Introduction of basic flight aerodynamics and Flight Mechanics

[Nilgiri]

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 tendency to return its equilibrium state (e.g. mean position) after being disturbed see figure below also 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 does not return to its equilibrium position. For example if you have balanced a stick on your hand and if 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 then the applied force is remove 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

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Good to see update after a long break

 

[war&peace]

Good to see update after a long break

yeah it took me sometime because i became occupied with a lot of new things and adjusting to the new place i.e. Sweden.

But I spotted a lot of typos so I must correct them.

 

[war&peace]

@CriticalThought @django Guys I started this thread in an attempt to impart the fundamentals of flight mechanics so that PDF members can appreciate the airplanes, missiles and UAVs in a more authentic way.

 

[Ebrahem]

@CriticalThought @django Guys I started this thread in an attempt to impart the fundamentals of flight mechanics so that PDF members can appreciate the airplanes, missiles and UAVs in a more authentic way.

ur efforts are much appreciated very helpful just w8ing for flutter part as i m currently working on this in my PHD i.e transient growth

 

[django]

@CriticalThought @django Guys I started this thread in an attempt to impart the fundamentals of flight mechanics so that PDF members can appreciate the airplanes, missiles and UAVs in a more authentic way.

I will have to role the years back to get into this one.Kudos and nice informative thread bro.

 

[war&peace]

ur efforts are much appreciated very helpful just w8ing for flutter part as i m currently working on this in my PHD i.e transient growth

Great...does your work involve experimental data too or it is only theoretical work....are you using some wind tunnel...

I will move to aeroelasticity but I think it may be a bit early for the common reader. Unsteady aerodynamics and aeroelasticity are my favorite topics. I'm especially interested in aeroservoelasticity as it helps me with flight controls and simulations.

Now we have got the basic idea of stability, it will be treated mathematically later on but i will my best to keep it as simple as possible but I can't make a promise to make it simpler than necessary and impart my audience the wrong or insufficient information so get your pens and notebooks ready and perhaps refresh your basic math knowledge....yeah may it is time dust off those K-12 physics and math books back from the closet... and if you have any difficulties there...drop a comment here and I will see if and how can I help you there.

It is right time to introduce a new term manoeuvrability related to stability. We often remark how manoeuvrable an aircraft is when are trying to point to its agility.. basically manoeuvrability means less stability. Military airplanes (aircrafts) are designed to perform manoeuvres and thus they need to be less stable..so much so the modern aircraft are designed unstable to the extent that they cannot be flown by the pilot unaided. They need flight computers to calculate the precise

General Dynamics designed one such airplane rather aircraft... and that's our F-16 yeah it was THE first airplane that couldn't be flown without a computer since it was unstable but in subsonic flow regime only. In supersonic flight it becomes stable again... it is a magic..until W&P (that's me) demystifies it. But to let you know that almost all dog fights and manoeuvres take place at subsonic speeds...Supersonic are used when an AC (aircraft) has to dash and run away or towards the target.

Performing manoeuvres at supersonic speeds is too dangerous and will destroy the airplane as the g-forces are very high ..higher than the structure and control surfaces can handle. So high manoeuvrability made F-16 the most formidable dogfighter to this day.

 

[war&peace]

Hey @Oscar what's your area of interest since I see you commenting a lot about AF though mostly your posts are related to policy/political aspects but I guess you have a sound technical background too. So your contribution will be appreciated and an addition to the little bucket of knowledge here.

@MastanKhan you can also contribute to this thread with your knowledge and suggestions/feedback.

 

[war&peace]

I will have to role the years back to get into this one.Kudos and nice informative thread bro.

I hope with your picture perfect memory, you may not need to role back at all or just a little. After all, I have Kept It Simple & Stupid (KISS) i.e. not much advanced mathematics so far..only some algebraic expressions where necessary.

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And talking of the stability and the airplanes, apart from fighterjets and some sport A/Cs especially for aerobatics, most of the fixed wing airplanes are designed to be stable.
Now we will get slightly more scientific and learn some more terms

 

[django]

I hope with your picture perfect memory, you may not need to role back at all or just a little. After all, I have Kept It Simple & Stupid (KISS) i.e. not much advanced mathematics so far..only some algebraic expressions where necessary.

