Is there some benefit to placing engines side by side as opposed to vertically (One above the other)? Question comes from seeing this http://www.luft46.com/mess/meschwlb.html
I’m not sure about other aircraft but the B747–and presumeably other Boeing aircraft–has its engines hung out on the wings because it helps stabilise the wing, preventing flutter.
Disclaimer: The above information comes via a book, the title of which I have forgotten, and over ten years vegetating in my gradually deteriorating brain matter. It may be wrong.
English Electric’s fighter Lightning in the 1950s and 1960s was built with an over/under placement of the engines. It was a decent plane, although never the top gun in the arsenal. I would guess that if it had proved to be significantly better than side-by-side, a few more planes would have been built with that design.
As a WAG, (I seem to have misplaced my aeronautical engineering degree), I would say that the current crop of multi-engine fighters use side-by-side because it makes placement of the landing gear, avionics, and any internal weaponry easier to shuffle into place. There are design constraints imposed by a plane’s cross-sectional dimensions. Since the wings are already making the plane “wide,” extending the engines out in the same direction reduces the problems with “adding” cross-section. Once that has happened, it is not that difficult to find nooks and crannies into which to tuck the gear. If you build the plane vertically, you either need to make the fuselage wider to hold all the “extras” (defeating whatever benefit you attempted to gain with a narrow fuselage) or you need to build a skyscraper of a fuselage to get everything into the plane.
I would also think that having engines that needed to be extracted from above and below the wings would make maintenance a nightmare, whereas in a side-by-side position, all your engine access points are simply right-handed and left-handed versions of the same feature from the top or bottom.
I welcome the corrections from the guys who actually do have engineering degrees.
tomndebb, you have all the basic reasons. Jet engines aren’t, however, made in mirror-image versions; both engines on a twin-engine fighter would be identical. Further, virtually everything a mechanic would need access to is along the bottom of the engine, and not only for accessibility - the external fuel and hydraulic stuff is there so a leak won’t spray onto hot metal and start a fire. That advantage disappears with a top/bottom layout, and maintenance access to the top engine becomes a nightmare.
There is an advantage to the layout in that an engine failure won’t produce asymmetric thrust that makes the aircraft harder to control. It will produce a pitch moment instead of a yaw moment, yes, but that’s much easier to handle. The yaw moment with both engines near the centerline isn’t that much, though, and engine reliability makes it much less of a consideration now than in the early years when the Lightning was designed. An uncontained failure (i.e. from a turbine disk burst) from one engine can easily take out an adjacent one no matter how they’re situated, though.
The EE Lightning didn’t have a direct top/bottom layout; the lower one was further forward than the upper one to minimize cross-section at the wing root (the area rule for supersonic aircraft). The upper one was no more maintainable for that, though, and required more access hatches in the body.
so was there any other reason for designing them in an “over/under” format other than in case of asymetrical thrust due to engine failure? I assume the the germans having designs for both side by side and over/under would have a reason for opting one over the other
Not really, no. Note the small number of planes that have ever been built that way. Remember that there was a lot of experimentation with engine locations when jets gave designers that added freedom - there were some with the engines on the sides of the nose, some with them buried in the wing roots, some in the vertical stabilizer roots, inlets on top, bottom, sides, wherever. Current conventional practice is conventional because it works best all around.
Just to add to the variety of possible engine placements, I’ve seen a Russian jet with the engines mounted above the wings. What’s up with that?
And for the truly bizarre, there’s this nice gallery of Russian strangeness.
actually Id heard of that design before. Apparently being able to fly that low means that you reach speed of sound with less energy than flying higher up. The last iteration of the design apparently had sufficient possibilities that western investment had started, the goal being to use it as an alternative to cargo freighters. This was all A LONG time ago, so no idea what happened.
The VFW-Fokker VFW 614 had the same above the wing engine placement. In the VFW’s case, it was to allow for a shorter landing gear and to prevent debris ingestion when using unpaved runways. I would imagine the Russian design is even more oriented towards use of unpaved runways. It does make the engine less accessible for maintenance and generates higher passenger noise levels, though, which is why it isn’t used much.
And another site in English. These are not true aircraft, and are referred to as “wing in ground effect” craft. They generally cannot fly much above the just-above-the-ground heights at which the ground effect air cushion provides significant amounts of lift, but can be quite fast and can carry tremendous loads. Like the resurrected airships, however, you have to wonder how well they would survive bad weather. I wouldn’t want to be flying that close to the ocean at that speed in a storm.