Introduction of basic flight aerodynamics and Flight Mechanics

[war&peace]

Slats: There are similar to the flaps but are located on the leading edge.

The tail unit: the tail unit consists of a horizontal stabiliser and an elevator

1. horizontal stabiliser: the horizontal surface like small wing near the tail of the airplane body (fuselage) these provide a counter moment to keep airplane levelled in the steady flight
2. elevator: The elevator are similar to the flaps but these attached to the trailing of the horizontal stabiliser and their function is provide the pitching moment (or can say these control the nose up or down motion of the airplane). We will discuss about these moments soon

Fuselage: This the main body of the airplane as shown the figure..this is where all the passenger cabins, pilot cabins, crew cabins and other facilities are located like kitchen and toilets. In case of fighter-jets, the fuselage has the radar and avionics mostly located inside nose cone if fuselage, then canopy where pilots sits and controls the airplane and through console and, then the engine also located inside the fuselage till the tail end where it exhausts.

The picture above defines the axes of the airplane namely xb, yb, zb called roll, pitch and yaw axes, corresponding moments L, M, N namely rolling, pitching and yaw-moments and their rates p,q, r

 

[war&peace]

All these concepts will be further revised and refined as per the need of the topic..Right now I'm trying to keep everything simple for newbies.

 

[war&peace]

Rolling Moment (L): This moment is about the longitudinal axis xb. It causes the airplane to rotate about it the centreline through the fuselage (xb) during a turn. Ailerons are used to generate this moments and the differential positions of ailerons on the left and right semi-wings generates the couple (Moment) about the xb and rotates the airplane and helps in turning. The rate of rotation about this axis is denoted by P.

So remember the rates since these are important for the design of controls and even important for understanding the flight.

 

[war&peace]

Pitching Moment (M): This moment is related to the nose up and down motion of airplane i.e. that means about the yb axis as shown in the figure click here. The elevators are used to generate this moment. One key information to keep in mind is that the origin of these axis is set at C.G (center of gravity).

One thing that should be kept in mind is that the airplane is a variable mass object because there is significant change in its mass due to fuel consumption during various phases of flight and thus the location of C.G changes during the flight but it is something very well known and accounted for in design and sizing during the preliminary design phase. Thus an airplane I will be discussing here can be described as variable mass rigid body object with 6 DoF ( we will get to this part so don't panic kiddos ) ...though in reality the airplanes are flexible objects but then that physical model will become too cumbersome however I will discuss those issue related to flexibility later on which are covered under the specialised field of aeroelasticity and aero-servo-elasticity to just name a few of them.

1. Buffeting
2. Divergence
3. Control reversal
4. Flutter

 

[SherDil]

@war&peace Zabardast Work....
Worth reading..... But i'll read it later .....

 

[war&peace]

@war&peace Zabardast Work....
Worth reading..... But i'll read it later .....

Thank you!! I appreciate it. It is not so difficult .. I wrote it for the dummies

 

[Nilgiri]

Thank you!! I appreciate it. It is not so difficult .. I wrote it for the dummies

I wanna see how many people actually "get" phasor diagrams for flutter when they come up when you are at level super saiyan on this thread

 

[SherDil]

Thank you!! I appreciate it. It is not so difficult .. I wrote it for the dummies

i am in a hurry. But i'll take your complement

 

[war&peace]

I wanna see how many people actually "get" phasor diagrams for flutter when they come up when you are at level super saiyan on this thread

I will feed them with pre-digested food

Of course flutter will be cumbersome for a lot of people since many confuse it with the resonance. However i will build it bit by bit and you are also welcome to contribute in this

 

[Nilgiri]

I will feed them with pre-digested food

View attachment 377277


Of course flutter will be cumbersome for a lot of people since many confuse it with the resonance. However i will build it bit by bit and you are also welcome to contribute in this

I'll let you guide the thread and I will interject annoyingly. I think this is the best plan

 

[war&peace]

So guys i was preoccupied with some stuff...so let's get back to our main topic which is about the airplane flight mechanics

So far we have discussed, discussed ailerons and elevators and their related rolling and pitching moments and rates, respectively...however there is another surface so lets discuss it

Rudder:
The large vertical surface at the tail end of the fuselage is called vertical stabiliser which provides the airplane the lateral or direction stability..However it is not rudder, the rudder is "moveable" control surface hinged to the vertical stabiliser and this surface can is used to provide the yawing moment that can turn the airplane nose to the right or left more like the steering system in your car but the difference between the airplane is that the former can just move in 2-dimensional space while the airplane moves in a 3-dimenional space with 6-DoF ( Degrees of Freedom, which I will explain shortly). However some of the DoF in the airplanes are coupled. Rolling and yawing motions/rotations are two such DoFs.

