What factors make some airplane easier to build than others?

Hi,

One final comment on fillets. My point is not that it is fillets can't be built (or are overly difficult to build) but rather that they are additional parts that need to be designed, have matching presses/tooling built for (for the left and right hand sides of a plane) and then have to be attached to a complex shape, typically connecting two separate large subassemblies (the main fuselage to the wing).

As an example, here are some links to Witold Jaworski's great work on 3D modeling of the SBD Dauntless (as shown on the ARC Forums website), and a 1/4 scale model of the SBD built by Micheal Fetyko (as shown on Model Airplane News website).
Image 1
Image 2

Here also is another from Witold Jaworski (from the Britmodeler site and the Airplanes in 3D website) showing the basic fuselage section shapes and wing locations that these fillets have to conform to. From this image you can see the complexity of the shapes, especially just aft of the wing trailing edge where the fuselage section bow in little before flaring out again, and then making a sharp turn along the bottom of the plane.
Image 3
Image 4

And finally from the image below (from the Blender Artist Website) you can see some of the panel lines of the fillet pieces, showing the need for several parts per side of the airplane to fillet each wing to the fuselage.
Image 5

In general, the point that I am trying to show here is not that fillets are a bad thing but only that planes with filleted intersections between the wing and fuselage can be more complex and/or time consuming to build than a plane with a 90 degree (or near 90 degree) connection, leading to a potentially more expensive build.

Additionally, operationally if trying to repair damage if you don't have the specific replacement parts on hand you would likely need to "beat to fit" a patch into place, to better match the complex geometry, leading to potentially moe man hours to keep a plane operational (in the event of damage), etc.

Regards

Pat

PS. Although it is not commonly mentioned all parts are typiclly built to within a certain tolerance, and fitting a complex shape to two different subassemblies (the wing and fuselage) can thus require a bit of "make it fit" type engineering to bring everything together.

pbehn said:

The goings on of the Air Ministry and the RAF are amusing at times. Did they do such an investigation into such things before they issued the first order for 300 Spitfires? Did anyone consider that if war did come they would need thousands per year not hundreds? Part of the design problem Mitchell had was fitting 8 Mgs into the design, the Bf 109 didnt have this issue. In choosing the design Mitchell did, the Spitfire could eventually fit canon in the wings with blisters, And then there is the thorny issue of gun heating, requested after the contract was placed. Oh and can we have a long range PR version please? Almost the moment war was declared they all sides realised they were blacked out from news about the opposition, especially their fleets, nothing could be more obvious or logical as a consequence of declaring war, yet no one did anything about it until it happened.

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I admit that I don't get the thrust of the posting, so breaking it down

Did they do such an investigation into such things before they issued the first order for 300 Spitfires?

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Clearly not to any great degree, but then they were interested in the performance and this was stunning for 1936, being far in advance of any other fighter of the time

Did anyone consider that if war did come they would need thousands per year not hundreds

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But this avoids the fact, that they did build thousands per year, so I don't see what your point is

Part of the design problem Mitchell had was fitting 8 Mgs into the design, the Bf 109 didnt have this issue.

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Again they did fit 8 Mgs into the Spit and as is well known went on to fit 2 x 20mm 4 x LMG, 2 x 20mm and 2 x HMG and finally 4 x 20mm.
I would argue that the 109 most definitely did have an issue. Initially they had 4 x LMG or 2 x 20mm FF and 2 x LMG. Then they went to 1 x cannon (20mm or 30mm) and 2 x HMG. Anything more had to be hung under the wings with a considerable impact on performance.
Or to put it another way. If the 109 didn't have this problem I would be interested to know how you would have fitted 8 x LMG into a 109, let alone 4 x 20mm

And then there is the thorny issue of gun heating, requested after the contract was placed

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Gun heating issues were not unique to the RAF and they were overcome

Oh and can we have a long range PR version please?

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And an excellent long range PR it was. Arguably one of the best PR aircraft of the war, combining range and performance and again, far better that the 109 equivalent

I admit to not understanding what this last point is. Its obvious that all sides wouldn't have detailed knowledge of their opponents equipment at the start of the conflict.

Almost the moment war was declared they all sides realised they were blacked out from news about the opposition, especially their fleets, nothing could be more obvious or logical as a consequence of declaring war, yet no one did anything about it until it happened

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Glider said:

I admit that I don't get the thrust of the posting, so breaking it down


Clearly not to any great degree, but then they were interested in the performance and this was stunning for 1936, being far in advance of any other fighter of the time

But this avoids the fact, that they did build thousands per year, so I don't see what your point is

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My point was the thinking and action is back to front. The RAF were the people responsible for the RAF and its future. Looking into how easily a Bf 109 is to make is academic, they should have been looking into how easy it was to make Spitfires especially since from 1936 they were involved in building a factory that ended up making it. The RAF initially ordered 600 Hurricanes but Hawkers tooled up for a minimum of 1,000. By the time war was declared 550 had been made. If you want mass production you dont talk of ordering 300, I know the order was quickly increased, but the first thing you need with mass production is an order to justify it. Initial production was so low that some were considering cancelling the contract, not helped by a requirement introduced that required the guns that were spread along the wing to be heated.

