Inverted V engines

[Work In Progress]

10 hours ago, krow113 said:

Fuel injection then as compared to now!?

You cant be serious. 

The info came to light as I researched the engines.

My information came to light over a number of decades of actually operating 1930s and 1940s designed and built inverted, upright, radial and horizontally-opposed aero engines. Most of them carbureted, a few fuel injected.

Why do you think a carburettor knows or cares whether the engine to which it is attached has upright or inverted cylinders?

 

[Work In Progress]

7 hours ago, Jure Miljevic said:

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

Jure

Not the Sablre, at least not in the usual Tempest / Typhoon installation. in that installation all the cylinders are horizontal, none of them points downwards.  It is referred to as an H configuration, but the H is on its side. I am not aware of any proposed or experimental installations with the engine rotated 90 degrees into an upright H, though there may be one that I've not thought of.

 

The X-configuration Vulture can be regarded as a combination of upright and inverted V-12 arrangements in terms of basic geometry, so it has some of the issues of inverted engines, such as having to be very careful before starting that you've eliminated pooled oil from the downward-pointing cylinders.

 

Radials are of course semi-inverted too, if you want to think of them that way, in that however you orientate them at least a hefty proportion of the cylinders point down. Unless it's in a non-aeroplane application like a helicopter, or a tank, in which case it might be at a completely different angle e.g.flat on the floor.

[Graham Boak]

You could perhaps claim the Vulture as Semi-inverted engine, but as it was basically two Kestrels then it was a modified design rather than one designed as such from the beginning.  The Sabre was a horizontally aligned H engine so irrelevant, but the earlier vertically aligned Rapier and Dagger were half inverted.

 

A lowered thrust line may make sense in an ungeared engine, although I'm not entirely clear why this should automatically  be true, but on a geared engine it is the gearing that produces the thrust line.  A lowered thrust line introduces a penalty in propellor diameter, and/or undercarriage length, so again I don't understand why this aircraft design feature should be driving engine design.  I feel that there is some engineering feature, specific to the engine, that inspired this, but no-one has suggested any yet.

Edited January 24, 2019 by Graham Boak

[Vlad]

43 minutes ago, Graham Boak said:

A lowered thrust line introduces a penalty in propellor diameter, and/or undercarriage length, so again I don't understand why this aircraft design feature should be driving engine design.

 

10 hours ago, bentwaters81tfw said:

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, 

 

22 hours ago, Vlad said:

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.

 

To the pilots out there, is my assessment about trim incorrect? Is this a valid aerodynamic justification for a lower thrust line? Or more precisely, a thrust line not above the CoG, and closer to the wing-line in a low-wing fighter.

[Jure Miljevic]

Hello

Thanks, I forgot Sabre is a horizontal H engine. Given it has no sump Vulture would perhaps be more of an inverted V than upright V? Otherwise, perhaps German decision to produce inverted V-engines had not been based on single major benefit this configuration has but on several minor advantages? Cheers

Jure

[Graham Boak]

A lower thrust line would indeed produce a reduction in trim change with power in low-wing aircraft, but not all aircraft are single-engined fighters with a low wing: not even in WW2, and especially not when these engines were being conceived in the early 1930s.  For most types with low wings used for a modern PPL, this is true, but what about the Cessna family?  Or indeed earlier light aircraft families such as the Miles, Percival or Auster lines, all of which benefit from a high thrust line despite an inverted engine.  (If you think I'm being parochial, how about Caudron or Bucker?)  To be honest, I don't think that minimising trim change was a particularly important factor in aircraft designs of the 1930s.  The pilot was expected to cope.  An example of the lack or interest in such features is the awkward position of the trim wheel in the Blenheim, behind the pilot's seat.  (Although with mid-mounted engines presumably the trim change with power was not a major feature anyway.)

 

Aircraft are designed to make use of the engines available, because engines have a much longer development time than airframes (at least until 4th generation jet fighters...).  Engines were designed to be fitted in as wide a range of aircraft roles as possible.   Any trend in engine design was therefore driven by consideration of thermodynamic, mechanical or other factors that affect the engine in isolation.  During the period we are discussing, high-powered engines were seen on fighter, bombers and transports.  Not all such designs made it off the drawing board, of course.  This was whether they were V, inverted V or radial;  low wing, high wing or mid wing.   The aircraft configuration was no doubt chosen with due consideration of the given engine configuration, and modified appropriately if required - or considered important.  It is only one factor in a number of (often competing) factors that go into the design for any given requirement.   Agreed that engine availability was one of the more important factors in national design trends, but we do not see any desire for high wing fighters with a high thrust line V engine,  although this would be predicted had the reasons given above for an inverted V been important.  An extreme case, but indicative of the need to consider all aspects of a design.

