Why is it that on most pax aircraft the tail section is made less smooth? The whole fuselage is made with countersunk rivets and then somewhere at the aft bulkhead they always start with normal rivets, e.g. on this picture Photos: Boeing 747-267B Aircraft Pictures | Airliners.net
why do manufacturers do that?
You'd probably have to ask the engineers to designed the aircraft in the first place, specifically, for each design. However, airflow separation has already occurred that far aft, and parasitic drag isn't much of an issue.
In come cases on the horizontal stab, you'll see raised rivets which are intended to energizethe boundary layer somewhat.
Providing the smoothest surface isn't always necessary, or desirable.
If you look to the back end of the 747, you'll find that it's not particularly smooth or aerodynamic, anyway.
Where there's no need to countersink or dimple surface skin, a raised rivet is perfectly acceptable, and that may appear to be the case. It may simply have been an issue of economy operation during contruction.
the tail section of the aircraft aft of the rear pressure bulkhead is unpressurised. because the skins do not need to withstand the loads caused by the pressurisation, they are by made of thinner aluminium. to rivet these thinner skins would require the holes to be dimpled because such thin skins can't be countersunk. the designers must have decided the extra expense wasn't justified on an area of the aircraft that didn't need to be perfectly aerodynamically clean. hench the easier, cheaper option of mush head rivets.
The area your picture shows is the APU Access Doors that consist of part of the Fire bay the APU sits in. This area not pressurised in flight but is built to withstand heat and is possibly not of aluminium construction. (I dont do 747's)
I dont know if the designers would be worried about the drag caused this far back at this end of the aircraft, however, the area is prone to vibration (as can be seen by the repair at the Right end) and it is possible that the whole cone may be replaced for repairs to be conducted. Repairs are much easier if there are no countersunk rivets - and Domed/Panhead/Universal rivets are much stronger/durable.
Is that a jacking point at the aft end....?
Its a long time since I worked on B747s and I only ask because the patch repair beside it is consistant with a jacking mishap......
As said it's to do with drag, once aft of the wing there is no advantage to flush riveting so the cheaper method of using dome headed rivets is employed, look at some of the lighter stuff such as Beech Baron 55P and you will see it.
When they built the spitfire, the prototype was flush riveted, split peas were then glued on each head and removed to get a compromise between drag and speed, they were removed until the ideal pattern was attained in flight tests.
Quote:
Ok, I thought it might be a tail steady point....
Think you are correct, tail steady point with strobe above. Years since I met a 747 freighter to wheel the contraption into position though.
Sorry but that's complete and utter rubbish. The rear jacking point/tail steady point for freighters is some way forward of that. Approximate stations for the strobe light is 2790 the rear jack point is sta 2590, which is some 200 inches or 16ft further forward in a far stronger built part of the a/c.
I did say its been a long time since I worked on B747s and I thought it looked a bit far aft, its jut the patch adjacent to it bear all the hallmarks of a jacking mishap.....
Only thing that scares me more than people who use big numbers, is people who can't or won't say sorry, I was wrong. No shame in not remembering anything ASKAP
Last edited by SeldomFixit : 1st November 2009 at 10:57.
A minor point.......although its not in the picture the OP linked to , its not a tail steady point ..........on the 747 it is a main jack point at the tail, as the other main jacking points are on the fwd spar area, close to the fuselage.......you jack the aircraft at these three points and then once in the air you can put a jack under the nose ....just to act as a steady.
Jacking a 747 is not for the squimish at heart! I remember watching the Fuselage skin ripple in giant aluminum waves as we hefted this flying pig off the ground. I couldn't watch it all the way through and had to walk out of the hangar until they got it up and all of the screams from the aluminum subsided.
By the way. Countersunk rivets are used in pressurized areas due to better sealing capabilities. Button head rivets are used in areas requiring superior strength fastening devices. Lots of Stresses induced in the tail of comventional aircraft. Aerodynamic improvements from Countersunk rivets are a secondary benefit.
3 main jacking points on a B747 - wing to body either side are main load bearers and tail. The freighter has a very seperate ball fitting, held in by a circlip I believe, that "steadies" the bum during loading ops.
There ARE AUX jacking points outboard on the wings.
Last edited by SeldomFixit : 2nd November 2009 at 07:39.
Reason: typos
Linear Mode
