Pres6_Drag2.pdf

8/10/2019 Pres6_Drag2.pdf

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6.1.3. Interference drag (cD,interference)

When two shapes intersect or are placed in proximity, their pressure distributions and

boundary layers can interact and result in a net drag of the combination that is higher

than the sum of the separate drags. This increment in drag is known as

"INTERFERENCE DRAG".

Interference drag occurs between theengine nacelle & wing,

engine nacelle & fuselage

wing & fuselage,

etc.


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Interference drag between engine nacelle & fuselage

Effect of the proximity of an engine nacelle to the rear pylon (part of fuselage) on the

CH-47 Chinook helicopter. Note that the interference drag can be as high as the drag

of the individual nacelle only. The nacelle has to be placed at least 0.5Dnfrom the pylon

to eliminate interference drag.

Engine nacelle

Pylon

Boeing Chinook CH-47


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Interference drag between (rotary) wing & fuselage

Effect of rotor hub proximity to pylon (which is part of the fuselage) on a helicopter. As

the distance between the rotor hub and pylon (Z/W) increases, the interference drag

decreases. In order to eliminate interference drag, Z/W must be larger than 0.7.

Pylon

Rotor hub

Robinson R44


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High-wing vs. Low-wing configuration

At the fuselage-wing juncture, a drag increment results as the boundary layers from the

two components interact.


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The larger the thickness of the interacting boundary layers, the larger the interference

drag.

Due to the nature of lift generation, the upper surface boundary layer on a wing is always

thicker than that over the lower surface. Hence,

high-wingconfiguration results in

thin-thin boundary layer interaction small cD,interference

and a low-wingconfiguration in

thick-thin boundary layer interaction highcD,interference

Therefore, a high-wing configuration has lower interference drag than a low-wing

configuration.


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Interference drag is largely affected by the angle between the wing and fuselage surfaces:

The smaller the angle, the larger the interference drag.

The sharper the corner, the larger the interference drag.

Filleting can help reduce interference drag.

Douglas DC-3

Fillet between fuselage and wing


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Filleting can help reduce interference drag.

F-16

There are no analytical methods for estimating

interference drag: experimental or CFD data are

required to determine it.


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6.1.4. Wave drag (cD,wave)

WAVE DRAG is a pressure drag arising due to the formation of shock waves. It is a result

of the pressure distributionover the body surface.

Flat plateCone

Net horizontal force due to pressure distribution is WAVE DRAG.


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There are a number of strategies reducing wave drag:

- sharp Leading Edge (LE)

- wing sweep

- "area-ruling" or "coke-bottling"

Sharp LE:

Sharp LE eliminates "detached" normal shock

waves, which are associated with large pressure

rise (through the wave) and large pressure drag.Attached oblique shock waves yield much smaller

pressure drag.


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Wing Sweep: (Already discussed in AERO 3002)

Wing sweep should be larger than the Mach angle.

Mach angle

Sweep angle


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Area Ruling:

Wave drag is analytically related to the second derivative (i.e. curvature) of the longitudinal

volume distribution of the aircraft.

The Sears-Haack plot minimizes curvature of the volume distribution for a given length and

internal volume:

Sears-Haack volume distribution: the ideal

volume distribution for minimum wave drag.


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Unfortunately, in real life it is impossible to match or even to approximate the Sears-Haack

curve. (Components such as wing, nacelles, etc. all introduce sharp breaks in the curvature

of the volume distribution).

However, by smoothing the volume distribution, the supersonic wave drag can be reduced

by as much as 50%. This leads to a coke-bottle like fuselage:

ORIGINAL AREA-RULED

Trying to resemble

Sears-Haack curve


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The "supersonic area rule" was used for decades by ballisticians, but first employed byRichard Whitcomb (supercritical airfoil, winglets) in the 1950's.

First application:

Convair F-102(could not break

sound barrier

until "area-ruling"

was applied

to the fuselage.

Coke-bottling


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6.1.5. Cooling drag(cD,cooling)

COOLING DRAG is the momentum loss of air used to cool the engine, oil, or other heatexchangers on the airplane.

Oil Cooler

P51 Mustang

Cooling drag can be expressed either as dragor power:

Some manufacturers provide engine power with the power lost due to cooling

drag subtracted. For a typical piston engine, the power lost due to cD,coolingcan

be as high as 6%.


