“AERODYNAMICS literature review”;
LITERATURE REVIEW
AERODYNAMICS
In recent years of research, most of the aircraft today are concern about aerodynamics in which it is a crucial element that need to be considered in creating an enormous amount lift with a better flight control. An aerodynamics is the study on how the air moves around the object (May, 2011). For aircraft, it is study how the air moves through the skin of the aircraft itself. Thus, anything that moves through air are considered react to aerodynamics.
Generally, for every aircraft, it will experiences of four forces which are lift, drag, weight and thrust as shown in Figure 2.1 below. These forces is the element to determine the attitude of the aircraft such as climbing, descending, and cruising. For this research, it more focusing on lift and drag force in order to determine the effectiveness of wing airfoil modification.
Figure 2.1 – Four Forces Acting on Aircraft (Brooksby, 2013)
COEFFICIENT OF LIFT AND DRAG
Normally, lift and drag are related to coefficient of lift and coefficient of drag whereby both are in dimensionless which normally affect the flight characteristic performance of an aircraft. Both coefficient of lift and drag are determined from the wind tunnel testing.
The coefficient of lift is denoted by CL. In calculation, this CL depends on the density of air around the airfoil, velocity of the airspeed and the wing area or reference area (Sadraey, 2013) as shown in Equation 2.1 where the CL consist all the complex dependencies and normally determined by the experimental test in wind tunnel.
However, the coefficient of lift also can be determined by the shape of the wing, and angle of attack. For example, as the shape of airfoil is cambered and the angle of attack increase, it will produced high amount coefficient of lift. But at a certain angle of attack, the coefficient lift will drop to produce wing stall due to increase in drag. The relation of coefficient of lift and drag can be shown in Figure 2.2.
Figure 2.2 – The Angle of Attack Related to Lift and Drag (Naegele, 2015)
Meanwhile, coefficient of drag is defined as CD which normally used to calculate the value of drag or resistance of an object in fluid state such as air or water. The equation also is quite similar to the coefficient of lift whereby it more focus on drag instead of lift as shown in Equation 2.2 In addition, the CD is frequently associated with a particular surface area. (McCormick B. , 1979)
The formula or equation for coefficient of lift and drag are as follow below.
Equation 2.1 – Coefficient of Lift Formula
Equation 2.2 – Coefficient of Drag Formula
Where;
CL = Coefficient of lift
CD = Coefficient of drag
L = Lift (lb or N)
D = Drag (lb or N)
𝜌 = Air density (slugs/ft3 or kg/m3)
V = Velocity (ft/s or m/s)
S = Reference Area (ft2 or m2)
Hence for this research, NACA 23012 airfoil was selected due its shaped capability to create an enormous amount of lift with low form drag. Thus, it will result a better coefficient of lift and drag value. However, the induced drag is still inevitable as the wing airfoil create a lift.
LIFT TO DRAG RATIO
The lift to drag ratio with equation L/D is the value of the lift force created by the wing airfoil which oppose to its drag force. This L/D commonly used to determine the relationship between lift and drag by dividing the coefficient of lift to the coefficient of drag, CL/CD (Vooren, 2007). Moreover, the L/D ratio will describe the efficiency of airfoil where a higher ratio of L/D are more efficient compared to lower ratio of L/D.
Besides that, the highest L/D ratio are occur at a specific coefficient of lift and angle of attack as shown in Figure 2.3 depending on the shape of the wing airfoil. When aircraft climbing at highest L/D ratio, it will producing minimum amount of total drag. However, any lower or higher angle of attack than highest L/D ratio produced, it will lead to reduction of L/D whereas increasing the total drag.
Figure 2.3 – Lift and Drag Coefficient Related to Angle of Attack (X-Plane, 2010)
Moreover, the L/D also can be used to determine the ratio and range of the glide. The glide ratio does not affect the weight of the aircraft cause it is depends on the aerodynamics forces acting on the aircraft (SKYbrary, 2015). For example, two different weight of aircraft has same L/D ratio and also same altitude are gliding to a same distance travel. However, the lighter aircraft will experience a longer time to reach the distance.
REYNOLD’S NUMBER
Reynolds number commonly denoted by Re is a dimensionless quantity which still being applied in fluids mechanic and heat transfer up to this modern technology century. It can be expressed by the Equation 2.3 whereby the ratio of inertial forces over viscous forces. The internal forces is determined by the multiplication of density, ρ maximum velocity relative to fluid, v and length or diameter of fluid, L whereby the viscous force is determined by dynamics viscosity, μ.
Equation 2.3 – Reynolds Number Equation.
