Transport aircraft layout

[KiwiBiggles]

A discussion in another thread about the characteristics of the Ju 52/3m vs Ju 252 vs C-47 lead me to thinking about just how different WW2 transport aircraft look compared to those of today. Nowadays it seems obvious that a military transport should have a high wing, an upswept rear fuselage with a loading ramp, and very short undercarriage tucked into the side of the fuselage. But, the Ar 232 aside, until the C-123 came along in the late 40s, all military transports, even purpose-designed ones, followed the general layout of most other aircraft.

So, was there any reason why the now-standard pattern for a military transport couldn't have been use earlier? The benefits seems self-evident. You only have to look at the much-publicized picture of a jeep being tortured into the side cargo door of a C-47 to realize how much better is a low horizontal fuselage with a rear ramp.

As an aside, can anyone think of any other class of aircraft where such an increase in utility came about through a change in design, not an advance in technology?

 

[tomo pauk]

Both Ju-52 and C-47 were once-civilian designs that were militarized. Air forces were trying to get as much as possible of those 'good enough' transport aircraft, without waiting for 'ideal' ones to be designed/built.

 

[kool kitty89]

A discussion in another thread about the characteristics of the Ju 52/3m vs Ju 252 vs C-47 lead me to thinking about just how different WW2 transport aircraft look compared to those of today. Nowadays it seems obvious that a military transport should have a high wing, an upswept rear fuselage with a loading ramp, and very short undercarriage tucked into the side of the fuselage. But, the Ar 232 aside, until the C-123 came along in the late 40s, all military transports, even purpose-designed ones, followed the general layout of most other aircraft.

The C-82 may have been late-war, but it's still notable with the high wing and clamshell rear cargo doors.

Also realize most of these aircraft were taildraggers, so the rear loading ramp configuration wasn't quite as obvious or convenient. (though Junkers managed it with the 252 and 290, even if they didn't use tricycle landing gear)

Both Ju-52 and C-47 were once-civilian designs that were militarized. Air forces were trying to get as much as possible of those 'good enough' transport aircraft, without waiting for 'ideal' ones to be designed/built.

Purpose-built military transports (and bombers) weren't consistently using high-wing layouts either, in spite of capacity advantages. The C-46 didn't go with either, though I don't think it or the C-47 were really prevented from being designed with rear loading ramps or had cargo capacity compromised because of the low wing. (the C-46 did get a particularly large cargo bay opening, though)

 

[Balljoint]

The Me 323 did a good job, i.e. big and drive on, if a bit slow and flammable when loaded with gasoline.

 

[davebender]

was there any reason why the now-standard pattern for a military transport couldn't have been use earlier?

Not in my opinion and that goes double for Luftwaffe as they rejected Ju-252 and Ar-232 for mass production. Both were mature designs with prototypes flying during 1941 to 1942. Both purpose built transports were a quantum leap over existing converted airliners such as C-47 and Ju-52. This isn't just a matter of range / payload. Ju-252 and Ar-232 could rapidly load / unload large equipment which would never fit through small side loading door of the converted airliners. Ar-232 could theoretically even airdrop entire load in a single pass, something many modern tactical cargo aircraft take for granted.

 

[GrauGeist]

There were some lessons learned from WWII that were adopted for modern transport, particularly in heavy lift...

 

[kool kitty89]

The Me 323 did a good job, i.e. big and drive on, if a bit slow and flammable when loaded with gasoline.

If you're including converted powered gliders, the Go 244 is a nice example similar in configuration to the C-82 too, but it's a lot smaller and had a pretty short range.

 

[Piper106]

One commercial design that might have been a good fit but which was not choosen for some reason was the Douglas DC-5.

 

[kool kitty89]

One commercial design that might have been a good fit but which was not choosen for some reason was the Douglas DC-5.

DC-3 had a higher volume cargo/passenger/troop capacity and existing large scale manufacturing and support infrastructure as well as already being in widespread use with many flying.

The DC-5 might have been an interesting candidate for a rear loading ramp. That said, the C-47's side cargo hatch was pretty damn big as it was, enough to squeeze a jeep through. (that 90 degree turn is the bigger problem, a read ramp allows for loading/unloading vehicles a lot more easily -C-46 has that problem too, if a bit less so with the even larger hatch)

 

[Shortround6]

Budd Conastoga

First flight 31 October 1943

And it was made of stainless steel.

 

[kool kitty89]

Budd Conastoga

First flight 31 October 1943

And it was made of stainless steel.

