The objective here is to explain the relationship between attitude and airspeed, and the effects on airspeed of changing the attitude. With practise, you will be able to maintain the desired airspeed, and to smoothly transition to another speed of your choice, with minimal reference to the ASI.
We have already seen in the lesson ‘Effect of the Elevator’ that attitude directly affects airspeed. As the stick is moved forward, the nose goes down, and speed increases. It is normally our goal to fly at the optimal speed for the circumstances. This requires the pilot to be able to maintain a steady speed, and to change speed as required. If we are trying to prolong the flight, and simply stay up, we will fly at the glider’s “minimum sink” speed, typically in the range of 35-40kts. If we want to go as far as we can, we will fly at the glider’s “best L/D”, typically around ten knots faster. If we really want to go places and expect good lift, we will aim to fly much faster, possibly all the way up to Vne. However, we must not fixate on the ASI. We need to maintain a good lookout, keep our head out of the cockpit, and hence learn to “fly by attitude”.
There are some important things to note:
- If we hold an attitude the speed will remain constant (in still air!).
- If we change attitude, the speed will change.
- This change in speed takes some time, most likely over several long seconds, after moving the stick. The glider will then settle at that speed and remain there as long as the new attitude remains unchanged.
It’s obvious then that we shouldn’t keep moving the stick to attain the desired speed while the glider is still accelerating (or decelerating). Instead, we must move the stick (gradually and smoothly) to where we think it will result in the attitude for the desired speed, and then wait for it to settle. After several seconds, the glider will have settled, and we can adjust again as needed.
Given that we need to maintain lookout, and that watching the ASI is not the way to achieve the desired speed, here’s an idea: If you want to accelerate by 10kts for example, move the stick forward to the estimated position for the new attitude, and use the intervening seconds to perform the lookout scan cycle. Once done, check the ASI and adjust. And repeat.
The demonstration shows the effect on the airspeed of raising the nose and holding the attitude. This is followed by lowering the nose to a couple of different attitudes to observe the impact on the airspeed, before raising it again to the first target. You will see that it can take quite a while for the airspeed to settle after changing attitude. You may also see that as speed increases it becomes more difficult to hold the new attitude – this is because the pilot is not using the trimmer. We will look at making life easier with the trimmer in the next lesson. Try it just with the stick first, and focus on holding the new attitude until the speed has settled. This demonstration is over flatlands, in calm conditions, flying a Standard Cirrus.
The video is best viewed in YouTube in HD in Full Screen mode, to easily see the on-screen messages and instruments during the demonstration. Use view, pause and rewind as needed to grasp the content and timing of the messages displayed, then focus on the action.
Performing the Exercise
In the exercise, first maintain the initial attitude, and check your speed. You’ll most likely need to move the stick back to slow down to 40kts. Move the stick and wait for the speed to settle. Note that it takes a little while. Adjust the attitude if necessary to achieve the target speed of 40kts. When you’ve done this, note the attitude by reference to the nose and the horizon – store this mental picture as “40kts”. Then move the stick forward to achieve a steady 50kts, and wait for it to settle. Adjust as needed, and when you are settled at 50kts, take another mental picture and store that as “50kts”. Now see if you can move between 40 and 50kts just by reference to the attitude, aiming to achieve this in one smooth movement of the stick each time. Then try 60kts…
Further Reading and References
Gliding From Passenger to Pilot, 2nd Edition: Page 86-87
BGA Instructors’ Manual, 4th Edition: Section 2, Chapter 7, Page 7-3