Aircraft design projects - part 6 docx

“chap06” — 2003/3/10 — page 169 — #27

Project study: electric-powered racing aircraft 169

20 30 40 50 60 70 80 90 100 110

Aircraft speed (m/s)

Aircraft drag (N)

Load factor = 4

3

2

1

0

500

1000

1500

2000

2500

Fig. 6.10 Drag and load factor versus aircraft forward speed

30 50 70 90 110

Aircraft speed (m/s)

(T–D) newtons

Load factor n= 1

2

3

4

1

2

3

4

Course pitch prop.

Fine pitch prop.

Intersection = max. speeds

–500

0

500

1000

1500

2000

2500

Fig. 6.11 (T–D) and load factor versus aircraft forward speed

6.7.2 Climb performance

As mentioned above, the difference between the thrust and drag curves, at a specific

speed, represents energy that is available for the pilot to either accelerate (kinetic energy

increase) or climb (potential energy increase) the aircraft. The excess force available

(thrust–drag) at various aircraft speed, and with the aircraft pulling ‘g’, is shown on

Figure 6.11. This figure also shows the advantage of fine pitch at low speed and coarse

pitch at high speed. Using all the available extra energy to gain height provides the

maximum rate of climb. Multiplying (T – D) by aircraft speed and dividing by aircraft

“chap06” — 2003/3/10 — page 170 — #28

170 Aircraft Design Projects

weight gives the max. climb performance of the aircraft at constant aircraft forward

speed (i.e. with zero acceleration).

The term [V(T −D)/W] is referred to as the specific excess power (SEP). At sea level

the maximum rate of climb versus aircraft speed is shown in Figure 6.12. Drag increase

in manoeuvring flight, as mentioned above, has a significant effect on the aircraft SEP.

Figures 6.13 and 6.14 illustrate the effect of choice of propeller pitch.

–30.0

–20.0

–10.0

0.0

10.0

20.0

30.0

40.0

30 40 50 60 70 80 90 100 110

Aircraft speed (m /s)

RoC =V (T–D)/Mg

Fine pitch

Course pitch

Max. speed

Max. speed

Fig. 6.12 Rate of climb versus aircraft forward speed

20 30 40 50 60 70 80 90 100

Aircraft speed (m /s)

V (T–D )/Mg

Max. speed

Load factor n= 4

1

2

3

–30.0

–20.0

–10.0

0.0

10.0

20.0

30.0

40.0

Fig. 6.13 Specific excess power (SEP) versus aircraft forward speed (fine pitch)

“chap06” — 2003/3/10 — page 171 — #29

Project study: electric-powered racing aircraft 171

30 40 50 60 70 80 90 100 110

Aircraft speed (m/s)

Load factor n= 1

2

3

4

Max. speeds

–15.0

–10.0

–5.0

0.0

5.0

10.0

15.0

20.0

25.0

30.0

V (T–D)/Mg

Fig. 6.14 Specific excess power (SEP) versus aircraft forward speed (coarse pitch)

6.7.3 Turn performance

Racing aircraft fly an oval circuit; it is therefore necessary to investigate the aircraft

turn performance in some detail to establish the optimum racing line. Good turning

performance will allow the aircraft to fly a tighter turn and therefore cover less distance

in the race. The pilot faces a dilemma. Pulling a tight turn will increase drag and

therefore reduce aircraft forward speed. This loss of speed will have to be made up

along the straights. Alternatively, flying gentle (larger radius) turns will maintain speed

but extend the race distance. Figure 6.15 shows the basic relationship between aircraft

forward speed, manoeuvring load factor (n) and aircraft turn rate. Tight turns (high ‘g’)

are achieved at low speeds. Race pilots do not like high ‘g’ and slow speed. They like

to fly fast and gentle.

To achieve a balance of forces on the aircraft in a turn, it is necessary to bank the

aircraft. The angle of bank is related to the aircraft load factor as shown in Figure 6.16.

Although the loads on the aircraft in a correctly banked turn are balanced, it is necessary

to instigate the turn from a straight and level condition and then to return to it. The

application of the control forces required to change these flight conditions creates

extra drag. To avoid these complications, a race could be flown in a fully balanced and

constant attitude if a circular, or near circular, path outside of the pylon was selected.

This would result in a much longer flight distance that would penalise the pilot unless

a higher average race speed could be achieved to offset this disadvantage. The best

strategy to adopt for the race is not obvious. Here lies the essence of good racing

technique.

Not all of the aircraft parameters can be considered in the performance analysis. For

example, sighting and aligning the pylons is an important element in successful racing.

The mid-fuselage cockpit position of the conventional layout may be regarded as less

effective than the forward position on the canard. Also, the canard control surface

may offer the pilot a reference line to judge his position more accurately. ‘Cutting a

pylon’ carries a substantial time penalty but flying a line that is too wide may present an

opponent with a passing opportunity. These are features that are difficult to assess in the