F.6/42-type fighter, but with Merlin power

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BarnOwlLover said:
As hinted at, it's simple math. 1480 miles divided by 2 equals 740 miles, which is actually more than I stated.
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This is what a full Hornet- the one with two Merlins - will do with half of it's fuel (230 imp gals in internally).
Half of a Hornet has just one Merlin to consume 230 imp gals (ie. more than what a clean P-51D has). A P-51D with 224 imp gals (269 US gals) was good up to 1500 miles; that is for best case, most economical speed, no allowances.
 
tomo pauk said:
This is what a full Hornet- the one with two Merlins - will do with half of it's fuel (230 imp gals in internally).
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To reinforce this, A "1/2" twin is usually going to have more drag the "single" because the single engine of the "1/2" is not only dragging around the engine nacelle but 1/2 the fuselage cross section. It makes for rather poor estimate.
In real life things get weirder, The P-51D had under 1/2 the flat plate area of the P-38J, only by a little bit. However the P-39, P-63 and P-40 all had more than twice the drag (flat plate area) of the "1/2" P-38. In part because they all had more than 1/2 the wing area. The wing is primary source of drag (unless the fuselage was real screwup).

A 1/2 Hornet would have about 181 sq ft of wing. Not much more than a Bf 109. Lots of luck putting 230imp gal of fuel in an airframe that size.

Some things do NOT scale well. Volume is a prime example. If you built an 8/10s "replica" of a Hornet (or any other airplane) you have about 51.2% of the internal volume.
You do have about 64% (?) of skin drag though

Using the Mustang as an example, the wing root was 104in long and with a 16% airfoil that gives a thickness of 16.64in to fit the fuel tank inside of. An 8/10s replica has an 83.2 in long wing root and max thickness of 13.3in. Except you can't scale down the wing skin thickness/structure, the tank wall thickness/self sealing material and so on. Net volume of the tank is less.

Basically as soon as the "1/2" Hornet is set up to actually be real airplane (220-240sq ft wing and a few other things) trying to estimate range based on the drag and fuel capacity of Hornet goes out the window.
 
Shortround6 said:
Basically as soon as the "1/2" Hornet is set up to actually be real airplane (220-240sq ft wing and a few other things) trying to estimate range based on the drag and fuel capacity of Hornet goes out the window.
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The point I was trying to make is that, while our brave fighter will have about half of the fuel the Hornet had, it will also have half of the number of engines for that fuel to be consumed. The fellow forum member accounted for fuel being halved (down to 230 imp gals), but not for number of engines that are being halved, too.
But, we can take a look at Spitfire VIII/XI (yes, the power will be higher on the Merlin 100 series, but we should cut the drag with the new fighter vs. the legacy airframe; if not, there is no point in the new fighter). Spit VIII per this doc does 1265 miles on 210 gals total = 120 gal internal + 90 in the drop tank - basically as draggy as it historically gets for this range.
See here for Spitfire IX HF - 1200-1355 miles with 255 gals (170 gal drop tank used), 900-980 miles with 175 gals (90 gal drop tank used).
Tempest V (a very unfair comparison, due to Tempest being much heaveir and bigger than a Merlin-powered fighter needs to be) with 248 gals fuel total example, 1210 miles range.

tl,dr - the fellow forum member understated the possible range for the 'half Hornet' with half the internal fuel by close to factor of 2.
 
tomo pauk said:
- the fellow forum member understated the possible range for the 'half Hornet' with half the internal fuel by close to factor of 2.
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You are quite right.
But the estimates seem to be a bit off due to speed.
The Data sheets are estimates (ALL/Most Spitfires cruised at 220mph at most economical regardless of size of drop tank?)
Mustang III with V-1650-3 cruises at 253mph at 20,000ft most economical and so does a Mustang III with V-1650-7 ??
Max weak mixture changes.
 
Well, then tell me how you'd design such an aircraft? That's part of why I started this thread. I'm trying not to pretend that I know everything. But I do know that, for instance, the Spitfire was draggy relative to the Mustang, while the Mustang was heavier. And, in theory, you shouldn't need a 2000-2500 hp for an interceptor fighter that also has decent range and carries 4 20mm cannons and such if the aircraft weighs less. And you do have to remember that aircraft that used 2000-2500 hp engines, due to the engines being larger and heavier, ended up being heavier due to power unit weight increases as well as reinforcements needed to carry said engine.

I don't think there's anything that would keep a fighter the size of a Merlin Spitfire from doing at least most of what F.6/42 demanded if designed properly.
 
