Power Control and Stabilty

Plane Stability - Plane Performance and Throttle Settings

What is your plane stability after you trim for a particular speed? A plane, with perfect stability, will maintain that speed whether the power if off or the power is increased. Right? 

Try this in actual flight at cruising speed in straight and level flight and you will discover what actually happens when you change the power setting. Cut the power and the typical plane will descend in a glide at a speed well above your cruising speed. The glide will steepen into a dive. Push the throttle wide open and the plane will settle on a different, slower speed as it climbs.

Just why does a plane speed up when you close the throttle and slow down when you open up the throttle? Seems contradictory to most pilots. 

Plane design and propeller blast is one reason. The tail surfaces ride in a place, on a plane, where the propeller blast hits them. In cruise mode, at a specific speed, the tail surfaces receive an extra wind in power on flight causing an extra down force on the tail surfaces. With power off that wind and extra force disappear. 

In designing a plane there is something called a thrust-line location involved in a planes development. It concerns the location of the propeller in relation to the rest of the airplanes design. Consider a Cessna, with a high wing, with its engine mounted low to provide good vision for a pilot. This design arrangement will cause a nosing up tendency when the pull of the propeller coupled with the backward pull of the high wings drag with power on. This effect disappears when the power is off.

A change in relative wind created by the wings at cruise speed with the power on and how it changes with power off. Remember that the tail surfaces reside in the downwash created by the surfaces of the wing while in flight. With power on, the propeller blast hits certain parts of the wing. These parts work in an extra sharp relative wind that sends an extra sharp downwash to the tail surfaces. This produces an extra down force on the tail surfaces. When the power is off this effect disappears too.

The effect becomes more pronounced the heavier and more powerful the plane. To a pilot the effect is important because under some conditions an airplane may want to stall it self. 

From a safety point of view this may occur during a go-around as you attempt to land when a plane attempting to take off the runway suddenly appears in front of your plane. 

If your plane, that cruises at 100 mph in straight and level flight, stalls at 45 mph and your approach speed is 70 mph after trimming properly you could stall the aircraft in a go-around. You open the throttle wide to climb away and re-enter the airport flight pattern. If you don't simultaneously re-trim or apply strong forward pressure on the stick the plane wants to climb like mad. It will climb into a power stall because you think the plane will climb at 70 mph. (no change in airspeed).  Instead, the plane will want to fly at 45 mph or less to quickly gain altitude. 

You must respond by applying strong forward pressure on the stick and/or re-trimming to lower the nose of your plane to a safe climbing attitude and the speed up to a safe 70 mph.

Each plane, in review, has a built in tendency to keep its own   angle of attack constant and to keep its speed constant regardless of how the the amount of power delivered by the plane may change. This is basic. This basic tendency is overcome by the effects of power on and off discussed. You, the pilot must be aware of these contrary tendencies and act accordingly.

Deer Season - A Tendency to "Hunt!"

A "Hunting" Tendency

Stability - static and/or dynamic. In the real world most planes have a tendency to hunt. If you let go of your planes controls it won't fly at a constant speed.

Instead, it oscillates up and down. Why?

The up and down flight path of an oscillating plane is an complicated interaction of speed changes, flight paths, directions of the relative wind, angles of attack, changing drags, changing propeller efficiencies, horsepower outputs, changing lifts and slight downward and upward g-loads.

All the above means the plane is trying to get back to its proper cruising speed and proper angle of attack. The plane "over controls" itself and doesn't quite succeed in steadying itself down after you released the controls.

Since your plane is trying to return to a proper cruising speed and angle of attack, the apparent unstable "hunting" doesn't mean the airplane itself is unstable.

If the airplane was actually unstable, in a dive it would continue to dive or it would stay in a climb until it stalled.

The plane is primarily stable. If an airplane can't steady itself it is dynamically unstable. The airplanes oscillations continue to increase in violence until the plane disintegrates. 

The majority of airplanes are dynamically stable where the oscillations become weaker and weaker. The presence of unstable air or other disturbances may prevent the dampening of the oscillations. 

Knowing the technical theory on "hunting" is not very important to a pilot. Stability is important to a pilot because of the way it affects the feel and behavior of the plane while he controls the plane. 

If the pilot is controlling the plane it will not oscillate. 

Tail of an Airplane - Misunderstood

Tail of an Airplane

The tail of an airplane is arranged to resist the diving tendency of a plane. It is not meant to hold the lighter end of an airplane up in the air while in flight.

The purpose of the horizontal fin of the tail is to hold it down. It is like a wing but set at a negative angle of attack so the air flow against the horizontal tail fin produces, in normal flight, a downward force.

