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Tuesday, 12 November 2013 10:56

Manage Your Energy! Featured

Written by  Snap&Roll
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"The way you manage the available energy is an  

important key in aerobatics"

When a plane is parked on the ground, it doesn't have any energy - or at least any that can be used.

A plane in movement has energy that can be utilized not only for flying and maintaining a certain altitude, but also for maneuvering.

For the purpose of understanding aerobatic flight, we say that an aircraft in flight basically has two types of energy. Added together, this is considered all the energy the aircraft has in any given moment.

 The two kinds of energy are kinetic, solely dependent and proportional to the square of the plane's speed, and potential, proportional to the plane's altitude.

An aerobatic pilot should understand how energy is gained and lost, and how these two energies are exchanged during the performance of figures in a sequence. The pilot can fly in a way that conserves the greatest amount of energy, or do the exact opposite and lose a great deal of energy.  Both techniques can be completely valid in different moments, but the ability to manage energy in this way requires a wide knowledge of these principles.

With the plane on the ground and the motor running, the plane gains energy burning fuel and providing thrust or traction through the propeller. Once the brakes are released, when we are burning more fuel, the aircraft gains more energy. In other words, the more gas we burn, the more energy we have to play with in the plane.

Whatever the speed of the plane, energy is lost through resistance or drag. The total drag of a plane can be determined by summing profile drag, which is always present, and induced drag, produced when the wing generates lift.

Like kinetic energy, profile drag is proportional to the square of the speed. Induced drag depends on the amount of lift the wing generates. The more lift, the more induced drag.

If the speed and altitude of the plane don’t change, the total energy is considered to be constant and in a state of equilibrium. When the engine is at its maximum power, this is the fastest the plane can fly without losing energy. When flying maximum speed at level flight, drag is equal to the plane's maximum power.

Assuming you have a plane at its maximum power, if the plane ascends and loses speed, it loses kinetic energy, but at the same time, gains potential energy. If the plane descends, it gains speed due to gravity and consequently increases its kinetic energy. However, it is losing altitude, causing its potential energy to decrease. As you can see, knowing how to manage both energies during an aerobatics sequence is essential to precisely flying each figure, with its specific shape, optimal speed, perfect altitude, and ideal position in the flight box.

During each aerobatic flight, drag changes constantly with the application of flight controls. Most times, drag will increase, especially with high 1G pitch, but also during rolls and rotations, due to the drag of aerodynamic surfaces (ailerons, rudder, and elevator).

"Good example of energy managment during 

aerobatics by Bob Hoover"

 

Energy Control Inside of a Figure

Each aircraft has its own features that generate drag and produce thrust. Knowing in detail the specific features of the plane you're going to fly allows you to manage your energy more precisely.  For example, the majority of modern aerobatic planes can perform a loop without losing altitude, ending the maneuver at a speed and energy-level greater than at the start. But with older planes or training planes with lower performances, the speed at the end of the maneuver will usually be lower than at the start.  In planes like the Cap 10B or the Decathlon, a speed reduction at the exit will occur, since the energy necessary to create lift to perform the maneuver is greater than that obtained by the power of the "small" engine.

For more limited planes, where there is a minimum entry speed that the pilot must maintain, a good technique for conserving energy inside of a figure is to be sure to not pull up the stick with more force than is really necessary. High G-values reduce the turning radius but drastically increase drag.

We should only close corners or make very small radii when we have an excess of energy.

 

Throttle control

Reducing the throttle represents the loss of an opportunity to generate energy. There are exceptions, specifically in sequences where it is necessary to reduce  power: in the de-acceleration before a spin; when reducing torque during a stall turn as the French usually do; or maybe before entering a snap roll that approaches your speed limit.  However, at all other times, holding back on power means a loss of energy and possibly flying the sequence at a lower altitude.

Although some pilots are in favor of holding back power at other times - for example, when making descending vertical lines - from the point of view of energy, unless you're going to exceed the engine RPM or the VNE, it is desirable to maintain the throttle at its maximum forward position.

