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I saw a Cirrus Jet on the ramp and was struck by how much its engine points upwards. I can't recall having seen this kind of positive thrust angle, and so wondered what the explanation was.

A picture of it in level flight shows that it's not an illusion because of sitting on the landing gear in a funny way. The engine really does have a 10-15degree upwards tilt. enter image description here

It looks like the exhaust nacelle might be angle a little back to the horizontal, although it's not clear to me if that's a thrust diverter or just allowing expansion of the flow as it exits: enter image description here

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    $\begingroup$ Don't confuse the apparent angle between the intake and exhaust with either engine alignment or thrust vector. Look at the L1011 or Boeing 727. $\endgroup$ Commented Apr 24, 2022 at 17:23

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There are two questions here: why so and how is it handled.

For the first one, I'll refer to the Jpe61's answer. Basically, it's just a reasonable compromise for a compact single-engine passenger jet.

As for the mitigation part, I happen to know some data.

The engine is mounted 8.53° nose up. This is a bit less than what it visually seems from the nacelle, due to the flattened intake. The engine is partially buried in the fuselage.

SF50 engine assembly
From SF50 PIM

The exhaust nozzle is tilted a whopping 14° with respect to the engine centreline (data from FJ33-5A IOI (Installation/Operation Instuctions)). This makes the thrust line pointing 5.47° down, thus overcompensating the engine alignment.

SF50
From the same link provided in the Koyovis's answer

As a result, the thrust line passes fairly close to the centre of mass.1 This creates minimal pitch disturbances due to thrust changes. SF50 was created largely for relatively inexperienced first-time jet pilots, and its handling must be fairly straightforward.


1 But still, it seems, slightly higher (esp. with respect to the drag centre). This is somewhat beneficial for speed handling.

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    $\begingroup$ Excellent answer. Where do the angle values come from? Service manual? $\endgroup$ Commented Apr 28, 2022 at 4:00
  • $\begingroup$ +1 for this answer $\endgroup$
    – Koyovis
    Commented Apr 28, 2022 at 5:21
  • $\begingroup$ Makes a lot of sense, especially for low speed/high thrust (go around) situation. Thumbs up for vectored thrust. $\endgroup$ Commented Apr 28, 2022 at 5:48
  • $\begingroup$ @Kenn, mostly from the standard engine and aircraft documentation (I mentioned PIM and IOI). I also have some data from the manufacturer (Cirrus). For the engine, it is interesting to know it is certified only up to 45° roll attitude, and even then only for 30 seconds. (Max 20° roll continuously). $\endgroup$
    – Zeus
    Commented Apr 28, 2022 at 7:33
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Just as engines fore of the CG are pitched down to create a nose-down pitch torque when thrust is added, rear mounted engines can be pitched up to create the same effect.

adding thrust adds speed, which adds lift, causing the plane to pitch up. Downward pitching thrust moment helps avoid power on stalls by helping to control this tendency,

The angling of the exhaust may have been added to fix an excessive pitching tendency.

One might note the exhaust stream across the tail may have been somewhat unpredictable in design, with either an increase in pitch down due to Coanda effect or a pitch up due to higher pressure in the exhaust stream than surrounding air. The engineers may have tweaked the exhaust angle to get it just right.

Finally, it should be noted that the angle of the intake is not necessarily directly in line with the combustion chamber and exhaust of the jet. Indeed, many jets have S-shaped inlets (which may help for birdstrikes). This is where the line of the nacelle may be a bit deceiving.

Shown here, in this reference, is the actual configuration of the air intake and engine.

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    $\begingroup$ It doesn't work that that. What matters is whether the thrust axis line passes above, on or below the center of mass for inertial reactions and the vertical aerodynamic center for trim reactions. The engine could be a mile out front or behind and thrust effects on pitch and trim are the same as long as its attachment to the airframe is rigid. That engine's thrust axis is made horizontal(ish) because the tailpipe is curved. Most of the thrust is generated by the flow accelerating through the nozzle. $\endgroup$
    – John K
    Commented Apr 24, 2022 at 17:28
  • $\begingroup$ I believe the Cirrus uses a technology that Williams calls “EXACT nozzle” which is a thrust vectoring system with no moving parts. $\endgroup$
    – Jim
    Commented Apr 24, 2022 at 17:59
  • $\begingroup$ As interesting as that is, it doesn't answer the question about why they mounted the motor that way. Think you can improve the answer? $\endgroup$ Commented Apr 25, 2022 at 15:59
  • $\begingroup$ @KennSebesta well, I keep thinking He 162. The intake may help reduce fuselage drag, while the compressor/combustor/turbine unit does its job more aligned with the fuse within the nacelle, and the exhaust is angled slightly upwards. It appears this model borrowed much from its nose mounted prop predecessors, with the jet mount preserving the "look" for their brand loyal customers. $\endgroup$ Commented Apr 25, 2022 at 20:30
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The engine points upwards indeed, but the main question is: how is the thrust vector pointing? If it extends to the Centre of Gravity there will be no pitch moments caused by thrust changes.

enter image description hereEdited photo from this link

The link provided by @RobertDiGiovanni shows the S-bend in the exhaust pipe, which will indeed point upwards from the engine mount axis - the hot exhaust stream needs to be kept away from the aluminium structure.

The thrust axis still looks a bit high to go through the CoG even if horizontal, which won't be much of an issue when considering for instance the low mounted jet engines on an A320.

