1 SIZING MATRIX AND CARPET PLOTS Serkan Özgen, Prof. Dr. Middle East Technical University, Dept. Aerospace Eng., Turkey 1. Introduction Aircraft design is an intellectual process that combines engineering knowledge, creativity and art. For this reason, in major companies aircraft design work is undertaken by Integrated Product Teams (IPTs) consisting of engineers, technicians, industrial design experts, managers each with different backgrounds and even cultures. Therefore, aircraft design is not done by one person aimed at a single goal but rather is an activity with multiple objectives. A successful design is technically sound, feasible, affordable, safe, reliable and aesthetically pleasing. When one looks at the History of Aviation and the airplanes that have become benchmarks like the Douglas DC-3, Cessna 172, Supermarine Spitfire, McDonnell Douglas F-4 and others, one notices that these were not the fastest airplanes of their class, nor they were the ones carrying the heaviest payload, nor they were the cheapest or aesthetically the most pleasant. These airplanes were a good combination of technical soundness, feasibility, affordability, safety, reliability and aesthetics built around realistic requirements. Those were optimum airplanes designed at the right time at the right place. Therefore, the task of a designer is to create a flying machine that is technically sound, safe, reliable, feasible and affordable. These objectives is to be kept in mind from the very beginning, namely the conceptual design phase. However, the designer is immediately faced with contradicting requirements to meet these objectives. For example a very safe airplane will probably not be feasible or affordable. Likewise, an airplane with a high technological level will be very expensive. This brings us to the concepts of trade and optimization. A good design is an efficient compromise of performance, safety, reliability, cost and aesthetics. This manuscript aims at outlining the basics of optimization of the performance and the weight of a light sportive airplane. The simple methodology explained is most relevant for the conceptual design phase where sizing and performance calculations constitute the major task. A well-optimized airplane is less likely to encounter unsurmountable weight and cost increases and performance deficiencies as the design progresses into preliminary and detail design phases. The four main ingredients of the presented method are weight estimation, aerodynamics, installed thrust and performance.
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SIZING MATRIX AND CARPET PLOTS
Serkan Özgen, Prof. Dr.
Middle East Technical University, Dept. Aerospace Eng., Turkey
1. Introduction
Aircraft design is an intellectual process that combines engineering knowledge, creativity and
art. For this reason, in major companies aircraft design work is undertaken by Integrated
Product Teams (IPTs) consisting of engineers, technicians, industrial design experts,
managers each with different backgrounds and even cultures. Therefore, aircraft design is not
done by one person aimed at a single goal but rather is an activity with multiple objectives.
A successful design is technically sound, feasible, affordable, safe, reliable and aesthetically
pleasing. When one looks at the History of Aviation and the airplanes that have become
benchmarks like the Douglas DC-3, Cessna 172, Supermarine Spitfire, McDonnell Douglas
F-4 and others, one notices that these were not the fastest airplanes of their class, nor they
were the ones carrying the heaviest payload, nor they were the cheapest or aesthetically the
most pleasant. These airplanes were a good combination of technical soundness, feasibility,
affordability, safety, reliability and aesthetics built around realistic requirements. Those were
optimum airplanes designed at the right time at the right place.
Therefore, the task of a designer is to create a flying machine that is technically sound, safe,
reliable, feasible and affordable. These objectives is to be kept in mind from the very
beginning, namely the conceptual design phase. However, the designer is immediately faced
with contradicting requirements to meet these objectives. For example a very safe airplane
will probably not be feasible or affordable. Likewise, an airplane with a high technological
level will be very expensive. This brings us to the concepts of trade and optimization. A good
design is an efficient compromise of performance, safety, reliability, cost and aesthetics.
This manuscript aims at outlining the basics of optimization of the performance and the
weight of a light sportive airplane. The simple methodology explained is most relevant for the
conceptual design phase where sizing and performance calculations constitute the major task.
A well-optimized airplane is less likely to encounter unsurmountable weight and cost
increases and performance deficiencies as the design progresses into preliminary and detail
design phases. The four main ingredients of the presented method are weight estimation,
aerodynamics, installed thrust and performance.
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2. Requirements
Each airplane is designed around a set of requirements. The key to the success of a design is a
set of realistic and consistent requirements. The requirements involve purpose and operation
of the aircraft, performance characteristics like speed, range, rate of climb, etc., and also
mission characteristics like payload, low observability, etc. The requirements may be set by
the customer, by safety and certification requirements or a combination of both. The sizing
process in the conceptual design phase is usually driven by performance requirements.
The requirements for the light sport aircraft for the VKI Short Course: UAVs & Small
Aircraft Design are given in Table 1. In addition to these “customer” requirements, the
designer may utilize additional requirements specified in the Certification Specifications. For
this airplane, the EASA Certification Specifications that may be applicable are: CS-23:
Certification Specifications for Normal, Utility, Aerobatic and Commuter Category
Aeroplanes [1]; CS-VLA: Certification Specifications for Very Light Aeroplanes [2]; CS-
LSA: Certification Specifications and Means of Compliance for Light Sport Aeroplanes [3].
Also FAR-23: Airworthiness Standards: Normal, Utility, Acrobatic, and Commuter Category
Airplanes [4] is applicable. Relevant CS and FAR performance requirements are given in
Table 2. In the current study, the light sport airplane will be designed and optimized according
to the “customer” requirements but the final optimized design will be checked against CS and
FAR requirements as well.
