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<strong>Conceptual</strong> <strong>Design</strong> <strong>Requirements</strong> <strong>of</strong> a <strong>Newly</strong> <strong>Developed</strong> <strong>UAV</strong><br />

<strong>as</strong> a Research Platform at KFUPM: Structure, Aerodynamics<br />

and Propulsion<br />

Amro M. Al-Qutub *<br />

King Fahd University <strong>of</strong> Petroleum and Minerals, Dhahran 31261, Saudi Arabia<br />

The objective <strong>of</strong> the present paper is to set the structure,<br />

aerodynamic and propulsion conceptual design requirements <strong>of</strong> a<br />

multi-role long endurance <strong>UAV</strong>. The aircraft is designed to be used<br />

<strong>as</strong> a research platform at King Fahd University <strong>of</strong> Petroleum and<br />

Minerals, Mechanical engineering department. The maximum<br />

payload and take<strong>of</strong>f weight were set at 10 kg and 65 kg, respectively,<br />

which are the main design constrains <strong>of</strong> the system. Further<br />

specifications include cruise speed <strong>of</strong> 220 km/hr, 8 hours endurance<br />

and a ceiling <strong>of</strong> 18,000 ft. This would allow the <strong>UAV</strong> to perform<br />

over 1,700 km <strong>of</strong> non-stop flight with maximum payload and without<br />

refueling. The present study w<strong>as</strong> b<strong>as</strong>ed on available information in<br />

the literature and published specifications <strong>of</strong> different <strong>UAV</strong> systems<br />

manufacturers. Results included systems weight distribution, and<br />

limitations on requirements <strong>of</strong> the aerodynamic performance and<br />

propulsion system. <strong>Design</strong> sensitivity analysis resulted in the<br />

definition <strong>of</strong> b<strong>as</strong>ic performance limitations to achieve the objective<br />

<strong>of</strong> the present <strong>UAV</strong>. This included high aerodynamic performance<br />

(L/D > 13) which requires a retractable landing gear configuration.<br />

Also, the propulsion system should be equipped with a pusher<br />

propeller driven by a 2 stroke IC engine (17 -22 hp) with specific<br />

power higher than 2.5 hp/kg and low SFC <strong>of</strong> 0.33 kg/hp.hr. System<br />

reliability is improved through the selection <strong>of</strong> higher engine design<br />

output. Also additional heat-sink system for engine head would<br />

further improve the service life during hot local se<strong>as</strong>on. The overall<br />

propulsion system performance will be enhanced with a high<br />

efficiency propeller (> 50%) during cruise. The structure should be<br />

made <strong>of</strong> light composite material to comply with limited aircraft dry<br />

weight, <strong>of</strong> 46 kg. Wings are separable to allow for testing different<br />

configurations and for the e<strong>as</strong>e <strong>of</strong> store and transportation. The<br />

survivability and reliability <strong>of</strong> data and system recovery is improved<br />

by a recovery system composed <strong>of</strong> a parachute and an airbag in c<strong>as</strong>e<br />

<strong>of</strong> emergencies.<br />

* Associate Pr<strong>of</strong>essor, Mechanical Engineering Department, aqutub@kfupm.edu.sa.<br />

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SSAS <strong>UAV</strong> Scientific Meeting & Exhibition, Jeddah, Saudi Arabia (June 6, 2006)<br />

Paper No. SSAS-2006-051


T<br />

I. Introduction<br />

HE Unmanned Aerial Vehicle (<strong>UAV</strong>) is a remotely piloted or self-piloted aircraft that<br />

can carry camer<strong>as</strong>, sensors, communications equipment or other payloads. <strong>UAV</strong><br />

technology and usage actually extends beyond Military and defense market. Many<br />

Universities around the world have their own <strong>UAV</strong> development program such <strong>as</strong><br />

University <strong>of</strong> Sydney, Clark University, Cranfield University, Georgia Tech. University 1 ,<br />

Berkeley University, Simon Fr<strong>as</strong>er University 2 , and others.<br />

For Example University <strong>of</strong> Sydney <strong>UAV</strong> fleet 3 is composed <strong>of</strong> five different flying<br />

<strong>UAV</strong>’s: KCEXP-series, Ariel (Figure 1), Brumby, T-Wing, and Bidule miniature Air<br />

vehicle. This besides all other <strong>UAV</strong> still under development such <strong>as</strong> The Solar powered<br />

High Altitude Endurance (HALE) meteorological <strong>UAV</strong>. The University <strong>of</strong> Sydney enjoys<br />

the funding from different organizations like Booing, and BAE.<br />

Figure1: Sydney University <strong>UAV</strong> examples: the Ariel (left) and the Brumby (right).<br />

