# Beside payload size, what precise factors are considered to choose the fuselages shape?

I am currently reading this very interesting book, which explains -I guess- most of the things you need to know in order to design aircraft. Yet it is not clear to me besides the payload size what other factors are precisely used in order to determine the exact shape and size of the fuselage. I know that aerodynamics are considered, but that's very broad...

I read that the conic lofting method is usually used to determine the exact shape of the fuselage. But I would need some more explanation to get a better understanding of how this is determined. In other words: I know my wing size, airfoil, payload size and would like to design the fuselage. How should I proceed?

Thanks

EDIT: Feel free to share the pragmatic approach

What bothers me is that I ended up finding a webpage with some interesting guidelines, but contains many constant values I have no I idea where they cone from: http://www.barnardmicrosystems.com/UAV/uav_design/guidelines.html

eg of such values: wingspan: 1.041, 0.382; step 3 length: 1.775

• What type/size of aircraft? Related: Chapter 6 of Design Of Aircraft or two thesis at TUDelft.
– mins
Commented Dec 15, 2017 at 19:21
• @mins type: fixed wing, low speed single engine. Ideally something along those lines: blog.tmcnet.com/robotics/Lockheed%20Martin%20Desert%20Hawk.jpg But maybe with the prop in front. I think having it at the fron may maybe make some stuff easier to understand and find out for me Commented Dec 15, 2017 at 19:36
• Don't increase drag too much for the engine you've probably already picked. So give it a boattail.
– user7241
Commented Dec 15, 2017 at 19:52

Assuming an engineering background, here’s a short list of factors affecting fuselage shape and size.

1. aerodynamic considerations
2. volume considerations
3. structural and elastic considerations
4. operational and interface considerations.

Aerodynamically, drag and stability are the major concerns.

Volume wise, payloads, subsystems, cargo, propulsion, etc must be considered. So it might take a few iterations for a preliminary design to converge at a feasible and optimum solution.

Structurally, elasticity of the fuselage is important, at least for weight efficiency, and general comfort. a bad shape can cause unwanted structural modes, which could risk failure in flight, or may cause difficulty in controls. If you were ever on a 747, you probably noticed how much it snakes towards the rear of the plane. The oval shape of the fuselage has parts to blame.

Finally, sometimes doors, openings, and structural interfaces dictate geometries of fuselages.

Additionally, geometric constraints such as tail strike prevention, or belly landing can be other reasons to shape the fuselage.

• I have an EE background. What good approach (techniques, alorithms, intuition, ...) would you suggest to look at if I want to investigate on point nr 1 in my current situation? The book I am reading doesn't AFAIK explain a generic way of investigating #1 Commented Dec 15, 2017 at 21:22
• Pages 109, 149 and 155 seem to cover the generals of the topic. The shape of the fuselage cannot be handled isolated from the general configuration of the aircraft. But for a small and not so expensive process I’d say go ahead and try to fit things inside and try to make it as small as possible. However, if you think it professionally, get an aircraft designer in the team so that your aircraft will (hopefully) not deconstruct mid air, due to a gust or due to some control inputs. :) Commented Dec 15, 2017 at 21:29
• indeed it is a very general and imho not pragmatic explanation. In my ideal world the book would contain smth like: step 1 do xyz because, step 2 do yxz because etc... Is there any book you'd recommend me reading in order to know how that's done professionally? Everything seems to be proportioned so there might be a way for me to get a proper understanding of everything without going too deep in aerodynamics, strength analysis and all that stuff... Commented Dec 15, 2017 at 21:40
• Basically I could make my payload fit in the fuselage, but I guess that for aerodynamical reasons and balancing I may have resize it a bit in order for the UAV to fly, which is why I think just making sure that the payload fits is probably only 20% of the work. So far I only have a wingspan of up to 1m (because that is the easies to build myself) and I have to size my fuselage to that. Yet according to what I found online there is no ratio between the fuselage size and wingspan/area. Which is why I'd need some guidelines Commented Dec 15, 2017 at 22:54