V-3420 vs X-3420

[KJ_Lesnick]

Which would have been easier to have been designed and worked, two V-1710's designed into a double V (as was done) or an X?

[Hobbes]

Coupling two 1710s is easier, because the only new part you need to design is the gearbox. For an X-engine you need a new block and crankshaft, and you need to redesign the bottom half to function upside-down

[KJ_Lesnick]

Hobbes

Understood.  Here's what I don't understand -- if the V-3420 was first designed in 1937, why wasn't it ready for the B-15 bomber which flew the same year?  Why didn't they just use the R-1830's until the new engine was ready?

[kitnut617]

Something to keep in mind, for some reason 'X' configured engines weren't very successful, and there were a lot of engines designed that way and none worked very well.

My theory is that technically speaking, an 'X' engine is really a multi-row radial which has four cylinders per row.  As many know, radials with an even number of cylinders per row just don't work properly ----- successful radials have always had odd number of cylinders per row (3, 5, 7 & 9) and successful multi-row radials were 14, 28 or 18, 36 cylinder engines.  I can't think of a 10 or 15 cylinder engine though.

[pyro-manic]

I don't think any five-cylinder radials ever had the "two-row" (or three or four for that matter) treatment. Don't see why it wouldn't have worked though.

KJ: "First run" does not mean "ready to fly" - using an experimental, unproven engine (four of them!) on an experimental, unproven (and unique - bigger, heavier than pretty much anything else of the era) aircraft is possibly not the best idea. Plus all the additional design and testing work might have been deemed not good value for money. If any more B-15s had been built then perhaps they would have had a better engine. The Y1B-20 (follow-on design from the -15) had R-2600s.

[Hobbes]

I don't think any five-cylinder radials ever had the "two-row" (or three or four for that matter) treatment. Don't see why it wouldn't have worked though.

It would have worked, the reason to go for a 9-cylinder single-row instead of a 10-cylinder 2-row is weight and complexity: one crankcase, one offset in the crankshaft rather than two etc.

[wuzak]

I don't think any five-cylinder radials ever had the "two-row" (or three or four for that matter) treatment. Don't see why it wouldn't have worked though.

Here's one: The Pratt & Whitney R-2060.

http://www.enginehistory.org/P&W/Museum/Recips/R-2060_020.jpg
http://www.enginehistory.org/P&W/Museum/Recips/R-2060_017.jpg
http://en.wikipedia.org/wiki/R-2060

Radial, liquid cooled, 4 row, 5 banks, 20 cylinder.

[wuzak]

Which would have been easier to have been designed and worked, two V-1710's designed into a double V (as was done) or an X?

When the USAAC came to Allison looking for a 1600hp motor they proposed Allison build an X-3420 using as many parts from the V-1710 as possible. Allison came up with an engine which had each pair of banks at the same 60° angle as the V-12, and 90° between the inner two banks.

A wooden mockup was built, but no operating engine.

When a new manager joined Allison he restructured their programs so they could concentrate on the core product - the V-1710 - which meant that the X-3420 was dropped, but he offered the V-3420 in its place at 2000hp.

The X-3420 was expected to be restricted to 2400rpm, compared to 2600rpm+ of the V-1710 of the time.

In both the X-3420 and V-3420 the angle between each pair of cylinder banks was maintained at 60° to allow the re-use of all the intake plenums etc.

Coupling two 1710s is easier, because the only new part you need to design is the gearbox. For an X-engine you need a new block and crankshaft, and you need to redesign the bottom half to function upside-down

In the case of the X-3420 vs V-3420 the only different requirement was the design of a new crankshaft, master and slave rods. The V-3420 had a new crankcase, a new reduction gearbox, a new supercharger and accesories section and supercharger drive.

Undoubtedly the crankshaft and rod design would probably be beyond at the time. Allison were a small company with not much in the way of excess design and development capacity.

As I said before the crank/rod system was expected to be the limiting factor in the X-3420. It was in the Rolls-Royce Vulture, though it was failures, rather than speed.

For what it is worth, the V-3420 debuted at 2300hp @ 2950rpm (0.7hp/ci) and at about the same time the Vulture V (used in Hawker Tornado prototype) was rated at 1950hp @ 3200rpm from 2600ci (0.75hp/ci).

