Short Stirling Engines - Hercules XI & Wright GR2600A5B

[Simon Thomas]

Does anyone have a copy of the performance curve for the Wright GR2600A5B fitted to the Short Stirling?

I am hoping to compare the performance curves of that and the Hercules XI.

In the book "Short Stirling, The First of the RAF Heavy Bombers", Pino Lombardi writes
However, Hercules engine production was accelerated to meet demands, and when three aircraft - N3657, N3711, and R9188 - were fitted with Wright Cyclone engines at Swindon, performance and fuel consumption trials to assess performance and fuel consumption produced disappointing results.

Mikec was kind enough to post the Wright Cyclone 14 Model C14A manual, which had some information including a curve for the GR2600A2A but unfortunately not a curve of the GR2600A5B. Wright R-2600-8-9-13 Hand Book of Orperating Maintenance Instructions
I have attached some information from that manual. The difference between the A2A and the A5B is the two speed supercharger and 90 vs 95 octane.

For reference, I have also attached the some information from the Bristol Hercules XI including the M and S performance curves.

Edit: stage/speed screwup fixed.

 

[tomo pauk]

I have attached some information from that manual. The difference between the A2A and the A5B is the two stage supercharger and 90 vs 95 octane.

Americans installed the A5B (R-2600-19 and -21) in the Vultee Vengenace, and the A5B-1 (a.k.a. R-2600-23) on the A-20s.
Please note that, apart from prototypes (eg. like the type powering the 1st Hellcat prototype), there was no 2-stage supercharged R-2600 - the A5B and A71 were 1-stage 2-speed supercharged.
FWIW (the 1st table might contain a typo or two, like the engine power at 29500 ft and at 2600 ft that was not achievable for the non-turboed -19):

 

[Bretoal2]

Confusion between 2-stage and 2-speed is very frequent...

 

[Simon Thomas]

Thanks Tomo - I often manage to muddle them up. It's not the first time I picked the wrong one, and it won't be the last.

I found the performance curve buried in a Boston IIIA manual in the tech area of this site. (Thanks to JimSan for originally making it available)

I haven't superimposed them, however it appears that although the GR-2600-A5B has a higher take off power rating and climb rating, when it comes to maximum continuous cruising weak mixture the Hercules is able to operate at higher rpm (2500 rpm vs 1850 rpm) and higher boost (0 vs 27 in) which equates to rather more power available during cruise.

 

[Simon Thomas]

Finally put it together. The variable spacing on the abscissa took a bit of time.
This graph shows max cruising power in weak mixture between the Hercules XI and the R-2600-A5B.

These were both run in the Stirling, and the results were the Stirling fitted with the R-2600 was slower, used more fuel and was significantly louder.

BSFC at 900 hp in weak mixture:
Hercules XI - 0.416 lb/hp.h
R-2600-A5B - 0.464 lb/hp.h

Info extracted from Bristol Hercules Manual and Boston IIIA manual (A.P. 2023C)

Edit:
Hercules XI 1870 lb
R-2600-A5B 1950 lb

 

[Bretoal2]

What a formidable engine the Hercules was ! It will never be said enough....

 

[Simon Thomas]

Comparison of the R2600-8 engine with the Hercules XI. The R2600-8 was developed at the same time as the XI, however for whatever reason the UK was not able or not willing to use it.
The max cruising power varied quite a bit with altitude. At the typical cruising altitudes of the Stirling between 15,000 ft and 18,000 ft, the XI had slightly more power. This was not by choice, it was meant to be higher but the aspect ratio and the excessive empty weight of the Stirling wing did not allow much higer. (Covered in another thread.)
The XI used quite a bit less fuel for all altitudes.

Fuel consumptions:
At 15,500 ft, both engines were at ~924 hp. The XI had a bsfc of 0.447 (FS) and the R2600-8 had a bsfc of 0.48 (MS)
At 19,000 ft, both engines were at ~876 hp. The XI had a bsfc of 0.448 (FS) and the R2600-8 had a bsfc of 0.54 (FS)

I was a little surprised for these two engines to be described as "completely different". My view is that they provide a reasonable comparison of very similar engines - except for the valve system. The power is rather similar, the big difference will come down to the supercharger. The fuel consumption favours the sleeve valve. As has been mentioned, these comparisons are only for this snapshot in time.

R-2800-8 info extracted from Avialogs SB2C1 manual.

 

[tomo pauk]

The max cruising power varied quite a bit with altitude. At the typical cruising altitudes of the Stirling between 15,000 ft and 18,000 ft, the XI had slightly more power.

Thank you for the graph.
What rpm and (max) boost was used for the R-2800-8 line?

 

[Simon Thomas]

The rpm was 2080 rpm, boost varied to maintain the max cruising power. Below is snip from full manual.

The bold lines on the MS and FS power curves in the pdf attached above show how the manifold pressure varies.

 

[tomo pauk]

The rpm was 2080 rpm, boost varied to maintain the max cruising power. Below is snip from full manual.

The bold lines on the MS and FS power curves in the pdf attached above show how the manifold pressure varies.

I've taken a peek on the charts, seems like that the 2100 rpm line for FS (ie. high gear)shows ~840 HP at ~21500 ft, and ~730 HP at 25000 ft.
FWIW:

 

[Simon Thomas]

The rpm lines after the pressure begins falling are hard to follow. The rpm spacing at the extreme RHS does not appear to be precise.

 

[tomo pauk]

The rpm lines after the pressure begins falling are hard to follow. The rpm spacing at the extreme RHS does not appear to be precise.

Yes, you are right.
Again, FWIW, I've merged the two pages of the pdf together, for the high gear: