In this document of Theory of Flight we are discussing some topics in detail which are: Intereference drag Induced drag Factors that Affects Induced Drag: Stabilator All these topics are discussed in detail hope you like it................

1. Project: Theory of Flights
Name: Waleed Imtiaz
Roll#: Bamm-f19-139
Section: 2/C
Email: bamm-f19-139@superior.edu.pk
Sir Shahid (TOF)

2. 1. Interference Drag:
Interference Drag is drag that is generated by the mixing of airflow streamlines between airframe
components such as the wing and the fuselage, the engine pylon and the wing or, in the case of a
military or other special purpose aircraft, between the airframe and attached external stores such
as fuel tanks, weapons or sensor pods.
Description:
Interference drag is generated when the airflow across one component of an aircraft is forced to
mix with the airflow across an adjacent or proximal component. If one considers two parts of the
aircraft that intersect at a particular point, such as the vertical and horizontal components of the
empennage, it is obvious where the point of intersection occurs. Each of these two components
generate high velocity (potentially transonic or even supersonic) airflow across their respective
surfaces. At the intersection of the two surfaces, there is less physical space for the airflow to
occupy resulting in the turbulent mixing of the two airflows and the production of a localized shock
wave. Due to this shock wave, the resulting total drag from the empennage is greater than the sum
of the drag produced individually by the vertical tail and the horizontal tail surfaces. Other
significant locations which generate interference drag include the wing/fuselage junction and the
wing/engine pylon or fuselage/engine pylon convergence.
Interference drag can be minimized by the appropriate use of fairings and fillets to ease the
transition between components. Fairings and fillets use curved surfaces to soften the transition at
the junction of two aircraft components. This, in turn, allows the airflow streamlines to meet
gradually rather than abruptly and reduces the amount of interference drag that is generated.
How Interference Drag Affects Your Plane's Performance:
Mixing Airflow:
Interference Drag is generated by the mixing of airflow streams between airframe components,
such as the wing and the fuselage, or the landing gear strut and the fuselage.

3. As air flows around different aircraft components and mixes, it needs to speed up in order to pass
through the restricted area. As the air speeds up, it requires extra energy. At the same time, it
creates turbulence, resulting in an increase in drag. The more acute (sharp) the angle, the greater
the interference drag that's generated.
Where You'll Find Interference Drag
You can typically find interference drag anywhere you find a sharp angle on your plane.
For example, look at where the fuselage and wing meet. Interference drag forms behind the trailing
edge of the wing adjacent to the fuselage. Airflow over top and underneath the wing mixes with
airflow around the fuselage, creating interference drag. If the wing was flying without an attached
fuselage, there wouldn't be interference drag at this location.
To reduce the drag, designers use fairings to ease the airflow transition between aircraft
components, like what you see in the picture below.

4. But interference drag isn't just limited to where the wings and fuselage meet. A Cessna 172 wing
strut, for example, has fairings around the base and top of the strut, where the strut meets the
fuselage and wing. Without fairings, these connections form noticeably acute angles, significantly
increasing interference drag.
So if there's drag produced by the strut connection points, why don't they just design the airplane
without struts? Well, they have. But it's not always ideal. The Cessna 177 Cardinal was developed
as a replacement to the Cessna 172 Skyhawk. The C177 doesn't have wing struts. Instead, it relies
on a cantilever wing for structural support. In general, cantilever wings weigh more, and are more
expensive than adding wing struts to the exterior of the airplane.
Retracting The Gear:
If you've flown airplanes with retractable gear, you should know that as you retract the gear, you
significantly increase interference drag. You might be thinking, "Wait a second, I thought
retracting the gear should decrease drag, not increase it?" Don't worry, you're not totally off.
As the landing gear retract into the fuselage, you're creating a progressively acute angle between
the fuselage and landing gear strut. And since tight, acute angles cause more interference drag than
wide angles, you'll be momentarily increasing drag as your gear retracts. The moment just before
the gear move into the fuselage is where the most interference drag is created.