The reason for this downward force is the tail operates in the downwash of air that flows off the upper surface of the wing.

You recall an airplane creates "lift" by pushing air mass downward that creates a force upward that equals the weight of the plane in level, cruising flight. The downwash flows down on the horizontal fin of the tail and pushes it down more than you realize.

You know, from just looking at a plane, the nose is heavier than the tail. The nose-heavy plane tends to "nose the plane down" while the horizontal tail stabilizer tends to "nose it up."

When you trim a plane for level flight at cruising speed the two forces exactly balance each other. That is exactly what we do when we trim a plane for a certain speed. We adjust the angle of the horizontal tail fin so, at the particular speed we want to achieve, the downward air mass force on the tail will exactly balance the downward pull of the force of gravity on the nose.

Navigation, Speed and Wind Drift

Navigation - Which is Better -Slow Plane or Fast Plane 

Remember, a large plane and a slow both drift with the wind in straight and level flight. 

Speed does make a difference in navigation. Weight, size and horsepower do not make a difference.

The Setup: 

Make a 100 mile flight to a destination with a 20 mph wind blowing across your course to your destination. You take off and point the nose of your plane directly toward your destination without an allowance for drift in your 100 mph cruise in your slow plane. You correctly determine it should take one hour to complete your flight. After an hour you look at the ground and your destination is nowhere in sight. In one hour of flying through the air your plane has drifted with the air 20 miles to the downwind side of your course. If it is a hazy day with visibility of 7 miles or less you will not see your destination.

If you attempt this same trip in, a 300 mph plane, point it directly toward your destination without compensating for drift and, after an hour, it too will drift exactly 20 miles.  But, at 300 mph your faster plane will arrive at the planned destination in 20 minutes. If the wind is 20 mph, then in 20 minutes your plane will drift in the wind a little less than 7 miles. Now, when you look out of your plane after 20 minutes you will see your destination off to one site easily visible. Your navigation is better when you can see your destination when you are supposed to see your destination. In a fast plane, even though it is drifting in the wind just as helplessly as your flight in a small plane, the drift is proportionately less important to the faster plane. The drift angle is less in the faster plane and that is why a faster plane is much easier to navigate. 

Training - The Figure of Eight

Illustration of Figure of Eight in Flight Training - Wind

In pilot training an instructor had you practice figures of eights over a conspicuous object on the ground. Some times this was incorporated into a steep turn exercise. For an illustration of wind effect a 20 to 30 degree bank was sufficient.

When you pass over an object you can identify from a distance start a 30 degree bank. Hold this 30 degree bank for 360 degrees. It is better to pick a direction to start the instruction heading, for instance , straight West, as you pass over the object, to begin the 30 degree bank for 360 degrees.

In calm air conditions, when you have completed 360 degrees of 30 degree bank, and are just beginning to head out on a due West course you would expect a complete circle was flown and the object you picked out to begin the turn was again right under your plane. You are correct.

In wind conditions it changes the flight path. If you hold the same 30 degree bank for 360 degrees you describe an exact circle through the air. The air in which you fly moves on with your plane in the "soup." Your path on the ground is not a perfect circle. It is more like the figure "6" at the moment you are again heading exactly due West. But, after starting your 30 degree constantly held bank and flying the bank for 360 degrees your position is not exactly over the object where you started your turn. Your plane is several hundred feet downwind form the object.

If you understand the concept of a turn in the wind above you understand the whole problem of drift in turns.

The drifting of the plane during the maneuver is not accompanied by any forces from any direction. It is accompanied, though, by confusing sensations for you, the pilot.

If you did not watch the ground you would not notice any wind effects. But, you can not help seeing the ground, while turning at a constant 30 degree bank for 360  degrees and noticing how the ground speeds up, now it slides sideways, now it slows up and now it slides to the other side.

How you react depends on where your attention is centered. If you are centered on your path over the ground you would notice how your path is pulled out of shape. The proper response to the changing path over the ground is to do nothing. Let the drift take place and make no effort to resist the illusions of what drift does and causes you to try to keep the plane in a perfect circle. 

You don't want to involuntarily try to keep the shape of the turn you started perfect. You may involuntarily try shallowing or steeping your turn or the danger of "holding rudder against the drift" and slipping or skidding during the parts of the turn  when you have the "wind" from your side.

If your attention is on flying a nice turn the slipping and skidding tendencies will be opposite. As you notice your planes apparent sideways sliding you will tend to skid as you turn from upwind to downwind and you will tend to slip while you turn downwind to upwind.

You have to learn to fly the plane by attitude and the feel in the seat of your pants. You have to disregard the strong impressions of sideways and improper motion the ground transmits to your eyes.