Structure and Aerodynamics of SAAB 340 Series Aircraft

 

The following report will contain information on the SAAB 340 series aircraft in regards to its structure, aerodynamics, propulsion, operation and safety throughout its history     (1970’s-2017)

 

Introduction

The SAAB 340 is a regional aircraft series designed by the Swedish aerospace manufactures with the desire to explore commercial airliners with 30-40 passengers in the 1970’s, originally named the SAAB 340A. The plane was selected to run with GE turboprop engines due to their inherent fuel economy compared to a turbofan or turbojet engines. The SAAB 340B was the first development of the SAAB 340A, the newer aircraft had more powerful engines, a larger tail plane and the crafts range and take-off weight where increased. The SAAB 340’s ceased production in 1998 with lack of interest from airliners and hence profits reduced. The SAAB 340 has many renditions the most common being the 340A, 340B, 340B plus including carbo, corporate and commercial interior designs.

Of note is that originally SAAB AB and Fairchild (an American company) co designed the 340, having the original name of the SF340, Fairchild designed wings, wing mount nacelles and empennage.

The SAAB 340 can be defined as a monoplane with a tractor engine setup. It has a tricycle wheel layout and a conventional tail. The wings are dihedral, low mount, tapered and cantilevered from the fuselage with a single spar. Some of these are further investigated below.

It would also be of note to mention the SAAB 340B (most common variant of SAAB 340’s) has seating typically for 34 passengers where their weight with baggage is 214lbs, a fact sheet with the aircraft data and statistics will be attached in the appendix.

Structure and Construction

The SAAB 340 uses common and unusual structural components during its 40+ year service, some of the features which has characterised the aircraft is its single spar wing design, cable strengthen tail plane and a metal bonded skin. These structural components used by SAAB AB to maintain the range of their aircraft whilst also increasing range and payload.

The SAAB 340 uses a single spar wing which inserted behind the leading edge which allows for the complexity of the wing to be reduced compared to a multi spar wing. This is beneficial in the construction of a regional aircraft due to the reduced in cost in regards to the difficulty in which it is to design and then build a multi spar wing. The single spar also allows for more free space in the wing interrupted only by the ribs which are designed to have holes in them to reduce weight and allow for usable space (fuel and control surface actuators) in the aircraft, see figure 3 in the appendix. The single spar in the tapered wing also allows for a slender trailing edge of the aerofoil which can be further explored in the aerodynamical section of this report.

The SAAB 340 uses a unique feature for a commercial airliner to use, a cable was installed from the top of the empennage to another, the unique design will be further explored below however the cable installed on the 340A is a unique structural component using the cables tension to give the wing added structural rigidity. The newer 340B and B plus don’t have the cable installed, which may be due to lighter and stronger longerons or ribs or better engineered surfaces like metal bonding which is used to cover over 40% of the aircrafts external surfaces.

The use of metal bonding in its surface skin reduces weight and increases the structural integrity of the aircraft. This increased rigidity in the surface allows for the aircraft to be designed with less internal bracing and material weight. By incorporating the structural benefits of an aluminium bonded skin into the aeroplane, SAAB AB and the Engineers can best maximise the range, interior space and payload.

The SAAB 340 is made up of common modern aircraft parts and features, the fuselage and wings are made from aluminium ribs, spars, longerons and stringers which although not unique take advantage of the lightness of aluminium and its structural strength compared to a point 3% carbon steel or an expensive composite equivalent. The SAAB AB engineers therefore had to make the most of cheaper but most effective materials to keep costs lows for consumers of the regional aircraft but still have a light and fuel efficient aeroplane to make regional distance with 30-40 passengers as per the original design specifications.

The structural aspects of the aircraft are important in making the SAAB 340 maximise its space and payload, the engineers therefore had to combine techniques like tension in cables and intelligently designed wings which will minimise weight but still comply with complex fluid flow around the fuselage and aerofoils as well as can be keep passengers safe and comfortable.

