Inverted V engines

[Vlad]

As a bit of a 109 nut I'm obviously fascinated with the inverted engine as well. I've read around and I can't find a definitive reason behind the unusual setup, but that's not what this thread is about.

 

It does seem there is a bucket list of little advantages it offers (better visibility, access, lower CoG etc.) without corresponding disadvantages, or counterpart advantages for the "right way up" setup. It just got me wondering, if it was handy for several (seemingly, to me, quite logical) reasons, why was it not done more?

[Graham Boak]

I suspect one reason is that designers who have experience with the ways of the upright V would prefer to continue developing something that they know how to deal with than something offering its own range of foibles of which they were unaware or at least inexperienced.  I do know that RR considered an inverted V at some stage in the Merlin pre-design stage and ruled it out: it would seem unlikely that they did it without good reasons.

 

However, just to address some of the ones you mention. 

There would seem to be no great advantage in a slightly lower cg when fitted to an aircraft. 

Given that most in-place servicing/tuning would be done working on the cylinder heads and sides, these would be easier to deal with standing up and working down, than sitting on your bottom trying to work upwards with your hands above your head. 

Having the various liquids dripping down into your eyes would seem to be a fairly good disadvantage.  For aircraft of this period, oil leaks were normal.

It isn't clear to me that (for example) the Bf109 had a superior view forward than the Hurricane.  Both had no vision directly ahead and required weaving when taxying.  For a multi-engine bomber the matter is irrelevant.

 

So the question perhaps should be turned onto its head.  if the inverted Vee offered any significant advantages, why was it only pursued in one nation whereas all other major engine producers preferred uprights?

[Vlad]

I guess you can make the same argument about technological inertia, and maybe inverted was a terrible idea but the Germans kept doing it because it was what they knew best. 😁

 

For access, the perceived advantage would be not needing a ladder to get to the cylinder heads, although you raise very good points about upward working. But to turn it around again, if it really would have caused a headache for mechanics to the extent you imply, did the Luftwaffe simply not care?

 

For view, I guess no fighter with a big V12 in front of the cockpit can be described as having a "good" view forward, but the Hurricane is a bit of an unfair comparison. It's a larger aircraft with a less well optimised fuselage cross-section, which allows for that slope of the top cowling. For say, Spitfire vs 109, there does seem to me to be a small advantage to the narrower and more sloped top cowl of the inverted setup, especially for view out of the forward quarter panels. Where a Spit pilot would be seeing a big flat cowling in the bottom half of his sight picture, or exhaust stubs and wide cylinder head covers to the sides, a 109 pilot has a few extra degrees of clear view over and around the nose. Definitely not enough to help taxiing, but maybe useful on an oblique target approach, or a deflection shot? It's also possible I'm making too much if a trivial difference here. 

[Graham Boak]

The Hurricane was not optimised for drag perhaps, but was deliberately optimised for the benefits of forward view.  I feel I'd have gone the other way myself, but that's relying upon hindsight.

 

Clearly both DB and Junkers believed that there as some advantage, previous German V engines having been upright (BMW).  

 

Looking postwar, we can't say anything about aircraft engines, but I can't think of a racing car engine that was an inverted V, yet a lower cg would have advantages there.  Even DB went to a straight eight in 1954.  However perhaps the problems of working on an inverted engine would have been more telling in such a case.

 

Re servicing, when it comes to a bomber you need step ladders anyway, and not all inverted Vees went into fighters.  Much of the normal engine maintenance on a Bf109 would have been at shoulder level, and as such not a great disadvantage if at all.  However, how high off the ground is an Fw190D's engine?  And in the end, how important is maintenance access considered anyway?  I can only think of the MB5 as a fighter designed for ease of maintenance, and the comments on it make quite clear how unique this was.

 

 

[Toryu]

The main reason obviously was armament. Upper fuselage machine guns and through-the-hub cannons work much better with inverted V engines because there is more room to stow them. When the Jumo 211 and the DB 601 were developed nose armament was the primary fighter weaponry.

[Vlad]

I always thought the 109 was designed with maintenance in mind to an extent (e.g. landing gear attachment allowing for removable wings) and I must have read somewhere that unaided cylinder head access via the inverted engine was part of this concept, although how true this is I don't know.

 

You are right that any potential advantages tend to evaporate once the whole aircraft has grown significantly larger i.e. the Dora or Ta-152.

 

I'd love to hear opinions regarding forward view, beyond the trivially obvious "it was bad for taxiing", from people who might have been lucky enough to sit in these aircraft for real (even statically). My own opinion on this is formed via flight simulators in virtual reality, which is about as good as it gets without access to the real thing.

 

@Toryu this makes sense, but the Soviets managed several designs with a hub cannon and/or top cowling armament on upright V engines. So did a few others, notably early P-40s. I suppose again, the 109 was a much smaller and tighter design so the optimisation mattered more?

