Sikorsky XR-7

[EEP1A]

Sikorsky XR-7 helicopter is one of the very few illusive USAAF “R” for rotary-wing/helicopters before the re-designation in 1947.
Usually, Sikorsky XR-7 helicopter is described as; “XR-6 developed with O-405-9 Not completed” like in the case of “United States Military Aircraft since 1908”, PUTNAM, by Gordon Swanborough & Peter M. Bowers.

This is strange because except for the sole XR-6 with Lycoming O-435-7, all the other 244 R-6 type helicopters produced (XR-6A, YR-6A, R-6A) were equipped with Franklin O-405-9 engines.

I have been very much interested in finding reliable information regarding XR-7 and recently found the answer at the official documents with comprehensive list of aircraft description, issued at the end and just after the WW II. The original document titled “MODEL DESIGNATION ARMY AIRCRAFT” (11th edition published by Commanding General Army Air Forces, January 1945) and (12th edition June 1946) can be seen from this address;

The followings are taken from those documents:

Description regarding Sikorsky XR-7;

(from page 64 of the 1945 edition)
MODEL XR-7
CONT. NO AC-35340
MFR. Sikorsky
QUT. 1
POWER PLANT MFR. Franklin
MODEL O-405
CHARACTERISTICS Same as the XR-6 except for engine change. (designation cancelled-redesignated XR-6A 2-12-43)

Page 90 of the 1946 edition has the same information as above plus, the original contract for XR-7 was for 1 but none completed. In the XR-6A section, with the same contract number AC-35340 as that of the XR-7, the original contract for XR-6A was for 4 but 5 were completed, which suggests that the sole XR-7 ordered was completed as XR-6A.
Attached photos are from SDASM archives;

Sikorsky XR-6A number 1 (43-28240) which might be the one originally ordered as XR-7.

Sikorsky XR-6 (43-47955) the only one with Lycoming O-435-7 engine.

Any comments are welcome.

EEP1A

 

[Stargazer]

First of all, congratulations on your research!

The elusive XR-7 has long been a mystery in my list (and of course there must be many of us) and as you well put it, all that we ever knew was that it was to be a development of the XR-6 with a Franklin O-405-9 engine. No serial numbers ever appeared, no sketch, not a hint of an explanation as to what that version might have been.

Now the notion that "XR-7" might have been the initial designation for what became the XR-6A is very interesting. It makes a lot of sense that the R-7 would have been a Franklin-powered development of the Lycoming-powered XR-6, hence the use of that engine on all subsequent R-6 variants. Someone must have figured that the two versions of the VS-316B were too close to each other to truly justify a new designator, and therefore XR-7 became XR-6A...

As far as I'm concerned, the case is solved! Thanks again for investigating the matter and sharing it with us.

 

[Jos Heyman]

Thanks to EEP1A for this information but I would be cautious to link the XR-7 to 43-28240.
I can see another scenario in which the XR-6A was originally intended to be fitted with the Lycoming engine and that 4 were ordered, along with a fifth with the Franklin engine ordered as XR-7. It was then found that the Lycoming engine somehow did not suit the XR-6 and -6A design and that the Franklin engine was chosen. That the Lycoming was not appropriate is probably the reason why the R-6B production version with Lycoming engines and to be built by Nash Kelvinator, was cancelled.
So, with the XR-6A to be completed with the Franklin engine there was no need foor the XR-7 designation and on 12 February 1943 the XR-7 was redesignated as XR-6A (that is fact, thanks to EEP1A).
Taking into account that, for instance 43-47953/47955 were - in a neat order YR-4B, XR-5, XR-6, it is possible that 43-28240/28244 were also in a neat order, ie 44-28240/28243 as XR-6A and 43-28244 as XR-7.
I have no foundation whatsoever for the above scenario and I could be totally wrong, but I present it as an alternative. The solution can only come (hopefully) when somebody inspects the formal records for aircraft with serials 43-28240/28244 and let us see on which record the XR-7 designation is crossed out.

 

[Stargazer]

Very interesting, Jos! I guess we'll have to wait until more evidence pops up before we can be assertive as to what really happened, but EEP1A and you sure have made great advances on solving this mystery.

 

[EEP1A]

STARGAZER2006 and Jos;

Thank you for your comments.

I completely agree with the scenario that “Someone must have figured that the two versions were too close to each other to truly justify a new designator..”

Regarding the settlement of which XR-6A was XR-7, I agree that we need more information.

Anyhow I believe that the external appearance of the XR-7 was identical to XR-6A (43-28240 to 244) because XR-7 was just redesignated to XR-6A.

Any information regarding XR-7 or XR-6A is welcome.

EEP1A