 

It seems sensible to me that the German approach will have originally been based on a number of perceived advantages rather than one large (and now invisible) one, but that doesn't mean that they were necessarily right to prefer this route in the first place.  Clearly it worked, but then so did upright V engines.  Upright Vs have continued to be designed in preference to inverted ones, and whereas this could have been covered by conservative custom and practice up to the 1950s, it's been a long time without any signs of a revival.  That's a bit of a hint that it was an interesting technical diversion of some value but not fully convincing.  

 

Perhaps somewhere in the RR Historical Trust there is a document explaining why they considered inverted engines, and why they did not take this route.

Edited January 24, 2019 by Graham Boak

[krow113]

lol

too funny

[Jure Miljevic]

Graham,

actually Polish fighter prototypes PZL P.1 and P.8 were both of high wing configuration with Hispano-Suiza V-engines and low thrust lines. Furthermore, both Focke-Wulf Fw 56 and Arado Ar 76 parasol light fighters/advanced trainers had Argus eight cylinder inverted V-engines with middle thrust lines. Although I agree with you that in general thrust line was of secondary consideration, one exception to the rule was another parasol Focke-Wulf Fw 159, one of fighter prototypes in German 1936 fighter competition.

Here is the photo of the second prototype Fw 159 V2 in flight, found here. Fw 159 had been powered by JuMo 210 G inverted V-engine which apparently had been installed as high as possible. Fw 159 ended distant third behind the Bf 109 and the He 112 and is today practically forgotten. Cheers

Jure

[Jamie @ Sovereign Hobbies]

On 1/24/2019 at 10:29 AM, Vlad said:

 

 

 

To the pilots out there, is my assessment about trim incorrect? Is this a valid aerodynamic justification for a lower thrust line? Or more precisely, a thrust line not above the CoG, and closer to the wing-line in a low-wing fighter.

 

Everything I have flown (conventional high and low wing monoplanes) has pitched up with increased power; a standard and desirable design feature of being in-trim in a positively stable aircraft. So long as the thrust line is half-way sensible, then a stable aircraft is trimmed to fly at a constant speed rather than a constant attitude so if trimmed to fly straight and level at any given airspeed, adding extra power will see the nose rise, the aircraft begin to climb and the airspeed stay the same. Depending on exactly how stable it may porpoise a little until it resettles at the same airspeed, but if left alone and not retrimmed then a stable aircraft will resettle at the same airspeed and in a steady climb. The opposite is true for reducing power.

 

The only aircraft I've ever heard pitching down with increased power have been those with extreme thrust lines compared to the centroid of net drag; e.g. the Supermarine Walrus. I've never even heard of it being remarked upon in relation to the PBY, whilst I've heard they do require care in other respects.

 

A lower thrust line will increase the pitch-up experienced under power, or rather, will make the aircraft more reactive to power changes. Whilst everything above remains true, a high(er) thrust line will somewhat counteract the aircraft's desire to pitch up with power increases. It will stabilise into a climb at the same speed, but will be slower in nosing up and will need to gain a few kts airspeed first. A low thrust line will encourage a more immediate pitch change.

 

As Graham Boak says though, on geared engines this is all fairly moot because the reduction gearing configuration can put the thrust line wherever you want it; vis Allison V-1710 variants:

 

 

 

 

[Work In Progress]

23 hours ago, Vlad said:

 

 

 

To the pilots out there, is my assessment about trim incorrect? Is this a valid aerodynamic justification for a lower thrust line? Or more precisely, a thrust line not above the CoG, and closer to the wing-line in a low-wing fighter.

 

When you are talking about the small difference between an upright or inverted V12 in a 109 or Spitfire, that particular effect does of course exist but it so small as to be immaterial alongside more important considerations.

It makes a material difference on something where the engine is a long way above the drag, and then you really have to fudge it by changing the angle of the thrustline. The Walrus is a great example of this, and the best visual example I can think of, because it's so extreme, is the Savoia S.55 . Pic borrowed from JWM's contribution to the S.55 thread over on Rumourmonger where there is news of the type being kitted by Dora, which is delightful news