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soon7x7
Topic Author
Posts: 2267
Joined: Thu May 04, 2006 10:51 am

Convair, DC8 Lifting Devices

Mon Apr 07, 2008 8:18 pm

Why did DC-8's and Convair 990's not have kreuger flaps while the 707 sported such features...was short field take off performance a consideration or was the gross weight of the Boing more substantial?...anyone?
 
boeing767mech
Posts: 806
Joined: Fri Dec 15, 2000 5:03 pm

RE: Convair, DC8 Lifting Devices

Mon Apr 07, 2008 11:03 pm

The CV990 had slats when it was first introduced. Then was modified with full wing Krueger flaps.

David

https://www.airliners.net/photo/Moder...19&prev_id=1281300&next_id=1273551

https://www.airliners.net/photo/Untit...20&prev_id=1279698&next_id=1273108
Never under-estimate the predictably of stupidty
 
miamiair
Posts: 4249
Joined: Thu Jul 15, 2004 9:42 pm

RE: Convair, DC8 Lifting Devices

Tue Apr 08, 2008 12:15 am

The DC-8s had slots that were located on both sides of each pylon.

I'll post a pic tomorrow from the office.
Molon Labe - Proud member of SMASH
 
Buzz
Posts: 694
Joined: Sun Nov 21, 1999 11:44 pm

RE: Convair, DC8 Lifting Devices

Tue Apr 08, 2008 5:41 am

Hi Soon7x7, Buzz here. Remember, there's no free lunch. The Kruger flaps add some weight, repair and maintenance problems and cost more than a plain wing. Somebody has to pay for it. And the aerodynamic advantage of Kruger flaps occurs at high angles of attack.

Douglas decided that the runways were long enough to get their DC-8's up and flying. Boeing's 707 team thought they were needed for shorter runways. In the end... we see both kinds of airplane operating off of 7 or 8 thousand foot runways.

The DC-8 leading edge slots were just inboard of the engine pylons, might have been interesting to see the wind tunnel / smoke tunnel models of those being developed: make a stable area of airflow as the stall creeps up from the trailing edge.

g'day
 
Blackbird
Posts: 3384
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RE: Convair, DC8 Lifting Devices

Tue Apr 08, 2008 6:42 am

Soon7x7,

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's.

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.

The CV-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]
 
soon7x7
Topic Author
Posts: 2267
Joined: Thu May 04, 2006 10:51 am

RE: Convair, DC8 Lifting Devices

Tue Apr 08, 2008 5:09 pm

Well looks like I got my answers...don't know where I have been, the pix of convair 990 W/LE Flaps are the first I've ever seen...thnx....I did know that the DC-8 at one point was modified with fixed slots (like stinson voyager style). But never saw DC-8 w/ Kreuger style...however nothing would surprise me 'cept the knowledge all of you have out there...Thnxmuch for the education...Gerard
 
aeroweanie
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RE: Convair, DC8 Lifting Devices

Wed Apr 09, 2008 3:51 am



Quoting Blackbird (Reply 4):
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.

The real big change from the -11 to the -12 was the 4% wing chord extension. This allowed a new, modified leading edge to be added that substantially improved the cruise drag characteristics of the aircraft. This is discussed in the late Dick Shevell's famous AIAA paper "Aerodynamic Bugs, Can CFD Spray Them Way?” (AIAA Paper 85-4067)

Regarding slats, the need for them first arose after the October 26, 1952 and March 3, 1953 Comet I overrun accidents at Rome and Karachi. In both cases, the pilot rotated too early and the wing partially stalled. As a result, the aircraft didn't climb or accelerate, leading to an overrun. It was found that the fix was to add leading edge slats, which raise the angle of attack the wing stalls at. Thus, the aircraft could be made tolerant of what is called an abused take-off.
 
Blackbird
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RE: Convair, DC8 Lifting Devices

Wed Apr 09, 2008 5:59 am

AeroWeanie...


