Weight estimation in Aircraft design.
Introduction before introduction:
This is one the lectures that i am taking in Wintersemester 2020. I found this course to be very interesting (Advanced aircraft design 1). However interesting this course maybe, the examination process is also differnt. Instead of a conventional written exam, this exam will take place like a Viva. So this will be a different experience. I have had some exams in this form in Germany, however this form of examination really questions your understanding. and your skill of explaining. And i have come up with this idea of writing a blog for each of the chapter we learn in class. This is a good idea to me because the course not just requires me to understand the subject matter but also learn how to express an idea in a spoken form. I need to brush on my speaking skills as well. I find this challenging task however also necessary. So having said that i would like to start my one of the early blogs. :)
Introduction:
So today we are going to learn about the weight estimation in aircraft design. So aircraft design is a tedious process which takes a lot of time and data. The initial phases of aircraft design is one of those phases where you have little criterias and lot of room to make changes however as we go down the line of aircraft design the wiggle room decreases ever so slightly. so the steps and kind of assuptions we make in the early stages helps us define the performance of the aircraft as a final product.
So what is weight estimation?
As the name itself explains, it is an act of estimating the weight of an aircraft. As i mentioned above the weight of the aircraft in its initial phase is a variable which habe little number of criterias which limit the values. however these numbers increase with the progression of the design phase.
Before going in theis topic further more. we need to understand what is aircraft weight and what do we mean when we say aircraft weight.
Aircraft weight is different at different phases of the flight. For example the weight of the aircraft decreases for a fuel burning engines changes with the distance. As the aircraft covers ground the fuel is burnt and the weight of the aircraft decreases so the landing weight of the aircraft is less than the weight during takeoff. here we need to consider some of the different weights of the aircrat.
1. MTOW- maximum takeoff weight is the weight of the aircraft during the take off phase. the weight of the aircrfat decreases with the journey. However this is the weight which is considered during the aircraft design process. and all the other factors like the runway length and engine thrust are decided with this value in mind.
2. OEW- Operational Empty Weight. is the weight of the aircraft which has all the systems and parts that are required for safe flight. this includes the airframe structure, propulsion plant, other systems like pneumatics and hydraulics, and also the operationa equipments.
So the weight of the aircraft that we are considering here is OEW+ Payload+ Fuel.
in a normal design of a short range aircraft the fraction of the fuel weight is about 25% however this is 45% for a long range aircrafts. This can also be seen in a different way. If we want to fly an aircraft for a longer distance then we need to increase the weight of the fuel and decrease the payload. and if we want to travel short distance, we could load less amount of fuel and more payloads anc complete the mission requirement.
Weight growth.
this is a phenoenon that takes place during the design phase of an aircraft. as the estimation of an aircraft is done in the initial phase with minimum criteria and the deisgn is not detailed, the design has a wiggle room to some extent. This wiggle room is filled in during the further steps of design and production steps.
This includes the suitability of the massproduction design. Use of better materials, or due to unavailability of certain parts or materials. addition of structures due after structural analysis.
However the weight growth should be under the limit of MTOW. because this is the limit that affects the propulsion plant thrust and also the cost of the whole project and also the performance of the aircraft.
Weight estimation methods:
there are basically 4 different methods or classes of weight estimation:
1. Class I
this method is based on the breakdown of the existing aircraft with similar mission requirements. So the weight of the aircraft is divided into different subgroups and represented in a fractional form with respect to MTOW. so if the fractional weight of a wing is 10% for an aircraft type then in the design phase of the same aircraft type the fraction is taken to be 0.1. This is under the assumption that the fraction remains the same for each aircraft type.
This is also the reason this method is called fractional method.
2. Class II
This method is based on the geometric design and V-N diagrams and also the emperical equations. Ans also some statistics.
3. Class II & 1/2
This method uses a simple idea which is by calculating the amount of material required to resist the appled loads. This method is better than the class I and is also design sensitive. So one could use this method to apply for different configurations.
4. Class III
This is basically based on FEM method
Conclusion:
So in thes graph you see the orange is the statistics and blue is the calculation. So we move down the desing process the estimation uses more calculation than statistics.
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