Stalling 7
Aircraft Behaviour Close to the Stall
Stall characteristics vary with different types of aircraft. However, for modern aircraft during most normal manoeuvres, the onset of stall is gradual. The first indications of a stall may be provided by any or all of the following:
•unresponsive flight controls,
•a stall warning or stall prevention device, or
•aerodynamic buffet.
The detailed behaviour of various aircraft types will be discussed later.
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Use of Flight Controls Close to the Stall
At low speeds normally associated with stalling, dynamic pressure is at a very low value and greater control deflection will be required to achieve the same response; also, the flying controls will feel unresponsive or “mushy”. If an accidental stall does occur, it is vitally important that the stall and recovery should occur without too much wing drop. Moving a control surface modifies the chord line and, hence, the angle of attack. An aircraft being flown close to the stall angle may have one wing that produces slightly less lift than the other; that wing will tend to drop. Trying to lift a dropping wing with aileron will increase its angle of attack, Figure 7.3, and may cause the wing to stall completely, resulting in that wing dropping at an increased rate. At speeds close to the stall, ailerons must be used with caution. On straight wing aircraft the rudder should be used to yaw the aircraft just enough to increase the speed of a dropping wing to maintain a wing’s level attitude. Swept wing aircraft basic stall requirements are designed to enable the ailerons to be used successfully up to ”stall recognition” (Page 148 and Page 154), but small amounts of rudder can be used if smoothly applied and co-ordinated with the ailerons.
15º |
22º |
Figure 7.3
Stalling
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7 Stalling
Stall Recognition
The aeroplane is considered stalled when the behaviour of the aeroplane gives the pilot a clear and distinctive indication of an acceptable nature that the aeroplane is stalled.
Acceptable indications of a stall, occurring either individually or in combination, are:
(1) |
A nose down pitch that cannot be readily arrested; |
(2) |
Buffeting, of a magnitude and severity that is a strong and effective deterrent to further |
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speed reduction; or |
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(3) |
The pitch control reaches the aft stop and no further increase in pitch attitude occurs |
Stalling |
when the control is held full aft for a short time before recovery is initiated. |
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Stall Speed
It is necessary to fly at slow speeds (high angles of attack) during take-off and landing in order to keep the required runway lengths to a reasonable minimum. There must be an adequate safety margin between the minimum speed allowed for normal operations and the stall speed.
Prototype aircraft are stalled and stall speeds established for inclusion in the Flight Manual during the flight testing that takes place before type certification.
“Small” aircraft (CS-23) use VS0 and VS1 on which to base the stall speed.
For “Large” aircraft (CS-25) a reference stall speed, VSR , is used.
•The reference stall speed (VSR ) is a calibrated airspeed defined by the aircraft manufacturer. VSR may not be less than a 1g stall speed. VSR is expressed as:
VSR ≥
VCLMAX
√ nZW
Where:
VCLMAX = Calibrated airspeed obtained when the load factor corrected lift coefficient is first a maximum during the manoeuvre prescribed in the starred bullet point on page 149.
In addition, when the manoeuvre is limited by a device that abruptly pushes the
nose down at a selected angle of attack (e.g. a stick pusher), VCLMAX may not be less than the speed existing at the instant the device operates.
nZW |
= |
Load factor normal to the flight path at VCLMAX |
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Stalling 7
Note: On aircraft without a stick pusher, VSR can be considered to be the same as the 1g stall speed (VS1g ). But it is impossible to fly at speeds less than that at which the stick pusher activates, so for aircraft fitted with a stick pusher, VSR will be 2 knots or 2% greater than the speed at which the stick pusher activates. (See Figure 7.4 and Figure 7.5 for an illustration of the designations of stall speed and stall warning).
From the “sample” aeroplane on Page 76, the speed at CLMAX was 150 kt. This can be considered as that aeroplane’s VCLMAX . At 1g, VSR would therefore be 150 kt.
•VCLMAX is determined with:
•Zero thrust at the stall speed.
•Propeller pitch controls (if applicable) in the take-off position.
•The aeroplane in other respects (such as flaps and landing gear) in the condition existing in the test or performance standard in which VSR is being used.
•The weight used when VSR is being used as a factor to determine compliance with a required performance standard.
•The centre of gravity position that results in the highest value of reference stall speed; and
•The aeroplane trimmed for straight flight at a speed selected by the manufacturer, but not less than 1.13VSR and not greater than 1.3VSR.
•*Starting from the stabilized trim condition, apply the longitudinal control to decelerate the aeroplane so that the speed reduction does not exceed one knot per second.
•In addition to the requirements above, when a device that abruptly pushes the nose down
at a selected angle of attack (e.g. a stick pusher) is installed, the reference stall speed, VSR , may not be less than 2 knots or 2%, whichever is the greater, above the speed at which the device operates.
VSR will vary with each of the above conditions. Additional factors which affect VSR are load factor, thrust in excess of zero and wing contamination. All these effects will be detailed later.
Density altitude does not affect indicated stall speed
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7 Stalling
Stall Warning
Having established a stall speed for each configuration, there must be clear and distinctive warning, sufficiently in advance of the stall, for the stall itself to be avoided.
(a) |
Stall warning with sufficient margin to prevent inadvertent stalling with the flaps and |
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landing gear in any normal position must be clear and distinctive to the pilot in straight |
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and turning flight. |
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(b) |
The warning may be furnished either through the inherent aerodynamic qualities of |
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the aeroplane or by a device that will give clearly distinguishable indications under |
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expected conditions of flight. However, a visual stall warning device that requires the |
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attention of the crew within the cockpit is not acceptable by itself. If a warning device |
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is used, it must provide a warning in each of the aeroplane configurations prescribed in |
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sub-paragraph (a) of this paragraph at the speed prescribed in sub-paragraphs (c) and |
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(d) of this paragraph. |
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(c) |
When the speed is reduced at rates not exceeding 1 knot per second, stall warning |
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must begin, in each normal configuration, at a speed, VSW , exceeding the speed at |
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which the stall is identified in accordance with Stall Recognition, on page 148, by not |
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less than 5 knots or 5% CAS, whichever is the greater. Once initiated, stall warning |
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must continue until the angle of attack is reduced to approximately that at which stall |
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warning began. |
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(d) |
In addition to the requirements of sub-paragraph (c) of this paragraph, when the speed |
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is reduced at rates not exceeding one knot per second, in straight flight with engines |
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idling and CG position specified on page 149, VSW, in each normal configuration, must |
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exceed VSR by not less than 3 knots or 3% CAS, whichever is greater. |
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(e) |
The stall warning margin must be sufficient to allow the pilot to prevent stalling (as |
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defined on page 148 - Stall Recognition) when recovery is initiated not less than one |
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second after the onset of stall warning in slow-down turns with at least 1.5g load factor |
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normal to the flight path and airspeed deceleration rates of at least 2 knots per second, |
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with the flaps and landing gear in any normal position, with the aeroplane trimmed for |
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straight flight at a speed of 1.3VSR , and with the power or thrust necessary to maintain |
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level flight at 1.3VSR . |
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(f) |
Stall warning must also be provided in each abnormal configuration of the high |
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lift devices that is likely to be used in flight following system failures (including all |
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configurations covered by Flight Manual procedures). |
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