6 Drag

Drag 6

Power Required (Introduction)

We will now consider the relationship between Thrust, Drag and Power. These sound like engine considerations which might be better studied in Book 4, but it has already been shown that Drag can also be referred to as ‘Thrust Required’ and you will now see that a similar relationship exists with ‘Power Required’ - they are both important airframe considerations.

• Thrust is a FORCE (a push or a pull), used to oppose Drag,

but Power is the RATE of doing WORK, or POWER =			WORK
			TIME
and	WORK = FORCE × DISTANCE
so	POWER must be	FORCE × DISTANCE
		TIME

For Power Required:

Which Force?

Drag.

Distance divided by time is speed.

Which speed?

The only speed there is - the speed of the aircraft through the air, True Airspeed (TAS).

Therefore: POWER REQUIRED = DRAG × TAS

•If an aircraft climbs at a constant IAS, Drag will remain constant, but TAS must be increased - so power required will increase.

It is necessary to consider power required when studying Principles of Flight because Work must be done on the aircraft to “raise” it to a higher altitude when climbing. Logically, maximum work can be done on the aircraft in the minimum time when the power available from the engine(s) is greatest and the power required by the airframe is least.

For easy reference, associate the word POWER with the word RATE. e.g. minimum rate of descent is achieved in a steady glide when the aircraft is flown at the minimum power required speed (VMP ).

These and other considerations will be examined more fully during the study of Aircraft Performance in Book 6 and Flight Mechanics in Chapter 12 of this Book.

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Drag 6

POWER
REQUIRED (kW)	6
(DRAG × TAS)	Drag

VMP

	THRUST REQUIRED
L / DMAX	or
	DRAG (kN)

TAS (kt)
	V MD

Figure 6.21

Figure 6.21 is drawn for sea level conditions where TAS = IAS and is valid for one particular aircraft, for one weight, only in level flight, and shows how a graph of TAS against ‘Power Required’ has been constructed from a TAS/Drag curve by multiplying each value of drag by the appropriate TAS and converting it to kilowatts.

The speed for minimum power required is known as VMP and is an Indicated Airspeed (IAS).

Note that the speed corresponding to minimum power required (VMP), is slower than the speed for minimum drag (VMD).

Effect of Altitude

An aircraft flying at VMD will experience constant drag at any altitude because VMD is an IAS. At altitude the TAS for a given IAS is higher, but the power required also increases because Power Required = Drag × TAS. So the ratio of TAS to Power Required is unaffected and VMP will remain slower than VMD.

This information primarily concerns aircraft performance, but the relationship of speed for minimum power required (VMP) and speed for minimum drag (VMD) is important for the study of rate and angle of descent in a steady glide, outlined in Chapter 12.

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6 Drag

Drag 6

Summary

Parasite Drag is made up of:

Skin friction drag.

Form (Pressure) drag.

(Skin friction drag plus Form drag is known as Profile drag.)

Interference drag.

Parasite Drag varies directly as the square of the Indicated Airspeed (IAS) - Double the speed, four times the parasite drag. Halve the speed, one quarter the parasite drag.

The designer can minimize parasite drag by:

Streamlining.

Filleting.

The use of laminar flow wing sections.

Flight crews must ensure the airframe, and the wing in particular, is not contaminated by ice, snow, mud or slush.

Induced Drag

Spanwise airflow generates wing tip vortices.

The higher the CL (the lower the IAS), the stronger the wing tip vortices.

Wing tip vortices strengthen downwash.

Strengthened downwash inclines wing lift rearwards.

The greater the rearward inclination of wing lift, the greater the induced drag.

Induced Drag varies inversely as the square of the Indicated Airspeed (IAS) - Halve the speed, 16 times the induced drag coefficient (CDi) and four times the induced drag (Di). Double the speed, one sixteenth the CDi and one quarter the Di.

The designer can minimize induced drag by:

Using a high aspect ratio wing planform.

Using a tapered wing planform with wing twist and/or spanwise camber variation, or incorporation of wing end plates, tip tanks, winglets or various wing tip shapes.

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Drag 6

Total Drag

Total drag is the sum of Parasite drag and Induced drag.

Total drag is a minimum when Parasite drag and Induced drag are equal.

At low IAS Induced drag is dominant.

At high IAS Parasite drag dominates.

The IAS at which Parasite and Induced drags are equal is called minimum drag speed (VMD).

As gross weight decreases in flight, Induced drag decreases, Total drag decreases and VMD decreases.

At a constant IAS, altitude has no effect on Total drag, but TAS will increase as density decreases with increasing altitude.

Configuration changes which increase the “Parasite Area”, such as undercarriage, flaps or speed brakes, increase Parasite drag, increase Total drag and decrease VMD.

Speed Stability

An aircraft flying at a steady IAS higher than VMD with a fixed throttle setting will have speed stability.

An aircraft flying at a steady IAS at VMD or slower with a fixed throttle setting will usually NOT have speed stability.

If an aircraft flying at a steady IAS and a fixed throttle setting within the non-stable IAS region encounters a disturbance which slows the aircraft, the aircraft will tend to slow further; IAS will tend to continue to decrease and Total drag increase.

Power Required

VMP the Indicated Airspeed for minimum ‘Power Required’ is slower than the minimum drag speed (VMD).

Maximum TAS/Power ratio (1.32VMP) occurs at VMD .

Drag 6

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