It should be noted that CS-23 and FAR-23 are applicable to Normal, Utility, Aerobatic and
Commuter Category airplanes. Normal, Utility and Aerobatic Category airplanes are those
with a certified maximum take-off weight of 5670 kg (12500 lb) or less, and those having a
seating configuration, excluding the pilot seats of nine or fewer. Commuter Category refers to
propeller-driven twin engine aeroplanes that have a seating configuration, excluding the pilot
seats of nineteen or fewer and a certified maximum take-off weight of 8618 kg (19000 lb) or
less. Certification Specifications for Normal, Utility and Aerobatic category is further divided
into two subcategories, namely airplanes heavier or lighter than 2722 kg (6000lb).
Certification requirements differ slightly between these three categories and only those
specifications corresponding to Normal, Utility and Aerobatic Category airplanes for a
certified maximum take-off gross weight of 2722 kg or less with a single reciprocating engine
are included in Table 2 because the Light Sport Aircraft designed according to the
requirements in Table 1 falls into this category due its weight, calculated below.
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Table 1. Design requirements for the VKI Short Course: UAVs & Small Aircraft Design.
Definition Light Sport Aircraft
General
Number of engines
Occupants (80 kg each)
1
2
Performances
Optimized for
Speed range (km/h)
Altitude (m)
Range (km)
Rate of climb (m/s)
Take-off run (m)
Stall speed (km/h)
Cruise
≈ 280
2400
1000
> 5
< 300
< 100
Useful weight
Luggage/crew member (kg) ≈ 10
Miscellaneous
Comfortable
Green
Safe
Cheap
Table 2. EASA and FAA design requirements and specifications
for Normal, Utility, and Aerobatic Category airplanes
(Wo ≤ 2722 kg/6000 lb, single reciprocating engine category only).
Definition FAA
FAR-23
EASA
CS-23
EASA
CS-VLA
EASA
CS-LSA
Applicability
Maximum take-off weight
Number of engines
Type of engine
Number of crew
Max. number of seats
2722 kg
one
reciprocating
2
9
2722 kg
one
reciprocating
2
9
750 kg
one
spark or
compression
ignition
2
0
600 kg
one
non-turbine
or electric
2
0
Performances
Stall speed, VSO1
Rotation speed, VR
Climb speed (@15m/50ft height), VCL
Climb rate or gradient @ VCL
Approach speed, VA
Climb gradient @ VA
61 kt
≥ VS12
≥ 1.2VS1
≥ 8.3 %
≥ 1.3VSO
≥ 3.3 %
113 km/h
≥ VS1
≥ 1.2VS1
≥ 8.3 %
≥ 1.3VSO
≥ 3.3 %
83 km/h
≥ 1.3VS1
≥ 2 m/s
≥1.3VS1
1:30
83 km/h
1 VSO: stall speed in the landing configuration. 2 VS1: stall speed obtained in a specified configuration. In this case, take-off configuration.
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3. The Baseline Design
An airplane is designed, which will be referred to as the Baseline Design hereafter, using
well-known methods outlined by Raymer [5], Roskam [6] and Anderson [7]. The Baseline
Design is accomplished following the steps:
i. Competitor study,
ii. First weight estimation,
iii. Airfoil and wing planform selection,
iv. Power-to-weight ratio (P/W) and wing loading (W/S) selection,
v. Refined sizing and a better weight estimation,
vi. Geometry sizing and configuration,
vii. Configuration sizing,
viii. Aerodynamics,
ix. Empty weight estimation using statistical component weight estimation method,
x. Installed and uninstalled thrust,
xi. Performance and flight mechanics.
Sizing and trade studies is the twelfth step and is the subject of this study. The configuration
of the Baseline Design is as follows:
Low-wing monoplane with conventional tail configuration,
Tricycle fixed landing gear,
Single naturally aspirated reciprocating tractor engine with a three blade constant
speed propeller,
Number of crew: 2 with side by side seating.
Figure 1 shows the three view drawing of the Baseline Design prepared using OpenVSP [8],
with its important design characteristics. Table 3 compares its performance characteristics
with the requirements given in Table 1. Data in Figure 1 show that the designed airplane falls
into the CS/FAR-23 Classification because of its weight but not the VLA or LSA
Classification. Also, the positive limit load factor selected for the design puts this airplane in
the Aerobatic Category. From Table 3, it can be seen that the airplane satisfies all the
“customer” requirements by a comfortable margin. The questions that remain are: Is this the
best airplane satisfying the requirements? Can a lighter, i.e. greener and less costly airplane to
acquire and operate be designed that can still meet the requirements?
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Characteristics Value
Length (m) 7.54
Wing area (m2) 11.2
Wing span (m) 10.0
Aspect ratio 9.0
Maximum take-off
gross weight (kg) 944.7
Wing loading, W/S
(kg/m2) 84.3
Wing airfoil NLF(1)-0115 [9]
Engine Lycoming IO-360-L
Propeller 3 blade/76” diam.
Engine power
(hp/kW) 160/119.3
Power-to-weight
ratio (kW/kg) 0.126
g-limits +6, -3
Figure 1. Three view drawing of the Baseline Design with important characteristics.
Table 3. Performance characteristics of the Baseline Design.