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Why aft CG increases TAS

Pink Panther

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Dear All,
This question might have been asked before but I couldn't find it.
Can someone explain me why exactely the TAS increases with an aft CG

Thank you, and a happy new year with many happy hours in the sky!
 

Cardinal

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The horizontal stabilizer usually generates downforce in conventional aircraft, to balance the moment caused by the CG being ahead of the wing's center of lift. Moving the CG aft reduces that nose-down moment, thus the horizontal stab doesn't have to work as hard, and it will generate less induced drag.

But since the horizontal generates downforce or negative lift, the wing has to lift the weight of the aircraft and cancel that negative lift. Thus the aft CG will mean less induced drag from the wing and the horizontal stab.

Application can be seen in the 747, MD11, and A330/340. They all have a tank in the vertical stab to which fuel can be transferred, providing not only extra capacity but helpfully moving the CG aft during cruise.
 

sstearns2

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.....

The previous response was good, but I'd like to add a little. As the CG moves aft the aircraft generally goes faster, but it becomes less stable (the phugoid damping decreases). The fly-by-wire systems on the newer aircraft automatically stabilize the aircraft and allow it to maintain good flying qualities with the more efficient aft CG.

The fly-by-wire fighters (e.g. F-16, F-117) fly with the CG so far aft that the aircraft would simply not be controlable without the computers.
 

InHot

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CG & True Airpspeed

Great thread so far, but I guess I've been out of the school house too long. I could dig out the books, but this is much easier. How about a down and dirty on "phugoid damping?"
 

SuperD

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InHot,

"Phugoid" damping refers to the pitch oscillations which slowly decrease in amplitude over time. When an aircraft is designed, two main types of stability are considered. First, static stability. This is the tendency for the aircraft to initially return to it's original attitide after being disturbed. For example, if turbulence pitches the nose up after being trimmed for staight and level, it is desirable for the nose to initially pitch down in an attempt to return to straight and level. This is called positive static stability. Is the nose stays where it is, it exhibits neutral static stability. If it keeps rising after the upset, then negative static stability exists (not very desirable in conventional aircraft).

Once positive static stability is attained, half the battle is won. Now we have to make sure that the aircraft returns to straight and level. As you know, we need postitive static stability to even consider the next type, which is dynamic stability. In our example situation, after the nose initially pitches down, it will overshoot the level flight attitude and pitch below until the airspeed increases enough to increase the tail-down force to bring the nose back up again. The nose will then overshoot straight and level again but the nose will not rise to the original position after the disturbance. This cycle repeats over and over with smaller and smaller overshoots until they are barely noticable and the aircraft is back in the original position. This is called positive dynamic stability. Neutral dynamic stability is when the nose rises and falls the same amount with each oscillation. Negative dynamic stability is when the nose rises and falls a greater amount with each oscillation (once again, not very desirable).

If our aircraft which is dynamically stable were to turn some smoke on, the path it traces would resemble a sine wave with decreasing amplitude but roughly constant frequency. The frequency would depend on the design of the aircraft (I.e. size of the horizontal stabilizer) and the loading of the aircraft (CG location), which brings us back to the original discussion. I'm sure everybody knows most of this stuff, so sorry to ramble. Hope this helps.

SuperD
 

sstearns2

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.....

To answer the 'what is phugoid damping' question...

The phugoid 'mode' was described in the previous post. Phugoid damping is a measure of how much the amplitude decays from peak to peak. As the CG move aft the damping decreases so the aircraft will need more occillations to return to straight and level flight. The phugoid mode has a period (time from peak to peak) of about a minute, so it is actually possible to fly and airplane with an unstable phugoid (negative damping). The aircraft will require a fair amount of pilot effort to fly with an unstable phugoid, but the benifit is a very responsive airplane that just goes where you point it. Most competition aerobatic aircraft are flown this way as well as some of the pre-fly-by-wire jets like the F-4.

Scott
 

AlbieF15

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FYI...

F16...true fly by wire. Aircraft would be unstable without computer interface. Same for Stealth F117.

F15...fly by wire system augments hydraulic system (CAS). Part of TR-1 flight for new Eagle guys is diabling CAS so they can see jet is quite comforable to fly. CAS acts as "power steering" and masks effects of stores...but otherwise not required for flight. It does, of course, improve roll and pitch authority.

F4...never drove it, but I think the Phantom guys will tell you it has NARY A BIT of Fly by wire. It is all hydromechanical.

Fly safe.
 

B-J-J Fighter

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Excellent Post

Great post guys, Im really learning a lot. Aerodynamics is always been hard for me to understand because its kind of hard to me to picture what is going on. Your doing a great job putting this stuff into laymens terns.

How about explaing the reason some a/c pitch up when flaps are applied and explain the different kinds of drag.
 

SuperD

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B-J-J,


I'll try and tackle your drag question. Basically, there are two main types of drag on an aircraft. They are parasite drag and induced drag.

