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ADVANTAGES OF DORNIER DO-335 CENTRE LINE THRUST (CLT) AEROPLANE COMPARED TO WING MOUNTED TWIN ENGINES

AEROPLANES OF THE SAME CLASSIFICATION AND

A PROPOSED DESIGN FOR CLT AEROPLANE

Norul Ridzuan Zakaria 1Norul Zafinas Zakaria

Malay Institute of Aero & Space Studies (IKAM)

Level 13, Plaza Masalam, Section 9 40100 Shah Alam, Selangor. IKAM290200@hotmail.com

1School of Aerospace Engineering

University Science Malaysia 14300 Nibong Tebal, Penang.

mixed_crush@yahoo.com

As presented at and appeared in the proceedings of Seminar on Engineering and Technology 2006, organised by Federation of Engineering Institutions of Islamic Countries and co-organised by

Institution of Engineers Malaysia, University Putra Malaysia and University Kuala Lumpur on September 4-5, 2006 at Putrajaya, Malaysia.

ABSTRACT

By the end of World War II, there was a ground attack and fighter bomber aeroplane tested by Luftwaffe or German Air Force, which has a revolutionary design known as Dornier Do-335. It has 2 engines mounted at its fuselage, which one at the front of its cockpit pulling the aeroplane forward and the other one at the rear of its cockpit pushing the aeroplane forward. This configuration, known as centre-line mounted twin engines or centre line thrust (CLT) has some advantages compared to the conventional twin engines layout, which the engines are mounted on the wing. The first part of this paper studies the advantages by identifying and comparatively measures the values of the advantages and explains how CLT aeroplane is better than wing mounted twin engines aeroplane of the same classification in performance, manoeuvrability and practicality especially for functioning as ground attack aeroplane. The second part of this paper uses the result of the study in the first part to propose a design for ground attack CLT aeroplane. KEYWORDS Centre line thrust (CLT), wing mounted twin engines, maximum speed, initial climb speed, wing span, front cross section area, drag, moment of inertia, manoeuvrability. INTRODUCTION In 1943, Luftwaffe was testing a new design for aeroplane known as CLT aeroplane. The two engines are mounted along the straight line, which goes across the centre of the aeroplane�s fuselage from front to rear when viewed from above. One engine is located in front of the aeroplane�s cockpit, which pulls the aeroplane forward, and another engine is located behind the cockpit, which pushes the aeroplane forward. CLT design was already familiar with seaplane since early 1920s. Dornier Wal, the CLT seaplane produced by Dornier were used to carry passengers and mails across Atlantic Ocean to North America and South America since mid 1920s, flown to North Pole on Amundsen-Ellsworth Polar Expedition in 1925, and produced under license in Spain, the Netherlands and Japan by end of 1920s. The aeroplane was so reliable that it became so popular for air forces and commercial airlines during that era (1). The effectiveness of CLT

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seaplane was further proven by Dornier Do-18, another CLT seaplane produced by Dornier in 1936 and performed similarly. Comparing a CLT aeroplane to a conventional wing mounted twin engines aeroplane, it can be understood that the former has advantages compared to the later design because centre line mounted twin engines design reduces the front cross section area and produces aerodynamically clean wing, which automatically minimize drag and increase aerodynamics. The design also eliminates the asymmetry problem when one engine fails. The Luftwaffe�s CLT aeroplane is called Dornier Do-335. It was to function as single seat ground attack and fighter bomber aeroplane for Luftwaffe. By 1945, the air force has successfully flight tested several units of Do-335 even though none were recorded engaged in any mission during World War II. This paper is the study on the advantages of Do-335 by comparing it to conventional wing mounted twin engines ground attack and fighter bomber aeroplanes of the same classification. From the study, a design for ground attack CLT aeroplane is proposed.

