Why Inverted Engines?

[QUOTE=Pondskater;1829945]

I’m a little puzzled by the suggestion that an inverted engine moves the propellor higher.

Nice photo 🙂

Not so much that it moves the prop higher…more that for a given prop diameter then the engine mass is lower (enhancing viz) and/or the landing gear can be shorter (less drag etc)

rgds baz

I’m a little puzzled by the suggestion that an inverted engine moves the propellor higher. I recall seeing something about the thrust line of an aircraft (propellor position) being related closely to the line of drag. So wouldn’t inverting an engine leave the prop at the same height, it effectively rotates the engine around the crankshaft? Or have I remembered it all wrong?

Anyway, thought you might like to see this – an good argument for why inventing the inverted engine could improve visibility.

There can be no doubt that (engineering wise) it is much easier to mount an inline engine in the ‘upright’ posn.It means that you can retain the ‘wet’ oil sump and you are probably less liable to suffer from spark plug fouling

DH obviously thought it was a worthwile thing to do for the Tiger Moth

It’s certainly an interesting question.

Some good posts here – thanks to all who have contributed. It seems to come down to questions of visibility, propellor clearance and weight distribution. Perhaps it was also a case of certain designers and engineers successfully running an inverted engine in their aircraft, and building on that success (or re-cycling the engineering that went into it) when it came to producing their next model. Why change something that worked first time around…
Or also a fashion for doing something a certain way at a particular time…

I think there’s a problem with the term ‘inverted’ implying that the other way up is correct, or of primary choice. (It’s a “leading question“, in a sense) Certainly that is generally the case, but it is notable that there have been a huge range of aero-engine layouts, the majority of which are a long way from the ‘conventional’ upright land-vehicle internal combustion engine where the original ‘conventional layout’ concept comes from – you need only look at the flat four and six, or radials, let alone more esoteric things like the rotary configuration, diesels, steam, or rockets, jets and turbines.

There are a number of aspects that are a basic with piston aero engines that show the origins in land-engines is well at a distance; twin magnetos as standard for instance.

For instance about 1916, the rotary was unarguably the best fighter (scout) engine layout; yet today it’s regarded as an odd (implying unsound) idea to the vast majority of the population.

Someone tried a four cylinder radial – didn’t work well, and modern theory states such approaches are doomed; but someone will always try it, and very occasionally they’re right.

The history of aero engine development from the very start is trying anything that may give a marginal advantage that may then lead to a greater, long term advantage. In a different world, the inverted layout may have delivered results it’s early adherents hoped for, and we would be asking why the odd ‘upright’ system was bothered with at all.

When you couple the experimental approach resulting in everything with the need for development to take a marginal opportunity to delivering a performance advantage, you can see why people persisted with systems beyond their best utility – tomorrow’s engine might deliver a big leap (the history of the aero rotary being a good example of experimentation, to performance advantage to over-use beyond that advantage when the negatives start).

Following that stage of experimentation would be the mature approach – the persistence of a configuration because that’s what we’ve got or that’s what we do – the “We’re here because we’re here because we’re here…” case.

I would add that in the case of aerobatic and high-performance combat aircraft, there was an acceptance of the shortcomings of a ‘conventional upright’ engine that should have been questioned earlier than it was; specifically poor performance in inverted or negative g flight. Let’s remember that while the reason that the Messerschmitts famously could dive away from the Hurricanes and Spitfires wasn’t because the DB engine was inverted, but by being an inverted design, many other assumptions – float carburettor etc. were also questioned and addressed.

Just some, I hope, different thoughts on the question.

Regards,

Wouldn’t rocker covers and gaskets (with a smear of stag compound) take care of that issue a little better than inverting the whole engine? 🙂

I did mean to reply to this post but forgot !
My last posting in the RAF was to UEF (university engineering flight) at Abingdon,we had 14 Bulldogs for London + Oxford UAS’s but we also operated 7 Chipmunks for 6AEF.
I am not saying that the Dripsy Majors were oily but we used to leave the Chippies out for as long as possible when they finished flying for the day,they then dumped gallons of dirty OMD370 over our Apron rather than ‘B’ Hangar’s floor.

