1. The document discusses factors that determine takeoff field length for aircraft, including critical speeds and how to calculate takeoff distance with and without engine failure. It also discusses how aircraft are designed to meet regulatory takeoff requirements. 2. Longitudinal stability and how location of the wing and horizontal tail surfaces relative to the center of gravity affect stability in different flight conditions such as with the nose up or down. 3. The document contrasts supercritical wings, which can delay shock waves to higher speeds, with laminar flow wings, and discusses how technologies like suction devices have been used to further improve supercritical wings on modern aircraft.

1. Aircraft Performance, Stability/Control, Compressibility
What are the factors in determining your takeoff critical field length for a certain runway? If after
doing initial calculations you find the runway is too short, what might you be able to do as the
pilot in command to be legal for takeoff?
Compare and contrast the longitudinal stability of a conventional aircraft in two conditions:
forward CG and aft CG? Discuss control forces and aircraft responses from trimmed condition if
you pulled back on the stick and let it go.
Compare and contrast a supercritical wing with a laminar flow wing? Discuss modern aircraft
that have utilized these wings. What are the enhancements and drawbacks?
Solution
Ans:
For example, In some cases aircraft take-off from San Jose and fly all the way to San Francisco
(about 40 miles) before making their first refueling stop. This is because the field length is
insufficient to take-off with full fuel in San Jose and the tanks are topped off at SFO where the
runways are longer. Since this kind of operating restriction is not desirable, the aircraft is
designed to meet take-off field length requirements for selected airports with full payload and
fuel.
This constraint often sets the aircraft wing area, engine size, or high lift system design.
The following speeds are of importance in the take-off field length calculation:
Vmu Minimum Unstick Speed. Minimum airspeed at which airplane can safely lift off ground
and continue take-off.
Vmc Minimum Control Speed. Minimum airspeed at which when critical engine is made
inoperative, it is still possible to recover control of the airplane and maintain straight flight.
Vmcg Minimum control speed on the ground. At this speed the aircraft must be able to continue
a straight path down the runway with a failed engine, without relying on nose gear reactions.
V1 Decision speed, a short time after critical engine failure speed. Above this speed,
aerodynamic controls alone must be adequate to proceed safely with takeoff.
VR Rotation Speed. Must be greater than V1 and greater than 1.05 Vmc

2. Vlo Lift-off Speed. Must be greater than 1.1 Vmu with all engines, or 1.05 Vmu with engine out.
V2 Take-off climb speed is the demonstrated airspeed at the 35 ft height. Must be greater than
1.1 Vmc and 1.2 Vs, the stalling speed in the take-off configuration.
Further information on these design speeds are given in the relevant sections of FAR part 25,
including those dealing specifically with take-off and also those dealing with control
requirements.
The calculation of take-off field length involves the computation of the distance required to
accelerate from a stop to the required take-off speed, plus a climb segment. Since the
acceleration distance is typically about 80% of the total distance, we first consider this portion.
The distance required to accelerate to the speed Vlo can be computed by noting that:
dV = a dt and dx = V dt = V/a dV
so:
If the acceleration is assumed to vary as: 1/a = 1/a0 + kV2 then:
So, we could either integrate the acceleration numerically or use an average value, computed at
.70 of the lift-off speed.
Ignoring the small speed change between lift-off and the 35 ft screen height, we can take Vlo =
1.2 Vs. Then, Vlo = 1.2 (2 W) / (rS CLmax).
With, a = F/ m = T-D / m (where T=Thrust, D=total drag including ground resistance, m=take-
off mass), the expression for acceleration distance becomes:
x = 1.44 W2 / (g r S CLmax (T-D))
This expression is not very useful directly because it is difficult to estimate the drag, and we
must add the climb portion of the take-off run. More importantly, commercial take-off distances
assume engine failure at the worst possible time. If the engine fails sooner, the pilot can stop in a
shorter distance. If the engine fails at a higher speed, the airplane can continue the take-off and
reach a height of 35 feet in a shorter distance. This worst time corresponds to the critical engine
failure speed VEFcrit. It is assumed that the pilot recognizes the engine failure and takes action a
short time* later, at which time the speed is called the decision speed, V1. At a speed higher than

