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Aerodynamics

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chanejane created the topic: Aerodynamics

Hi Bob
Refering Exe A3
Question 6 . The greatest amout of induced drag would be produced by applying:
(a) high angles of attack at high speed
(b) high angles of attack at low speed
(c) low angles of attack at low speed
(d) low angles of attack at high speed

my answer is BRAVO. Bob tait study guide says that answer is ALPHA.

my reason for choosing bravo is that at low airspeed high AOA induce drag increases, high airspeed low angle of attack induce drag decreases.

Exe A4
Question No 8 refer diagram attached.
For an aircraft to achieve maximum endurance in level flight it must be flown
(a) at speed S
(b) slower than speed S in any wind conditins
(c) slower than speed S if tailwind exits
(d) faster than speed S if a tailwind exits

my answer is ALPHA , study guide answer is BRAVO.
reason for chooslng Alpha .... flying at minimum power to achieve maximum range.


Can u plse clarify this.
tks
JC
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  • John.Heddles
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John.Heddles replied the topic: Aerodynamics

Q1 The greatest amount of induced drag would be produced by applying:

(a) high angles of attack at high speed
– what’s left usually is the one – certainly will be higher CL. But, let’s be careful of a g-stall in this situation

(b) high angles of attack at low speed – reject due probably lower CL than (a)

(c) low angles of attack at low speed – reject due low CL

(d) low angles of attack at high speed – reject due low CL

my reason for choosing bravo is that at low airspeed high AOA induce drag increases – good thought, high airspeed low angle of attack induce drag decreases – you were looking at (d) as the problem alternative ? Perhaps best re-read Bob’s notes.

The basics – induced CDi depends on CL squared (and some other bits and pieces). You are looking for the option with the highest CL.

This is the reason the fast jet folks do a run and break approach into the circuit .. the low level, high bank angle, high g 180 turn washes off heaps of speed by the time the aircraft rolls out on downwind (and minimises the exposure time to ground fire in hostile situations). In years gone by, the delta-winged Mirage, in a max rate roll (around 400 deg/sec if my recollection is correct – only had the one ride in a dual seater at ARDU), would lose around 100KIAS in one roll .. drag is a pretty potent force.


Q2 For an aircraft to achieve maximum endurance in level flight it must be flown

(a) at speed S
– for piston/props, min drag is relevant to range rather than endurance – reject it

(b) slower than speed S in any wind conditions – for piston/props, min power relates to max endurance and, in general, we will see min power speed a bit slower than min drag (the sums suggest around 76% min drag speed). Looks like a good option

(c) slower than speed S if tailwind exits – tailwind is irrelevant to endurance - reject

(d) faster than speed S if a tailwind exits – wind is irrelevant to endurance - reject

reason for chooslng Alpha .... flying at minimum power to achieve maximum range.

Fuel flow generally relates to engine power for a piston/prop so you are looking for the min power to get min fuel flow and max endurance.

Generally, for piston/props we are looking to maximise

CL/CD for range .. so fly for best L/D

CL to the 3/2 power/CD for endurance.

I think you need to re-read Bob’s notes again, perhaps ? You are confusing power and drag and need to sort out the significance of each in your mind.

Engineering specialist in aircraft performance and weight control.
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chanejane replied the topic: Aerodynamics

Thanks John.
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chanejane replied the topic: Aerodynamics

I have another question.
Exe A5 question 16
The IAS at which the stall occurs during a 75% bank level balanced turn would increase over the level flight stalling speed by approximately.
(c) 100%
how did u get to 100%
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bobtait replied the topic: Aerodynamics

At 75° of bank the load factor is 4. The stalling speed increases by the square root of the load factor so the stalling speed increases by the square root of 4, which is 2. If you increase a number by a factor of 2, you have increased it by 100%.
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chanejane replied the topic: Aerodynamics

Thank u so much.
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