SOURAV7414
Sep 12, 2007, 06:51 AM
When is the total drag maximum

firmbeliever
Sep 12, 2007, 12:30 PM
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The basic forces (http://www.auf.asn.au/groundschool/index.html)
1.6 Drag
Drag is the resistance of the atmosphere to an aircraft pushing through it and the resistance depends on:-
(a) the streamlining of the aircraft body
(b) I. The excrescences attached to the airframe
ii. Turbulence at the junctions of structural components
iii. The cooling airflow around the engine
(c) the roughness of the surface skin
(d) the 'wetted' area I.e. The amount of surface exposed to the airflow
(e) the density of the air
(f) the speed of the airflow
(g) the angle of attack.

These components of drag are classified in several ways and we will look at them in the aerofoils and lift module.
Part of the air resistance, item (g) the induced drag, is a consequence of angle of attack, being very high, maybe 70% of the total, at the high aoa of the minimum controllable airspeed, but decreasing as speed increases being possibly less than 10% of the total at full throttle speed.
However the balance of the air resistance, known as parasite drag, increases as speed increases until the total air resistance equals the maximum thrust that can be produced.

You can see from the diagram that parasite drag is directly proportional to dynamic pressure [½rV²] while induced drag is inversely proportional to it.

Thus in normal straight and level flight air resistance is high at both minimum and maximum airspeeds and lowest at some mid-range speed where, as resistance is at a minimum, the thrust required to maintain constant height will also be at a minimum: consequently that is the speed – Vbr – which provides maximum range. If drag is at a minimum then the lift/drag ratio will be at a maximum, consequently this is also, or rather very close to, the best engine-off glide speed – Vbg.

Air density [and thus air resistance] decreases with altitude so the parasite drag component for a given airspeed decreases with altitude while the induced drag component increases because the wing has to fly at a greater aoa to produce the lift required.

The standard expression for total aircraft drag is very similar to the lift equation:

(Equation #1.2) Total drag = CD × ½rV² × S newtons

Where CD is the total drag coefficient and the ratio of total aircraft drag to dynamic pressure. CD increases as aoa increases.

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