3. V1, the pilot must continue the take-off; at a lower speed he or she must stop.
The commercial take-off problem is very complex, involving acceleration on all engines,
acceleration with one engine inoperative, deceleration after engine failure, and climb with one
engine inoperative. This means that the design of spoilers, braking system, and rudder will affect
the FAR take-off field length.
The preliminary design computations, therefore, include correlation of the primary design
parameters with actual demonstrated performance. The correlation parameter is closely related to
that which appears in the simple analytical analysis on the previous pages. Examples of the
correlations for take-off field length with engine failure are shown in the figure below. The
propeller data is much more uncertain due to variations in propeller efficiency.
The FAA take-off field length in some cases may be set, not by the field length based on engine
failure, but on the all-engines operating performance. If the all-engines runway length multiplied
by 1.15 exceeds the 1-engine-out field length, the larger value is used. For four-engine aircraft
the all engines operating condition times 1.15 is usually critical.
Fits have been made to the FAR field length requirements of 2,3,and 4 engine jet aircraft vs. the
parameter:
W is the take-off gross weight (lbs).
Sref is the reference wing area (sq ft).
s is the ratio of air density under the conditions of interest which might well be a hot day in
Denver or another high altitude airport.
CLmax is the aircraft maximum lift coefficient in the take-off configuration.
T is the total installed thrust (all engines running). It varies with speed and must be evaluated at
70% of the lift-off speed which we take as 1.2 Vs. The variation of thrust with speed shown here
may be used for this calculation if detailed engine data is not available.
For 2 engine aircraft: TOFL = 857.4 + 28.43 Index + .0185 Index2
For 3 engine aircraft: TOFL = 667.9 + 26.91 Index + .0123 Index2
For 4 engine aircraft: TOFL = 486.7 + 26.20 Index + .0093 Index2
Since for four engine aircraft, the all-engines operating (with 15% pad) case is critical, one may
use this fit for the all-engines operating case with 2 or 3 engines as well. Note that the 15%

4. markup is already included.
2.
In designing an aircraft, a great deal of effort is spent in developing the desired degree of
stability around all three axes. But longitudinal stability about the lateral axis is considered to be
the most affected by certain variables in various flight conditions.
Longitudinal stability is the quality that makes an aircraft stable about its lateral axis. It involves
the pitching motion as the aircrafts nose moves up and down in flight. A longitudinally unstable
aircraft has a tendency to dive or climb progressively into a very steep dive or climb, or even a
stall. Thus, an aircraft with longitudinal instability becomes difficult and sometimes dangerous to
fly.
Static longitudinal stability or instability in an aircraft, is dependent upon three factors:
1. Location of the wing with respect to the CG
2. Location of the horizontal tail surfaces with respect to the CG
3. Area or size of the tail surfaces
In analyzing stability, it should be recalled that a body free to rotate always turns about its CG.
To obtain static longitudinal stability, the relation of the wing and tail moments must be such
that, if the moments are initially balanced and the aircraft is suddenly nose up, the wing moments
and tail moments change so that the sum of their forces provides an unbalanced but restoring
moment which, in turn, brings the nose down again. Similarly, if the aircraft is nose down, the
resulting change in moments brings the nose back up.
The CL in most asymmetrical airfoils has a tendency to change its fore and aft positions with a
change in the AOA. The CL tends to move forward with an increase in AOA and to move aft
with a decrease in AOA. This means that when the AOA of an airfoil is increased, the CL, by
moving forward, tends to lift the leading edge of the wing still more. This tendency gives the
wing an inherent quality of instability. (NOTE: CL is also known as the center of pressure (CP).)
Figure 4-20 shows an aircraft in straight-and-level flight. The line CG-CL-T represents the
aircrafts longitudinal axis from the CG to a point T on the horizontal stabilizer.
The CL in most asymmetrical airfoils has a tendency to change its fore and aft positions with a
change in the AOA. The CL tends to move forward with an increase in AOA and to move aft
with a decrease in AOA. This means that when the AOA of an airfoil is increased, the CL, by
moving forward, tends to lift the leading edge of the wing still more. This tendency gives the
wing an inherent quality of instability. (NOTE: CL is also known as the center of pressure (CP).)
Longitudinal static stability involves maneuvering (traditionally called "maneuvering stability"
or "man-stab") and non-maneuvering (traditionally called "long-stab" or "long-stat") flight.
Both arise from your airplane's pitch response to an angle of attack change, but they are assessed
differently. Man-stab is your plane's response during accelerated flight (when you're pulling

