Weight and Balance - A Personal Delima - Safety

Center of gravity and Weight Balance of an Aircraft

The point at which the entire weight of an airplane would balance perfectly. The center of gravity affects the stability of the plane and, for safe flight, the plane must remain within specified limits established by aircraft manufacturers while in flight.

When the weight of a plane is at or below the allowable limits for its type and configuration and its center of gravity is within the allowable range and both will remain so or the duration of the flight, the plane is said to be within weight and balance.

The configuration refers to whether the plane is parked, moving on the ground, take-off, landing for example.

You may define maximum weights for different situations. Maximum weight may be higher, for large aircraft, in take-off configuration than for maximum landing weight. (weight is lost from fuel burned.)

Fuel burned may affect the center of gravity. In a Beechcraft Bonanza, the center-of-gravity may move toward the rear of the plane as fuel is used since the fuel tanks slope downward from fore to aft in the wings. Depending on other load considerations you have to make sure your center-of-gravity stays within the allowable range for the entire flight.

Since safety is the theme it does matter when weight and balance are incorrect in fixed wing aircraft. 

Placing the center of gravity of a plane outside the allowed range can lead to a unavoidable crash. If the plane is outside of its acceptable fore-aft axis the plane may pitch uncontrollably down or up. This excessive pitch change could occur in all phases of flight or certain phases where it doesn't occur on takeoff but could move outside of its fore-aft axis before you land. Calculations of center of gravity must take those possibilities into account. One example is the rear-most baggage compartment of the Beech Craft V-Tail Bonanza. It is placarded for 275 pounds. If you carry three adult passengers and your-self the fore-aft center of gravity may be close to the aft limit of the envelope. If your flying to reach a vacation spot you may pile golfing equipment into that placarded 275 pound baggage compartment. 

That was a fatal mistake. On take-off the plane would nose up with severe control problems. If you were successful in combating control of your aircraft during takeoff the  control of the plane in flight would  become worse as fuel moves the center of gravity further aft. This is a big killer of pilots that are not careful about weight and balance.

If the weight of the aircraft is out of range (heavier than the maximum weight) the plane may not achieve or sustain controlled, level flight. Excessive take-off weight may make it impossible to take off within available runway lengths, or it may completely prevent take-off. If you do manage to take-off, without incident, it may slow your rate of climb and/or make climbing beyond a certain altitude difficult or impossible. Refer to prior article dealing with Mountain Flying for the dangers with an under-powered plane and/or over gross aircraft in mountainous territory.

With you always in flight safety,

Jim  

What Notion is about as Healthy as that a Rattlesnake makes a Pleasant Pet?

How about the idea that the purpose of the rudder is to steer the airplane around the corner? No way.

The rudder gives the pilot the means to combat the adverse yaw effect of the ailerons. This is what a coordinated turn is all about. That is why we use rudder on going into a turn and why we use rudder when coming out of a turn. If the ailerons had no adverse yaw a plane wouldn't need a rudder.

It would be great if a student pilot had a early and clear concept of what an airplane had a rudder for. Think of all the skidding and slipping it would eliminate. Think of the fatalities it would save.

Remember your steep turn training. It was easy to enter with slight rudder involvement. Recovering was another thing. 

Banking out of fast, low angle of attack flight into a steep turn there is not much adverse aileron yaw and the need for rudder is less. Unbanking out of slow, high angle of attack flight, there is much adverse aileron yaw and much need for rudder. Your correct response will prevent a "broken neck" so to speak.

There is only one way to get out of a steep turn quickly and safely. This is especially important if the recovery from the steep turn has begun to sour.

First, to reduce the angle of attack, let the stick or wheel come forward to unstall the wings and giving the ailerons less adverse yaw. This enables the wing to lift itself.

Second, now apply a lot of top rudder (left rudder in a right turn and vice-versa).

Third, begin to apply high aileron to roll the airplane out of the steep bank.

Lesson today, "Stick Forward" to reduce angle of attack.

Here's to flying safely!

Jim

Torque or Slipstream? - Left Turning tendency

The Real Reason

The left turning tendency occurs when the propeller pushes back a stream of air mass which spirals as it streams to the rear.

With its spiraling motion the air mass hits the vertical fin at a slight angle. This pushes the tail to the right and yaws the nose to the left. Heres where the fun begins.

The propeller hits a small particle of air mass at the right -hand side of the planes nose. The impact of the propeller sends it backwards  and downward along the right side of the fuselage where it crosses underneath the pilots seat to the left side of the fuselage where it flows upward (cork-screw) and backward along the left side of the tail. It is then ready to cross over the top of the tail back to the right side; but, it finds its path blocked by the tail fin. The air mass particle impacts the fin with a force and shoves the tail fin to the right.

At the same time, in the beginning, another air mass particle is caught by the propeller on the left side of the planes nose. The impact sends the air mass particle speeding backward along the left side of the fuselage  and slightly upward. It crosses over the top of the cockpit and flows downward and backward along the right side of the tail. It is then ready to cross underneath the tail back to the left side. But, it runs into a problem. There is no tail fin to block the air mass particle. It exerts no force on anything and flows away behind the plane.

The net result of these two circulating air mass particles is a resultant shove of the tail fin to the right and a yaw of the plane to the left.

In aircraft design the tail fin is set at a slight angle so to line it up with the actual line of direction of the air flow at the tail in cruising, level flight.  The true reason for off setting the tail fin is not to make a force but, rather, to keep a force from being made.

In slower flight, as the tightness of the spiral of the air mass particle increases, the sideways shove of force on the tail fin increases. If the design angle offset is not sufficient, the left turning tendency continues and you must hold right rudder.

In gliding flight this tendency doesn't appear because the propeller is not creating a slipstream. The offset built into the tail fin is not necessary and, in fact, yaws the nose of the aircraft to the right. The pilot, in a glide, now holds slight left rudder.

You could abolish the left turning tendency by putting as much tail fin underneath the tail as there is atop the tail. The problem is the tail fin underneath would interfere with the conventional landing gear of a tail dragger. New plane design, with tricycle landing gear, have allowed the tail fin arrangement mentioned above to appear in modern planes.

The H-tail is here and slanted engines have eliminated left turn tendencies that plagued older aircraft.

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.