Suppose you are doing a stall turn, holding back the throttle on a vertical descent of 300 ft., pulling some 3Gs and then applying maximum power. This creates a radius of approximately 200 ft. and an altitude of 500 ft. below the initial height when you finish.

Now, if we repeat the same scenario but with power applied, the 300 ft. descent occurs more quickly because we are using power and the plane is accelerating. A radius of 200 ft. requires higher Gs, because the speed is greater. It is inevitable that when the figure is complete, the plane is going faster than it would with the throttle held back. The plane's kinetic energy will be greater than in the first scenario, giving us more total energy to perform the following maneuver.

 

But what should you do if you approach the VNE or maximum G-limit?

Due to the characteristics of some planes and having a very reduced speed margin, it is very easy during the vertical descent to approach the VNE or G-limit on the level off. In these instances, given that you should always be inside the safety limitations of your plane, if you want to complete the maneuver with the greatest possible kinetic energy, the solution would be to keep the throttle forward (if the plane is variable-pitch at full power or fixed pitch at maximum rpm) and shorten the time or distance of descent, keeping an eye on the anemometer in order to not exceed the VNE and avoiding pulling the stick sharply during the pickup.

There are pilots who believe that during a stall turn, for example, the vertical line of ascent has to have the same longitude as the vertical of the descent section. This is completely incorrect! According to Section 6 of the FAI Sporting Code, Part 1 of aerobatic flight, the only thing the judges evaluate is whether the lines, going up and coming down without rotations, are completely vertical, 90 degrees exactly with the horizon, and that the plane doesn't slide to one side, deducting 1 point for every 5 degrees off from its theoretical position.  They don't score the longitude of the sections, which means that the ascent line can be longer than the descent. The only slightly different case is if there are rotations during the lines. In this case, the longitude of the section before the rotation should be the same as the longitude of the section after the rotation. If there are rotations in the ascent portion and in the descent, the longitude of the ascent does not have to be the same as that of the descent.

When flying training planes, during the descent we can begin the rotation as soon as possible in order to not have to unnecessarily extend the second section (after the rotation), to avoid exceeding the VNE or G-limit during the level off.

When we fly the competition sequence between figures, we have sections or upright lines that allow us to manage energy. If the section between figures is at a high speed, like after a stall turn, the plane flies faster than it would during upright and level flight at maximum power.  In this situation, it is better to begin the next figure as soon as possible, especially if it is a vertical ascent line, to make sure you have enough energy to complete the figure.

In some figures that end at low speeds, like a half loop and half roll, we can elongate the final section in order to accelerate the plane and have more energy for the following figure. It is true that if the sequence is designed correctly, figures that require less energy are usually grouped with figures that have a low-speed exit, and higher energy figures with figures that have high-speed exits. So for example, after a half loop-half roll, we might have a snap roll, and after a stall turn, a humpty bump.

Unknown sequences are a different matter. In these, the pilot's skill is tested and pushed to the limit by complex figures and impossible unions of figures. In these sequences, we easily find that after a figure where the plane exits with little energy, one has to perform a figure that requires a lot of initial energy. Here, the pilot's skill and tact with the plane are crucial.

Before flying any sequence, no matter what it is, but especially during Unknown sequences, we should have a good energy plan. That means, specifying the high speed sections and planning how to fly one figure after another as quickly as possible to achieve good flight harmony. Memorizing where the low speed sections are to accelerate the plane and get the most out of the maneuver. In this way, we create a compact sequence of the highest equilibrium possible, sure to be rewarded in the scores given by the judges.

 

References::

Basic Aerobatics. Geza Szurovy-Mike Goulian

Advanced Aerobatics. Geza Szurovy-Mike Goulian

Skydancing.

Better aerobatics. Alan Cassidy

Read 5373 times Last modified on Friday, 17 January 2014 14:51
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