Expert: Ray Wilkinson - 11/16/2006

Question
Hi Mr Wilkinson, I hope everything it's ok.  I live in Puerto Rico, the very first "big" Island you will encounter coming from Europe.  I am very into someday building a BD4, I like it very much, because it is fast and strong and boxy.
I am 26, but I hope to get it in a couple of years with the help of God.  I am always dreaming of that aircraft and everything I want to put in it.  You probably imagine what I am talking about: Instruments and Avionics and Engine Monitor and Camcorder Display and Warnings and Switches and Fuses and Controls, and Luxury Leather Upholstery and Colors and Markings and Air Conditioner and Bezel Lights and Power Plant -Lycoming io-390x-, which is most powerful 4 cylinder in the market *40,000 dollars* delivering 210hp at 2700 rpm...  I mean like I said is very nice to dream of these, and the best part is that I have a very nice plan to make it happen in a few years *becoming a Warrant Officer Rotary Wing Pilot *6 years contract* in the US Army, and building and selling a couple of nice log homes...  The total cost of the BD4, I estimate, 150,000 at the most... And that's cheap, if you think of every detail *specially strengh and speed and comfort and state of the art cockpit *yeah*...  
    But what I needed to know is what do I need to do in order to make that fuselage pressurized, or at least as close as pressurized as it can get?  Mr Wilkinson, I want to thank you for taking the time to answer me this question.  You will be contributing to fulfill somene else's dream.  Also, Mr Wilkinson, in the far future I really will like to build a High Wing, bi motor *IO-390-X **** which I forgot to tell you that was design for experimental aircraft and I dont think they are selling it as mass production yet, they have an aerobatic version too***  so, one engine on each wing *like I said, that's 210hp on each wing, Wao!  to me that must be crazy!
I wanted to be an aircraft for the pilots *2* and 4  more passangers...with retractable landin gear...  What do you think Mr Wilkinson?  Do you think you can make me a little scratch drawing of how the aircraft would look *according to your expertise*, I prefer if the wings look like a bird, just as the DC3...  I mean I just would like to have a rough idea of the dimensions...  Sorry, but I still do not have all these books related to design, in which case I would have a much clear idea of the concept.  Well Mr Wilkinson, thank you again,,  Respectfullly, Julio Muņoz  

Answer
Hi Julio

This is a tricky question to answer. The fuselage of an aircraft needs to be designed from the outset to hold pressure, because the load on the fuselage skin due to pressurisation can be large. This is therefore one of the first questions the designer will need to address when an aircraft is first conceived. It is very difficult or impossible to modify an aircraft after it has been built. Also, there are systems that need to be installed to provide the pressuriation, and these need also to be allowed for during the design, and doors and windows also need to be designed differently.

You used the term 'as close to pressurised as I can get', but I'm not sure how to interpret this, as there's no half measure here (although you can pressurise to different levels). Civil airliners are generally pressurised to 8000 feet - that is, the cabin pressure inside the aircraft is the same pressure as an unpressurised aircraft flying at 8000 feet, even if the aircraft is flying at 35000 feet. Military aircraft are generally pressurised to higher cabin altitudes. It sin't worth putting in a very small amount of pressurisation, as the costs outweigh the benefits.

The load on the skin due to the pressure for an airliner at 35000 feet is about 100kg per square metre, which puts a big tensile load on the skin, and it also changes the stress distribution on the skin.

Your other question - about the shape of the aircraft - is also not easy to answer. To design an aircraft requires a lot of knowledge and understanding about aerodynamics, structures and materials. It's way beyond my capability, as I know a reasonable amount about structures and a bit about materials, but little about aerodynamics (certainly not to the point where I could design an aircraft), which is why aircraft are generally designed by teams of specialists. We have our final-year students design an aircraft, but they are in groups with each student taking on a specialist area, and they don't complete the whole design in the six months that they have, just get the design concepts sorted out. There are some good books on the subject, but they may not be easily available outside the UK.

I hope this helps.

Regards

Ray