Glider said:

Again they did fit 8 Mgs into the Spit and as is well known went on to fit 2 x 20mm 4 x LMG, 2 x 20mm and 2 x HMG and finally 4 x 20mm.
I would argue that the 109 most definitely did have an issue. Initially they had 4 x LMG or 2 x 20mm FF and 2 x LMG. Then they went to 1 x cannon (20mm or 30mm) and 2 x HMG. Anything more had to be hung under the wings with a considerable impact on performance.
Or to put it another way. If the 109 didn't have this problem I would be interested to know how you would have fitted 8 x LMG into a 109, let alone 4 x 20mm

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My point was that it is very easy to say the Bf 109s wings were easy to make, the Bf 109 didnt meet the RAFs requirement for 8 heated guns, finding room for the guns and U/C in a thin wing was a major part of why Mitchell designed the wing he did,

Glider said:

And an excellent long range PR it was. Arguably one of the best PR aircraft of the war, combining range and performance and again, far better that the 109 equivalent
I admit to not understanding what this last point is. Its obvious that all sides wouldn't have detailed knowledge of their opponents equipment at the start of the conflict.

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. The P/R Spitfire was excellent, but why did it take the declaration of war to realise they needed it? How did they think they were going to fight the war? They didnt have a P/R aircraft that could return and get the pictures developed but they did have two squadrons of Defiant turret fighters, its hard to figure out what their thought processes were.

pbehn said:

My point was the thinking and action is back to front. The RAF were the people responsible for the RAF and its future. Looking into how easily a Bf 109 is to make is academic, they should have been looking into how easy it was to make Spitfires especially since from 1936 they were involved in building a factory that ended up making it. The RAF initially ordered 600 Hurricanes but Hawkers tooled up for a minimum of 1,000. By the time war was declared 550 had been made. If you want mass production you dont talk of ordering 300, I know the order was quickly increased, but the first thing you need with mass production is an order to justify it. Initial production was so low that some were considering cancelling the contract, not helped by a requirement introduced that required the guns that were spread along the wing to be heated.



My point was that it is very easy to say the Bf 109s wings were easy to make, the Bf 109 didnt meet the RAFs requirement for 8 heated guns, finding room for the guns and U/C in a thin wing was a major part of why Mitchell designed the wing he did,

. The P/R Spitfire was excellent, but why did it take the declaration of war to realise they needed it? How did they think they were going to fight the war? They didnt have a P/R aircraft that could return and get the pictures developed but they did have two squadrons of Defiant turret fighters, its hard to figure out what their thought processes were.

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Fair points I agree. The only one I would comment on is the last one. No one realised the need for a long range high performance PR aircraft, so the RAF were no better, or worse, than anyone else.

Glider said:

Fair points I agree. The only one I would comment on is the last one. No one realised the need for a long range high performance PR aircraft, so the RAF were no better, or worse, than anyone else.

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When you look at the thinking and effort put into Dowdings CAC system and all the "tech" they had to develop from scratch it is almost impossible to believe that the same organisation didnt see the need for a PR aircraft, but they didnt. They had a whole command called Bomber Command with no way of telling where their bombs were falling, this included a dedicated night time bomber and dedicated night bomber squadrons.

I think the change in operational environment took a lot of people by surprise.

Some people built a lot of photo recon planes but it turned out they needed a lot more than just a long range airplane with a camera.
It seems at quick look that some people assumed that light bomber could do the job and maybe it could in 1936-39 (depending on locale) but once the photo recon planes had to penetrate radar monitored airspace even quick airplanes were useless against fast fighters/interceptors. Recon planes that had to deep penetrate areas that had spotter systems gave some problems but the Japanese did use special high performance aircraft to stay out of trouble.
Once the idea that the recon plane could slip in, tootle about and take pictures and slip out without being spotted went away speed became the patron saint of recon planes.
Do 17s were replaced by Bf 110s and the Luftwaffe were having big problems figuring out how may of their strikes were doing. The Blenheim had gone from fasted bomber in the world to target in about 3-4 years.

