How Center of Gravity Affects Flight | Tail Down Force | Aircraft Stability

Summary

This script delves into the balance of forces on an aircraft during straight and level flight, focusing on the equilibrium between lift and weight. It explains the concept of the center of lift and center of gravity (CG), and how their positions relative to each other affect the aircraft's stability and stall characteristics. The video discusses how a forward CG enhances stability but increases drag, while a rearward CG improves efficiency and reduces stall speed, albeit at the cost of stability. The script also touches on the practical implications of CG placement on takeoff and the importance of adjusting elevator input accordingly.

Takeaways

• ✈️ In straight and level flight, the lift force equals the weight force, and these forces are in equilibrium.
• 🔄 The center of lift (center of pressure) is roughly at the midpoint between the leading and trailing edges of the wing.
• 🎢 The center of gravity is typically in front of the center of pressure, enhancing the aircraft's stability.
• 📉 If the center of pressure is behind the center of gravity, the aircraft will have a tendency to pitch down.
• 🛫 The horizontal stabilizer creates a tail down force to counteract the pitch down tendency.
• 📈 Increasing the angle of attack can increase lift, which is necessary if the center of gravity moves forward.
• 📉 A forward center of gravity increases stability but can also increase stall speed and reduce efficiency.
• 🚀 A more rearward center of gravity can improve aerodynamic efficiency and potentially allow for faster speeds.
• 📉 The placement of the center of gravity affects the stall speed, with a forward CG leading to higher stall speeds.
• 📏 The center of gravity is defined relative to a reference point, often near the engine firewall, and can be adjusted by moving weight.
• 🛣️ On the ground, the aircraft's nose heaviness due to a forward center of gravity requires additional back elevator input during takeoff.

Q & A

What are the four forces acting on an aircraft in straight and level unaccelerated flight?

The four forces acting on an aircraft in straight and level unaccelerated flight are lift, weight, thrust, and drag.

Where is the center of lift, or center of pressure, located on an aircraft?

The center of lift, or center of pressure (CP), is roughly at the midpoint between the leading and trailing edges of the wing and can move around in flight.

What is the significance of the center of gravity (CG) in an aircraft?

The center of gravity (CG) is the pivot point of the aircraft, similar to balancing a model airplane on a finger. It's typically located in front of the center of pressure for stability.

Why is it important for the CG to be in front of the CP in a well-loaded aircraft?

Having the CG in front of the CP ensures stability by creating a pitch-down tendency when the CP is behind the CG, which is counteracted by the tail down force (TDF).

How does the horizontal stabilizer help maintain level pitch attitude in an aircraft?

The horizontal stabilizer maintains level pitch attitude by exerting a downward force on the tail, known as tail down force (TDF), which resists the pitch-down movement.

What happens to the required lift force when the CG is moved forward in an aircraft?

When the CG is moved forward, the pitch-down tendency increases, requiring greater tail down force and, consequently, a higher angle of attack and more lift to maintain equilibrium.

How does the position of the CG affect the stall characteristics of an aircraft?

A forward CG increases stability and protects the aircraft in a stall by causing the aircraft to pitch down more rapidly when the lift decreases, aiding in stall recovery.

What is the trade-off of having a more rearward CG in terms of aerodynamic efficiency and stall speed?

A more rearward CG can increase aerodynamic efficiency and allow for faster flight speeds by reducing the required tail down force and angle of attack, but it can also make the aircraft less stable and more difficult to recover from a stall.

How does the placement of the CG affect the stall speed of an aircraft?

A forward CG increases the stall speed because it requires a larger tail down force and a higher angle of attack, while a rearward CG decreases the stall speed by reducing the angle of attack.

What are 'arms' in the context of aircraft loading and how are they measured?

Arms are positions on the long axis of an aircraft, measured as distances in inches from a predetermined reference point, often near the engine firewall, with positions in front of this point having negative measurements.

Why is the CG's position relative to the main wheels important during takeoff in a tricycle gear aircraft?

A well-loaded aircraft typically has its CG in front of the center of pressure, which also places it in front of the two main wheels in a tricycle gear. This makes the aircraft nose-heavy on the ground, requiring extra back elevator input to pitch up during takeoff.