DEFINITION Centre line thrust (CLT) aeroplane Aeroplane with its 2 engines mounted along the straight line, which goes across the centre of the aeroplane�s fuselage from front to rear. Wing mounted twin engines aeroplane Aeroplane with its 2 engines mounted at its wing. Ground attack aeroplane Aeroplane specially designed and equipped with weapons to destroy targets on the ground. Fighter bomber aeroplane Aeroplane specially designed and equipped with weapons to destroy aeroplane in the air and targets on the ground. OBJECTIVES The objectives of this study are as follows: 1. To list and measure the advantages of Do-335 over wing mounted twin engines aeroplanes of the

same classification.

2. To demonstrate that CLT aeroplane is better than wing mounted twin engines aeroplanes for functioning as ground attack and fighter bomber aeroplane.

3. To propose a design for ground attack CLT aeroplane. METHODOLOGY

1. List the engines and specification of Do-335.

2. Provide an illustration of Do-335, which clearly show that it is a CLT aeroplane.

3. Select wing mounted twin engines aeroplanes of the same classification as Do-335, and list their engines and specification. Give a reason for the selection of each aircraft.

4. Provide an illustration of one of the selected wing mounted twin engines aeroplanes, which clearly show the difference between the aeroplane and CLT aeroplane. Provide another illustration of another selected wing mounted twin engine aeroplane with different design.

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5. Compare and calculate the difference between the maximum speed and initial climb speed of Do-

335 and the selected aeroplanes. Explain the significance of high maximum speed and initial climb speed for the aeroplanes.

6. Compare and calculate the difference between the values of wing span of all the aeroplanes.

Explain the significance of shorter wing span for the aeroplanes.

7. Draw a front cross section area of Do-335 and an imaginary wing mounted twin engines Do-335, known as Do-335WME. Estimate the front cross section area of Do-335 and Do-335WME. Compare and calculate the difference between the front cross section area values of both aeroplanes. The estimation is done by the following method: ! Front cross section area drawings of each aeroplane are prepared with the same scale. ! Horizontal and vertical lines with the same distance between each of them are drawn across

the drawings producing squares with the same size. ! The area is estimated by counting the number of squares, which cover the drawings. Explain the significance of small front cross section area for ground attack and fighter bomber aeroplanes.

8. Using simple physics, describe other advantages of CLT aeroplane compared to wing mounted

twin engines aeroplane.

9. Prepare front, side and top view drawings of a proposed ground attack CLT aeroplane. Prepare another drawing of the aeroplane with its engines, seats, fuel tanks & landing gears visible. Name and describe the aeroplane.

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RESULTS 1. Engines and Specification of Do-335 Engines and Specification of Do-335 (2)

1 Engines Daimler-Benz DB-603G 2 Horse power 1900 3 Maximum speed 765km/h 4 Initial climb speed 1400m/min 5 Wing span 13.80m

2. Illustration of Do-335

An illustration of Do-335 (3).

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2. Wing Mounted Twin Engines Aeroplanes of the Same Classification as Do-335 and Their Engine and Specification

There are 8 models of wing mounted twin engines aeroplane of the same classification as Do-335 selected for this study. They are:

1. Heinkel He-219 2. Messerschmitt Me-410 3. Lockheed P-38 Lightning 4. Grumman F7F Tigercat 5. FMA IA-58 Pucara 6. Rockwell OV-10 Bronco 7. Grumman OV-1 Mohawk

He-219 is a fighter bomber aeroplane of Luftwaffe during World War II. The aeroplane is selected because it was using the same engines used by Do-335. Its engines and specification are listed in the following table:

Engines and Specification of He-219 (4)

1 Engines Daimler-Benz DB-603G 2 Horse power 1900 each 3 Maximum speed 670km/h 4 Initial climb speed 550m/min 5 Wing span 18.50m

Me-410 is a fighter bomber aeroplane of Luftwaffe during World War II. The aeroplane is selected because it was using the same engines used by Do-335. Its engines and specification are listed in the following table:

Engines and Specification of Me-410 (5)