Aerodynamics

The SAAB 340, like all modern aircraft, is designed to minimise drag whilst maximising lift. The SAAB AB aircraft uses intelligent and mandatory aerodynamics and aerodynamical systems on their aircraft; a unique tail plane, parabolic nose and fuselage shape and the wings shaping plus engine integration.

The SAAB 340 has a conventional empennage, where the vertical stabilizer has two parts, highlighted in figure 4 of the appendix. SAAB AB has incorporated this design in the 340 and their 2000 series aircraft. The two stages in the vertical stabiliser helps to create a more stable and streamlined laminar flow structure around the stabiliser and hence allows for the rudder to be more effective in controlling the airflow around it.

The SAAB 340 has a typical airliner style parabolic nose cone which leads into a straight circular fuselage (minus the ‘V’ shape wing mount nacelles on the bottom of the aircraft, shown on figure 6). The parabolic nose cone is highly efficient in having the air flow move around the aircraft without inducing large quantities if air resistance and drag. Further SAAB AB have slimmed the rear of the plane into a point, typical of modern airliners in an effort to have the air flow coming off the aircraft in a low drag streamlined structure.

The SAAB 340 uses a tapered, cantilevered, dihedral wing (as mentioned above) and has an equally cantilevered dihedral horizontal stabiliser both of which have aerodynamic features of no winglets, small cross sectional height as well as the wings having been integrated with the engine. The lack there of winglets on the SAAB 340 highlights how the engineers have been able to reduce the mixing of air from the top and bottom of the aerofoil. The usual effect of winglets will create a barrier, not allowing for the high and low speed air flow to create drag and vortexes. SAAB AB have been able to sculpt their aerofoils so that the need for a winglet has been reduced, a technique used by Boeing, on the 747 for example.

The SAAB 340 has also integrated their GE turboprop engines into the wing design. This is seen in figure 6 of the document particularly in the front view, where the engine has been “slotted” into the wing and has allowed for the turboprop to be elegantly and aerodynamically efficiently positioned to reduce the drag which can occur from having an engine hang off the wing.

This fitted setup also gives the operational functionality of the aircraft as the retractable undercarriage is also integrated into the engine area. This innovative solution for SAAB AB engineers has allowed for the aircraft to have a retractable undercarriage but keep stability on a runway, giving the SAAB 340 aerodynamically efficient flight and hence fuel efficient flight too.

The SAAB 340B+ differs from the 340B due to its extended wingtips which allows for it to be more aerodynamically efficient in more humid environments and to be able to take off on shorter runways, this variation in wing design of the different models gives the aircraft additional enhancements is rural and regional areas where the flying environment isn’t ideal.

Propulsion

The SAAB 340 uses GE turboprop engines with a 4-propeller prop from Dowty Rotol or Hamilton Sundstrand, differences in noise given in figure 5 of the appendix. The SAAB 340B uses the GE CT7-9B, pictured in figure 6 of the appendix, which has an 1870 shp rating [5].

Figure 8 a cutaway of the CT7-9B turboprop [6]

The turboprop engine used by the 340 is highly efficient engine with high range for its size. The engine can fly a maximum 470nm with a maximum total weight in excess of 29000lb (take off) on under 6000lb of fuel. The two engines can thrust the SAAB 340B and B Plus at 283 knots at an altitude of 25000 feet. Further information on range and payload is in figure 9 of the appendix.

The SAAB 340 has used different engines over the years, originally running CT7-5A2 turboprops the change to a CT7-9B led to an increase in the SHP of the engines, more power, and an increase in fuel efficiency.

More so, it is of current note that the SAAB 340B has had recent engine and propeller failures with REX region airlines in Australia. The failure of a Turboprop engine on the 23^rd of march 2017 and a propeller falling off mid-flight on the 17^th of march 2017. However due to intelligent engineering on behalf of SAAB AB the two planes could fly on one of the turboprops and make a safe emergency landing. Thus, highlighting both the aerodynamic design quality of the aircraft and how the GE CT7-9B engine can still power the aircraft with only half available thrust.

