767 Forwad tilting Bogeys?
Thread Starter
Joined: Apr 2003
Posts: 109
Likes: 0
From: TTPP
767 Forwad tilting Bogeys?
Some years ago I red somehere--actually Airliners magazine the (Q&A) section- On why the 767 landing gear was designed to tilt forward instead of the usual rearward. I remember an explanation vaguely to the following effect:
When the 757/767 was designed during the Persian Gulf fuel shortages of the late 70s and early 80s when a efficient Airliner was the 'the foremost attribute to the design' (Isn't it always) ; Incorporated into that design was a centre of gravity that was slightly more rearward than normal, so that it achieved an aoptimum and efficient Angle of attack in flight- during cruise most probably. The forward-tilting bogeys were designed to offset this moment when not in cruise and in 'dirty' configuration.
Could som more techno-savvy person (s) enlighten me please.
Much Appreciated
When the 757/767 was designed during the Persian Gulf fuel shortages of the late 70s and early 80s when a efficient Airliner was the 'the foremost attribute to the design' (Isn't it always) ; Incorporated into that design was a centre of gravity that was slightly more rearward than normal, so that it achieved an aoptimum and efficient Angle of attack in flight- during cruise most probably. The forward-tilting bogeys were designed to offset this moment when not in cruise and in 'dirty' configuration.
Could som more techno-savvy person (s) enlighten me please.
Much Appreciated
Joined: Jul 2000
Posts: 114
Likes: 0
From: Robin Hood country.
C2C
Ref the 767. That would seem to be a techno savvy post; forward tilting trucks do keep the Cg within aft limits in landing configuration. The aft Cg can also result in a pitch-up tendency during landing when the spoilers extend - which occurs just prior to the autobrakes taking their sample grab causing the nose to pitch down again. Aft Cg equals fuel efficiency as the horizontal stab needs to supply a counteractive upward force which is additive to lift from the wing (the stab supports some of the aircraft weight so the wing doesn't have to do as much), the disadvantage is a reduction in stability i.e. the aircraft becomes susceptible to over control. Of course C2C, if your question was as to how Cg effected fuel burn then an affirmation as to why the trucks tilt forward hasn't helped much.
Edited for IB57
IronButt. It certainly can be nibbling the edge, ever noticed how touchy the pitch can be at times when landing? Have to push forward once the sink rate has been checked to do a Boeing roll - similar technique but for a different reason when landing the Sky Pig way back in the Simmons' days. All too high tech for me too really, have to confess that I got most of it from the back of a breakfast cereal box.
Ref the 767. That would seem to be a techno savvy post; forward tilting trucks do keep the Cg within aft limits in landing configuration. The aft Cg can also result in a pitch-up tendency during landing when the spoilers extend - which occurs just prior to the autobrakes taking their sample grab causing the nose to pitch down again. Aft Cg equals fuel efficiency as the horizontal stab needs to supply a counteractive upward force which is additive to lift from the wing (the stab supports some of the aircraft weight so the wing doesn't have to do as much), the disadvantage is a reduction in stability i.e. the aircraft becomes susceptible to over control. Of course C2C, if your question was as to how Cg effected fuel burn then an affirmation as to why the trucks tilt forward hasn't helped much.
Edited for IB57
IronButt. It certainly can be nibbling the edge, ever noticed how touchy the pitch can be at times when landing? Have to push forward once the sink rate has been checked to do a Boeing roll - similar technique but for a different reason when landing the Sky Pig way back in the Simmons' days. All too high tech for me too really, have to confess that I got most of it from the back of a breakfast cereal box.
Last edited by XL5; 6th May 2006 at 05:35.
Joined: Jun 2001
Posts: 3,780
Likes: 0
From: Surrounded by aluminum, and the great outdoors
all too hi tech for me....boeing tech rep yrs ago said it was due to the config of the center tanks, the gear simply wouldnt fit forward tilting....if the tilt opf the gear is required to keep the cg in limits, aren't we a bit on the edge to begin with?
Thread Starter
Joined: Apr 2003
Posts: 109
Likes: 0
From: TTPP
Originally Posted by XL5
C2C
. Of course C2C, if your question was as to how Cg effected fuel burn then an affirmation as to why the trucks tilt forward hasn't helped much.
Edited for IB57
IronButt. It certainly can be nibbling the edge, ever noticed how touchy the pitch can be at times when landing? Have to push forward once the sink rate has been checked to do a Boeing roll - similar technique but for a different reason when landing the Sky Pig way back in the Simmons' days. All too high tech for me too really, have to confess that I got most of it from the back of a breakfast cereal box.