Induced drag is that drag caused by the production of lift. We all know about the pressure differences between the upper and lower surfaces of the wing. The air "spills" over the tips to fill in the void caused by the lower pressure above, thereby creating wingtip vortices. Looking at the wing spanwise (at the tip looking towards the fuselage), as the flow comes up and over the tips, it imparts a downward velocity on the flow leaving the trailing edge. This downward velocity component has the effect of tilting the lift vector rearwards (since lift is perpendicular to the relative wind). So, the lift from the wing now has a vertical component (to offset weight) and a horizontal component opposite the direction of flight. This horizontal component is drag. This drag force is not constant though. It depends on the lift coefficient (angle of attack), the planform shape of the wing, and the dynamic pressure (density and speed) of the airflow. Remember the FAA's "heavy, clean, and slow"? Well, the heavier the aircraft and the slower the airspeed, the higher the AOA will have to be to sustain the necessary lift. Flaps produce vortices of their own that tend to dissipate the main vorticies, so a clean wing will usually produce greater wake turbulence, but it varies with each wing and flap structure. "Ground effect" is something that we're all familiar with. As the aircraft approaches the pavement, the ground tends to dissipate the vortices, causing the lift vector to tilt forward. This reduces the drag and increases the lift at the same time. Some people will tell you that it is a "cushion of air" that causes ground effect. No such thing.

Basically, parasite drag is everything that is not induced drag. One exception is wave drag, but that's for another time. Parasite drag consists of skin friction drag (air viscosity), form drag, and several other types of less-significant drag. Form drag is the result of the shape of the aircraft and the size of the wake it produces. Flat plates held perpendicular to the airflow will exhibit lots of form drag, but very little skin friction drag. The opposite is true if you align the plate with the airflow. The classic example is sticking your hand out the window in a moving car. If your palm is forward, you will have much more drag (mostly form drag) than if your palm is down (mostly skin friction, but a much lesser value).

As a pilot, it's important to know when the aircraft will have the most drag. Parasite drag increases with airspeed (exponentially I might add!) and induced drag will decrease (mostly due to the lower angles of attack found at higher speeds). But as the aircraft slows, the parasite drag will decrease but the induced will increase. When the induced drag far overshadows the parasite drag, this is called the "backside of the power curve", meaning that more power will be required to sustain level flight as the aircraft slows.

Anyway, enough for now. This is a good discussion, let's have some more answers!

SuperD
 

sstearns2

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Flaps

Flaps generally cause the nose to rise because of an increase in downwash on the tail. It's not that the wing is making any more lift with the flaps down, but that the lift is redistributed so more of the lift (and thus downwash) is made inboard right in front of the tail and less outboard. The wing itself actually wants to pitch nose down with flap extension, but the effects of downwash are usually stronger. There are aircraft that pitch nose down with flaps, but I can't think of any right now.

Scott
 

MetroSheriff

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It's not that the wing is making any more lift with the flaps down, but that the lift is redistributed so more of the lift (and thus downwash) is made inboard right in front of the tail and less outboard.
Huh? Redistributed lift and downwash???

I wont swear to it, but....I vaguely recall something about flaps increasing the camber, AOA, and on some types, (i.e. Fowler) wing area resulting in an increse in total lift produced by the airfoil. The resultant shift of the center of pressure in relation to the CG of the aircraft causing a pitch-up or pitch-down tendancy, but maybe I'm wrong.
 
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ifly4food

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Re: Flaps

sstearns2 said:
It's not that the wing is making any more lift with the flaps down, but that the lift is redistributed so more of the lift (and thus downwash) is made inboard right in front of the tail and less outboard.
Huh? The flaps do increase lift by increasing the angle of attack. They also add drag.
Not sure about the tail theory.

My understanding is that the force of drag upon the flaps acts above (or below) the CG of the airplane, thus rotating the airplane around that axis, causing a pitch up (or down).
 

sstearns2

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Drag

The previous post on drag was good, but here's a little different way of looking at it (the way I think of it anyway).

The way I think of it, there are 3 kinds of drag...

1- Parasite or 'skin friction' drag.

2 - Induced drag or the drag caused by making lift in whatever direction.

3 - Wave drag or the drag cause by air decellerating thru the speed of sound (shock waves).

They are all really the same basic thing, the acceleration of the air in some direction. Skin friction drag is caused by the entrainment of air in the boundary layer (the layer of air right next to the skin of the airplane). Basically skin friction drag is caused by the air your just dragging along with you as you fly along. Induced drag is caused by the acceleration of the air in some direction to make lift in the opposite direction. The wing accelerates the air down to make lift up.

Imagine there is a box of still air, then a glider flies thru the box of air. Now the box of air has a velocity component in the direction of the flight of the glider caused by the glider dragging (acclerating) air along with itself and there is a vertical velocity component caused by the need of the glider to accelerate air down in order to make the needed lift. The air will also be rotating some amount because of 'wing tip vorticies'. (I hate the word wing tip vortex and disagree with the book explainations, but that's another tirade.)