Operation Requirements

The SAAB 340 requires a runway, crew and a form of refuelling at destination/departure. Due to the SAAB 340’s design the aircraft has been deliberately constructed with regional and rural travel in mind where infrastructure is low and basic hence the 340 has built in facilities such as stairs and extra additions for the craft to cope and work in weather of the extremes and on runways made of gravel and dirt.

The SAAB 340B has extras such as the gravel pack and longer wingtips which allow for it to operate in humidity and heat without larger runways or advanced asphalt airports [8].

The SAAB 340B requires, at SL with optimal flaps and at ISA, a runway of 4220ft (1.3km) however at less optimum environments the runway needed can be 6690ft (2.02km).

The SAAB 340 does require 2 pilots and one flight attendant can be also apart of the crew. Die to the small seating numbers the SAAB 340 doesn’t have room for more than one attendant. The floor plan is shown below highlighting where the crew is stationed on a typical 340 layout.

Figure 10 SAAB 340 layout as intended by SAAB AB [4]

Safety Record

The SAAB 340 has a rather good incident record with only four flights ending in fatalities, however this is considered four times to many. The below table makes note of the four fatal incidents of the SAAB 340 [9]

Fatal Incidents
Year	Airline and Flight	Fatalities	Reason
1994	KLM city hopper 433	3	The Engine Stalled and the Aircraft struck the ground, the Aircraft broke upon impact
1998	Formosa Airlines	13	The Aircraft has electrical faults and the Crew was disorientated, the plane crashed in the ocean
2000	Crossair 498	10	The Aircraft banked to steep causing a spiral dive and resulted in the aircraft crashing
2011	SOL Líneas Aéreas 5428	22	The Aircraft whilst flying at FL179 began to pick up ice accretion and by the time the plane descended to FL140 the ice was to server causing the crash

The SAAB 340 has had successfully operated for many years with only few fatal accidents which is a testament to the engineering behind the aeroplane however aircraft isn’t without incident. Most recently Australian regional aircraft carriers Rex regional airlines has had two engine issues. Firstly, on the 17^th of march 2017 the propeller fell off the turboprop engine, followed by the 23^rd where the engine on the starboard side came up with warnings causing it to land.

These two recent examples showcase how even in extreme circumstances where 50% of the planes engines have been rendered useless the pilots can fly the regional airliner safely to an airport to land. Some other notable instances of the SAAB 340B having mid-air issues include a recent Western Air plane crash at Grand Bahama International Airport where following take off the plane experienced issues with its undercarriage and was forced into an emergency landing where the undercarriage then gave way, the intelligent design of fuselage floor and turboprop-wing integration allowed for the plane to stop without causing any major injuries to the passengers [11].

Conclusion

The SAAB 340 series was designed in the 1970’s as a cost effective and effect aircraft for SAAB AB to explore commercial aviation, this venture led to one of the most successful regional airliners with more units being sold than any other during its 40-year service history. The structural, aerodynamic and propulsion features although common place in aviation demonstrate the design of the 340 as a simple and effective flyer.

The 340 series has basic crew and infer structure requirements however this has allowed for it to fly from city to city and onto the county without changing the undercarriage. Coupled with it relative susses in terms of safety the SAAB 340 is a proven reliable aircraft.

Appendix

Figure 3 of SAAB 340B, note the ribs and spar in the wing [3]

Certified Noise Levels
Dowty Rotol	Hamilton Sundstrand
Fly-Over	78.4	78.2
Side-line	85.9	86.2
Approach	91.8	90.1

Figure 5 giving the noise differences between the two propeller companies [4]

