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Groundschool – Theory of Flight
Revision 49 — page content was last expanded 11 February 2014.
The take-off sequence in a light aircraft is the most critical of all normal flight procedures. All the engine's available performance must be employed during the acceleration and initial climb — leaving no power in reserve — and there is no potential energy of excess height or excess momentum available. Thus, during take-off, the pilot's options are extremely limited.
Prior to take-off, it is essential to check the aircraft, airfield and atmospheric conditions to determine if take-off can be undertaken safely, how the take-off and climb-out will be conducted and to have a predetermined emergency plan.
(For pre-take-off communication procedures see 'Radiotelephony communications and procedures')
Take-off procedures and techniques vary according to aircraft type: seaplane or landplane, tailwheel configuration — tractor or pusher engine; nosewheel configuration — tractor or pusher; flap equipped; canard configuration; delta-winged; powered parachute; or weight-shift aircraft. Some procedures should be specified in the pilot's operating handbook for that aircraft.
In this module, we will look at the common factors to be considered in the execution of the take-off for the normally configured, three-axis, nosewheel or tailwheel aeroplane.
There are differing take-off procedures or techniques, or combinations thereof, applicable to particular airfield conditions:
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The diagram indicates possible cumulative effects of some take-off conditions on TODR. But as explained in section 11.6, 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 conducted safely. The golden rule is "If you have ANY doubts, don't fly".
The most favourable conditions for optimum take-off performance at MTOW are:
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 aircraft'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. See 'Engine failure after take-off' and 'The turn back, possible or impossible — or just unwise?'.
There are web versions of two CASA Advisory Circulars on this site: Operations at non-controlled airfields and Safety during take-off and landing. Both these documents should be read in conjunction with this module.
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Ultralight aircraft also tend to have a much higher power-to-weight ratio than their general aviation counterparts. For example, at MTOW, the two-seat 110 hp Cessna 152 and Piper Tomahawk both weigh 1670 lb and have a power loading of 15 lb/hp; whereas a two-seat amateur built aircraft acceptance category ultralight equipped with an 80 hp engine will have a power loading of 12.5 lb/hp, and only 10 lb/hp if fitted with a 100 hp engine. A single-seat CAO 95.10 ultralight fitted with just a 60 hp engine will have a power loading of 11 lb/hp. The lower the power loading, or the higher the power-to-weight ratio, the greater and faster the reaction will be to the engine/propeller effects.
Full application of compensating rudder may be required at the start of the ground roll. 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 should be 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 resulting 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.
For example, if the aircraft's attitude is pitched up the upper rim of the propeller disc is forced back while the lower rim is pushed forward. The precession moment is moved 90° clockwise* to the applied force so the upper rim becomes the disc's right side (looking from the rear) and the reaction moment is directed to the rear tending to yaw the aircraft to the right, i.e. during the period the aircraft is being rotated about its lateral axis the gyroscopic precession effect is also trying to rotate the aircraft about its normal axis. Similarly if the aircraft is pitched down the precession effect prompts a yaw to the left. Conversely if the aircraft is strongly yawed to the left the nose tends to pitch up; if yawed to the right the nose tends to pitch down. There is no gyroscopic precession effect when the aircraft is rolled about the longitudinal axis.
*Note: assuming a clockwise-rotating (viewed from behind the aircraft) tractor or pusher propeller.
The magnitude of the gyroscopic moments induced by the rotating propeller are dependent on the rate of change in aircraft pitch or yaw, the rotational speed of the propeller and its moment of inertia. The precessive forces are transferred via the shaft to the propeller speed reduction unit or direct to the engine crankshaft, bearings, crankcase and mountings.
Sport and recreational aircraft generally have a high power-to-weight ratio and the engines apply unusually high rpm to the propeller. The most prevalent example of the gyroscopic effect in such aircraft is in the early stages of a taildragger's take-off run should the pilot shove the control column forward to raise the tail and accelerate. At this stage airspeed is low so the aerodynamic forces generated by the airframe are also low and have a decreased ability to counter the gyroscopic effects. The pitch down causes the aircraft to yaw to the left so the pilot must anticipate this action by applying compensating rudder as the tail is lifted.
