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Introduction of basic flight aerodynamics and Flight Mechanics

Discussion in 'Air Warfare' started by war&peace, Jul 5, 2016.

  1. war&peace

    war&peace ELITE MEMBER

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    In a steady (unaccelerated) and level flight, an airplane is in a total balance i.e. four major forces cancel each other
    L=W means levelled flight ( balance in vertical direction)
    , T=D means no acceleration or deceleration i.e. uniform speed (balance of forces in horizontal direction)
    where L is the lift force generated by lifting devices i.e. wings mainly, and/or canards and/or fore-wing and also the tail but that is normally aimed to counter or provide the moment for pitching motion. The lift force balances the weight
    W = m*g

    L=CL* q *Sref, Lift is an aerodynamic force i.e. force due to the flow of air

    q= 0.5*rho*U^2 dynamic pressure

    CL is coefficient of the lift CL= CL0+CLa*a
    CLa
    basically the slope of the cl curve and CL0 is the intercept... and for flate plate CLa =2*pi and it is dCL/da as i said it is just the slope.

    Now the question comes to mind is how does the wing produces lift, so the simple explanation is considering a 2-D sections of the airplane wing (it is a 3D object/surface) called airfoil as shown in the figure blow ( taken fron internet) So when the air flows past an airfoil it divides into two paths, above and below the airfoil. If there is angle between the chord of the airfoil and incoming wind, the lengths of paths are different and that causes pressure difference between the lower and upper surfaces of the airfoil. The upper surface normally has lower pressure and thus known as suction side while the lower surface has a higher pressure or positive pressure. This pressure difference generates the lift

    View attachment 315681
    In figure below you can see the chord, leading edge and trailing of an airfoil.
    View attachment 315684



    Here is the airfoil with forces

    View attachment 315678


    Now, in D=T, is an aerod. force in horizontal direction ( or parallel to the wind speed while the lift is at right-angles as shown in the figure above) for a levelled flight and is known as drag that simply means air resistenace and T is the thrust...which provided by the engine or specifically by the jet
    D =Cd* q *Sref
    D = T
    holds only when the airplane is flying with uniform or constant speed but if thrust goes up, it will accelerate and vice versa. So you can see drag as a negative effect/force and normally the designers try to minimize it or they try to optimise the lift to drag ratio.
    As we discussed before, the angle between the direction of airflow i.e wind speed and the chord is called angle of attack (denoted by greek letter alpha or briefly AoA)...so the aerodyn. forces in the wind coordinates are life and drag while in body fixed (airfoil) coordinates are axial and normal forces with cofficients Cx and Cz as showin the figure above and these can be transformed into each other by simple vector or trigonometric analysis i.e. drawing the components of the forces one coordinate and equating them with the forces in the other coordinate and vice-versa and the AoA is only angle needed in this simplified case.
    L = N*cos(a) - T*sin(a)
    D = N*sin(a) + T*cos(a)

    A resultant force R is just obtained by the vector addition of axial and normal forces.
    Now as soon as the aircraft will depart from this levelled and steady flight the balance will be disturbed and net force won't zero and components of the forces will appear and that's where trigonometric ratios come into play.
    However a wing is a 3D object so the flow over it is 3D...which is not necessarily a good thing as it results in the 3D effect and can reduce the flight efficiency of an airplane. What happens is that the streamlines on the suctions side of the wings get contracted while the on pressure side reverse happens and this differential resultsi n vortex shedding and vortex is rotating flow structure in the wake and effect anything that it interacts. We will see it in more detail when we discuss the wing, canard, wing-tip devices (i.e. wing lets and wing tip mounted weapons). Now lets move on to more flight mechanics. So look at the picture below most probably inspired by our favorite F-16 :) . Here it is necessary to understand that how the body forces and the aerodynamic act on the body of the airplane...Due to a complex configuration of an airplane the pressure distribution around it changes however the resultant of the aerodynamic forces seems to act through a point called center of pressure while weight of the airplane acts through center of the gravity (cg) and normall there is a distance between these two points and thus it gives rise to a couple (moment) i.e. tendency to rotate about its spanwise or pitching axis.

