MTBF of X. If all else is equal:

* An airplane powered by one of these engines has an MTBF of 1X. The engine is a single point of failure.

* If an airplane with two of these engines can maintain altitude (i.e. keep flying) on one engine, then it is twice as reliable as the single-engined airplane. The twiin has an MTBF of 2X, since either engine can get the plane safely to its destination.

* But if an airplane with two of these engines CANNOT maintain altitude on one engine, then it is HALF as reliable as a single-engined airplane with the same powerplant. If either engine fails, the plane goes down. So the plane's MTBF is 1/2X

So if an airplane with four of these engines could maintain height on one engine, it would have an MTBF of 4. But that usually is not the case. More engines for a given installed power usually mean more structural and power plant weight, higher drag, higher fuel consumption, and thus still more weight. So the aircraft MTBF for a four-engined airplane that can maintain height on two engines is exactly the same as for a twin with the same engines. A more typical four-engined airplane that could maintain height on three engines would have an MTBF of 1.5X.

Above four engines, the value of additional units starts to run into diminishing returns. The weight and drag of more engines requires more of them to be operating in order to maintain height. The MTBF of the airplane starts to approach unity.

Now we need to consider the value of X, the engine's MTBF compared to the time required to complete a given flight over a given route. A short MTBF doesn't matter if the distance you need to fly is short enough so that you can take off and land before it runs out. But it matters alot if you are flying across a continent and even more if you are flying over an ocean with no intermediate landing grounds. In the early days of commercial and military aviation, engine MTBF was short but flight times were mostly shorter still. So, given the perennially high cost of aviation engines, single engines were the norm (most airplanes that had twins had them because they needed the power, not because they could fly on one engine). In the 1930s, as long, transcontinental and overseas routes became more common, engine MTBF and outputs increased, just not enough. So airlines and regulators started to require multiple engines for all but the shortest services. In the 1980s, engine MTBF finally became long enough to allow safe, twin-engined trans-Atlantic flights, at which point classics like the four-jet Boeing 707 and 747 gave way to twins, like the Boeing 767 and the various Airbus models.

So now lets return to the case of the 120 Hispano-Suiza 12Z. Even if we ignore the 12Z's reputation for poor reliability (low MTBF), it's hard to see how the Breguet could be more than an amusement for a designer's idle hour. Think of all the structure weight, engine weight, and drag created by all those engines, compared to a smaller number delivering the same power. Think about the likely MTBF of all the extra gear trains, shafts, bearings, clutches, and what all needed to gang those engines together on shared propeller shafts. Then ask how many engines are needed to keep all that structure and all those mechanicals at operating altitude? Once you know, you can use the MTBF of the engine and drive train to calculate the MTBF of the airplane. You can then compare the result to the required time of flight for an Atlantic route. I doubt that the numbers would be encouraging. If the airplane could maintained height on 80 engines, it would have been no more reliable than a four-engined airplane.: 1.5X. If it needed more than 80, the airplane MTBF would be significantly worse.

I suspect that airplanes with six or more engines are relatively rare because they just aren't practical. The size of the Do-X vastly outstripped the power available from contemporary engines. So its designers added engines until they could get it off the ground--just. Like other 1930s giants, the aircraft were novelties, experiments built in small numbers (if at all) only because they could be. They weren't practical propositions. Practical aircraft of this size had to wait until four engines could provide the required power.







.
 

dan_inbox

ACCESS: Top Secret
Senior Member
Joined
Sep 3, 2006
Messages
872
Reaction score
419
* If an airplane with two of these engines can maintain altitude (i.e. keep flying) on one engine, then it is twice as reliable as the single-engined airplane. The twiin has an MTBF of 2X
Don't think so. The probability p(NOT-A AND not-B) is not 2 x p(NOT-A), even when p(A)=p(B).
The MTBF would be twice IF you started the 2nd engine only when the first fails, which is not the case.

Probabilities are notoriously counter-intuitive.
 

hesham

ACCESS: USAP
Senior Member
Joined
May 26, 2006
Messages
27,652
Reaction score
4,622
From AFM09 magazine,

the Breguet XXX or Br.30.
 

Attachments

  • 1.png
    1.png
    209.5 KB · Views: 24
  • 2.png
    2.png
    255.1 KB · Views: 19
  • 3.png
    3.png
    341.2 KB · Views: 20
  • 4.png
    4.png
    4.2 MB · Views: 23

Archibald

ACCESS: Above Top Secret
Senior Member
Joined
Jun 6, 2006
Messages
5,141
Reaction score
3,025
Breguet Br. 851
192 m wingspan and 1.000 tons

How practical would a 15-engine aircraft be? With so many separate components, I'd expect multiple engine failures per flight, just as a matter of statistics. So the remaining engines would have to be powerful enough to maintain altitude with X number of props feathered and the consequent extra dead
weight and drag. Thoughts?
It was to be powered by 120 Hispano-Suiza 12Z motors, placed in fifteen octuple mounts along the wing. A nightmare!

Whaaaaaat ? If that's not a joke, then... THE HORROR.


...for the record, the 12Z never worked properly. Just ONE engine, on ONE ground bench or ONE D-520 modified airframe.
Not only did the 12Z never worked in a ruined 1946 France, but it wasn't working either in 1939 France - it was deeply flawed (like pretty much everything in 1939 France from politics to AdA procurement to the QGQ to the very senile and complacent Generals, but that's another story).

120 * 12Z... better not to think about it. Dope was good, at Breguet !
 
Top