I cannot believe that Cirruses and Upright Dripsy’s were any more oil tight and most of them were at least partially uncowled hence my comment about oily windscreen.

rgds baz

Am now home amongst my books 🙂

From Bramson/Birch ‘Tiger Moth Story’
1982 Airlife edition on P 28

Has a short piece on the genesis of the Inverted version of the ‘Dripsy’ engine…

The idea of turning the Gipsy engine upside down emanated from the desire to improve the pilots view ahead.
With an upright engine ,such as the Cirrus or Gipsy 1,the cylinders and valve gear tend to obscure the pilots vision along the nose,and although the engine position was lowered in some later (pre tiger) Moths there is a limit to how much this can be done.Because the Propeller must have adequate ground clearance with the tail up.
The inverted engine provided the practical answer to all 3 problems…Improved view for pilot,adequate ground clearance for prop and a thrust line in the correct posn for stability

In a lowish-powered fixed-gear plane you could fit shorter undercarriage legs to the aircraft by inverting the engine and so mounting the prop higher.
This translates to less drag and a few knots more on the ASI.

Or in a higher powered aircraft you could bend the wing to suit, hence the shorter legs on the crank winged Corsair 😀

Some good posts here – thanks to all who have contributed. It seems to come down to questions of visibility, propellor clearance and weight distribution.

Personally I’m not swayed by the weight distribution point at all, there are too many other exceptions for that to hold true for me.

The increased prop clearance given by inverted inline engines on smaller aircraft, and thus allowing for shorter, lighter, and more sturdy undercarriages, and the easier maintenance issues on less well equiped GA arifields, makes perfect sense.

With German V engines,
Visibilty, slight advantages on some single engine fighters yes.
Prop clearance, again slight advantage on some types (BF110 maybe?), but this can be achieved quite effectively with the output gearbox and prop shaft postioning, irrespective of engine positioning.
Ease of maintenance, none.
Really it seems to me that it was just the way the Germans did things with their V engines, the FW190D and TA152 series, and HE177 in particular really doesn’t offer up any other explanation?

As mentioned above it is quite possible that the Germans chose the inverted V route on the basis of thinking that was what RR was going to do after seeing a mock up some time.

Despite popular myth that the DB was a superior engine it in fact was a strange mix of advanced tech – direct into cylinder fuel injection – and out of date design such as needle roller bearings on the big-ends. The DB605 in the 109G was 35 litres, and comparable in weight and size to the Griffon of 37 litres, rather than the 27 litres of the Merlin. The Jumo 211 and 213 especially were more advanced engines, both being 35 litres.

Truth be told the DB engines would have been better off being upright and not inverted. Amongst the many issue with the engine, was a weakness with the big-ends, and aeration of the oil causing big-end failure, a rod out the side of the block, and fire feed by hot oil. Quite a number of Luftwaffe pilots were lost to big-end failure, and resultant fire, and it has continued to this day in the DB605 powered Bouchons, fortunately without loss. Also the excessive oil splash from the big-ends was responsible for the asymmetric compression ratios. More oil would reach one bank due to the rotation of the crank and this would be enough to alter the detonation limit requiring a slightly lower compression ratio.

The direct fuel injection was also as much of a disadvantage as an advantage, in that the engines lost much benefit from the lowering of intake temperature buy having the fuel injected into the eye of the supercharger. This was why water injection was used to boost power below the rated altitude of the engine. The other method of ensuring no detonation was to make the mixture very rich. This lead to a trail of black smoke and many a combat report contains claims for damage or loss on this basis when in fact no damage was done. The direct fuel injection was not as efficient as the injection type of carburettor used in Allied aircraft reducing the range of the aircraft.