Outlines

00:00

🛫 Principles of Aircraft Stability and Center of Lift

This paragraph discusses the equilibrium of forces acting on an aircraft during straight and level flight. It introduces the concept of the center of lift (or pressure) and the center of gravity, explaining how these points affect the aircraft's balance and stability. The paragraph also explores how changes in the center of gravity can necessitate adjustments in the angle of attack to maintain lift. The relationship between the center of gravity and the center of pressure is highlighted, emphasizing the importance of their relative positions for aircraft stability and stall characteristics.

05:02

📏 Impact of Center of Gravity on Stall Speed and Efficiency

The second paragraph delves into how the position of the center of gravity (CG) impacts stall speed and the efficiency of an aircraft. It explains that a forward CG increases stability and raises stall speed, while a rearward CG can improve aerodynamic efficiency and potentially allow for higher speeds. The concept of 'arms' is introduced to describe the positioning of weight along the aircraft's longitudinal axis. The paragraph also touches on the practical implications of CG position on the ground, particularly during takeoff, and the need for adjustments in elevator input to achieve a smooth climb after liftoff.

Keywords

💡Equilibrium

Equilibrium refers to a state where opposing forces or influences are balanced, resulting in no net change. In the context of the video, it is used to describe the state of an aircraft when the lift force is equal to the weight force, resulting in straight and level unaccelerated flight. The script mentions that the center of lift and center of gravity are critical points that help maintain this equilibrium.

💡Center of Lift

The Center of Lift, also known as the Center of Pressure (CP), is the point through which the total lift force of an aircraft can be considered to act. The script explains that it can move around in flight but is generally thought to be at the midpoint between the leading and trailing edges of the wing. It's likened to the point where a string would be attached to a model airplane for hanging purposes.

💡Center of Gravity

The Center of Gravity (CG) is the point at which the weight of an aircraft can be considered to act. It's described in the script as the pivot point of the aircraft, akin to balancing a model airplane on your finger. The video emphasizes that the CG is typically in front of the CP, which contributes to the aircraft's stability.

💡Pitch

Pitch in aviation refers to the rotation of an aircraft around its lateral (wing-to-wing) axis. The script discusses how the position of the CP relative to the CG affects the aircraft's pitch. If the CP is behind the CG, the aircraft will have a tendency to pitch down, which is counteracted by the horizontal stabilizer.

💡Horizontal Stabilizer

The Horizontal Stabilizer is a part of the aircraft's tail that helps maintain level flight by exerting a downward force on the tail, known as Tail Down Force (TDF). The script explains that it resists pitch-down movements, allowing the aircraft to maintain level pitch attitude.

💡Tail Down Force

Tail Down Force (TDF) is the downward force exerted by the horizontal stabilizer on the tail of the aircraft. As described in the script, TDF is crucial for balancing the pitch-down tendency caused by the aircraft's weight distribution and is part of the forces that need to be counteracted by lift.

💡Angle of Attack

Angle of Attack is the angle between the oncoming airflow and a reference line aligned with the aircraft's wings or rotors. The script mentions that by increasing the angle of attack, an aircraft can increase lift. It's a critical parameter that affects both lift and drag, and it's directly related to the aircraft's stall characteristics.

💡Stall

A stall occurs when the wing of an aircraft ceases to produce enough lift to support the aircraft's weight, leading to a rapid decrease in flight speed and altitude. The script uses the concept of a string being cut to illustrate the stall, where the aircraft pitches down due to the loss of lift, which is related to the position of the CP.

💡Stability

Stability in aviation refers to an aircraft's tendency to return to its original state after being disturbed. The script explains that having the CG in front of the CP contributes to a stably loaded aircraft, making it more resistant to upsets and easier to control.

💡Induced Drag

Induced drag is the drag created as a byproduct of generating lift. The script points out that increasing the angle of attack to compensate for a forward CG increases lift but also increases induced drag, which is a fundamental concept in aerodynamics where there is always a trade-off.

