takeoff and climb

After taxying to the holding point of the runway in use, the aircraft is aligned to about 45 to the runway and towards the wind. Aligning the aircraft in this manner ensure that the prop wash created during the full power tests does not damage an aircraft that may be behind you. The  pre-takeoff check list is accomplished. The checks are always enumerated in the pre-flight check list which should always be available in the cockpit. Checks will include engine functions, and fuel. With the brakes full on, the engine is run up to high revs, usually 2200 rpm, and each magneto is cut in turn. This will result in an RPM drop which should normally not exceed 125 RPM. The carb heat is tested, which should show a small drop in RPM. If the aircraft is complex, the propeller is recycled twice by increasing pitch sufficiently to reduce the RPM by at least 100. The engine RPMs are then dropped to tick-over to ensure that even running is experienced. The fuel boost pump is switched on and if the airfield is controlled, permission to line up is requested.

First degree of flap is usually applied, and the elevator trim adjusted to neutral. When this is completed, and clearance is given, the airplane is taxied to the centre  of the runway and aligned with it. The throttle is opened fully to start the takeoff run (also called take off roll). During this takeoff run, the control wheel, or stick, is usually held in the neutral position, but the rudder pedals are used to keep the airplane on the runway's centreline. If the aircraft has a castoring nosewheel, small dabs of differential brake will be required until sufficient airspeed has been attained to give rudder authority.

The sudden increase in engine power will place and uneven pressure on the empennage. This will result in a tendency to yaw, which must be counteracted using the rudder pedals. Some aircraft offset the engine installation to reduce the effect. Most engines rotate clockwise, which will produce a yaw to the left and require right rudder. Anticlockwise engines (some older British types for instance) will produce the opposite effect. A large power decrease will cause the aircraft to yaw in the opposite direction.

If a crosswind is present, the control wheel is held towards the wind to prevent the windward wing lifting.

As takeoff airspeed is approached, gentle back pressure on the control wheel raises the elevator which causes the airplane's nose to pitch upward slightly. This lifts the nose wheel off the runway
(see fig. below).


stages of a takeoff

Once the nose wheel is off the runway, the more right rudder will probably have to be applied to counteract the left-turning tendency which is greater once the aircraft leaves the ground. As the airplane lifts clear of the runway, the pilot varies the pressure on the control wheel. First, pressure is relaxed slightly to gain airspeed while still in ground effect (additional lift provided by compression of air between the airplane's wings and the ground). As airspeed increases to the best rate-of-climb airspeed, back pressure on the control wheel is adjusted to maintain that airspeed until the first desired altitude is reached. (Best rate-of-climb airspeed provides the most altitude for a given unit of time.) Once the runway is clear, the undercarriage is retracted (if the aircraft is complex) and flaps are returned to neutral (clean) at about 300 feet.

Short field take-off

In a short field take-off the aim is to accelerate as fast as possible, be airborne well before the boundary, to clear obstacles near the boundary while climbing at the maximum angle of climb, and to maintain reasonable safety margins. Thus we are not so concerned with protecting the undercarriage.

The procedure is to maintain a more or less level minimum drag attitude i.e. 4 or 5 aoa (with a nose wheel held just above the bumps) throughout the ground roll until Vx is reached; then rotate directly to a 12 aoa and climb away at Vx until obstacles are cleared; then reduce aoa to continue the climb at Vy or a higher speed. The ground roll is longer but the acceleration is greater, because rolling friction is normally less than induced drag at a low aoa: you reach Vx in a shorter distance and the TODR is less. The aircraft is subject to all the engine effects but an abnormal P-factor turning tendency should be anticipated after the lift-off rotation.

As in normal take-off the procedure may vary a little if the aircraft is fitted with flaps that can be set to a position which provides increased lift without a significant increase in drag. The recommended flap setting for a short field take-off may vary from that for other take-off conditions, as the flap position that facilitates minimum ground roll may decrease climb performance. There are some suggestions that flaps should not be lowered to the take-off position until the aircraft is nearing lift-off speed (so that the initial acceleration is faster) but the slight advantage provided by this can be dramatically offset by inadvertently lowering the flaps past the take-off position. Better to set the flaps when doing the pre-take-off checks where there is time to double check the selected position.

There may be a suggestion that an aircraft equipped with brakes is run up to full power at the start of take-off, while holding on the brakes; but generally it is probably better to smoothly run up to full power whilst the aircraft is rolling; there being less chance of stone damage to the propeller, and it is easier to prevent a swing developing. Swings and swing correction reduce the acceleration, and it is better to allow time at the beginning of the ground roll to get the aircraft firmly under control.

Obviously a take-off into wind is highly desirable, unless runway slope and rising terrain dictate otherwise, and the ground roll should be started as close to the boundary fence as reasonably possible. The procedure described above is for a hard, dry surface or for short, dry grass. If the surface is soft or the grass is long and wet then the rolling friction may exceed the induced drag at medium aoa or the slippery surface may make directional control difficult. In such cases it may be better to get the wheels off early and fly in ground effect until Vx is attained, as in the soft field technique. If there are any doubts about the take-off conditions then stay on the ground.

