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Know-It-All Q&A - Fixed-Wing Airplane

Q. What requirements are there for an ultralight or light plane to be modified with skis?

A. Ultralight vehicles operated under FAR 103 can be modified with skis provided they don’t increase the empty weight above the 254-pound limit. There’s no extra weight allowance for skis. No vehicle or pilot authorization is required.

Experimental light-sport aircraft (E-LSA) and experimental amateur-built aircraft (E-AB) may be flown on any skis provided certain conditions are met. The modification would be considered a major change, and the owner must comply with the aircraft’s operating limitations dealing with a major change. Typically, this requires the aircraft be returned to Phase 1 flight testing limitations, and upon completion of testing, a logbook endorsement must be made certificating the aircraft is safe and controllable through its normal range of operations.

Special light-sport aircraft (S-LSA) can be equipped with skis only with the approval of the aircraft manufacturer. Ski type and method of installation must be done in compliance with the written instructions from the aircraft manufacturer.

Standard category aircraft and S-LSA can only fly on skis approved by the FAA under the aircraft’s Type Certificate (TC), Supplemental Type Certificate (STC), or FAA field approval. Passengers may not be carried unless the pilot has made three takeoffs and landing with skis in the previous 90 days.

Q. Is there a reference for technical information about installing skis?

A. Yes, the FAA publishes Advisory Circular 43.13-2B. Chapter 5 in this AC provides details of ski installations. A copy of the AC can be downloaded from the Internet by clicking here.
Airplanes

Q. Is there a reference for technical information on the operation of ski-equipped aircraft?

A. Yes, the FAA publishes a Skiplane and Seaplane flying handbook, FAA-H-8083-23. Chapter 7 in this handbook provides operational details of skiplanes. A copy of this handbook can be downloaded from the Internet by clicking here.

Q. What’s the function of flaps?

A. Flaps work primarily by changing the camber of the airfoil since deflection adds aft camber. Flap deflection doesn’t increase the critical (stall) angle of attack, and in some cases flap deflection actually decreases the critical angle of attack.

Deflection of trailing-edge control surfaces, such as the aileron, alters both lift and drag. With aileron deflection, there is asymmetrical lift (rolling moment) and drag (adverse yaw). Wing flaps differ in that deflection acts symmetrically on the airplane. There is no roll or yaw effect, and pitch changes depend on the airplane design.

Pitch behavior depends on flap type, wing position, and horizontal tail location. The increased camber from flap deflection produces lift primarily on the rear portion of the wing. This produces a nosedown pitching moment; however, the change in tail load from the downwash deflected by the flaps over the horizontal tail has a significant influence on the pitching moment. Consequently, pitch behavior depends on the design features of the particular airplane.

Flap deflection of up to 15 degrees primarily produces lift with minimal drag. The tendency to balloon up with initial flap deflection is because of lift increase, but the nosedown pitching moment tends to offset the balloon. Deflection beyond 15 degrees produces a large increase in drag allowing a steeper approach path.

Q. What effect does CG location have on performance?

A. The effect of the position of the CG on the load imposed on an aircraft’s wing in flight is significant to climb and cruising performance. An aircraft with forward loading is “heavier” and, consequently, slower than the same aircraft with the CG further aft.

With forward loading, “nose-up” trim is required in most aircraft to maintain level cruising flight. Nose-up trim involves setting the tail surfaces to produce a greater down load on the aft portion of the fuselage, which adds to the wing loading and the total lift required from the wing if altitude is to be maintained. This requires a higher AOA of the wing, which results in more drag and, in turn, produces a higher stalling speed.

With aft loading and “nose-down” trim, the tail surfaces exert less down load, relieving the wing of that much wing loading and lift required to maintain altitude. The required AOA of the wing is less, so the drag is less, allowing for a faster cruise speed. Theoretically, a neutral load on the tail surfaces in cruising flight would produce the most efficient overall performance and fastest cruising speed, but it would also result in instability. Modern aircraft are designed to require a down load on the tail for stability and controllability.

Q. What is the crab method for control on a crosswind approach to landing?

A. The crab method is executed by establishing a heading (crab) toward the wind with the wings level so the airplane’s ground track remains aligned with the centerline of the runway. This crab angle is maintained until just prior to touchdown, when the longitudinal axis of the airplane must be aligned with the runway to avoid sideward contact of the wheels with the runway. If a long final approach is being flown, the pilot may use the crab method until just before the roundout is started and then smoothly change to the wing-low (sideslip) method for the remainder of the landing.