💡Aerodynamic Efficiency

Aerodynamic Efficiency refers to how well an aircraft cuts through the air with minimal drag and energy loss. The script suggests that moving the CG rearward can reduce the required tail down force and induced drag, leading to increased aerodynamic efficiency and potentially higher speeds or better fuel economy.

💡Stall Speed

Stall Speed is the minimum speed at which an aircraft can maintain level flight without stalling. The script explains that the position of the CG affects the stall speed, with a forward CG increasing the stall speed due to the need for a higher angle of attack to counteract the increased tail down force.

Highlights

Aircraft in straight and level unaccelerated flight experiences equilibrium of forces.

Lift force is equal to weight force in equilibrium.

Center of lift (CP) is roughly at the midpoint of the wing.

Center of gravity (CG) is the pivot point of the aircraft.

CG is typically in front of CP for a well-loaded aircraft.

CP behind CG causes a pitch-down tendency.

Horizontal stabilizer resists pitch-down movement with tail down force (TDF).

Lift must match weight and TDF to maintain equilibrium.

Increasing angle of attack increases lift.

Moving CG forward increases pitch-down tendency and requires more lift.

CG and CP in different locations increase stability but decrease efficiency.

Stall occurs when lift decreases rapidly at critical angle of attack.

Forward CG helps in recovering from a stall.

Rearward CG is more fuel-efficient and allows for faster flight.

CG placement affects stall speed; forward CG increases stall speed.

Arms are defined based on distance from the engine firewall.

CG position affects the aircraft's balance and stability on the ground.

After takeoff, it's important to adjust elevator input for a smooth climb.