In a lowish-powered fixed-gear plane you could fit shorter undercarriage legs to the aircraft by inverting the engine and so mounting the prop higher.
This translates to less drag and a few knots more on the ASI.

Some good posts here – thanks to all who have contributed. It seems to come down to questions of visibility, propellor clearance and weight distribution. Perhaps it was also a case of certain designers and engineers successfully running an inverted engine in their aircraft, and building on that success (or re-cycling the engineering that went into it) when it came to producing their next model. Why change something that worked first time around…
Or also a fashion for doing something a certain way at a particular time…

Why Inverted Engines?

Changing tack ever so slightly, two Lockheed aircraft, one high wing and the other low wing with the same basic engine & reduction gearbox but one is inverted. Yes the Hercules and Electra/Orion. Reasons for the fit in these cases, IMHO, are self-evident.:confused::)
Bob

I’m by no means an expert, but from an anecdotal point of view I’d always thought inverted engines were more hassle. You have to pre-flight the engine carefully to avoid hydraulic lock (a la radial) and you have to be careful about not fouling the plugs when the engine is at lower RPM’s (eg. when taxying). I have heard it suggested that part of the reason for the large number of Bf109 undercart collapses was due to pilots having to keep the revs up to keep the plugs clear, therefore leading to some fairly brisk taxi speeds and higher risk of collaps under sideloads and over bumps etc.

Looking at the DB 605 engined Spit with its lower thrust line, it presumably had a smaller diameter prop than the Merlin version. So, did the DB 605 prop turn at a faster RPM than the Merlin?

Mention has also been made of the low thrustline of the Ju 87, was this the reason it had a cranked wing? Remember the FU4 Corsair with its big prop needed a cranked wing to keep it undercarriage short (and strong for carrier landings).

The DB Spitfire had the cowling of the Bf 110, air scoop of a Bf 109 G, redesigned engine support (reworked and fitted to the Spitfire bulkhead), the DB 605 A configured as a standard Bf 109 G series one, with standard G-series propinstallation and standard 109 G-series propblades of 3 metres diameter.
As they wanted to find out the difference in performance, I doubt they changed anything regarding the engine but the enginemount (obviously necessary), and the complete electrical system and instruments, as the German ones worked on 24 Volts, British ones on 12 Volts. I read, changing the electrical system was the main time consuming work necessary. Max. continuous rpm was 2.300, take-off rpm 2.800.

Michael

Looking at the DB 605 engined Spit with its lower thrust line, it presumably had a smaller diameter prop than the Merlin version. So, did the DB 605 prop turn at a faster RPM than the Merlin?

Particularly the DB engined Spitfire but DB props in general always seem to me to be much wider in chord than a DH/Rotol prop. I have absolutely no idea about prop aerodynamics but could it be to transmit the power with a smaller diameter?

Later Mosquitos (and Lancasters?) had paddle blade propellors. Were they correspondingly smaller diameter or was it a more powerful Merlin variant? I’ve never noticed an obvious difference in photos or read it anywhere.

Primarily the cranked wing allows greater clearance for larger external stores to be slung beneath the fuselage, while keeping the undercarriage at a reasonable length, not so much to keep fuselage diameter down (ie Stuka could have been mid winged, like Barracuda, Avenger etc), however one sideline benefit is being able to have the prop centre line lower in the nose.

The cranked wing also allowed a low-drag 90 deg join between the wing and the fuselage, permitting a smaller diameter fuselage and simpler external shape at the join. This was all part of the package.

Looking at the DB 605 engined Spit with its lower thrust line, it presumably had a smaller diameter prop than the Merlin version. So, did the DB 605 prop turn at a faster RPM than the Merlin?

Mention has also been made of the low thrustline of the Ju 87, was this the reason it had a cranked wing? Remember the FU4 Corsair with its big prop needed a cranked wing to keep it undercarriage short (and strong for carrier landings).

I hope you haven’t shown it to children, or people of a nervous disposition?

Moggy

I have the original Barraccuda artwork used on the Frog model box 😀