Piston-to-turboshaft helicopter conversions


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Jens was first-up with a turboshaft conversion, the Sikorsky S-58T. Here's a tentative list of unbuilt and built piston-to-turboshaft conversions ...



Bölkow Bo 103 (4-cyl Agusta GA.70 V) [Evol] to Bo 103B (90 shp BMW turboshaft)*
[* presumably this engine was the 66kW/90hp BMW/MAN/MTU 6012]



Bölkow Bo 104 (2 x NSU Wankel rotaries) [Evol] to (2 x MAN 6022-A3s)*
[* Originally by BMW, the MAN 6022-A3 was derived from the CH-53G's APU]

Bölkow P 309 (piston type?) [Evol] to (turboshaft type?) Gersdorff: Ludwig Bölkow und sein Werk p91

Bristol 171 (Leonides 173) [Evol] into Bristol 203 (1 x RR Gnome) and 214 (2 x Gnome)

Bristol 173/191 (2 x Leonides Major) [Evol] into 192 (2 x Napier Gazelle)

Hiller UH-12E5 (Lycoming VO-540) [Evol] into UH-12E5T (Allison 250-C20B)
[Similar to Soloy conversion, proposed for US Army New Training Helicopter program]

Kamov Ka-26 (2 x M-14V-26 radials) [Evol] into Ka-128 (Turboméca Arriel 1D1)

Piasecki H-21 (1 x R-1820-103) [Conv] to turboshaft (3 x Turboméca Turmo III, 750 shp max, 600 shp continuous) AFM N°10, 6/7 2006

Piasecki YH-16 Transporter (2 x P&W R-2180 Twin Hornets) [Conv] to YH-16B Turbotransporter (2 x 3700 shp Allison T56) http://www.aero-web.org/specs/piasecki/yh-16a.htm

Vertical Aviation Technology 260L/Sikorsky S-52 (Lycoming VO-435) [Conv] to Hummingbird 260T (mockup installation only, Allison 250-C18) FRW 7/1988 p32 & JAWA 1988-89 p495

Yakovlev Yak-24 (2 x ASh-82Vs) [Evol] into Yak-24P (2 x 2700 shp AI-24V)


Bell 47G (Lycoming VO-435) [Conv] by Soloy (Allison 250-C18) [German aviation magazine] Aircraft (Ger. magazine) # 27 753

Bristol 191 (Belvedere predecessor, 2 x Leonides Major) [Conv] into Bristol 191/2 (192 test rig, 2 x Napier Gazelles)

Enstrom F-28, 1 x 225 hp Lycoming HIO-360 [Evol] to Spitfire Mk 1, 1 x 420 hp Allison
-- F-28 deriv. by Spitfire Helicopter Company of Lynnfield, Massachusetts

Enstrom 280FX, 1 x 225 hp HIO-360 [Conv] to 280FXT, 1 x 420 hp Allison 250-C20W
-- led to similar 3-seat TH-28 for US Army, 6 built before program cancelled
-- led to 4-seat civil/military Enstrom 480 series, 285 shp Allison 250-C20W

Harbin Z-5/Xuanfeng (HS-7/ASh-82V) [Conv] to Z-6 (PT6T-6 Twin Pac)

Hiller UH-12D and 'E (Lycoming VO-540) [Conv] by Soloy into UH-12E3T (Allison 250-C20B)

Hiller 360 (Franklin/Lycoming) [Evol] into 12L-4 test-bed (PT6A)

Kaman K-225 (1 x 225 hp Lycoming O-435-A2) [Conv] to 1 x 175 shp Boeing 502-2
-- First turboshaft-powered helicopter, December 1951

Kaman K-240 (HTK-1), 1 x 240 hp O-435-A2) [Conv] to to HTK-2, 2 x 225 shp Boeing YT50-BO-2
-- NB: some sources list a rating of only 210 shp for the Boeing 502-2/YT50-BO-2s

Kaman K-600 (HOK-1) 600 hp P&W R-1340-28 [Conv] to Lycoming XT53, led to H-43B

Kaman K-600 (H-43B) [Evol] 1 x 849 shp Lycoming T53-L-1B turboshaft

Kamov Ka-26 (2 x M-14V-26 radials) [Evol] into Ka-126 (1 x Mars TV-O-100) and [UNBUILT] Ka-128 (Turbomeca Arriel 1D1)

Kamov Ka-26 [Evol] into Ka-226 (2 x Allison 250-C20s, optional Progress AI-450, Turboméca Arrius 2G, or Klimov VK-800)

Piasecki YH-16 Transporter (2 x P&W R-2180 Twin Hornets) [Evol] to YH-16A (2 x 1800 shp Allison T38) http://www.aero-web.org/specs/piasecki/yh-16a.htm