[Toryu]

One afterthought: There was not much that needed working upwards on an inverted V. For instance, the spark plugs, which needed frequent changes, are on the outside of the cylinder bank, both on the Merlin and on the DB. This means that the British fitters had to work 'upwards' while the Germans stood on a low stool and worked downwards!

[FinnAndersen]

I know that the question was about inverted V engines, but much  of the comment applies also to the single row inverted engines, of which the British used quite a lot.

 

So the question could be shifted to "Why would you want to make the engine inverted at all?"

 

I remember flying with such engines (in a D.H. Dove) where the joke always were that you checked the petrol and topped up the oil. Boy, they leaked.

 

/Finn

 

 

[Corsairfoxfouruncle]

Sadly all engines of that era leaked. 20 years ago i did some volunteering at an aviation museum here in Chicago. We had multiple types from WW2 and later, P-51, TBM, B-25J, and a Sea Fury among others. One day while giving a tour a woman asked me why there were big sheet metal tray’s underneath all the planes ? I explained that most reciprocating engines wether Radial or V-12 would leak a little. However i half jokingly said that the trays were there to tell us when the engines were empty. If we didnt see any oil we knew it was time to refill them.  The look she had on her face was worth it all. 

[Graham Boak]

I've heard the same comment about Phantom integral wing tanks.  If they weren't leaking they were empty.

 

Given that aircraft engines of the period were not specifically designed for fighters but for all types of aircraft, any advantages of view or armament placing were purely fortuitous.

 

I feel that the reason for light aircraft moving from upright in-lines such as the Cirrus to inverted types such as the Gipsy were to gain the benefits of a higher thrust line, hence better ground clearance, with better forward view and (I suspect) less oil being thrown back at the occupants.  A smaller undercarriage could also have been a gain, and the exhaust was better placed (and probably shorter).

[Vlad]

The thrust line point is interesting because once you get bigger geared engines the relationship is reversed, with inverted having a lower thrust line.

 

Is this a possible advantage in other ways though? A lower thrust line should mean less downward pitch moment from the thrust, and less trim change with varying power settings.

[Toryu]

1 hour ago, Graham Boak said:

Given that aircraft engines of the period were not specifically designed for fighters but for all types of aircraft, any advantages of view or armament placing were purely fortuitous.

Nearly all high performance engines of the pre-jet age were developed with a view to fighters, and even the early jet engines.

[Graham Boak]

That needs strongly qualifying.  For a long time after WW1 it was considered that fighters should be light and agile, which ruled out the larger engines of the day such as the Lion and Buzzard.  When smaller more powerful inlines did appear, the Curtiss D-12 was in the Fox bomber, and the Kestrel In the Hart.  Even the Merlin first saw service in the Battle.  The two-stage 60 series was intended  for the high altitude Wellington, with the Spitfire fit very much an afterthought.  In Germany the DB 600 series appeared on the He111 before any fighter.  Thinking that they might be used in fighters is one thing, designing them with that as a preference is another.

 

It may be that the Hispanic Suiza designed for the engine cannon was an exception, but remember that this was a Hispanic Suiza cannon, and still an upright V layout.

[Tail-Dragon]

Here's a thought, on a conventional V-type engine, once the reduction gear is attached to the front of the engine, it makes the thrust line much higher. That allows the engine to be mounted much lower, less bulk obscuring forward vision.

One other thought - on a radial engine, one of the problems is hydraulic lock,. When the engine sits, oil can seep past the rings in the lower cylinders and pool in the combustion chamber. This can cause damage when the engine is cranked on starting (it can blow the cylinder clean off in worst cases!) This is why radials are pulled, or cranked through before starting. In an inverted V-12, you could multiply this problem by 12!

[Jamie @ Sovereign Hobbies]

Something which affected radials that any inverted cylinders could be prone to is hydraulic locking the cylinders due to oil leakage into the combustion chamber.

 

That's why radials are turned over by hand or on the starter motor with fuel and mags off to clear the lower cylinders out.

 

In addition, priming to start can, depending on inlet valve port and manifold geometry, be simplified if fuel can gravity feed down the valve port.

 

Oil scavenging (these are all dry sump engines, of course) is very straight forward with an erect V as you still have a convenient pan to collect crankcase oil and allow the scavenge pump to pump it back to the reservoir.

[dogsbody]

On ‎1‎/‎22‎/‎2019 at 9:41 AM, Vlad said:

 

@Toryu this makes sense, but the Soviets managed several designs with a hub cannon and/or top cowling armament on upright V engines. So did a few others, notably early P-40s. I suppose again, the 109 was a much smaller and tighter design so the optimisation mattered more?

 

Most of the Soviet upright inline engines were based on Hispano engines that has been imported from France before WW2.

