The Boeing 707 ventral fin story

[b377]

Here's another one for you wise men.

I believe some European customers for the 707 insisted on a ventral fin (VF) for certification purposes ( BOAC, Luft ) but was, I believe, adopted by US operators as well 300/400 ? series for better yaw performance. But not all US operators used it.

However the VF was not universally used and later 707 production did not use it. Was this to do with flight control improvements or euro regulators being overly demanding and it wasn't really that esential?

[411A]

Ventral fins were required during US certification to provide greater yaw stability...they came in two sizes, 13 inch and 39 inch, and were eliminated on later aircraft when the vertical stabilizer height was increased.
Note also that all aircraft that had ventral fins installed also had parallel type yaw dampers as standard fit.

[forget]

Ventral fins were required during US certification

Surely it was the CAA (ARB) which insisted on the ventral fin before clearing BOAC's aircraft.

[b377]

Was the VF removed from older 707s when they had their vertical stabilier modified to the taller one? However I think I've seen BOAC 400s with the long vert stab and a VF.

[BelArgUSA]

707 Ventral Fins

b377 -
If I ever meet you, you will owe me a few lagers...!
Scratching my old head here, hoping not to tell you wrong.
Hopefully some other friends will assist as well.
Ok - 707-720 can have eiher...
xxx
(1) Large ventral fin... no other way to define it -
It was found on ALL old "straight pipe" 300 and 400s.
Also found on the "early 300B with fan engines" with 17 flaps takeoff setting.
And finally on 100s which had JT3C engines... ex. TWA 707-131
Purpose - improve lateral stability, and prevent over-rotation.
Note that the very first 300s had a shorter tail fin.
UK CAA required a taller fin (read D.P. Davies) + VF for lateral stability improvement.
Boeing made it standard on all early 300s and non-fan 100s...
xxx
(2) Small ventral fin -
Was found on all 720, and 100 with fans -
Purpose - limit damage to fuselage in case of over-rotation.
Stall warning (stick shaker) did not operate early enough.
xxx
(3) NO ventral fin -
Only on 300B-Adv, 300BAH (B-Adv-Heavy) and 300C.
All of these were airplanes with full set of leading edge flaps, and 14 flaps for takeoff.
Stick shaker operated much prior reaching over-rotation attitude.
By the way - these got changed from "parallel" to "series" Yaw Damper.
xxx
Are you giving me an oral exam for 707/720 TR...?
My type rating is somewhat rusty, sorry.
Hope you passed me with colors.
xxx

Happy contrails

P.S. - Thanks 411A - Forgot the "size" of these ventral fins.

[b377]

It would be a pleasure BelArgUSA - after all education seldom comes for free.

cheers or should I say, hasta luego ?

[BelArgUSA]

The 720...

Do you know why the baby 707 is called the 720...?
Goes back to 1960 or thereabouts.
xxx
Early days of 707 and DC8 sales war.
PanAm had ordered both - initially.
American had decided to order the 707, no DC8... So did TWA.
And United opted for DC8... So did Eastern, and Northwest.
xxx
Then 1 or 2 years passed -
The DC8 was found to cruise at slower speed. Nickname "DC-Late"...
Boeing offered the shorter range 707-020 to airlines.
xxx
United got attracted to order 29 of these 707-020. But problem...
How will the stockholders react... Did UAL make a mistake with DC8s...?
United signed the order, but asked Boeing to "change designation to 720"....
And Eastern ordered a few 720 as well.
Initially, before delivery - American called theirs "707-023"...
xxx

Happy contrails

[b377]

No I did not .. but then again I never saw much logic in Boeing's numbering:

727 three engines
737 two engines
707 four engines
747 four engines ( got it right there !)
717 two engines ( adopted child)

....

Avianca operated two of the early 720s right into the late 80s

[dixi188]

I believe the C135 (KC135) was originally called Boeing 717.

True of False?

[Cremeegg]

BelArg

What a pleasure it always has been to read your posts and now you're retired I'm delighted to see you posting more frequently. We can all learn from your wisdom.