King Fahd University <strong>of</strong> Petroleum and Minerals (KFUPM) is encouraging<br />

multidisciplinary projects where different department contribute and integrate a system in<br />

a specific technology field. KFUPM encouraged faculty to work towards a vision that may<br />

contribute to the well fear <strong>of</strong> the surrounding society and the enhancement <strong>of</strong> the<br />

dissemination <strong>of</strong> knowledge. The development <strong>of</strong> <strong>UAV</strong> in KFUPM h<strong>as</strong> the following<br />

mission:<br />

1. Develop know-how <strong>of</strong> <strong>UAV</strong> for local applications<br />

2. Provide experimental platform for multi-disciplinary research related to <strong>UAV</strong><br />

applications<br />

3. Provide a tool for academic excellence in different are<strong>as</strong>.<br />

The specific <strong>UAV</strong> should satisfy the following:<br />

• able to accommodate for equipments related to multi-disciplinary research and<br />

development work.<br />

• safe to operate<br />

• reliable for local environment<br />

• comply with local regulation <strong>of</strong> communication<br />

Building a <strong>UAV</strong> demands a v<strong>as</strong>t knowledge <strong>of</strong> b<strong>as</strong>e vehicle platforms, flight dynamics,<br />

control theory, and real-time s<strong>of</strong>tware in a network environment. The flight system enables<br />

the aircraft platform to serve <strong>as</strong> the autonomous agent. It requires sophisticated navigation<br />

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SSAS <strong>UAV</strong> Scientific Meeting & Exhibition, Jeddah, Saudi Arabia (June 6, 2006)<br />

Paper No. SSAS-2006-051


sensors and integration methodologies for accurate and reliable autonomous operation.<br />

Key <strong>UAV</strong> system technologies were identified to be in: airframes; propulsion units;<br />

autonomous flight controllers; launch and recovery; navigation and guidance; selfprotection;<br />

ground control stations; payloads; and data communication, storage,<br />

processing, and dissemination (Information Technology). Key system technologies cover<br />

the following items:<br />

• <strong>UAV</strong> airframe, structure and aerodynamics<br />

• Propulsion unit<br />

• Control system (Flight control, Propulsion control)<br />

• Telemetry and ground station<br />

• Instrumentation for performance evaluation and control<br />

• Support system<br />

The objective <strong>of</strong> the present paper is to study the specific conceptual design and<br />

performance requirements <strong>of</strong> the <strong>UAV</strong> structure, aerodynamics and propulsion system.<br />

II. General <strong>Requirements</strong><br />

1) Structure and Aerodynamics general requirements:<br />

• Composite structure + Aluminum alloy<br />

• Payload <strong>of</strong> 10 kg with gross take<strong>of</strong>f weight <strong>of</strong> 65 kg<br />

• Reinforced<br />

• Landing gears retractable<br />

• Support up to 5g’s<br />

• Internal fuel tank and equipments<br />

• Conventional high wing configuration for stability<br />

2) Flight general requirements:<br />

• Take <strong>of</strong>f and landing on paved ground<br />

• 160-220 Km/h cruise speed<br />

• 8+ hours endurance<br />

• Ceiling 6 km (18000 ft).<br />

3) Propulsion general requirements:<br />

• Internal combustion engine (IC) high performance<br />

• Propeller driven with minimum diameter and high efficiency during cruise<br />

• If possible electrical starter + alternator<br />

• Withstand up to 45 o C, and 55 o C for short duration during take<strong>of</strong>f<br />

• High engine efficiency with low SFC<br />

• High power/weight ratio<br />

In the design ph<strong>as</strong>e both engine efficiency and power/weight ratio have to be optimized for<br />

minimum system weight.<br />

B<strong>as</strong>ically the payload is the most important part in the design ph<strong>as</strong>e since it sets the<br />

limitations on possible missions the <strong>UAV</strong> can perform. For b<strong>as</strong>ic research and surveillance<br />

purposes 10 kg payload is considered sufficient since continuous developments in<br />

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SSAS <strong>UAV</strong> Scientific Meeting & Exhibition, Jeddah, Saudi Arabia (June 6, 2006)<br />

Paper No. SSAS-2006-051


electronics avails higher performance and lower weights sensors, camer<strong>as</strong> and processors.<br />

As shown in Figure 2, compared to conventional <strong>UAV</strong>'s, the payload-to-gross weight ratio<br />

% Payload/gross Weight<br />

70<br />

60<br />

50<br />

40<br />

30<br />

20<br />

10<br />

PAYLOAD vs Endurance<br />

0<br />

3 4 5 6 7 8 9 10<br />

Endurance (h)<br />

Figure 2: Payload vs Endurance <strong>of</strong> available Known Conventional<br />

<strong>UAV</strong>’s ( 4)<br />

Ceiling (feet)<br />

80,000<br />

70,000<br />

60,000<br />

50,000<br />

40,000<br />

30,000<br />

20,000<br />

10,000<br />

Present <strong>UAV</strong><br />

Service ceiling <strong>of</strong> <strong>UAV</strong>'s<br />

0<br />

0 10 20 30 40 50 60<br />

Endurance (h)<br />

Present <strong>UAV</strong><br />

Figure 3: Ceiling vs. Endurance <strong>of</strong> available Known Conventional<br />