Wow, stainless steel. Looks like it probably needed R-2600s or R-2800s for reasonable power/weight.

Not in my opinion and that goes double for Luftwaffe as they rejected Ju-252 and Ar-232 for mass production. Both were mature designs with prototypes flying during 1941 to 1942.

There's also the Fw 206, which seems to have been canceled outright in 1939 with no prototypes built. It seems to have been in the works early enough to be useful and closer to the DC-3/C-47 than anything else in German development at the time and a better direct replacement for the Ju 52 than anything else on the table. (similar size, 2 1000 HP class Bramo 323 engines planned, design derived from the Fw 200)

 

[drgondog]

C-82, the C-119 were the first for AAF and then USAF - and then C-124. The C-82 actually enterered service before WWII ended.

 

[nuuumannn]

Aircraft layout really depends on what you want to do with the load and in WW2 time, piston engines just weren't powerful enough for such a layout to be exploited. Here's why. Putting a big hole in the bum of an aircraft requires a stiff fuselage with the wing box above the hold, if you want an uninterrupted space. All this adds drag. The Conestoga, Caribou and C-123 stand out in the design respect, the latter two being used as tactical airlifters in service, for which the ideal layout is best for rapid loading, but the problem is performance. The C-123's piston engines were supplanted by jet engines, as was the C-82 and Dollar 19, to enable them to carry beefier loads into shorter strips, and the Caribou was very slow, making it a rather easy target in a fire zone. The advent of turboprops meant faster transit times, greater loads and short field capability beyond what piston engined equivalents could achieve. The C-130 is the stand out example here.

So that's tactical lifters. Strategic lifters today can be seen in both configurations, but all are gas turbines, usually pure jets. The configuration adopted by the Ju 52, DC-3 etc, with a big side door remains in service today in high speed strategic airlift; the KC-135s, the various airliner conversions used in civil and military applications etc follow on from the likes of the C-54, the Avro York etc. The opening nose has been in use since he 40s also in the likes of the Bristol Freighter, C-87 an C-124 but again, speed and height is the factor; these piston engined aircraft were slow and at high airfields suffered exponentially. The C-82s - and Dollar 19s needed extra help from stove pipes. The 747 freighter continues the opening nose trend - an original design feature as Boeing intended the basic design to compete with the strategic airlifter requirement fulfilled by the C-5. The C-5 and Antonov 124/225 and C-141 and Ilyushin Il-76 have the configuration being discussed, but they are gas turbine powered, thus enabling enormous loads to be sent anywhere at high speeds and altitudes and as close to the battlefield as possible to strips that'll take them.

So, to sum up, it wasn't really until the advent of gas turbine engines that the configuration really came into its own and performance/weight restrictions weren't as stringent. Sure, you can take a C-123 into a smaller strip than a C-130, but only with a limited load by comparison. If you want it to get out again with that load, you might need to sling those stove pipes under the wings; a feature of 1940s and 50s American piston engined aircraft; the B-36, the P-2 Neptune, the C-123 etc, which became even heavier in service than their designers initially intended.

 

[Koopernic]

Adding a loading ramp or 'trappo-klappe' as was done in the ju 252, Ju 352 and Ju 290 seems a good solution that could work with both tail draggers and tricycle under carriage aircraft.

The angle of incline used on these aircraft was quite steep due to the length of the undercarriage which would have added some awakwardness but nowhere near as much as a side door on a tail dragger. Nevertheless photographs show outsized loads such as small trucks, half tracks, long artillery pieces loading into these aircraft. The Germans hardly changed the tail of these aircraft to make it longer and flatter to allow a longer 'ramp', they simply added the ramp into the form of a fairly standard fuselage.

Had the C-130 not existed I think that Lockheed could, if asked, have added a tail ramp to the Lockheed Electra and come up with a reasonable and much cheaper transport. Only the portion of the aircraft aft of the wings needs to be re-designed. The positioning of the floor in mid fuselage on modern jets such probably makes such an idea awkward, the foor would need to be lowered.

During the failed Stalingrad airlift ground handling and rough field issues became paramount. He 111 pressed into service out performed He 177 for instance with many of the larger aircraft experiencing takeoff issues. Issues of short field performance, undercarriage ground pressure, rough field taxiing became critical to deliver supplies as did being able too of load quickly. The new generation of Luftwaffe transports that quite cleverly addressed these issues was initiated in 1938 but by the time they were ready about 3.5 years latter it was considered too late to ramp them up to production. Maybe that was right or wrong.