Moreover, the used of Reynolds number is carried out in dimensional analysis of fluid and heat transfer situation where it specify the different flow section either laminar or turbulent flow (Falkovich, 2011). Mostly the laminar flow will occur at low Re in which the viscous forces are at highest value. Meanwhile, the turbulent flow will occur at high Re whereby the internal forces tend to create eddy turbulence and vortices which is unstable airflow. In addition, the Re also can be utilise to determine the resemblance between two different situations of fluid flow.
For this research, the Re will be used in order to determine the characteristics flow of laminar and turbulent flow at which point of the wing airfoil potentially occur. It was known that laminar flow will occur at small cross section area of tube while the turbulent flow will occur at large cross section area of tube. (Brenner, 1983)
NACA 23012 AIRFOIL
Previously, in the early years of aviation, there was a United States of America government agency known as National Advisory Committee for Aeronautics, NACA that pioneered methods to describe basic airfoil shapes for standardization. And recently, most aircraft used an airfoil shape on wing section are referring NACA airfoils in order to generate more lift and better aerodynamic efficiency.
In 1930s, the "families" of airfoils was created and experiment by NACA whereby NACA four-digit and five-digit series are the most successful airfoils in their research. The airfoil comprised of a "basic thickness form" – a symmetrical "teardrop" shape-superimposed on a "camber line" from which the profile adopted most of its aerodynamic characteristics, such as the amount of lift it created at zero angle of attack, and the strength of the "pitching moment" or diving tendency that cambered served to produce (Garrison, 2009). Most of these airfoils are still relevant nowadays whereby the NACA four-digit series airfoils is easier to fabricate.
However, this research will be used of NACA five-digit series airfoils in which NACA 23012 has same thickness with NACA four-digit series but different camber line and numbering system or letter as shown in Table 2.2. This NACA 23012 was designed by Eastman Jacobs, are the most extensively used airfoil of all time where is it a combination that related to lift, low drag and pitching moment’s characteristics improvement. Table 2.1 below show advantages and disadvantages of NACA 5 digit.
Advantages Disadvantages
1. Higher maximum lift coefficient.
2. Low pitching moment.
3. Roughness has little effect. 1. Poor stall behaviour
2. Relatively high drag
Table 2.1 – Advantages and Disadvantages of NACA 5 digit.
Table 2.2 – NACA 23012 Design Specification
Digits Letter Example Description
1 L 2 This digit controls the camber. It indicates the designed coefficient of lift (CL) multiplied by 3/20. For example L=2. Thus, CL = 0.3
2 P 3 The position of maximum camber divided by 20. For example P = 3. Hence, maximum camber is at 0.15 or 15% chord
3 Q 0 0 = normal camber line, 1 = reflex camber line
4 & 5 XX 12 The maximum thickness as percentage. For example XX = 12. So, the maximum thickness is 0.12 or 12% chord.
The numerical research has proved that NACA 23012 is the one of the most airfoil to produce a relevant high maximum lift and low profile drag which resulting a surprisingly high speed range index value. Besides that, it also have a mild value of pitching moment which is suggested for high lift test (Eastman, 1936). Figure 2.4 shows the NACA 23012 airfoil design shape that has been generate by using NACA 5-digit equation which have small or medium camber and moderate in thickness.
Figure 2.4 – NACA 23012 Airfoil Design (Airfoils Tools, 2015)
MAGNUS EFFECT
Generally, Magnus effect is commonly well known for its application on the spinning ball as shown in Figure 2.5 below. However, it has been attracted attention in aviation industry whereby the concept is a potential successful to be implement in wing development. This concept introduced by Gustav Magnus where it produced a high lift coefficient compared to the airfoil lifting design.
Figure 2.5 – Magnus Effect Concept (Wikipedia, 2015)
This Magnus effect or also known as Magnus force can produced huge amount in magnitude rather than available wing lifting force. The magnitude can be define as a function of spinning rate, flight velocity and geometry of the body.
Previously, there are many type of Magnus rotor investigation that has been done to determine the aerodynamic efficiency which usually in terms of L/D ratio as shown in Figure 2.6. The Magnus rotor are capable to produce lift at low airspeed while spinning at high rates.
Figure 2.6 – Lift-Drag Ratio Various Type of Magnus Rotor (Seifert, 2012)
Hence, for this research, the Magnus effect concept are occur due to the installation of horizontal lift fan. The horizontal lift fan is installed in the 1/3 of wing chord airfoil with modification of NACA 23012 airfoil. As the horizontal lift fan rotate downward, there will be a more positive ambient pressure close to the boundary layer of upper surface airfoil which increase the velocity the airflow. An increase in airflow, it will lead in producing the lift of the wing airfoil.in an upward direction.
ROTATING CYLINDER
The flow around the circular cylinder is quite fascinating whereas it mostly study the fluid or air flow that is related to the relationship between fluids with a moving or rotating solid surface as shown in Figure 2.14. This rotating cylinder is related to the generation of lift forces by the effect of Magnus effect. The lift can be increasing as the rotation or speed of the rotating cylinder is increased with steady condition whereas the drag can be considered almost completely diminish (Alcantara, 2014).
