Forces in a climb – 20/04/07
difference between the current power requirement and power available – the excess power – can be used to accelerate the aircraft or climb, to accelerate and climb, or perform any manoeuvre which requires additional power.

if the pilot has potential power available and opens the throttle the thrust will exceed drag – utilise that extra thrust to accelerate to a higher speed while maintaining level flight.

Or opt to maintain the existing speed but use the extra thrust to climb to a higher altitude. The rate of climb (altitude gained per minute) depends on the amount of available power utilised for climbing, which depends in part on the airspeed chosen for the climb.

If an aircraft is maintained in a continuous full-throttle climb at the best rate of climb airspeed the rate of climb will be highest at sea level and decrease with altitude as engine power decreases.

It will eventually arrive at an altitude where the excess power available for climb reaches zero.

All the available power is required to balance the drag in level flight, and there will be only one airspeed at which level flight can be maintained and, below which, the aircraft will stall = absolute ceiling.

forces aligning the angle of climb with the line of thrust. In fact the line of thrust will usually be 4° to 10° greater than the climb angle. The climb angle is the angle the flight path subtends with the horizon.

The relationships in the triangle of forces shown is:-
Lift = weight × cosine c
Thrust = drag + (weight × sine c)

In a constant climb the forces = equilibrium but now thrust plus lift = drag plus weight.

lift is less than weight!

It is power that provides a continuous rate of climb, but momentum may also be used as a temporary energy exchange expedient.

A very important consideration, particularly when manoeuvring at low level at normal speeds, is that the steeper the climb angle the more thrust is required to counter weight.

Forces in a descent
If an aircraft is cruising at, for instance, the maximum 75% power speed and reducing the throttle to 65% power, the drag now exceeds thrust and thus 2 options

1 maintain height allowing the excess drag to slow the aircraft to the level flight speed appropriate to 65% power

2 maintain the existing speed and allow the aircraft to enter a steady descent or sink. The rate of sink (a negative rate of climb or altitude lost per minute) depends on the difference between the 75% power required for level flight at that airspeed and the 65% power utilised.

If I move forward on the control column to a much steeper angle of descent, while maintaining the same throttle opening, the thrust plus weight resultant vector becomes greater, the aircraft accelerates with consequent increase in thrust power and the acceleration continues until the forces are again in equilibrium.

Actually it is difficult to hold a stable aircraft in such a fixed angle “power dive” as the aircraft will want to climb –

When I close the throttle completely, there is no thrust, the aircraft enters a gliding descent and the forces are then as shown in my diagram below. In the case of a constant rate descent the weight is exactly balanced by the resultant force of lift and drag. From the dashed parallelogram of forces shown it can be seen that the tangent of the angle of glide equals drag/lift.

L OOK OUT

A LTITUDE

I NSTRUMENT CHECK 1 – ALT / 2 – DI
ALT – POWER – TRIM TO FLY STEADY STRAIGHT AND LEVEL AND AT SELECTED AIRPSEED.
climbing and descending – PAT
P OWER

A LTITUDE
T RIM

Deceleration: – L R A L P I
L OOK OUT
R EDUCE POWER TO DESIRED RPM (1st sound) for desired speed
A ALTIDUDE
L OOK OUT
P ROGRESSIVELY ADJUST ALT TRIM
I NSTRUMENT CHECK 1 – ALT / 2 – DI

Acceleration: – L I H T L P I I
L OOK OUT
I NCREASE POWER TO RPM (1st sound) for desired speed
H OPLD ALT- needs pressure to counteract thrust of propwash
T TRIM
L OOK OUT
P ROGRESSIVELY ADJUST ALT TRIM
I NSTRUMENT CHECK 1 – ALT / 2 – DI
I NDICATOR AIR SPEED QUICKLY CROSS CHECK 1 – ALT / 2 – DI
SO WHAT SHOULD THE HORIZON DATUM LOOK LIKE AT DIFFERENT SPEEDS WHILST FLYING STRAIGHT AND LEVEL AND IN BALANCE –AND AT DIFFERENT AIRPSEEDS