The wing-low method will compensate for a crosswind from any angle, but more importantly, it enables the pilot to simultaneously keep the airplane’s ground track and longitudinal axis aligned with the runway centerline throughout the final approach, roundout, touchdown, and after-landing roll. This prevents the airplane from touching down in a sideward motion and imposing damaging side loads on the landing gear.

Q. My approaches to landing are consistently high. How can I correct this?

A. When the final approach is too high, if equipped, lower the flaps as required. Further reduction in power may be necessary, while lowering the nose simultaneously to maintain approach airspeed and steepen the approach path. When the proper approach path has been intercepted, adjust the power as required to maintain a stabilized approach. When steepening the approach path, however, care must be taken that the descent does not result in an excessively high sink rate. If a high sink rate is continued close to the surface, it may be difficult to slow to a proper rate prior to ground contact.

Q. How should power be set during approach to landing?

A. Power can be used effectively during the approach and roundout to compensate for errors in judgment. Power can be added to accelerate the airplane to increase lift without increasing the angle of attack; thus, the descent can be slowed to an acceptable rate. If the proper landing attitude has been attained and the airplane is only slightly high, the landing attitude should be held constant and sufficient power applied to help ease the airplane onto the ground. After the airplane has touched down, it will be necessary to close the throttle so the additional thrust and lift will be removed and the airplane will stay on the ground.

Q. Why do students practice steep turns?

A. The objective of the maneuver is to develop the smoothness, coordination, orientation, division of attention, and control techniques necessary for the execution of maximum performance turns when the airplane is near its performance limits. Smoothness of control use, coordination, and accuracy of execution are the important features of this maneuver.

The steep turn maneuver consists of a turn in either direction, using a bank angle between 45 and 60 degrees. This will cause an overbanking tendency during which maximum turning performance is attained and relatively high load factors are imposed. Because of the high load factors imposed, these turns should be performed at an airspeed that does not exceed the airplane’s design maneuvering speed (VA). The principles of an ordinary steep turn apply, but as a practice maneuver the steep turns should be continued until 360 degrees or 720 degrees of turn have been completed.

Q. What is meant by ballooning on landing?

A. During landing, if your sink rate is faster than it should be, there’s a tendency to increase the pitch attitude too rapidly. This can start the airplane climbing and is known as ballooning. Ballooning can be dangerous because the aircraft altitude is increasing, airspeed is decreasing, and the airplane is rapidly approaching a stall.

When ballooning is slight, a constant landing attitude should be held and the airplane allowed to gradually decelerate and settle onto the runway. When ballooning is excessive, it’s best to execute a go-around immediately; do not attempt to salvage the landing. Power must be applied before the airplane enters a stalled condition.

Q. While taxiing in windy conditions, what position should my flight controls be in?

A. Based upon the wind direction and speed you should place the flight controls in a position to maximize control of the airplane while taxiing. The following diagram shows the recommended control surface position.

Flight controls position during taxiing.
Flight controls position during taxiing.

Q. What is maneuvering speed (Va)?

A. On January 18, 2011, the FAA published a Special Airworthiness Information Bulletin clarifying Va. The following summarizes key parts of this bulletin.

The design maneuvering speed (Va) is the speed below which you can move a single flight control, one time, to its full deflection, for one axis of airplane rotation only (pitch, roll, or yaw), in smooth air, without risk of damage to the airplane.

Experimental airplanes may not have a published Va; however, they’ll still have some maximum maneuvering speed associated with the maximum structural design loads. Therefore, the pilot should be aware of what speed this is and adhere to the limitation. Va is valid for operation at the gross weight stated, which is typically at max gross weight. It’s especially important to note that Va decreases as the airplane weight decreases. At first, this may seem counterintuitive.

When the airplane is subjected to an external force, such as the aerodynamic force from a control surface, the airplane responds by accelerating (rotational acceleration) about one of the airplane’s axes. This was stated many years ago in Newton’s second law of motion. The law states that when an object of mass “m” is acted upon by a force “F,” it will undergo acceleration “a” in the same direction as the force, more simply stated in the widely known equation “F = ma” which can be rewritten as “a = F/m.” Rewritten this way, it’s clear for a given control force “F,” as the airplane weight “m” decreases, then the acceleration “a” will increase. This higher acceleration gives rise to higher loads on the airplane structure.

