How Center of Gravity Affects Flight | Tail Down Force | Aircraft Stability
Summary
TLDRThis 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
🛫 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.
📏 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.
Mindmap
Keywords
💡Equilibrium
💡Center of Lift
💡Center of Gravity
💡Pitch
💡Horizontal Stabilizer
💡Tail Down Force
💡Angle of Attack
💡Stall
💡Stability
💡Induced Drag
💡Aerodynamic Efficiency
💡Stall Speed
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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