Why are there practically no three engined aircraft in GA?

Odd numbers of engines, where engines >=3 is generally very rare.

Lots of 2 engine planes, only a few 3, lots of 4 engine planes, just 1 five engined aircraft, a handful of 6 engine designs, just one 7 engine aircraft, and a handful of 8 engine aircraft. You have to put at least one engine on the centreline with an odd number of engines, and the centreline is probably the hardest place to put an engine if you’ve got a choice.

alioth wrote:

You have to put at least one engine on the centreline with an odd number of engines, and the centreline is probably the hardest place to put an engine if you’ve got a choice.

Lots of single engine aircraft which both have an odd number of engines and place them on the centerline The most efficient placement of an engine is actually inside the fuselage, almost no drag added. Just compare airplanes where the same fusealge exists as a twin and a single and compare total HP and performance / fuel efficiency.

Cost. three “powerful” engines are less efficient and more costly to maintain than two “even more powerful” engines, so unless you absolutely cannot get two “even more powerful” engines, you would never do that.

For transatlantic ops (especially on more southerly routes) and for trans-pacific ops, a third and possibly fourth engine may still be useful because using twins may not be viable on some routes.

MedEwok wrote:

p

Surplus = unnecessary cost

The money spent on the unecessary engines is probably better spent in (customer): flying, (manufacturer): making other parts of the plane better, such as deicing / weather radar / avionics

Traditionally, tri-engined aircraft were designed when the engine options were limited and the only way to achieve a given amount of power was to add another engine. There were limited scaling opportunities for engines and that’s not been the case for about 50 years in turbines. Today the same engine model will come in many different power configurations.

And with ETOPS even the big airlines are now twin engines. I still think a 4-engined aircraft can legally fly longer than any ETOPS without having an alternate, but I could be wrong about this. 777’s regularly have 330min ETOPS operations these days. I doubt we’ll ever see any new 3 or 4-engined planes made, except for future electric ones perhaps.

AdamFrisch wrote:

And with ETOPS even the big airlines are now twin engines. I still think a 4-engined aircraft can legally fly longer than any ETOPS without having an alternate, but I could be wrong about this. 777’s regularly have 330min ETOPS operations these days. I doubt we’ll ever see any new 3 or 4-engined planes made, except for future electric ones perhaps.

I am out of the dispatch business for a while but what I have heard on that subject is that the future will no longer be twin engine only but also for 3 or 4 engined planes in terms of EROPS planning. Which makes perfect sense too. Of course, limits for 3 or 4 engined planes can be more relaxed (if that is possible) but there are more factors than just engine. Decompression, fire, just to name two of the most brutal ones.

In GA, 3 engines don’t make much sense as they only add an additional cost factor nobody will be ready to pay. Looking at the development in commercial aviation, the trend is definitly back to 2 engines only, the only 4 engine planes in production are the Super heavies.

As I remember, the only reason for the Trislander configuration was that no appropriate 350-400 HP piston engines were available to replace the 260 HP engines on the Islander. That would have been the logical development.

I spotted these Italian 3 engined aircraft in the Vigna di Valle Aircraft Museum 25Ks North of Rome.

The museum is very worth visting if your in Rome it is on the shore of Lake Bracciano and was/is a seaplane base. I took about a 100 pictures and haven’t named them all yet. The museum also houses what should have been the winner of the Schneider Trophy (MC72) with back to back Fiat 24 cylinder engine driving contra rotating propellors.