Even ground manoeuvring may induce unfavourable gyroscopic effects – swinging the aircraft around with a burst of power plus rudder/brake places high loads on the propeller shaft. For an example of the possible longer term effects on the propeller shaft see 'The Fox story – gyroscopic loads' also see matching engine and propeller. You can read a little about gyroscopic effect in Spitfires and Seafires; the gyroscopic effect is also utilised in some advanced aerobatic manoeuvres in aircraft with powerful engines and large propellers, the Lomcevak end-over-end tumble and inverted spin was the first.
P-factor is dependent on thrust and is 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; 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 using 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 — there is only propeller drag. However, should the throttle be opened suddenly during the ground roll while the tailwheel is on the ground, there may be a prompt P-factor reaction. wheelbarrowing'.
It is very important in such aircraft to identify any departure from the planned heading at a very early stage of the 'swing' and take prompt, corrective action — but not to the extent of over-correcting. 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 inertial effect into a very rapid 180° movement, which often causes wingtip and undercarriage damage, and occurs at speeds between 5 and 25 knots. At low speeds and/or in light winds, the inertial effect is stronger than any weathercocking action.
There are occasions when it is necessary for a pilot to induce a groundloop, usually when aborting a take-off and nearing the boundary fence or something solid at speed — or after a misjudged approach and landing. The groundloop is induced by applying full rudder and brake on the appropriate side.
The swing effect is exacerbated if a tailwheel aircraft is 'short-coupled'; i.e. the moment arm between the tailwheel and the main wheels (or the fin and the cg) is short, and thus the tailwheel friction moment is less than it might be. Such aircraft swing very rapidly.
The inertial effect requires that taxiing techniques for tailwheel aircraft differ from those for nosewheel aircraft. 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 inertial effect is always trying to straighten up the ground path (wind conditions permitting). However, with a tailwheel aircraft, once a turn is initiated the inertial effect will keep the turn going — and possibly tightening — until the pilot takes definite action by using opposite rudder or brake to halt the turn.
The inertial effect of the cg position relative to the main wheels is relevant when landing; see the rebound effect. spanwise pressure gradient' section of the 'Aerofoils and wings' module we saw that induced drag was a consequence of lift generation, and the associated wingtip vortices increase the momentum imparted to the downwash. As the centre of each vortex is a little inboard of the wingtip, the vortices also have the effect of reducing the effective wing span, the effective wing area and probably the effective aspect ratio.
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, so induced drag is less than normal and the centre of each vortex moves outboard a little with the potential for a little more lift. The phenomenon is ground effect and mainly — because of the drag reduction — produces faster acceleration on take-off (which can be very useful) and slower deceleration on landing (which generally is not useful). It can only occur when the lower surfaces of the wings are much less than one full wingspan distance from the surface. The closer the airborne aircraft is to the surface, the greater the reduction in induced drag. A light aircraft that maintains height with the wing under-surface about one-quarter wing span above the ground, might experience a 30–40% reduction; at low speeds, this would amount to a 15–20% reduction in total drag. A 50% reduction in induced drag might be achieved if the wing height is equivalent to one-tenth of wing span, which may be possible in a low-wing aircraft and if the pilot has a very steady hand.
Induced drag is normally a much greater force than the wheel/tyre rolling friction on a smooth, dry surface. If flying in ground effect and utilising maximum available power, then when a disturbance causes the aircraft to lift further away from the ground, the induced drag will be restored immediately with a consequent decrease in airspeed, decrease in lift and substantial sink towards the ground. Similarly, if maintaining a constant low velocity in ground effect (i.e. not accelerating, which is poor energy management practice but can readily occur in an underpowered or overweight aircraft, or when attempting take-off in high density altitude conditions) the aircraft may not break out of the ground effect because as the control column is pulled back, the induced drag increases, velocity slows, lift decreases and the aircraft sinks back into ground effect. If the aircraft cannot be accelerated it may end up tripping over the boundary fence, unless the throttle is closed and the aircraft landed.