[b377]

Are you giving me an oral exam for 707/720 TR...?

BelArg

I think the you and others with similar knowledge should put it in a book for the rest of us and the future world. That halcyon era of aviation will soon pass into oblivion once the protagonists of that era move on to a different plane.

[Rainboe]

VC10 was still Queen of the Skies! Came in 2 models, Standard and Sooper. Such a nice machine to fly. Trouble was the engines went quiet a couple of hours before the 707's did. But the noise! That crackle!

[galaxy flyer]

I believe the C135 (KC135) was originally called Boeing 717.

True of False?

I believe True, the KC had a smaller diameter fuselage than the later 707. The airlines wanted 6-abreast in coach and the KC fuselage wouldn't allow it. But by the time the DC9-whatever came along, nobody remembered the original 717, so they used that number. Engines never had anything to do with Boeing's system.

GF

[b377]

Love the VC10 tooo, saw it fly over head once a week. Distinctive sound ? You bet. Smoke too.

Never flew in it

Grey RAF tankers ? Not the same are they ?

[forget]

Ref BelArg.

What a pleasure it always has been to read your posts .....

Seconded! Enjoy your retirement. Flying (not Prune) has lost a genuine 'been there - done that'.

[Jumbo Driver]

Originally Posted by Rainboe

VC10 was still Queen of the Skies! ... ... But the noise! That crackle!

Spot on, Rainboe !!

However, my recollection of the VC10 is that the faster you went, the quieter it became ... (inside ...) ...

JD

[echo-romeo]

Its many years since I've read about the Boeing 707 lower ventral fin but I felt the above explanations were missing a vital ingredient so I got some of my old books out to double check.........

The early B707's without the additional lower ventral fin could achieve attitudes on take-off that were greater than the minimum unstick attitude under certain conditions - so essentially the wing drag was much greater than the early engines could overcome - and the wing was essentially in a stalled attitude - so an early rotation on take-off would prevent further acceleration - and the aircraft would never get airborne. I also seem to remember that early Comets had this same problem and there was a take-off crash at Rome Ciampino (LIRA / CIA) caused by this phenomena.
Combined with directional take-off engine out problems on the B707 as mentioned above where the ventral fin gave a increase fin area and more control it also geometrically prevented the early B707 from getting into the minimum unstick attitude - I think all modern jet aircraft are geometrically restricted in this manner although I understand that modern wing design and more powerful engines make the problem unlikely. I've seen many minimum unstick test films such as the B747 with its tail on the runway still getting airborne. So no lower ventral fin but the shape of the lower rear fuselage providing the geometric restriction.

I'm not an aerodynamics or aircraft design expert so apologies if my explanation is confused or less than perfect. Further reading on this matter is Handling the Big Jets by D.P. Davies 3rd Edition " Mis-handled Rotations" Page 181-184. Various Boeing 707 histories also discuss this problem. Later Boeing 707's had different wings and more powerful engines as well as taller vertical stabilisers and improved rudder boost.

I hope this answer provides some further interest and will no doubt attract a few corrections and further arguments.

Cheers

[411A]

...and improved rudder boost.

DP Davies, and he alone, insisted Boeing improve the rudder power system on the original long-bodied 707.
Fix...or else.
Full marks for this fine gentleman, for if he had not, many folks would have surely died in accidents.
Davies had the big stick as regards certification, and he made it work. In spades.
Captain David P. Davies...a class act, in many ways.
Obtain a copy of his book...'Handling the Big Jets' and you will be informed about the rather poor handling qualities of early jet transport aircraft.
And, MUCH more.

[b377]

GF ... Engines never had anything to do with Boeing's system.

Too right GF never saw a Boeing plane with zero, one, five, six or seven engines ... except in photoshop

The late Malcomb Stamber of Boeing 747 fame, gested about it in one of his videos, thought I'd borrow it. Intelectual property clarified.

[GlueBall]

Don't forget to include the special Braniff edition: B707-200 series!