Why are aircraft engines slightly tilted down?

[ddanu9015]

Hey Mates.. Have you noticed that aircraft engines are slightly tilted down ?


https://cimg4.ibsrv.net/gimg/pprune.org-vbulletin/602x403/main_qimg_96f6c6e87f3da8ed85a63a8103fe2a29_92cb71cab67824b4d 05baa07d37348873d1334e4.jpg

[KRviator]

Paging tdracer ...

But till he arrives, I am going to say that because aircraft typically cruise a few degrees nose-up, with the engine thrustline tilted downwards slightly on the ground, this places the engines thrust vector completely horizontal in the cruise thereby eliminating any 'wastage' of fuel that would otherwise be converted into in an unnecessary vertical component if they were indeed parallel to the waterline...

[MechEngr]

There is upwash as the subsonic plane moves through the air because the air is being shoved forward. See Figure 4.3 in https://www.aircraftspruce.com/catalog/pdf/13-08723.pdf and https://www.youtube.com/watch?v=6UlsArvbTeo&t=1s


The majority of the explanations in that link are suspect as the engine core is clearly parallel to the fuselage. It is only the inlet that is tilted.

[EDLB]

Aircraft body is tilt up a bit in level flight. In addition you compensate with the thrust vector downward, if the engines are forward of the CG that the engines below create a nose up force.
With props at the nose it is the same, they are tilted downward a bit too, to compensate for nose up in high throttle settings. In optimum you want a neutral reaction for different throttle settings.

[megan]

The inlet has a downward cant but I think the engine and exhaust are parallel to the waterline. Perhaps on the ground all that engine weight hanging from a beam ahead of the wing leading edge gives it a bit of a droop, the pods do dance about in turbulence, the wing too is a flexible structure.


https://cimg2.ibsrv.net/gimg/pprune.org-vbulletin/2000x1333/los_angeles_exteriors_and_landmarks_2020_5306c0961fc554dfd21 336b3f566b4abe853923a.jpg

https://cimg6.ibsrv.net/gimg/pprune.org-vbulletin/960x883/gallery_full_01_520cdbdd92fd85d63b3f8d9c196a9a76861f5890.jpg

[pilotmike]

[QUOTE=ddanu9015;11535238]Hey Mates.. Have you noticed that aircraft engines are slightly tilted down ?

Why are aircraft engines slightly tilted down? (https://aeropeep.com/why-are-aircraft-engines-slightly-tilted-down/)
QUOTE]
That's only when viewed resting on flat ground. When viewed in flight they are correct. - thankfully some clever Aeronautical Engineers saw to that!

[DaveReidUK]

Next: "Why do wings bend downwards on the ground?"

https://cimg2.ibsrv.net/gimg/pprune.org-vbulletin/301x200/a380_363f7ad22bf0e565d94da5c60e4bfa59f2312fca.jpg

:O

[B Fraser]

Ann Hedral (Dai's sister) will be along shortly.

[kriskross]

There is a fair bit of weight hanging off them. They go the other way when lift works its magic. See a You Tube on an aircraft wing taking off.

[DaveReidUK]

Ann Hedral (Dai's sister) will be along shortly.

I think you'll find she's Anya, used to work for Tupolev.

[ChrisJ800]

If the engines were mounted above the wings and thrust line then they would need to be tilted up. If they are in the thrust line like a Comet then neutral thrust line and if like most airliners when mounted below thrust line then down tilt needed as if they were not tilted there would be a big upward tilt on thrust being applied. Typed after lots of wine beer and sake so seems factually correct right now.

[Dave Gittins]

Next: "Why do wings bend downwards on the ground?"

https://cimg2.ibsrv.net/gimg/pprune.org-vbulletin/301x200/a380_363f7ad22bf0e565d94da5c60e4bfa59f2312fca.jpg

:O
Because they are flexible cantilever beams and carrying the weight of the engines. When they generate lift, they "bend" straight. If the engines weren't there they'd bend upwards and would need to be much stronger cantilevers. The weight of the engines however gives them "bending relief".

