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Hawker Audax and Hart modifications for pre-war service in Singapore?

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  • Sid Guttridge
    Rank 1 Registered User

    Hawker Audax and Hart modifications for pre-war service in Singapore?

    I have noticed from The K File by J. J. Halley that Hawker Audaxes and Hawker Harts sent to Singapore in the second half of the 1930s were given some modifications that earned the addition of (Singapore) to their official designation.

    Does anyone know what modifications were made to the aircraft and why?

    Was it, perhaps, related to the humidity of the tropics?

    Were any other types similarly converted?

    Many thanks,

  • WebPilot
    Rank 5 Registered User

    Tropicalised versions generally had improved engine cooling systems, dust filters and survival equipment. Most types that served in more extreme environments had such mods.


    • Sid Guttridge
      Rank 1 Registered User

      Hi WebPilot,


      However, if the Singapore Audaxes and Harts got the same conversions as, say, the Indian or Southern Rhodesian Audaxes, then one has to wonder why they got a specific sub-type designation.

      It seems possible that there was something specific to the Audax (Singapore) that distinguished it from the others.

      Many thanks,



      • DonClark
        Toujours propos

        Hart, Hind, Audax variants and mods is a rich field indeed. Variants were very closely related, such that a Hart could be built as an Audax, or Audax completed on the line as eg Hector and so on.

        However, as far as the Audax (Singapore) is concerned, Halley The K File: The RAF of the 1930s and Mason Hawker Aircraft Since 1920 both agree:
        Audax K3720 was sent out as a prototype fitted with a Kestrel V DR (derated to 510 hp) in 1935, followed by a "small batch" for the SSVAF in 1936.
        According to Mason the Mark I Audax engine was a Kestrel I B of 525hp, but see my post below*.

        In RAF service o'seas, it was also common for Audax top engine cowls to be very heavily vented, larger radiators fitted, and de-rated Kestrel Vs installed.
        Whether all were built as Audax (India) or so modified in service, cannot readily say.

        Likewise, the Hart (India) and the Hart (Special) versions were given the Kestrel X DR (derated to 510hp) and the larger radiator, while noting that the Hart (Special) aircraft were in fact...Audaxes.

        That's apart from
        the LP balloon tyre option,
        whether or not a tail wheel fitted in place of skid, or
        removal of the message hook eg in FTS use,.
        And so on.

        By the by, I didn't spot a Hart (Singapore) mention in either Halley or Mason.
        Last edited by DonClark; 22nd October 2018, 11:32. Reason: serial!
        Don Clark


        • John Aeroclub
          Rank 5 Registered User

          Perhaps there were changes to the carburetors and other changes to the engine due to the excessive humidity of Singapore?



          • DonClark
            Toujours propos

            Air Publication 1404 A Vol I (1932) The Hart Aeroplane: Kestrel I B Engine or Kestrel X (de-rated) Engine has nothing to say about the Hart (India) or the Audax (Singapore), however, it does include an Appendix for the Hart (Special) which describes the aircraft differences in detail, if not the engines. May be of some interest:

            1. The Hart (Special) aeroplane is an Audax aeroplane modified to carry out, as far as possible, the duties of a Hart day bomber.

            2. The design of the Hart (Special) aeroplane is almost identical to that of the Hart day bomber thus permitting the general use of Air Publication 1404A for the Hart (Special) aeroplane; the differences between the two aeroplanes are dealt with in this Appendix.

            Equipment differences
            3. The equipment of the Hart (Special) aeroplane differs from that of the Hart day bomber by the following listed items :
            (i) The omission of
            (a) Oxygen installation
            (b) Provision for photographic duties
            (c) Flotation gear
            (d) Marine equipment.

            (ii) The addition of
            (a) Desert equipment; fixed items are provided on Audax aeroplanes incorporating modification No Audax/319
            (b) The standard Audax 12 V20 Amp hr accumulator

            (iii) Replacement of
            (a) Non-braked undercarriage by a braked undercarriage
            (b) Radiator by a tropical radiator
            (c) Oil cooler by a tropical oil cooler
            (d) Wheel tyres by heavy duty tyres,

            (a)The bomb release gear is electro-magnetically operated,
            (b) The engine is a Kestrel X (De-rated)."

            In contrast to the notes in Mason referred to above*, AP1404A states unequivocally, more than once, that the Kestrel I B and Kestrel X (derated) are each of 510hp. Halley (op cit) agrees with Mason, however: Kestrel I B 525 hp!