Soft field take-off

Soft field procedures may be applicable to muddy, waterlogged or long/wet grass surfaces. The prime aim in a soft field take-off is to reduce the extremely long ground roll, accepting to become airborne with less than adequate initial airspeed safety margin while utilising ground effect for fast acceleration. The following procedure should not be used in turbulent or gusty conditions, as the possibility of a stall after lift-off is increased.

In very soft conditions the usual technique is always to keep rolling, i.e. do not taxi to the take-off position and then stop to do the take-off checks, they should be completed beforehand. When lined up open the throttle fully and smoothly whilst holding the control column back. Use of a maximum lift flap setting is usually highly recommended. As the elevators become effective the nose of a nosewheel aircraft will rise; if a taildragger the elevator pressure should be sufficiently relaxed so that the tail wheel is held off the surface but the aircraft should remain firmly in a tail down attitude. As ground speed builds start relaxing the back pressure and the aeroplane will lift itself [or more likely lurch and stagger] from the surface at its minimum unstick speed [Vmu] and at an aoa very close to the stalling aoa, so it is vulnerable to turbulence and mishandling. Also P-factor and slipstream effect may come into play at this time and it is important to keep the wings level with aileron and stop any turn with opposite rudder to negate any cross-controlled skid.

The pilot must then smoothly reduce aoa to 5 or 6 and hold the aircraft just above the surface, in ground effect, so that it accelerates at the maximum possible rate. Gyroscopic effect may be significant during the pitch down to the smaller aoa, which must be anticipated with rudder. The aircraft is rotated after Vy (or Vx if there are obstructions) is attained to break it out of ground effect, held for a few moments to ensure it will accelerate, and then climb-out is commenced. At the initial rotation the aircraft will slow as induced drag increases substantially and rapidly, firstly, because of the restoration of the normal induced drag as it pulls out of ground effect, and secondly because of the increased aoa. The aircraft is likely to sink back to the surface if rotation occurs before sufficient speed is built.

The TODR for a soft field take-off will be considerably longer than that for a normal take-off. It is most unwise to attempt take-off from an airfield that is both short and soft.

Coping with significant crosswind

During the initial stages of the ground roll in any type of take-off with a significant crosswind component the aircraft will tend to weathercock into wind, pivoting around the main wheels. There are lateral stresses on all wheels in contact with the ground during the roll. The lateral control of the aircraft is then very much dependent on adequate tyre contact with the surface, so if the surface is slippery a crosswind take-off may not be advisable. As the aircraft accelerates the relative wind velocity (combining the ambient wind velocity, the aircraft's own forward speed and the slipstream velocity) over the tailplane surfaces, will have an increasing headwind component and a (relatively) decreasing crosswind component. Thus it is normal to start the ground roll with a large rudder deflection to counter weathercocking, with the deflection decreasing as speed builds.

It is usually advisable to also raise the into-wind aileron to prevent the into-wind wing from rising, particularly if gust induced; the inclined lift vector because of the rising wing will tend to turn the aircraft away from the wind. Be aware that if the into-wind aileron is raised, whilst you are countering the weathercocking with rudder, then you must be operating cross-controlled, which will cause the aircraft to sideslip into wind if you should get airborne in that condition.

The aileron deflection is decreased as speed builds, but in strong crosswinds it may be advisable to actually lower the into-wind wing so that the aircraft is rolling just on the into-wind main wheel. The lift vector is then inclined from the vertical and has a lateral component which counteracts the effect of the crosswind, the aircraft line of roll being kept straight by the friction of that into-wind wheel. If the angle is correctly judged there should be no stress on the wheel. As the aircraft is being lifted off the ailerons should be returned to neutral and the wings levelled.

To provide an additional safety margin the aircraft should be held on the ground for a higher than normal lift-off speed, plus if conditions are gusty, add 50% of the wind gust speed in excess of the mean wind speed e.g. wind speed 10 knots gusting to 20 knots, add 5 knots to the lift-off airspeed. If the aircraft does become prematurely airborne for any reason then, rather than let the wheels bump down again, hold the aircraft off the ground, accelerate in ground effect and use the soft field take-off technique.

After becoming airborne the aircraft will drift away from the heading, so to mark a tidy and controlled departure, the aircraft must be gently turned, onto a new heading that will compensate for the drift and the 'track made good' will follow the extended line of the ground roll, at least until the aircraft reaches 500 feet agl; at which height regulations allow a turn in the circuit direction.

It can be that the crosswind either amplifies or reduces the slipstream and other effects. It may be wise to consider taking off in a direction that takes advantage of that counter effect even if that entails taking off with a tailwind component. Also there is no rule that says you must always take-off aligned with the centre of the runway or strip, if crosswind conditions warrant it plan your ground roll at an angle across the strip edge to edge.

Causes of take-off accidents

One or more of the following factors are commonly causal in take-off accidents:

exceeding weight and balance limitations

failure to set elevator trim at the correct position for the airframe configuration

over-controlling during the ground run and at lift-off

premature lift-off

climbing too steeply after lift-off

failure to calculate the TODR and particularly the effects of high density altitude

failure to abandon take-off early enough when it is apparent that airfield surface conditions preclude a safe departure

using an excessive bank angle in a climbing turn.