5. Gs). Long-stab describes your airplane's initial tendency following a deviation from its trimmed
airspeed.
Say you've trimmed the plane for 100 knots in straight and level flight. You'd expect to hold a
little back-stick to fly 90 knots and a little forward-stick to fly 110 knots, if your airplane exhibits
positive longitudinal static stability. Holding back-stick implies that if you let go the plane would
initially tend to accelerate back to its initial trimmed airspeed and vice versa for the forward-
stick, faster airspeed case.
If you have to push the stick to maintain 90 knots, your airplane would be statically unstable at
that flight condition. The implication here is that if you release your push, the plane's initial
tendency would be to decelerate more, moving further away from its trimmed airspeed. Clearly
an undesirable, nonintuitive situation.
If you released the stick and the plane remained at 90 knots or 110 knots when it was initially
trimmed for 100 knots, it would appear to exhibit neutral static stability. I say "appear" because
other factors-like control system friction-might be overpowering your plane's static stability.
3.
A supercritical wing of the variety in which the compression shock is stabilized is being
improved by providing a suction device, substantially along the entire wing span, right at the
chord station of the upper-wing surface where intercepting the sonic line. Specific rules
concerning the suction and suction slit are given. A small chord station range may be covered by
a movable slot or several parallel-running slots. The surface contour may be modified right at a
slot to, thereby stabilize the compression shock further. The invention shifts buffet onset to larger
off-design Mach numbers and angles of attack.
In accordance with the preferred embodiment of the invention, it is suggested to improve a
supercritical airfoil as per the objects, particularly as per the specific object, by providing suction
along the upper surface of the wing at the location of formation and development of a shock in
order to suck up a portion of the boundary layer. This partial sucking-up of the turbulent
boundary layer shifts the shockinduced boundary layer separation toward a larger shock. Shock
point migrations, resulting otherwise from the separation, and in-stationary shock oscillations
(buffeting) are thus suppressed in an extended off-design range, as compared to situations which
would exist if the suction were not provided for at the specified location. Thus, the invention
stabilizes the transsonic flow pattern across the wing beyond the range attained by mere
profiling. One may also say that the invention shifts the (ultimately inevitable) buffet onset to
larger off-design Mach numbers and larger angles of attack, resulting in correspondingly larger
lift coefficients and lower drag coefficients in Mach number and angle of attack ranges in which,
otherwise, the performance would drop drastically. The design number for the lift/drag ratio is
improved accordingly.

6. Airfoil design has improved dramatically in the past 40 years, from the transonic "peaky"
sections used on aircraft in the 60's and 70's to the more aggressive supercritical sections used
on today's aircraft.
Continuing progress in airfoil design is likely in the next few years, due in part to advances in
viscous computational capabilities. One example of an emerging area in airfoil design is the
constructive use of separation. The examples below show the divergent trailing edge section
developed for the MD-11 and a cross-section of the Aerobie, a flying ring toy that uses this
unusual section to enhance the ring's stability.
Subtle manipulation of aircraft aerodynamics, principally the wing and fuselage boundary layers,
can be used to increase performance and provide control. From laminar flow control, which
seeks to reduce drag by maintaining extensive runs of laminar flow, to vortex flow control
(through blowing or small vortex generators), and more recent concepts using MEMS devices or
synthetic jets, the concept of controlling aerodynamic flows by making small changes in the right
way is a major area of aerodynamic research. Although some of the more unusual concepts
(including active control of turbulence) are far from practical realization, vortex control and
hybrid laminar flow control are more likely possibilities.
Structures
Structural materials and design concepts are evolving rapidly. Despite the conservative approach
taken by commercial airlines, composite materials are finally finding their way into a larger
fraction of the aircraft structure. At the moment composite materials are used in empennage
primary structure on commercial transports and on the small ATR-72 outer wing boxes, but it is
expected that in the next 10-20 years the airlines and the FAA will be more ready to adopt this
technology.
New materials and processes are critical for high speed aircraft, UAV's, and military aircraft, but
even for subsonic applications concepts such as stitched resin film infusion (RFI) are beginning
to make cost-competitive composite applications more believable.
Propulsion
Propulsion is the area in which most evolutionary progress has been made in the last few decades
and which will continue to improve the economics of aircraft. Very high efficiency, unbelievably
large turbines are continuing to evolve, while low cost small turbine engines may well
revolutionize small aircraft design in the next 20 years. Interest in very clean, low noise engines
is growing for aircraft ranging from commuters and regional jets to supersonic transports.
Multidisciplinary Optimization
In addition to advances in disciplinary technologies, improved methods for integrating
discipline-based design into a better system are being developed. The field of multidisciplinary
optimization permits detailed analyses and design methods in several disciplines to be combined

7. to best advantage for the system as a whole.
Although a specific technology may provide a certain drag savings, the advantages may be
amplified by exploiting these savings in a re-optimized design. The figure to the right shows how
an aircraft was redesigned to incorporate active control technologies. While the reduced static
margin provides small performance gains, the re-designed aircraft provides many times that
advantage. Some typical estimates for fuel savings associated with "advanced" technologies are
given below. Note that these are sometimes optimistic, and cannot be simply added together.