            I don't have a Kestrel manual, nor are RR engines my field. Someone with RR expertise may be able to shed more light on the precise differences between Kestrel marks/variants as in Hart, Hart (India), Hart (Special), Audax (Singapore) and their Kestrel X (De-rated) 510hp engine vs the Kestrel I B 510hp.
            Last edited by DonClark; 22nd October 2018, 05:41.
            Don Clark


            • Lingo Dog
              Rank 1 Registered User

              As is usual on this forum, a definitive answer on virtually subject. Well done!


              • DonClark
                Toujours propos

                Well, thanks, though at least for the Audax (Singapore) more indicative than definitive.

                The list of Hart (Special) inclusions/exclusions at least gives context, which suggests (though without certainty) that possible changes for an Audax (Singapore) might include eg
                exclusion of desert equipment
                restoration of photographic equipment.

                The level of Hawker (and other) type variants detail at the time was both minute and profuse, and not that well recorded in readily available accounts today.

                On carburettors:
                As suggested above, Kestrel notes are likely to be of assistance if required. For example, in the case of the Mercury VIII and IX, AP 1491B gives highly detailed instruction on a wide range of adjustments, for various carburettor types, for starting, running and tuning. These were not of a kind to redesignate an aircraft type, in my mind. Still, open to correction and really not my field.
                Last edited by DonClark; 21st October 2018, 19:12.
                Don Clark


                • DonClark
                  Toujours propos

                  At the risk of overkill, a little more on...

                  The Straits Settlements Volunteer Air Force
                  "The Straits Settlements Volunteer Air Force (SSVAF) was created on 25 March 1936"

                  "On 10 August 1936, it was announced that D. S. E. Vines, a former RAF Squadron Leader, would be appointed first commander of the SSVAF.
                  The volunteers spent this period training in Hawker Audax and Avro Tutor planes."
                  Sources: Straits Settlements Volunteer Air Force citing Straits Times

                  Audax (Singapore) K3720 was taken on SSVAF charge 25 Mar 1936, wearing unit no 15 and serving with them until at least Sep 1939 before return to the Air Depot Drigh Road (Karachi), later serving with 5 Sqdn in 1941. SOC Dec 1941.
                  Source: Halley The K File

                  The immediate pre-war SSVAF period was later recalled by one Mowbray Garden* in these terms:
                  "In the Spring of 1939 [we] were accepted for commissions in the SSVAF, which was stationed at Seletar.
                  This unit had two Avro Tutors, two Hawker Harts and eight Hawker Audaxes and we flew them under the surveillance and guidance of regular RAF flying personnel"
                  Source: Shores et al Bloody Shambles Vol I.

                  Whether the 8 aircraft of 1939 were all Audax (Singapore), cannot say.
                  In Aug 1940 the SSVAF was absorbed into the MVAF, with a very mixed bag of civil aircraft.
                  Apparently no Audaxes then remained on the roster.
                  Source: Malay Volunteers Group

                  *F/Lt Mowbray Garden DFC (RAFVR 76482) was decorated in October 1946 for his service with 243 Squadron (Buffalo I) in 1941/42, among those under the London Gazette general citation
                  "in recognition of gallant and distinguished services rendered during the period of operations against the Japanese in Malaya and the Netherlands East Indies terminating in March, 1942"
                  Source: London Gazette, Maton Honour The Air Forces.
                  Last edited by DonClark; 22nd October 2018, 06:54.
                  Don Clark


                  • anneorac
                    Ex-Pat Scottish Member

                    This view of K3720 at Seletar appears to show a rather standard looking Audax with a naturally aspirated Kestrel. Sadly it's impossible to tell from this angle if it was fitted with twin fuel tanks like the India versions of the Hart & Audax. Complete speculation on my part but I wonder if the Singapore aircraft copied the Osprey in using stainless steel in an effort to combat the humid and salty conditions found in the region?



                    • DonClark
                      Toujours propos

                      Brilliant photo, Anneorac. Interesting suggestion re the Osprey & stainless steel.

                      However, according to Halley/K File "a few Ospreys were built with stainless steel fuselages to counter the effects of corrosion at sea".
                      Among the 130-odd built, a quick skim of the a/c list found no further clue as to which or how many.
                      The BAE Osprey summary shows 3 Osprey III trials aircraft (S1699 to S1701) and "some" of the remaining Mark IIIs.

                      As for K3720 itself, the Halley/K File entry reads:
                      "Deld to 2 ASU 22.1.34; Hawkers 5.1.35 for mod to Audax (Singapore); PD 21.2.35; AD Seletar 12.3.35; SSVAF25.3.36 later coded 15 [...]"
                      To me all that says it was built as an Audax airframe from the start - thus not stainless steel.
                      After a couple of skims I simply couldn't spot any other SSVAF Audax airframes, either...some one more attentive may!