Engine failure after take-off [EFATO]

Pilots should always be prepared for the possibility that the engine will lose partial or total power during the take-off and climb out; or, for that matter, at any other time during flight. When such an event occurs the cardinal rule is to fly the aeroplane, which initially implies quickly getting the nose down into the right attitude for an appropriate airspeed, either Vbg or Vmd depending on circumstances. Some say the second and third edicts should also be 'fly the aeroplane' and 'fly the aeroplane'. ( However if a partial power loss is accompanied by extreme vibration or massive shaking of the aircraft then it is just as important to get the engine completely shut down.)

Precautions when taking off towards rising terrain

Take-offs should always be planned so that they do not cause nuisance to others but it is also prudent to avoid taking off in a direction that takes you close to structures, trees, masts and powerlines unless you are sure that the aircraft will clear them by whatever safety margin you consider acceptable within the existing atmospheric conditions.

A take-off toward rising terrain is not something that should be undertaken without a thorough check of all conditions, even if such a take-off has previously being undertaken at a particular location without incident. Density altitude, wind and other conditions may be such that another take-off will result in a 'controlled flight into terrain' incident.

Ascertaining terrain height

The height of the terrain above the airfield or more particularly the angle of climb needed to safely clear it has to be ascertained, by whatever means available, to confirm that the aircraft's rate of climb will more than outmatch both the increasing terrain height and the effect of air downflow from the slope.

A simple way to judge the angle of climb needed is to extend your arm fully with the fingers bent so that your extended line of sight, including the bottom edge of your little finger, is horizontal. The width of each finger is around 2 and the width of the palm is around 10.

For an example we will have a look at  'farmer Jones's' airstrip on that hot summer afternoon. Here there is but one grass strip, 1000 feet in length and oriented north/south. Northward, and starting near the end of the strip, the terrain has a 1 in 10 slope rising toward an extensive crescent ridge with an elevation 1000 feet above the airstrip. Using the 1-in-60 rule we can calculate that a 1 in 10 slope equates with an angle of slope of 6.

Ascertaining angle of climb needed

We established our aircraft's practical rate of climb at sea level in standard ISA conditions as 850 feet per minute and Vy = 65 knots or 6500 feet per minute. (One knot is near enough to 100 ft/min so to convert knots into feet per minute just multiply by 100). Then using the 1-in-60 rule we can estimate our aircraft's sea level angle of climb in nil wind conditions, thus:- 850/6500 60 = 8. Also note that the ratio of vertical speed to forward speed is about 1:8.

But we are not operating in sea level ISA conditions and Vy is only an indicated airspeed not a true airspeed. TAS is close to 1.5% greater than IAS for each 1000 feet of density altitude, so at our density altitude of 5280 feet TAS is (1.5 5.28) % = 8% greater = 65 1.08 = 70 knots or 7000 ft/min. Also our practical rate of climb will be reduced by 10% per 1000 feet density altitude (= 52.8%) to 400 ft/min and the ratio of vertical speed to forward speed has been reduced to 1:18.

Using the 1-in-60 rule the angle of climb in nil wind conditions, is then:- 400/7000 60 = 3.4. Comparing the climb slope with the terrain slope of 6 we can see that it is impossible to outclimb the terrain, in fact the impact point will not be very far from the end of the strip.

But what would be the climb angle if we chose to climb at Vx, which should provide a ratio of vertical speed to forward speed 10% -15% better than Vy. If Vx then provided a ratio of 1:15.5 the climb angle, using 1-in-60, would be nearly 4, which would extend the impact point a little further up the slope.

Effect of wind on angle of climb

A reasonably steady horizontal headwind makes some difference to the angle of climb. Let's say that headwind is 15 knots which would have the effect of reducing the aircraft's Vy ground speed by 1500 ft/min to 5600 ft/min, so the angle of climb would be 400/5600 60 = 4.3. However winds that cross over slopes are not horizontal, they may have a substantial vertical component so the gain because of the reduction in forward ground speed may be more than offset by a reduction in vertical speed, in fact the downflow rate of sink can easily exceed the aircraft's rate of climb, in which case a "controlled flight into terrain" is inevitable.

Limiting climbing turns during take-off

The accelerated stall happens when the airspeed at which an aircraft will stall depends on the wing loading and, as a consequence of providing the centripetal force for the turn, wing loading increases as angle of bank increases. For instance, the wing loading increases slowly up to a bank angle of 30 where it is 15% greater than normal after which it increases rather rapidly, being 41% greater at a 45 bank angle. Bank angles exceeding 20 25 should not be made at low levels including take-off and landing operations.

The wing loading increase in the turn is provided by an increase in
CL which in itself is brought about by an increase in aoa. We also know that the lift coefficient increases in direct relationship to increase in angle of attack. Now what will happen if we are climbing at Vx and decide to quickly turn away from rising terrain or an approaching aircraft, using a 45 bank angle, while still climbing? We know from the table that to maintain a 45 level or climbing turn, wing loading and thus aoa, must increase by 41% and that the aoa at Vx is probably around 12, so that a 41% increase will take the aoa to 17 and the aircraft will stall.

Full power stalls in a balanced climbing turn tend to result in the outer wing stalling first, because of the higher aoa of the outer wing, with a fairly fast wing and nose drop (particularly so if the propeller torque effect is such that it reinforces the roll away from the original direction of turn and the aircraft is a high wing configuration) and likely to result in a stall/spin situation that any pilot lacking spin recovery experience may find difficult to deal with. If the climbing turn is being made with excessive bottom rudder then the lower wing might stall first with the consequent roll into the turn flicking the aircraft over. Recovery from a stall in a climbing turn is much the same as any other stall ease the control column forward to about the neutral position, stop any yaw, level the wings and keep the power on.