Transcripts

we already know that when our aircraft

is in straight and level unaccelerated

flight the four forces acting on our

aircraft are said to be in equilibrium

for example our lift force moving

upwards is equal to the weight force

moving downwards but it can be a bit

vague thinking of these forces in

general terms like this

where do these forces actually move

through the aircraft

all of the lifting forces of the

aircraft can be thought of as having a

center marked here as the center of lift

or center of pressure cp it can move

around in flight but can be thought to

be at roughly the midpoint between the

leading and trailing edges of the wing i

always think of the center of lift as if

our airplane were a little model and we

fixed a string to the top of it so we

could hang it from our ceiling the point

the string attaches to is where our

force lifting the airplane up acts

through all of the weight of the

aircraft can be said to act through the

center of gravity marked here

this is like the pivot point of the

aircraft if we wanted to swing the

aircraft up and down this point wouldn't

move or this is where we would balance

the model aircraft on our finger

it's no accident the center of gravity

is in front of the center of pressure

a well-loaded aircraft as we'll see will

have its weight distribution such that

this is true

what this means though is that if the

center of pressure the string we're

pulling the aircraft up through is

behind the center of gravity the point

the aircraft rotates through the

aircraft will have a tendency to pitch

down as these forces counteract each

other

in fact the stronger the lift and weight

the greater this pitch down tendency

becomes

an aircraft can maintain level pitch

attitude thanks to its horizontal

stabilizer which resists the pitch down

movement by exerting a downward force on

the tail which we call tail down force

or tdf so a well-loaded and designed

aircraft will have these three forces

distributed something like this

notice though that in addition to weight

moving downwards through the cg we also

have a second tail down force creating a

downwards force this adds to the

effective weight of the aircraft which

will have to be counteracted with lift

so in order to maintain equilibrium our

lift will have to match not only the

weight moving through the cg but the

tail down force as well so it'll have to

become larger we can increase lift by

increasing angle of attack so as we fly

through the relative wind we may have an

angle of attack of let's say 2 degrees

we know that if we increase weight we'll

need to increase lift by increasing

angle of attack

but if we don't increase weight and

instead move that center of gravity

forward a bit let's say by having a

backseat passenger come squeeze up front

with us that pitch down tendency will

become greater and will need greater

tail down force to balance it this will

require more lift and hence a higher

angle of attack why is this a good thing

to have the cg and cp in different

locations

well consider what happens in a stall

we pitch up to our critical angle of

attack a stall is a rapid drop in lift

if the center of pressure is the point

we attach the string to the top of the

aircraft we might think of the stall

like we're cutting that string

without much of that lift the aircraft

rotates around its forward center of

gravity pitching down

decreasing angle of attack and helping

to break that stall

this is a stably loaded aircraft so we

fly with the center of gravity in front

of the center of pressure to increase

our stability what's the catch

as we said with this forward cg our

angle of attack has to be a bit higher

to counteract the extra tail down force

increasing angle of attack adds drag

specifically induced drag remember you

can't get something for nothing in

physics so we might be flying around 90

knots with this distribution of forces

and angle of attack we could

theoretically fly at a zero angle of

attack if we moved some weight rearwards

and brought the cg right on top of the

center of pressure

this would require almost no tail down

force and our drag would be reduced for

the same power setting we could fly

faster

so a more rearward center of gravity

could be more fuel efficient and or

allow us to fly faster in a small enough

aircraft i can actually pick up a few

extra knots of airspeed just by sliding

my seat backwards so what's the catch

here well we've sacrificed stability for

aerodynamic efficiency it'll be easy to

pitch up to our stall angle of attack

now that we're not as nose heavy when

the aircraft does stall though that lack

of weight up front will mean the

aircraft won't pitch down as easily in

the stall recovering from the stall will

be difficult and may take more elevator

authority than we have to accomplish

making the aircraft much less stable

what about stall speeds we know the

color coding on the airspeed indicator

shows us that the bottom of the green

arc is our stall speed in a specified

configuration which we can see in the

aircraft poh under limitations

if we look at the green arc section it

tells us that the stall speed will be at

the bottom of this arc when we have our

most forward cg allowable so does the

placement of our cg affect stall speed

yes it does let's see what that looks

like with the cg as far forward as we're

allowed to go

once again we'll need a larger tail down

force and hence a greater angle of

attack we've already moved closer to our

stall angle which might give you a hint

right there if we pitch up to 18 degrees

we'll hit the stall just as our speed

decreases to the bottom of the green arc

now let's move the cg all the way back

to the rear limit our angle of attack

decreases we're further away from that

stall angle this means that we have more

room to go to pitch up and diminish

speed before the aircraft will stall our

stall speed decreases and we'll find it

somewhere below the green arc so that

forward cg makes us more stable and

protects us in a stall and it also makes

us less efficient and or slower and

brings our stall speed up

where we load the weight into the plane

then matters

we could define positions on the long

axis of the plane called arms based on

their distance in inches from a

predetermined point in the cessna that

predetermined point happens to be near

the engine firewall so we say that's at

zero inches and is the reference datum

any other position on the aircraft is

measured as its distance from this point

anything in front of this point will

have a negative number let's think of

the aircraft as a big seesaw with a

balance point or fulcrum in the middle

if we load two 100 pound weights onto

the plank one at the zero inch point and

the other at the 200 inch point they'll

both be exactly 100 inches from the

fulcrum which is at the 100 inch point

this point is our center of gravity

if we swap one of these weights for a

heavier one

at the same position in order to balance

this plank now that fulcrum our center

of gravity will need to move closer to

the heavier weight if we want to bring

the cg back to the 100 inch point we

could just swap the other way for 150

pound one or we could slide this weight

way out till it's hanging over the edge

this will also have the effect of moving

the cg point back to its original

position

so the lighter weight placed further out

from center will have just as much

ability to turn the plank as the heavier

weight placed closer in if this is

confusing consider this aircraft found

on the ramp sitting on its tail after a

snowfall the relatively light snow

sitting on top of the horizontal

stabilizer far from the aircraft's

center has a much greater ability to tip

it back than that same weight would

closer in

one last consideration with center of

gravity because a well-loaded aircraft

has its cg in front of the center of

pressure we usually find the cg to be in

front of the two main wheels in a

tricycle gear aircraft this doesn't

really matter in flight the aircraft

rotates about its cg in the air

but on the ground like when we're on the

take off roll on the runway the aircraft

must rotate about its main gear

this causes it to be nose heavy we need

extra back elevator input to counteract

that forward cg and pitch up

once the aircraft is off the ground the

aircraft is free to rotate about its cg

this same extra amount of elevator input

will cause us to pitch up more for this

reason after the aircraft has left the

ground it's often important to relieve

some of the back elevator pressure to

allow for a smooth climb out

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