5. It's something to keep in mind when you're flying out of hot, high-density altitude airports.
If you're just above the ground and beginning your climb, you'll reduce your climb performance
in the process of bringing the gear up. On hot days, wait a few extra seconds and climb up to a
higher altitude before bringing up the gear.
Fortunately, the performance loss will only last as long as it takes for the gear to fully retract.
As the angle between airframe components shrinks, interference drag increases. That's why you
see fairings placed around most airplanes where the sharp angles meet.
2. Induced Drag:
Induced Drag is an inevitable consequence of lift and is produced by the passage of an aero foil
(e.g. wing or tail plane) through the air. Air flowing over the top of a wing tends to flow inwards
because the decreased pressure over the top surface is less than the pressure outside the wing tip.
Below the wing, the air flows outwards because the pressure below the wing is greater than that
outside the wing tip. The direct consequence of this, as far as the wing tips are concerned, is that
there is a continual spilling of air upwards around the wing tip a phenomenon called ‘tip effect’ or
‘end effect’. One way to appreciate why a high aspect ratio for a wing is better than a low one is
that with a high aspect ratio, the proportion of air which moves in this way is reduced and therefore
more of it generates lift.
For the wing more generally, the streams of air from above and below the wing are flowing at an
angle to each other as they meet along the trailing edge of the wing. They combine to form vortices
which, when viewed from the rear, rotate clockwise from the left wing and counter clockwise from
the right. The tendency is for these vortices to move outwards towards the wing tip joining together
as they do so. By the time the wing tip is reached, one large wig tip vortex has formed and is shed.

6. Most of these vortices are of course completely invisible but, in very humid air, the central core of
a vortex may become visible because the air pressure within its center has reduced - and has
therefore cooled - sufficiently for condensation to occur. A higher wing loading in a turn will also
increase the strength - and the degree of reduced pressure - so that visible vortex cores are even
more likely during turns. If close up to these vortices, they can also sometimes be audible!
Most of the air flowing off the top of a wing - ‘downwash’ - continues more or less horizontally
towards the empennage because it is balanced by a corresponding up wash in front of the wing
leading edge. In contrast, the upwards air movement which leads to vortex ‘consolidation’ at the
tip is just outside the tip whereas the corresponding downward movement is just at the extremity
of the wingspan so that the net direction of airflow past the wing is downwards. The lift created
by the wing - which is by definition at right angles to the airflow, is therefore inclined slightly
backwards and thus ‘contributes’ drag - induced drag.
Although there must always be at least some induced drag because wings have a finite thickness,
design attempts wherever possible to reduce this flow. A required wing area can be achieved using
different wing span-to-chord ratios (aspect ratios). The larger the wing aspect ratio, the less air
disturbance is created at the tip. However, for most aircraft, there are both practical limits to
maximum wing span for ground maneuvering and structural issues which mean that eventually,
the weight penalty to adequately strengthen a long thin wing becomes excessive. The fact that
aircraft carry most of their fuel in the wings is also a factor in wing design. Typical transport
aircraft aspect ratios range between 6:1 and 10:1.
Other ways to reduce induced drag and tip vortex strength in a wing design are also based upon
reducing the quantity air movement upwards at the wing tip by aiming to generate relatively more
of the lift away from tips. Wing taper towards the tip assists this as does wing twist. The Boeing
767 is an example of a twisted wing. The inner wing is set at a higher Angle of Attack (AOA) than
the outer wing and thus generates proportionately more lift whereas the tip, at a very small Angle
of Attack generates very little. Winglets (shark lets) have also become popular, both the usual up-
turned versions and the older Airbus A320 series two-way ‘wingtip fence’ versions. Well-designed
winglets can prevent about 20% of the airflow spillage at the tip - and therefore 20% of the induced
drag.
Induced drag and its wing tip vortices are a direct consequence of the creation of lift by the wing.
Since the Coefficient of Lift is large when the Angle of Attack is large, induced drag is inversely
proportional to the square of the speed whereas all other drag is directly proportional to the square
of the speed. The effect of this is that induced drag is relatively unimportant at high speed in the
cruise and descent where it probably represents less than 10% of total drag. In the climb, it is more
important representing at least 20% of total drag. At slow speeds just after takeoff and in the initial
climb, it is of maximum importance and may produce as much as 70% of total drag. Finally, when
looking at the potential strength of wing tip vortices, all this theory on induced drag must be
moderated by the effect of aircraft weight. Induced drag will always increase with aircraft weight.

7. Drag Equation:
A pilot should not be able to calculate Drag with help of this formula. However, he must
understand the variables that go into it:
 An increase in velocity will result an increase in Drag. Generally speaking, the faster you
fly the more Drag exists.
 An increase in Density will result in an increase in Drag that is one of the reasons airplanes
fly at a higher altitude to decrease Drag and to increase flight performance.
 Drag Coefficient (CD) is a number that depends on the shape of an object and also increases
with angle of attack. An increase in Angle of Attack will increase Lift and Drag
 Increasing the overall Surface Area will increase Drag. Increasing the wing surface area
will increase Drag.
Induced Drag effect with Speed:
 Induced Drag behave differently when increasing airspeed.
 Induced Drag decreases if airspeed is increased.