Even a 30 banked climbing turn at Vx will produce an aoa of 14, very close to the stall aoa and providing no margin for even minor turbulence or slight mishandling. The margin which you should always have in hand, to cope with such events, is 3 or 4, which indicates that, when climbing at Vx, turns should not be contemplated. Even when climbing at the Vy aoa [ around 8 ], until a safe height has been gained, turns should be limited to about 20 to allow an additional margin should wind shear be encountered in the climb out and the nose lowered a little for the turn.

Factors affecting safe take-off performance

Apart from the pilot's condition and capability, take-off performance is limited by the following constraints, all of which should be carefully assessed within pre-take-off procedure to establish whether a safe take-off is viable.
 

  • Aircraft weight and balance. The critical nature of aircraft weight and balance at take-off should be considered.
     

  • Standard take-off distance. TOD should always be expressed as the total distance required to accelerate from a standing start and clear an imaginary screen which is 50 feet, or 15.2 metres, high. The ground roll is that first part of the TOD where the aircraft's weight is supported, or partly supported, by the undercarriage; sometimes people incorrectly refer to the ground roll as the TOD, ignoring the fact that the distance covered from the lift-off point to clear 50 feet may be longer than the ground roll. It is not unknown for an under-powered aircraft to be able to lift off but then be unable to climb out of ground effect; as discussed below.

    TOD is officially expressed as the take-off distance required [TODR] to clear the 50 feet screen. Note that these standards require that the operating conditions associated with a particular TODR will be specified in approved aircraft take-off performance charts. These conditions are pressure altitude, temperature, runway slope and surface, and wind velocity.

    'Short, dry grass' is taken to mean grass less than 100 mm long.

    Unless the manufacturer's take-off performance figures are published as an approved performance chart within the aircraft's Flight Manual, or comparable document, then such figures should be treated as unverified sales claims. In the absence of any specified conditions in an unapproved performance chart, assume that sea level ISA, nil wind and smooth, dry runway are the basis for the published data.
     

  • Stopping distance required. The distance required to reach flight speed and then bring the aircraft a halt, should be known. It may be necessary to abandon the take-off soon after lift-off, due to doubtful engine performance or other event this is particularly important in short field or 'hot and high' take-offs. If take-off and landing distance [over a 50 feet screen] charts are available then the total distance needed to take-off, abandon take-off at 50 feet, land and bring the aircraft to a halt is just the sum of the charted take-off and landing distances required.
     

  • Airframe condition. An airframe in a battered or dirty condition, or which sports unnecessary or non-standard accoutrements, will increase drag and retard acceleration, lengthen TODR and reduce climb performance.
     

  • Engine age, condition and operating temperatures. An engine which is incapable of producing its designed rpm and torque will reduce acceleration, lengthen TODR and reduce climb performance. The engine manufacturer's instructions regarding warm-up procedures should be followed, to ensure appropriate temperatures and pressures are established, before the engine is subject to the stresses of take-off power; otherwise the potential for an 'engine failure after take-off' is greatly increased.
     

  • Propeller condition and pitch. Chipped leading edges or scored blades, apart from being dangerous due to the possibility of delamination or fracture, will adversely affect thrust output. Blade pitch at a coarse setting, a 'cruise' setting, will reduce acceleration and climb performance.
     

  • Tyre pressure. Under-inflated tyres increase the rolling friction, decrease the acceleration and will add maybe10% to the ground roll.
     

  • Airfield dimensions and slope. The usable length of runways or strips must be known, as well as the degree of slope. Taking-off upslope will reduce acceleration and lengthen the ground roll as thrust must also overcome a force equal to the aircraft weight the sine of the angle of slope, in addition to drag and rolling friction. The ground roll will increase by about 15% for each 2% of upslope. Runway slope can be measured by taking an altimeter reading at each end, divide the elevation difference by the runway length (in feet) and multiply by 100 to get the approximate slope percentage.
     

  • Airfield surface and surrounds. A short dry grass or rough gravel surface might add 10% to the ground roll compared to that for a smooth sealed surface. Wet or long grass might add 50% to the ground roll and a soft or waterlogged surface might double the ground roll. Surface water and/or wet grass can lead to aquaplaning and loss of directional control. The height of obstructions and local terrain must be known.
     

  • Airfield density altitude. A critical factor which is often not correctly assessed, the density altitude has a major effect on engine output, propeller performance and lift generated. Thus it affects acceleration, TODR and climb performance to such an extent that on 'hot and high' airstrips an aircraft may be incapable of safe take-off and climb-out.
     

  • Wind velocity and turbulence. After weight and balance and density altitude; wind strength, direction, gradient, downflow, gust intensity, surface turbulence and the potential for wind shear events are normally the major considerations in take-off performance for a properly maintained aircraft.

At left an indication of possible cumulative effect of some take-off conditions on TODR; but as explained below the take-off distance required can be much greater.

The pilot in command of an aircraft must assess all the foregoing factors and conditions to ascertain the cumulative total distance required for take-off and obstacle clearance and then judge if the take-off can be safely conducted. The golden rule is "If there are any doubts, don't fly".

