7AAD0038 Flight Mechanics - University of Hertfordshire

Assignment - Analysis of the downwash at the tail of an aircraft

Learning Outcome 1: Develop a knowledge and understanding of aircraft response under a wide range of flight conditions.
Learning Outcome 2: Describe or analyse an aircraft's typical dynamic behaviour under a wide range of flight conditions.
Learning Outcome 3: Investigate the static stability and control characteristics of an aircraft under a wide range of conditions.

Assignment Brief: The aim of this assignment is to reinforce student knowledge of aircraft dynamics and static stability through practical experience with a flight simulator.

The assignment involves working through the exercises identified on the attached sheet and gathering the necessary data to answer the specific questions. You will then write and submit a short report in accordance with the requirements given in the attached briefing sheet.

Flight Mechanics: Flight Simulator Assignment

You will carry out three exercises. The first is a familiarisation exercise and should not be included in your report.

The second and third exercises are flight tests to look at various aspects of aircraft stability and control and should comprise the content of your report (sections A and B).

Exercises for section A and B need to be carried out by students working in pairs or groups of up to four students. If you would like help finding a group of students to work with, please contact a member of the module teaching team.

Familiarisation exercise

The purpose of the familiarisation exercise is to:

• ensure that you are familiar with the basic controls of the aircraft in pitch, roll and yaw.
• demonstrate and provide hands-on experience of trimmed flight - i.e. straight and level (S&L) flight requiring no continuous force applied to the control stick.

Section A: Longitudinal static stability investigation

You will carry out an exercise to investigate the longitudinal static stability of the aircraft. The aim is to investigate how centre of gravity (CG) position affects the longitudinal stability of the aircraft and determine the necessary trim settings for different speeds and altitudes.

Task

A.1. Select a suitable range of CG's to test (minimum of five, and more preferably) covering both forward and aft of the default CG position.
A.2. Attempt to establish longitudinal trim by using the longitudinal elevator/trim controls for speeds in the range of 60
- 120 knots.
A.3. Assess the level of static stability by perturbing the aircraft gently out of trim and noting its behaviour.
Collect sufficient data to plot the following:
• Change in longitudinal trim control settings against airspeed, VT, and lift coefficient, CL, for different CG positions.
• Change in longitudinal trim settings against different CG positions.
A.5. Using your data or plots, calculate the CG margin for each test case, as well as the neutral point for your aircraft.

Section B: Phugoid flight test exercise

The purpose of the phugoid exercise is to:
(a) Initiate a phugoid and record the relevant data.
(b) Demonstrate the effect of CG position on phugoid period and damping characteristics.
(c) Deduce the aerodynamic drag coefficient in the given flight condition from time to half amplitude.

Phugoid Response Test - In Straight and Level Flight
The phugoid can be initiated by implementing a sudden dive or climb to achieve a target pitch angle, and then releasing the controls again. The characteristics of the phugoid vary with the position of the CG.

Task
B.1. Initiate the aircraft with a CG position of 10%. Establish longitudinal trim at 10,000ft and a speed of 250kt EAS and 75mm throttle as above.
B.2. Apply 15o nose down or nose up pitch and release the stick. Allow aircraft to respond. Switch on the event marker
B.3. Allow the aircraft several minutes to respond in order to obtain several cycles. Switch off the event marker
B.4 Repeat the above steps for CG positions of 20% and 30%.

Phugoid Analysis for Aircraft Disturbed from Straight and Level Flight

Carry out the following analysis for THREE CG positions.
(i) Determine the period of the motion by plotting TAS against time.
(ii) Determine the time to half amplitude from the equation:

t_1/2 = Δt log_e2/log_e(x₁/x₂)

where x₁, x₂ are amplitudes Δt seconds apart on TAS against time plot.

(iii) Compare the measured period against the theoretical period prediction given by:

T_theory = 0.4528V_TAS m/s = 0.2331VTAS kts

(iv) Calculate C_L for the aircraft in the steady flight condition.

(v) Calculate C_D using the formula:

μ = - g/U C_D/C_L

where µ is the real root of the characteristic equation.

(vi) Use your value of μ to determine a value of the natural undamped frequency.

Attachment:- Analysis of the downwash at the tail of an aircraft.rar