1. A fighter aircraft has built-in instability level of 4% (SM = -0.04) for most part of its flight in subsonic flights (M < 1). Then the following statement describes nature of its longitudinal stability. Retain correct option from amongst underlined words by striking off wrong ones: In supersonic flight (M >1) at same CG as in subsonic flight, the aircraft will have: increased instability of 21% OR substantially large static margin (SM) of 21% OR nominal SM of 4% Why? Draw sketches to support your choice for M < 1 & M > 1 showing CG, NP and instability level or SM as applicable on MAC.
0.04
0.21
Instability level = 4% : CG behind NP for M< 1 Aft shift in NP from M< 1 to M> 1 approximately 0.25 Shift in NP SM changes from -0.04 for M< 1 to +0.21 for M>1
CG
0.25 NP: M< 1
NP:M> 1
2. An aircraft with built-in longitudinal stability has a Canard surface located ahead of its wing instead of a horizontal tail behind the wing. Stall angle of the Canard is given to be 16°, lower than wing stall angle of 18°. Then two aerodynamic aerodynamic features of the aircraft with with Canard describing Trim Drag and Stall are one amongst the choices given below. Retain correct option by striking off wrong wrong ones: The aircraft has trim drag benefit OR trim drag penalty OR same trim drag as compared to aft tail configuration which is stable. Illustrate your answer by suitable sketches. LC Canard Load LW Wing Lift
CG W = LW + LC
W
Lower Wing lift results in reduction in induced drag compared to aft tail
Also the aircraft stalls at 16° OR 18° OR does not stall at all. Explain why? Aircraft does not stall at all! The moment Canard stalls the aircraft nose drops and this prevents the canard from stalling. Page 1/5
3. Neutral Point (NP) of an aircraft is often seen as aerodynamic property of the aircraft. Nature of aerodynamic flow over an aircraft flying near ground is different than the flow it experiences when it flies up and away from the ground. Then the following statements bring out essential differences in the two flows and their effects. Strike out wrong answers retaining correct ones: Retain correct option by striking off wrong ones: When flying close to ground wing downwash field at tail increases OR decreases. The horizontal tail contribution to aircraft stability increases OR decreases. Accordingly NP of the aircraft moves forward OR aft in ground effect. Explain with suitable sketches changes in the flow at the tail with and without ground effects and corresponding pitching moment characteristics.
Tail experiences a reduced downwash field due to image system of wing tip vortex pair near ground and the NP shifts aft making the SM higher in ground effects. 4.
A subsonic aircraft is trimmed in level flight at some altitude h 1 at an angle of attack α = 4° using an elevator deflection of δ e1. The aircraft with same CG and weight flying at higher altitude of h 2 (> h1) needs higher α of 8° to fly level and uses elevator of δ e2 to trim. For manoeuvring the same aircraft at α = 6° at the altitude of h 1, the elevator required is δ e3. For these three flight conditions one or more of the following is/are correct. Given δe0 is elevator angle intercept value extrapolated for α = 0 case a) δe2 = 2 δe1 – δe0 and δe2 > δe3 b) δe2 = 2 δe1 + δe0 and δe2 < δe3 c) δe2 = 2 δe1 – δe0 and δe2 < δe3 d) δe2 = 2 δe1 + δe0 and δe2 > δe3 Draw δe Vs α variation for above three flight conditions to explain your choice
Page 2/5
5.
A transport aircraft has positively cambered wing and symmetric airfoil for its aft tail. The AC of the wing for this aircraft is found to be located behind the aircraft CG. For such an aircraft, ONE or MORE of the following is/are correct a) both fuselage and wing contributions to longitudinal stability are negative b) both fuselage and wing contributions to longitudinal stability are positive c) fuselage contribution to longitudinal stability is negative and the wing contribution positive d) fuselage contribution to longitudinal stability is positive and the wing contribution is negative CM Support your choice by suitable plots/sketches Fuselage
α CG
AC Wing
Wing
Fuselage contribution for stability is always negative. For aircraft CG ahead of wing AC the wing contribution for stability is positive
6.
Trim tab is provided at trailing edge of elevator. One of the following statements is correct: a) trim tab is deflected in the same direction as the elevator to relieve pilot load by reducing net control surface hinge moment to zero b) trim tab is deflected in the direction opposite to the elevator to relieve pilot load by reducing net control surface hinge moment to zero c) to provide additional lift for maneuverability d) none of the above Draw suitable sketches to explain your choice
Tab is deflected opposite to elevator deflection to trim elevator hinge moment. (stick force zero )
Page 3/5
7.
The CG of a transport aircraft at take off was at 10% MAC (behind leading edge of the mean aerodynamic chord). After 5 hours of flight, the CG location moved to 13% MAC. Further the CG moved to 8% MAC location after 8 hours of flight, prior to landing. The stick fixed neutral point NP of the aircraft is at 16% MAC. Then ONE or MORE of the following is/are correct: a) static margin SM initially reduces after take off and increases at landing b) static margin SM initially increases after take off and decreases at landing c) assuming cruise C L to be nearly same, the elevator angle (δ e) to trim the aircraft after 5 hours of flight, increases and then δ e decreases in cruise segment prior to its descent for landing d) assuming cruise C L to be nearly same, the elevator angle (δ e) to trim the aircraft after 5 hours of flight, decreases and then δ e increases in cruise segment prior to its descent for landing Support your choice in δ e Vs CL curve for a range of CG or any other sketch or plot.
8%
10% 13%
16%
Time TO SM
CG:13hr TO
5hr
NP
6%
5 hr
13hr
3%
8%
δe
NP : 16%
5hr 13hr 8.
TO
CL
Elevator control power (C mδe) is related to tail efficiency factor (ηT) and ONE or MORE of the following parameters: a) lift curve slope of the tail a T b) tail volume coefficient V H c) down wash gradient dϵ/dα at the tail d) flap effectiveness of elevator τ Write down the expression of elevator control power also giving proper sign Cmδe = -ηTVHaTτ < 0 Page 4 of 5
9. Directional stability C nβ due to wing location (high/low) and wing sweep are described below. Choose your correct answer(s) from the following: a) high wing is stable and sweep back is unstable b) high wing is unstable and sweep back is stable c) low wing is stable and sweep back is unstable d) low wing is unstable and sweep back is stable Draw suitable sketches bringing out wing flow affecting the above
10.
Fighter aircraft carry large and heavy drop tanks (with associated drag) mounted on pylons below the wings symmetrically on either side. Sometimes these aircraft fly with asymmetric flight configuration with large drop hanging below the wing on one side. Assuming right wing to be clean we have one of the following correct: a) aileron on the heavier side has to be deflected upwards and rudder to be deflected opposite side of heavy wing for flying with wings level and with zero side slip b) aileron on the heavier side has to be deflected upwards and rudder to be deflected towards heavier wing for flying with wings level and with zero side slip c) aileron on the heavier side has to be deflected downwards and rudder to be deflected to the side of heavy wing for flying with wings level and with zero side slip d) aileron on the heavier side has to be deflected upwards and rudder to be deflected towards heavier wing for flying with wings level and with zero side slip
Show rolling moment and yawing moment imbalance in asymmetric flight configuration and control Up actions required to trim these moments . RM Down