The most favourable conditions for optimum take-off performance at MTOW are:

  • a pilot who follows the rules and the recommended procedures
  • an aircraft in very good condition and fitted with a 'climb' or variable pitch propeller
  • a surface that is dry, smooth and level or with a slight downslope
  • a low density altitude i.e. low elevation and low temperature
  • a smooth full headwind of reasonable and constant velocity.

You should not only be concerned that the take-off is conducted safely, it should also be accurately controlled, beginning with taxiing, so that alignments, headings, attitude and airspeeds the 'numbers' are properly maintained throughout. The take-off should take advantage of the aircraft's and engine's maximum rate of climb capability to reach the threshold height and it should look well executed to an informed observer standing behind the aeroplane's take-off point. In addition you must have pre-established plans to safely cope with partial or total power loss, occurring at any stage of the take-off sequence.

Engine effects and aerodynamic phenomena

There are some engine effects, plus aerodynamic and inertia phenomena, which will be noticeable at take-off, but both their existence and the extent of their effect are dependent on the configuration of the aeroplane. Tailwheel aeroplanes are particularly subject to these phenomena, which can cause difficulties to any pilot who is inexperienced in the slow speed handling of such aeroplanes.

The lower the power loading, or the higher the power-weight ratio, the greater, and faster, the reaction will be to the engine effects.

The helical slipstream

The propeller blades produce a rotating slipstream tube with a diameter equal to that of the propeller disc and a helicity that increases as forward speed increases. If the propeller rotates clockwise, when viewed from behind the aircraft, the slipstream tube will also rotate clockwise. Now imagine that the engine is mounted in the nose, as with the Jabiru, then the slipstream will be rotating clockwise around the fuselage and anything mounted below the fuselage will experience increased pressure on the right side, from the slipstream striking it at an angle, and anything mounted above the fuselage will experience higher pressure on the left side. The significant surfaces mounted above the fuselage are the fin and rudder and the increased pressure on their left hand side will tend to push the tail to the right i.e. in nil wind conditions the aircraft will want to swerve to the left; particularly in the early stages of the take-off run when the slipstream counts for practically all the airflow around the fin and rudder. The swing direction would be reversed for aircraft with the propeller rotating anti-clockwise.

Full application of compensating rudder may be required at the start of the ground roll, but the helical effect lessens as the aircraft accelerates (because the angle at which the slipstream meets the vertical surfaces lessens and also the rudder becomes increasingly effective) so rudder pressure is decreased as the take-off roll progresses. Slipstream effect is not so apparent in the landing ground roll because normally the throttle is closed.

However if the engine is mounted above the fuselage the rotating slipstream tube will be higher relative to the fin and rudder and the swing effect may be lessened or reversed: aircraft with a pusher engine mounting are subject to the same effect. Before you fly any aircraft it is advisable to determine which way the aircraft will swing and how to control the swing.

The helical slipstream will also meet the horizontal stabiliser at an angle but the resultant effect is difficult to determine or distinguish.

When a tailwheel aircraft has all wheels on the ground, as in the early part of the take-off ground roll, the slipstream may be deflected by the airfield surface so that the effect on the fin and rudder may vary between the tail-down and tail-up positions.

Propeller torque effect

The reaction torque of a propeller rotating under power attempts to rotate the aircraft about the propeller shaft; of course it is prevented by the resistance of the wings and undercarriage, however, at the beginning of the take-off run the torque may be sufficient to increase the friction on one tyre and thus cause the aircraft to pull towards that side. The effect is there in the early stages of take-off but may not be apparent as such, because it reinforces the swing tendency initiated by the helical slipstream. (The propeller torque on some very high-powered piston engine fighter aircraft has been such that at full power the aircraft tended to hop sideways down the runway. In such aircraft the engine was not opened up to full climb power until airborne.)

Gyroscopic precession effect

Any applied torque which tends to alter the direction of the axis of a spinning gyroscope causes the direction of the axis to move slowly [precess] 90 to the applied force and in the direction of rotation. A fast rotating propeller disc acts as a gyroscope and the precession effect may be noticeable as a slight vertical nose movement when a level turn is initiated in flight, but it is particularly apparent when the tail of a tailwheel aircraft is raised during take-off; when it may cause a fast acting swing to the left for a clockwise rotating propeller. The pilot must anticipate this action by applying compensating rudder as the tail is lifted. The same might apply when the tail is lowered during the landing run, with a swing in the opposite direction, but it is unlikely as normally the engine would be idling.

Gyroscopic precession effect is dependent on the rate of change in pitch or yaw, the rotational speed and the moment of inertia of the propeller.

P-factor

P-factor, or asymmetric disc effect or asymmetric blade effect, occurs when the thrust line is not aligned with the flight path i.e. when flying with a high angle of attack. As the propeller disc is then inclined to the relative airflow a down-going propeller blade has a greater component of forward velocity than an up-going blade, thus the down-going blade generates just slightly more thrust than the up-going blade. For a clockwise rotation more thrust is then generated on the right hand side of the disc which again reinforces the slipstream, torque and gyroscopic induced tendencies for such aircraft to swing left during take-off.

