Know-It-All Q&A - Weight Shift Trikes
Q. I want to learn more about trikes. What do you suggest?
A. The FAA publishes Weight-Shift Control Aircraft Flying Handbook, FAA-H-8083-5. This handbook is an excellent reference for students and pilots flying weight shift aircraft. It can be downloaded free of charge by clicking here.
Finding a weight shift flight instructor is another excellent way to learn more. You can search for weight shift flight instructors using the EAA Sport Pilot Instructor Database. This extensive list can be searched by clicking here.
EAA publishes a training guide for someone interested in flying an ultralight trike under FAR 103. This training guide is available free of charge by clicking here.
Q. How would you define a weight shift trike?
A. Weight-shift control (WSC) aircraft means a powered aircraft with a framed pivoting wing and a fuselage controllable only in pitch and roll by the pilot’s ability to change the aircraft’s center of gravity (CG) with respect to the wing. Flight control of the aircraft depends on the wing’s ability to deform flexibly rather than on the use of control surfaces.
Q. What effect does the carriage have in pitch stability?
A. The wing design is the main contributing factor for pitch stability and moments, but the carriage design can also influence the pitching moment of the WSC aircraft. For example, at very high speeds in a dive, a streamlined carriage would have less drag and, therefore, a greater nose-up moment because of less drag. The design of the carriage parts can have an effect on aerodynamic forces on the carriage, resulting in different moments for different carriage designs.
The drag of the wing in combination with the drag of the carriage at various airspeeds provides a number of pitching moments, which are tested by the manufacturer - a reason the carriage is matched to the wing for compatibility. Each manufacturer designs the carriage to match the wing and takes into account these unique factors.
Q. What concerns do soft short-field takeoffs create?
A. It should be emphasized that the WSC aircraft is different from most aircraft. The high wing creates a high center of gravity in which the front wheel can bog down in soft fields and flip the WSC aircraft forward. The propeller in the back pushing down on the front wheel also contributes to this unique situation. This is a limitation for WSC aircraft that should not be ignored. WSC aircraft that land in soft fields or sand may not be able to take off. There is a wide variation of manufacturer designs with the least preferable being a skinny, high pressure, highly loaded front tire. WSC aircraft with large wide tires that can be operated at low pressure are designed for operation in soft and rough fields.
Q. What are some advantages of weight shift trikes?
A. Simplicity in the control system, interchangeable wings, easy to transport and store, short takeoff and landing performance, and broad speed range. Trikes offer the most birdlike flight; when holding the control bar the pilot is in fact holding the wing. This offers an element of connection to the air that any other aircraft can’t duplicate. Trike wings offer exceptional stability; many trikes can be operated safely without the need for an airspeed indicator.
Q. What is a whip stall-tuck-tumble?
A. A WSC aircraft can get to a high pitch attitude by flying outside its limitations or flying in extreme/severe turbulence. If the wing gets to such a high pitch attitude and the AOA is high enough that the tips stall, a whip stall occurs. In a WSC wing, most of the area of the wing is behind the CG (about three-quarters). With the tips and aft part of the wing having the greatest drag, and the weight being forward, an immediate and strong nose-down moment is created and the WSC nose starts to drop. Since both the relative wind and the wing are rapidly changing direction, there is no opportunity
to reestablish laminar airflow across the wing.
This rotational momentum can pull the nose down into a number of increasingly worse situations, depending on the severity of the whip stall. The phases that can result, depending on the severity:
Phase 1 - Minor whip stall results in a nose-down pitch attitude at which the nose is at a positive AOA and the positive stability raises the nose to normal flight.
Phase 2 - If the rotational movement is enough to produce a vertical dive, the aerodynamic dive recovery might raise the nose to an attitude to recover from the dive and resume normal flight condition.
Phase 3 - The rotational momentum is enough to bring the nose significantly past vertical (the nose has tucked under vertical), but could still recover to a vertical dive and eventually resume a normal flight condition.
Phase 4 - The rotational momentum is severe enough to continue rotation, bringing the WSC wing into a tumble from which there is no recovery to normal flight, and structural damage is probable.
Whip stall to tumble phases and sequence.