Sikorsky S-52 (Franklin O-425-1) [Conv] into Sikorsky S-52-5 (Turboméca Artouste II) JAWA 1954-55 p295

Sikorsky S-52/H-18 (Franklin O-425-1) [Conv] into Sikorsky XH-39/S-59 (Continental XT51-T-3 Artouste II) http://www.aerofiles.com/sik-s59.jpg http://www.aerofiles.com/sik-h39.jpg

Sikorsky S-55 (R-1340, R-1300) [Conv] into Helitec/Aviation Specialties S-55T (TSE 331)

Sikorsky S-55 (R-1300) [Conv] into Vertical Aviation Technology Whisper Jet S-55QT (TSE 331)

Sikorsky S-58 (R-1820-84) [Conv] by Sikorsky or Orlando into S-58T (PT6T-3 or PT6T-6)

Sikorsky S-58 (Westland import, R-1820-84) [Conv] to Wessex testbed (1 x Napier Gazelle NGa.11)

Westland WS-55 Whirlwind (R-1340-40, R-1300-3, Leonides Major) [Conv/Evol] into Whirlwind Mk3 (T58/RR Gnome)

Just found when I browsed through the pityful few issues otf the Aviation Francais Magazine :
A planned conversion of the Piasecki H.21 with three Turboméca Turmo III engines.
Thanks Jens!

Does AFM give any other details? I entered it as "Turmo III" but I am wondering if we can assume 750-800 shp Turmo IIIBs (as per SE.3200 Frelon?). [750 shp max, 600 shp continuous.]

If Piasecki/Vertol wanted the option of safe running on two turbines, the combined power of the Turmo IIIBs would seem about right for replacing the 1425 hp Wright recip.
The article just speakes of "Turmo III"engines, giving power output as 750hp (max).
(3-view from AFM N°10, 6/7 2006)


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Thanks Jens! H-21 triturbine added.

I've also added the Piasecki YH-16A and YH-16B (thanks boxkite). http://www.piasecki.com/yh-16.htm
Could this '(work)horse with no name' be a YH-16 derivative ???

SOURCE: magazine Der Flieger 10/1952 (p201)


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Yes, I believe so.

From: http://www.aero-web.org/specs/piasecki/yh-16a.htm

Quoting from J.P.Spencer "Whirlybirds: A History of the U.S. Helicopter Pioneers", 1998

"This tragic accident caused the H-16 program to be scrapped, preempting the sixty-nine-passenger YH-16B Turbotransporter (a conversion of the YH-16 then in progress), which would have flown with two 3700shp Allison T56 engines. It also preempted Frank Piasecki's vision of interchangeable under-body pods for the rapid transport of differing loads such as field operating rooms, communications centers, and mobile repair centers. A tall stilt landing gear had already been designed to let the YH-16B accommodate such pods."
Another unbuilt helicopter that may qaulify for this thread is the Enstrom 280 Eagle which would have been the Model 280L powered by an Allison 250-C20B turboshaft.
Enstrom also built (in 1988) the Model 280FXT with Allison 250-C20W turbine. This prototype was a rebuilt of the first Model 280FX. It formed the basis for the Model 480 (TH-28)
Could someone put 'helicopter' in the subject since there's already a huge discussion on piston to turboprop conversions, mainly regarding airplanes?
How about the Kaman HOK-1 with the R-975 becoming the H-43 with a T-55?
Some belated updates (and corrections to this thread). Also two questions ...

Hood: "unbuilt" Enstrom Eagle. Wasn't Eagle the sole PoC 280FXT conversion?

Frank: I've listed the K-600/HOK-1 conversion with XT53. Was 'T-55' a typo or is this another variant we need to know about?
I've come across a reference to a Bristol 171 Turmo while digging through files at the National Archives.
It was mentioned on a Bristol graph showing the workload of the design department in 1955-56 (effects of cancelling the Type 191).

Sadly, I don't think anything else surfaced while I was looking at Scyamore files. I'm guessing that might have been what became the Type 203 with the Gnome.
Brief mention in Flight of 1959 of the Piasecki [X]H-21D having flown with a pair of T58s:

... and T58s have now flown in the Sikorsky S-62, Vertol H-21D,
Sikorsky HSS-1F (S-58) and in a Westland Whirlwind.


Plus earlier 1957 reference:
Another twin installation has been made in a Vertol H-21D, the Navy
having undertaken this on behalf of the Army who are the sponsors;
this aircraft should have flown last April but has not yet done so.