Figure 2.7 – Rotating Cylinder Boundary Layer (Hall, 2015)
Flettner rotor is one of the application of the rotating cylinder. This rotor normally rotates at high speed in related to low airspeed will produced enough lift for aircraft attitude such as take-off and landing. However, there will be two gyroscopic effect which affect the aircraft lateral motion which are precession and nutation (Seifert, 2012).
This Equation 2.4 show the calculation of lift in Magnus effect on rotating cylinder.
L=r × V × G
Equation 2.4 – Magnus Effect Lift Formula
From the Equation 2.4, it can be determine that the lift produced per unit length, L are equal to the multiplication of density of the flow, r velocity of the flow, V and the strength of the vortex establish by rotation, G. Thus, the lift is directly proportional to velocity, density and strength of the vortex (Thom, 1926).
The torque induced precession is the orientation of rotation axis is change due to the principle of conservation of angular momentum. This is lead a rotating cylinder resist to change its orientation due to the angular momentum. Meanwhile, the notation is based on a slightly irregular motion of the rotation axis due to the external force which may occur in aircraft when yaw and roll angles are oscillate at the same time.
Meanwhile, the effect of lift can be determine on the design concept of the rotating cylinder. For example, the design of Flettner rotor with different endplate as shown in Figure 2.15. It shows that the lift can be produced in an enormous value as the size of the endplate also increase. Thus, the lift is proportional to the size of the endplate.
In most scenario, the Flettner rotor is considered as the best trade-off between the consumption and the aerodynamics efficiency. Thus, it has been utilizes in the aeronautics application in for development of future aircraft conceptual design.
Figure 2.8 – Flettner Rotor with Different Endplate Size. (Seifert 2012)
AIR COMPRESSOR/BLOWER
The air compressor is widely use nowadays throughout the world. By utilizing current technology, this air compressor are more efficient whereby it can generate an accurate amount of pressure needed with a simple adjustment compared to previous century. It because it consist of motor which generate by either mechanical or electrical to convert into potential energy stored in the tank which is pressurized air or pneumatic air. The compressed air stored in the tank can be used for various purpose of application. Figure 2.7 below shows one example of compress air purpose.
Figure 2.9 – Portable Air Compressor (Elliot, 2006)
Thus, compressed air produce a very powerful utility in term of high pressure produce with reliable and safe (Elliot, 2006). For this research, in terms of airflow, the compress air will produced a greater amount of laminar airflow distance travel or high velocity as pressure being applied is increase. The variable pressure will be set in response to change angle of attack.
In addition, with related to the Newton’s third law, and Magnus effect, as the compressed air being applied to the horizontal lift fan in wing airfoil there will be an equal reaction produced oppose the applied pressure which will generate the airfoil to move forward. As angle of attack are increase, the airfoil will create lift. In the same time, the pressure blower applied to the horizontal lift fan will experience a great amount of Magnus effect produce. This is because, the lift of Magnus effect are depending on the rotation of the horizontal lift fan. As higher amount of pressure being applied to the lift fan, more rotational will create by the lift fan.
CROSS FLOW FAN
The cross flow fan or known as transverse fan as shown in Figure 2.8 has been investigated and experimental for many years that normally utilize for low speed aircraft. It consist of 2 dimensional widespan propulsor that are installed within the wing structure to provide huge amount of thrust and lift along the wingspan (Kim, 2003). The cross flow fan has been utilize nowadays to maximize the lift produce and reduced the drag. It consist of two type which are fan wing and propulsive wing.
Figure 2.10 – Cross-Flow Fan (Hi-Tech Blowers Inc, 2015)
FAN WING
It is a mechanical device that utilize cross-flow fan in the wing for distributed propulsion and augmented wing lift at a very low flying speed. A cross-flow fan are installed at leading edge of wing will transfer the potential energy from the engine to the kinetic energy along wing span as shown in Figure 2.9. Thus, a lift will be created from the vortex and rearward acceleration of huge amount of airflow whereas lead to short take-off and landing with silent or quiet and efficient short-haul heavy lift capability (Seyfang, 2012).
Figure 2.11 – Fan Wing Location in Aircraft (Seyfang, 2012)
Moreover, it has no sudden stall at high angles of attack. It because of the airflow are not separated as the wing is still blown. Surprisingly, it also has no asymmetry if the engine failure whereby each fan are connected to a single cross-shaft so that the power of the engine are distributed in balance. Last but not least, it also capable to auto rotate just like helicopter for emergency landing purposes to develop lift at low speed.