<strong>UAV</strong>’s 4<br />

is average, however the endurance places the present <strong>UAV</strong> among the top <strong>of</strong> the group 4 .<br />

The ceiling is among the majority <strong>of</strong> <strong>UAV</strong>’s <strong>of</strong> its endurance category, Figure 3.<br />

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SSAS <strong>UAV</strong> Scientific Meeting & Exhibition, Jeddah, Saudi Arabia (June 6, 2006)<br />

Paper No. SSAS-2006-051


A. Aerodynamics and Structure<br />

B<strong>as</strong>ed on the objective <strong>of</strong> the project the <strong>UAV</strong> is a long endurance (8 hrs +) with<br />

re<strong>as</strong>onable cruise speed <strong>of</strong> about (220 km/hr). Moreover, take<strong>of</strong>f should be possible either<br />

from a launcher or a runway. This imposes a grate challenge in designing the<br />

aerodynamics and the structure <strong>of</strong> the aircraft. Endurance requires:<br />

− high aerodynamic performance (high L/D, lift to drag ratio)<br />

− Relatively Large fuel tank ( to improve weight ratio )<br />

− Strong structure with high strength to weight ratio materials.<br />

− Low thrust specific fuel (high efficiency propulsion system)<br />

Endurance<br />

To estimate the endurance <strong>of</strong> an aircraft we may start with the m<strong>as</strong>s reduction <strong>of</strong> fuel<br />

during level steady flight:<br />

dm ⎛ D ⎞<br />

= −TSFC<br />

∗ ⎜ ⎟(<br />

m • g)<br />

(1)<br />

dt<br />

⎝ L ⎠<br />

Where TSFC is the thrust specific fuel consumption <strong>of</strong> the propulsion system, m is the<br />

m<strong>as</strong>s <strong>of</strong> the vehicle, and g is the gravitational acceleration. Assuming constant L/D and<br />

TSFC through the flight, the endurance <strong>of</strong> the vehicle can be calculated <strong>as</strong>:<br />

1 ⎛ L ⎞ 1 mo<br />

Δ t = ⎜ ⎟ ln<br />

(2)<br />

g ⎝ D ⎠ TSFC mb<br />

Where mo is the take<strong>of</strong>f weight and mb is the fuel empty weight <strong>of</strong> the <strong>UAV</strong>.<br />

Take<strong>of</strong>f requirements<br />

The <strong>UAV</strong> h<strong>as</strong> to be able to take<strong>of</strong>f from a runway, or from a launcher. Taking <strong>of</strong>f from<br />

a runway requires landing gear system that reduces L/D if extended during flight. So,<br />

retractable landing gear should be used to maintain high aerodynamic performance. The<br />

major penalty in this c<strong>as</strong>e is incre<strong>as</strong>ed structural weight (by only few percent). A<br />

re<strong>as</strong>onable take<strong>of</strong>f velocity <strong>of</strong> the <strong>UAV</strong> is about 25 m/s. B<strong>as</strong>ed on the objective <strong>of</strong> the<br />

project, the maximum payload is 10kg. Referring to figure 3, the aircraft take<strong>of</strong>f gross<br />

weight can be chosen <strong>as</strong> 65 kg, which corresponds to payload to gross weight ratio <strong>of</strong><br />

15.4%. To calculate the wing platform area, we may use the lift equation:<br />

ρ 2 = V C S<br />

(3)<br />

2<br />

L L<br />

With plain flap system and large <strong>as</strong>pect ratio (>9) <strong>of</strong> the wing, maximum lift<br />

coefficient can reach a value over 1.4. Using equation (3), and considering the take<strong>of</strong>f<br />

speed to be grater than stall speed by 20%, the required wing area for the <strong>UAV</strong> for take<strong>of</strong>f<br />

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SSAS <strong>UAV</strong> Scientific Meeting & Exhibition, Jeddah, Saudi Arabia (June 6, 2006)<br />

Paper No. SSAS-2006-051


will be 1.66 m 2 at standard atmosphere. Higher atmospheric temperature and/or altitude<br />

would require slightly higher take<strong>of</strong>f speed due to reduced density. For example at 50 C o ,<br />

sea level, the take<strong>of</strong>f speed will be 26.5 m/s, only a 6% incre<strong>as</strong>e compared to standard<br />

atmosphere conditions.<br />

Structure<br />

The Present <strong>UAV</strong> h<strong>as</strong> a conventional aerodynamic configuration with pusher-type<br />

propeller See figure 4. The main fuselage h<strong>as</strong> a maximum diameter <strong>of</strong> about 350 mm to<br />

contain camer<strong>as</strong> similar to the BAI 5 <strong>as</strong> well <strong>as</strong> large fuel tank and Payload:<br />

