POWER-OFF GLIDE PERFORMANCE

The gliding performance of an airplane is of special interest for the single-engine airplane
in the case of powerplant failure or malfunc­tion. When a powerplant failure or malfunc­tion occurs, it is usually of interest to obtain a gliding flight path which results in the mini­mum glide angle. The minimum glide angle will produce the greatest proportion of glide distance to altitude loss and will result in maximum glide range or minimum expendi­ture of altitude for a specific glide distance.

GLIDE ANGLE AND LIFT-DRAG RATIO. In the study of climb performance, the forces acting on the airplane in a steady climb (or glide) produce the following relationship:

sin y~

where

7 ^ angle of climb, degrees T= thrust, lbs.

D=drag, lbs.

W^lbs.

In the case of power-off glide performance, the thrust, T, is zero and the relationship reduces to:

sin y= —

By this relationship it is evident that the mini­mum angle of glide—or minimum negative climb angle—is obtained at the aerodynamic conditions which incur the minimum total drag. Since the airplane lift is essentially equal to the weight, the minimum angle of glide will be obtained when the airplane is operated at maximum lift-drag ratio, (L/D)mo;e. When the angle of glide is relatively small, the ratio of glide distance to glide altitude is numeri­cally equal to the airplane lift-drag ratio.

glide distance, ft. glide altitude, ft.

glide ratio = (L/D)

Figure 6.6 illustrates the forces acting on the airplane in a power-off glide. The equilibrium of the steady glide is obtained when the sum­mation of forces in the vertical and horizontal directions is equal to zero.

In order to obtain maximum glide ratio, the airplane must be operated at the angle of at­tack and lift coefficient which provide maxi­mum lift-drag ratio. The illustration of figure 6.6 depicts a variation of lift-drag ratio, LjD, with lift coefficient, CL, for a typical airplane in the clean and landing configurations. Note that {LjD)max for each configuration will occur at a specific value of lift coefficient and, hence, a specific angle of attack. Thus, the maximum glide performance of a given airplane configu­ration will be unaffected by gross weight and altitude when the airplane is operated at (L(D’)max. Of course, an exception occurs at very high altitudes where compressibility ef­fects may alter the aerodynamic characteristics. The highest value of (LjD) will occur with the airplane in the clean configuration. As the airplane is changed to the landing configura­tion, the added parasite drag reduces (LjD)max and the CL which produces (L/D)imi will be in­creased. Thus, the best glide speed for the landing configuration generally will be less than the best glide speed for the clean configuration.

The power-off glide performance may be appreciated also by the graph of rate of descent versus velocity shown in figure 6.6. When a straight line is drawn from the origin tangent to the curve, a point is located which produces the maximum proportion of velocity to rate of descent. Obviously, this condition provides maximum glide ratio. Since the rate of descent is proportional to the power required, the points of tangency define the aerodynamic condition of (L/D),mx.

FACTORS AFFECTING GLIDE PER­FORMANCE, In order to obtain the mini­mum glide angle through the air, the airplane must be operated at {LjD)maz. The subsonic (LjD)max of a given airplane configuration will occur at a specific value of lift coefficient and angle of attack. However, as can be noted from the curves of figure 6.6, small deviations from the optimum CL will not cause a drastic reduction of {LjD’) and glide ratio. In fact, a 5 percent deviation in speed from the best glide speed will not cause any significant reduction of glide ratio. This is fortunate and allows the specifying of convenient glide speeds which will be appropriate for a range of gross weights at which power-off gliding may be encoun­tered, e. g., small quantities of fuel remaining.

An attempt to stretch a glide by flying at speeds above or below the best glide speed will prove futile. As shown by the illustration of figure 6.6, any CL above or below the optimum will produce a lift-drag ratio less than the maximum. If the airplane angle of attack is increased above the value for {LJD)mazt a tran­sient reduction in rate of descent will take place but this process must be reserved for the land­ing phase. Eventually, the steady-state condi­tions would be achieved and the increased angle of attack would incur a lower airspeed and a reduction in {LjD) and glide ratio.

The effect of gross weight on glide performance may be difficult to appreciate. Since (LjD)maz of a given airplane configuration will occur at a specific value of CL, the gross weight of the air­plane will not affect the glide ratio if the air­plane is operated at the optimum CL. Thus, two airplanes of identical aerodynamic con­figuration but different gross weight could glide the same distance from the same altitude. Of course, this fact would be true only if both airplanes are flown at the specific CL to produce (LjD)№>iU.. The principal difference would be that the heavier airplane must fly at a higher airspeed to support the greater weight at the optimum CL. In addition, the heavier airplane flying at the greater speed along the same flight path would develop a greater rate of descent.

The relationship which exists between gross weight and velocity for a particular Cb is as follows:

CconstantC‘)

where

Vx = best glide speed corresponding to some original gross weight, Wi H2=best glide speed corresponding to some new gross weight, W%

As a result of this relationship, a 10 percent increase in gross weight would require a 5 per­cent increase in glide speed to maintain (L/Z>)mea.- While small variations in gross weight may produce a measurable change in best glide speed, the airplane can tolerate small deviations from the optimum Cu without signif­icant change in(L/D) and glide ratio. For this reason, a standard, single value of glide speed may be specified for a small range ol gross weights at which glide performance can be of importance. A gross weight which is con­siderably different from the normal range will require a modification of best glide speed to maintain the maximum glide ratio.

