Hello all, first post but long-time lurker.

I am working on a technical documentary of sorts about the Boeing 707 and some of the information I need has been difficult to find, so I have some questions I'm hoping can be answered here.

1. The Boeing 707 received an extension to its tail fin as well as a ventral fin early in its service life to increase directional stability. I have read at least two conflicting reports as to why this was done, but no definitive data.

The two cases I have heard are:

a. The extensions were devised after the first Boeing 707-220 for Braniff crashed during a training flight during which the training pilot severely exacerbated dutch roll motion to the point of structural failure
b. The extensions were required by the British for their Boeing 707-420 aircraft because of poor directional stability in the engine out go-around situation at low airspeeds

Does anyone know specifically which scenario it was, or if it was something else entirely?

2. Does anyone happen to know which line numbers were the first to receive the modifications above?

Most all aircraft were retrofitted after the fact, but I could use information about which production aircraft rolled out of the factory with the modifications already done.

3. What was the actual reason that Boeing built the -138 for Qantas?

I have heard conflicting accounts of this one as well.

a. Qantas had inspected the Comet 4, DC-8, and 707. The Comet had a range too short for the needs of the airline, the DC-8 was too big and Douglas would not modify it, so they asked Boeing to make a 707 of the original length planned when the 707 was announced, to which they agreed.
b. Qantas had ordered the 707 with the intention of operated the aircraft with it's original length of 127 ft 10 in. Pan Am and other airlines petitioned Boeing to stretch the aircraft for more passenger load, but this shortened the range of the aircraft beyond what Qantas could use, so Boeing built the Qantas aircraft to original spec and all other -120 aircraft to the new lengthened spec.

And finally

4. Is there a comprehensive documentation of the different test programs accomplished by the 367-80, with dates?

The ones I am aware of (though unfortunately lacking much specific detail or more than a single source for each) are listed below:

Tested the wing glove used on later 707 models (still there today)
Tested very high deflection flaps (70 degrees?) (Blown flaps?)
Tested several airfoil shapes
Tested reversed (upside-down) leading-edge slats on horizontal tail-plane
Tested engines used on later 707 models
Outfitted for testing with a single JT8D and Boeing 727 flaps and slats

Any information about these topics would be very appreciated!

Chris

Those questions are pretty specific, you might want to try Boeing media relations.

The ventral fin was added at the requirement of the British ARB, D.P.Davies wrote about it in his book, “Handling the Big Jets”

GalaxyFlyer wrote:

The ventral fin was added at the requirement of the British ARB, D.P.Davies wrote about it in his book, “Handling the Big Jets”

Indeed - there's an audio interview with D.P. Davies on the internet recorded back in 1992, part of which deals with his certification issues with the 707.

1: b. Sourced in several places. Not only was the fin height added and the ventral fin added, they also boosted the rudder more because the rudder force with OEI was high. See also this: http://adastron.com/707/qantas/Q-bellows.htm
2. Not 100% sure, but my guess is one of the early BOAC frames.
3. b. See here: http://adastron.com/707/qantas/138-length.htm Also, "Legends and Legacy" from Peter Serling mentions that AA and PA campaigned for the 10-foot stretch.
4. Good question....

I sent you a PM, I've got a bunch of other cool tidbits you might want to know about.

DH106 wrote:

GalaxyFlyer wrote:

The ventral fin was added at the requirement of the British ARB, D.P.Davies wrote about it in his book, “Handling the Big Jets”

Indeed - there's an audio interview with D.P. Davies on the internet recorded back in 1992, part of which deals with his certification issues with the 707.

Thanks to you both! I own this book already but have not read it cover-to-cover. I will take another look through it. I will also check for the interview.

LH707330 wrote:

1: b. Sourced in several places. Not only was the fin height added and the ventral fin added, they also boosted the rudder more because the rudder force with OEI was high. See also this: http://adastron.com/707/qantas/Q-bellows.htm
2. Not 100% sure, but my guess is one of the early BOAC frames.
3. b. See here: http://adastron.com/707/qantas/138-length.htm Also, "Legends and Legacy" from Peter Serling mentions that AA and PA campaigned for the 10-foot stretch.
4. Good question....

I sent you a PM, I've got a bunch of other cool tidbits you might want to know about.

Thanks for the more detailed response. I will be in touch with you for sure if I have other questions, which I undoubtedly will

After doing some more digging, I was able to answer more thoroughly my fourth question.

The high deflection angle flaps were something I noticed in a photo of the aircraft in flight. The image below appeared to show flaps at a much higher deflection angle than I had ever seen.



Turns out that I was correct. The book "STOL Progenitors" has some great detailed information on the systems involved.

https://www.google.com/books/edition/ST ... frontcover

The single-slot flaps broken in two sections from the original 707 were abandoned and in their place, a simple unbroken hinged plain flap spanning 68% of the wing was installed with what was called Boundary Layer Control, or BLC. High-pressure air was redirected from the engines to be blown over the flaps to dramatically increase the lift characteristics. These flaps could be deployed at angles as high as 85 degrees, but ultimately angles higher than 55 were discovered to be disadvantageous due to flow separation, even with the BLC active. Further, the flap modifications made it impossible to retract the landing gear, so the airplane only ever flew with this configuration in slow-flight.

This book also answered my question about the tail-plane slats, which were installed to give greater trim capability at the low airspeeds tested in flight with the advanced flaps. The lowest speed tested successfully was 75 knots on a 6 degree glidepath, but apparently required the engines to be run at 96% N2, and also required the outboard thrust reversers to be available in-flight to control the aircraft in an engine-out scenario. I would imagine the margin for failure was one of the nails in the coffin that prevented this design from being used on any of the 707 variants, or other aircraft of its class.

Based on what I could find, NASA later changed the design of the plain flaps to be a simple split design (closely resembling but not quite a single-slotted flap) with three split segments, and did further testing with BLC and even Direct Lift Control. The upgraded flap segments could be used as ailerons and even deflect upwards to negative angles, like glider flaps almost. I believe this is the configuration that still exists on the aircraft as it sits today at the Smithsonian, perhaps with the BLC components removed during the restoration in the 90s. Details here:

https://ntrs.nasa.gov/citations/19690029434

I will update further if I can find more information. Some very cool stuff here! Never knew just how far they pushed this airplane.

chris341 wrote:

3. What was the actual reason that Boeing built the -138 for Qantas?

I have heard conflicting accounts of this one as well.

a. Qantas had inspected the Comet 4, DC-8, and 707. The Comet had a range too short for the needs of the airline, the DC-8 was too big and Douglas would not modify it, so they asked Boeing to make a 707 of the original length planned when the 707 was announced, to which they agreed.
b. Qantas had ordered the 707 with the intention of operated the aircraft with it's original length of 127 ft 10 in. Pan Am and other airlines petitioned Boeing to stretch the aircraft for more passenger load, but this shortened the range of the aircraft beyond what Qantas could use, so Boeing built the Qantas aircraft to original spec and all other -120 aircraft to the new lengthened spec.

I don’t know how accurate this is, but one of the common explanations I have heard quoted is that it related either to the length of the runway at Nadi in Fiji (a key tech stop point for crossing the Pacific) or because the runway had a “hump” in it at that time which the longer fuselage aircraft couldn’t handle.

https://blogs.crikey.com.au/planetalkin ... rd-rumble/

Good luck with your documentary.