Engine Angle on Tail-Jets ?
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Join Date: Jun 2001
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Engine Angle on Tail-Jets ?
Never having been involved in tail-jets (apart from sitting in the back!) I am puzzled when I look at most of them. It looks to me as if the engines are "pointing upwards". 
To my mind, this suggests that a fist-full of extra power would rotate the aircraft nose-down (due to a downward thrust vector at the tail). With the engines in the more "conventional" position under the wings, an increase of thrust give a nose-up moment, which I would think to be a desirable feature - in most cases, when we want a lot of thrust, it's because we want to go upward (TO, GA, GPWS)
So why are most tail-jet engines oriented this way? Or is it just an illusion caused by the shape of the engine housing?
To my mind, this suggests that a fist-full of extra power would rotate the aircraft nose-down (due to a downward thrust vector at the tail). With the engines in the more "conventional" position under the wings, an increase of thrust give a nose-up moment, which I would think to be a desirable feature - in most cases, when we want a lot of thrust, it's because we want to go upward (TO, GA, GPWS)
So why are most tail-jet engines oriented this way? Or is it just an illusion caused by the shape of the engine housing?
Join Date: Sep 2002
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Often the engine has a slight nose-up orientation because it's aligned to the local flow for cruise conditions.
If you assume a near-zero AoA at cruise, with the fuselage level, then the downwash behind the wing will mean that the local flow there is angled down. So to minimize engine drag its best to align the nacelle with the flow i.e. slightly nose-up/tail-down.
Still, the thrustline of a sidemounted engine is often closer to the cg (albeit above) than a podded engine's, and sometimes the magnitude of the nose-up moment from the latter may be a design issue.
If you assume a near-zero AoA at cruise, with the fuselage level, then the downwash behind the wing will mean that the local flow there is angled down. So to minimize engine drag its best to align the nacelle with the flow i.e. slightly nose-up/tail-down.
Still, the thrustline of a sidemounted engine is often closer to the cg (albeit above) than a podded engine's, and sometimes the magnitude of the nose-up moment from the latter may be a design issue.
