Saro/Blackburn Segrave

A handful of examples of the Saro/Blackburn Segrave twin-engined light touring monoplane were built in the early 1930s, mainly by Blackburn at Brough. Some were license-built by Piaggio as the P.12, but I don't know how many (Wikipedia suggests just two).

The last Blackburn-built Segrave was particularly interesting, in that it was the testbed for the patent Blackburn-Duncanson tubular light alloy wing spar, which also doubled as the fuel tank. Following Duncanson's research into the idea, he teamed up with Blackburn to commercialise it. The Segrave design tapered from root to tip and the biggest-diameter centre section doubled as the main fuel tank. Despite having to be mashed into an existing two-spar wing, with extensive extra bracing, it resulted not only in greatly increased stiffness but also in such a large weight saving that an extra passenger seat could be added to the Segrave. Source: Flight, 9 Aug 1934, pp804-5.

Sadly, the Segrave failed commercially and the tubular-sparred "Segrave II" was the last one to be built.

Interestingly, it was just about the same time, ca. 1933-4 that the Blohm & Voss (aka Hamburger Flugzeugbau) new chief designer Richard Vogt made the tubular metal spar-cum-fuel-tank something of a trademark, though his were steel rather than light alloy. Coincidence?

I wonder also, did Blackburn stick with Duncanson's tubular spar in later types or did they revert to conventional construction?

Nice Info my dear Steelpillow.

Duncanson developed his tubular monospar system while employed by Goster and it featured in their High Speed Mail Carrier project in 1932. Unfortunately the poor financial state of Gloster led to many of the design staff quitting and Duncanson left to join Blackburn. After the spar was proven in the modified Seagrave it was developed further for the Blackburn HST10 transport. Unfortunately this failed to find a market and when Duncanson was killed in a car accident in 1936 his spar system was not taken further.

Thank you, that is most interesting. Vogt is said to have brewed up his system while on his flight back to Germany from Japan in 1933, to take up his new job at Hamburger Flugzeugbau. It sounds as if Duncanson beat him to it, not only in conception but also in first flight - the Ha 136 didn't fly until the Autumn of 1934, some months after the Segrave II.

I can't find any online info about the Gloster high speed mail plane project. Do you have a link by any chance?

Duncanson patented his tubular spar system in 1928
Gloster High Speed Mail Carrier is in part 2 of an article "Folland's forgotten monoplanes" in The Aviation Historian, Issue 10

Thank you again!

Thought this thread needed some pictures so here goes. First up is the original Saro Segrave Meteor I.

Next is the the two Blackburn CA18 Segrave I airframes which stayed as that variant.

Next up is two pictures of the sole example of the CA20 Segrave II, which had originally been a Segrave I, with one showing it partially clothed and the other in full splendour in the air.

And finally two pictures of the Italian cousin, the Piaggio P.12.

Nearly forgot the final design to use the Duncanson wing, some pictures of the completed but never flown Blackburn HST10, which also was known as the Type B9. First shot shows construction underway.

Next two shots show the B9/HST10 on the ground just prior to the taxi trials which were due to start on July 7th 1936. However events led to the cancellation of these and the airframe eventually went to the Loughborough College of Aeronautics, where it was broken up in 1946.

I wonder if a light bomber based on the wing would have enjoyed any success in WWII?

It's funny you said that Grey Havoc. When looking at the profile view of the BR/HST10, I particularly noticed that the tail reminded me of the Blenheim or Beaufort, whilst the nose shows that this shape was retained for the Botha clearly. But looking at the success of the Botha, or lack thereof, may cause you to think whether using the Duncanson wing would have just been an effort to make the proverbial silk purse out of a sows ear!

Yes, it would have been interesting to see what Duncanson could have done with the Botha, or at least the requirement that led to it!

Grey Havoc said:

I wonder if a light bomber based on the wing would have enjoyed any success in WWII?

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There were indeed studies by Blackburn for military variants:
C.A.21B H.S.B.T.10 - Coastal reconnaissance/bomber transport landplane derivative of the H.S.T.10. with mid upper and ventral gun positions. Two 365 h.p. Napier Rapier VI powerplants.
H.S.T.20 - Bomber; enlarged version of H.S.B.T.10. Two 630 h.p. Napier Dagger M.1 powerplants.
(From Blackburn Aircraft Since 1909, A.J. Jackson (Putnam, 1989))

Looking at that C.A.21B H.S.B.T.10 Coastal Recce/bomber transport variant caused me to look at the second pic I posted of the B9/HST10, and I noticed that just ahead of the fin there is a round panel similar to that used prior to a turret being fitted. However this seems far too close to the fin to be the actual location, and I would have expected something closer to the Anson/Oxford location for the upper turret. Not sure where that ventral location may have been though.
As per usual I have got to playing with paintshop to see what I could come up with a possible scenario for the first military variant. The profile view from the Aeroplane GA has been used as a basis. As I stated also being used is the turret from an Anson I. I have surmised that it would have been expedient to have fitted some kind of gondola arrangement underneath. This could house the bomb aimer position, the bomb bay and the rear ventral gun position. Below the main profile view is a scrap view showing this gondola. I cannot stress more that this is merely my imaginings of what this variant could have looked like and does not (as far as I know) have any basis for what the actual project could have looked like.

VictorXL188 said:

I cannot stress more that this is merely my imaginings of what this variant could have looked like and does not (as far as I know) have any basis for what the actual project could have looked like.

View attachment 694987

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Actually the dorsal gun position was located between the frames aft of the door. It did not have a cupola, just a folding windshield. The bombs were held in a ventral panier, no bomb aimers panel under the nose.

Tubular wing spars were revived by American engineer Jim Bede (1933 - 2015). He started (1961) by installing tubular main wing spars in his BD-1 which evolved into the Grumman American Trainer. The tubular wing spar also served as a fuel tank(s). I have flown the later Grumman American Cheetah variant.
Bede went on to design more than a dozen kit-planes most of which had tubular spars. Many of them also had symmetrical airfoils. While symmetrical airfoils may not have as high a lift-to-drag ratio, they vastly simplify construction/parts-count since the same pocket ribs could be installed on the left wing or right wing.
Amusingly, while the diminutive BD-5 had a tapered wing planform, it retained a constant-diameter tubular wing spar. This forced the wing roots to have a shallower finesse ratio than the wing tips (12% and 18%?). Bede hoped that this would tame stall characteristics but the last BD-5 that I saw had thick leading edge cuffs to increase the leading edge radius.