PFVA63 said:

Hawker Hurrican was easir to build and easier to repair in some cases than the Supermarine Spitfire because its wings were simpler in form than those on the Spitfire, where individual panels could potentially have curvature in two directions, making it more time consuming to both manufacture and replace in the event of damage.

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But on the other hand, the Hurricane started out with fabric covered wings that significantly hurt performance. A lot of the structure was built out steel tubes that to be cut to exact length, then ends pressed exactly square, then fitted into fairly complex little fastener/joiner gizmos and tightened, all to aeronautical standards of quality. Much of the fuselage was built of lots and lots of little pieces of wood carefully nailed and glued in best WW1 fashion. Then cover in fabric, then dope fabric. All of that was actually pretty fiddly and time consuming work. It was relatively economical because it was old legacy technique with cheap labour and a decade of experience behind it.

The Spitfire had a lot of complicated structure but part of that was because it was more sophisticated and high performance aircraft - almost all metal, stressed skin. Assembling all the pieces was a fiddle but making many of the pieces was basically - put material in press, hold down switch, wait a moment, remove shaped component, send for riveting. A lot of manufacturing work could have been eliminated by either compromising the performance, or handling, or taking another six months on detailed design. But the RAF wanted a plane that was fast, climbed well, handled well, took off and landed well, and they wanted it yesterday.

pbehn said:

The goings on of the Air Ministry and the RAF are amusing at times. Did they do such an investigation into such things before they issued the first order for 300 Spitfires? Did anyone consider that if war did come they would need thousands per year not hundreds?

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Order was placed in 1936 when the plan was to hurriedly grow the entire RAF to 1750 aircraft in 124 squadrons (fighters, bombers, trainers, transports, everything). So the choice was along the lines of:

1. Order 600 Hurricanes and Spitfires right now, enough to kit out 1/6 of the entire airforce even allowing for 100% spare/attrition reserve. Move on to thinking about what to order in 1937 and hope that the 1938 designs will incorporate some useful lessons from Hurri & Spit experience.
2. Send supermarine and hawker off to redesign their aircraft to make better use of (a currently non-existent but plans for construction being drawn up) aircraft mass production infrastructure, and maybe order a bunch in 1937 depending on how well they do. Skip over the fact that nobody in the world has any experience of manufacturing modern military aircraft in these kinds of numbers so marking their homework will be a challenge. Meanwhile order 600 biplanes of some sort right now to make up the numbers, and wonder what will be learned for 1939 designs if the monoplanes do make it into service in 1938.

b0ned0me said:

But on the other hand, the Hurricane started out with fabric covered wings that significantly hurt performance. A lot of the structure was built out steel tubes that to be cut to exact length, then ends pressed exactly square, then fitted into fairly complex little fastener/joiner gizmos and tightened, all to aeronautical standards of quality. Much of the fuselage was built of lots and lots of little pieces of wood carefully nailed and glued in best WW1 fashion. Then cover in fabric, then dope fabric. All of that was actually pretty fiddly and time consuming work. It was relatively economical because it was old legacy technique with cheap labour and a decade of experience behind it.

The Spitfire had a lot of complicated structure but part of that was because it was more sophisticated and high performance aircraft - almost all metal, stressed skin. Assembling all the pieces was a fiddle but making many of the pieces was basically - put material in press, hold down switch, wait a moment, remove shaped component, send for riveting. A lot of manufacturing work could have been eliminated by either compromising the performance, or handling, or taking another six months on detailed design. But the RAF wanted a plane that was fast, climbed well, handled well, took off and landed well, and they wanted it yesterday.

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This. I was going to say that you should compare the structures of the DC-3 and the Amiot 143. Douglas had some of the most advanced manufacturing in the world (oddly, thanks to a draft-dodging French engineer). Their hydraulic presses cut and bent metal in a single operation, which was insanely efficient and allowed them to sell the DC-3 at prices lower than the production cost of some competing aircraft that had inferior performance.

For the D.520, you have many similar factors. It was also contrasted to the M.S.406, which combined multiple production techniques in a single aircraft.

tomo pauk said:

Whose job was to make sure that guns function between SL and service ceiling - costumer's or manufaturer's?

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I tend to blame the set dresser, not the costumer.

Mitsubishi Ki-46 specification issued 12 December 1937, top speed required 373 mph, first flight November 1939 but top speed was around 335 mph, an engine upgrade in 1941 brought the speed to 375 mph.

The western air forces had done a significant upgrade to their long range reconnaissance units in the second half of the 1930's by making reconnaissance versions of their latest twin engined bombers, the fighters caught up and passed the bombers, plus radar was around. The workload on the pilot of a single seater aircraft to navigate and fly for several hours is significant, hence the preference for at least a navigator.

