Flight Envelop - Cơ học và tính năng tàu bay | Học viện Hàng Không Việt Nam
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Môn: Cơ học và tính năng tàu bay (ĐHKL01)
Trường: Học viện Hàng Không Việt Nam
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Học viện Hàng Không Việt Nam – Khoa Khai thác HK
Học viện Hàng Không Việt Nam – Khoa Khai thác HK Phần 1 Phần 2 Phần 3 Phần 4 Phần 5 High Buffet onset Flight Airspeed Altitude speed chart Envelope Limitation Envelope shock stall
Học viện Hàng Không Việt Nam – Khoa Khai thác HK
1. High speed shock stall
Constant altitude acceleration
Consider an aircraft in constant altitude acceleration phase. In level
flight, the lift balances the weight. The following can thus be written: 1 L
.S.V ².C W . m g 2 L
In cruise flight, on the airliners, the speed is generally expressed in
Mach number. To be able to monitor the occurrence of transonic
phenomena via the Mach number. P V .
a M RT .M M
Học viện Hàng Không Việt Nam – Khoa Khai thác HK
1. High speed shock stall
Constant altitude acceleration
The previous lifting equation can therefore be written as:
P.S.M².CLm.g 2 1.4 0, 7 .
P S.M ².C . m g L
As the altitude remains constant during acceleration, the pressure P also remains constant.
To provide the same lift during acceleration, the aircraft must fly with:
M ².C const L
Học viện Hàng Không Việt Nam – Khoa Khai thác HK
1. High speed shock stall
Effect of Mach on the polar curve
Up to Mach 0.7, we can consider a single polar. Beyond this number,
a "collapse of the polar curve" is observed: the drag coefficient
increases whereas the lift coefficient decreases. C 0.7 L 0.75 0.8 0.82 0.84 CD
Học viện Hàng Không Việt Nam – Khoa Khai thác HK
1. High speed shock stall
Effect of Mach on the polar curve
Consider an aircraft at Mach 0.5, at equilibrium and flying at a certain
angle of attack. It is characterized by a point on the polar curve (point
N). From the operating point N, during a level acceleration, the aircraft describes the red curve: CL B: "low" speed stall 0.7 B 0.75 N 0.8
A: high speed "shock" stall 0.82 A 0.83 0.84
Học viện Hàng Không Việt Nam – Khoa Khai thác HK 2. Buffet onset chart
Curve CLmax.M² = f(M)
For a given wing, at a given altitude, there is a maximum lift for each
Mach number, reached for the maximum lift coefficient C . This Lmax
maximum lift has the following expression: m
L ax P.S.CLmax.M² 2 The term C
.M² represents the maximum wing lift capacity; beyond Lmax
this capacity, stall occurs. The curve C
.M² = f(M) represents the limit of the flight envelope Lmax
for aerodynamic (lift) reasons. It is called "buffet onset chart".
Học viện Hàng Không Việt Nam – Khoa Khai thác HK 2. Buffet onset chart
Curve CLmax.M² = f(M)
Note: buffeting is a vibration phenomenon annunciating stall at low
speeds as well as high speeds.
Representation of the C .M² C = Cst L Lmax B buffet onset chart for a subsonic wing: For M < 0,7 : C = Lmax Cst – the curve C .M² is a Lmax parabola. A
For M 0,7 : the curve C .M² starts Lmax to increase then collapses. M
Học viện Hàng Không Việt Nam – Khoa Khai thác HK 2. Buffet onset chart C .M² Coffin corner L Mach range
From the flight point A, the aircraft can: Accelerate in level flight: M C B
increases up to the "shock stall" A
point (B); as CL.M² must remain constant, CL must decrease to
accelerate at constant Mach, the
pilot has to reduce its angle of attack 1 M
by pushing the stick forward. Slow down in level flight: M
decreases up to the "low-speed
stall" point (C); in this case, the pilot
has to increase the angle of attack by pulling the stick back.
Học viện Hàng Không Việt Nam – Khoa Khai thác HK 2. Buffet onset chart Aerodynamic ceiling In the lift equation: 0, 7 .
P S.M ².C . m g L
If: Altitude ↑ P decrease C ↑ L
Conclusion: The more the altitude increases, the more CL must increase.
The limit is reached for C
(point D), which corresponds to the Lmax
maximum flight altitude for the selected mass and speed: this value is
the aerodynamic ceiling and is written Z . max
Học viện Hàng Không Việt Nam – Khoa Khai thác HK 2. Buffet onset chart Aerodynamic ceiling Absolute ceiling C .M² L Z D The curve can be graduated in altitude D: A
aerodynamic ceiling at a
given Mach, mass and load factor 1 M
Học viện Hàng Không Việt Nam – Khoa Khai thác HK 2. Buffet onset chart Maximum mass
In the lift equation: 0, 7P.S.M ².C m.g L
If: Mass ↑ C ↑ L
Conclusion: The more the altitude increases, the more CL must increase.
The limit is reached for C
(point D), which corresponds to the Lmax
maximum mass possible for the selected altitude and Mach. As
observed previously, the curve can be graduated in mass.
Học viện Hàng Không Việt Nam – Khoa Khai thác HK 2. Buffet onset chart Maximum mass C .M² L m n D The curve can be graduated in mass or in load factor A M 1
D: maximum mass at a given Mach, altitude and load factor
Học viện Hàng Không Việt Nam – Khoa Khai thác HK 2. Buffet onset chart
We defined a set of curves giving the limits of the flight envelope for
aerodynamic reasons. For example, at a given mass and altitude, if
the aircraft accelerates, stall will occur for a Mach value displayed on the curve.
At a given Mach, a pilot cannot select any altitude, or carry out too close manoeuvres.
At a given altitude or for a given mass, a pilot cannot select a
Mach number too high or too low.
Học viện Hàng Không Việt Nam – Khoa Khai thác HK 2. Buffet onset chart C .M² L Z m n M Mstall
Học viện Hàng Không Việt Nam 2. Buffet onset chart
Học viện Hàng Không Việt Nam – Khoa Khai thác HK
3. Flight envelope (or service envelope, or performance envelope)
The airplane must exhibit acceptable flying qualities at the most critical
loading within the ranges of speed and altitude for which certification is requested.
− The airline pilot is provided with a safe operational flight
envelope (bounded by certificated limits) that has been
thoroughly explored during flight testing.
− Aerodynamic (stall) limits, Structural (g) limits and Dynamic pressure (q) limit.
Học viện Hàng Không Việt Nam – Khoa Khai thác HK
3. Flight envelope (or service envelope, or performance envelope) + "q" Limit Stall limit Structural Limit n V -
Note: Flight within the “envelope” is possible
without stalling or damaging the aircraft.
(Safe operating region = inside the box)
Học viện Hàng Không Việt Nam – Khoa Khai thác HK
3. Flight envelope (or service envelope, or performance envelope)
Học viện Hàng Không Việt Nam – Khoa Khai thác HK
3. Flight envelope (or service envelope, or performance envelope)
Aerodynamic (stall) limit
o The aerodynamic limit is a “lift limit” and occurs where the aircraft stalls
o Varies as a function of the square root of weight and load factor
o When stalling while above 1 g, it’s known as an “accelerated stall 2 n W V stall S C L m ax