Propeller<br />

Fuselage<br />

Camer<br />

a<br />

Figure 4: B<strong>as</strong>ic configuration <strong>of</strong> the <strong>UAV</strong>.<br />

The <strong>UAV</strong> is to be made from composites (gl<strong>as</strong>s fiber and/or Carbon fiber) and<br />

Aluminum alloy, to provide high strength to weight ratio. Skin parts and some <strong>of</strong> the<br />

skeleton supposed to be made from composite material while hard points and joints to be<br />

mainly from aluminum alloy for improved reliability. Part <strong>of</strong> the wing is supposed to be<br />

separable during storage and transportation for convenience <strong>of</strong> use and handling. For the<br />

systems contained in the fuselage they have to abide by the dimensional restrictions and<br />

maintain a re<strong>as</strong>onable static margin for stability purposes. Systems layout in the fuselage is<br />

illustrated in figure 5. Engine, landing gear and camera are supposed to have fixed<br />

locations. Fuel will be stored in multiple tanks, close to center <strong>of</strong> gravity for stability<br />

during flight and flexibility <strong>of</strong> payload <strong>as</strong> well <strong>as</strong> static margin consideration. The payload<br />

bay should contain flexible mount system for different parts and configuration <strong>of</strong><br />

communication, control and payloads. The camera is (which is a part <strong>of</strong> the payload)<br />

mounted at the bottom <strong>of</strong> the aircraft <strong>as</strong> shown in figure 5. Recovery Parachute and airbag<br />

h<strong>as</strong> to be on the center <strong>of</strong> gravity.<br />

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SSAS <strong>UAV</strong> Scientific Meeting & Exhibition, Jeddah, Saudi Arabia (June 6, 2006)<br />

Paper No. SSAS-2006-051


Fuel<br />

Tanks<br />

Propulsio<br />

n<br />

Cruise performance<br />

Figure 5 : Layout for Main systems in the Fuselage<br />

The <strong>UAV</strong> is expected to operate at a ceiling altitude <strong>of</strong> 18,000 ft. At the cruise speed <strong>of</strong><br />

220 km/hr the lift coefficient is CL = 0.31 (at gross weight). This is a re<strong>as</strong>onable lift<br />

coefficient for high aerodynamic performance. Lower speed will produce higher endurance<br />

and this can be optimized according to the type <strong>of</strong> the <strong>UAV</strong> mission. Moreover, the cruise<br />

speed <strong>of</strong> the present <strong>UAV</strong> is considered among the highest in its category <strong>as</strong> can be seen in<br />

table 1 6,7 .<br />

Table 1: Different <strong>UAV</strong> performance and power requirements 6,7 .<br />

<strong>UAV</strong> Gross<br />

Weight<br />

(kg)<br />

Parachute Payload bay<br />

Ceiling<br />

(ft)<br />

Airbag<br />

Endurance<br />

(hrs)<br />

Camera<br />

Main Landing Gear Nose Gear<br />

Cruise<br />

speed<br />

(km/hr)<br />

Power<br />

(hp)<br />

From Table 2 the weight to power ratio <strong>of</strong> <strong>UAV</strong>'s <strong>of</strong> similar category <strong>as</strong> the proposed is<br />

in the range <strong>of</strong> 2.4 to 5.2 kg/hp. The variation <strong>of</strong> designed hp on propulsion system<br />

depends on mainly on:<br />

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SSAS <strong>UAV</strong> Scientific Meeting & Exhibition, Jeddah, Saudi Arabia (June 6, 2006)<br />

Paper No. SSAS-2006-051<br />

W/P ratio<br />

(kg/hp)<br />

Huntair 127 17,000 7.5 184 38 3.3<br />

Raptor 854 65,000 8 - 50 17.1<br />

Prowler 91 21,000 6 100 38 2.4<br />

STM-5B 114 16,000 6 144 25.5 4.47<br />

Pectre II 100 23,000 6 220 26 3.85<br />

Skyeye 355 18,000 10 126 46 7.72<br />

Hunter 727 15,000 12 162 120 6.06<br />

Model 410 818 30,000 12 184 160 5.11<br />

Shadow 272 17,000 14 129 52 5.23


− Weight <strong>of</strong> the aircraft<br />

− Flight altitude<br />

− <strong>Design</strong> clime rate<br />

− Flight speed<br />

In the present work it is proposed to use an engine with 17 to 22 hp, which corresponds<br />

to a weight to power ratio <strong>of</strong> 3.82 to 3. This will put proposed <strong>UAV</strong> among the top <strong>of</strong> the<br />