                      The one certain Audax (Singapore) change is the Kestrel V DR (in place of the Kestrel IB): that plus "tropicalisation".
                      However, the only definitive ref I have for "tropicalisation" is that for the Hart (Special) noted above (post #6).
                      The BAE histories for the Hart and Audax only show that Hart (India), Hart Special), Audax (India), Audax (Singapore) etc were "tropicalised" but without detail.

                      From other summaries it seems there was variation as to exactly which "tropicalisation" elements were added or excluded from subtype to subtype, depending on theatre and role, like the heavily vented top cowl of 4FTS Audaxes at Abu Sueir in 1939.
                      Open to correction and addition if anyone can come up with a good Audax ref.

                      The forum software froze and smashed this account as I tried to amplify it just now. Re-written, I think I'll leave this one now.
                      Last edited by DonClark; 22nd October 2018, 10:58.
                      Don Clark


                      • anneorac
                        Ex-Pat Scottish Member

                        There are a couple of poor quality photos on Flight Global Archive which show SSVAF Audaxes. Have a look at this page and the page before.


                        According to Putnam's Hawker Aircraft since 1920, the Osprey moved over to stainless steel construction late in 1932. It is however a little hazy on details. Where's a copy of a post 1932 AP1431 when you need one?

                        192 FLIGHT. FEBRUARY 23, K^Q S.S.V.A.F. Audaxes at Mergui aero drome, Burma. The natives squatting in the foreground are Burmese priests. opening of its own training depot in June, 1933. Not


                        • DonClark
                          Toujours propos

                          Excellent, Anne. I have trouble searching the Flight Archive.

                          The pic is good enough to show serials for 2 of the three aircraft: K5142, K3720 and the third obscured. This takes us back to Halley to find

                          25 Audax (India) del by Glosters Aug to Oct 1935 of which
                          "K5141 Deld to Hawkers 11.5.36 for cv to Audax (Singapore); R&J Parks 16.7.36; AD Seletar 14.8.36 SSVAF 16.9.36 coded 2 [...]"
                          "K5142 Deld to Hawkers 11.5.36 for cv to Audax (Singapore); R&J Parks 16.7.36; AD Seletar 14.8.36 SSVAF 4.11.36 coded 3 [...]"
                          "K5167 Deld to Hawkers 30.6.36 for cv to Audax (Singapore); R&J Parks 27.7.36; AD Seletar 7.9.36; SSVAF 4.5.37 [...]"

                          All I could spot. That would seem to be that: started life as Audax (India) and thus usual airframe, not stainless.

                          Although Mason doesn't say, I wonder whether the Audax (India) originally had the Kestrel X DR 510hp, as did the Hart (India). In that case, the Audax (Singapore) Kestrel V DR 510 hp would v likely be the main difference, though how a Kestrel V and X differ, DR or otherwise, I know not.

                          Just for amusement, Mason's Audax tech data shows Kestrel IB as 530hp, vs 525hp for Hart Kestrel IB, and not as the 510hp shown in AP1404.
                          If right, why these small diff in hp mattered I'm unsure, perhaps to do with desirable rpm in the conditions.
                          Likewise, the need to fit the derated Kestrel V or X, at 510hp. Not my field.

                          However...AP1431 post 1932: For those who may not know, at UK National Archives AIR 10 holds specific Air Publications for the Audax and for Kestrel IB, V and X X(DR) engines.

                          Anyway my hat off to you, Anne: and that's all from me, troooly.
                          Last edited by DonClark; 22nd October 2018, 22:52.
                          Don Clark


                          • John Aeroclub
                            Rank 5 Registered User

                            I had a play with the photo put up by Anne and the serial by the way is K3720.

                            I would suggest that the compression ratio of the lower rated engines suited the denser humid air conditions of the Far East. The extra perforated top cowls found on some Hawker types in the Indian theatre are more often found on aircraft originally destined for the drier near east. The stainless steel Osprey wings are also different in many ways from the pure Hart / Audax machines and as the erks knew them "An Awdax is a Art wiv an ook".

                            Unfortunately there are a number of errors in the other wise excellent Mason book such the Hartebees having only one front gun when they had two and despite being treated as distinctly different type, both it and the Australian Demon were both Demons with slightly varying Army co-op fit.

                            Last edited by John Aeroclub; 23rd October 2018, 13:07.


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