Climbing and Descending Lesson

It is important to know the forces acting on an aircraft during a steady climb. The image below looks a lot more complicated than it is. We recommend that you practice drawing this up.

From the above image, we can make the following points:

  • To maintain constant airspeed, thrust must equal drag plus the rearward component of weight (T = D + RCW).
  • Lift is less than weight, and our rate of climb (altitude gained over a given period of time) is proportional to excess power available over that required for straight and level flight at a given airspeed.

Airspeed: One of the biggest factors to affect the climb is airspeed. There are typically three types of climbs that pilots perform:

  1. Best rate (VY): This is the greatest height attained over time.
  2. Best angle (VX): This is the best height attained for a given horizontal distance (used for obstacle clearance).
  3. Cruise climb: This is the quickest way to destination. It gives better visibility and engine cooling.

Weight: If an aircraft is flying level at a given angle of attack, for example, that for best L/D ratio (about 4°), and at a known weight and IAS, when the weight is reduced, for example, by the aircraft dropping its load of agricultural ‘Super’, the lift must also be reduced to balance the new weight. To maintain optimum conditions with the angle of attack at 4°, the speed must be decreased until lift falls to the same value as the new weight of the aircraft. In fact, the new (lower) required IAS will be proportional to the square root of the change in weight. Any increase in weight means an automatic increase in the rearward component of weight during a climb. To maintain airspeed, the angle of climb needs to be decreased; therefore, there will be a reduction in the rate of climb.

Flaps: Because the use of flap gives improved control and safety at low speed, flap can sometimes be used to advantage in the climb. Lowering flaps increases lift and drag in an indirect proportion, i.e., a reduction in airspeed, along with a reduction in the lift-to-drag ratio. To maintain airspeed, the angle of climb needs to be decreased, which in turn means a reduction in the rate of climb. It is important to note that the use of flap will not improve an aircraft’s performance in a steady climb. Flap can be used for take-off though this is normally retracted as soon as practical.

Wind: Wind has no effect on the rate of climb. It does, however, alter the distance covered across the ground. In doing so, wind changes range, angle of climb, and groundspeed. Climbing into wind is slower over the ground and at a steeper angle of climb than climbing into still air is. Climbing with the wind is faster over the ground but at a shallower angle of climb than climbing in still air is.

Looking at the above image, in climbing 500 feet in one minute, the aircraft travels a certain distance over the ground. When the aircraft is flying into headwind, it covers less distance over the ground than in still air but still climbs 500 feet in one minute. When an aircraft has tailwind, it climbs 500 feet in one minute and covers more distance over the ground; in other words, the aircraft has a shallower climb. An aircraft climbing at a given rate of climb per minute therefore covers different distances over the ground. These distances depend on whether the aircraft is climbing into still air or has either headwind or tailwind. However, the aircraft still attains the same vertical height in a particular time.

Effect of altitude on climbing: As we climb, our engine loses power and, therefore, loses its excess power, which is what our aircraft climbs on. Therefore, we get a decrease in climb performance as we increase altitude, as explained in the previous chapter.

Descent is the general term used to describe the condition of flight in which the aircraft decreases its height. The descent can be carried out using some power, or it can be a power-off descent (a glide). The term glide usually refers to a power-off descent, when the rate of descent and airspeed are relatively low. A dive has a high rate of descent and high airspeed, usually with power on.

We will look at the forces involved in three different descents:

  1. Glide descent
  2. Glide descent with flap
  3. Powered descent

In a glide, the forces are balanced, as they are in every steady condition of flight, but as the engine is providing no thrust, there are only three forces to balance: lift, drag, and weight. The lift and drag combined produce a resultant force acting vertically upwards, which balances the weight to maintain a state of equilibrium. Refer to the diagram above—you can see that the angle between the flight path and the horizontal is equal to the angle between the lift and resultant, or weight 1 and weight. This angle is called the gliding angle.

The gliding angle at any given airspeed is directly related to the lift-to-drag ratio. The higher the lift-to-drag ratio (i.e., the less the drag), the flatter the gliding angle. The lower the lift-to-drag ratio (i.e., the greater the drag), the steeper the gliding angle.

LET US CONSIDER THREE AIRCRAFT:

AIRCRAFT A – L/D Ratio of 2:1

For every 100 metres of height loss, the aircraft travels 200 metres horizontally over the ground.

AIRCRAFT B – L/D Ratio of 8:1

For every 100 metres of height loss, the aircraft travels 800 metres horizontally.

AIRCRAFT C – L/D Ratio of 10:1

For every 100 metres of height loss, the aircraft travels 1000 metres horizontally.

If the aircraft started its glide at 400 metres in still air conditions, then it would travel horizontally (10 × 400) 4000 metres.