The effective angle of attack of the horizontal stabiliser is also affected, mainly by the changing angle of the wing downflow. This might be evident as an uncommanded but slight pitch-up or down when leaving or entering ground effect.
The same effect generally applies to seaplanes and amphibians for water take-offs and landings, so 'ground effect' should be more properly termed 'surface effect'.
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sections 3.3 and 3.4. However, we will run through an example for an airstrip — 'Olly's Folly' — located at an elevation of 2000 feet. Under ISA conditions, the standard temperature and pressure at that height is 11 °C and 942 hPa respectively. We will do density altitude calculations for a cold winter morning in a high pressure system, and a hot summer afternoon in a low pressure trough. Remember that each 1 °C variation from ISA is roughly equivalent to 120 feet variation in density altitude.
(a) 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:
–11 × 120 = –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.
(b) 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 +24 × 120 = +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 RA-Aus 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."
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type of windsock. 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 runway, 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, because cover may be voided.
Taking off with a significant crosswind component makes it more difficult to keep aligned with the selected path — because 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 90° crosswind component in both take-off and landing. For a very light aircraft, the demonstrated crosswind component limit may be 10–12 knots, beyond which there is insufficient rudder authority to counter any adverse movement. If the crosswind limit is not known, you can assume that it is less than 25% of Vso. (FAR Part 23.233 requires that all aircraft have safe handling characteristics with a direct crosswind component not less than 0.2 Vso.)
There are also various techniques to be learned for positioning the ailerons, elevators and rudder — depending on aircraft configuration, wind strength and wind direction — while taxiing and during the ground roll. While 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 aircraft are fairly unstable when turning while taxiing. Turns made at speeds much above walking pace may result in a wingtip ground strike.
Estimating the headwind or tailwind componentIn some crosswind take-offs, you may 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 — the square of the headwind or tailwind component equals the square of wind velocity minus the square of the crosswind component.
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Normal take-off — nosewheel three-axis aircraftLet's imagine a nosewheel undercarriage aircraft (having lined up in the chosen direction and ensured that the nosewheel has trailed in the fore and aft position) just starting its take-off run, with the throttle being smoothly advanced to maximum power. The airframe will be in a level attitude and, if the wings have a 4° angle of incidence, the angle of attack will also be 4°. The aircraft's total weight is supported on the main wheels and the nosewheel. The rudder will be held deflected in a position to counter the initial slipstream and torque effects, with applied rudder pressure reducing as acceleration progresses.
Ground roll. As the ground roll accelerates — because thrust is greater than the rolling friction plus total drag — the airflow velocity increases. At a speed perhaps 20% below Vs1 the elevators should have enough authority so that a little back pressure on the control column will provide sufficient up-elevator to raise the nosewheel from the surface, and increase the aoa by 2 or 3 degrees to 6 or 7°.
This may slow the acceleration rate slightly but the reasons for getting the nosewheel off the ground earlier than really necessary — and holding it there — are:
As rolling speed builds, so does airspeed and lift. If you allow the aoa to increase beyond 6–7°, a flight velocity may be prematurely reached and the aircraft will lift itself off at an airspeed slightly above stall speed. In this condition, any slight turbulence or mishandling will cause a loss in lift and the aircraft will settle back again, or maybe just one wing drops and it hops about on one wheel. Obviously not a tidy departure; you, not the aircraft, must be in command of the take-off — and you must maintain alignment with some selected reference point throughout the take-off.
Rotation. Unless the aircraft manual, or flight school procedures for students specify otherwise, the usual technique is to hold the aircraft at 6–7° aoa until airspeed builds up to a lift-off speed [Vlof] 15–20% above Vs1, then apply further back pressure to rotate the airframe around the main wheels to an aoa of around 12°, and the aircraft will lift off smoothly and commence to climb away with sufficient airspeed in hand to deal with minor turbulence. Anticipate that P-factor effect will cause the aircraft to turn. Do not wait so long that the aircraft flies itself off; you, not the aircraft, should be in command. The increase in induced drag, which is greater than the removal of the rolling friction, will slow the acceleration rate. So, as the initial climb progresses, ease the stick forward until Vy is reached (at an aoa around 8°) and maintain maximum rate of climb at that speed until the planned threshold height is reached. For some aircraft it may be advisable to use Vtoss rather than Vy until a safe height is reached. In gusty wind conditions, it may be prudent to delay rotation until airspeed is perhaps 10% higher than normal.
