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Flight Performance - Level 3

TakeOff and Landing

    i_t = Thrust Incidence Angle

    D = Total Aerodynamic Drag

    F_rw = Total Runway Resistance

    F_ex = Excess Thrust

with:

   

  

where:

= Coefficient of friction associated with the nose wheels.

= Coefficient of friction associated with the main wheels.

Also,

where:

N_x = Longitudinal Load Factor

g₀ = 32.174 feet/second²

V_g = Ground Speed

Collecting terms:

1. Requiring the summation of forces in the Z-axis to be zero.  ( note:  The positive Z-axis direction is perpendicular to the runway and pointing towards the top right of the page.)
   



2. Requiring the summation of moments about the Y-axis to be zero. ( note:  The Y-axis in this case is perpendicular to the page and coming out of the page.)  We will take moments about the main wheels, since the aircraft will pitch about the main wheels during the takeoff or landing ground roll. We will ignore any pitch dynamics during the ground roll. We will ignore any moment caused by the vertical component of gross thrust.

What we now have is three equations with three unknowns for purposes of simulating a takeoff or landing ground roll. It is assumed that one has a thrust and drag model for the lift, drag, gross thrust, and engine drag terms in the above equations.

The three unknowns are the two normal forces on the wheels (F₁ and F₂) and the excess thrust (F_ex).  Of course, the primary parameter of interest is the excess thrust from which we can compute the derivative of ground speed. Once we have the excess thrust, we can differentiate the ground speed derivative to obtain speed and distance versus time.

Collecting the three equations:

 

 

 

Rearranging the equations:

 

We will define the terms in the square brackets in each one of the equations as A₁, A₂ and A₃, respectively.  Then we can rewrite the three equations in matrix form as follows: 

Here the matrix equation is of the form C F = A, where the bold letters represent vectors and C is the three-by-three matrix. We need to solve for F.

During the course of flight test, we measure excess thrust (F_ex). However, the thrust and drag may be unknown, or at least not known precisely. Therefore, we may need to iterate between the above equation and the solution of the above equation. The A₁ term is thrust minus drag minus the runway component of weight.

 The above matrix relationship can be solved by multiplying both sides by the inverse of the square matrix, C, namely C^-1, as long as the determinant of C is not zero.  Thus

 

Developing a Takeoff Simulation

Usually, the designer will provide an initial estimated model for lift and drag as a function of angle of attack (a ). Normally, a is zero during the ground roll and that is why it was not included in the above general equations. The thrust incidence angle, i_t , is always usually either zero or small. Only the most precise simulations will typically account for a separate tail and body drag, so we can ignore D_t, the drag of the tail, in many cases. Accounting for tail lift and drag becomes more important when modeling braking performance to determine the load distribution on the main gear and the nose gear.

For takeoff performance, a value of 0.015 is usually assumed for the rolling coefficient of friction (m ). In addition, a point mass model will be assumed with all the forces acting through the center of gravity of the aircraft. With these assumption, the three equations reduce to two equations, namely:

where mF=F_rw.

Combining the above two equations by substituting for F leads to:

The above equation can be used in two ways:

(a) first, to solve for excess thrust, i.e.,

or, (b) second, to solve for thrust minus drag, i.e.,

We know (or assume values for) the other variables.

From the first equation, we can compute the excess thrust during the ground roll of the aircraft. One would be provided models for net thrust drag and lift. The drag and lift models would be in the form of drag and lift coefficients versus angle of attack. Typical model formulations are as follows:

where:

M = Mach number

H = Pressure altitude

T_a = Ambient temperature

a = angle of attack

h_AGL = Aircraft wing height above ground level

The fuel flow is required in order to account for weight change during the takeoff. The parameter h_AGL is needed to account for ground effect. The above are just typical model forms. They may also include Reynolds number terms. In addition, the engine is usually not at 100% thrust at brake release so a thrust spool up factor needs to be supplied.

 

 Ground Effect

The following plot is typical of a relationship defining the decrease in drag due to lift in ground effect.

A very simplified model that approximates an F-16 in military thrust was created to illustrate takeoff simulation. The model constants and equations are as follows:

S = 300. = reference wing area (feet²)

b = 35. = wing span (feet)

AR = 4.0 = b²/S = Aspect Ratio

h_w = 5.0 = height of wing above ground while aircraft on the ground (feet)

W_ts = 25,000. = start gross weight (pounds)

F_no = 10,000. = thrust at zero Mach (pounds)

F_nslope = 5,000 = slope of thrust vs. Mach (pounds)

K_Fn0 = 0.65 = thrust factor at zero time

t = 2.0 = thrust time constant (seconds)

We assume that the overall thrust factor increases from its zero time value via the formula

     and if K_Fn >1, then K_Fn = 1.

The equation for the net thrust for this model becomes:

and the weight of the fuel can be related to the thrust by:

where sfc = thrust specific fuel consumption.

The percent of out of ground effect drag is computed from (see previous graph)

under the condition that X_GE =1.0 , if X_GE >1.0.