Gliding at an angle of attack for the largest or maximum L/D ratio will give the least drag and, therefore, the flattest glide.

The application of flap increases drag, which decreases airspeed. In order to maintain the best L/D ratio speed, the nose of the aircraft must be lowered, which increases the angle of descent. As a result, the range and endurance decreases.

Applying more power increases thrust, which increases airspeed. The nose of the aircraft must, therefore, be raised to maintain our best L/D ratio speed. By raising the nose of the aircraft, the angle of descent decreases, which decreases the rate of descent and increases the range. If the engine-propeller is producing power, then the thrust force will help overcome part of the drag force.

The result is that the aircraft with power applied will have a shallower descent angle and a lower rate of descent than in a power-off glide. With sufficient power, the glide angle may be zero, i.e., the aircraft will fly level. With even more power, the aircraft may climb. If you are sinking beneath the desired flight path, the correct procedure is to apply power and not just raise the nose (which will simply worsen the situation—with a steeper glide). Adding thrust helps counteract drag; the weight component required to counteract drag is less; hence, a shallower flight path is possible. Any change in power will require some small adjustment to the nose attitude. If the desired airspeed is to be maintained, use power and attitude together to give you the desired performance.

There are several factors that affect the glide angle:

  • Airspeed
  • Power
  • Flaps
  • Weight
  • Wind

Power and flaps we have already covered. We will now look at weight and wind.

Airspeed

During a glide, a similar but not identical relationship exists between IAS and ANGLE of ATTACK as in level flight. Therefore, adjusting the speed to correspond to the optimum angle of attack (largest L/D ratio) will allow the best gliding performance to be obtained. When the speed is changed from the correct glide speed, the angle of glide becomes steeper as the L/D ratio is reduced. A higher speed increases profile drag while lowering the L/D ratio. A lower speed increases the induced drag due to the higher angle of attack, which decreases the L/D ratio. Therefore, to cover the greatest distance on the glide, it is important to use the correct speed. A pilot of a light aircraft normally has no means of measuring angle of attack, but does have an airspeed indicator that indicates the IAS. At any given weight, the angle of attack and the IAS are exactly related. For example, let us assume that an aircraft with a weight of 2000 lb (908 kg) has the following angles of attack at the speeds shown.

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All the pilot knows is that 70 knots is the best climb and best glide speed. This is then an indication that 70 knots is the best angle of attack speed and will give the largest L/D ratio. At any IAS greater than 70 knots, the aircraft has a lesser angle of attack and, thus, a smaller L/D ratio. Therefore, for every foot of height lost in the glide, the aircraft travels less horizontal distance over the ground.

Weight

Aircraft weight does not affect the glide angle. An increase in weight increases the forward component of weight and, therefore, airspeed. In turn, airspeed increases lift and drag. The descent angle and range remain the same while the rate of descent increases. If the weight is less, then you can achieve the best glide angle by flying at a slightly lower glide speed (i.e., the best glide speed decreases with a decrease in weight).

At an angle of attack for the best L/D ratio and, therefore, the best glide, airspeed is lower, but the glide angle remains the same. This also means that the rate of descent for the aircraft when light will be less (i.e., the aircraft will glide the same distance through the air but will take longer to reach the ground). A light aircraft can be a very distinct advantage when gliding for range with the wind, but a disadvantage when gliding into wind. Gliding at a higher airspeed means that the aircraft is exposed to a headwind for a shorter time; therefore, the aircraft will glide further.

Wind

Wind has no effect on rate of descent; it does, however, have an effect on range. As you can see below in the image, headwind decreases the ground distance travelled (range), while tailwind increases the ground distance travelled. With headwind, the speed for maximum range should be increased, which means you spend less time in the air to be affected by the wind.

Discuss the following questions in class now.

  1. Draw the forces in a climb
  2. What is VX?
  3. What is VY?
  4. How would a sudden increase in Weight affect the airspeed of an aircraft in a Climb?
  5. How does Flap affect the Rate of Climb of an aircraft?
  6. How is the Apparent Angle of Climb affected by a Tail and Headwind components?
  7. Draw the Power Curves – label the important points
  8. Define Absolute Ceiling
  9. Define Service Ceiling
  10. Draw the Thrust Curve – label the important points
  11. Why is the speed for Best Range faster than the speed for Best Endurance?
  12. Draw the forces in a Glide Descent
  13. Weight is supported by what Force or Forces in a Glide Descent
  14. What Angle of Attack do we need to fly at for Best Range
  15. How is Range affected by an increase in Weight?
  16. How is the Glide Angle affected if we increase Weight?
  17. If we lower Flap what happens to Range?
  18. How do we know when we are flying at the Best Lift / Drag Ratio?
  19. An Aircraft Flight Manual (AFM) gives the Best Glide Speed – what is the importance of this speed?
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