Weights	lb’s	Flight
Max Take Off	29000	Max Cruise Speed		283 kt
Max Landing	                 28500	Max Operating Altitude		25000 ft
Max Zero Fuel	26500
Operational Empty	19000
Max Payload	7500
Max Fuel	5690
Airfield Performance
Sea Level	ISA	ISA+10°C	ISA+20°C
Take Off Weight (lb)	29000	29000	29000
Take Off Distance (ft)	4220	4460	4680
5000 ft above MSL	ISA	ISA+10°C	ISA+20°C
Take Off Weight (lb)	29000	29000	28180
Take Off Distance (ft)	5245	6215	6690
Sector Performance
Sea Level	100nm	200nm	300nm	400nm		500nm
Take of Weight (lb)	27995	38400	28750	29000		29000
Block Fuel (lb)	610	1000	1330	1630		1920
Block Time (min)	34	56	79	103		129
Passengers with Baggage	34	34	34	34		33
5000 ft above MSL	100nm	200nm	300nm	400nm		500nm
Take of Weight (lb)	27920	28180	28180	28180		28180
Block Fuel (lb)	595	975	1295	1580		1865
Block Time (min)	33	55	77	101		126
Passengers with Baggage	34	34	32	31		30
Specifications	Payload Range	Long Range Cruise		Long Range Cruise
Length	64 ft 9 in	34 Passengers	445nm		470nm
Height	22 ft 11 in	Maximum Payload	370nm		400nm
Wing Span	70 ft 4 in
Propeller ground clearence	1 ft 8 in	Engine
Cargo Door (HxW)	51 in x 53 in	SHP at Sea Level	1870
Sill Height, Passenger Door	5ft 4 in
Sill Height, Cargo Door	5 ft 6 in

Below is the SAAB 340B Fact sheet, Figure 11 [4]

References

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[20] ‘SAAB Regional Aircraft, Supporting Your Future’ brochure from SAAB AB website

PART TWO: aircraft data and graphs

Manufacturer	Aircraft	MTOW	max range	no. pass	ref no.
Airbus	A300-600	375900	4050	266	1
A310	361600	4350	220	2
A318	149900	3100	107	3
A319NEO	166400	4200	124	4
A320	172000	3300	150	5
A321	206100	3200	185	6
A330-200	529100	7250	247	7
A330-300	529100	6350	277	8
A330-800NEO	533500	7500	257	9
A330-900NEO	533510	6550	287	10
A340-200	606300	6700	261	11
A340-300	609600	7300	277	12
A340-500	837800	9000	293	13
A340-600	837800	7800	326	14
A350-800	546700	8200	280	15
A350-900	617300	8100	325	16
A350-1000	681000	7950	366	17
A380	1272000	8200	544	18
Boeing	707-120	248000	2607	181	19
717-200	110000	1440	106	20
727-100	170000	2703	131	21
737-700	154500	4400	149	22
737-800	172500	4000	189	23
737-900	187679	3200	215	24
747-8	987000	8000	410	25
757-200	255000	3910	228	26
777-8	242630	8700	375	27
777-9	759000	7600	425	28
777-200LR	766000	8555	317	29
777-200ER	656000	7065	313	30
777-300ER	775000	7370	396	31
787-8	484002	7355	242	32
787-9	557000	7635	290	33
787-10	557000	6430	330	34

(Graphs on page 14)

From the above data, we can calculate the pound/nautical miles for each aircraft, by doing this we can then multiply the results by 11 000nm to give the MTOW for that aircraft to fly that distance. By averaging the results, we can get a weight of 856728.7lbs. working is shown below in aircraft order as the table on page 14 (A300-600 to 787-10).

weight/range	x11000
92.81481481	1020963
83.12643678	914390.8
48.35483871	531903.2
39.61904762	435809.5
52.12121212	573333.3
64.40625	708468.8
72.97931034	802772.4
83.32283465	916551.2
71.13333333	782466.7
81.4519084	895971
90.49253731	995417.9
83.50684932	918575.3
93.08888889	1023978
107.4102564	1181513
66.67073171	733378
76.20987654	838308.6
85.66037736	942264.2
155.1219512	1706341
95.12850019	1046414
76.38888889	840277.8
62.89308176	691823.9
35.11363636	386250
43.125	474375
58.6496875	645146.6
123.375	1357125
65.2173913	717391.3
27.88850575	306773.6
99.86842105	1098553
89.53828171	984921.1
92.85208776	1021373
105.156038	1156716
65.80584636	723864.3
72.9535036	802488.5
86.6251944	952877.1
average=	856728.7

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