Therefore, as the airplane weight decreases, the allowable maneuvering speed must also decrease to ensure that the airframe isn’t damaged. Pilots may remember from their written exam that VA-NEW = VA √ (WNEW/WMAX-GROSS) as the way to calculate the corrected (new) maneuvering speed due to operating at a weight less than the maximum gross weight. Note: This formula is for calculating the Va change about the pitch axis; however, it can be used for all axes.

Recommendations
The FAA wants to clarify that operators should know what the maneuvering speed is and to caution pilots on what to avoid by adhering to the information described above. We recommend the following for maneuvering at, or even below, Va:

  • Do not apply a full deflection of a control, followed immediately by a full deflection in the opposite direction.
  • Do not apply full multiple control inputs simultaneously, i.e., pitch, roll, and yaw simultaneously, or in any combination thereof, even if you are below Va.
  • Reduce Va when operating below gross weight, using the following formula:

VA-NEW = VA √ (WNEW/WMAX-GROSS)

Q. How can I make a good transition during roundout while landing in a crosswind?

A. Generally, the roundout can be made like a normal landing approach, but the application of a crosswind correction is continued as necessary to prevent drifting.

Since the airspeed decreases as the roundout progresses, the flight controls gradually become less effective. As a result, the crosswind correction being held will become inadequate. When using the wing-low method, it’s necessary to gradually increase the deflection of the rudder and ailerons to maintain the proper amount of drift correction.

Do not level the wings; keep the upwind wing down throughout the roundout. If the wings are leveled, the airplane will begin drifting and the touchdown will occur while drifting. Remember, the primary objective is to land the airplane without subjecting it to any side loads that result from touching down while drifting.

Q: Why do some airplanes have a yaw string?

A: The yaw string is the most effective, yet least expensive, slip/skid indicator; it’s made from a piece of yarn mounted in the free airstream in a place easily visible to the pilot, best used outside the propeller slipstream. The yaw string helps you coordinate rudder and aileron inputs. When the controls are properly coordinated, the yarn points straight back, aligned with the longitudinal axis of the airplane. During a slipping turn, the tail of the yaw string will be offset toward the outside of the turn. To center the yaw string in a slipping turn, add pressure to the rudder pedal that is opposite the tail of the yaw string. During a skidding turn, the tail of the yaw string will be offset toward the inside of the turn. To center the yaw string in a skidding turn, add pressure to the rudder pedal that is opposite the tail of the yaw string.

The Wright brothers used a yaw string on their 1902 glider. This was their first flight instrument and helped them master controlled flight.

Q. Why do airplanes have trim systems?

A. The airplane is designed so that the primary flight controls (rudder, aileron, and elevator) are streamlined with the nonmovable airplane surfaces when the airplane is cruising straight and level at normal weight and loading. If the airplane is flying out of that basic balanced condition, one or more of the control surfaces is going to have to be held out of its streamlined position by continuous control input. The use of trim tabs relieves the pilot of this requirement. Proper trim technique is a very important and often overlooked basic flying skill. An improperly trimmed airplane requires constant control pressures, produces pilot tension and fatigue, distracts the pilot from scanning, and contributes to abrupt and erratic airplane attitude control.

Because of their relatively low power and speed, not all light airplanes have a complete set of trim tabs that are adjustable from the cockpit. In airplanes where rudder, aileron, and elevator trim are available, a definite sequence of trim application should be used. Elevator/stabilator should be trimmed first to relieve the need for control pressure to maintain constant airspeed/pitch attitude. Once a constant airspeed/pitch attitude has been established, the pilot should hold the wings level with aileron pressure while rudder pressure is trimmed out. Aileron trim should then be adjusted to relieve any lateral control yoke/stick pressure.

A common trim control error is the tendency to overcontrol the airplane with trim adjustments. To avoid this, the pilot must learn to establish and hold the airplane in the desired attitude using the primary flight controls. The proper attitude should be established with reference to the horizon and then verified by reference to performance indications on the flight instruments. The pilot should then apply trim in the above sequence to relieve whatever hand and foot pressure had been required. The pilot must avoid using the trim to establish or correct airplane attitude. Airplane attitude must be established and held first, and then control pressures trimmed out so that the airplane will maintain the desired attitude in “hands-off” flight. Attempting to “fly the airplane with the trim tabs” is a common fault in basic flying technique even among experienced pilots.

Q. What is meant by maneuvering speed?

A. Maneuvering speed (Va) is the maximum speed where full, abrupt control movement can be used without overstressing the airframe. 