Some nosewheel aircraft may have a tendency to pitch up rather rapidly during rotation and the pilot must be ready to arrest this with forward pressure on the control column.
Climb-out. Do not hold the aircraft down to build up speed beyond Vy and then pull up steeply — it displays poor airmanship and is extremely dangerous. Airspeed in a 'zoom' climb like that will drop off very quickly — possibly faster than the pilot can pitch the nose down — which may lead to an irrecoverable departure stall. Take-off procedure may vary a little if the aircraft is fitted with flaps that can be set to a position that provides increased lift without a significant increase in drag. There are other factors involved — see 'Practice good energy management in the take-off!'.
Unless stated otherwise in the Pilot's Operating Handbook or engine notes, maintain full throttle until a safe height is reached. The initial climb speed maintained would normally be Vy but if a fixed-pitch cruise propeller is fitted then an airspeed higher than Vy may be more effective. After the initial climb a higher 'enroute climb' airspeed may be the optimum choice to reduce sector time and to maintain engine temperatures within the manufacturer's specified bounds; full power and low airspeed will 'cook' an aero-engine.
Also, a lower pitch angle during climb improves forward visibility.
Normal take-off — tailwheel three-axis aircraftTailwheel aircraft are subject to all the effects mentioned in section 11.3, and have a fairly predictable mode of behaviour at ground speeds between 5 and 25 knots. They will want to swing and gyrate, and these movements must be anticipated and promptly corrected by the pilot.
Ground roll. At the start of the ground roll (again having lined up in the chosen direction and ensured that the tailwheel has trailed in the fore and aft position) the fuselage of a 'taildragger' is naturally pitched up at an angle of maybe 10–12°. Combined with the angle of incidence this means that the aoa at the start of the roll will be close to the stall aoa of about 15°; some aircraft with a high angle of incidence may be past the stall aoa.
As the throttle is being smoothly and fully opened at the start of the ground roll, slipstream and torque effects will be at their greatest. Consequently, normal procedure is to start the ground roll with compensating rudder applied, and with the elevators held in the fully up position to put pressure on the tail wheel. The friction of the tailwheel will assist in taming the initial convolutions, particularly if the tailwheel is steerable. However, the high aoa implies consequent high drag and slow acceleration.
The tailwheel is the weakest part of the undercarriage, so there is a need to relieve the loads on it as early as possible, particularly if the airfield surface is rough. Also the sooner the generation of lift begins to reduce tyre friction the faster the aircraft accelerates. Thus the requirement is to get the tailplane up reasonably soon so that, firstly, aoa is reduced to 6 or 7° or less; and thus the aircraft is able to pick up her skirts and run. Secondly, the lower the ground speed at which the aircraft's tail is raised, the gentler will be the swing from the ensuing gyroscopic effect. Thirdly, the sooner a near-level (i.e. slightly tail-down) attitude is achieved, the sooner the building P-factor effect is negated. However, remember that gyroscopic effect is also dependent on the rate of change of attitude in pitch, so ease the stick forward rather than shoving it forward. (Propeller surface clearance must be maintained so be careful on non-prepared strips.) Then, as lift-off speed is reached, rotation and climb-out proceeds as for a nosewheel aircraft.
Taildragger enthusiasts sometimes refer to the appearance of the aircraft during take-off — when the pilot holds it in a level, minimum drag, maximum acceleration attitude — as being 'on the step'; the term is borrowed from seaplane pilots.
At rest the seaplane's centre of buoyancy is usually under the forward limit of the aircraft's centre of gravity, while the location of the vortex-creating transverse step in the hull or float/s is usually just to the rear of the centre of buoyancy.
