Blackbird From , joined Dec 1969, posts, RR: Reply 3, posted (5 years 4 months 4 weeks 1 day 14 hours ago) and read 2361 times:
Simple, couldn't get an answer. So I kept asking.
The question kind of had to do with the screw-driver MD-80 tail and 777 tail. I was told by somebody that the DC-8's tail was in a way a screwdriver type design, but I'm not exactly sure I understand unless you mean a triangular cross section. But I'm still wondering if the 2D type MD-80 or 777 would work equal or better.
113312 From United States of America, joined Apr 2005, 537 posts, RR: 1 Reply 4, posted (5 years 4 months 4 weeks 1 day 3 hours ago) and read 2315 times:
Screwdriver type tail cone? McDoug called it a Beaver Tail. First Gen jetliners didn't put much emphasis on a tail cone other than to try to fair out the fuselage smoothly. In later years, additional research was done to the flow and drag in this area particularly for tail mount engine designs, such as the MD-80, which induce flow at the tail with the engine thrust. Lessons learned from that airplane were applied to other designs such as the MD11.
Blackbird From , joined Dec 1969, posts, RR: Reply 6, posted (5 years 4 months 4 weeks 17 hours ago) and read 2256 times:
Quote: Screwdriver type tail cone? McDoug called it a Beaver Tail. First Gen jetliners didn't put much emphasis on a tail cone other than to try to fair out the fuselage smoothly. In later years, additional research was done to the flow and drag in this area particularly for tail mount engine designs, such as the MD-80, which induce flow at the tail with the engine thrust. Lessons learned from that airplane were applied to other designs such as the MD11.
Regarding the designation of the type of tailcone use, I'm glad to know the correct designation! Thanks,
Are you sure about that early commercial jets didn't put much emphasis on the tailcone? I was told by one person, (although I have not been able to substantiate or verify this; which by the way, is the reason I'm asking questions here to verify) that the DC-8's tail-cone was highly efficient and while conical (or more like rounded triangle) behaved much like a screw-driver/beaver-tail tailcone. Is that true or not?
The same person also told me that the beaver-tail was largely incorporated into the MD-80 because it had a different tail design (Again, not entirely able to substantiate this -- other than what you said about the exhaust flow over the tailcone; but the MD-11 and 777 don't have engines on the tail, although they do use different tailcones than the DC-8, honestly, I don't know)
As i've mentioned a while ago, it is useless to just look at just one component (in this case the tail cone) to ask why it is shaped like it is and how it could be improved or be compared with other configurations. It is the sum of all the aircraft's aerodynamics (and engineering compromises or even paradoxes) which determine the final shape of any aircraft. What i mean is that sometimes one would like to sculpture a great aerodynamic shape but may need compromise for other than aerodynamic reasons. The DC-8 is no exception to that rule, certainly not when it was designed with 1950s technology.
May i suggest you write McBoeing a letter and throw in all your charm and kindly request from them all the proprietary aerodynamic and engineering data regarding the DC-8? And then do the same for the MD-80, MD11 and B777, the aircraft you want to compare the "8" with.
Only then you will find the answer as to why the tail cone on the "8" is as it is and only then you will get a factual answer as to how it compares to other aircraft. Without such data it remains a guessing game.
Starglider From Netherlands, joined Sep 2006, 644 posts, RR: 44 Reply 12, posted (5 years 4 months 3 weeks 3 days 20 hours ago) and read 2079 times:
Quoting Blackbird (Reply 11): would a blade-cone have worked on an older fuselage like that?
As an educated guess without going into the numbers, perhaps it would work on the DC-8 since it did not have an APU in that area (or anywhere else for that matter) and basically already has somewhat blade-cone features. So if there is no other mechanism or structure in that area requiring the existing tail cone contours, there would be no argument not to pinch it into screwdriver cone if that proved to be an advantage. Provided the tapering starts aft of the horizontal stabilizers/elevators to avoid altering those to prevent a gap between the fuselage and the elevators. Aerodynamically that would probably amount to extending the tail cone somewhat beyond the current aft most contour which means slightly more surface area.
Question then is if the improved aerodynamics, as is claimed by installing a screwdriver tail cone on current aircraft types, weigh up against a slight increase in drag due to increased surface area compared to the original DC-8 design. Maybe a (slight) reduction in base drag at the trailing edge of the pinched cone tips the scale in favor of such alteration.
Again this is just an assumption. To really find out would require wind tunnel testing of a complete aircraft model and compare it with the original design data.
Base drag or base pressure drag is related to vigorous unsteady motions in the near wake of an aerodynamic body due to vortex shedding (in this case at the DC-8 aft fuselage). At the tail cone area the near wake flow aft of the cone, the lift-drag-ratio decrement depends on how blunt the contours of the cone are. Boeing's blunt B737 tail cone is a good example whereby it is visible what measures were taken to counter this problem by adding vortex generators in this area. If more blunt, base pressure is lower compared to ambient pressure which means an increase in drag (suction drag) which is referred to as base drag. This phenomena results in a considerable drag increase at transonic speeds if no measures are taken to reduce this base drag. Comparing the DC-8 tail cone with the one on the B737 gives me the impression that the cone on the DC-8 is more efficient than on the B737 because it is a clean design and more pinched and it does not require additional measures such as vortex generators.
