P!mp my Blenheim

[Shortround6]

Manual for the MK V says max dive angle for using the 500lb bomb is 55 degrees.
You are up against the weird British system of holding the bomb bay doors closed with elastic cords and depending on the bomb's weight to overcome the cord tension and push the doors open.

Perhaps not a deal breaker in itself but unless you really change the bomb bay and the doors you are limited to a pair of 500lb bombs which are not exactly either ship or bridge killers. Not like a JU 87 or 88 with 1100lb bombs or larger.

Bolt the doors closed and use external bomb carrier/s?

British could have gotten a lot more mileage out of the Blenheim if they used it for some of the roles it was more suited for, The MK IV was intended as a maritime recon version, Pushed aside by the Botha and Beaufort. Maybe not as good as they were in theory (no 4th creman) but better than the Anson.

If you are going to build a fighter Blenheim then build a fighter Blenheim, give it a gun pack that doesn't look like it was made by home handyman out of plywood using a hand held circular saw. ( the ability to tip down for loading and servicing the guns was rather neat but come on, they built hundreds of these things and never improved the aerodynamics)
Get rid of the turret on the fighters, clip wings? fit constant speed props? improve fit and finish?
You might have been able to add 15-20mph to the fighter versions and while still in trouble against 109s or 110s the bombers would have a much harder time getting away from them.
Stick 2-4 guns in the nose of the night fighters in addition to a streamline belly pack?
add 1-3 guns in the wing?
Get the Blenheim fighter up to a 6-8 gun fighter instead of a 5 gun fighter?

 

[Admiral Beez]

Stick some guns underwing that will lob a shell through a panzer 1 to 3. Yes a hs129. Fast and manouvrable it will kill the german 3 rd armee in its tracks. No Duinkerken, no breakthrough un attended.

I like the ideal of a Blenheim CAS aircraft. Better engines, bathtub armour, a pair of hull-mounted belt fed, high ammunition capacity 20mm cannons plus underwing bombs that will peel any German tank. Or drum fed gun pods, leaving the bomb-bay for ordnance. Can we operate it with a single crewman? Rear gunner won't hit anything anyway and CAS assumes air superiority - but do we need navigator?

 

[Admiral Beez]

British could have gotten a lot more mileage out of the Blenheim if they used it for some of the roles it was more suited for,

Carrier ops? If the 56.25 ft width can be folded down to <33 ft, the Blenheim's 42.5 ft length will fit on Indomitable's forward lift and the Implacables'. If that's too ambitious, the Outrageous' lift dims of 47-by-46-foot should be doable. Now, if we can fit it on a carrier, and take the place of some fighters and Stringbags, can we find its purpose?

Perhaps carrier capability is flight deck only, like the Potez 565. The Blenheim Mk II was optimised for long range reconnaissance - two or three of those folded-up onboard with the rest of the FAA squadron ashore at the closest RNAS may be useful. Strip off the dorsal turret, keep the three crewmen to support observations, communication and navigation, fill the bomb bay with fuel cells, but keep the weight down for deck ops.

 

[Shortround6]

It's purpose was light bomber, maritime reconnaissance/anti-sub/strike (although two versions may be needed) interim night fighter. long range recon, and a few other roles.

Trying to design folding wings, and other oddball major changes is really pushing it.

Unless you can pull a real miracle the Mercury engine was as good as it was going to get. The British had set up at least one shadow factory to make the Mercury in the mid 30s.
The Perseus doesn't offer more than small single digit percent of increase in power and may be less reliable. The Taurus went nowhere, you are out of British engines that will fit and available. Swiping Pegasus engines from bombers that can carry 4000lbs of bombs (or more) to fit to a bomber carrying 1000lbs (stretch to 1500-2000?) doesn't seem like a good use of resources.