After water-taxying to the line-up position, the first part of a seaplane take-off involves getting the aircraft past the 'hump' speed where the aircraft is displacing the maximum amount of water, thereby creating the maximum hydrodynamic drag. (As with air resistance, water resistance also increases in proportion with the aircraft velocity squared.) The throttle is opened, while holding the control column right back, so that the thrust power (including the vertical component of the thrust line) combined with increasing lift from the high aoa quickly lifts the forward portion of the hull/floats above the surface and the hull or floats are 'ploughing' the water, i.e. pushing the water aside. The hump speed might be around 20–30 knots.
The second part of the take-off is to to minimise hydrodynamic drag by getting the aircraft operating as a planing hull where it is supported by the hydrodynamic reaction of the water, rather than just pushing the water aside. After attaining hump speed the control column is eased forward to reduce aoa and induced drag and then some back-pressure is applied. So, rather than ploughing through the water and unable to accelerate due to the very high hydrodynamic drag, the aircraft is riding (hydroplaning/aquaplaning) on the deepest part of the hull forward of the step in the hull or floats, total drag (hydrodynamic plus aerodynamic) is greatly reduced allowing the aircraft to accelerate 'on the step' to a speed where wing lift can both break the adhesive action of the water and support total aircraft weight. When on the step the vortices induced by the step break up boundary layer flow and reduce water adhesion to the hull.
The aircraft accelerates on the step until rotation speed is reached but unlike a runway take-off, hydrodynamic drag will increase during rotation because more of the hull/float surface is 'wetted' and the aircraft is pushing more water aside. Sufficient thrust must be available to overcome the increase in both the hydrodynamic drag and the induced drag at rotation, otherwise the aircraft can't lift off the water.
The 'step-taxying' term describes fast taxying with less than lift-off power. 'Ground' effect is still pertinent in waterborne operations.
The procedure is to maintain a more or less level minimum drag attitude — i.e. 4–5° aoa (with a nosewheel held just above the bumps) throughout the ground roll until Vx is reached — 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 that 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, because 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 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. It is better to set the flaps when doing the pre-take-off checks, when 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 better to smoothly run up to full power while the aircraft is rolling. There is 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. I suggest you read the article 'Tree's a crowd' in Flight Safety Australia September-October 2002 issue.
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 while holding the control column back. Using a maximum lift flap setting is usually highly recommended. As the elevators become effective, the nose of a nosewheel aircraft will rise. With a taildragger, the elevator pressure should be relaxed sufficiently so that the tailwheel is held off the surface but the aircraft remains firmly in a tail-down attitude. As ground speed builds, start relaxing the back pressure and the aircraft 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, so 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 5–6° 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.
The following is an extract from an RA-Aus incident report:
The pilot intended to conduct a trial instructional flight from a grass strip in excess of 250 metres in length. Due to recent rain the strip was soft and several solo take-offs had been carried out, each clearing the fence at the end of the strip by 25–30 m.
After some test runs with the passenger on board the pilot's 'gut feeling' was to abandon the exercise but he elected to take-off using a short field technique. The aircraft accelerated until the nosewheel lifted off the ground and then slowed, with the nosewheel sinking back onto the ground. Because he still had what he believed to be sufficient speed in hand the pilot tried to make it over the fence — and didn't. The damage to occupants was minor but the aircraft was a write-off.
The pilot identified the cause of the accident as lack of experience in operations from wet fields. In his words the aircraft was 'basically stuck to the field'
Coping with significant crosswindDuring 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 and pivot 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, and decrease the deflection 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 while 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 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 that counteracts the effect of the crosswind; the aircraft line of roll is 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, return the ailerons to neutral and level the wings.
To provide an additional safety margin, hold the aircraft on the ground for a higher-than-normal lift-off speed. If conditions are gusty, add 50% of the wind gust speed in excess of the mean wind speed; e.g. if wind speed is 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, gently turn the aircraft onto a new heading to 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 it means 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.