1 Engines Daimler-Benz DB-603G 2 Horse power 1900 each 3 Maximum Speed 620km/h 4 Initial Climb Speed 650m/min 5 Wing span 16.40m

P-38 is a fighter bomber aeroplane in the operation during World War II. The aeroplane is selected because it was a twin engines fighter bomber aeroplane with single seat similar to Do-335. Its engines and specification are listed in the following table:

Engines and Specification of P-38 (6)

1 Engines Allison V-1710 2 Horse power 1475 each 3 Maximum speed 666km/h 4 Initial climb speed 870m/min 5 Wing span 15.85m

F-7F is a fighter bomber aeroplane in operation at the end of World War II. The aeroplane is selected because it has the closest performance to that of Do-335. Its engines and specification are listed in the following table:

Engines and Specification of F-7F (6)

1 Engines Pratt & Whitney R-2800 2 Horse power 2100 each 3 Maximum speed 700km/h 4 Initial climb speed 1380m/min 5 Wing span 15.70m

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IA-58 is a modern ground attack aeroplane. The aeroplane is selected because it is a modern twin engines ground attack aeroplane still in operation. Its engines and specification are listed in the following table:

Engines and Specification of IA-58 (6)

1 Engines Turbomeca Atazou XVI G 2 Horse power 1022 each 3 Maximum speed 500km/h 4 Initial climb speed 1080m/min 5 Wing span 14.50m

OV-1 is a modern reconnaissance, which also function as ground attack aeroplane. The aeroplane is selected because it is a modern twin engines ground attack aeroplane still in operation. Its engines and specification are listed in the following table:

Engines and Specification of OV-1 (6)

1 Engines Lycoming T53-L-7 2 Horse power 1400 each 3 Maximum Speed 465km/h 4 Initial Climb Speed 11.03m/min 5 Wing span 14.63m

OV-10 is a modern reconnaissance, which also function as ground attack aeroplane. The aeroplane is selected because it is a modern twin engines ground attack aeroplane still in operation. Its engines and specification are listed in the following table:

Engines and Specification of OV-10 (6)

1 Engines Garrett AiResearch T76-G-10/12 2 Horse power 715 each 3 Maximum Speed 452km/h 4 Initial Climb Speed Not available 5 Wing span 12.19m

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4. Illustration of 2 of the Selected Wing Mounted Twin Engines Aeroplanes with Different Design

An illustration of IA-58 (7).

An illustration of OV-10 (8).

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5. Comparison between the Maximum Speed and Initial Climb Speed of Do-335 and the Selected Aeroplanes

Comparison between the Maximum Speeds of Do-335, He-219, Me-410, P-38, F-7F, IA-58, OV-1 and OV-10

No Aeroplane Maximum speed % difference compared to the maximum speed of Do-335

1 Do-335 765km/h 0 2 He-219 670km/h 12.4 slower 3 Me-410 620km/h 18.9 slower 4 P-38 666km/h 12.9 slower 5 F-7F 700km/h 8.5 slower 6 IA-58 500km/h 34.6 slower 7 OV-1 465km/h 39.2 slower 8 OV-10 452km/h 40.9 slower

Comparison between Initial Climb Speeds of Do-335, He-219, Me-410, P-38, F-7F, IA-58 and OV-1 No Aeroplane Initial climb speed % difference compared to

the initial climb speed of Do-335 1 Do-335 1400m/min 0 2 He-219 550m/min 60.7 slower 3 Me-410 650m/min 53.6 slower 4 P-38 870m/min 37.9 slower 5 F-7F 1380m/min 1.4 slower 6 IA-58 1080m/min 22.9 slower 7 OV-1 1103m/min 21.2 slower

The Significance of High Maximum Speed and Initial Climb Speed

High maximum speed and initial climb speed are very important for the aeroplanes to perform ground attack or fighter bomber missions, which include reaching the target in time, escaping from enemy interceptor aeroplanes, avoiding from being shot by enemy and returning to base safely.