In order to achieve the optimum capability of the wing, a wind tunnel test have been conduct to determine the aerodynamic efficiency as shown in Figure 2.10 below. Thus, it have been confirmed the key of the successfulness of the wing is the ratio of fan blade tip speed to aircraft speed which known as Tip Speed Ratio, TSR which increase the lift and huge potential to hover and fly vertically (Thouault et.al, 2010).
Figure 2.12 – Fan Wing Wind in Tunnel Testing (Seyfang, 2012)
The design of fan wing blade is a crucial element, whereas it has been tested in a huge number of design concept with consideration of blade sizes, blade number, blade cambers, and blade setting angles. The blade design will be compare in CFD where it indicates the wing blade design complex airflow. The design also will be test in a simple water channel at different TSR in order to determine the air flow pattern for each design as shown in Figure 2.11.
Figure 2.13 – Fan Wing Airflow (Seyfang 2012)
PROPULSIVE WING
It is a mechanical rotating device that constructed with thick wing section with a cross-flow fan to develop a greater thrust and better flight control as shown in Figure 2.12. This type of design is suitable to be used in any scale of aircraft, from small UAV to large cargo aircraft. The propulsive wing is capable to increase the lift while maintain the airflow as it rotates. Besides that, it also can decreasing the drag and preventing form stall attitude due to its location installed at trailing edge of the wing.
This type of airfoil, has the ability to ingest and maintain the airfoil without consideration of angle of attack. Thus, it can operate over 45 degree while producing lift coefficient more than 10 at take-off and landing. In addition, due to its combination of propulsor, flow control device and thickness aspect ratio, this type of wing will provide a compact and cost effective short take-off and landing for cargo which can carry 3 times more the payload weight with less noise.
Figure 2.14 – Propulsive Wing Structure (Propulsive Wing, 2010)
As the cross-flow operates to rotates, it draw the air from the upper wing to the lower wing surface and remove the air towards the trailing edge. At this situation, the wake turbulence is almost completely eliminated due to the propulsive wing airfoil design which increase the aerodynamic characteristics efficiency such as lift (Kim, 2003)
Figure below shows the streamline flow when the propulsive wing at higher angle of attack. It can be seen that the boundary layer reattachment as the fan is rotating. Thus, there will be at least no turbulent on the wing airfoil design whereas it will create an enormous amount of lift.
Figure 2.15 – Boundary Layer at Higher Angle of Attack when the fan is off (a) and fan is on (b) (Propulsive Wing, 2010)
DUCTED AND UNDUCTED FAN
DUCTED FAN
Up until this era, there are still the same in considering the type of engine fan, either ducted or unducted fan. The ducted fan is a type of a propeller that are mounted in the cylindrical shroud, casing or duct. This type of propeller is promising in reducing the losses of thrust from the propeller tips (Hovey, 1974). Besides that, it also provide a high static thrust propulsion system which include a safety features attributes of the cylindrical shroud or duct that enclosed the rotating fan as shown in Figure 2.16. It consist more number of blades with a shorter length which is capable to operate at higher rotational speed.
Figure 2.16 – Ducted Fan (ebay, 2015)
This ducted fan usually have an odd number of blades because to prevent from resonance in the shroud. This type of fans is always be offers in VTOL aircraft application and form other purposes of low speed design for hovering purposes. There are few advantages and disadvantages of the ducted fan as shown in Table 2.3.
Table 2.3 – Advantages and Disadvantages of Ducted Fan
Advantages Disadvantages
Efficient producing thrust at low speed and high static thrust. 1. Less efficient at cruise
Quieter rather than propellers by shield the noise of blade 2. Complex duct design with requirement of high RPM.
Capable to allow limited amount of thrust vectoring 3. Produce drag and stall at higher angle of attack.
UNDUCTED FAN
Unducted fan is a new design with higher efficiency where it has no cylindrical shroud or duct as shown in Figure 2.17. It has a design with large number of short and highly twisted blade even though the diameter is small. The design itself is more focus on the performance and speed of the engine with economy fuel saving (Otis, 2002).
Figure 2.17 – Unducted Fan (Wikipedia, 2015)
Generally, it consist 2 number of row of the blade configuration whereby it are not suitable for the VTOL application purposes for thrust vectoring. The design itself might be the potential risk of passenger leaving to fly with other ducted fan because their perception of risk and choice of prospective passengers (Cameron, 2012). The advantages and disadvantages of the unducted fan are shown in Figure 2.15 below.
Table 2.4 – Advantages and Disadvantages of Unducted Fan
Advantages Disadvantages
Fuel Economy. 1. Excessive noise pollution as it rotates
Provide safe operational at tips speed higher than conventional blades because prevent shockwave. 2. Dangerous due to not incorporated with enclosed shrouding which may harm if engine failure.
Capable to achieve 30:1 bypass ratio where it can achieve higher value of Mach number