And talking of stability and the airplanes, apart from fighterjets and some sport ACs especially for aerobatics, most of the fixed wing airplanes are designed to be stable.
Now we will get slightly more scientific and learn some more terms

So far so good.Kudos bro

 

[war&peace]

Now we need to get familiarised with the concept of center of gravity (cg), so i will define it as the point on body through which its weight appears to act. More formally and physically sound definition is that it is the point at the net of mass moment of all the particles is zero so mathematically,

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An airplane is like a system of particles with different masses located at different positions and the centre of gravity as combined effect of all these mass components can be found. So what are those components I think we discussed those earlier briefly but here we go, in a fighterjet these are engine(s), wings, pylons, canopy and pilot compartment, tails, control surfaces and their sub systems.. each system is made from different material and thus weighs differently so we can use the above relation to calculate the overall center of gravity (cg). Now if I go a bit deeper and philosophical, there are two differents centers: the center of mass (cm) and center of gravity (cg) and first equation describes the center of mass while the center of gravity is calculated by multiplying 'g' (the acceleration due to gravity) and on the Earth and for the size of an airplane there are no significant gravity gradients so 'g' is constant for all practical purposes i.e.

 

[Nilgiri]

In supersonic flight it becomes stable again... it is a magic..until W&P (that's me) demystifies it. But to let you know that almost all dog fights and manoeuvres take place at subsonic speeds...Supersonic are used when an AC (aircraft) has to dash and run away or towards the target.

It's not a surprising phenomenon given then number of things that reverse fundamentally (specifically what tends to accelerate/decelerate w.r.t flows and thus applied forces) from subsonic regime to supersonic.

Simplest version of that is to see a converging or diverging nozzle and have subsonic and supersonic flow run across it and see what the accelerations are (this also opens up why say rocket nozzles are designed to first converge and than diverge). Have we talked about this yet anywhere in the thread? Its an interesting subject...and opens up large chapter of (isentropic, largely "static") fluid dynamics. Inherently it explains why often aircraft good at subsonic regimes are pretty lousy in supersonic and vice versa....and aircraft that have to be good at both have to make compromises in both regimes for it.

BTW we should all work together here on some aeroelasticity development/talk/calc when its appropriate. I did model a whole bunch of stuff back in the day (even in such basic matlab-simulink modules I developed) but my career has taken me away from it for quite a long time, it will be good to revisit again and refresh.

 

[war&peace]

It's not a surprising phenomenon given then number of things that reverse fundamentally (specifically what tends to accelerate/decelerate w.r.t flows and thus applied forces) from subsonic regime to supersonic.

Simplest version of that is to see a converging or diverging nozzle and have subsonic and supersonic flow run across it and see what the accelerations are (this also opens up why say rocket nozzles are designed to first converge and than diverge). Have we talked about this yet anywhere in the thread? Its an interesting subject...and opens up large chapter of (isentropic, largely "static") fluid dynamics. Inherently it explains why often aircraft good at subsonic regimes are pretty lousy in supersonic and vice versa....and aircraft that have to be good at both have to make compromises in both regimes for it.

BTW we should all work together here on some aeroelasticity development/talk/calc when its appropriate. I did model a whole bunch of stuff back in the day (even in such basic matlab-simulink modules I developed) but my career has taken me away from it for quite a long time, it will be good to revisit again and refresh.

Good to see you here. I'm trying to build it in a step by step manner and keeping things simple because it is not a formal undergrad class of students with right background. Here people with different backgrounds and levels constitute the audience. Flow through convergent-divergent nozzle will be explained at a later stage.
Right now I'm just establishing the basic of stability before I dive into controls but it will be a shallow dive and gradually increase the depth ..what do you say buddy?

 

[war&peace]

Continuing from post#40...

Centre of gravity can easily be calculated for regular shapes and mathematical relations are available for them but in case of an airplane, it is lot more difficult for the following reasons

1. Airplane is not a primitive or a simplified shape
2. It is made from materials with different densities, and those are located at different places with cutout and void in between them
3. Airplane has a variable mass (fuel, payload)

So for these reasons, it is a quite involved process to determine the location of CG and also to establish its variation as the mass changes due to the fuel consumption.

 

[war&peace]

Thus the range of change in location of CG should be known and the controls should adequately provide the response needed over the whole range.

 

[Nilgiri]

Thus the range of change in location of CG should be known and the controls should adequately provide the response needed over the whole range.

You can more easily see the range of trim (on horizontal stabiliser) afforded for large (normally cargo) aircraft (and also to afford greater regimes regarding all the effective moment arms and also the massive shift in lift vector with high lift flaps on wing) because of this :

Somewhat smaller aircraft normally have it too just have to zoom into the interface:

Essentially keeps elevator deflection more efficient too (and within its own operable envelope) for much larger windows too regarding payload, speed, altitude of the operation etc.

@Signalian you may like this thread.