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Designing an aircraft

Old 26th Nov 2019, 03:02
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Question Designing an aircraft

This is going to sound like a stupid question, but how do engineers start off designing an aircraft?

Do you look at the cabin capacity you want then build an airframe around it, followed by engines that suit the AUW? For things like fighters, do they start with the weapon system and design around that?

Where do you actually start?
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Old 27th Nov 2019, 15:47
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Originally Posted by evilroy View Post
Where do you actually start?
I don't have a intelligent answer for you, but I know of a couple helicopter models that started as 2 saw-horses with 2 "unbreakable" ventilation ducts laid across them, from which they built the entire aircraft around those ducts. It's only my opinion, however, when those "unbreakable" ducts eventually do break, it seems like you have to disassemble the entire aircraft to replace them.

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Old 28th Nov 2019, 18:30
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Originally Posted by evilroy View Post
This is going to sound like a stupid question, but how do engineers start off designing an aircraft?

Do you look at the cabin capacity you want then build an airframe around it, followed by engines that suit the AUW? For things like fighters, do they start with the weapon system and design around that?

Where do you actually start?

You're right...almost. You start by deciding what it is you want the beast to do! i.e. Do you want to fly one pilot to prove a concept or try to build a two-crew commercial cargo carrier / high altitude strategic bomber? By doing this you can decide the amount of weight you want to carry and the distance and speed you want to go - and then start working out the machinery to do that....

Good luck!
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Old 30th Nov 2019, 10:34
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Alternatively, take a look at the B Ae A.T.P................. and how not to design an aircraft ...the perfect case study for budding design engineers !
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Old 2nd Dec 2019, 08:11
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Originally Posted by evilroy View Post
This is going to sound like a stupid question, but how do engineers start off designing an aircraft?

Do you look at the cabin capacity you want then build an airframe around it, followed by engines that suit the AUW? For things like fighters, do they start with the weapon system and design around that?

Where do you actually start?
I used to teach this very subject in a university, and there is no unanimity of opinion, but I'll tell you how I taught it.

You start by defining four things, upon which the design will rest.

(1) What is the role of the required aeroplane?: for example required range, payload, ceiling, runway length requirements...

(2) Understand thoroughly the regulatory environment that you are designing into - in particular the airworthiness and operating regulations, as except for some minor issues (and in those invariably on the basis of "equivalent safety", nobody will vary them for your design.

(3) Understand thoroughly the operating environment. So temperatures, pressures, external threats (whether that's enemy action or weather), solar radiation, skills of pilots and mechanics who'll be operating it.

(4) Understand thoroughly both the available present technology, and the direction of travel and timely delivery of new technology developing. This will include a lot of stuff like the masses and physical sizes of the various major components (lots of graph plotting gets done).

From these four, you can start to produce an approximate shape and initial table of the major components and their masses and configurations. A principle called "Figure of Merit" is generally used to determine preferred engine, fuselage materials, wing structure and so-on from the various options available to the designer. This then gets put together into an initial "conceptual design".

This design is then analysed and the strengths and weaknesses determined in detail. From that analysis, you then iterate towards a new design. About this point you bring some professional pilots and mechanics onto the design team to advise on best practice issues. It'll probably go through 4-6 iterations "on the drawing board" (nowadays in reality all in computer design systems such as CATIA).

When you think you are happy with the outline design, you take it to the market and confirm demand and potential acceptability. At that point you build a suitable sized team, and start designing it in detail, testing sample "bits" (wind tunnel models, wing structures...) and commissioning the detailed research needed to support it.

You then build one or more prototypes, do some necessary ground testing then (developmental) flight test them. From this, you will inevitably find deficiencies and areas needing improvement - that is what "Test Pilot Opinion" is all about.

You then start full production, the first entering (operational) flight test. On the basis of the OT&E process, operating data and training regimes are developed.

Then it finally enters service.

I'll bookmark this thread so happy to expand on stuff later.

The most commonly used university textbook on aeroplane design at the moment is this by the way: RAYMER AIRCRAFT DESIGN: A CONCEPTUAL APPROACH (personally I'm not a huge fan, but it's very widely used). I grew up on this book:
Amazon Amazon
which is a lot better, but unfortunately somewhat out of date now and the author's estate refuse permission for anybody to update it.

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Old 9th Dec 2019, 22:18
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Thanks for all the replies!
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Old 10th Dec 2019, 01:11
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Key principle of successful aircraft design (or any business for that matter) that is so often forgotten:-

Product costs 1 to make and sold for 1.01 = success
Product costs 1 to make and sold for 0.99 = failure

It doesn't matter how good your design was if you can sell it for a surplus.
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Old 10th Dec 2019, 13:02
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Originally Posted by Dan Dare View Post
Key principle of successful aircraft design (or any business for that matter) that is so often forgotten:-

Product costs 1 to make and sold for 1.01 = success
Product costs 1 to make and sold for 0.99 = failure

It doesn't matter how good your design was if you can sell it for a surplus.
In aviation, that ceased to be true years ago, if it ever was.

The price you sell it for is a tiny part of the lifecycle cost. So if you sell the product for 50p that cost 1 to build, add to that 2 in R&D to develop it, but then the through life net income off supporting the product is 10, you've won. That is the reality of most modern aviation products, despite initially selling it at a loss.

There's also the sadly real fact that often a company is in the doldrums between major sales, and will build and sell products deliberately at a loss for two reasons: one that it's cheaper in the short term to build and sell at a loss than to lay off the workforce and decomission plant, and two that it needs to maintain the skillset until the next large product sale.

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