6. Wing spans of Do-335 and the Selected Aeroplanes and Their Comparison

Comparison of Wing Spans of Do-335, He-219, Me-410, P-38, F-7F, IA-58 and OV-1

No Aeroplane Wing span % difference compared to the wing span of Do-335

1 Do-335 13.80m 0 2 He-219 18.50m 34.1 longer 3 Me-410 16.40m 18.9 longer 4 P-38 15.85m 14.8 longer 5 F-7F 15.70m 13.8 longer 6 IA-58 14.50m 5.1 longer 7 OV-1 14.63m 6.0 longer

The Significance of Shorter Wing Span

Shorter wing span is very important for the aeroplanes to perform ground attack or fighter bomber missions, because it reduces the probability of being seen or detected and shot by enemy since the radar signature on the aeroplane from the ground is reduced.

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7. Drawing of Front Cross Section Area of Do-335 and Do-335 WME

Estimated Front Cross Section Area of Do-335 and Do-335WME and Their Comparison Estimated Front Cross Section Area of Do-335 and Do-335WME and their Comparison

No Aeroplane Estimated front cross section Area

% difference compared to the estimated front cross section area of Do-335

1 Do-335 242 units 0 2 Do-335WME 316 units 30.6 bigger

The Significance of Small Front Cross Section Area

Front cross section area of an aeroplane is related to drag as the following equation:

D = Cd X ½(dV2) X A (D = drag, Cd = drag coefficient of the aeroplane, d = density of the air, V = velocity of the aeroplane, A = area) Therefore the drag, which negatively affects the performance of an aeroplane, will be small if the front cross section area of the aeroplane is small too. By having small front cross section area, an aeroplane can have better performance due to the small drag it experiences in flight. A ground attack or fighter bomber aeroplane with small front cross section area also has lower probability to be seen or detected and shot by enemy since the radar signature from the front or rear is reduced.

Do-335 A = 242units

Do-335WME A = 316units

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8. Other Advantages of CLT Aeroplane Compared to Wing Mounted Twin Engines Aeroplane

1. When an aeroplane turns, it rotates with its axes of rotation located along its fuselage. If most of its mass is located at the axes of rotation, less energy is required to cause its rotation, but if there are significant masses a distance away from the axes of rotation, more energy is required to cause its rotation. For a CLT aeroplane, most of its masses, which are its engines are located on the axes of rotation, while for a wing mounted twin engines aeroplane, significant quantity of its masses, which are its engines are located a distance away from the axes of rotation.

When an aeroplane turns or rotates along its fuselage, there is a phenomenon known as moment of inertia, or resistance against the rotation.

For a CLT aeroplane with a cylindrical fuselage such as Do-335, the equation for the estimation of the moment of inertia is:

I = ½ mr2

(I = moment of inertia, m = the mass, which are the masses of its engines since they contribute to most of the masses of the aeroplane, r = the radius of its fuselage).

For a wing mounted twin engines aeroplane, the equation for the estimation of the moment of inertia is:

I = 2(mr2)

(I = moment of inertia, m = the mass, which is the mass of each of its engines since they have the same mass, r is the distance of its engines from the centre of its fuselage).

Comparison of the 2 equations shows that a wing mounted twin engines aeroplane need at least 4 times more energy than that of a CLT aeroplane to overcome its moment of inertia to turn or rotate along its fuselage, if both aeroplanes use the same engines.

Therefore, a CLT aeroplane can turn faster than a wing mounted twin engines aeroplane does, if both of them have the same engine performance. This capability to turn or rotate at a significant speed is known as manoeuvrability, is very important for ground attack and fighter bomber aeroplane to outmanoeuvre enemy aeroplane and avoid from being shot.