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6.1.5. Cooling drag(cD,cooling)

cD,coolingis highly configuration dependent and no general method exists for its calculation.

The performance analyst needs to consult with the engine manufacturer to assess

cD,cooling.


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6.1.6. Trim drag (cD,TRIM)

The horizontal tail/canard produces lift to balance the airplane around its pitching axis.

Any drag increment attributed to the generation of this lift is called TRIM DRAG.

Most often, TRIM DRAG is equal to the induced drag of the tail.

It can be determined from:

tw

tw

LxlLxM

WLLF

)(.:0

:0


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6.1.6. Trim drag (cD,trim)

Di

ttL

Lt

L

Ltt

trimD c

eARc

eARc

c

c

S

Sc

2

.

.,

The total trim drag in terms of geometry:

And in terms of CG (Centre of Gravity) location:

where subscript tcorrespond to horizontal tail values.

22

,

ttDi

trimD

e

e

b

b

l

x

l

x

c

c


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6.1.6. Trim drag (cD,trim)

And plotting this second equation shows that

negative TRIM DRAG can be obtained

for small positive centre-of gravity locations.

Trim drag is typically in the order of 1-2%

of total airplane drag in cruise.

Source: McCormick (1995)


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6.2. Drag reduction

The drag break-down for a jet transport clearly shows that the 2 major contributors to

total drag are:

- skin friction drag

- induced drag75% of total drag

Hence, drag reduction techniques aim at reducing these two categories of drag.



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6.2. Drag reduction

Induced Drag Reduction: Winglets

The winglet was invented by Richard Whitcomb (supercritical airfoils, area-rule).

The basic idea is to create a forward pointing "negative drag":

Flow leakage around wing tip

Induced velocity acts not

only downwards but also inwards.

This will change the resultant

AOA acting on winglet

Winglet has cambered airfoil,which at an AOA creates lift

Forward component of this lift is

"negative drag".



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6.2. Drag reduction

Design guidelines for winglets:

- low profile drag

- low interference drag (larger than 90 deg angle and/or fillet between tip and

winglet)

- high winglet AR for increased efficiency

- location close to T.E. where viis larger

- larger loading at the wing tip increases v iand winglet efficiency

Typical induced drag reduction: ~25%

Winglet geometry used for first

generation jet transport with

higher wing loading around tip

(Boeing 707).



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6.2. Drag reduction

Design guidelines for winglets:

For 2ndgeneration jets, the loading is lower towards the wing tip than for 1stgeneration

jets. Therefore, applying the above guidelines for winglet design are even more important

to maximize the drag reduction effect:

Effects of winglets on drag for

1stand 2ndgeneration jets.



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6.2. Drag reductionTypical winglet design for a modern jet transport. Due to the lower wing loading towards the tip, the winglet has

to be well optimized to achieve useful amounts of drag reduction.

Large angle between wing and winglet for

reduced interference drag

Airbus A340


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6.2. Drag reduction

Skin Friction Drag Reduction: Laminar Flow Control (LFC)

Laminar B.L., which has less skin friction drag than turbulent B.L. (sec. 6.1.1), can be

promoted for example by suction of the boundary layer through slots on the surface:



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6.2. Drag reduction


Advantages: - Range increase of about 30%.

- Fuel saving of about 30%

Problems: - swept L.E. destabilizes B.L

- need for fences or chordwise suction slots

- technology: pumping power, double skin, extra weight



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6.2. Drag reduction


Practicality test: Jetstar LFC Flight Test Program

(Jul 1985 - Feb 1986)

- Lockheed Jetstar (4 engine executive jet)

- 4 scheduled flights a day

- Atlanta, Pittsburgh and Cleveland locations- low maintenance system



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6.2. Drag reduction

Drag Cleanup

Even without fancy and expensive techniques, many small drag items can be reduced by

paying attention to details, i.e. ensuring

- smooth surfaces

- protuberances streamlined or avoided

- tight seals around wheel wells, door openings, cutouts

Example: Wind tunnel test - adding all necessary items increased total airplane drag by 65%!!



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6.3. Total airplane drag

See the Pres6.3_TotalAirplaneDrag file.

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