With the following parameters defined, namely,

    Cl_min = 0.05 = lift coefficient corresponding to minimum drag

    C_dmin = 0.0500 = minimum drag coefficient

then the drag coefficient (C_d) is computed as follows:

With the definition of the initial pressure altitude for the runway, for example,

H_C = 2,300 feet = initial pressure altitude

then the ambient pressure ratio (d ) is as follows:

and we may relate it to the standard day value at sea level by

where: 

P_a = ambient pressure, and

P_aSL = ambient pressure standard day sea level = 2116.117 pounds/foot²

Using the aspect ratio and the angle of attack, then the lift coefficient (C_L) is as follows:

The angle of attack is held to zero during the ground roll until a rotation speed is reached. This rotation speed (in this simulation example) is at a calibrated airspeed of 100 knots. Upon reaching the rotation speed, the typical takeoff will rotate to some given angle of attack. Then, that angle of attack is held until the aircraft generates enough lift such that lift is greater than the weight and the aircraft lifts off the runway. The angle of attack profile used in this example simulation is as follows:

where we assume that

The angle of attack (a ) is limited to a pre-determined value.   In the example that value is 13 degrees.  In the numerical integration, 13 degrees angle of attack is reached at 143 knots calibrated airspeed. The lift first exceeds weight at airspeed of 156 knots. The aircraft (or the simulated aircraft) will lift off the ground when lift is greater than the weight.

Lift and drag are computed as follows using the mach number definition and ambient pressure ratio:

Finally, the last terms in our model are for the runway resistance. We will assume zero runway slope and a runway coefficient of friction, m = 0.015.   Then,

under the condition that F_rw = 0. for L > W_t since the aircraft will become airborne.

Now, the equation (previously derived above):

will reduce to

Also, the longitudinal load factor N_x was related to the excess thrust through the total gross weight by

where

 

During the ground roll, the h-dot term is zero.  During the air phase, the normal load factor equation is used, i.e.,

where

and g = flight path angle.

 

From the N_X , N_Z and g equations we can integrate to find ground speed (V_g) and geometric height (h).  All of the forces, however, are functions of airspeed and pressure altitude.  We have assumed a standard atmosphere for temperature (in degrees Kelvin)

We can now find the true air speed using

where:

V_t = true airspeed

V_w = wind speed

if the wind speed were nonzero.  We will assume wind speed equals zero in the example.

 

The following equations relating the aircraft's calibrated velocity, V_C,   to other parameters are given here for the sake of completeness, but whose derivation may be found elsewhere:

From the speed of sound and the mach number:

where

The compressible dynamic pressure, q_C, may be calculated, knowing the ambient pressure and the mach number, by

The calibrated velocity of the aircraft with respect to a standard sea level day is then given by

 

 A plot of thrust, drag plus the runway resistance terms and excess thrust versus calibrated airspeed is shown in the following:

 

The following plot is a blow-up of just the drag from just before rotation to about 60 feet above the ground. This illustrates the changes in slope of drag versus speed as the fixed alpha is achieved and as the aircraft is no longer in ground effect. Of course, these are idealized computer simulations so one would not see such clearly defined effects in flight test data.

We can numerically integrate the equations to provide a plot of distance versus calibrated airspeed or height versus calibrated airspeed. By scaling the distance by a factor of 100, one can present both the longitudinal and vertical distance on one plot as follows.

Units for y axis: 100 feet for distance (blue), seconds for time (purple)
and feet for altitude (green)

 

 Effect of Runway Slope

Using the pseudo-F-16 model, the effect of runway slope, the values of time and distance as a function of runway slope (in degrees) is shown in the following table. The average acceleration is computed as follows:

where:

t = time at liftoff (seconds)

d = distance at liftoff (feet)

Slope	distance	time	acceleration	% from zero
-1.0	3001	22.6	11.75	4.52%
0.0	3131	23.6	11.24	0%
0.5	3164	24.0	10.99	-2.29%
1.0	3247	24.6	10.73	-4.56%
2.0	3403	25.8	10.22	-9.06%

As can be seen, the effect of runway slope for this particular model is about 4.5% per degree of runway slope.  For a typical light aircraft, the effect of runway slope is at least twice that amount, due to the much smaller thrust to weight ratio of the typical light aircraft.  Although the percentage change in acceleration is about the same for a positive or negative runway slope, one must take into account the fact of having a negative absolute rate of climb at liftoff for a negative slope runway.  For instance, for a liftoff at 100 knots ground speed with a negative 1.0 degree slope runway, the absolute rate of descent is about three feet/second.  The rate of climb (or descent) with respect to the horizontal plane is given by:

 

Effect of Wind on Takeoff Distance

Again using the same pseudo F-16 model, the following plot illustrates the effect of wind

Y axis: percent change in liftoff distance

 

Idle Thrust Decelerations

To assist in the development (or verification) of a Takeoff and Landing simulation, idle thrust decelerations may be performed. One would accelerate the aircraft on the runway to some high airspeed. Then, cut the throttle to idle and allow the aircraft to freely decelerate. We can solve for drag (D) in the above equation

and then put D into coefficient form.

where:

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Updated: February 23, 1999