However, P-factor is dependent on thrust and proportional to forward speed, so it is not a significant factor in the initial part of the ground roll for a tailwheel aircraft, even though the axis of the airscrew disc is inclined to the horizontal; but it will become increasingly apparent as the ground roll progresses if the aircraft's tail down attitude is maintained. P-factor may also become apparent as higher velocities are reached just before and after lift-off, if a high aoa is employed at those stages. P-factor may cause the aircraft to yaw when flying level with high power at high angles of attack.

P-factor has little or no effect on a tailwheel aircraft during the landing ground roll because, normally, when the throttle is closed no thrust is produced, only propeller drag. However should the throttle be suddenly opened during the ground roll whilst the tail wheel is on the ground, anticipate a prompt P-factor reaction.

Inertial effect of centre of gravity position

If the aircraft's centre of gravity is behind the main wheels, as it must be in a tailwheel undercarriage aircraft, then any ground swerve initiated by the helical slipstream, gyroscopic effect, torque, a crosswind, wind gust, deflating tyre or rough ground will tend to be reinforced by the inertia of the aircraft, applied through the cg position, tending to pivot around the main wheels. When the cg, of the loaded aircraft, is in front of the main wheels, i.e. a nose wheel undercarriage, the aircraft's inertia will lead to self-correction of the swing; provided there is no excessive weight on the nose wheel. The cg inertial effect is usually much more likely to cause real difficulties when a tailwheel aircraft is slowing i.e. on landing, rather than when accelerating. Be aware that there are circumstances where the cg inertial effect also applies to nosewheel aircraft.

It is very important in such aircraft to identify any departure from the planned heading at a very early stage i.e. within 2 or 3, and take prompt corrective action but not to the extent of overcontrolling. The pilot must recognise the swing, stop it, correct the heading and then halt the correction. Over correction is exacerbated by a hard, smooth runway surface. A groundloop is a swing that has been accentuated by the inertia effect into a very rapid 180 movement, often causing wing tip and undercarriage damage and tending to occur at speeds between 5 and 25 knots. At low speeds and/or in light winds the inertia effect is stronger than any weathercocking action.

There are occasions when it is necessary for a pilot to induce a groundloop, usually when nearing the boundary fence at speed after a badly misjudged landing. The groundloop is induced by applying full rudder together with full brake on that side.

The swing effect is exacerbated if a tailwheel aeroplane is 'short-coupled' i.e. the moment arm between the tail wheel and the main wheels (or the fin and the cg) is short and thus the tail wheel friction moment is less than it might be. Such aircraft swing very rapidly.

The inertia effect requires that taxiing techniques for tailwheel aeroplanes differ to those for nosewheel aeroplanes. A turn initiated by rudder or brake in a nosewheel aircraft will stop as soon as the pilot removes rudder or brake pressure, because the inertia effect is always trying to straighten up the ground path (wind conditions permitting). However with a tailwheel aircraft once a turn is initiated the inertia effect will keep the turn going and possibly tightening until the pilot takes definite action, by use of opposite rudder or brake, to halt the turn.

Ground effect

In the aerofoils and wings module we saw that induced drag was a consequence of lift generation and the associated wing-tip vortices increase the momentum imparted to the downwash. When an aircraft is flying very close to the airfield surface, during take-off and landing, the formation of the vortices is partly impeded by the proximity of the ground and induced drag is less than normal.

The phenomenon is ground effect and implies faster acceleration on take-off (which can be very useful) and slower deceleration on landing (which generally is not useful) but it can only occur when the lower surfaces of the wings are less than one full wingspan from the surface. The closer the airborne aircraft is to the surface the greater the induced drag reduction. A light aircraft with a wingspan of 10 metres, holding height with the wing under-surface two metres above the ground, might experience a 30% 40% reduction which, at low speeds, would amount to a 15% 20% reduction in total drag.

It should be borne in mind that induced drag is normally a much greater force than the wheel/tyre rolling friction on a smooth, dry surface. Also if flying in ground effect utilising maximum available power and a disturbance causes the aircraft to lift further away from the ground, the induced drag will be immediately restored with a consequent decrease in airspeed, decrease in lift and substantial sink towards the ground. Similarly if maintaining a constant low velocity whilst in ground effect, i.e. neither accelerating or decelerating [and which is poor energy management practice but can readily occur in an underpowered or over-weight aircraft or when attempting take-off in high density altitude conditions] it may be found that the aircraft will not break out of the ground effect because, as the aircraft is pulled up, the induced drag increases, velocity slows, lift decreases and the aircraft sinks back into ground effect. If the aircraft is not able to be accelerated it may end up tripping over the boundary fence, unless the throttle is closed and the aircraft landed.

Calculating density altitude

Cold winter morning: temperature is 0 C and by setting 1013.2 on the altimeter pressure setting scale we read off the pressure altitude as 1600 feet. [We remember, of course, to then reset the scale to local or area QNH].

The temperature of 0 C is 11 C less than ISA so the density altitude variation due to temperature variation is (minus)11 120 = minus 1320 feet.
So density altitude = pressure altitude temperature variation = 1600 1320 = 280 feet

Thus the aircraft should perform well at take-off, close to its rated sea level capability.


Hot summer afternoon: temperature is 35 C and by setting 1013.2 on the altimeter pressure setting scale we read off the pressure altitude as 2400 feet.

