## Chapter 5 Drag

Drag defined

Drag is the component of the aerodynamic force that is parallel to the relative wind and retards the forward motion of the aircraft.

Drag

The Drag equation is:

The coefficient of drag is the ratio of the drag pressure to the dynamic pressure.

Drag like lift is proportional to the dynamic pressure of the air and the area on which it acts.

The equation is much like the lift equation except that it measures the force in the stream-wise direction or parallel to the flow.

Drag

When talking about drag, there is a problem when determining the area to be used in the equation.

Drag is being generated by a three dimensional body but drag is only proportional to 2 dimensions of the body (length x width).

Use fuselage shape example Top, Side and Front.

Drag

As a standard most bluff bodies (fuselage, engine nacelles or landing gear use the projected frontal area or max cross section.

For wing or tail surfaces, the planform or top view is used.

Sometimes engineers will use the wetted area or the area that would get wet if the whole airplane were dunked in water.

Laminar Flow airfoils

When comparing an older 4 digit airfoil to one of the newer 6 series airfoils there is a noticeable decrease in drag coefficient called the drag bucket.

At low AOA the drag remains extremely low because the laminar flow remains laminar so far back.

At high AOA because of the sharper leading edge the CD goes up higher than the 4 digit airfoil.

Lift to Drag Ratio

The lift to drag ratio is a measure of efficiency.

A higher L/D is more desirable and the equation is:

L/Dmax

1. Highest point on the curve

2. The most efficient alpha

3. Min drag

4. Best glide

5. Max endurance (jet powered)

6. Max range (prop powered)

7. Max climb angle for Jets

Glide Ratio

Glide ratio is numerically equal to the L/D ratio

15/1 is both glide ratio and L/D ratio

Gross Weight does not effect glide performance.

Glide ratio is always the same

At a higher gross weight, V must be higher to achieve the L/Dmax α

What is the Glide Ratio for the B-19?

Induced Drag

Induced drag is drag generated by the production of lift or more accurately by the production of wingtip vortices.

The DI formula is:

or

Terms pg 65

Average chord – tapered wing take all chord lengths and average them

Aspect ratio = span (b)/chord (c)

high = glider wing

low = jet fighter wing

Induced Drag

Low pressure on top and high pressure underneath induces a vortex to form at each tip, causing a downward push on the air leaving the trailing edge.

This downward component known as downwash, causes the airstream to depart at an angle downward from the incoming air.

The lift vector being perpendicular to the flow, is now tilted backward at half the downwash angle.

Induced Drag

This means some lift is being generated opposite to the flight path this rearward component is by definition drag.

Induced drag is influenced by the CL and aspect ratio.

It increases directly as the square of CL and inversely as the aspect ratio.

Induced Drag

At low speed and low aspect ratio (short wings) induced drag is greatest.

Induced drag varies inversely with the velocity squared.

Ground effect

Ground effect usually happens when within one wingspan of the ground.

The surface actually helps destroy the downwash generated by the wingtip vortices and thus forces the lift vector more to the vertical thereby reducing drag.

Above 1 span length there is little or no ground effect

Ground effect

At 3/10 span length there is a reduction of 20% induced drag

For our planes that’s 9 feet up

At 1/10 span length there is a reduction of 50% induced drag

For our planes that’s 3 feet up

Ground effect

There is also a change in the effective angle of attack. Because of the altered downwash, an angle of attack increase is the result

Pitching moments develop downward for an aircraft entering ground effect because of the wings downwash not being able to help the tail generate lift downward.

Ground effect

Pitching moments develop upward for the aircraft leaving ground effect and may cause an increase in angle of attack such that the corresponding increase in drag may cause the aircraft to settle.

There may be an increase in static pressure if the ports are below the wing. This will result in a decrease of airspeed the closer the plane gets to the ground or water.

Parasite Drag

Parasite drag is the drag caused by protuberances and increases directly with the velocity squared.

The DP formula is:

Parasite Drag

Skin Friction – Drag caused by rivets, dirty surfaces, effects boundary layer

Form Drag – caused by the shape of the surface

Interference – collision of boundary layers of different surfaces

Leakage – pressure differences inside and outside the plane, like cracks in door seals

Profile – drag with regards to moving helicopter rotors

Definitions pg 72

Total drag

Total drag is the sum of induced drag and parasite drag

Drag Curves

The total drag curve has limitations

1. Parasite drag will be increased as stall alpha is reached because more frontal area is exposed

2. In high speed flight, wave drag may form as a result of compressibility.

The Total drag formula is:

Wing Loading

Power Loading