. Of course C2C, if your question was as to how Cg effected fuel burn then an affirmation as to why the trucks tilt forward hasn't helped much.
Edited for IB57
IronButt. It certainly can be nibbling the edge, ever noticed how touchy the pitch can be at times when landing? Have to push forward once the sink rate has been checked to do a Boeing roll - similar technique but for a different reason when landing the Sky Pig way back in the Simmons' days. All too high tech for me too really, have to confess that I got most of it from the back of a breakfast cereal box.
The "techno-savvy" explanation seemed so much cooler though still hoping its true though. Seems an awfully elaborate eexplanation to conjure up. These bogeys are small tho. compared to the rest of the airframe. Besides what's all this "sky pig" stuff about?Cheers
Joined: Mar 2000
Posts: 463
Likes: 7
From: Deepest Essex.
I once head the forward tilt explained as part of the alternate extension mechanism. The trunnions in the wing, on which the gear swings, are toed out slightly ( toward the wingtip ) at the front. Thus, when the gear legs free-fall during alternate extension, aerodynamic forces help push them aft, helping the side and drag braces on each leg to be locked. If the bogey was level, or tilted rear wheels down, then the fuselage would have to have been bulged outward to accomodate the gear in it's retracted state. Not ideal drag-wise at all!
In their retracted state, they would lie further forward, thus countering any reduction in cruise trim drag achieved by pushing the c.g. aft.
That said, the 767 gear is a wonderful piece of engineering, in spite of the curious little flap on the after part of the wing where the cam on the retaction jack breaks the surface during retraction!
The forward tilt even allows a range of feeble excuses from the flight deck on the odd occasion the touchdown isn't as sweet as one would normally like!
In their retracted state, they would lie further forward, thus countering any reduction in cruise trim drag achieved by pushing the c.g. aft.
That said, the 767 gear is a wonderful piece of engineering, in spite of the curious little flap on the after part of the wing where the cam on the retaction jack breaks the surface during retraction!
The forward tilt even allows a range of feeble excuses from the flight deck on the odd occasion the touchdown isn't as sweet as one would normally like!
Joined: Jun 2001
Posts: 3,780
Likes: 0
From: Surrounded by aluminum, and the great outdoors
Hahaha...sky pig...yeah I did that in the 80's for Simmons...the bandit in "the soo" then the "pig" aka "ghetto sled" in Marquette...remember Spanky's??? recent name I heard for the Shorts...."Irish Concorde" best one yet!!!
Joined: Jul 2000
Posts: 114
Likes: 0
From: Robin Hood country.
It took a week for the phone calls to go up and down the chain of hierarchy. Riff-raff such as myself only have access to those persons having access to other persons able to give something definitive. The answer didn't change from that of three years ago.
The 767 landing gear was designed aft for CG (hence commercial) considerations, because of this rearward placement the trucks must tilt forward to accommodate gear stowage. The forward tilt does move the CG without which a change would have to be made in the CG loading limitation. So, spin that if you wish.
The 767 landing gear was designed aft for CG (hence commercial) considerations, because of this rearward placement the trucks must tilt forward to accommodate gear stowage. The forward tilt does move the CG without which a change would have to be made in the CG loading limitation. So, spin that if you wish.
Joined: Jul 2005
Posts: 580
Likes: 0
From: OZ
Originally Posted by XL5
It took a week for the phone calls to go up and down the chain of hierarchy. Riff-raff such as myself only have access to those persons having access to other persons able to give something definitive. The answer didn't change from that of three years ago.
The 767 landing gear was designed aft for CG (hence commercial) considerations, because of this rearward placement the trucks must tilt forward to accommodate gear stowage. The forward tilt does move the CG without which a change would have to be made in the CG loading limitation. So, spin that if you wish.
The 767 landing gear was designed aft for CG (hence commercial) considerations, because of this rearward placement the trucks must tilt forward to accommodate gear stowage. The forward tilt does move the CG without which a change would have to be made in the CG loading limitation. So, spin that if you wish.
So you are saying if the 767 without the center hydraulic system it has a C of G limitation due to the fact that there will be no pressure to position the truck.. or the aircraft must be loaded in a configuration to allow for center hyd sys loss...
Does not sound right to me.
Joined: Jun 2001
Posts: 3,780
Likes: 0
From: Surrounded by aluminum, and the great outdoors
xl5 might be on to something...the actual location of the gear on the aircraft might be due to cg requirements and when placed there, might have to be forward tilting to fit into the belly...the tilt itself having no relation to the c.g. just the physical mounting point of the gear..???? anyway it definately requires a different technique to extract a "greaser" from a 767 than a 757...the obvious excuse...i did a "Boeing approved" landing as per the FCTM