The Hurricane metal wing, a quote,

"As regards the second difficulty, that of the stressed skin all metal wing (File No. 522507/36), the prototype had fabric wings, but it had always been hoped that production aircraft would have the new stressed skin wings. In these wings the metal covering was made a part of the weight bearing aerial structure. The importance if the stressed skin lay in its weight saving capacity. Fabric covered wings had already reached the limit of weight allowed and the stressed skin wings saved no less than 70 pounds. Investigations by the firms into this question had been started as early as July 1935. When the production specification was drawn up however it was decided that the Hurricane should in the first place be fitted with fabric covered wings, which would be interchangeable with metal ones. At this stage the technical state of the stressed skin wings was still experimental and not progressed further than the design stage. The position was aggravated by the fact that Hawkers were working on two other monoplanes for the Air Ministry at the same time, the P.4/34 [Henley] and the F.9/35 [Hotspur], both of which were expected to have metal wings. The construction was the same in each case, the difference being in the armament. In June 1936 it was estimated that the test wing would be ready by September but "stiffness" problems arose which involved a new design and it was necessary to earmark a special group of men in the drawing office for metal wing development. In December it was agreed that the P.4/34 should be fitted with the first pair of metal wings. This decision naturally reacted adversely on the Hurricane and in October 1937 it was considered that the new wings could not come in before the 301st aircraft. It was not until March 1939 that the first pair of production stressed skin wings were received from Glosters and fitted to production aircraft at Brooklands. Shortly afterwards flight tests were completed and the wings were cleared for operational use."

The Spitfire and Hurricane were not expected to last any longer in service than previous fighters, their successors, like the Typhoon and Tornado, designed to make best use of twice the engine power were under development. You could add things like the Defiant, the turret considered to reduce the big problems of accurate deflection shooting at the new high speeds.

The Gauntlet had 228 built, the Gladiator 746, including 207 for export, 50 Sea versions and 22 Sea Interim, in 1939 Gladiators were 320 out of 1,357 fighters built in Britain, along with 586 Hurricanes and 435 Spitfires.

In December 1939 the RAF put priority into developing the Spitfire, like the proposed mark III, versus the Hurricane.

Shortround6 said:

I think the change in operational environment took a lot of people by surprise.

Some people built a lot of photo recon planes but it turned out they needed a lot more than just a long range airplane with a camera.
It seems at quick look that some people assumed that light bomber could do the job and maybe it could in 1936-39 (depending on locale) but once the photo recon planes had to penetrate radar monitored airspace even quick airplanes were useless against fast fighters/interceptors. Recon planes that had to deep penetrate areas that had spotter systems gave some problems but the Japanese did use special high performance aircraft to stay out of trouble.
Once the idea that the recon plane could slip in, tootle about and take pictures and slip out without being spotted went away speed became the patron saint of recon planes.
Do 17s were replaced by Bf 110s and the Luftwaffe were having big problems figuring out how may of their strikes were doing. The Blenheim had gone from fasted bomber in the world to target in about 3-4 years.

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The Ar234 was originally conceived as a fast recon aircraft.

That it was able to be transformed into a bomber was a bonus.

elbmc1969 said:

This. I was going to say that you should compare the structures of the DC-3 and the Amiot 143. Douglas had some of the most advanced manufacturing in the world (oddly, thanks to a draft-dodging French engineer). Their hydraulic presses cut and bent metal in a single operation, which was insanely efficient and allowed them to sell the DC-3 at prices lower than the production cost of some competing aircraft that had inferior performance.

For the D.520, you have many similar factors. It was also contrasted to the M.S.406, which combined multiple production techniques in a single aircraft.

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Just got back from Le Bourget. The structure of the Yak-9 is a freaking nightmare ...

The metal tube structure is quite complex and seems to be bolted in some places and welded in others. Douglas would have been stamping out parts for a semi-monocoque that would have assembled far faster than this mess and would have been lighter and stronger. Combined with the attachments for 2x4s (the spars of the horizontal and vertical tails), it doesn't inspire confidence.

elbmc1969 said:

Just got back from Le Bourget. The structure of the Yak-9 is a freaking nightmare ...

The metal tube structure is quite complex and seems to be bolted in some places and welded in others. Douglas would have been stamping out parts for a semi-monocoque that would have assembled far faster than this mess and would have been lighter and stronger. Combined with the attachments for 2x4s (the spars of the horizontal and vertical tails), it doesn't inspire confidence.

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That sounds like a really great project plane.

Shortround6 said:

Finns and Russians figured things out before the war. They removed all traces of lubricant and accepted the increase wear.

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I read an account of a Soviet POW who demonstrated to a Luftwaffe squadron how to boil their MGs to remove all the lube.