8. 3. Factors that Affect Induced Drag:
The size of the lift force:
Because induced drag is a component of the lift force, the greater the lift, the greater will be the
induced drag. Lift must be equal to weight in level flight so induced drag will depend on the weight
of the aircraft. Induced drag will be greater at higher aircraft weights. Certain manoeuvres require
the lift force to be greater than the aircraft weight. The relationship of lift to weight is known as
the ‘Load Factor’ (or ‘g’). For example, lift is greater than weight during a steady turn so induced
drag will be higher during a steady turn than in straight and level flight. Therefore, induced drag
also increases as the Load Factor increases.
Induced drag will increase in proportion to the square of the lift force.
The speed of the aircraft:
Induced drag decreases with increasing speed (for a constant lift force). This is because, as speed
increases, the downwash caused by the tip vortices becomes less significant, the rearward
inclination of the lift is less, and therefore induced drag is less.
Induced drag varies inversely as the square of the speed.
The aspect ratio of the wing:
The tip vortices of a high aspect ratio wing affect a smaller proportion of the span so the overall
change in downwash will be less, giving a smaller rearward tilt to the lift force. Induced drag
therefore decreases as aspect ratio increases (for a given lift force). The induced drag coefficient
is inversely proportional to the aspect ratio.
It can be seen that the relationship for the induced drag coefficient, (CDi), emphasizes the need of
a high aspect ratio wing for aeroplane configurations designed to operate at the higher lift
coefficients during the major portion of their flight, i.e. conventional high speed jet transport
aircraft.
The effect of aspect ratio on lift and drag characteristic. The basic aerofoil section properties are
shown on these plots, and these properties would be typical only of a wing planform of extremely
high (infinite) aspect ratio. When a wing of some finite aspect ratio is constructed of this basic
section, the principal differences will be in the lift and drag characteristics - the moment
characteristics remain essentially the same.
The effect of increasing aspect ratio on the lift curve is to decrease the wing angle of attack
necessary to produce a given lift coefficient. Higher aspect ratio wings are more sensitive to
changes in angle of attack, but require a smaller angle of attack for maximum lift.
It can be seen that at any lift coefficient, a higher aspect ratio gives a lower wing drag coefficient
since the induced drag coefficient varies inversely with aspect ratio. When the aspect ratio is high,

9. the induced drag varies only slightly with lift. At high lift coefficients (low IAS), the induced drag
is very high and increases very rapidly with lift coefficient.
The lift and drag curves for a high aspect ratio wing, show continued strong increase in CL with α
up to stall and large changes in CD only at the point of stall.
Continuing to increase aspect ratio is restricted by the following considerations.
Very high aspect ratio wings will experience the following:
Excessive wing bending moments: which can be reduced by carrying fuel in the wings and
mounting the engines in pods beneath the wing.
Reduced rate of roll (particularly at low airspeed): This is caused by the down-going wing
(only while it is actually moving down) experiencing an increased effective angle of attack. The
increased effective angle of attack is due to the resultant of the forward TAS of the wing and the
angular TAS of the tip. The higher the aspect ratio, the greater the vertical TAS of the tip for a
given roll rate, leading to a greater increase in effective angle of attack. The higher the effective
angle of attack at the tip, the greater the resistance to roll. This phenomenon is called aerodynamic
damping and will be covered in more detail in later chapters.
Reduced ground clearance in roll during take-off and landing.
4. Stabilator:
A stabilator, sometimes referred to as an all-moving tail, is a fully movable aircraft horizontal
stabilizer. In this type of installation, the entire horizontal tail surface is responsive to pilot control
wheel or control stick inputs. This is in contrast to the more common elevator control movement
associated with both a fixed or a trimmable horizontal stabilizer. Stabilators are most commonly
found on high speed military combat aircraft where they are used to enhance maneuverability and
to eliminate the Mach tuck caused by shock wave formation behind the elevator hinge line of a
conventional tail. Stabilators are also installed in some light, general aviation aircraft.
Stabilators are designed to pivot about their aerodynamic center and, as a consequence, very little
pilot effort is required to make a control input. This amount of effort does not vary with airspeed
or angle of attack. To prevent over-controlling, the stabilator on a light aircraft is fitted with an
anti-servo tab on its trailing edge. Control input causes the anti-servo tab to deflect in the same
direction as, but further than the stabilator. This additional deflection induces an aerodynamic force
which resists the pilot input. In most cases, the anti-servo tab also acts as a trim tab.
Supersonic aircraft are not fitted with anti-servo tabs. In older aircraft, the potential for over-
controlling due to light control forces was addressed by resistance force generated by springs or
hydraulic pressure. In modern military jets, fly-by-wire controls moderate the control inputs to
prevent over-controlling.

