Napier Dagger: what would be the limts

[tomo pauk]

Napier's designs rarely catch any limelight, so let's talk about the Dagger and what should be it's upper limits.

 

[Shortround6]

In regards to?

It was a 1027cu in (16.8l) engine.

It's short stroke of 95mm allowed a piston speed of 2625fpm at 4200rpm.

It used a compression ratio of 7.5 to 1. with a 6lb max boost on 87 octane fuel. the small cylinders and high RPM help with this.

It does make 0.97 hp/cu. in. (59.5hp/L) already.

It has a dry weight of 1390lbs which is 70lbs more than a Wright R-1820G205 and 70lbs less than a P&W R-1830S3C4-G both of which have 2 speed superchargers.

It has a BMEP of 183 compared to the Merlin X (1130hp) 181 BMEP.

I am not sure where you can go with it. it's cooling was marginal already. Hereford units report only starting the engines for ground warm up with the air-scoops pointing into the wind and several engine failures due to the oil not being sufficiently warmed up through out the engine before parts of the engine were over heating in ground running. Old Sargent's tales or?????

A Bristol Pegasus was as streamlined as a barn but gave similar power for 1135lbs. and only had 36 valves for it's 9 cylinders compared to 48 valves in the Dagger and 18 spark plugs to 48.

 

[tomo pauk]

The 'upper limits' would be the limit for power output
Were the exhaust noise high pitch issues solvable (instalation of longer exhausts maybe)? There is too many 'maybes' and 'ifs' (like maybe they should've copied de Havilland's approach, or Isota Fashini's), but the air cooling of a long engine seem like an art of it's own.

 

[Shortround6]

With all air-cooled engines the engine is only as powerful as the Hottest cylinder because that is the one that will suffer detonation first. Getting equal airflow over a row of 6 cylinders was really tricky. baffling has to work at both climb speeds and all out level flight. Splitting the air flow into 6 equal parts as it turns 90 degrees is not easy.

The radials proved more adaptable to higher powers because of the angle between the cylinders allowed for increased finning on later models without changing the bore centers. Going from 6lbs boost to 12lbs boost means an increase of roughly 28% in fuel and air going through the engine for a 28% rise in heat needing to gotten rid of by the fins. you don't net 28% to the prop because of higher supercharger power needs and other factors.

 

[wuzak]

I wonder if Napiers considered doing a larger liquid cooled version, but still with poppet valves?

I think the capacity of it (and the Peregrine) were stumbling blocks to getting the power necessary for future needs.

I also wonder if blocks from existing engines could be used to make an H like it. If Rolls-Royce were to follow the concept they could have started with 4 Kestrel blocks, with twin cranks, but geared together.

Such a twin Kestrel, based on the XXX would give about 1440hp for about 2000lbs (no duplication of accesories and superchargers).

A twin Peregrine would be about 1700hp for similar weight.

As the Kestrel/Peregrine had the same bore as the Sabre, but a longer stroke I wonder what it could do with the Sabre's stroke. With that (giving 2238ci) and using the same BMEP and piston speed as the Peregrine I @ 885hp @ 3000rpm the H-Peregrine would give 1770hp @ 3475rpm.

Up that to Merlin XX piston speeds and BMEP (1480hp @ 3000rpm) you'll get 2540hp @ 3800rpm.

 

[Shortround6]

Ah, Wuzak, twin Kestrel/Peregrine? Isn't that a Vulture?

I know, twin cranks geared together vs common crankshaft. it eliminates the crankshaft problem but at the cost of of a bigger, heavier crankcase.

 

[wuzak]

Ah, Wuzak, twin Kestrel/Peregrine? Isn't that a Vulture?

I know, twin cranks geared together vs common crankshaft. it eliminates the crankshaft problem but at the cost of of a bigger, heavier crankcase.

And a bigger frontal area no doubt - definietly higher (or wider if arranged horizontally).

 

[yulzari]

I noted elsewhere that De Havilland did a study for the Air Ministry and concluded the only real cooling issue was a lack of exit capacity. I suspect this was on the Hector not Hereford but the Hector performed fine in towing laden Hotspur gliders about and that is a definite low air speed high power worst case situation. Professional Irish and semi-professional Auxiliary Air Force maintenance staff hated it's complexity which I suspect may have been why conscript bomber squadron staff had more trouble with it but the hereford installation might have been worse than the Hector one. As Formula 1 engine suppliers will tell you, poor installation gets publicly blamed upon the manufacturer.

It's greatest potential was probably that it was in production already so was a viable actual available choice.

Halford's aim was to progressively increase it's power by both raising the boost and by raising the rpm and it was exploiting rpm that was the raison d'etre for the Dagger.

 

[tomo pauk]

A twin Kestre/Peregrinel sounds like a baby V-3420. Now, the twin Merlin...oh, boy, would the 3000-3500 HP Typhoon/Temepst be a sight to behold back in 1943-45
A different layout, with independent engines, looks like a good choice for a carrier capable plane? Following the trail of P-24, or maybe DB's coupled engines.

 

[Shortround6]

I noted elsewhere that De Havilland did a study for the Air Ministry and concluded the only real cooling issue was a lack of exit capacity. I suspect this was on the Hector not Hereford but the Hector performed fine in towing laden Hotspur gliders about and that is a definite low air speed high power worst case situation. Professional Irish and semi-professional Auxiliary Air Force maintenance staff hated it's complexity which I suspect may have been why conscript bomber squadron staff had more trouble with it but the hereford installation might have been worse than the Hector one. As Formula 1 engine suppliers will tell you, poor installation gets publicly blamed upon the manufacturer.

It's greatest potential was probably that it was in production already so was a viable actual available choice.

Halford's aim was to progressively increase it's power by both raising the boost and by raising the rpm and it was exploiting rpm that was the raison d'etre for the Dagger.

Once again, a reminder that the problems with the Dagger VIII could be much different than the Dagger IIIM in the Hector. Not only did peak power go from 825hp to 1000hp, for a 21% increase in "net" heat rejection but the actual indicated horsepower (power developed in the cylinders) of the VIII was even higher. The RPM means higher internal friction and the higher boost supplied by the supercharger means more power to drive the supercharger, so the gross cooling load is higher than the net cooling load.
They may have tried increasing the finning on the VIII and/or tried something else to help the cooling but an installation that worked fine with the IIIM engine would be rather in adequate for the VIII.

 

[Readie]

Great thread chaps. I have learnt a great deal.
Keep the information coming.
John

 

[wuzak]

A twin Kestre/Peregrinel sounds like a baby V-3420. Now, the twin Merlin...oh, boy, would the 3000-3500 HP Typhoon/Temepst be a sight to behold back in 1943-45
A different layout, with independent engines, looks like a good choice for a carrier capable plane? Following the trail of P-24, or maybe DB's coupled engines.

Rolls-Royce did do a proposal of an H-24 Merlin, using the Merlin 61 as a basis. It was, like the P24, two complete and inependent engines on the one crankcase. It had two complete two stage superchargers (so a 4 stage 4 speed supercharger using the logic sometimes applied to the P24) with aftercoolers. It was heavy - heavier than two Merlin 61s, and soemwhere in the region of 3500-4000lbs (1588-1814kg). Even so, it was lighter and more powerful than the Eagle 22 H-24.