Please remember, in Sept of 1939 the Blenheim was in service in larger numbers than another aircraft the RAF had. Also remember that the Beaufort and Beaufighter were both in the works. First flight by the prototype Beaufighter was July 17th 1939. All you are doing with a pimped out Blenheim is trying to buy time for the Beaufighter to show up in quantity. Do a bit more damage to the opposing forces and keep more of your crewmen alive to help man the new, better bombers when they show up. It is never going to be war winner on it's own with either guns or bombs.

go back and look at the Data sheets Tomo posted. This is not a heavy airplane, It has less than 2000hp even running 9lbs boost on 100 octane fuel at low altitude. However it is not a small airplane either. It is almost the same size as an A-20. but a whole light lighter.

trying to stick in Merlins adds about 1200lbs minimum, just for the engines and cooling systems. That is a lot of weight in an under 10,000lb empty weight airplane.

 

[Admiral Beez]

On the Blenheim, how did they get the fifty or so aircraft from Britain to Malaya? We're they partially flown there? That would have exhausted a lot of engine TBO (time between overhauls).

Here's one being uncrated at Singapore.

 

[tomo pauk]

Manual for the MK V says max dive angle for using the 500lb bomb is 55 degrees.
You are up against the weird British system of holding the bomb bay doors closed with elastic cords and depending on the bomb's weight to overcome the cord tension and push the doors open.
Perhaps not a deal breaker in itself but unless you really change the bomb bay and the doors you are limited to a pair of 500lb bombs which are not exactly either ship or bridge killers. Not like a JU 87 or 88 with 1100lb bombs or larger.
Bolt the doors closed and use external bomb carrier/s?

I'd go with removing the bomb bay doors all together when big bombs are carried, especially if intention is to dive-bomb.

This might be of interest - Blenheim bomb loads in Finnish service, from Wikipedia:
Seventh series, VII (BL-191..BL-195), six Finnish-made Blenheim I bombers, was cancelled in 1944.
Series I with doorless bomb bays could carry 800 kg bomb load in the bomb bay and up to 100 kg on wing cells. Series II, V and VI could carry 800 kg load on bomb bay and 172 kg on wing cells and fuselage racks. Series III and IV had the original RAF bomb bays and racks and could carry only 454 kg (1000 lb) load on bomb bay and 72 kg (200 lb) on wing cells. The bomb bays, bomb bay doors and bomb racks of various series were modified on major overhauls to host bigger bombs.

If you are going to build a fighter Blenheim then build a fighter Blenheim, give it a gun pack that doesn't look like it was made by home handyman out of plywood using a hand held circular saw. ( the ability to tip down for loading and servicing the guns was rather neat but come on, they built hundreds of these things and never improved the aerodynamics)
Get rid of the turret on the fighters, clip wings? fit constant speed props? improve fit and finish?
You might have been able to add 15-20mph to the fighter versions and while still in trouble against 109s or 110s the bombers would have a much harder time getting away from them.
Stick 2-4 guns in the nose of the night fighters in addition to a streamline belly pack?
add 1-3 guns in the wing?
Get the Blenheim fighter up to a 6-8 gun fighter instead of a 5 gun fighter?

A fighter Blenheim is a tall order, unless it is a night fighter?
I'd certainly remove the turret (less drag & weight) and have the wings clipped (better rate of roll, a tad less drag). Perhaps the radar operator can be located behind the pilot, so armament can be in the fuselage (using blast tubes) and not under fuselage? improvement of fit & finish is always a good thing, so is a better prop.

 

[tomo pauk]

...
trying to stick in Merlins adds about 1200lbs minimum, just for the engines and cooling systems. That is a lot of weight in an under 10,000lb empty weight airplane.

Merlin might be a better investment on the Beaufort or Hampden.
A Twin Wasp on Blenheim?
Peregrine? Granted, this will not solve much, perhaps just cater for night fighter role?
Install two pairs of .303s instead of new fuel tanks when going from Mk.I to Mk.IV for the (night) fighter role?

The 'fighter Blenheim IV', kindly provided by Neil Stirling:

 

[yulzari]

Looking around at the period 'spare' engine production capacity there is always the Dagger. If you ignore Napier's cooling duct advice and hire De Havilland for that item.

 

[tomo pauk]

Looking around at the period 'spare' engine production capacity there is always the Dagger. If you ignore Napier's cooling duct advice and hire De Havilland for that item.

Swapping two Mercuries with two Daggers adds 800 lbs to the hypothetical Blenheim just for difference in dry engine weight.

 

[swampyankee]

Trying to keep the changes within reason.....
1) There's not a heck of a lot of choice within the British engine makers' catalogues: the RR Peregrine (885 hp; which would probably lead to another aircraft with orphaned engines), the Taurus (1,140 hp; hopefully the Blenheim has good OEI characteristics), and the Dagger (794 hp) The Peregrine is about 200 lb heavier than the Mercury (plus 200 lb or so for the cooling system), the Taurus about 300 lb heavier, and the Dagger about 400 lb heavier.. Going outside the UK, possible engines would be the R-1535 (820 hp; about 100 lb heavier; the weight comparisons are all to the Mercury), the Wright R-1820 (1,000 hp; about 200 lb heavier), the Pratt R-1830 (1,000 hp, about 300 lb heavier), the Hispano-Suiza 12Y (the -32/33 was about 100 lb heavier than the Mercury and producing about 950 hp. The cooling system would add about another 200 lb or so)

In my not-so-humble opinion, the Hispano-Suiza and Dagger are the worst two choices, the latter because it's both much heavier (close to 40% heavier) and somewhat less powerful (if we're going to get a heavier engine, we need significantly more power); the former because manufacturing would have to be organized well before WW2 starts. The R-1535 would be the closest to a drop-in installation. If there was demand, it could probably be boosted to produce 925 hp without too much redesign (there wasn't, so P&WA had no reason to try). I think the best options would be R-1535, Taurus, R-1830, R-1820, Peregine, Dagger, 12Y.

Since the Blenheim is pretty much in the same category as the Battle as a bomber, i.e., less than useful in daylight, my three options would be

1. A training aircraft. Modify the fuselage so there can be a pilot and instructor (probably in tandem, with the instructor behind and above the trainee). Since this aircraft would not be operating in a combat area, it would not need armor, self-sealing fuel tanks, etc. It would need modern communications, full IFR instruments, and heat and deicing (it would be operating in Canada!). This variant wouldn't need any more power.
2. A short-range maritime patrol aircraft. This would require armor and self-sealing tanks, increased fuel capacity, radar, forward-firing armament (enough to suppress the AA gunners on a submarine), and depth charges. This would require significant more power, and would probably be the most difficult.
3. An interim night-fighter. This would get the uprated R-1535. The radar observer would have to sit way back possibly where the gunner used to be, and the armament would be in the nose, under the pilot. This would also need armor, self-sealing tanks, heat, and deicing.

 

[Shortround6]

The Canadians built Bolingbrokes (basically Blenheim IVs) with both R-1535s and one with a lower powered R-1820s. Results were less than stellar.

The Canadian planes were being built with Mercury XV engines which offered 905hp for take-off and 995hp at 9250 ft on 100 octane fuel. Still gave 840hp at 14,000ft.

Please keep these numbers in mind when considering alternative engines. Same engines were used in most Blenheim IVs.

The R-1535 Twin Wasp juniors used in 15 Bolingbroke IVWs was rated at 825hp for rake-off. take off was less than spectacular and single engine performance was so poor that any operational aircraft had their bombload reduced to 500lbs to help compensate.

The R-1820 came in when there was a shortage of 100 octane fuel for training aircraft and the Mercury XV engine lost a considerable amount of take-off power operating on 87-93 octane fuel available to the training program in Canada. One airframe was fitted with Cyclone GR-1820-G3Bs rated at 900hp (2350rpm) for take-off. This marginally improved take-off but their larger diameter restricted the pilots view and other performance details don't seem to be public? testing started June 29th 1942 and continued for some time. Larger diameter engines might cause more drag? The GR-1820-G3B was only rated for 820hp at 8800ft max continuous (2100rpm) and had a single speed supercharger.

yes later R-1820s can do better and a two speed supercharger will help both take-off an altitude performance. But they are heavier than the GR-1820-G3B.

For the R-1535 fans, P & W delivered 1269 of them in 1940 and just 22 in 1941, production ended in Feb 1941.

The R-1830s, at least the ones making 1100-1200hp for take-off went 1400lbs or better, the older ones at 1000hp or less may be a bit lighter but then what is the point?

 

[ThomasP]

Dagger VIII ran around 1000 BHP at 4200 rpm and +5 lbs boost at 8750 ft (87 octane), or 1115 BHP at 4400 rpm and +7 lbs at 7500 ft (100 octane). At 14,000 ft power was 820 BHP at 4200 rpm or 920 BHP at 4400 rpm both at FT. Assuming no increase in drag, it would only give about +15 mph below 14,000 ft and about +10 mph above, but it might be more useful in terms of load carrying.

 

[tomo pauk]

To me, the ultimate Blenheim needs to be powered with Merlin.

 

[Shortround6]

Even a Merlin III is 400 heavier than the Mercury and the cooling system is almost another 300lbs. So Merlin IIIs would be over 1400lbs total (can't use the existing propellers.)

Blenheims, as built, had a rather large flaw. They increased the take-off weight between the MK I and the MK IV by 2000lbs but the landing weight only went a little bit, if at all.
That is the reason for the fuel dump pipes under the outer wings for the fuel tanks in the outer wings (not fitted on MK Is). To rapidly get the weight down in case of an emergency landing right after take off.
Take-off weight went up even more on the MK V, Not sure what the landing weight was or what they did to beef up the landing gear.

The Blenheim was not a particularly low drag airplane. The fighter version (data above in Tomo's post #27) had that gun box designed by railroad apprentices that cost 10-15mph in speed. One rather modified MK I did get to 290mph when worked on and tested by Cottons photo recon crew. But the amount of putty, sanding and painting was not practical for production.

Clipping the wings, fitting some streamline fairings, modifying the landing gear doors (not done by the PR crew) deleting the turret, using 100 octane fuel (already done in service) and using constant speed propellers instead of two pitch props would all help, pick and chose at you wish. Those fuel dump pipes might be worth a few mph by themselves.
Many Blenheim fighters in service carried one or two racks for four 40lb bombs which certainly didn't help. Also may not show up on some tests/data cards.

Some aircraft (Like the Skua) got the sleeve valve Perseus engine because all Mercurys were allocated to the Blenheim program at the time, this did change.
The Mercury never got a two speed supercharger.
The Blenheim did seem to suffer from the let's build a lot of them and not change the production line syndrome while they worked on and waited for better aircraft which wound up being late or never showing up at all (or wouldn't do the job either when they they did show up). Rather similar to the he 111

 

[tomo pauk]

Even a Merlin III is 400 heavier than the Mercury and the cooling system is almost another 300lbs. So Merlin IIIs would be over 1400lbs total (can't use the existing propellers.)

The 'no free lunch' rule applies as ever. OTOH - we'de have an increase of ~25% engine power above 9000 ft, or perhaps 30% increase if we account for better exhaust thrust. A slight decrease of drag can help, too.
When Dornier moved from Do 17Z to 215, they have had less of power increase than what I'm suggesting here.

Blenheims, as built, had a rather large flaw. They increased the take-off weight between the MK I and the MK IV by 2000lbs but the landing weight only went a little bit, if at all.
That is the reason for the fuel dump pipes under the outer wings for the fuel tanks in the outer wings (not fitted on MK Is). To rapidly get the weight down in case of an emergency landing right after take off.
Take-off weight went up even more on the MK V, Not sure what the landing weight was or what they did to beef up the landing gear.

FWIW:
The 'light weight' (no ammo/bombs/fuel/crew) of Mk.I was 9335 lbs (data from April 1941), that of the Mk.IV was 10600-11700 lbs (two data sheets give two figures, also from 1941 - depending on protection for crew and/or fuel tanks?). 'Light weight' of Mk.V was Max weight was ~12000 lbs.

'Tare weight' (no guns, radios, what else? no turret?) of Mk.I was ~8080 lbs, went up to 9240-9790 lbs for Mk.IV, and again to 10700 for the Mk.V.

'Max weight' went from 12500 lbs for the Mk.I to 17000 lbs for the Mk.V.

(no wonder the Mk.V was dog's breakfast)