ppl_05_e2

Thin aerofoils (thickness/chord ratio about 7%) are best for high speeds. The Concorde had a thickness/chord ratio of only about 3%.

The wing section on the Piper PA28 Warrior, a popular training aircraft, is a laminar fow aerofoil (see Figure 4.9). (NB-the Warrior Pilot’s Operating Handbook describes the wing section as laminar fow, and it is certainly slimmer than the original PA28 Cherokee wing, but in cross-section is not as fne as the lower right diagram). This type of aerofoil, typically, has a thickness/chord ratio of about 12% and is so designed that the point of minimum static pressure (point of maximum camber) on the wing is at 50% chord so that the airfow is speeded up gradually to the half way point of the wing chord, giving the aircraft good performance in the cruise at small angles of attack, but poor characteristics at the stall. Laminar fow wing sections generate good lift for low drag, and are often used on high performance sailplanes. The principal disadvantage of laminar fow wings is that they are sensitive to slight changes in wing contour. Therefore, laminar fow wings, especially the leading-edge area of the wing, must be kept free of contamination from insects, raindrops, and the like at all times,. Contamination on the surface of laminar fow wings can easily break up the laminar airfow and cause aircraft handling problems for the pilot.

The different types of aerofoil mentioned above, with their different values for thickness/chord ratio, maximum thickness, maximum camber, and the position of these latter two along the chord, turn or defect the airfow to different extents.

That, in turn, will affect the air velocity and pressure distribution around the wing.

Figure 4.10 The lift characteristics of three different types of aerofoil.

Consequently, each different type of aerofoil will have a different Coeffcient of Lift and Coeffcient of Drag for any given angle of attack. This situation is illustrated for Coeffcient of Lift in Figure 4.10.

AIRFLOW THROUGH A VENTURI TUBE.

In our examination of airfow, we must take a look at what is known as the venturi effect. In the chapter on Lift, you learnt about the Bernoulli Principle and the Bernoulli Equation. One of the most common examples of the practical application of the Bernoulli Principle is the venturi tube. The most common type of venturi tube, used in practice, is a tube which narrows to what is called the throat, and then gradually widens out again, as shown in Figure 4.11, overleaf. As you have already learnt, if we assume – as we do throughout this book – that air is incompressible at speeds well below the speed of sound, the rate of mass fow through the venturi tube must

Laminar flow

wings are sensitive to

slight changes

in contour, and must be kept free of contamination, especially the leading edge.

In a venturi

tube, where the velocity of air

is highest, its

static pressure will be lowest.

The venturi tube creates low static pressure at its throat.

be constant. Therefore, as the cross-section of the tube decreases, the velocity of the airfow will increase, reaching its maximum velocity at the throat. Bernoulli’s

Equation tells us that where velocity is greatest, static pressure will be lowest. So, as the airfow approaches the throat, static pressure will fall, and increase again as the tube cross section increases in diameter and the airfow velocity decreases.

A venturi tube, then, is used to create a decrease in pressure, lower than that of the atmosphere. Carburettors employ the venturi effect for their basic function. On older aircraft, venturi tubes are also used in the operating systems of some fight instruments (such as the direction indicator or artifcial horizon) which operate on the so-called “partial vacuum” system. You can read about both these uses of venturi tubes in the Aeroplanes (General) volume, in this series of manuals.

The venturi tube principle may also be

Figure 4.12 Venturi Tube fitted to Auster J6. used to create areas of high airfow velocity; for example, in a wind tunnel.

The speed of the free-

stream airflow, around an

aircraft in flight, is the same as the aircraft’s true airspeed.

MEASURING AIRSPEED.

It is convenient for you to note at this point that when an aircraft is in motion, either on the ground, during the take-off and landing roll, or in the air, the relative airfow is used to measure the speed of the aircraft through the air.

Now, the speed of the free-stream airfow around an aircraft in fight is, naturally, equal to the forward speed of the aircraft relative to the air, referred to by pilots as the aircraft’s true airspeed.

You learnt in the chapter on Lift that the dynamic pressure exerted by a moving, ideal fuid on a body is the pressure exerted by virtue of the velocity and density

Figure 4.13 The speed of the free-stream airflow is the same as the aircraft’s true airspeed.

of the fuid. Therefore, we may reasonably deduce that an aircraft’s airspeed is a function of the dynamic pressure exerted by the airfow it encounters in fight. You may remember the following equation for dynamic pressure, which can be extracted from Bernoulli’s Equation:

Dynamic pressure (Q) = ½ ρ v2

You may also recall that the total pressure in a fuid in motion, horizontally, is given by the relationship:

The instrument which displays to the pilot an aircraft’s indicated airspeed is known, simply, as the airspeed indicator.