 

[Koopernic]

double post

 

[Shortround6]

Part of figuring out what makes a good transport is figuring out what you want it to do. AND what you are willing to spend to get it.

Payload is important and here with early transports (low powered) we run into the volume problem, People are high volume/low weight "cargo". If you want to carry a lot of people you need a fat fuselage and that ups the frontal area and drag. If you want to carry heavy cargo you need a stronger floor than the a plane carrying people needs. If you want ramps for vehicles you need a heavier structure than a people carrier. You want landing gear that will handle rougher landing strips than a C-47 it will cost weight. You want short landing and take-off? it means big wing or high lift devices which cost weight and drag.

With limited power you run out of options real fast. 2000-2400hp doesn't really give you a lot of good options to get much trickier than the C-47. A C-47 could carry about 8000lbs worth of crew, fuel and cargo at normal gross weight and anther 5,000lbs at max overload. Change (increase) the empty weight by 600lbs and that could be 100 gallons of fuel or about 1 hour of flight time at cruising speed at a given take-off weight. C-47s had added around 1500-2000lbs of empty weight over the early DC-3 commercial aircraft.

Tactical transports that can land on rough fields and deliver vehicles and guns to the paratroops are all well and good but they are not the most efficient transports for general purposes. They will have lower cruising speeds and shorter ranges for the same weight cargo as the general purpose transport. And until you start using a lot of power the size vehicles and guns they can deliver are somewhat limited. Or the range is. An American 2 1/2 ton truck weighed just over 10,000lbs for the short version with gas and oil but no crew or cargo. A C-46 with a pair of 2000hp engines was rated for a 'normal' payload of 18,562lbs. with a crew of 3 and 1400 gallons of gas it could carry 9052lbs. Not enough to carry the truck even if you could get it in and that is at a gross weight of 48,000lbs. The R-2800s sucked down 190 gals an hour at at 67% power so you can trade payload for range. During WW II the C-46 was operated at gross weights of 52,000lb and even 60,000lbs on occasion (and under what for restrictions?) However from rough runways in some temperature conditions a C-46 at 50,000+ pounds could need 6000ft of runway to get airborne.

Granted the paratroopers may not need 2 1/2 ton trucks but you either need several different types of air transports or biasing the fleet to tactical needs can compromise it's over all utility. Even a transport built to deliver 3/4 ton trucks directly to the front lines is going to be different than one built to deceiver general cargo 15-30 miles behind the lines.

As planes got more powerful some of these limits got blurry or went away. An early C-130 had more power than a B-50 bomber (let alone a B-29). Again even early turbo props changed things tremendously for the aircraft designers. The engines in the early (1950s) C-130 offered more take-off power than the P W R-4360 28 cylinder radial, weighed about 1/2 as much and could run at much higher max continuous power settings while offering better fuel economy at the high power settings.

The DHC-4A is about 20 years newer than the C-47, has 250hp more per engine (about 20% more power) and is 8 mph slower at a similar gross weight and with a similar payload.

 

[tyrodtom]

They made a air transportable version of the 2 and 1/2 ton truck, the frame was bolted together right behind the cab.

It could be split in half, each half carried on a separate aircraft.

 

[Shortround6]

Another comparison The 4000hp C-46 vs the 4600hp C-123 of 12-15 years later. The C-123 weighed about the same empty and had a higher gross weight, It's top speed and cruising speed were both about 20mph slower than the C-46 despite the higher power. Please note that the C-123 used "C" series P&W R-2800s which needed less airflow for the same cooling as the "B" series engines in the C-46.

Ramps and easy loading are nice to have, in some cases very nice, but they are not free.

I don't have enough numbers to know if using GP (General Purpose) transports to tow assault gliders for tactical use is better overall than using assault transports year round as GP transports or not. A lot may depend on the percentage of varius types of missions done over a given time period.
The US (and NATO) had the Luxury of not really worrying about fuel shortages when buying post war transport aircraft. they also had engines with better power to weight ratios even before the turbines showed up.

 

[GrauGeist]

One aspect of the loading ramp is a reduction of time on the ground by the transport...

Side (door) unloading of heavy equipment (vehicles, pallets, etc.) would also require additional manpower

At a forward area that's under threat of enemy action, being able to get the transport in, unload and then get clear of the area is a huge plus: you're getting materials to the front quicker and you're reducing the probability of the aircraft being caught on the ground in the event of attack.

 

[blueskies]

Lockheed L-142 design, early 1943