The weight they have to carry is loaded along the length of the wing. If all the lift had to be transferred to the fuselage at the wing root, they'd have to be built like a brick outhouse. I image Comets, 1-11s, Tridents and VC10s had much stronger wings to resist bending than most Boeings.

But look at the 787 which has a decided upward bend in flight.

[DaveReidUK]

If the engines were mounted above the wings and thrust line then they would need to be tilted up. If they are in the thrust line like a Comet then neutral thrust line and if like most airliners when mounted below thrust line then down tilt needed as if they were not tilted there would be a big upward tilt on thrust being applied. Typed after lots of wine beer and sake so seems factually correct right now.

I'm struggling to see how angling the engines by a few degrees in either direction would make any appreciable difference to the pitch moment about the CofG that results from application of thrust.

That's entirely separate from the thrust-line-in-the-cruise issue discussed above.

[DogTailRed2]

So that when the examiner climbs in the engine to inspect it his pen doesn't roll to the back.

[pilotmike]

If all the lift had to be transferred to the fuselage at the wing root, they'd have to be built like a brick outhouse.

Is there another way of 'transferring all the lift to the fuselage', without the use of bracing struts, for example?

[DaveReidUK]

Is there another way of 'transferring all the lift to the fuselage', without the use of bracing struts, for example?

And if all the lift were transferred to the fuselage, what would stop the wings and engines from plummeting to earth ?

:O

[TotalBeginner]

I don't think it's really 'angled down' as such. It's just the bottom half of the intake is set further back than the top. This is so that at high angles of attack the lower lip doesn't shroud the oncoming air and disrupt it resulting in a possible compressor stall.

[tdracer]

The inlet has a downward cant but I think the engine and exhaust are parallel to the waterline. Perhaps on the ground all that engine weight hanging from a beam ahead of the wing leading edge gives it a bit of a droop, the pods do dance about in turbulence, the wing too is a flexible structure.


Megan basically has it - the engine centerline is generally parallel to the aircraft 'waterline' (although some aircraft installations do have a slight angle - it's considerably less than what you see at the inlet). The front of the inlet is 'drooped' to better align with the airflow at a typical cruise AOA. This gives the inlet a slightly better pressure recovery at cruise (and hence slightly better fuel burn). And inlet pressure recovery is a huge design consideration - especially at cruise - since it has an outsized effect on fuel burn (reduced ram drag and increased thrust).
While the droop does help a bit with preventing inlet separation at high AOA (takeoff) - that's not the primary reason why it's there. Inlet lip separation at very high AOA (actually prevention of the separation) is a major design criteria for the inlet and is addressed at the overall inlet design stage, since the absolutely last thing you want to have happen if there is an over-rotation at TO is for the engines to surge due to inlet separation. The original JT9D installation on the 747 was particularly prone to inlet separation at high AOA, and there were a couple close calls when outboard engine's surged due to a takeoff over-rotation (outboard engines were more prone due to the wing flex).

Years ago, Pratt came in with a proposal to do a 'cambered' inlet instead of a drooped inlet (basically incorporating the inlet droop over the entire length of the inlet, rather than the typical droop design which incorporates it over a short length of the inlet). Claimed they could get something like a half a percent to one percent improved fan efficiency. Our inlet people looked at it and said 'no measurable improvement', but Pratt persisted with a bunch of ground testing they claimed validated the concept. Then they did some back-to-back flight testing (A310 IIRC) and couldn't measure any change. They quietly dropped the concept after that :rolleyes:.

[terrain safe]

Next: "Why do wings bend downwards on the ground?"

https://cimg2.ibsrv.net/gimg/pprune.org-vbulletin/301x200/a380_363f7ad22bf0e565d94da5c60e4bfa59f2312fca.jpg

:O
Gravity innit. 'Cos they're closer to the centre of the earth than when they're flying.