Wave drag is caused by the sudden deceleration of air thru a shock wave. Air cannot decelerate thru mach 1 smoothly, it will be going Mach 1.5 for example and the hit the shock wave and slow to Mach 0.75 (or so, I don't have a mach table in front of me) in a fraction of an inch. Basically there is a big wall of air being dragged along behind the shock wave that causes a huge amount of drag.

I hope this make some amount of sense.

Scott
 

bobbysamd

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Flaps

I cast my vote with Metro Sheriff and IFly4Food. Flaps indeed add lift. Isn't that why so many aircraft require extended flaps for takeoff (and landings, of course)? When flaps are extended, camber changes, the center of pressure moves and, hence, the pitch change. Bernoulli lift. Relative wind hits the extended flaps and causes a pitching moment. Newtonian lift.

Very stimulating discussion, folks. Better that taking a ground school refresher or digging out the books. Great posts, too, on drag and aircraft stability (one of my favorite aerodynamic subjects :) ).
 
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chperplt

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I wont swear to it, but....I vaguely recall something about flaps increasing the camber, AOA, and on some types, (i.e. Fowler) wing area resulting in an increse in total lift produced by the airfoil. The resultant shift of the center of pressure in relation to the CG of the aircraft causing a pitch-up or pitch-down tendancy, but maybe I'm wrong.
Metrosheriff is the big winner.

You are correct.
 

ifly4food

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Re: Drag

sstearns2 said:
The previous post on drag was good, but here's a little different way of looking at it (the way I think of it anyway).

The way I think of it, there are 3 kinds of drag...

1- Parasite or 'skin friction' drag.

2 - Induced drag or the drag caused by making lift in whatever direction.

3 - Wave drag or the drag cause by air decellerating thru the speed of sound (shock waves).

They are all really the same basic thing, the acceleration of the air in some direction. Skin friction drag is caused by the entrainment of air in the boundary layer (the layer of air right next to the skin of the airplane). Basically skin friction drag is caused by the air your just dragging along with you as you fly along. Induced drag is caused by the acceleration of the air in some direction to make lift in the opposite direction. The wing accelerates the air down to make lift up.

Imagine there is a box of still air, then a glider flies thru the box of air. Now the box of air has a velocity component in the direction of the flight of the glider caused by the glider dragging (acclerating) air along with itself and there is a vertical velocity component caused by the need of the glider to accelerate air down in order to make the needed lift. The air will also be rotating some amount because of 'wing tip vorticies'. (I hate the word wing tip vortex and disagree with the book explainations, but that's another tirade.)

Wave drag is caused by the sudden deceleration of air thru a shock wave. Air cannot decelerate thru mach 1 smoothly, it will be going Mach 1.5 for example and the hit the shock wave and slow to Mach 0.75 (or so, I don't have a mach table in front of me) in a fraction of an inch. Basically there is a big wall of air being dragged along behind the shock wave that causes a huge amount of drag.

I hope this make some amount of sense.

Scott
Dude! I think you need to lay of aerodynamic theory for a while.
Parasite drag is direct contact of leading edge surfaces with the air and friction. Did you know that there are over 200lbs/in2 on every surface at 250KIAS?

Induced drag is a by product of lift. Wake vortices are a great example (whether you believe in them or not... but I hope you do). It is what you described as parasite... dragging air behind you.

Mach drag? I won't worry about that until my airline buys a Concorde!

Who do you fly the Brakillya for?
 

sstearns2

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....

Previous post....

>The flaps do increase lift by increasing the angle of attack. They >also add drag.


Flaps increase the ability of the wing to make lift. In order to fly staight and level lift has to equal wieght. The weight of the aircraft doesn't increase with flap extension, so the amount of lift made in straight and level flight is the same with the flaps up or down.

Scott
 

ifly4food

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Re: ....

sstearns2 said:
Previous post....

>The flaps do increase lift by increasing the angle of attack. They >also add drag.


Flaps increase the ability of the wing to make lift. In order to fly staight and level lift has to equal wieght. The weight of the aircraft doesn't increase with flap extension, so the amount of lift made in straight and level flight is the same with the flaps up or down.

Scott
Your logic is flawed. It has nothing to do with weight.
Lift is a combination of speed, angle of attack, and the wings lift coefficient. Flaps allow you to maintain a high value of lift at a lower than otherwise airspeed. In essence, they change the coefficient of lift of the wing (by increasing camber) and increase the angle of attack (by lowering the trailing edge of the wing).
The total lift doesn't change in relation to the weight; you just get that value at a lower airspeed with flaps.

I recommend the book Aerodynamics for Naval Aviators where you can research these issues. It's also a great cure for insomnia
 
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