Q. What seating position should I maintain while turning?
A. The pilot’s posture while seated in the trike is very important, particularly during turns. It affects the interpretation of outside visual references. Pilots shouldn’t lean away from the turn in an attempt to remain upright in relation to the ground; they should ride with the aircraft. This should be a habit developed early so that the pilot can properly learn to use visual references.
Q. Do I need a pilot certificate to fly a two-seat trike?
A. The FAA considers a two-seat trike an aircraft, and the trike must be registered with an N number and have an airworthiness certificate. To carry a passenger the pilot needs a sport pilot certificate with weight-shift privileges. A student pilot certificate holder is allowed to solo a two-seat trike if they have met pre-solo requirements and have a solo endorsement from a weight shift-qualified certified flight instructor.
Q. Where is the center of gravity (CG) on a trike?
A. The CG is the theoretical point of concentrated weight of the aircraft. It’s the point within the weight-shift-control (WSC) aircraft about which all the moments trying to rotate it during flight are balanced. The most obvious difference in the CG for a WSC aircraft is the vertical position compared to an airplane, as it’s always lower than the wing. Pilot’s Handbook of Aeronautical Knowledge accurately states the CG is generally in the vertical center of the fuselage. The same is true for the WSC aircraft. However, the WSC wing is higher above the fuselage/carriage, and since most of the weight is centered in the carriage, the CG is well below the wing.
In a two-seat WSC aircraft, the second seat is typically behind the pilot’s seat and the CG is usually located close to the rear passenger seat. Therefore, the CG location doesn’t change significantly with a passenger. Fuel tanks are typically located near the vertical CG so any difference in fuel quantity doesn’t significantly change the CG fore and aft with different fuel quantities.
For level flight, the CG is directly below the wing/carriage attachment point known as the hang point, and the propeller thrust line is typically designed to be near the vertical position of the CG.
Q. What happens to trim speed as weight increases?
A. Similar to airplanes, sailplanes, and PPCs, increasing weight creates increases in speed and descent rate. However, the weight-shift-control (WSC) aircraft has a unique characteristic. Adding weight to a WSC aircraft creates more twist in the wing because the outboard leading edges flex more. With less lift at the tips, a nose-up effect is created and the trim speed lowers.
Therefore, adding weight can increase speed similar to other aircraft, but reduce the trim speed because of the increased twist unique to the WSC aircraft. Each manufacturer’s make/model has different effects depending on the specific design. The stall speed increases as the weight or loading increases, so some manufacturers may have specific carriage/wing hang-point locations for different weights. Some require center of gravity (CG) locations to be forward for greater weights so the trim speed is well above the stall speed for the wing.
Q: Should I be using power, or should the throttle be at idle during landing?
A: The weight-shift-control (WSC) aircraft has a good glide ratio, and normal landings can easily be done with the power at idle. It’s a good practice to master the landings with the throttle at idle so that the glide angle, speeds, and descent rates become habit and part of a normal routine. This is helpful so that, if there’s an engine failure, the pilot is accustomed to landing with minimum power and is able to spot-land the WSC aircraft for emergency conditions at or beyond a specified point. As a general practice for normal landings in calm conditions or a slight headwind, the throttle should be brought back to idle at the start of the base leg for landings.
Title 14 FAR,section 91.119, Minimum Safe Altitudes: General, is an important safety precaution and states: “Except when necessary for takeoff or landing, no person may operate an aircraft anywhere below…an altitude allowing, if a power unit fails, an emergency landing without undue hazard to persons or property on the surface.” This allows long final approaches “with power when necessary,” but overall, it’s important to be no lower than an altitude from which you can glide to a safe landing area.
It should be noted that the power is above idle for some landing situations, such as:
- students first learning to land; a slower rate of descent is the result of higher power settings. In this case, the landings would be done with a target farther down the runway so a safe landing could always be made with engine failure.
- shallower descent angle if directed by air traffic control, or a longer final approach is required.
- high winds and/or turbulent conditions requiring a higher energy level.
For landings where throttle is required, the foot throttle is typically used so the hands can stay on the control bar while approaching the ground for this critical phase of flight. However, the hand/cruise throttle may be set above idle for specific situations as required by the pilot.