2. If one of the engines on a wing mounted twin engines aeroplane fails during flight, there

will be lost of symmetry in energy distribution, because the energy is only provided by an engine away from the centre of mass, while the other engine, which is also away from the centre of mass, becomes a mass, which contribute to absolute weight. Extra energy is required to overcome the loss of symmetry and power. If one of the engines on a CLT aeroplane fails during flight, there will be no lost of symmetry in energy distribution, because energy is still produced from the engine located along the centre of mass. Extra energy is required only to overcome the lost of power. Therefore, the performance of a wing mounted twin engines aeroplane will be reduced more than that of a CLT aeroplane, if both of them face an engine failure.

3. If one of the engines on a wing mounted twin engines aeroplane fails during take off or

landing, there will be a tendency for the aeroplane to turn towards the side where the failed engine is, because the force on the side where the engine is delivering power will be pushing forward, but there will be more resistance on the other side since the wheels are on the ground. If one of the engines on a CLT aeroplane fails during take off or landing, the aeroplane will continue moving at the direction piloted, but with less momentum.

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4. The engines of a wing mounted twin engines aeroplane generate more torques than the engines of a CLT aeroplane because there are located away from the fuselage. Therefore a wing mounted twin engines aeroplane need bigger or more fin for stability and manoeuvrability compared to a CLT aeroplane with the same engines. A CLT aeroplane can be designed with smaller fin due to its stability during flight.

8. Design of a Ground Attack CLT Aeroplane

Front view of No-335.

Side view of No-335.

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Top view of No-335.

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Front view of No-335 with engines, seat, fuel tanks and landing gears visible.

Side view of No-335 with engines, seat, fuel tanks and landing gears visible.

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Top view of No-335 with engines, seat, fuel tanks and landing gears visible. The aeroplane is named as, No-335 and designed based on the design of Do-335. The most noticeable difference from that of Do-335 is the aeroplane�s cockpit is located at the front of both its engines, while Do-335�s cockpit is located at the rear of its front engine. The other physical difference is the wing of No-335, which is slightly swept-back, while that of Do-335 is straight. The most noticeable similarity between both aeroplanes is that both of them have similar designed tail fins. The objective of the design of No-335 is to maximise the visibility from the cockpit by locating it forward of the front engine and the centre of gravity, which is located at a point between the engines, so that the wing can be mounted backward of the cockpit, and to locate the centre of gravity exactly at the centre of the aeroplane�s fuselage. This design enables the pilot to view downward from the cockpit without any obstruction of the wing and maximize the stability of the aeroplane. Do-335 lacks these advantages. The front engine and front propeller are located slightly lower than the rear engine and rear propeller, similar to that of Do-335. The power from both engines, are transferred to their respective propellers via long shafts. The shaft, which links the front propeller to the front engine, lies below the cockpit, while the shaft, which links the rear propeller to the rear engine, goes through the tail section. If one of the engines fails, the aeroplane can fly without any lack of stability, but with less power.

To effectively function as fighter bomber in today�s military mission, the only engines suitable are jet engines. Therefore, No-335 is only suitable to function as ground attack aeroplane. The aeroplane is proposed to be equipped with 2 units of engines with performance similar to that of Daimler-Benz DB-603G and its dimensions are also similar to that of Do-335. However, No-335 will have better performance than Do-335 because of the new and advanced materials and processes employed in its construction.

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CONCLUSIONS 1. CLT aeroplane has better performance than wing mounted twin engines aeroplanes, which use the

same engines. Do-335 can fly 12.4% faster than He-219 and 18.9% faster than Me-410, and can climb 60.7% faster than He-219 and 53.6% faster than Me-410 although all 3 aeroplanes use the same engine.

2. CLT aeroplane has better performance than wing mounted twin engines aeroplanes, which use

different engines, but are in the same classification. Do-335 can fly 12.9% faster and climb 37.9% faster than P-38, even though both aeroplanes are single-seat twin engines fighter bomber during World War II. Do-335 can also fly 8.5% faster and climb 1.4% faster than F-7F, which has the closest performance to that of Do-335. Besides that, Do-335 can fly 34.6% faster than IA-58, 39.2% faster than OV-1 and 40.9% faster than OV-10, and climb 22.9% faster than IA-58, and 21.2% faster than OV-1 although IA-58, OV-1 and OV-10 are modern ground attack wing mounted twin engines aeroplanes still in operation.

