An argument in favor of multi-engine design

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BBerson

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I think the DC-1 was designed to prove that a twin could be as safe as the Fokker Trimotor with one engine failed.
 

blane.c

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I think the DC-1 was designed to prove that a twin could be as safe as the Fokker Trimotor with one engine failed.
That may be true, seems logical from the standpoint of the era of the designs.

Many of the arguments from early aviation had validity for there time and of course some will always be true, not all hold water today because things have progressed. I personally question the validity of a modern well thought triple being in any real sense less safe than a twin. Of course if you just want to cob some motors onto something that is another matter entirely.
 

BBerson

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Three engines is probably not less safe, just less efficient. The trend in commercial aviation is fewer engines as engines get more reliable.
 

Pops

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Designing a good mulit engine aircraft is easy. Just look at what Cessna did on the 411 and do the opposite will get you close.
 

BJC

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"Can a C411 fly on one engine?"

"No."

"What is the second engine for?"

"When one engine quits, the second one gets you to the site of the crash."
 

Swampyankee

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Three engines is probably not less safe, just less efficient. The trend in commercial aviation is fewer engines as engines get more reliable.
The other reason is that a twin has much more surplus thrust before loss of an engine, which permits a steeper and faster climb out; this reduces the noise footprint on the ground. If your design needs 50,000 lb of thrust to climb, a twin will have 100,000 lb of thrust with both engines; the three-engine aircraft will have 75,000 lb of thrust with all three, and the four-engine one only 67,000 lb of thrust with all four.
 

blane.c

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The other reason is that a twin has much more surplus thrust before loss of an engine, which permits a steeper and faster climb out; this reduces the noise footprint on the ground. If your design needs 50,000 lb of thrust to climb, a twin will have 100,000 lb of thrust with both engines; the three-engine aircraft will have 75,000 lb of thrust with all three, and the four-engine one only 67,000 lb of thrust with all four.
If you need 50,000 lbs of thrust to climb then maybe 75,000 lbs of thrust is enough for your needs in a homebuilt design. Maybe you don't want the additional cost of the extra 25,000 lbs of thrust and or maybe you would like the plane to require less control input in the advent of an engine out.

The four engine planes I flew would climb on two engines, not that you would ever want to, but you would have the same total thrust as the Twin engine.
 

BBerson

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The other reason is that a twin has much more surplus thrust before loss of an engine, which permits a steeper and faster climb out; this reduces the noise footprint on the ground. If your design needs 50,000 lb of thrust to climb, a twin will have 100,000 lb of thrust with both engines; the three-engine aircraft will have 75,000 lb of thrust with all three, and the four-engine one only 67,000 lb of thrust with all four.
Why does the twin have so much surplus thrust?
 

blane.c

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Why does the twin have so much surplus thrust?[/QUOTE

Because in order for a multi engine to work like you would expect it to (fly and continue to climb if necessary after an engine failure) it must have an enough power to also compensate for the drag of the dead engine and the yaw inputs required to fly straight with asymmetrical thrust. This is a gob of additional power.
 

harrisonaero

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Here's a relevant question- which aircraft engine (non auto conversion) has the highest horsepower to cost ratio? Assume experimental and available used and/or rebuilt.
 

bmcj

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Being a fan of gliders, I have trouble justifying the need of even one engine on an aircraft, though I can see the utility in having one engine for two airfcraft. :laugh:
 

bmcj

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For those who are advocating two engines driving one prop through some sort of gearing mechanism, I can recall one very popular aircraft that used that approach, gearing four single cylinders into one output shaft, driving a single propeller:

Cub1.jpg
 

blane.c

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The Cri-Cri type motor-mount could place two engines on a lot of existing one engine designs. Properly thought through (you may also need a bigger vertical stabilizer and or rudder for example) this could still be the easiest way to have some redundancy in power because you do not have to completely design a whole aircraft. There are some aircraft that have put a pusher type engine above the cabin area to add a second engine as this alleviates some of the weight and balance issues but it also adds a pitching moment around the center of gravity that must be learned to be dealt with. The DoubleEnder a highly modified Super Cub type puts both engines on essentially a pod above the cockpit in a tractor pusher configuration. I don't see that a yawing motion caused by an engine failure of a side by side engine arrangement is going to be any harder to learn to correct for or any more severe than learning to correct for the pitching motion of an overhead engine arrangement. Video of the DoubleEnder shows control with engine out to be rather docile, I suspect that a side by side arrangement would be similar and the engines would be easier to work on.
 
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