Q. What are flaperons?

A. A flaperon is a type of control surface that combines aspects of both flaps and ailerons. In addition to controlling the roll or bank of an aircraft like conventional ailerons, both flaperons can be lowered together to function much the same as a dedicated set of flaps would.

The pilot has separate controls for ailerons and flaps. A mixer is used to combine the separate pilot input into this single set of control surfaces called flaperons. The use of flaperons instead of separate ailerons and flaps can reduce the weight of an aircraft. The complexity is transferred from having a double set of control surfaces (flaps and ailerons) to the mixer.

Some designs that incorporate flaperons mount the control surfaces away from the wing to provide undisturbed airflow at high angles of attack or low airspeeds. Common light planes that use flaperons are the Challenger and Kitfox.

Q. What is a ground loop?

A. A ground loop is an uncontrolled turn during ground operation that may occur while taxiing or taking off but especially during the after-landing roll. Drift or weathervaning doesn’t always cause a ground loop, although these things may cause the initial swerve. Careless use of the rudder, an uneven ground surface, or a soft spot that retards one main wheel of the airplane may also cause a swerve. In any case, the initial swerve tends to make the airplane ground loop, whether it’s a tailwheel type or nosewheel type.

Nosewheel-type airplanes are somewhat less prone to ground loop than tailwheel-type airplanes. Since the center of gravity (CG) is located forward of the main landing gear on these airplanes, anytime a swerve develops, centrifugal force acting on the CG will tend to stop the swerving action.

If the airplane touches down while drifting or in a crab, the pilot should apply aileron toward the high wing and stop the swerve with the rudder. Brakes should be used to correct for turns or swerves only when the rudder is inadequate. The pilot must exercise caution when applying corrective brake action because it’s very easy to overcontrol and aggravate the situation.

Q. What orientation should a bolt be installed?

A. Standard practice is to insert all aircraft bolts either with the bolt threads down, pointing outboard or aft whenever possible. This is done in case the nut should somehow come off; the forces of gravity and inertia will tend to keep the bolt in place.   

Q. What’s the purpose of steep turns?

A. The objective of this maneuver is to develop the smoothness, coordination, orientation, division of attention, and control techniques necessary for the execution of maximum-performance turns when the airplane is near its performance limits. Smoothness of control use, coordination, and accuracy of execution are the important features of this maneuver.

The steep turn maneuver consists of a turn in either direction, using a bank angle between 45 to 60 degrees. This will cause an overbanking tendency during which maximum turning performance is attained and relatively high load factors are imposed. Because of the high load factors imposed, these turns should be performed at an airspeed that doesn’t exceed the airplane’s design maneuvering speed. The principles of an ordinary steep turn apply, but as a practice maneuver the steep turns should be continued until 360 or 720 degrees of turn have been completed.

An airplane’s maximum turning performance is its fastest rate of turn and its shortest radius of turn, which change with both airspeed and angle of bank. Each airplane’s turning performance is limited by the amount of power its engine is developing, its limit load factor (structural strength), and its aerodynamic characteristics.

Q. I’m having trouble with the roundout prior to landing. What can I do?

A. Starting the roundout too late or pulling the elevator control back too rapidly to prevent the airplane from touching down prematurely can impose a heavy load factor on the wing and cause an accelerated stall. Suddenly increasing the angle of attack and stalling the airplane during a roundout is a dangerous situation since it may cause the airplane to land extremely hard on the main landing gear, and then bounce back into the air. As the airplane contacts the ground, the tail will be forced down very rapidly by the back-elevator pressure and by inertia acting downward on the tail.

Recovery from this situation requires prompt and positive application of power prior to occurrence of the stall. This may be followed by a normal landing if sufficient runway is available – otherwise the pilot should execute a go-around immediately.

If the roundout is late, the nosewheel may strike the runway first, causing the nose to bounce upward. No attempt should be made to force the airplane back onto the ground; a go-around should be executed immediately.

Q. Does weight affect the glide ratio of an airplane?

A. The glide ratio of an airplane is the distance the airplane will, with power off, travel forward in relation to the altitude it loses. For instance, if an airplane travels 10,000 feet forward while descending 1,000 feet, its glide ratio is said to be 10 to 1.

The glide ratio is affected by all four fundamental forces that act on an airplane (weight, lift, drag, and thrust). If all factors affecting the airplane are constant, the glide ratio will be constant. Wind is a very prominent force acting on the gliding distance of the airplane in relationship to its movement over the ground. With a tailwind, the airplane will glide farther because of the higher ground speed. Conversely, with a headwind the airplane will not glide as far because of the slower ground speed.

