Technically the Boeing 367-80 (The B-707 proof-of-concept prototype)
didn't have any leading-edge devices initially. As the plane got bigger, was widened from five-abreast to six-abreast, it's wing-loading was increased. Since the are just inboard of the number 1 and number 4 engines tend to be prone to stalls more than other parts of the wing. For this reason they added kreuger-flaps there.
Back in those days leading-edge devices if they were used at all were virtually never used on airliners. If you used 'em, you used it only when needed. Modern day we fit slats to everything that we can.
Either way the DC-8 was originally designed as a clean-wing to the best of my knowledge. However, the problem came first from the pylon design (the part that goes over the top portion of the wing)
which acted like vortex-generators to energize flow over the wing. They turned out to produce more drag than initially expected and they negated lift by producing relatively high pressure zones over them, and also produced drag which they partially tried to deal with by adding camber to the tops of the pylons. It didn't work too well.
To combat the high-pressure zones where they were at their worst, at low-speed, they fitted fixed-slots (not slats)
to the pylons with doors to close over them at high-speed. The low-pressure zone from the slot negates the high-pressure zone from the wing-pylon. If sized right, they produce some lift too! Looking at the wing from most angles you will not see the slot and it looks like just wing with no LED
The baseline DC-8-11 when fitted with slots (it didn't have them)
and an extra 3-foot wingspan addition increased the maximum takeoff weight by 8,000 lbs (The DC-8-12)
. The slots and bigger-wing were used on every other DC-8 model except the -11, though technically every DC-8-11 was later converted to DC-8-12's which had slots.
In terms of takeoff performance, the takeoff and landing-speeds are about the same for both the B-707 and DC-8 and I would assume distance would be in the same ball-park (especially considering the B-707-320 and DC-8-33 weigh about the same)
The baseline Convair CV
-880 model had no leading-edge devices at all. It had a better thrust/weight ratio than either Boeing or Douglas design (although unimpressive by today's standards)
though, had a lower-drag fuselage, wing and engine-pods and accelerated pretty well. It's wing was also proportionately (albeit thinner) bigger than the B-707's as well. It's takeoff and landing performance were not bad when moderately loaded (Takeoff run was around 5,000 to 5,500 feet increasing to over 7,000 feet when fully loaded, and landing run could be accomplished in 5,350 feet)
even despite it's very high takeoff and landing speeds.
-880M though had outboard slats and inboard kreuger-flaps -- its trailing-edge flaps also deflected generally a bit more (20/30/40/50 deg on the CV-880 vs 22/33/44/55-deg on the CV-880M)
and actually could be extended at higher speeds. (260 kts vs 245)
The plane was also structurally sturdier, carried extra fuel, and more powerful engines. (it's thrust/weight ratio was about the same as the regular CV-880M)
It also had a powered-rudder the regular CV
-880 didn't have (The original CV-880, who's rudder was manually/aerodynamically-controlled had insufficient leverage due to insufficient airspeed, and it's position on the shorter fuselage could not restore heading following an outboard engine failure at V1. Full rudder could stop the heading-change but not fully restore the original heading unless the pilot turned the control column activating the spoilerons which would provide the extra yaw to do the job)
The Convair 990 was based on the CV
-880M and was designed with the idea of cruising at Mach 0.91 with transcontinental range and a greater capacity than the regular -880. To achieve the speed they used a higher wingsweep which also added 250 extra square feet, 4 Kuchemann carrots which provided supercritical effects and carried extra fuel, and turbofanned CJ
-805-derivatives. To achieve the extra range, the turbofans played a role here too, but the plane also carried drastically more fuel (CV-880M = 193,000 lbs about, CV-990 = 239,000)
. To carry the extra capacity, the plane was 10-feet longer.
Originally they wanted to use slats outboard and kreuger-flaps inboard to the best of my knowledge (I could be wrong here -- it could be full span)
in conjunction with larger double-slotted flaps, however the edges of the slats produces turbulent flow which reduced pitch-control, something that wasn't evident in the wind-tunnel, they instead went with full-span electrically-driven kreuger-flaps. There were also problems with the pylons as well, which they fixed.
[Edited 2008-04-07 23:45:59]