An aircraft’s

true airspeed is a function of

the dynamic

pressure acting on the aircraft in flight.

Total Pressure =

Static Pressure

+ Dynamic

Pressure.

Dynamic Pressure, Q, = ½ ρ v2

The detailed function of the airspeed indicator is covered in our ‘Aeroplanes

(General)’ book. Here we will examine only those theoretical aspects of measuring airspeed which are relevant to the Principles of Flight.

Basically, the airspeed indicator (ASI) is an instrument whose operating system “subtracts” static pressure from total pressure, in the air fowing around the aircraft in order to give the pilot an indication of the dynamic pressure, Q, of the airfow.

Q = ½ ρ v2 = Total Pressure - p

Figure 4.15 An Airspeed Indicator (ASI) indicating 95 knots.

The ASI is calibrated in such a way that the pilot reads from the instrument the indicated airspeed of his aircraft. When the pilot corrects indicated airspeed by taking into consideration the actual density of the air (ρ), he will obtain the aircraft’s true airspeed. The indicated airspeed in Figure 4.15 is 95 knots.

The total pressure of the airfow is measured by an open tube, called a pitot tube, whose open end faces the oncoming free-stream airfow. A hole or holes (known as static vents) facing

“side-on” to the airfow, so that no air fows into the static vents, sense the static pressure within the airfow. The static vents may be ftted in the fuselage of an aircraft, as shown in Figure 4.16, or they may be incorporated into the circumference of a pitot tube to form a pitot-static tube, as depicted in Figure 4.17. The device is named after 18th century French engineer and inventor

Henri Pitot (pronounced ‘pee-tow’).

Figure 4.16 A Static Vent.

The total pressure sensed by the pitot tube (sometimes known as pitot

pressure) is fed to one side of a pressure-measuring device, (usually a sensitive diaphragm) within the airspeed indicator (ASI) system while the static pressure sensed by the static vents is fed to the other end of the ASI’s pressure-measuring device. This arrangement is illustrated in Figure 4.18.

The pressure difference sensed by the ASI is the difference between the total pressure in the relative airfow and the static pressure within the relative airfow. This difference is, of course, a measure of the dynamic pressure, Q, of the airfow, ½ ρ v2. The dial of the ASI can, therefore, be calibrated to read, the speed of the airfow, v, directly, for a given value of ρ.

The ASI

“subtracts” static pressure from

total pressure to

allow dynamic pressure to be indicated on the ASI instrument face.

In calibrating the ASI, the value for ρ is assumed to be the air density in ISA sea-level conditions. We will indicate this value of air density by the symbol ρSL.

We have also said that the indicated airspeed, as read directly from the ASI, and corrected for instrument and position error is called calibrated airspeed. We will represent indicated airspeed (which we assume to be equal to calibrated airspeed) by the symbol vi.

Now, in the expression for dynamic pressure ½ρv2, v is the true airspeed.

But vi will be the true airspeed only where ρ = ρSL

Therefore, ½ρv2 = ½ ρSL vi2

And so, when ρ = ρSL , r = 1, and true airspeed, v, is the same as indicated airspeed, vi. But ρ is rarely, if ever, exactly equal to ρSL. When the aircraft is operating at altitude, it may be assumed that ρ is almost always less than ρSL. Consequently,

r and will be less that 1. Therefore, at altitude, true airspeed, v, is invariably greater than indicated airspeed.

At sea-level, then, where local air density will be very close to ISA air density, the indicated airspeed read directly from the ASI may be taken to be close to the true airspeed. But, because local air density decreases with increasing altitude, largely because of the decrease in pressure, invariably, air density at altitude, especially above 2 000 ft, will be less than the ISA sea-level air density. Therefore, at altitude, true airspeed will be greater than indicated airspeed.

Consequently, when a pilot calculates his true airspeed during his navigation preparations for a cross-country fight, at a planned indicated airspeed of, say, 110 knots and a planned cruising altitude of 3 000 ft, he will know that he can expect the true airspeed to be higher than his planned indicated airspeed. It is true airspeed that he must use for his navigation calculations.

As we have said you will learn how to carry out these calculations in the volume

Navigation & Radio Aids. However, it is important that you understand the airfow and Principles of Flight considerations behind your navigation calculations.

Where air

density is less than the ISA

sea level value,

indicated airspeed will be lower than true airspeed.

In your

navigation flight planning, the difference

between indicated airspeed and true airspeed must be taken into account.