The P-51H already proves this is possible; excellent performance and range, solid carrying capacity — and it's still arguably a couple thousand pounds heavier than an aircraft with a ~1700 lb engine needs to be — largely, I suspect, owing to its size. Moving to a smaller aircraft would probably come at the cost of some range, as the reduction in drag probably wouldn't be enough to offset the reduction in internal fuel volume, but less range than a P-51H still leaves you plenty of room to be better than the Tempest, which is the aircraft we're trying to match/beat here.

As concerns wing area, I don't think 181 is massively too small, provided it's a well designed wing. That's around the same size as a the wing on the Yak-9, which is around the weight we're aiming for here, and the Yak-9 was known to be a pretty agile aircraft, and notably had a lower wing loading than a Tempest V. You may want to expand the wing a little bit, but you don't necessarily need a Spitfire/Mustang sized wing to have a suitably maneuverable aircraft.

This document may be of help if anyone wants to think about the half-Hornet concept in greater depth. The weights are pretty easy to ballpark, the areas are maybe a bit trickier. http://www.wwiiaircraftperformance.org/Hornet/Hornet_F1_weight.jpg
 
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A few questions do emerge.
Like, who is 'you' in this case? Any feedback on the actual number of engines on the "half Hornet"?
 
The Hornet was using two engines to haul the same guns/ammo as the Big Hawkers did, some adjustments to ammo. You can't scale the pilot (225lbs with parachute and dingy), you don't really want to scale down the pilot protection/ammo protection. Some yes, but not 50%. You have to use nearly the same size cockpit/instrument panel. You need to use the same radio/s.

Take a look at the weight chart. Between the structure and the powerplants 63.8% of the clean gross weight is already accounted for. Now there may be a few things the designer can do with the structural weight, but if he is using a pair of Merlin 130s there isn't much wiggle room.
There is often a disconnect between aircraft designer "Payload" and user "Payload". And you have airstaff users and pilot/group commander users.
Aircraft designer includes a lot of the "stuff" like self sealing fuel tanks/radios/armor as "payload". Stuff that has to carried but doesn't affect the construction of aircraft and it's basic design. Some times the designer has to beef up the structure due to increased weight, or sometimes they have to make the wing bigger due to field requirements (landing and take-off).

Now please note the Hornet would NEVER have been approved in 1938 for several reasons, one of which was that the tire pressure was too high and it would need larger tires, which would have needed larger nacelles which would have needed.............................................
BarnOwlLover said:
I don't think there's anything that would keep a fighter the size of a Merlin Spitfire from doing at least most of what F.6/42 demanded if designed properly.
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Most designers had around 30% of of gross weight to fit in the "payload" high 20s to low 30s. Then you are playing trade offs. More fuel means less guns/ammo for example. This assumes other things stay the same, Like take-off distance, climb to 50ft and landing distance for instance. Which it didn't, Field length grew by leaps and bounds during the war.
Please note that fighters of a given country were required to do combat maneuvers at a specified gross weight. Doing combat maneuvers at higher gross weights (like with drop tanks or bombs or rockets) often made the wings break or fall off or the plane would decide to go into an inverted spin or some other awkward situation.
In 1942/43 nobody really knew what the fuel situating would be in late 1944/45. They knew that 150 fuel was coming, they were testing it. What they didn't know was what it exactly it would consist of and/or they didn't know how much of it they could get compared to 100/130 fuel (remember in late 1942 they were changing the formulations for 100/130 to increase production. )
Toluene makes a great fuel additive for high performance fuel, it is also a prime component to TNT
 
150 grade fuel is a red herring. Allies can start installing the water/alcohol injection system to their aircraft for extra boost and power.
It is Germans that need to up their game, and again they need to improve their supercharges 1st - even a 160 grade fuel will not help the Fw 190A-8 vs. a Merlin Mustang on 130 grade, or a P-47 with 130 grade and ADI.
 
Having guestimated some of the numbers based on that document, I'm not sure a "half-Hornet" really offers much after all.

The wings, let's say we're alright with pretty much half the area, assuming weight scales more or less directly there, we're looking at less say ~1100 lbs to be a little on the conservative side. Fuselage we'll shorten by 20% to 28 ft; this still includes the cockpit, which will weigh the same, so it won't scale directly, maybe more like ~10%. Let's call it an even 500 lbs. Tail unit, I really don't know, let's say another 10% off that, conservatively, 270 lbs. Nacelle structure's we're doing away with more or less entirely, but you'll still need one set of engine bearers, so let's say 50 lbs (about 2/3 the weight of one nacelle). Powerplant we'll cut about in half, 3230 lbs; fuel supply we'll do the same since it only has to supply one engine, 245 lbs, everything else about the same so you come out to a tare weight of 6560, add another ~2600 on for pilot ammo and fuel (assuming half the volume of the Hornet, which may be generous, but doesn't seem that insane).

That's... almost identical to the P-51H (and 1000 lbs heavier than a Merlin Spit). Drag is harder to estimate, but given the very similar sea-level speeds of the Hornet F.1 and the P-51H (albeit with the Mustang making about 150hp more), I think it's safe to assume they'd be similar in that respect too.

Now, I still think, either of these designs — the lightweight mustang or the theoretical half-Hornet — gets you a damn fine tactical fighter. The P-51H can take off with about as much ordinance as a Tempest, has much better range, probably better performance, whilst also offering significantly better fuel economy, much easier maintenance and I would imagine cheaper production costs, and you have yourself a winner. Moreover, the Merlin clearly had much more room for further development than either the Centaurus or the Sabre, given the boost limitations of the sleeve valves.

A middle-weight option based on the Griffon could be interesting as well — Spiteful, MB5, CA-15, Tempest III, all contenders — but I'm not sure if they would really offer much that a similarly well optimised Merlin design wouldn't, and even here, I'm not sure there's as much room for development.
 
Just pointing out that designers in 1942/43 needed a crystal ball and saying they should have used an engine that used 150 octane fuel because that's what they had in late 1944/45 kind of misses the point.
So does relying on WEP power and water injection. There were extra maintenance procedures that had to used and shortened overhaul lives. If you don't have a choice then by all means use it. If you do have a choice and engine A needs water injection and engine B will give you the performance you want without it which do you choose?
The P-47 was an extreme example but the late Ds carried 248 lbs of water. That is equal to over 40 US gallon of fuel (granted you need a self sealing tank.)
Most US planes carried under 1/2 that amount. If the Ki-84 had used all gasoline instead of water what would it's range have been?
Commercial aircraft started using it right after the war. But they understood it's use better after thousands of hours of war time use and they usually had hours of cruising flight after the "wet" take-off to get rid of any residue (water in the oil will evaporate with time). Airlines were also a hard bunch. Water injection let them take-off with higher gross weights using smaller engines for increased payloads (more revenue or longer range). Did it pay for itself?
British and especially the Americans had somewhat different requirements than the Germans and Japanese. The Americans had to send every engine thousands of miles to a combat front. They would do what they needed but increasing the provision for spare engines was not looked at with favor just from the shipping perspective. No one thing was the deciding thing but a number of them.
The fuel the British were using in 1944 was not what the US was testing/planning on in the US in 1944. The US used the British fuel but the US was trying to get to 115/145 fuel for better performance in cruise conditions (looking for long range bombers or for use against Japan) rather than 100/150 fuel for sprint ratings. Sometimes the extra 5 points needed different materials and when you are talking about hundreds of thousands of gallons per day even small amount percentages of compounds turns into tons per day.
 
RR had several Merlin prototypes with water injection and they also had some with "intercoolers" (or charged coolers on single stage engines" the different versions were usually rated the same for the same basic engine (supercharger and gear ratio). These were on the single stage engines when they were looking for higher altitude.
The Packard -9 got water injection but that was a little late for for 1942/43 design.
 

I'm not sure what is wrong with Allied engines running on 130 grade fuel, Merlin included.
Choosing an engine B is not always the option - it will take some time before P-51 has a 2-stage Griffon in the nose, let alone the 2-stage R-2800. So people were keen to extract every bit of power from the existing lines of engines, even if the cost was increased maintenance or engine life duration.


P-63 carried 25 gals of ADI mixture. Not sure about the P-51H.
Ki-84 will 1st need to have at least the 'starting' 100 oct fuel before we think of removing the ADI tank.
A Spitfire with 184 US gals as the Ki-84 carried internaly would've been very useful.
 
True but the British F.6. /42 specification was for a new fighter and had the Sabre, Centaurus and Griffon engines a possible candidates.
Picking an uprated Merlin in 1942/43 is a long shot to get the desired performance.
The Merlin was running 18lbs of boost at the time and needed several modifications for the 25lb boost ratings. Combat emergency may have been different but the 100 series engines got new crankcases, the new crankshaft with the new oiling system and strengthen cylinders and other changes. If you had sized your airplane (smaller than it's competitors) to use the new Merlin and the new Merlin had failed you are stuck. Either the new improvements don't work or the new fuel is not commonly available.

Now it turns out that the Sabre wasn't done with it's tale of woe in 1942/43 and the Centaurus was running late.
 

I'd suggest that we leave aside the 150 grade fuel and the associated +25 psi boost for the moment.

Spitfire and Mustang were just the right size, there is no need to go smaller. Spitfire have had the drag problem (Mustang have had the weight problem, but Spitfire's problem was bigger in hindsight), so this is where our new fighter will need to do much better. Merlin 60s or 70s will do for a long-range fighter armed with 4 Hispanos doing 440+ mph.
 