I believe there was also a twin-T53 H-21 project but can't determine if it ever made it to hardware stage. Oh, this site claims it was c/n B-167 that was converted back to piston after testing!


Likewise B-168 converted back after T58 testing:


One or other was used for stability augmentation testing in 1959:

Sud S-58 Bi-Bastan conversion, Flight 1964


Powered with the 1,900 h.p Turbomeca Bi-Bastan, a coupled version of the single-shaft turboprop, the conversion included lengthening the nose by 2ft 3in to maintain the c.g. position with the lighter engine.

As this is a shaft turbine, the exhaust is in the nose and the engines are reached by opening clamshell doors.

The aircraft underwent full flight trials, but there is no current information about its status.

Earlier 1962 reference notes that it was also intended for the H-21 ( sensibly, since the French also used that ) but no evidence it flew in that airframe:


Some searches of French web pages indicates that the S-58 re-engining was a contingency at a time of budget restrictions, when there was a risk that the SA.330 project would not proceed. When the contract for that type was finally issued in 1964, the S-58 project fell from priority and was relegated to a commercial package offered to S-58 operators.

Initially the SA.330 was sketched as using the same Bi-Bastan engine, so not all the engineering was wasted.


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Agusta A.115, 1962. License-built Bell 47J fitted with Astazou II, quite sporty with 480 shp on tap. Hovering ceiling OGE 3,500 metres! Only one conversion.

Apophenia said:
Westland WS-55 Whirlwind (R-1340-40, R-1300-3, Leonides Major) [Conv/Evol] into Whirlwind Mk3 (T58/RR Gnome)

Also one Whirlwind was fitted with the Blackburn Twin-Turmo ( BnTC.1 ) rated at 980 shp. Re-converted to Alvis Leonides engine after testing and delivered to the RAF as XJ466.

The Twin-Turmo was based around a pair of Blackburn-built Turmo 600s driving a common final-stage. Installation in the Whirlwind was at 35 degrees to integrate with the original gearbox. Description and diagram here:

Late in their lives, a few RAF Whirlwinds were retrofitted with Rolls Royce Gnome turbine engines.

Westland Wessex production started with Napier Gazzelle engines, but most production Wessex had a pair of RR Gnomes in the nose. Gnome was a T58 turbo shaft engine built under license from General Electric (USA). T58 was standard power for later S-61 Sea King helicopters.

Later, Sikorsky converted some military-surplus S-58s to S-58T configuration by installing Pratt & Whitney of Canada PT6A-3 Twin-Pacs. S-58T can be recognized by the twin, rectangular intakes on top of the nose. During the 1970s (?) some one suggested converting a fleet of SE Asian (Thai ?) military S-58s to S-58T configuration.
1,800 horsepower (combined) P&WC PT6A-3 Twin Pacs power Bell 212 Twin Huey's and a variety of other, twin-engined helicopters.
Soloy has retrofitted a bunch of Hiller UH-12E and Bell 47 helicopters with Allison/Rolls Royce 250-C20B turbine engines. Most of these conversions dust crops.
Soloy installs the same Allison/RR 250 engine as installed in Jet Ranger, Hughes 500, etc. factory-fresh airframes.

Robinson offers the (new production) R66 helicopter with an RR300 300 horsepower turbine. From a distance, the R66 resembles the earlier, piston-powered Robinson 44 with 4 seats, 2-bladed rotors, Lycoming O-540-F
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Here's one that didn't fly. Orlando Helicopter had various fanciful ideas for going beyond the basic S-58T re-engining, such as the Viking noted in the 1988-89 Jane's.

Much like Mil did to make the initial Mi-8 from the Mi-4...


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Sud S.58 turbine conversion model, International Helicopter Museum, Weston Super Mare, 2018


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Direct conversion of the Sikorsky S-56 by Westland, re-engined with Napier Elands, proposed August 1954 (ref. The Aeroplane July 11, 1958). Engines to be located fore-and-aft in more streamlined nacelles on the tips of the S56 stub wings.
In the lower left of this GE advert from Avweek there is a small but rare photo of the Sikorsky HSS-1 with twin T58 installation. First flew 30 January 1957.


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Brief mention in Flight of 1959 of the Piasecki [X]H-21D having flown with a pair of T58s:


Plus earlier 1957 reference:


I believe there was also a twin-T53 H-21 project but can't determine if it ever made it to hardware stage. Oh, this site claims it was c/n B-167 that was converted back to piston after testing!

Likewise B-168 converted back after T58 testing:

One or other was used for stability augmentation testing in 1959:

Are there any photographs of the converted Piasecki XH-21D turbine airframes ? (I'm curious as to intake/exhaust arrangement)

Cheers, Joe

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