Base pressure drag is even more evident on supersonic aircraft, where base drag, depending on aerodynamic shape of the aircraft, can result in a considerable drag increase up to approx Mach 1.3 which results in a significant lift-drag-ratio decrement when compared to speeds of Mach 0.8. After M 1.3 the negative pressure slowly shifts to a higher pressure with increasing Mach number. Above Mach 2.5 it can result in a favorable increment of about 2 percent in terms of lift-drag-ratio.
But that is a different story for another time so back to the DC-8 example. A different class of flows is obtained by eliminating the vortex-shedding instability with a splitter plate in the near wake. This increases the base pressure and reduces drag, the mechanics in the near wake is now governed by unsteady motions (turbulence) in the separated free shear layers.
Essentially, the pinched or screwdriver tail cone functions as such a splitter plate.
Quoting Blackbird (Reply 13): BTW: I'm not sure if you could answer this one, but how much surface area could possibly be increased as a result from such a tail modification?
Not much, most likely it would result in a slight lengthening beyond the existing cone trailing edge and the upper contour changed in a straight line aft as an extension of the aft fuselage where it pinches and ends at the perpendicular sharp trailing edge. The bottom contour could be an extension of the fuselage bottom contour ala B777 or MD11. Alternatively it could be shaped like the MD80 where the bottom contour changes to a horizontal line parallel to the upper contour. Either way this would increase surface area when compared to the original cone design. Only after testing these changes against the original design (wind tunnel and/or CFD) will give you a definitive answer to which shape is the most efficient.
Starglider From Netherlands, joined Sep 2006, 644 posts, RR: 44 Reply 16, posted (5 years 4 months 3 weeks 1 day 23 hours ago) and read 1988 times:
Quoting Blackbird (Reply 15): Where's the tail-running light? Is that the one that lights up the vertical-stab?
No, those are logo-lights and are installed on the upper surface of the horizontal stabilizers with the beam pointed to the vertical stabilizer left and right surfaces (some aircraft have the logo lights at the wing tips with the beam pointed to the respective vertical stabilizer surfaces).
The tail running light is a white light at the aft end of the tail cone as part of the navigation light system (which also includes a red light at the left- and a green light at the right-wingtip) enabling to determine an aircraft's relative visual position/direction. Additionally, not on the DC-8 back then but some of the current airliners also have a high intensity recognition light (strobe light) in near the white navigation light at the tail.
Starglider From Netherlands, joined Sep 2006, 644 posts, RR: 44 Reply 18, posted (5 years 4 months 3 weeks 1 day 19 hours ago) and read 1972 times:
No problem at all. As an example, look up a close-up picture of a B777 pinched tail-cone and look at the top and bottom corners of the trailing edge. No reason for it to have been a problem fitting a light back then if such a configuration would have been chosen for the DC-8 design.
Blackbird From , joined Dec 1969, posts, RR: Reply 19, posted (5 years 4 months 3 weeks 17 hours ago) and read 1941 times:
Makes sense... the Convair 880 used a 2D tailcone... it wasn't really a blade (it had some bluntness in the back)
A blade cone would have lengthened the tail... would it have lengthened the aircraft (The horizontal stab sweeps back a lot so I'm not sure if it would increase the overall length or not) significantly? How much would it have lengthened about if you guessed?
Starglider From Netherlands, joined Sep 2006, 644 posts, RR: 44 Reply 20, posted (5 years 4 months 2 weeks 5 days 6 hours ago) and read 1883 times:
Quoting Blackbird (Reply 19): How much would it have lengthened about if you guessed?
Depends on the how much the tail-cone taper would be changed. Looking at a plan view (top-view) drawing of the DC-8 i draw this conclusion: Since the taper of the aft fuselage along the root of the horizontal stabilizers is a good reference and, as mentioned before should not be altered, leaving the taper ratio unchanged the apex of the pinched tail-cone would be about 2 ft beyond the current overall dimensions dictated by the aft tips of the horizontal stabilizers. So yes, it would have lengthened the aircraft somewhere between 1 to 2 ft.
But it remains a guess. True dimensions and shape would have to be tested.
Starglider From Netherlands, joined Sep 2006, 644 posts, RR: 44 Reply 22, posted (5 years 4 months 2 weeks 4 days 7 hours ago) and read 1836 times:
Quoting Blackbird (Reply 21): Wow, the tailcone would with a blade-tip have extended all the way past the back of the tips of the swept-horizontal stabilizer?
Yes. Between 1 or 2 ft beyond existing overall dimensions. Perhaps even a little bit more if the taper ratio were to be reduced near the end of the tail-cone. All hypothetically and depending in rigorous testing of course . . . . .