10. How it works:
At the rear of the fuselage of most aircraft one finds a horizontal stabilizer and an elevator to
provide stability and control of the up-and-down, or pitching, motion of the aircraft nose. On many
fighter planes, in order to meet their high maneuvering requirements, the stabilizer and elevator
are combined into one large moving surface called a stabilator. Because the stabilator moves, it
varies the amount of force generated by the tail surface and is used to generate and control the
pitching motion of the aircraft. There is usually a stabilator on each side of the fuselage and they
work in pairs; when the right stabilator goes up, the left stabilator also goes up. This slide shows
what happens when the pilot deflects the stabilators.
The stabilator is used to control the position of the nose of the aircraft and the angle of attack of
the wing. Changing the inclination of the wing to the local flight path changes the amount of lift
which the wing generates. This, in turn, causes the aircraft to climb or dive. During takeoff the
stabilators are used to bring the nose of the aircraft up to begin the climb out. During a banked
turn, stabilator inputs can increase the lift and cause a tighter turn. That is why stabilator
performance is so important for fighter aircraft.
The stabilators work by changing the angle of attack of the horizontal stabilizer. As described on
the inclination effects slide, changing the angle of attack of an airfoil changes the amount of lift
generated by the foil. With greater downward deflection of the leading edge, lift increases in the
downward direction. With greater upward deflection, lift increases in the upward direction. The
lift force (F) is applied at the center of pressure of the the stabilator which is some distance (L)
from the aircraft center of gravity. This creates a torque
T = F * L
on the aircraft and the aircraft rotates about its center of gravity. The pilot can use this ability to
make the airplane loop or dive.
On most aircraft, the horizontal stabilizer and elevator are separate pieces with the elevator being
connected to the stabilizer by a hinge. These aircraft rely on changing the shape of the tail airfoil
to produce a change in the down force for control. On some aircraft, the pitch stability and control
is provided by a canard which is a horizontal surface placed forward of the center of gravity (a tail
in the front). The Wright brothers 1903 flyer used a forward elevator for pitch control.
What is the Difference Between a Stabilator and an Elevator?
The stabilator and elevator are two very effective pieces of aerodynamic machinery. They are both
found at the rear of an aircraft and both serve a similar purpose. Despite this, there are distinct
differences between these two components of the empennage. An aircraft elevator is an example
of a flight control surface, or an aerodynamic device which allows an operator to control the
aircraft's altitude. It, along with the horizontal stabilizer, maintains the pitch, lift, and angle of
attack of an aircraft. The aircraft stabilator, colloquially referred to as an all-moving or all-flying
tail, is a one-hundred percent adjustable aircraft stabilizer. Essentially, the stabilator is a 2-in-1

11. device that performs the duties of both the horizontal stabilizer and elevator. Hence the name,
stabilator.
Engineers from The New Piper Aircraft Co. have stated that, because the stabilator has a tidier
design and provides a larger surface for pitch control, it is more effective in allowing for smoother
ascension and descension than the classic stabilizer/elevator combination. Another feature of a
stabilator, called the antiservo, is an additional flap at the rear of the stabilator. The antiservo’s job
is to make the aircraft stabilator less sensitive and help it stay in the optimal position. The trim tab,
another feature of an aircraft’s tail section, moves parallel to the stabilator at a greater pace. The
result is that the effort required to move the yoke, or steering wheel, heightens relative to airspeed
and control deflection. This is a safety measure that increases control along the longitudinal axis
and stops the pilot from over controlling.
Stabilators were partially developed as military parts and are now found on virtually all combat
aircraft. This is due to the weight balance stabilators ability to continue controlling pitch through
a variety of flight speeds, including supersonic flight. Non-delta winged supersonic aircraft use
stabilators because conventional elevators can allow shock waves to form. Shock waves strongly
diminish the effectiveness of elevators, thereby causing a dangerous aerodynamic phenomenon
called mach tuck. Mach tuck will cause the nose of an aircraft to pitch downward when air flows
past the wings at supersonic speeds.
Although there is a significant difference in the design and construction of a stabilator versus an
elevator, they both essentially perform the same task of maintaining control of the plane’s nose.
Regardless of whether a pilot is operating an aircraft with a stabilator or an elevator, they likely
won’t feel a great difference in control.