The temperature of 35 C is 24 C greater than ISA so the density altitude variation due to temperature variation is (plus) 24 120 = plus 2880 feet.
So density altitude = 2400 + 2880 = 5280 feet

Thus the aircraft will perform poorly at take-off, probably at less than 70% of its rated sea level capability.

The following is an extract from an RAAus incident report:
"I was attempting to take-off in a paddock approximately 140 metres in length. Due to the hot [35 C] conditions the aircraft did not get enough lift which resulted in the main wheels catching the top wire of the boundary fence. The aircraft was slowed and struck the ground in a nose-down position. The wire snapped allowing the aircraft to bounce approximately 20 feet in the air. I cut the power and landed the aircraft to the left to miss another fence. This caused the left wingtip to strike the ground before coming to a stop. I walked away from the accident."

The aircraft manufacturer provided the following information:
"... the take-off distance to safely clear a 15 metre obstacle is 213 metres in ISA sea level conditions."

Rule of Thumb #1

    In the absence of manufacturer supplied data the effect of density altitude on TODR (for a dry, smooth and level surface) can be roughly estimated:-

"In nil wind conditions, for each 1000 feet that the pressure altitude exceeds sea level add 10% to TODR, then for each 10 C that the airfield temperature exceeds 0 C add a further 10%."

e.g. the 'Olly's Folly' hot day situation, the aircraft manufacturer's standard sea level TODR is 250 metres.

Pressure altitude is 2400 feet: 250 1.24 = 310 metres.

Temperature is 35 C: 310 1.35 = 419 metres TOD.

Then add a further 10% margin for random events = 460 metres estimated TODR, but this is for a dry, smooth and level surface, if the surface is long grass with a 2% upslope then you might have to add another 50% to TODR making it nearly three times the manufacturer's standard distance!

Remember that all the factors mentioned above relating to surface, slope, pressure, temperature, airframe and engine condition are cumulative and the runway length is finite.

 

Rule of Thumb #2

    In the absence of manufacturer supplied data the effect of density altitude on maximum rate of climb at Vy can be roughly estimated:-

Let's say our aircraft's manufacturer states the rate of climb at sea level in standard ISA conditions is 1000 feet per minute at Vy. However manufacturers' standard sea level rates of climb are usually based on an aircraft in factory new condition, flown by a very accurate pilot in the most benign atmospheric conditions. The manufacturer's standard should be downgraded by a factor that represents an adjustment for general engine, propeller, airframe and other conditions say 15%, thus the practical rate of climb at sea level in standard ISA conditions should be regarded as 850 feet per minute at Vy.

"The practical rate of climb at Vy should be reduced by 10% for each 1000 feet of density altitude."

e.g. At a density altitude of 5000 feet a 50% reduction to 425 ft/min.

Effect of wind

Wind direction, strength and variability is usually assessed by observing the airfield windsocks, which indicate the direction and variability and provide some idea of the surface speed. Indication of wind speed will vary with the type of windsock. A common type indicates a speed of 15 knots or greater when it is horizontal and about 7 or 8 knots when drooping at 45. The Bureau of Meteorology area forecast should provide an indication of the overlying gradient wind.

Take-off into wind!

There are several reasons why an aircraft, operating from reasonably flat terrain, should normally take-off directly into wind or as close to that as possible when operating from defined runways or strips. If an into wind take-off coincides with an upslope then a little calculation should be done to ascertain whether a downslope tailwind take-off is preferable. You may find some 'one-way' airstrips where a combination of airfield slope and rising terrain at the high end mandates take-off in one direction only, no matter what the wind direction. If you intend operating into such strips check the aircraft insurance policy carefully, cover may be voided.

  • The ground [rolling] speed for take-off is lower. The airspeed during the ground roll equals the ground speed minus the (head or tail) wind speed component. (It is a little confusing but it is conventional to treat a headwind component as a negative value and a tailwind component as a positive value). Thus if the aircraft is rolling at 30 knots into a 10 knot headwind the airspeed = 30 ( 10) = 40 knots. If it was rolling at 30 knots with a 10 knot following wind (a tailwind) the airspeed = 30 (+10) = 20 knots.
     

  • It is easier to keep straight because of the aircraft's increased directional stability due to the higher airspeed.
     

  • The take-off ground roll is shorter.
     

  • The into-wind climb-out will be steeper and provide better obstacle clearance. [But the rate of climb, i.e. time to height, is not dependent on wind direction.]
     

  • The vertical wind profile is such that the wind velocity changes encountered during the climb are most likely to be an increase in wind speed coming from ahead of the aircraft, thus providing a momentary increase in lift should any vertical shear effects be encountered.
     

  • If the engine should fail after take-off the aircraft can readily land into wind thus reducing impact force, because the ground speed is reduced quite significantly at light aircraft speeds.
     

  • It is safer to conform to an accepted traffic pattern, which is always based on take-off into wind, or as near as runway direction allows.

Estimating the crosswind component of the wind velocity

When operating from defined airstrips or runways the chances of the wind direction corresponding exactly with the strip alignment are low, thus most take-offs have an element of crosswind. Also local gusts and eddies usually cause the wind strength and direction to alter during take-off.

However taking off with a significant component of crosswind makes it more difficult to keep aligned with the selected path as the aircraft will try to weathercock into the crosswind and increases the possibility of one wing lifting during the ground roll. Lateral forces may stress the undercarriage.