 

 

Chris

 

[Work In Progress]

14 hours ago, FinnAndersen said:

I remember flying with such engines (in a D.H. Dove) where the joke always were that you checked the petrol and topped up the oil. Boy, they leaked.

 

/Finn

It's not just what seeps out, they can easily burn a litre an hour

[Toryu]

OK - so, where do we stand now? Most suggestions why inverted Vs were chosen have been turned down, and a lot of reasons have been found why it was not a good choice. The original question, however, has not been answered: Why did the German aero engine manufacturers go for the inverted lay-out? After all the Jumos and DBs were excellent engines that served in very adverse climates. And their predecessor V-12 was an upright construction (BMW VI). Hence the decision was neither due to nostalgia nor ignorance.

It would be great if a technically competent reader could enlighten us.

[Procopius]

Some interesting information pertaining to this matter is here: https://ww2aircraft.net/forum/threads/inverted-v-engine-vs-v-engine.29473/post-797148

[Vlad]

50 minutes ago, Toryu said:

The original question, however, has not been answered: Why did the German aero engine manufacturers go for the inverted lay-out?

 

Actually, that was not exactly my question. I was simply after more information, and several disadvantages were listed, which all make sense, so overall this has been a very productive thread even though we may never know the real, definitive answer to the German practice.

 

I don't think the advantages have been categorically dismissed, only shown to be perhaps situational or trivial. That doesn't mean the designers at Junkers and DB in the 1930s saw them that way as well (or perhaps they saw others we don't know about).

 

And none of the identified problems are insurmountable, as you say these engines were very successful.

 

So where we stand I think is possibly a case of perceived advantages leading down one path and technological inertia doing the rest once the initial reason was either lost or became less relevant. The main thing I take away from this is that the advantages or disadvantages of either upright or inverted were not in themselves great enough to warrant a complete upheaval of established technology, design and fitting practices.

[krow113]

Visibility , armament , and fuel injection capability were the factors . 

[Work In Progress]

Fuel injection makes no difference either way to the practicability of an engine being inverted or upright. All the hours I've flown propelled by inverted engines (e.g. Blackburn Cirrus Minor in Austers, Gipsy Majors in Moths and Chipmunks, Gipsy Queens in Rapides, Renault PO.4 in Stampes, and Walters in Zlins), those engines have been carbureted. 

 

I definitely buy the field-of-view point when it comes to V-12s, though.  I would be exaggerating if I said it means the difference between not being able to see the runway in the three-point configuration, and not being able to see the the whole airfield. But not by as much as you might think. The view ahead out of a tail-down Buchon is MUCH worse than the view out of a DB-engined 109 (disclaimer: never flown either, but you only need to sit in them to see that).

 

Even without the V configuration, just looking at in-line fours and sixes, an inverted engine substantially improves the forward view. You can see a lot more of interest and importance out the front of an inverted-engine Moth than one of the earlier upright-engine variants.

Edited January 23, 2019 by Work In Progress

[krow113]

Fuel injection then as compared to now!?

You cant be serious. 

The info came to light as I researched the engines.

[bentwaters81tfw]

The main purpose of inverting the engine is to alter the thrust line. This was part of my syllabus in engines and airframes for my PPL

It allows for airframe development, the visibility and the ability to arrange weapons around the engine is secondary. As aero engines are designed to run 'dry sump' the oil scavenge system was extensively developed, and of course the Germans utilising fuel injection systems were also a further development.

[Jure Miljevic]

Hello

I would say both V and A configurations, as it had (has?) been known in German language speaking countries, have their merits and shortcomings. Germans stuck with inverted V, yes, but as one of the posters on the page Procopius linked to said, this configuration was forced upon designers from above. It was similar during the WWI when IdFlieg, aiming for higher degree of standardization, from 1914 to late 1916 ˝encouraged˝ production and development of six cylinder in-line engines at the expense of every other configuration. Rapp produced their own 146 h.p. Rp.IV, but these eight cylinder V-engines were all exported to Austro-Hungary, as well as a handful of license built D-35 V-engines, designed originally by Ferdinand Porsche from Austro-Daimler. By 1917 some German companies started to develop V-engines similar to Hispano-Suiza V-8 but I believe only Benz Bz.IIIb (8 cylinders) and Bz.VI (12 cylinders) had been produced before armistice. The former had been installed in five aircraft of three different types and I believe the latter was intended for airships anyway. Rapp became BMW in mid-1917 and concentrated on over-sized, over-compressed upright six cylinder in-line engines BMW IIIa and BMW IV. I have no idea if BMW V even existed but, as Graham and Toryu pointed out, BMW VI was another V-engine. If there was technological inertia behind inverted V-engines in Germany, it is difficult to see what it had been based on.

BTW, how about Napier Sabre and RR Vulture? Could those two be called semi-inverted engines? Cheers

Jure