    View attachment 315707

    Thus the nose of the airplane in the picture about will go up and would keep going up if not arrested by some measure so this configuration is unstable and needs active input by the pilot or the flight computer to keep it flying but it gives an advantange to the airplane i.e. it is ready to maneouvre more easily... a pretty important characteristics for the fighterjets especially during the kutta-fight :). However the passenger and cargo airplanes are designed to be inherently stabl and that means the Cp is moved aft of the c.g.
     
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  2. touela

    touela FULL MEMBER

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    I tried, I mean I really really tried, but I cannot understand it... way to technical for me... do you have a lectura for Dummies example :undecided:
     
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  3. war&peace

    war&peace ELITE MEMBER

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    Oh I am trying my best to write it in an easy to understand way.
    Can you please point out what you do not understand? Your feedback is valuable for me.
     
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  4. war&peace

    war&peace ELITE MEMBER

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    Some terms that are necessary to understand the airplane mechanics. Airplane flies through the air. Air is a fluid and the study is motion is known as aerodynamics. Some aerodynamics terms are defined as below

    Mach Number: It basically measures how fast an object flies through the air relative to the speed of sound 'a' which is under normal conditions is 340 m/s (1224 km/hr or 661 kts), mathematically
    Mach = speed of object / speed of the sound
    so an object/airplane flying at exactly the speed of sound will have a Mach = 1. so airplane flying at Mach 1.6 is flying 60% faster than the speed of sound. This way you can easily figure speed of an airplane or missile so for example a BM flying through the air at Mach = 7.0 will have a speed of 8568 km/ hr that mean in 1 hr it can reach a target at distance of 8568 km but ballistics tells that an ICBM travel at much higher speeds than this thus considerably shorter times to reach such a distance. However the speed of sound is not constant are varies with the temperature of the air can be calculated using the following formula
    a = sqrt( gama * R * T)
    where a, gama, R = 1.4 and T are speed of sound, specific heat ratio for air, universal gas constant for air, absolute temperature respectively.

    @Khafee @Doordie @Sarge @Djinn @MastanKhan
     
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  5. Signalian

    Signalian PDF THINK TANK: ANALYST

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  6. The SC

    The SC ELITE MEMBER

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    Aircraft and Rocket Flight

    Airships and balloons owe their ability to ascend and remain aloft to their inflation with a gas lighter than air; this is an application of Archimedes' principle of flotation, i.e., that a body immersed in a fluid (liquid or gas) is buoyed up by a force equal to the weight of the fluid that it displaces. Aircraft, which are heavier than air, are able to remain aloft because of forces developed by the movement of the craft through the air. Propulsion of most aircraft derives from the rearward acceleration of the air. It is an application of Newton's third law, i.e., that for every action there is an equal and opposite reaction. In propeller aircraft the forward motion is obtained through conversion of engine power to thrust by means of acceleration of air to the rear by the propeller. Lift is obtained largely from the upward pressure of the air against the airfoils (e.g., wings, tail fins, and ailerons), on whose upper surface the pressure becomes lower than that of the atmosphere. In jet-propelled aircraft, propulsion is achieved by heating air that passes through the engine and accelerating the resultant hot exhaust gases rearward at high velocities. Rockets are propelled by the rapid expulsion of gas through vents at the rear of the craft. The high speeds that are produced by jet and rocket engines have brought about substantial changes in the science of flight.

    http://encyclopedia2.thefreedictionary.com/Principles+of+flight
     
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  7. Nilgiri

    Nilgiri BANNED

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    Is there a specific question anywhere in the thread out of interest? I would be happy to try answer them.

    I see some good theory being presented to start.

    Maybe I should get my stability and control notes out and scan them to post here...that class was insane heh.
     
