Use of exhaust thrust in radial engines in late 30s

[fortrena]

Even though specially-designed exhaust stacks which produced some thrust and increased speed by a small amount (4 ot 5 %, or slightly more?) were mounted on fighter aircraft powered by Vee type liquid-cooled engines (German Bf / Me 109, British Hurricane and Spitfire for ex.) before the start of the Second World War , fighter aircraft powered by radial air-cooled engines (French MB 152 and American P-36 for ex.) were not so equipped until later, with the Fw 190, perhaps, being the first.

Given the potential gains in speed, was there a technical reason behind this non use?

Is it possible that the Hughes H-1 racing aircraft was fitted with specially-designed thrust-producing exhaust stacks?

The following report may prove interesting... https://apps.dtic.mil/sti/pdfs/ADA801032.pdf

 

[Justo Miranda]

Ki-43-IIb "Otsu" (Mark 2b)

Fuel cooling system added on some examples. Both exhaust pipes were angled backwards for slightly increased thrust.

Nakajima Ki-43 Hayabusa - Wikipedia

en.wikipedia.org

 

[Justo Miranda]

Renard R-37

 

[rinkol]

Even though specially-designed exhaust stacks which produced some thrust and increased speed by a small amount (4 ot 5 %, or slightly more?) were mounted on fighter aircraft powered by Vee type liquid-cooled engines (German Bf / Me 109, British Hurricane and Spitfire for ex.) before the start of the Second World War , fighter aircraft powered by radial air-cooled engines (French MB 152 and American P-36 for ex.) were not so equipped until later, with the Fw 190, perhaps, being the first.

Given the potential gains in speed, was there a technical reason behind this non use?

Is it possible that the Hughes H-1 racing aircraft was fitted with specially-designed thrust-producing exhaust stacks?

The following report may prove interesting... https://apps.dtic.mil/sti/pdfs/ADA801032.pdf

Another early implementation concerned the Bramo 323 engines installed in the FW 200.

I have the sense that this is something that fell through the cracks - the engine manufacturers had exhaust arrangements they were happy with and the airframe manufacturers accepted what they were given.

 

[fortrena]

A very valid point. Especially so in the case of 1930s Bristol radial engines whose exhaust gases went into a a collector ring at the front of the NACA-type cowling.

That said, one has to wonder why air forces or air ministries, realising the gains in speed made seemingly possible by the introduction of specially-designed exhaust stacks on Vee-type engines, in the late 1930s, seemingly without too much complication, did not turn to radial engine makers and asked why they were not looking into this.

Re. the U.S., the widespread use of radial engines on airliners may perhaps have played a small role. Having a group of exhaust pipes on the side of an engine which faced the passenger cabin might have (significantly?) increased the noise level - an issue which plagued the civilian and military Canadair North Star, the Merlin-powered versions of the Douglas DC-4 / DC-6.