Beside The Tail Of A Turbojet 707

MrFord · Tue Apr 03, 2001 1:04 pm

I noticed in many photos of first-generation Boeing 707 that these planes had a "tail extender" or someting like that at the far back of the plane. There's an example :

Click for large version

Photo © Eduard Marmet

But on newer, fan-powered 707B of C series, this extension is absent :

Click for large version

Photo © Eduard Marmet

Is anybody know what's the utility of this device ?
Thanks

MrFord

FDXmech · Tue Apr 03, 2001 2:08 pm

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.

XFSUgimpLB41X · Tue Apr 03, 2001 2:10 pm

That looks to be some sort of a strake.. like you see on the Lear 45's. They provide airflow stability and help reduce drag as far as i know.

MrFord · Tue Apr 03, 2001 2:53 pm

Thanks a lot for the response FDXmech and XFSUgimpLB41X !

That's make a logical sense about the stability increase on yaw axis.

skwpilot · Tue Apr 03, 2001 4:03 pm

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.

Here is a photo of the above mentioned TWA 707:

Click for large version

Photo © Ted Quackenbush

Happy flying!

Skwpilot

VC-10 · Tue Apr 03, 2001 4:40 pm

The British Air Registration Board required the fitting of the ventral fin before the 707 was allowed on the British register.

Greeneyes53787 · Wed Apr 04, 2001 1:34 am

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 (?).

Greeneyes

TomH · Wed Apr 04, 2001 8:34 am

MrFord,
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.
TomH

OPNLguy · Wed Apr 04, 2001 9:01 am

For whatever it's worth, check out the following thread, down about 10 messages into it ...

https://www.airliners.net/discussions/tech_ops/read.main/5790/

Cheers...

Beside The Tail Of A Turbojet 707

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