[DaveReidUK]

Megan basically has it - the engine centerline is generally parallel to the aircraft 'waterline' (although some aircraft installations do have a slight angle - it's considerably less than what you see at the inlet). The front of the inlet is 'drooped' to better align with the airflow at a typical cruise AOA. This gives the inlet a slightly better pressure recovery at cruise (and hence slightly better fuel burn). And inlet pressure recovery is a huge design consideration - especially at cruise - since it has an outsized effect on fuel burn (reduced ram drag and increased thrust).
While the droop does help a bit with preventing inlet separation at high AOA (takeoff) - that's not the primary reason why it's there. Inlet lip separation at very high AOA (actually prevention of the separation) is a major design criteria for the inlet and is addressed at the overall inlet design stage, since the absolutely last thing you want to have happen if there is an over-rotation at TO is for the engines to surge due to inlet separation. The original JT9D installation on the 747 was particularly prone to inlet separation at high AOA, and there were a couple close calls when outboard engine's surged due to a takeoff over-rotation (outboard engines were more prone due to the wing flex).

Years ago, Pratt came in with a proposal to do a 'cambered' inlet instead of a drooped inlet (basically incorporating the inlet droop over the entire length of the inlet, rather than the typical droop design which incorporates it over a short length of the inlet). Claimed they could get something like a half a percent to one percent improved fan efficiency. Our inlet people looked at it and said 'no measurable improvement', but Pratt persisted with a bunch of ground testing they claimed validated the concept. Then they did some back-to-back flight testing (A310 IIRC) and couldn't measure any change. They quietly dropped the concept after that :rolleyes:.

I vaguely recollect from aerodynamics lectures many, many years ago that around 2 degrees was a typical engine thrust setting angle. I have no idea whether that has changed over the years.

[fdr]

If the engines were mounted above the wings and thrust line then they would need to be tilted up. If they are in the thrust line like a Comet then neutral thrust line and if like most airliners when mounted below thrust line then down tilt needed as if they were not tilted there would be a big upward tilt on thrust being applied. Typed after lots of wine beer and sake so seems factually correct right now.

Chris, was that retsina? Or Soju?

The engine installation is to maintain a minimum nacelle drag condition in the most appropriate operating condition, and to gain as near as possible axisymmetric inflow (W0) into the nacelle and tonthe face of the fan and core, (W2) where it is split onto bypass mass flow W13 and core flow W2.1 for a turbojet, skip the bypass, hold the mayo. Why that has some importance is that the fan is fluffing about at around 50Hz more or less for big un's more for baby ones TFE, 195Hz etc... and that is of interest as the fan is fixed pitch ( more or less) and therefore every degree of inflow misalignment with the shaft axis doubles the effective AOA change at the the frequency of the shaft.. and that causes unstable pitching aero effects. As the fan blades are transonic at around .25r, that means the shock is unstable... as the blades are also sonic at around .76-.8r, then the sonic bow oblique shock is also unstable. The official term is something related to ablutions and sandwiches. The majority of vibration on propellers and fans has nothing to do with mass balance and everything to do with dynamic pitch effects. Why is this of interest to me? I have an STC program that stops that nonsense and the aero means it gains a high order efficiency, enough to shock the OEM who observed their engine doing very odd figgerz.

cyclic loads = bad

Control authority can be improved with a change of thrust line, particularly if it is way above or below the vertical aerodynamc center. Most manuals will say that is related to the CG, but it is actually aerodynamic not inertial. For inertial play with locke numbers etc.

the closer the thrust line passes the AC, the less trim effect will arise from a change in thrust settings. To that extent, your timendrinking is not wasted at all. This is not always the case, the C130 shafts are set slightly downwards but a HU16 is upwards. A B747, B737 A330 etc slightly downwards. To be difficult, the engines as often as not have a toe inwards which increases the thrust line offset but makes people happy, in that case the interaction with the fuselage and nacelle is changing the local flow conditions enough to get a bit of pigeon toe going.

[rigpiggy]

Look at the top tanks on a lear.23, then look at a 35. The tip tanks were positioned so at cruise attitude the top tanks were directly in line worth airflow.