smc-3724-70-Pressure

ENGINEERING, AE 2025 HSC 24c

Describe the operation of a mechanical altimeter. Use a labelled diagram to support your answer. (4 marks)

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A mechanical altimeter measures altitude by detecting changes in atmospheric pressure.
Air enters through a static vent and acts on a sealed aneroid capsule inside the instrument.
As altitude increases, external air pressure decreases, causing the capsule to expand.
This expansion drives a mechanical linkage connected to a pointer on the display dial, indicating altitude.

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A mechanical altimeter measures altitude by detecting changes in atmospheric pressure.
Air enters through a static vent and acts on a sealed aneroid capsule inside the instrument.
As altitude increases, external air pressure decreases, causing the capsule to expand.
This expansion drives a mechanical linkage connected to a pointer on the display dial, indicating altitude.

♦♦ Mean mark 38%.

ENGINEERING, AE 2025 HSC 17 MC

An aircraft is flying at a cruising altitude where the external atmospheric pressure is 20 kPa. Inside the aircraft's fuel tank, a column of aviation fuel has a density of 800 kg/m³ with a height of 1.5 m.

What is the pressure at the bottom of the fuel column?

10 940 Pa
20 813 Pa
32 000 Pa
300 800 Pa

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\(C\)

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\(P_{\text{hydrostatic}}= \rho g h= 800 \times 10 \times 1.5 = 12\ 000\ \text{Pa}\)

\(P_{\text{total}}= P_{\text{atmospheric}} + P_{\text{hydrostatic}}= 20\ 000 + 12\ 000 = 32\ 000\ \text{Pa}\)

\(\Rightarrow C\)

ENGINEERING, AE 2024 HSC 8 MC

The cross-section of a tapered tube is shown.

What are the pressure readings at both gauge \(A\) and at gauge \(B\) as the air flows through the tube?

\begin{align*}
\begin{array}{l}
\rule{0pt}{2.5ex} \ & \rule[-1ex]{0pt}{0pt} \\
\rule{0pt}{2.5ex} \textbf{A.} \rule[-1ex]{0pt}{0pt} \\
\rule{0pt}{2.5ex} \textbf{B.} \rule[-1ex]{0pt}{0pt} \\
\rule{0pt}{2.5ex} \textbf{C.}\rule[-1ex]{0pt}{0pt} \\
\rule{0pt}{2.5ex} \textbf{D.}\rule[-1ex]{0pt}{0pt} \\
\end{array}
\begin{array} {|c|c|}
\hline
\rule{0pt}{2.5ex} \textit{Pressure at gauge A} \rule[-1ex]{0pt}{0pt} & \textit{Pressure at gauge B} \\
\hline
\rule{0pt}{2.5ex} \text{High} \rule[-1ex]{0pt}{0pt} & \text{High} \\
\hline
\rule{0pt}{2.5ex} \text{High} \rule[-1ex]{0pt}{0pt} & \text{Low} \\
\hline
\rule{0pt}{2.5ex} \text{Low} \rule[-1ex]{0pt}{0pt} & \text{Low} \\
\hline
\rule{0pt}{2.5ex} \text{Low} \rule[-1ex]{0pt}{0pt} & \text{High} \\
\hline
\end{array}
\end{align*}

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\(B\)

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According to Bernoulli’s principle, as the air velocity increases through the tapered section, the pressure must decrease.
This results in high pressure at the wide entrance (gauge A) where velocity is low and low pressure at the narrow exit (gauge B) where velocity is high.

\(\Rightarrow B\)

♦ Mean mark 46%.

ENGINEERING, AE 2023 HSC 6 MC

The landing gear on an aircraft uses a hydraulic braking system. A force of 60 N is applied at the master cylinder with a piston diameter of 12 mm.

What is the force at the brake calliper with a piston diameter of 42 mm ?

4.9 N
17.14 N
210 N
735 N

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\( D \)

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Pressure is the same in both master cylinder and brake calliper \(\big(P = \frac{F}{A}\big) \)
Convert units: \(12\ \text{mm} = \dfrac{12}{1000} = 0.012\ \text{m, 42 mm}\ =\dfrac{42}{1000} = 0.042\ \text{m} \)
Using \(\dfrac{F_2}{A_2} = \dfrac{F_1}{A_1} \):
\(F_2 = \dfrac{F_1 \times A_2}{A_1} = \dfrac{60 \times \ \pi \times 0.021^2}{\pi \times 0.006^2} = 735\ \text{N} \)

\(\Rightarrow D \)

♦ Mean mark 45%.

ENGINEERING, AE 2017 HSC 6 MC

The diagram shows an aerofoil.

Which condition needs to be achieved for lift to occur?

Pressure 1<Pressure 2
Pressure 1= Pressure 2
Pressure 1> Pressure 2
Pressure 1+ Pressure 2=0

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`A`

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For a net upwards force, the force from pressure below (pushing up) must exceed the force from pressure above (pushing down).
Therefore Pressure 1<Pressure 2.

`=A>`

ENGINEERING, AE 2017 HSC 26a

The flaps of an executive jet are controlled using a hydraulic system. A force of 1 kN acts on the 40 mm diameter master piston.

What force would need to be developed to move the flap if the slave piston has a diameter of 100 mm? (3 marks)

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`6.25\ text{kN}`

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`P=F/A`

`1000/(pi xx 20^2)`	`=F_S/(pi xx 50^2)`
`F_S`	`=(1000 xx pi xx 50^2)/(pi xx 20^2)=6250\ text{N}=6.25\ text{kN}`

ENGINEERING, AE 2019 HSC 10 MC

The diagram shows the airflow in a venturi.

What does this diagram illustrate?

Coriolis effect
Pascal's principle
Dynamic pressure
Bernoulli's principle

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`D`

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Bernoulli’s principle states that the higher the pressure is, the lower the velocity will be.
This is illustrated in the diagram as it shows that there is a decreased pressure in the thinner, higher velocity section.

`=>D`

♦ Mean mark 55%.

ENGINEERING, AE 2020 HSC 22d

An air gauge contains air at 750 kPa (gauge) while atmospheric pressure is at 100 kPa. The air at absolute pressure operates a piston of 15 mm diameter.

Calculate the force the compressed air exerts on the piston. (4 marks)

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`150.45\ text{N}`

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`P_(ga)=750\ text{kPa}, \ P_(atm)=100\ text{kPa}, \ D=15\ text{mm}`

`P`	`=P_0+rhogh`
`P_(abs)`	`=P_(atm)+P_(ga)= 750 + 100= 850\ text{kPa}`

`P_(abs)`	`=F/A`
`F`	`=P_(abs)xxA`
`A`	`=(piD^2)/4=(pixx0.015^2)/4=1.77xx10^(-4)\ text{m}^2`

`:.\ F=850xx10^3xx1.77xx10^(-4)= 150.45\ text{N}`

♦♦ Mean mark 40%.

ENGINEERING, AE 2021 HSC 9 MC

A pitot tube supplies two pressure readings, total pressure and static pressure.

These pressure readings are then used to determine the

sealed pressure.
dynamic pressure.
standard pressure.
hydrostatic pressure.

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`B`

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Dynamic Pressure = Total − Static

`=>B`