The loop is the “grandfather” of all aerobatic manoeuvres. Since it is the easiest to do and takes the least time to learn, it is usually the first manoeuvre that I teach. Before aerobatic training is commenced, the student should have undergone a thorough course in spinning.
Diameters of the loop
The diameters of loops that are flown in various aircraft differ considerably. For example, loops in slow flying Chipmunks and Tiger Moths are very tight and short in duration. Only a few seconds elapse between the time that the pitch up for the manoeuvre is commenced, until the aircraft is back in straight and level flight.
Faster aircraft, built purposefully for aerobatics, such as the Zlin 50, Pitts Special, Extra 300 and the Super Decathlon, all fly loops that are greater in diameter and that take a longer time to accomplish. The pilot therefore has more time to observe the many aerodynamic principles that come into play throughout the execution of the manoeuvre.
As part of the “HASEL” checks, aerobatic power as per the aircraft’s handbook must be set. A good “lookout “must always be exercised and a clearing turn should be flown before entry to the manoeuvre to ensure that there are no other aircraft nearby.
The dive angle
The dive angle for the manoeuvre is important. If it is too shallow, the aircraft will use up an excessive amount of height before reaching its speed for a loop.
If the dive angle is too steep, there is a good chance that the entry speed will be exceeded and that an unnecessary expenditure of height will have occurred.
The instructor will therefore point out where the front windscreen coaming should be placed in relation to the horizon so as to achieve the dive angle for an optimum rate of acceleration in relation to height loss.
As the aircraft accelerates, depending on the direction of rotation of the engine, a rudder input is required to counteract the effect of an offset tail fin and to keep the aircraft in balance with the “ball” centred.
Looking both left and right and making sure that both wingtips are equidistant from the horizon, will help to ensure that a straight loop is flown.
Once the required speed for the loop has been acquired, the pull up is commenced
Flying a rounded loop
The amount of g required to fly a reasonably rounded loop in most aerobatic aircraft is anywhere between 3.5 g and 4.5 g. If the loop is not pulled tight enough, an egg shaped manoeuvre will result. There will also be a larger than normal decay of airspeed during the first half of the manoeuvre.
Reaching the top, with the aircraft inverted, the pilots will feel a “lightness in their pants” that is caused because there is less g. If the loop is pulled too tightly, the speed over the top of the loop will be somewhat higher than it would have been with a loosely pulled loop, but the angle of attack of the wing could be very close to its stalling angle.
Nearing the top of the loop, the head must be tilted back in order to look for the opposite horizon and to ensure that the wings are still level.
Since the speed is decaying very rapidly, but with the engine at full power, the slipstream effect due to the helical movement of air from the propeller becomes progressively more pronounced. The nose of the aircraft will tend to yaw in the opposite direction to when it was diving and an application of the opposite rudder will be required to keep it straight.
Sight of the opposite horizon is important as it enables the pilot to look for a point or a line feature that will assist him or her in keeping the aircraft straight, and secondly, it will help the pilot to establish the rate at which the aircraft is pitching so as to achieve a nice, round loop.
If the pitch rate is increased too much, then with the combination of low airspeed and a high angle of attack, the aircraft will “judder” on the fringes of the stall. An immediate relaxation of the back-pressure on the stick would be required to remedy the situation, as the application of further “up elevator” will cause the aircraft to stall and “flick” out of the manoeuvre.
The second half of the loop
As the aircraft accelerates on the “back side” of the loop, the application of rudder changes back to what is required in a dive. This keeps the aircraft balanced. The wings must be kept level so as to ensure that the direction of the exit from the manoeuvre is the same as it was on the entry.
With the increase in speed it will necessary to increase the back pressure on the stick and to once again pull between 3.5g and 4.5 g in order not to lose too much height on the recovery. The manoeuvre is completed when the aircraft is back in straight and level flight.
In learning to fly the loop, you will also be introduced to high g and experience fairly high forces on your body. Enjoy them and get used to the feeling! This is just the beginning!
Disclaimer: The information provided in these tutorials is suggestive of reasonable operating philosophies. It should not preclude sound airmanship and proper decision making and should apply to the appropriate sphere of operation being addressed.