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In general, are there any differences between the engines on a multi-engine aircraft? Are engines designed to be specifically for the left or right wing, or inboard or outboard for a 4-engine aircraft?

What about the tail engine on a tri-jet like the DC-10? Is it the same as the two wing-mounted engines?

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    $\begingroup$ I don't know about jets, I'm guessing they're usually pretty much the same. There are some props with counter-rotating engines (to avoid having a critical engine) though which are obviously somewhat different. I don't know if they're mirrored completely or how it's done. $\endgroup$ – falstro Apr 20 '15 at 10:25
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    $\begingroup$ One classic example of two different engines was the Rutan Voyager: en.wikipedia.org/wiki/Rutan_Voyager $\endgroup$ – jamesqf Apr 20 '15 at 18:15
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    $\begingroup$ For modern jet engines, the engines are identical , but the engine mounts to the aircraft may be different. The most obvious difference would be "mirror image" mounts for left and right tail-mounted engines, or for the center engine of a 3-engine poane. With 4-engines, the inner and outer engine mounts may be different. For flight testing a new engine design, sometimes a mix of "new" and "old" engines of completely different types are used to minimize risk and provide reliable backup for the engine being tested. $\endgroup$ – alephzero Apr 20 '15 at 20:56
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    $\begingroup$ I'd also mention the A380 where only the inner engines are equipped with reverse thrust. Though the engines themselves may be the same, this is a significant difference, at least more than mounts. $\endgroup$ – sweber Apr 20 '15 at 21:18
  • $\begingroup$ The Rutan Boomerang, en.wikipedia.org/wiki/Rutan_Boomerang, also has two different engines. From what I heard, it wasn’t for any particular design considerations, he just happened to have two engines that he wasn’t using. $\endgroup$ – JScarry Feb 10 '17 at 15:52
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Generally yes, but not always. If you include the APU as an engine, the answer would need to be different, but I understand your question concerns just the engines used for propulsion.

For jets, the direction of rotation doesn't make much of a difference, so the same engines can be used on all stations. However, with propellers the swirl does influence flying characteristics, especially at low speed, so special left- and right turning versions of turboprop and piston engines are available.

Maintenance and logistics become much simpler if only one type of engine is used. In the past, not so much emphasis was placed on this, so some airplanes used different types of engines. Examples were:

  • Convair B-36B, which combined piston and jet engines in the same airframe
  • Junkers G-38, which used a Junkers L88 inboard and a Junkers L8 outboard. Later, the outboard engines were also changed to the L88. This can be seen from the number of propeller blades: If the outer engines drive a two-bladed propeller, it is the earlier version with the L8.
  • Hawker-Siddeley Trident 3, which was almost a four-engined jet, because it had a small, tail-mounted RB-162 engine in addition to the three tail-mounted RB-163 Spey to provide thrust on takeoff.
  • Rutan Voyager, which used a bigger front and a smaller rear engine.
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    $\begingroup$ One difference may be 3-engine jets: the middle engine is mounted on the vertical stabilizer and the fuel exhaust tube is somewhat longer and slightly curved. It's just a question whether you consider this as a "different engine", but I think that if you count clockwise/anti-clockwise, you should also count this. $\endgroup$ – yo' Apr 20 '15 at 12:22
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    $\begingroup$ @yo': Normally the tail engine is the same and only enclosed in different cownling with special intake duct (intake has cold air, so it's easier to change then exhaust, which has hot gasses). $\endgroup$ – Jan Hudec Apr 20 '15 at 13:24
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    $\begingroup$ @yo': As for clockwise/counter-clockwise, there are two options. The engine can be reversed, in which case basically all parts are mirrored, which makes the engine very different, or there can just be a different gearbox, in which case the engine itself is the same. Both options are used somewhere. $\endgroup$ – Jan Hudec Apr 20 '15 at 13:25
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    $\begingroup$ @JanHudec An example of right/left handed engines is the P-38: "Counter-rotation was achieved by the use of "handed" engines, which meant that the crankshaft of each engine turned in the opposite direction of its counterpart." From en.m.wikipedia.org/wiki/Lockheed_P-38_Lightning $\endgroup$ – Ralph J Apr 20 '15 at 15:38
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    $\begingroup$ About the Trident, are you referring to the "booster" engine on the Trident 3? That was not an APU, and I can't find anything about the APU providing thrust. $\endgroup$ – fooot Apr 20 '15 at 17:04
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The place to find the true answer is in the Type Certificate Data Sheet. This document will tell you exactly what engines can be installed on an aircraft.

For example on the DC-10-10:

3 General Electric CF6-6D, CF6-6D1, CF6-6D1A, CF6-6K or CF6-6K 2 Turbofan Engines. (CF6-6D and CF6-6K engines may be intermixed in accordance with Appendix XXII of the applicable FAA Approved Airplane Flight Manual. CF6-6D1 and CF6-6D1A engines may be intermixed in accordance with page 2.1 of Section IVB of applicable FAA Approved Airplane Flight Manual.)

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As this image shows the Navy flew a plane with two propeller engines and two jet engines. Read more about it here.

Navy P2V in flight

From wikipedia:

Beginning with the P2V-5F model, the Neptune became one of the first aircraft in operational service to be fitted with both piston and jet engines. The Convair B-36, several Boeing C-97 Stratofreighter, Fairchild C-123 Provider, and Avro Shackleton aircraft were also so equipped. To save the weight and complexity of two separate fuel systems, the jet engines on the P2Vs did not burn jet fuel- they burned the same fuel as the piston engines: 115–145 Avgas. The jet pods were fitted with intake doors that were kept closed when the J-34s were not running to prevent them from windmilling, allowing for economical piston-engine-only long-endurance search and patrol operations. In normal U.S. Navy operations, the jet engines were run at full power (97%) to expedite and assure all takeoffs, then shut down when the aircraft reached a safe altitude. Also, the jets were started and kept running at flight idle during low-altitude (500 feet during the day and 1,000 feet at night) anti-submarine and/or anti-shipping operations at sea as a safety measure in case one of the radials developed problems.

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    $\begingroup$ The picture is good but this is otherwise pretty light on details. Adding more would help improve this. $\endgroup$ – fooot Apr 21 '15 at 0:09
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    $\begingroup$ We encourage users to post at least a basic amount of information, because if the link dies in the future, the post becomes almost useless. Providing more than just a link also makes it more worthy of upvotes. $\endgroup$ – fooot Apr 21 '15 at 0:32
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    $\begingroup$ Ah. Ok, then I'll update with more info. $\endgroup$ – Scooter Apr 21 '15 at 0:53
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Usually the Engines on the same plane are of the same model (or type). As mentioned in an answer earlier the rotation direction of jet turbines doesn't affect handling, however, the engines installed on either sides must have same thrust as to make the handling easier.

One example for differing engines however is A380, where the inboard engines are equipped with thrust reverser and outboard engines don't have them. There are several reasons for doing this, not having thrust reverser makes engine comparatively simpler to build and maintain.

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The "Super 27" replaced the outer engines of the Boeing 727.

The problem with re-engining a trijet is the necessity of re-engineering the center engine's casing and intake, wspecially if an S-duct is involved. That is so complicated that re-engine plans typically left that engine alone.

So it was with the Super 27 with a swap to the more powerful JT8D-217. I have jeard of schemes to put even more modern engines on the outer pylons, but cannot find the link.

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