Variations in weight don’t affect the glide angle, provided the pilot uses the correct airspeed. Since it’s the lift-over-drag (L/D) ratio that determines the distance the airplane can glide, weight will not affect the distance. The glide ratio is based only on the relationship of the aerodynamic forces acting on the airplane. The only effect weight has is to vary the time the airplane will glide. The heavier the airplane, the higher the airspeed must be to obtain the same glide ratio. For example, if two airplanes having the same L/D ratio, but different weights, start a glide from the same altitude, the heavier airplane gliding at a higher airspeed will arrive at the same touchdown point in a shorter time. Both airplanes will cover the same distance – only the lighter airplane will take a longer time.

Q. What is a stall?

A. A stall occurs when the smooth airflow over the airplane’s wing is disrupted and the lift degenerates rapidly. This is caused when the wing exceeds its critical angle of attack. It can occur at any airspeed, in any attitude, with any power setting.

The practice of stall recovery and the development of awareness of stalls are of primary importance in pilot training. The objectives in performing intentional stalls are to familiarize the pilot with the conditions that produce stalls, to assist in recognizing an approaching stall, and to develop the habit of taking prompt preventive or corrective action.

Intentional stalls should be performed at an altitude that will provide adequate height above the ground for recovery and return to normal level flight. Although it depends on the degree to which a stall has progressed, most stalls require some loss of altitude during recovery. The longer it takes to recognize the approaching stall, the more complete the stall is likely to become, and the greater the loss of altitude to be expected.

Q. How does ground effect affect my airplane?

A. Ground effect must be considered during takeoffs and landings. For example, if a pilot fails to understand the relationship between the aircraft and ground effect during takeoff, a hazardous situation is possible because the recommended takeoff speed may not be achieved. Due to the reduced drag in ground effect, the aircraft may seem capable of takeoff well below the recommended speed. As the aircraft rises out of ground effect with a deficiency of speed, greater induced drag may result in marginal initial climb performance. In extreme conditions, such as high gross weight, high density altitude, and high temperature, a deficiency of airspeed during takeoff may permit the aircraft to become airborne but be incapable of sustaining flight out of ground effect. In this case, the aircraft may become airborne initially with a deficiency of speed and then settle back to the runway.

If, during the landing phase of flight, the aircraft is brought into ground effect with a constant angle of attack, the aircraft experiences an increase in lift and a reduction in the thrust required, and a floating effect may occur. Because of the reduced drag and power-off deceleration in ground effect, any excess speed at the point of flare may incur a considerable float distance. As the aircraft nears the point of touchdown, ground effect is most realized at altitudes less than the wingspan.

Q. Pitch or power: Which controls airspeed?

A. No discussion of climbs and descents would be complete without touching on the question of what controls altitude and what controls airspeed. The pilot must understand the effects of both power and elevator control working together during different conditions of flight. The closest one can come to a formula for determining airspeed/altitude control that is valid under all circumstances is a basic principle of attitude flying which states:

“At any pitch attitude, the amount of power used will determine whether the airplane will climb, descend, or remain level at that attitude.”

Through a wide range of nose-low attitudes, a descent is the only possible condition of flight. The addition of power at these attitudes will only result in a greater rate of descent at a faster airspeed.

Through a range of attitudes from very slightly nose-low to about 30 degrees nose-up, a typical light airplane can be made to climb, descend, or maintain altitude depending on the power used. In about the lower third of this range, the airplane will descend at idle power without stalling. As pitch attitude is increased, however, engine power will be required to prevent a stall. Even more power will be required to maintain altitude, and even more for a climb. At a pitch attitude approaching 30 degrees nose-up, all available power will provide only enough thrust to maintain altitude. A slight increase in the steepness of climb or a slight decrease in power will produce a descent. From that point, the least inducement will result in a stall.

Q. What is wing washout?

A. Many airplanes are designed with wing washout. It is a condition where the wing root has a greater angle of incidence than the wingtips. Airplanes are designed this way so that the wings will stall progressively outward from the wing roots to the wingtips.

When a wing exceeds the critical angle of attack, the wing will stall; the wing roots will exceed the critical angle first and stall before the wingtips. The wings are designed in this manner so that the aileron control will be available at high angles of attack and slow airspeed. This gives the airplane more stable stalling characteristics.

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