All aircraft should have a demonstrated velocity limit for the crosswind component in both take-off and landing and, for a very light aircraft, that demonstrated crosswind component limit may be 10 12 knots, beyond which there is insufficient rudder control authority to counter any adverse movement. If the crosswind limit is not known you can assume that it is less than 25% of Vso.

There are also various techniques to be learned for positioning the ailerons, elevators and rudder depending on aircraft configuration and wind strength and direction during the ground roll and whilst taxiing the aircraft. Whilst taxiing the aircraft will always tend to weathercock into wind and there are techniques for taking advantage of that when turning in breezy conditions. Be aware that, due to the high cg and narrow wheel track, all light aeroplanes are fairly unstable when turning whilst taxiing. Turns made at speeds much above walking pace may result in a wingtip ground strike.

 

Easy calculation for determining the crosswind component

    Once you have determined your take-off direction and estimated the wind velocity:

    1.   Estimate the wind angle. i.e. If you intend taking off towards the north and the wind is coming from the north-east or north-west then the wind angle is about 45.

    2.   The crosswind component is the windspeed multiplied by the sine of the wind angle. However a reasonable approximation of the crosswind component is made if you multiply the wind angle by 1.5 and apply the result as a percentage [to maximum 100%] of the wind speed.

e.g. Wind speed 15 knots, wind angle 45:
Crosswind component = 45 1.5 = 67.5% of 15 = 10 knots

    3.   If the wind angle is 60 or more consider the full wind speed as the crosswind component. i.e. wind speed 15 knots, wind angle 60 then crosswind component = 15 knots.

 

Estimating the headwind or tailwind component

In some crosswind take-off circumstances you will need to estimate the headwind or tailwind component of the wind velocity. The headwind or tailwind component of a crosswind is not the wind velocity minus the crosswind component. It is, of course, the square of the headwind or tailwind component which equals the square of wind velocity minus the square of the crosswind component.
 

Easy calculation for determining the headwind or tailwind component

    Once you have determined your take-off direction and estimated the wind velocity:

    1.   Estimate the wind angle. i.e. If you intend taking off towards the north and the wind is coming from the north-east or north-west then the wind angle is 45.

    2.   The headwind component is the windspeed multiplied by the cosine of the wind angle. However a reasonable approximation of the crosswind component is made if you deduct the wind angle from 115 and apply the result as a percentage (to maximum 100%) of the wind speed.

e.g. Wind speed 15 knots, wind angle 45:
Headwind component. = 115 45 = 70% of 15 = 10 knots

    3.   If the wind angle is 30 or less consider the full wind speed as the headwind component. i.e. wind speed 15 knots, wind angle 25 then headwind component = 15 knots.

If the wind angle exceeds 90 from your intended take-off direction then of course you will have a tailwind component. In which case use the acute angle which the wind subtends with your take-off direction, e.g. if the wind is from the south-east or south-west when taking-off towards the north the acute angle is 45 and exactly the same calculation as above is made to determine the tailwind component.

 

 

Easy calculation for determining the headwind or tailwind effect on ground roll distance.

If you know the nil wind take-off ground roll for a particular aircraft you can estimate the take-off ground roll for various headwind components, with the same airfield surface conditions.

The take-off ground roll = the nil wind ground roll [ (lift-off speed wind speed) /lift-off speed ]

For example if an aircraft has a ground roll of 100 metres before reaching the normal lift-off speed of 40 knots, what would be the take-off ground roll into a headwind of 5 knots?"

The take-off ground roll = 100 [ (40 5) / 40 ]

        = 100 0.875 = 100 0.765 = 76 metres.

What would it be with a tailwind of 5 knots?

The take-off ground roll = 100 [ (40 + 5) / 40 ]

        = 100 1.125 = 100 1.265 = 126 metres.

As you can see there is a significant difference [50 m] in ground roll in light winds. If the wind speed components involved were 10 knots the ground roll would be 56 metres into a headwind and 156 metres with a tailwind.

climbing

The performance in in climb is dictated by the engine power. Every aircraft design has a best angle of climb (Vy) which is the airspeed that will give the maximum increase in height in a given time.  The best angle of climb (Vx) will give the maximum height gain over the shortest distance. This is used primarily to clear obstacles on take-off. The data is available in the aircraft pilot operating handbook.

Climbs to other and higher altitudes are made at airspeeds determined by the pilot, until the desired cruising altitude is reached.

Upon reaching cruising altitude, the airplane's pitch attitude is reduced and the airplane accelerates to cruising speed. The power is reduced and adjusted to maintain the selected cruising speed. Almost simultaneously, the pilot adjusts the elevator and possibly the rudder to keep the airplane at the desired altitude and heading (direction). The aircraft is then re-trimmed.

wind

If the aircraft is climbing into the wind, the angle of climb will be greater. The rate of climb is not affected.

engine

Under the full power settings of climb, engine temperatures should always be monitored.

dipping and weaving

The nose of the aircraft during climb will obscure vision. It is therefore advised to dip the nose or weave every 500ft to ensure there is no traffic confliction.

aircraft ceiling

As the aircraft climbs to higher altitudes, the power output of the engine decreases. This is particularly so in the case of normally aspirated engines. The rate of climb will gradually decrease. The ceiling of an aircraft is defined as when the rate of climb decreases to 100 feet per minute.