<strong>UAV</strong>'s list in terms <strong>of</strong> engine power and performance. Actually the main re<strong>as</strong>ons <strong>of</strong> that<br />

are the high cruise speed and the high operating temperature in the local environment,<br />

which is part <strong>of</strong> the objective for this project. To calculate the endurance <strong>of</strong> the <strong>UAV</strong> the<br />

weight ratio h<strong>as</strong> to be known. Table 2 is a list <strong>of</strong> most recent <strong>UAV</strong> engines specification<br />

available in the world market that can be used for the present project 8-11 . B<strong>as</strong>ed on present<br />

technology, the propulsion system is expected to weigh about 9 kg, including engine,<br />

propeller and generator.<br />

Table.2: Some <strong>UAV</strong> Engines available in the international market.<br />

Engine<br />

Output<br />

Weight Power /Weight ratio<br />

(Brand)<br />

(hp)<br />

(kg)<br />

(hp/kg)<br />

A200B<br />

17 5.1 / 6 kg (est.)<br />

2.83<br />

Quadra<br />

300W Gen.<br />

3W-200i B2TSQS<br />

20 4.8/6 kg (est.)<br />

3.67<br />

3W Motoren<br />

W.Gen<br />

313<br />

22 8<br />

2.75<br />

Zanzotterra<br />

W. 300w Gen.<br />

L275 E<br />

25 7.7 / 9 kg (est.)<br />

2.78<br />

Limbach<br />

W. Gen<br />

AR741<br />

37<br />

10.7 /12kg (est.)<br />

3.1<br />

<strong>UAV</strong> Engine Ltd. (rotary)<br />

W. Gen.<br />

B<strong>as</strong>ed on the calculated size <strong>of</strong> wing and fuselage, the airframe <strong>of</strong> the <strong>UAV</strong> is expected<br />

to have a weight between 10 and 16 kg, depending on design and material used for the<br />

construction <strong>of</strong> the system. Table 3 provides the design weight <strong>of</strong> different systems in the<br />

<strong>UAV</strong>.<br />

Fuel consumption can be estimated using figure 6, which is an example for typical<br />

turboprop engine 12 . From this figure we can use a conservative thrust specific fuel<br />

consumption <strong>of</strong> 0.06 kg/N.hr. Using equation 2 with the <strong>as</strong>sumption <strong>of</strong> having an<br />

aerodynamic performance <strong>of</strong> L/D=15, the <strong>UAV</strong> endurance will be 11.14 hrs. This satisfies<br />

the objective <strong>of</strong> the project.<br />

Airframe Propulsion Control &<br />

communication<br />

Table 3: Weights <strong>of</strong> different system in the <strong>UAV</strong>.<br />

Landing<br />

Gear<br />

Recovery<br />

System<br />

Payload<br />

(Max.)<br />

Fuel<br />

with<br />

max<br />

payload<br />

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SSAS <strong>UAV</strong> Scientific Meeting & Exhibition, Jeddah, Saudi Arabia (June 6, 2006)<br />

Paper No. SSAS-2006-051<br />

Gross<br />

Weight<br />

12 kg 9 kg 3 kg 4 kg 4 kg 10 kg 23 kg 65 kg


Figure 6: Typical Engines TSFC (12)<br />

The calculated endurance is b<strong>as</strong>ed on several <strong>as</strong>sumptions regarding structural<br />

technology, aerodynamic technology and propulsion system performance. Figures 7 to 9<br />

Are the results <strong>of</strong> sensitivity analysis to illustrate the effect <strong>of</strong> each <strong>of</strong> the mentioned<br />

parameter on the endurance <strong>of</strong> the <strong>UAV</strong>. The total structure m<strong>as</strong>s includes airframe,<br />

propulsion, communication and control, landing gear, and recovery system. Reducing the<br />

structure weight to 26 kg will result an incre<strong>as</strong>ed endurance <strong>of</strong> over 15 hrs. If all typical<br />

values <strong>of</strong> aerodynamic performance and propulsion performance are met, the total<br />

structural m<strong>as</strong>s h<strong>as</strong> to be limited to 36 kg in order to meet the 8 hrs flight endurance Figure<br />

7. Aerodynamic performance also proved to be critical for endurance. Incre<strong>as</strong>ing<br />

aerodynamic performance will incre<strong>as</strong>e the endurance. The limit to meet the objective here<br />

is L/D ≥13, if all other variables are typical. Reducing the (TSFC) improves the flight<br />

endurance linearly, Figure 9. Improving the TSFC down to 0.045 will incre<strong>as</strong>e the<br />

endurance to 14.85 hrs. B<strong>as</strong>ed on available technology, the TSFC h<strong>as</strong> to be maintained<br />

below 0.06 during cruise conditions. Figure 10 illustrates the effect <strong>of</strong> different payload<br />

m<strong>as</strong>s on flight endurance. Reducing payload to 5 kg (half <strong>of</strong> max.) will incre<strong>as</strong>e flight<br />

endurance to 12.3 hrs, a 1.2 hrs incre<strong>as</strong>e over full payload (10kg). Detailed discussion on<br />

the propulsion system is at the next section.<br />

In conclusion, there are three main parameters that affect the flight endurance:<br />