3. CLT aeroplane has shorter wing span than wing mounted twin engines aeroplanes, which use the

same engine. Do-335 has 34.1% shorter wing span than He-219 and 18.9% shorter wing span than Me-410 although all 3 aeroplanes use the same engines.

4. CLT aeroplane has shorter wing span than wing mounted twin engines aeroplanes, which use

different engines, but are in the same classification. Do-335 has 14.8% shorter wing span than P-38 even though both aeroplanes are single-seat twin engines fighter bomber during World War II. Do-335 also has 13.8% shorter wing span than that of F-7F, which has the closest performance to that of Do-335. Besides that, Do-335 has 5.1% shorter wing span than IA-58, 6.0% shorter wing span than OV-1 and X% shorter wing span than OV-10 although IA-58, OV-1 and OV-10 are modern ground attack wing mounted twin engines aeroplanes still in operation.

5. CLT aeroplane has smaller front cross section area than wing mounted twin engines aeroplane

therefore the former has smaller drag and better aerodynamics than the later. A CLT aeroplane can have its front cross section area 30% smaller than that of wing mounted twin engines aeroplane of the same engines.

6. CLT aeroplane can have more manoeuvrability than wing mounted twin engines aeroplane if both

of them have the same engine performance. 7. The performance of wing mounted twin engines aeroplane will be reduced more than that of CLT

aeroplane if both of them face an engine failure. 8. CLT aeroplane is safer to fly, land and take off when 1 of its engines fails, compared to wing

mounted twin engines aeroplane. 9. Aeroplane designers and engineers can design and produce better performance and smaller twin

engines aeroplane if they design and produce a CLT aeroplane instead of wing mounted twin engines aeroplane with the same engines.

10. CLT aeroplane is more effective and less vulnerable as ground attack aeroplane compared to wing

mounted twin engines aeroplane.

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RECOMMENDATIONS 1. Aeroplane manufacturers are recommended to consider manufacturing CLT aeroplanes, rather

than the conventional wing mounted twin engines aeroplanes if they have plan to manufacture twin engines aeroplanes, especially if they plan to manufacture twin engines ground attack aeroplane.

2. Aerospace engineers and researchers are recommended to further the study on the advantages of

CLT aeroplanes over wing mounted twin engines aeroplane by testing scaled models of both designs in a wind tunnel, so that the drag coefficient of each model can be determined and compared. From the research, better designs for CLT aeroplane can be produced.

3. Aerospace designers are recommended to incorporate the CLT concept in designing various types

of aeroplane, including passenger aeroplane with advanced twin turboprop engines besides ground attack aeroplane.

FUTURE PLAN The authors plan to produce a 3 dimensional AutoCAD drawings and scaled models of the proposed CLT aeroplane in this study. More studies will be done on the drawings and models.

REFERENCES 1. David Oliver, Wings Over Water, Greenwich Editions, London, UK, 2003. 2. www.warbirdsresourcegroup.org/LRG/do335.html, June 23, 2006. 3. www.umt.fme.vutbr.cz/~ruja/modely/podklady/Dornier/Do-335/9.jpg, July 29, 2006. 4. www.warbirdsresourcegroup.org/LRG/he219.html, June 23, 2006. 5. www.warbirdsresourcegroup.org/LRG/me410.html, June 23, 2006. 6. Paul Eden and Soph Moeng, The Encyclopaedia of World Aircrafts, Silverdale Books, Leicester,

UK, 2002. 7. www.combataircraft.com/aircraft/fia58_vl.jpg, August 7, 2006. 8. www.fas.org/irp/program/collect/ov-10.htm, August 4, 2006.