FDXmech From United States of America, joined Mar 2000, 3251 posts, RR: 37 Reply 1, posted (12 years 8 months 1 week 6 days ago) and read 1715 times:
These are referred to as "ventral fins" and were used in earlier models of the 707 to increase stability around the yaw axis(directional stability). This was part of an improvement package that among other things increased the effectiveness of the rudder system due to during early training flights, several incidents resulted in loss of control resulting in such violent manuevers as to cause one or more engines to rip off the pylon.
Skwpilot From United States of America, joined Feb 2001, 60 posts, RR: 1 Reply 4, posted (12 years 8 months 1 week 5 days 22 hours ago) and read 1685 times:
I was looking around through some of the 707 photos and I managed to find a photo of a TWA 707-331B which is a bit of a rarity. It is a JT-3D powered example with the abrieviated ventral fin (slightly shorter than the origional). FDXmech is correct about the yaw stability being the reason for it's installation. Most jets with highly swept wings suffer from a problem called "Dutch Roll". This occurs when a jet develops a yawing motion out of phase with a tendency to roll back and forth. It was difficult for the pilot to correct and led to some serious problems. You will also notice that all later model had(and most earlier ones modified with) an extension to the vertical stabilizer as well.
Greeneyes53787 From United States of America, joined Aug 2000, 844 posts, RR: 0 Reply 6, posted (12 years 8 months 1 week 5 days 13 hours ago) and read 1659 times:
I believe that both USAF SAM 26000 and 27000 aircraft had the dorsal fins and turbofans. Possibly those same planes first came with turbojets. I remember Eisenhower flying in a KC137 (military 707) with turbojets. I don't remember if it had the fin.
Convair 880s had trouble during training as an outboard engine was shut off or throttled back. The asymetrical thrust was too much to overcome using the rudder alone. But as far as the dutch roll challenge in the 880- this was a sticky wicket until it was discovered that snapping the airbrakes for a quick jolt solved it every time.
I believe the 990 greater length and a taller vertical fin reduced the asymetrical thrust potential considerably.
Probably the Boeing dorsal fin doubled as a skid at rotation (?).
TomH From United States of America, joined May 1999, 960 posts, RR: 2 Reply 7, posted (12 years 8 months 1 week 5 days 6 hours ago) and read 1621 times:
Perhaps FDXmech was thinking of the Beoing 720, which was 15 feet shorter than the 707-3XX series. All, if not most of the 720s were produced with the ventral strake, likely a compensation for the shorter fuselage.