− Structure design and material<br />

− Aerodynamic design<br />

− Propulsion system performance<br />

Structure and aerodynamics are to be optimized in the design process. Propulsion<br />

performance is considered <strong>as</strong> a major design constrain.<br />

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SSAS <strong>UAV</strong> Scientific Meeting & Exhibition, Jeddah, Saudi Arabia (June 6, 2006)<br />

Paper No. SSAS-2006-051


In conclusion the following are the aerodynamic and structure specifications:<br />

Aerodynamic:<br />

− High aerodynamic performance L/D>13<br />

− Pay load = 10 kg max<br />

− Gross weight 65 kg<br />

− Conventional configuration<br />

− Take<strong>of</strong>f speed = 25 m/s (standard atmosphere)<br />

− Wing Area 1.6 m 2 approx.<br />

− Max cruise 220 km/hr<br />

− Endurance 8 - 12 hrs.<br />

− Max ceiling 18,000 ft<br />

− Flight range > 1,800 km<br />

Structure:<br />

− Made <strong>of</strong> composites (fibergl<strong>as</strong>s-carbon) with Aluminum Alloy.<br />

− Separable wings.<br />

− Retractable landing gear<br />

− Recovery system (parachute + airbag)<br />

− Total Structure weight < 36 kg<br />

− Multiple Fuel tanks<br />

− Payload rack for flexibility <strong>of</strong> mounting with min dim. <strong>of</strong> (200X200X400) mm.<br />

Flight Endurance ( hrs )<br />

16<br />

14<br />

12<br />

10<br />

8<br />

6<br />

4<br />

2<br />

0<br />

26 29 32 35 38 41<br />

Structure m<strong>as</strong>s ( kg )<br />

Figure 7: Effect <strong>of</strong> Structure M<strong>as</strong>s on Flight Endurance<br />

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SSAS <strong>UAV</strong> Scientific Meeting & Exhibition, Jeddah, Saudi Arabia (June 6, 2006)<br />

Paper No. SSAS-2006-051


Flight Endurance ( hrs )<br />

Flight Endurance ( hrs )<br />

18<br />

16<br />

14<br />

12<br />

10<br />

8<br />

6<br />

11 13 15 17 19 21<br />

Aerodynamic Performance (L/D)<br />

Figure 8: Effect <strong>of</strong> Aerodynamic Performance on Flight Endurance<br />

18<br />

16<br />

14<br />

12<br />

10<br />

8<br />

6<br />

0.045 0.05 0.055 0.06 0.065 0.07 0.075<br />

TSFC ( kg/ N.hr )<br />

Figure 9: Effect <strong>of</strong> thrust Specific Fuel Consumption on Flight Endurance<br />

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Paper No. SSAS-2006-051


Flight Endurance ( hrs )<br />

15<br />

14.5<br />

14<br />

13.5<br />

13<br />

12.5<br />

12<br />

11.5<br />

11<br />

10.5<br />

10<br />

Propulsion system<br />

The objective <strong>of</strong> the project requires a reliable and efficient propulsion system<br />

specifically during cruise condition (180 - 220 km/hr) with a ceiling <strong>of</strong> 18,000 ft. In<br />

addition, the <strong>UAV</strong> h<strong>as</strong> to be practical and e<strong>as</strong>y to maintain.<br />

Type <strong>of</strong> System<br />

10 8 6 4 2 1<br />

Payload M<strong>as</strong>s ( kg )<br />

Figure 10: Effect <strong>of</strong> Payload M<strong>as</strong>s on Flight Endurance, Fixed Fuel m<strong>as</strong>s <strong>of</strong> 23 kg.<br />

For the <strong>UAV</strong> application similar to present <strong>UAV</strong> category, propulsion systems are<br />

usually limited to either turbojet or propeller driven by an IC engine. Propeller <strong>UAV</strong> are<br />

more popular due to lower cost and higher efficiency at relatively low speeds (Ma


Engine <strong>Requirements</strong><br />

The engine h<strong>as</strong> to be able to operate in the local environment. So the b<strong>as</strong>ic<br />

requirements are:<br />

Cruise<br />

- Power to meet take<strong>of</strong>f at 9,000 ft altitude (Abha-Saudi Arabia)<br />

- Low fuel consumption to meet endurance requirements<br />

- Tolerate high temperature and dusty environment<br />

- Tolerate long periods <strong>of</strong> flights >20 hrs.<br />

- Tolerate high g forces during catapult launch.<br />

At level flight with a speed <strong>of</strong> V, the required propulsion power can be calculated <strong>as</strong>:<br />

Power P<br />

⎛ D ⎞<br />

= m g ⎜ ⎟ V<br />

(4)<br />

⎝ L ⎠<br />

For cruise at 18,000 ft and 220 km/hr, with gross weight <strong>of</strong> 65 kg and minimum L/D<br />