The 4% leading-edge extention was added, to my knowledge after all the first DC-8 models (DC-8-10 / DC-8-20 / DC-8-30 / DC-8-40) were already in service due to the fact that drag levels were higher than initially predicted due to the fact that the leading-edge was too blunt, (Original wing-design was tested at very low Reynolds numbers which are a function of scale where turbulence levels are very low, at higher reynolds numbers turbulence and drag become higher.) drag was too high, and fuel consumption was undesirable. The DC-8-10, and DC-8-20 (and possibly some lighter DC-8-30's) simply made-do with the extra drag. The heavier -30 and -40 models featured a 1.5-degree flap droop which increased lift, and lowered AoA and managed to reduce drag a tad.

Of course this is only a temporary solution to the problem. The real solution is a wing-redesign, which Douglas developed. It entailed a 4% chord-length increase, a sharper and re-profiled leading-edge. Slatted versions featured redesigned slats to work with the new leading-edge to my knowledge. It was first tested on a DC-8-43 to my knowledge--To demonstrate the effectiveness of the design, they actually dove it through the sound-barrier! Recovery required both elevator and stab-trim, and the plane had a 5,000 pound water-ballast in the aft section of the plane, but it did it.

The 4% leading-edge re-design which also allowed for tanks to be fitted in the revised leading-edges, was applied to virtually every single DC-8 design retroactively, and eventually became a standard feature.


The only Comet to the best of my knowledge to have slats was the prototype. Even despite having light wing-loading, the wing was not exactly the optimum low-speed design. Because of a worry that it might stall badly, they fitted slats to the outboard-wing. However, the Comet was found to have reasonably acceptable stalling characteristics and they were deleted from the prototype.

The reason the Comet stalled under the conditions that it did was because pilots flying the plane practiced improper rotation, and possibly the attitude-indicator used. In the days of propeller planes it was common for the pilot to pull the nose gear off the ground a little bit before reaching takeoff speed (Sometimes the angle was increased all the way up to critical AoA). The Comet was very underpowered, and unlike propeller planes didn't have airflow from the props going over the wing to augment lift -- Taking the nose to critical alpha would slow her down and as she'd slow AoA would exceed critical and stall; the turbulence and drag produced by this would drag the plane down and keep her from accelerating. Overrotation was also possible especially at night-time where the horizon is harder to ascertain exactly (which was only aggravated by the plane's responsive hydraulic-controls which had no dynamic-feel to them), especially considering the attitude-indicator was not marked in degrees.

During the first accident, the plane was right next to takeoff speed but not quite fast enough and the pilot rotated too early, and he actually got into the air! He was flying too slow at too high an angle of attack and stalled. Whether the airflow into the intake was disrupted or not, I'm not sure, but the pilot lowered the nose quickly to try to correct but it was too late and the plane came back down on the runway. For one reason or another, the pilot lowered the nose, throttled the engines back and hit the brakes trying to abort the takeoff. Apparently he was beyond V1 and ran off the end of the runway. The plane was totalled, but nobody died.

The second time, the pilot rotated too early, and apparently due to a lack of reference with the horizon and poor attitude-indicator, got the nose up pretty steep. In fact he dragged the tail-skid on the ground a bunch of times leaving a huge hail of sparks. Once he realized he was stalling, he lowered the nose. I'm not sure if he got it on the ground or kept it slightly off the ground, but in either case the plane began re-accelerating and just as it was about to either reach takeoff speed the plane ran off the end of the field, struck a drainage ditch and broke-up. Everybody onboard died. In this case to the best of my knowledge the wing-stalling also disrupted the airflow into the intakes resulting in loss of thrust.

After these two accidents, they revised pilot training placing more emphasis on keeping the nose on the ground until rotation speed is reached. Additionally for an added measure of safety, they re-designed the wings leading edge which may have borrowed some characteristics from the Comet 2's wing-design which featured a more cambered leading-edge. It stalled at a higher alpha and improved safety. I'm not sure what they did with the attitude-indicator, but the hydraulic controls were revised with q-feel to provide feedback.
 
miamiair
Posts: 4249
Joined: Thu Jul 15, 2004 9:42 pm

RE: Convair, DC8 Lifting Devices

Wed Apr 09, 2008 10:56 am

Here's a pic/sketch of the outboard slot.

The upper panel is fixed at the trailing edge and is pulled down at the leading edge and a door opens on the lower leading edge creating the slot.

http://i30.photobucket.com/albums/c325/miamiair/DC8-SLOT_2.jpg
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