And on the inclination angle of the engine: on a small aeroplane, where to mount the single jet engine so that the intake is unencumbered, yet the exhaust has a small moment arm relative to the CoG? I reckon they did a pretty good job there.

-----Update-----

@Zeus' answer has a more accurate thrust vector direction, turns out to be even closer to the CoG.

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  • $\begingroup$ There's an interesting nuance here. While it is true that there's some kind of a reorientation of the stream in the exhaust section, the momentum transfer required to push the airstream up will push the tail down. $\endgroup$ Commented Apr 25, 2022 at 15:58
  • $\begingroup$ BTW, as interesting as that is, it doesn't answer the question about why they mounted the motor that way. Think you can improve the answer? $\endgroup$ Commented Apr 25, 2022 at 15:58
  • $\begingroup$ @KennSebesta The orientation of the engine and that of the exhaust (which determines thrust direction) are two different entities. On a small aeroplane, where to mount the single jet engine so that the intake is unencumbered, yet the exhaust has a small moment arm relative to the CoG? I reckon they did a pretty good job there. $\endgroup$
    – Koyovis
    Commented Apr 26, 2022 at 1:39
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    $\begingroup$ I happen to know "how the thrust vector is pointing", but it was too long for a comment so I wrote a separate answer using the same picture... $\endgroup$
    – Zeus
    Commented Apr 28, 2022 at 1:53
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As I read it, the question is asking about the engine installation: why is it placed in the way it is?

The reason is overall simplicity. The placement of the engine avoids the need to use ducting or a pylon. If the engine was placed such that it was aligned with the longitudinal axis of the aircraf, it would either need to be installed on a pylon to lift it above the fuselage like in the case of Flaris LAR 1, or it would need to be placed inside the fuselage, necessitating ducting like in the case of Stratos 716X

Flaris LAR 1 Flaris LAR1 (image from company website)

Stratos 716X Startos 716X (image from company website)

Installing the engine on a pylon rises the thrust vector higher, adding the pitch moment changes due to changes in thrust. Pylon also adds structural weight, as the pylon itself, but also as reinforcements necessary for the fuselage. Installing the engine inside the fuselage adds structural complexity and may complicate the design of the pressure vessel.

While the Cirrus SF50 configuration is the simpliest possible of the feasible single engine applications (installing the engine under the fuselage won't work for a number of reasons), there are two problems with it: thrust vector angle and intake flow stability at high AoA flight.

In the case of Cirrus SF50 the issue with thrust alignment has been tackled by using thrust vectoring: the exhaust is diverted more horizontal as it exits the nozzle. The airflow around the fuselage has obviously been found not to be a critical issue, but whether or not it poses limitations to the operating envelope of the aircraft is not known to me, finding this out would require some research.

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  • $\begingroup$ The question seems to be about (alleged) thrust angle, not installation. "Vectored" thrust answers that. "Airflow around the fuseluge" ... let's say you want more headroom in your cabin? We know from wings (especially at higher AoA) some flow separation can occur past the apex of the curve. That's why the intake is mounted there. Marine creatures have a dorsal fin there for similar reasons: drag reduction. Strange but true. A solid form (such as a winglet) can have less (overall) drag than no tip at all (at higher AoA). $\endgroup$ Commented Apr 26, 2022 at 11:00
  • $\begingroup$ @RobertDiGiovanni the headline and the first sentence refer "turbine" and "engine". $\endgroup$
    – Jpe61
    Commented Apr 26, 2022 at 11:44
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    $\begingroup$ Perhaps more accurately described as "nacelle". Anyways, the Flaris LAR1 looks quite similar. Appreciate your pics. $\endgroup$ Commented Apr 26, 2022 at 12:49
  • $\begingroup$ @RobertDiGiovanni the LAR1 is a good comparison for the SF50, as it portrays "the other" solution for the top-of-fuselage installation. Same concept, different interpretation. $\endgroup$
    – Jpe61
    Commented Apr 26, 2022 at 15:45
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I believe the jetpipe is aligned with the long axis of the aircraft. Doing this while keeping the engine following the curvature of the fuselage results in the thrust centerline closer to the center of gravity of the aircraft. I suspect this eliminates a lot of undesirable pitch action during throttle changes, similar to that found in Lake amphibious aircraft with the engine mounted high off CG.

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  • $\begingroup$ If so, then why not mount the tail higher on the body, so that the tail boom would have been flatter on top and the engine would have had fewer bends? $\endgroup$ Commented Apr 24, 2022 at 16:35
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    $\begingroup$ @Kenn Sebesta Probably Wind Tunnel and CFD studies showed such a design would have produced too much drag and added excess weight. $\endgroup$ Commented Apr 24, 2022 at 18:07
  • $\begingroup$ As the engine is tilted, thrust vector is pointed slightly downwards, and as the engine behind the CoG, the engine thrust will induce a pitch moment. I do not know if there is some kind of structure inside the exhaust nozzle to vector the thrust to aligh it more with the longitudinal axis, this might mitigate the pitch moment. $\endgroup$
    – Jpe61
    Commented Apr 24, 2022 at 19:36
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    $\begingroup$ @Jpe61, the nozzle actually even overcompensates the engine tilt and points the exhaust up. I added an answer to illustrate. $\endgroup$
    – Zeus
    Commented Apr 28, 2022 at 2:02

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