<strong>of</strong> 11, the required propulsion power will be Powerp = 3.54 kW ( 4.74 hp ). Typically, IC<br />

engines lose about 7% <strong>of</strong> output power per 3000 ft altitude. Taking this into consideration<br />

and <strong>as</strong>suming that the engine will operate at 67% <strong>of</strong> rated power during cruise, also<br />

<strong>as</strong>suming propeller efficiency <strong>of</strong> 60%, the required engine shaft power at standard<br />

atmospheric conditions should be about 20 hp, to meet the cruise condition. The extra 33%<br />

<strong>of</strong> engine power is for maneuver and climb requirement.<br />

High altitude take<strong>of</strong>f<br />

For take<strong>of</strong>f at 9,000 ft the power available will be 79% <strong>of</strong> the rated engine power. To<br />

maintain a re<strong>as</strong>onable weight to power ratio <strong>of</strong> less than 5.2 kg/hp, the engine rated shaft<br />

power should be > 15.82 hp. This is even less than the engine power requirement for cruise<br />

condition.<br />

Propeller Efficiency<br />

B<strong>as</strong>ed on endurance calculations, the required TSFC < 0.06 kg/N.hr. Most <strong>UAV</strong> engine<br />

suppliers claim specific fuel consumption (SFC) for their product <strong>as</strong> less than 0.33<br />

kg/hp.hr. Diesel engines recently developed for aviation application can reach SFC <strong>as</strong> low<br />

<strong>as</strong> 0.19 13 . The TSFC is affected by both SFC and propeller efficiency (ηp) :<br />

V<br />

TSFC = 0.<br />

00134 SFC<br />

(5)<br />

η<br />

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SSAS <strong>UAV</strong> Scientific Meeting & Exhibition, Jeddah, Saudi Arabia (June 6, 2006)<br />

Paper No. SSAS-2006-051<br />

P


Typical propeller efficiency can be <strong>as</strong> high <strong>as</strong> 85%, Figure 11. Usually each propeller<br />

design is b<strong>as</strong>ed on a given flight speed. <strong>Design</strong>ers should select propeller according to<br />

specific design requirement:<br />

- Power matching with engine<br />

- efficiency at cruise<br />

- static and low speed thrust<br />

- noise<br />

- diameter<br />

Propeller Efficiency (%)<br />

TSFC (kg/N.hr)<br />

80<br />

60<br />

40<br />

20<br />

100 200 300 400 500<br />

Flight Speed MPH<br />

Figure 11: Typical Propeller Efficiency (12)<br />

0.08<br />

0.07<br />

0.06<br />

0.05<br />

0.04<br />

0.03<br />

0.02<br />

0.01<br />

0<br />

0.4 0.5 0.6 0.7 0.8<br />

Propulsion Efficiency<br />

Figure 12: Effect <strong>of</strong> Propulsion Efficiency on Thrust Specific Fuel Consumption<br />

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Paper No. SSAS-2006-051


For the present project the propeller selected should have maximum efficiency close to<br />

a speed <strong>of</strong> 200 km/hr. Figure 12 illustrates the required TSFC variation with propulsion<br />

efficiency for airspeed <strong>of</strong> 220 km/hr and SFC <strong>of</strong> 0.33 kg/hp.hr.<br />

From Figure 12 it is clear that to meet the endurance requirements the propulsion<br />

efficiency during cruise condition should be ηp > 45 %. The recommended efficiency <strong>of</strong><br />

propeller in the present study is 50% or better at cruise condition. This is consider a<br />

re<strong>as</strong>onable estimate compared to low speed propellers. 13,14 Altitude and flight speed will<br />

change the propeller size and configuration to meet the efficiency requirement. So, more<br />

than one propeller type should be used to meet different altitudes and flight speed for<br />

optimum performance. The propulsion requirements are <strong>as</strong> the following:<br />

a) Engine :<br />

− 20 -25 hp Two stroke engine with:<br />

− SFC < 0.33 kg/hp.hr (at cruise)<br />

− Power to weight ratio > 2.5 kg/hp (including generator)<br />

− Ability to operate to temperatures up to 45 o C<br />

− Ability to operate in dusty environment (equipped with intake filter)<br />

− Able to with stand axial acceleration > 10 g.<br />

− Operate efficiently at max altitude 18,000 ft<br />

− Self priming ignition system with min. 100 W generator.<br />

b) Propeller: :<br />

− Pusher type<br />

− Meets engine power<br />

− 70-80 cm diameter, minimum possible size<br />

− Efficiency ≥ 50% at cruise condition<br />

III. Conclusion<br />

The major objective <strong>of</strong> the multi-role <strong>UAV</strong> is to be capable <strong>of</strong> long endurance (8-12<br />

hrs) flights with a maximum payload <strong>of</strong> 10 kg. The maximum expected cruise speed is 220<br />

km/hr, and the ceiling is 18,000 ft. This would allow the <strong>UAV</strong> to perform over 1,700 km<br />