The turbulence from the wingtip vortices of aircraft at high angles of attack is particularly strong and a function of aircraft weight. For aircraft taking off, a high aoa is initiated at the start of rotation and continues through climb-out. The wake vortices sink and drift with the wind, and may take several minutes to dissipate. Thus, light aircraft must practise caution when departing behind another aircraft of similar weight, more so if it is a significantly heavier aircraft, as the turbulence will readily roll the aircraft on to its back or worse. When following a very large aircraft note the runway position where the aircraft rotated, wait perhaps two minutes for the wake to dissipate a little, aim to be airborne well before the noted runway position and, where there is any crosswind component, maintain a line along the upwind side of the runway. For a little more information see Aircraft wake vortices in the 'Microscale meteorology and atmospheric hazards' module.
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A take-off towards rising terrain is not something that should be undertaken without a thorough check of all conditions, even if such a take-off has previously been 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 heightThe 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 another look at the 'Olly's Folly' airstrip on that hot summer afternoon. Here there is 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 towards 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 neededWe 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. lee side downflow in the meteorology section.
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accelerated stall, finding that 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. The table in section 2.8 shows that wing loading increases slowly up to a bank angle of 30° — where it is 15% greater than normal — after which it increases rather rapidly — where it is 41% greater at a 45° bank angle. We then concluded that turns involving bank angles exceeding 20–25° should not be made at low levels — including take-off and landing.
The wing loading increase in the turn is provided by an increase in CL, which 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. There will be 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 is likely to result in a stall/spin situation — which 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 provide no margin for even minor turbulence or slight mishandling. The margin you should always have in hand to cope with such events is 3 or 4°. This 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.
Further readingThe online version of CASA's magazine Flight Safety Australia contains articles relating to take-off that are recommended reading. Look under 'Take-off and landing' in the 'Further online reading' page.
The next module in this Flight Theory Guide discusses landing an aircraft together with the associated circuit and approach to landing.
Things that are handy to know
• The description taildragger is used as a generic term applied to all tailwheel aircraft. However, this is not strictly correct; a true taildragger is an aircraft equipped with a tail skid rather than a tailwheel. Aeroplanes so equipped are usually not fitted with main wheel brakes and they are designed for operation from grass airfields where all take-offs and landings can be made into wind. Such aircraft have little resistance to swinging if operated from a sealed, smooth surface.
• Many tailwheel aircraft will have a steerable tailwheel, which improves the aircraft performance during crosswind operations and makes ground handling easier in windy conditions, particularly if not equipped with differential braking. The steerable tailwheel is usually linked to rudder movement and the rudder pedals in some way. But the tailwheel aircraft may have a disconnect feature that allows the tailwheel to fully castor, thereby improving manoeuvring when parking. The steering mechanism may automatically disconnect when weight is off the tailwheel, in which case a spring or other device returns the tail wheel to a low-drag fore and aft alignment. A nosewheel aircraft may be similarly equipped with a steerable nosewheel.
Stuff you don't need to know
• One of the most successful fighter aircraft of the 1914–18 war was the Sopwith Camel, fitted with a 130 hp rotary engine. The liquid-cooled engines of the day were very heavy and the rotary was designed to utilise air cooling of the cylinders, thus producing a lighter engine for fighter aircraft. The engine rotated around the crankshaft, which was fixed to the airframe. The propeller was bolted to the crankcase so that the engine and propeller rotated as a unit and, because of the flywheel effect, ran very smoothly at normal cruise settings around 1200–1300 rpm.
However, as can be imagined, the torque and gyroscopic effects were extreme and such that the aircraft turned very sluggishly to the left but was lightning fast in a turn to the right. If a 90° turn to the left was required it was faster to initiate a 270° turn to the right. In a left turn the aircraft wanted to climb, while in a right turn the gyroscopic effect pushed the nose down. The aircraft was very unstable, hence very manoeuvrable.
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Groundschool – Flight Theory Guide modules
| Flight theory contents | 1. Basic forces | 1a. Manoeuvring forces | 2. Airspeed & air properties |
| 3. Altitude & altimeters | 4. Aerofoils and wings | 5. Engine & propeller performance | 6. Tailplane surfaces |
| 7. Stability | 8. Control | 9. Weight & balance | 10. Weight shift control | [11. Take-off considerations] |
| 12. Circuit & landing | 13. Flight at excessive speed | 14. Safety: control loss in turns |
| Operations at non-controlled airfields | Safety during take-off & landing |
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