View Full Version : 747 Engine architecture

28th May 2008, 13:54
This has been bothering me for a while...

When I look at pictures taken out over the wing of a 747, I cant help but notice the outer engines appear to be toe-ed in slightly.

I think it can be seen quite clearly on this picture:
Photos: Boeing 747-422 Aircraft Pictures | Airliners.net (http://www.airliners.net/photo/United-Airlines/Boeing-747-422/1180321/L/&tbl=photo_info&photo_nr=32&prev_id=1195947&next_id=1174507)

Is it just a trick of the eye or are the outer engines in fact angled inwards?

I am unable to find anything on here or google about this subject, and with no definate figures, I wouldnt dare to estimate the angle and work it out - but I would have thought the outer engines at full power must create quite a significant crushing force on the pylons or wing-roots.

If anyone could shed any light on this subject it'd be great to hear the reasoning - or it'd be nice to put my mind at ease if it is just my eyes playing tricks on me.

28th May 2008, 14:07
Is it an attempt to counteract the wing tip vortices?

28th May 2008, 14:17
747 inboard and outboard engines have a two degree tow in. Something to do with dynamic stability I think.

28th May 2008, 14:26
That would make sense too, it would increase stability about the yaw axis.

28th May 2008, 14:26
So - to calculate the "crushing force":

sin(2 degrees) is about 0.03

Highest rated engines on 747 are about 60000# Fn

Lateral component due to toe-in = 60000 x 0.03 = 1800#

(Pretty minor compared to other loads on the wing)

I'm thinking also that this tends to reduce the yawing moment during assymetrical engine ops.

Notso Fantastic
28th May 2008, 15:06
The airflow under the wing flows slightly outboard as the wingtip vortex draws air from the high pressure area under the wing and swirls it upwards at and to the rear of the tip. I understood that to maximise cruise efficiency, you angle the outboard engine in a bit so that it is exactly in line with local airflow under the wing at cruise (for those aeroplanes that spend a long time in cruise). There is no point in having an engine that does not have a thrustline in line with the airflow at the engine location. Therefore, the inner engine is angled slightly less. Whatever, in turbulence, you should see them waggling and nodding whilst the wing is flapping up and down. Rather disconcerting.

28th May 2008, 15:11
Thanks for the replies folks!

Its an interesting subject and is great to hear the reasoning!

Nepotism, I can understand angines engled inwards would increase stability, but the whole reason I noticed this subtle design feature is how the inboard and outboard engines DON'T appear to be aligned - which would be confirmed by Notso Fantastic.

So, my next question is, are engines (slightly) toe-ed inwards on twin engined jets? and are the outboard engined toe-ed in also on all other 4 engine jets?

Thanks again folks

gas path
28th May 2008, 18:37
The toe-in is to take account of the bow wave of the airflow off the nose. The intakes are also angled to present the airflow at a better angle in the cruise.
The pylons (on later a/c) also 'droop' to help with the presentation of the engine/intake to the local airflow and to help with the rotation 'mush'.
BTW this was most noticeable on one of QF's 747 classics that had a -400 pylon installed in the no3 position, after it was damaged when the r/h WLG failed on taxi in FCO.:8
edit to add. The triple 7 also has 'toe-in' on the engine/pylon as well as a quite pronounced angle on the intake for the same reasons as above.

29th May 2008, 04:32
Just a minor correction is was a CX "Classic" pylon which had the "pod nod" which was robbed for the QF "Classic".

Twitter n Bisted
29th May 2008, 05:43
Taken from my training notes...

Each of the Engines are mounted with a 2-degree toe-in to take
advantage of the bow-wave effect on airflow at the Nose of the

29th May 2008, 08:49
Don't worry Twitter n Bisted, jb2_86_uk won't believe you as it doesn't work with his theory.

I am just writing to Boeing now to tell them they got it wrong.:ok:

29th May 2008, 09:04
"So, my next question is, are engines (slightly) toe-ed inwards on twin engined jets? and are the outboard engined toe-ed in also on all other 4 engine jets?"

Whilst a mind is a terrible thing to lose...I seem to recall (from a long, long, time ago) that the MD-80's engines were toe'd out 1 1/2 (the intakes being farther out than the tail pipes). But my notes are long gone and as I said in the beginning - a mind is a terrible thing to lose! Someone here will have the twinjet answer and correct me quickly, I am sure.

29th May 2008, 09:53
Nepotism, I was not implying you were wrong. If I knew the reason, there would be no need for me to have even started the thread! But, as I have stated several times now, my whole curiosity on this topic started after seeing these pictures with the engines appearing to be out of alignment with each other. Also, so far you are the only poster who has claimed catagorically that all four engines are toe-ed in by the same angle.

TnB, can you clarify that by 'Each of the Engines' you are refering to all 4, or just the outer engines.

I appreciate all the input from the posters. I can happily claim to have been educated yet again by the ppruners!! :ok:. Next time when someone asks me why jet engines are toe-ed in I can give them a full and technical answer... although the situation hasnt yet arose!

Tis just a shame you cant go a day on here without being goaded into a handbags-at-dawn debate.

Spanner Turner
29th May 2008, 10:26
Below is from the 747 maintenance manual (Before you ask, YES, I checked both the 747 Classic and the 747-400, they're both the same)

(5) Nacelle abbreviations
Nac BL Nacelle Buttock Line. Nacelle Buttock Line 0.0 for
inboard engines is 2 degrees inboard from Wing Buttock
Line 470.0.

Nacelle Buttock Line 0.0 for the
outboard engines is 2 degrees inboard
from Wing Buttock Line 834.0.

767 engines are angled inboard 1 degree.


30th May 2008, 08:18
Thanks Spanner Turner, thats great info! :ok:

30th May 2008, 19:13
I'm with Notso F,

The reason for the inward engine cant is the local airflow
on the underside of the wing will spread slightly outwards.
On the topside it will spread inwards. Finite wing theory.

You will also notice the flaptrack fairings are canted outwards.
A couple of example shots;




22nd Mar 2009, 17:05
http://www.airliners.net/photo/British-Airways/Boeing-747-436/1500476/L/ (http://www.airliners.net/photo/British-Airways/Boeing-747-436/1500476/L/)

Is the outboard engine angled upwards as compared to the inboard as well?