Yawing moment (N):
The moment generated by rudder control surface is called yawing moment denoted by N. It causes the nose of the airplane to deflect left or rift depending in which direction the rudder is deflected but since the motion is about vertical axis z_b passing through the c.g so if rudder is moved clockwise, it will generate a ccw moment and vice-versa.. and the rate at which this rotation is generated is called yaw-rate and it is denoted by r.

 

[war&peace]

Degrees of Freedom (DoF):
The degrees of freedom basically represent the directions an object can move including both translations and rotations. Since an airplane can translate in three directions (X, Y, Z) or namely longitudinal, transverse, vertical and three rotations about these axes means an airplane has a total six DOFs.

 

[gambit]

In movies, a take off is usually portrayed as when the pilot pulls back on the stick/yoke and the aircraft becomes airborne.

That is not true.

If the aircraft is of the conventional type landing gear, meaning have a tail wheel, pulling back on the stick/yoke will mean more force on the tail wheel into the ground, creating drag, essentially like turning on the brake, and slowing the aircraft down. Do this hard enough and there will be a take off crash.

If the aircraft is of the tricycle type landing gear, meaning have a nose wheel, pulling back on the stick/yoke may disrupt lift over the wings, resulting in not taking off at all. Perhaps even a take off crash.

In real life, before take off, all the flight control surfaces are preset. On the take off roll, any pilot command is to keep the aircraft centered on the runway. As ground speed increases, lift is generated and the aircraft will become airborne on its own. Once there is sufficient clearance, the pilot can throttle up and command a slight tail up movement to gain more altitude.

Movie type take offs are more for dramatic storytelling effects than it is about real life.

 

[war&peace]

In movies, a take off is usually portrayed as when the pilot pulls back on the stick/yoke and the aircraft becomes airborne.

That is not true.

If the aircraft is of the conventional type landing gear, meaning have a tail wheel, pulling back on the stick/yoke will mean more force on the tail wheel into the ground, creating drag, essentially like turning on the brake, and slowing the aircraft down. Do this hard enough and there will be a take off crash.

If the aircraft is of the tricycle type landing gear, meaning have a nose wheel, pulling back on the stick/yoke may disrupt lift over the wings, resulting in not taking off at all. Perhaps even a take off crash.

In real life, before take off, all the flight control surfaces are preset. On the take off roll, any pilot command is to keep the aircraft centered on the runway. As ground speed increases, lift is generated and the aircraft will become airborne on its own. Once there is sufficient clearance, the pilot can throttle up and command a slight tail up movement to gain more altitude.

Movie type take offs are more for dramatic storytelling effects than it is about real life.

hmmm I welcome your comment, though I have not discussed the take off or landing part yet but of course things are exaggerated in movies and most of the time, scientific laws are sacrificed for the sake of entertainment and that's the reason I rarely find SciFi interesting though some of them present good ideas like Martian.

But coming back to basic flight mechanics and airplane design, the sizing and structure of an airplane is always a compromise between the structural strength and aerodynamic forces. No airplane can be designed to weigh like a tank (weight per unit area) and fly.. So airplanes are flexible structures, especially the commercial airliners and heavy cargo airplanes are build upon a wireframe instead of using solid metal... thus they are limited in what they can do.. as we know that lift is an aerodynamic force and directly depends on the wind speed squared, an airplane runs on the runway to achieve a certain wind speed and throttle is stick is push to max before starting the final run up and releasing the breaks so that max speed is achieved within the length of the runway and as soon as a certain speed has been achieved the pilot pushes the flaps down to increase the camber and surface area and thus increasing the lift at the same wind speed which comfortably makes the airplane airborne.

 

[war&peace]

Lets talk about stability and then controls.
Well there are three types of systems (1) stable (2) unstable ..... any guess ... no yeah (3) neutrally stable... Though the words are kind of self-explanatory but we need to define them clearly

1. Stable System: A system is considered stable that has the tendency to return its equilibrium state (e.g. mean position) after being disturbed (see figure below). Take the example of a pendulum which starts to oscillate if disturbed but finally it will stop and return to its equilibrium position
2. Unstable System: contrary to the stable system, an unstable system does not return to its equilibrium position. For example if you have balanced a stick on your hand and someone disturbs it, it will immediately depart from the equilibrium position and fell down. A type of inverted pendulum. (See fig below)
3. Neutrally Stable System: a system that neither returns to its original position nor becomes unstable rather attains a new equilibrium state when the applied force is removed, is called neutrally stable system. An object lying on a flat surface if disturbed moves to a new position and gets stops there, then that is the new equilibrium position. (See fig below)