The Vulture used 4 equally spaced banks - and was very compact. It weighed more, however, but would have received a lighter (by about 200lbs) reduction gear had it continued.

[wuzak]

http://www.enginehistory.org/Gallery/JimBuckel/Allison%20V-3420%20for%20Fisher%20P-75A%20Eagle%201-18.JPG

[wuzak]

For comparison:

Vulture (from RRHT)

Length overall: 87.625in (2,226mm)
Width overall: 35.8in (909mm)
Height overall: 42.175in (1,071mm)
Weight: 2,450lb (1,111kg)

V-3420 (From Wiki)

Length: 100.0 in (2,540 mm)
Width: 56.0 in (1,422 mm)
Height: 34.0 in (864 mm)
Dry weight: 2,600 lb (1,180 kg)

The Vulture had approximately 20% less frontal area than the V-3420 - though they are slightly different shapes. The Vulture also was lighter, though the early V-3420s could be around 2,300lbs (1,043kg) they only had a single speed single stage supercharger drive whereas the Vulture had a 2 speed drive.  The Vulture would have received a new reduction gearbox had it continued, saving maybe 200lbs (90kg).

By the time of cancellation the Vulture had run to 2500hp on a number of occasions. Some report even as much as 3000hp - but RRHT could not confirm that.

Allison had done an X engine - albeit one designed by Wright Field.

The X-4520
http://www.enginehistory.org/Allison/X-4520/Allison_45201.jpg
http://www.enginehistory.org/Allison/X-4520/Allison_45202.jpg

That one may have side by side rods, rather than the master rod/slave arrangement that caused so many problems in the Vulture.

Rolls-Royce built an X-16 (Eagle XVI) at the same time as they built the F (Kestrel). It too had side by side rods, and the top cylinders offset from teh bottom ones.

[KJ_Lesnick]

When was the X-4520 built, and how much HP did it crank out?

[wuzak]

The first Allison was Manuf. No. 1, AAC S/N 25-521, the X-4520, a 12-cylinder air-cooled 4-bank "X" configured engine designed by the Army Air Corps and built by the Allison Engineering Company in 1925. The Air Corps serial identifies the Federal Government Fiscal Year in which the funds that purchased the engine were acquired, followed by the sequential number of the engine in the total number of engines purchased by the Army during that year. 25-521 says that this was the 521th engine purchased with FY 1925 funds.

http://www.enginehistory.org/allison.shtml


From Dan Whitney, Vees for Victory:

Power: 1200hp @ 1800rpm
Bore: 5.75" (146.1mm)
Stroke: 7.25" (184.2mm)
Capacity: 4518ci (74.0l)
Weight: 2800lb (1270kg)
Length: 108" (2743mm)
Width: 60" (1524mm)
Height: 53" (1346mm)

Compression Ratio: 4.9:1 (remember, the fuel wasn't so good)
Prop Reduction Ratio: 2:1

The X-4520 incorporated a "rotary induction" system - which looked very much like a centrifugal compressor, but was there to get even mixture distribution among all the cylinders, rather than compress the air for more power. The impeller was driven by 5:1 step up gears (so, prop was doing 900rpm, impeller 9000rpm).

Connecting rods were fork and blade type, the upper cylinders using one set, and the lower cylinders another set. The lower cylinders were offset to the rear relative to the upper cylinders.

The X-4520 was delivered in 1927, but was not tested until 1932 becuase neither the Air Corps or Allison had the facilities to test the power levels. In 1932 it recorded 1323hp @ 1900rpm, but testing was stopped quickly due to a piston "sticking". It was determined that the engine was unsuitable for use due to inadequate cooling.


Whitney also references a "X-Liberty" developed during WW1:

The Liberty X-24 was an expedient way for McCook Field to obtain a 750bhp engine during the middle of WW I. The War ended before teh engine was ready for flight, but following the war the development of the inverted portion of the crankcase and cylinders became the basis for the inverted Liberty 12.

[wuzak]

The first 1000hp rated engine was also an X, the Napier Cub