<strong>of</strong> non-stop flight with maximum payload and without refueling. Compliance to<br />

specifications limits <strong>of</strong> different systems is expected to <strong>as</strong>sure achievement to the<br />

objectives <strong>of</strong> this project. The present study w<strong>as</strong> b<strong>as</strong>ed on available information in the<br />

literature and published specifications <strong>of</strong> manufacturers. <strong>Design</strong> constrains are identified<br />

<strong>as</strong>:<br />

− Available engine performance and specifications<br />

− Available control system specifications (size – weight-power)<br />

− Communication System<br />

Other major specifications can be met through careful design and execution. B<strong>as</strong>ed on<br />

design sensitivity analysis the <strong>UAV</strong> is expected to have a conventional aerodynamic<br />

configuration with pusher propeller driven by a 2 stroke IC engine with > 20 hp output for<br />

improved reliability. Special heat sinks for engine heads are recommended for improved<br />

reliability <strong>of</strong> the system during hot local se<strong>as</strong>on. Also, the structure should be made <strong>of</strong> light<br />

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SSAS <strong>UAV</strong> Scientific Meeting & Exhibition, Jeddah, Saudi Arabia (June 6, 2006)<br />

Paper No. SSAS-2006-051


composite material to comply with limited aircraft dry weight <strong>of</strong> 36 kg. This would<br />

provide a tolerance <strong>of</strong> 4 kg for the structural components <strong>of</strong> the aircraft in the design and<br />

construction ph<strong>as</strong>e compared to the typical design in table 3. Wings should be separable<br />

for e<strong>as</strong>e <strong>of</strong> store and transportation. Moreover, aerodynamic performance (L/D) have to be<br />

grater than 13, which requires a retractable landing gear configuration. In addition, the<br />

engine h<strong>as</strong> to have high specific power > 2.5 hp/kg, with low SFC< 0.33 kg/hp.hr. Also,<br />

the propeller should have high efficiency during cruise (> 50%). Survivability <strong>of</strong> the <strong>UAV</strong><br />

is improved by a recovery system composed <strong>of</strong> a parachute and an airbag in c<strong>as</strong>e <strong>of</strong><br />

emergencies.<br />

Acknowledgment<br />

The author acknowledges the support <strong>of</strong> King Fahd University <strong>of</strong> Petroleum and<br />

Minerals during this research.<br />

References<br />

1 Johnson, E. N., Hart, M. G., and Christophersen, H.B., “Development <strong>of</strong> an Autonomous Aerial<br />

Reconnaissance System at Georgia Tech,” AIAA's 1 st Technical Conference and Workshop on<br />

Unmanned Aerospace Vehicles, Systems, Technologies, and Operations, 20-23 May 2002<br />

2 Hennessey, C., “Autonomous Control <strong>of</strong> a Scale Airplane,” School <strong>of</strong> Engineering Science, Simon<br />

Fr<strong>as</strong>er University, 14 April 2000<br />

3 Wang K.C." <strong>UAV</strong> <strong>Design</strong> Activities in a University Environment" presented at the 9th Australian<br />

International Aerospace Congress, Canberra, Australia, 6-8 March 2001 (PDF 4.0 file)<br />

4 Amro M. Al-Qutub and Sami El Ferik "System Definition for Unmanned Aircraft Development"<br />

KFUPM Internal Report Jun, 2003.<br />

5 BAI Aero systems. Payloads and Sensors. www.baiaerosystems.com/payloads.html<br />

6 <strong>UAV</strong> forum: http://www.uavforum.com/index.shtml<br />

7 Association for Unmanned Vehicle Systems International: http://www.auvsi.org/<br />

8 <strong>UAV</strong> Engines Ltd.: http://www.uavenginesltd.co.uk<br />

9 Zanzottera Engines: http://www.zazoterteraengines.com<br />

10 Limbach Engines: http://www.limflug.de<br />

11 3W Modellmotoren GmBH Engines: http://www.3w-modellmotoren.com<br />

12 Hill and Peterson "Mechanics and Thermodynamics <strong>of</strong> Propulsion", Addition-Wesley, 2 nd edition,<br />

1992<br />

13 Shaila L. J<strong>as</strong>zlics and Rianer Stemme " Possibility <strong>of</strong> the Next Generation <strong>of</strong> High-Altitude Long-<br />

Endurance Unmanned Aerial Vehicles" Technical Publication/ HALE-<strong>UAV</strong>.<br />

14 F 2 Craig D. Paxton, Peter J. Gryn, Eerisa Hines, Ulises Perez, and Ge-Cheng Zha " High<br />

Efficiency Foreword Swept Propellers at Low Speed" AIAA 2003-1069.<br />

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Paper No. SSAS-2006-051

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