In this case the Spitfire also has the problem of not really being able to carry much external ordnance for what I believe were structural reasons. That might not have been important to the original F.6/42 specification, but it was a major quality of the aircraft that specification produced, and in general an important quality in a front line fighter by the time any aircraft made to that specification would have entered service.
 
When it comes to ordinance, the Spitfire from the Mk V onwards was capable of carrying up to 1500 lbs of bombs (1 500 lb under each wing and fuselage), though usually 1000 lbs was carried (500 under fuselage, 250 under each wing). To a large degree, weight of payload doesn't matter a ton if delivery is accurate (just as how a large payload can compensate somewhat for accuracy errors).

Also for 4 20mm cannons, what are we looking at in terms of ammo capacity? And what are we looking at for range on internal fuel and what conditions (namely speed) is that range achieved? The Ki-84 carried 150 rpg for it's cannons, and had a range of over 1000 miles. The caveat being that this range was achieved with a cursing speed of 175 mph. For bomber escort, you're probably going to want more speed as far as cruse.

IMO, 700-800 miles range on internal fuel wouldn't obviously set the world on fire, but is a good deal better than most Spitfires.

Also, the P-51H was tested with two specs. One being an escort fighter (9550 lbs TOW) and interceptor (8450 lbs TOW).

I guess maybe I should ask what is the weight cut off between where a two stage Merlin will make an effective single seater powered by a single engine, and when something like the Griffon should probably take over?
 
Isn't this a bomb-capable, increased fuel, 4x20mm, Merlin-powered Spitfire? Bombs were done, four cannons were trialed, and increased internal fuel was fitted in later marks.



 
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BarnOwlLover I would say the objective is specific excess thrust on par with (or ideally superior to) a Tempest V across the flight envelope, as that's obviously the aircraft the closely related "Tempest Light Fighter" specification was aiming to improve on. Because it's so difficult to compare drag between different air frames at different speeds and altitudes, let's ignore that for now (as well as prop efficiency which we'll just assume is similar), and just talk about power to mass. We'll compare empty weight, simply because that allows us to control for differences in fuel load, which as you mentioned, will obviously vary based on mission requirements.

The Tempest V has an empty weight of 9885 lbs — let's just say we've got a malnourished pilot and round that up to a nice even 10k. I'm struggling to find numbers for the Sabre IIB, so we'll look at the Sabre V, which there are reports of later IIBs being boosted pretty close to anyhow:
2300hp at takeoff
2600hp WEP low
2300hp WEP high
1715hp low cruise
1575hp high cruise.

Compare those numbers to a Merlin 130, which we'll take as more or less representative of the Merlin 100 series that we'd be planning our hypothetical fighter around. In parentheses are the hypothetical max weights our multi-role Merlin fighter (with hungry pilot) would have to come in under to match or better the power/mass ratio of the Tempest V under the given engine settings (which again, in the case of WEP, are pretty ambitious for the Sabre IIB).
2000 take off (8696)
2070 WEP low (7961)
1850 WEP high (8043)
1250 low cruise (7289)
1190 high cruise (7555)

Compare that to the ~6600 lbs of an empty P-51H, and you can see the Mustang is actually gonna have a pretty substantial advantage in power to weight ratio across the flight envelope. But of course what we really have to consider isn't power, but excess thrust — that is, how much thrust is left after countering drag. Prop efficiency is going to have a big impact on that, especially at lower speeds for cruise and take off, but again let's just assume that's more or less comparable between the two at all speeds. Given the Mustang's superior max sea level speed, we can surmise that at most speeds and altitudes, the P-51H is probably going to have more excess thrust than the Tempest (though keep in mind this is for an aircraft running ADI, so about 150 hp more than our Merlin 100 will be developing, at least initially). That means it's actual advantage may be even greater than the power-mass figures would suggest. At ~5800 lbs, a similarly engined Mk VIII Spit would have an even bigger advantage in power to mass, but here factoring in drag would count against you, rather than for you, probably to the point of nullifying most of that weight advantage when we start talking about excess thrust/weight.

The lightweight Mustang pretty clearly demonstrates that the potential was there for a Merlin-powered fighter to meet this specification, given sufficient development of the powerplant. Was that development foreseeable in 1942, when this specification was written? Evidently not, but it's not impossible to imagine that if certain people in certain positions has had a bit more foresight, things might have gone differently. The lightweight Mustang concept itself wouldn't be formally proposed for another year however, whereas there had been at least some thought given to a lower-drag Spitfire in the form of the Mk III (though this in itself probably wouldn't have gone far enough), so if the impetus had been there to pursue this type of fighter, it might just as easily have taken that direction.
 
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