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  8. war&peace

    war&peace ELITE MEMBER

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    Thanks for the appreciation.
    However no shortcuts :p: I mean I'm authoring myself instead of copy pasting material from any online website as I making it customised for this particular audience and here the examples will be pdf specific ones like F-16, JF-17 etc :-)

    But i will appreciate anyone who would like to contribute and controls is a nice subject..which I studied with Baba Roskam (IYKWIM) so you can chip in with you content at the right time but try to make it interesting, simple and part of this story
     
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  9. Nilgiri

    Nilgiri BANNED

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    Thats fine. I appreciate such threads and tag me when you feel I can contribute. Same goes to anyone else that has questions for any of the content you post (but it has to be specific....not just I dont understand! hehe)

    I certainly will not be copy and pasting really from my notes, but presenting my own analysis...with the notes as the grounding where needed. I just had some good pictures and plots in that, that may be useful for this thread to scan.

    Our professor actually was quite a good one for that course, it wasnt usual regurgitating but the notes themselves were in-class derivation (you were kind of forced to attend as a result lol) with blanks to fill in etc (but everything else typed out so you dont have to wear out your hand). No other prof did it that way, I quite enjoyed the format....like a mystery novel with in class detective work.
     
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  10. war&peace

    war&peace ELITE MEMBER

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    Angle of attack:
    I defined this equation in my first post OP...
    CL= CL0+CLa*a
    a in the equation above is the angle of attack, however this equation is about the airfoil as describe in first post...but I want to define another important parameter here that is the angle of attack of the airplane, it would be more robust definition if i define wind and body axes first but I would skip that part for the sake of simplicity and define the angle of attack (aoa) as the angle with the relative wind and centreline (chord) of the airplane wing..i.e. flight direction.. usually the more the angle of attack, more the lift but it is upto a limit called stall.

    Stall:
    The stall is basically the loss of lift force. It happens as the angle of attack increases, the flow starts to separate over the upper surface of the airfoil (airplane wing) and at certain angle of attack, the flow separation line moves forward over the surface of the airfoil and zero lift is generated and thus the airplane fall under influence of gravity until it nose dives and air speed over the wings increases so much so that the wing generates lift again and stops it from plunging any further but in this process an airplane can loose a lot of altitude ...thus if stall happens at lower altitudes, the airplane can hit the ground and be destroyed..
     
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  11. war&peace

    war&peace ELITE MEMBER

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    yes of course, may be you can start with basics of stability theory... and then we introduce the concepts of controls, transfer functions and Laplace transform, but we keep to simple LTI at least initially but later on, we can go for multi-variable controls and state space modelling. Also concepts of roots, poles, s-plane, Nyquist criteria..etc and then will ease into avionics and signals and address basic principles of radars and communications :) it will be a fun ride.. :)
     
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  12. war&peace

    war&peace ELITE MEMBER

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    So lets continue here... we discussed stall but we avoided the math ...do you want the math, please comment otherwise we continue...
    Now a few more definitions are necessary
    1. Indicated airspeed
    2. True airspeed
    3. Pressure-altitude
    4. Temperature model of the atm
    But i think first we should discuss something about the airplane geometry, configuration, parts and their functions... for educational purpose we normally use a generic airplane model / body diagram. show below
     
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  13. war&peace

    war&peace ELITE MEMBER

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    serveimage-4.jpeg
     
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  14. Nilgiri

    Nilgiri BANNED

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    It may be useful to add C of G and aerodynamic centre to the diagram at some later point for stability analysis.

    How detailed do you want to go btw? We can resolve a pretty detailed force diagram and compare it to a simpler "summed up" one.
     
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  15. war&peace

    war&peace ELITE MEMBER

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    Description (Please refer to figure above) :
    Wing: Main part the produces lift. the tip to tip straight distance is called wingspan. the semi-span is half of the wingspan. The front edge of the wing called leading (LE) edge while the rear edge is called trailing edge (TE)
    Flaps: These are high-lift devices ...which basically increase the camber and surface area when deploy. basically used during the landing and takeoff phases of the flight to provide extra lift at low wind speeds.
    Ailerons: These are the surfaces attached to the wing normally near the wing tips on the trailing edge. These are deployed in differential so the effect is the rolling moment i.e. rotate the airplane about its longitudinal. axis used when the airplane changes its direction and banks on any side.

    Excellent idea but I will add a clean image with less details then cg, ac and even ec can be added with definition of axes and moments.
     
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