getting to grips with aircraft performance 如何掌握飞机性能 1(29)

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“JAR/FAR 25.115
(a) […] from 35 ft above the takeoff surface at the end of the takeoff distance [to the end of the takeoff path]”
NET Flight Path = Gross takeoff flight path minus a mandatory reduction.
“JAR/FAR 25.115
(b) The net takeoff flight path data must be determined so that they represent the actual [Gross] takeoff flight path reduced at each point by a gradient equal to:
.
0.8% for two-engine aeroplanes

.
1.0% for four-engine aeroplanes”

Net Gradient = Gross Gradient - Gradient Penalty

Table C7: Values of Gradient Penalties
The gradient penalty between the net and the gross flight path must be taken into account during the first, second, and final takeoff segments (Figure C17).

4.2.2. Obstacle Clearance during a Straight Takeoff

“JAR–OPS 1.495
(a) An operator shall ensure that the net take-off flight path clears all obstacles by a vertical distance of at least 35 ft.”
As an example, the minimum required climb gradient during the second segment must be 2.4% for a two-engine aircraft. But, as per regulation, the net flight path must clear any obstacle by at least 35 feet (Figure C17). This may sometimes require the second segment gradient to be greater than 2.4% and, consequently, the Maximum Takeoff Weight may have to be reduced accordingly. This is a case of obstacle limitation.
4.2.3. Obstacle Clearance during a Turn
Once again, the obstacle clearance margins during a turn differ between JAR and FAR regulations. The FAR regulation doesn’t consider any additional vertical margin during a turn, as the bank angle is limited to 15o. The following rule is then purely JAR-OPS:

“JAR-OPS 1.495
(c)(2) Any part of the net take-off flight path in which the aeroplane is banked by more than 15° must clear all obstacles […] by a vertical distance of at least 50 ft.”

4.2.4. Loss of Gradient during a Turn
During a turn, an aircraft is not only subjected to its weight (W), but also to a horizontal acceleration force (Fa). The resulting force is called “apparent weight” (Wa), and its magnitude is equal to the load factor times the weight (nz.W).

Considering the above Figure C18, the load factor (nz) can be expressed versus the bank angle (Φ) as follows:
1
n=
z cosφ

So, as soon as the aircraft is banked, the load factor becomes greater than one. This induces a loss of climb gradient, as the climb angle can be expressed as follows (refer to the “Climb” chapter) :
Thrust 1
γ
% =.
nz .Weight L/D

“AMC OPS 1.495 (c)(4) The Aeroplane Flight Manual generally provides a climb gradient decrement for a 15° bank turn. For bank angles of less than 15°, a proportionate amount should be applied, unless the manufacturer or Aeroplane Flight Manual has provided other data.”
　
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