The effect of altitude on glide performance is insignificant if there is no change in (LjD’)max. Generally, the glide performance of the major­ity of airplanes is subsonic and there is no noticeable variation of with altitude.

Any specific airplane configuration at a partic­ular gross weight will require a specific value of dynamic pressure to sustain flight at the Q, for (Z/D)TOoJ,. Thus, the airplane will have a best glide speed which is a specific value of equivalent airspeed (EAS) independent of altitude. For convenience and simplicity, this best glide speed is specified as a specific value of indicated airspeed (lAS^} and compressibility and position errors are neglected. The prin­cipal effect of altitude is that at high altitude the true airspeed (TAS) and rate of descent along the optimum glide path are increased above the low altitude conditions. However, if (L/D)maj is maintained, the glide angle and glide ratio are identical to the low altitude conditions.

The effect of configuration has been noted pre­viously in that the addition of parasite drag by flaps, landing gear, speed brakes, external stores, etc. will reduce the maximum lift-drag ratio and cause a reduction of glide ratio. In the case where glide distance is of great im­portance, the airplane must be maintained in the clean configuration and flown at (L/D") .

The effect of wind on gliding performance is similar to the effect of wind on cruising range. That is, a headwind will always reduce the glide range and a tailwind will always increase the glide range. The maximum glide range of the airplane in still air will be obtained by flight at (L(D’)1Ma – However, when a wind is present, the optimum gliding conditions may not be accomplished by operation at (L/D’)max. For example, when a headwind is present, the optimum glide speed will be increased to obtain a maximum proportion of ground dis­tance to altitude. In this sense, the increased glide speed helps to minimize the detrimental effect of the headwind. In the case of a tail­wind, the optimum glide speed will be reduced to maximize the benefit of the tailwind. For ordinary wind conditions, maintaining the glide speed best for zero wind conditions will suffice and the loss or gain in glide distance must be accepted. However, when the wind conditions are extreme and the wind velocity is large in comparison with the glide speed, e. g., wind velocity greater than 25 percent of the glide speed, changes in the glide speed must be made to obtain maximum possible ground distance.

THE FLAMEOUT PATTERN. In the case of failure of the powerplant, every effort should be made to establish a well-planned, stabilized approach if a suitable landing area is available. Generally a 360° overhead ap­proach is specified with the approach begin­ning from the ‘ ‘high key’ ’ point of the flameout pattern. The function of a standardized pattern is to provide a flight path well within the capabilities of the airplane and the abilities of the pilot to judge and control the flight path. The flight handbook will generally specify the particulars of the flameout pattern such as the altitude at the high key, glide speeds, use of flaps, etc. Of course, the par­ticulars of the flameout pattern will be de­termined by the aerodynamic characteristics of the airplane. A principal factor is the effect of glide ratio, or (L/D)mai, on the alti­tude required at the high key point at the beginning of the flameout pattern. The air­plane with a low value of (L/D) will require a high altitude at the high key point.

The most favorable situation during a flameout would be for the airplane to in posi­tion to arrive over the intended landing area the altitude for the high key point. In this case, the standard flameout pattern could be utilized. If the airplane does not have suffi­cient glide range to arrive at the landing area with the altitude for the high key point, it is desirable to fit the approach into the lower portions of the standard flameout ap­proach. If it is not possible to arrive at the intended landing area with sufficient altitude to ‘‘play” the approach, serious considera­tion should be given to ejection while suffi­cient altitude remains. Deviations from a well-planned approach such as the standard flameout pattern may allow gross errors in judgment. A typical error of a non-standard or poorly executed flameout approach is the use of excessive angles of bank in turns to correct the approach. Because of the great increase in induced drag at large angles of bank, excessive rates of descent will be incurred and there will be further deviations from a desirable flight path.

The power-off gliding characteristics of the airplane can be simulated in power on flight by certain combinations of engine power setting and position of the speed brake or dive flap. This will allow the pilot to become familiar with the power-off glide performance and the flameout landing pattern. In addition, the simulated flameout pattern is useful during a precautionary landing when the powerplant is malfunctioning and there is the possibility of an actual flameout.

The final approach and landing flare will be particularly critical for the airplane which has’ a low glide ratio but a high best glide speed. These airplane characteristics are typical of the modern configuration of airplane which has

low aspect ratio, sweepback, and high wing loading. Since these airplane characteristics also produce marginal flare capability in power – off flight, great care should be taken to follow the procedure recommended for the specific airplane.

As an example of the power-off glide per­formance of an airplane with low aspect ratio, sweepback, and high wing loading, a best glide speed of 220 knots and a glide ratio of 6 may be typical. In such a case, the rate of descent during the glide at low altitude would be on the order of 3,700 FPM. Any deviations from the recommended landing technique can­not be tolerated because of the possibility of an excessive rate of descent. Either premature flare or delayed flare may allow the airplane to touch down at a rate of descent which would cause structural failure. Because of the mar­ginal flare characteristics in power-off flight, the best glide speed recommended for the land­ing configuration may be well above the speed corresponding to the exact maximum lift-drag ratio. The greater speed reduces induced drag and provides a greater margin for a successful power-off landing flare.

In the extreme case, the power-off glide and landing flare characteristics may be very criti­cal for certain airplane configurations. Thus, a well-planned standard flameout pattern and precise flying technique are necessary and, if very suitable conditions are not available, the recommended alternative is simple: eject!

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