Q. What do you call the part of the trike that the pilot and passenger sit in?
A. The part of a trike you sit in is referred to as the carriage. The carriage is a completely separate structure from the wing. Without the wing, the carriage can be driven around if needed. Most of the weight and cost of the weight-shift-control aircraft is in the carriage. There’s a wide range of carriage designs from the most simple and basic open trikes to the more sophisticated and complex trikes that integrate cowlings and offer a number of adjustments for the pilot and passenger, resulting in comfort and less fatigue during flying. Generally, the more complex the trike, the more it costs and weighs, and the more power it requires for similar wings.
Q. What is the purpose of the fins on trike wheel pants?
A. The wing is a significant factor in the design of yaw stability, but the carriage can be a large factor also. If the area in front of the center of gravity (CG) is greater than the area in back of the CG, and the wing yaws to the side, then the front would have more drag and create a moment to yaw the weight-shift-control (WSC) aircraft further from the straight flight. Therefore, fins are sometimes put on the carriage wheel pants as needed so the carriage also has a yawing aerodynamic force to track the WSC aircraft directly into the wind.
Since the carriage has such a large effect on yaw stability, the carriage is matched to the wing for overall compatibility. A manufacturer designs the carriage to match the wing and takes into account these unique factors of each design.
Q. Do your arms get tired holding on the bar all the time?
A. Straight-and-level flight requires almost no application of control pressures if the aircraft is properly trimmed and the air is smooth. For that reason, pilots must not form the habit of constant, unnecessary control movement. Pilots should learn to recognize when corrections are necessary and then make a measured response easily and naturally.
Common errors in the performance of straight-and-level flight are:attempting to use improper reference points on the aircraft to establish attitude
- forgetting the locations of selected reference points
- too tight a grip on the flight controls resulting in overcontrol and lack of “feel”
- improper scanning and/or devoting insufficient time to outside visual reference (head in the flight deck)
- fixation on the nose (pitch attitude) reference point only
- unnecessary or inappropriate control inputs
- failure to make timely and measured control inputs when deviations from straight-and-level flight are detected
- inadequate attention to sensory inputs in developing feel for the aircraft.
Q. How does roll control work in a trike?
A. Generally, it’s thought that the wing remains level and the weight shifts to the side to initiate a turn. Another way to look at how the weight-shift-control (WSC) wing rolls is to examine the carriage and the wing momentfrom the carriage point of view. For example, the center of gravity hangs far below a wing weighing one-eighth of the carriage weight. When the control bar is moved to the side, creating a moment about the carriage/wing hang point, the carriage stays vertical and the wing rotates around the carriage. Therefore, there are two rolling moments that both contribute to the WSC trike rolling into a bank:
- the pilot creating the force on the control bar, rotating the wing about the carriage/wing hang point
- shifting weight to one side of the wing, thus warping the wing to aerodynamically change the lift on each side, as in airplane roll control.
Q. Why is a weight-shift-control (WSC) aircraft called a “flex wing”?
A. Even though the airfoil sections are rigid, the WSC aircraft is called a “flex wing” for two reasons. First, it’s designed so the outboard leading edges flex up and back when loaded. The flexing of the outboard section of the wing also allows load relief because the tips increase twist and decrease angle of attack (AOA) – the greater the weight, the greater the flex and wing twist. This flexing allows the WSC aircraft to automatically reduce loads in unstable air, providing a smoother ride than a rigid wing. Since the wing flexes and reduces the load for a given AOA at the root chord, WSC aircraft can’t obtain loads as high as those obtained by a rigid wing. This flexing of the outboard leading edges also assists in initiating a turn.
Second, the wing is designed to flex as it changes twist from side to side for turning, historically known as wing warping. WSC wing warping is similar to what the Wright Brothers did on their early aircraft, but they did it with wires warping the wing. The WSC aircraft uses no wires and warps the wing by shifting the weight.This flexibility is designed into the wing primarily for turning the aircraft without any movable control surfaces like the ailerons and rudder found on an airplane.
Q. What material is the wing fabric made from?
A. Sail material is a combination of polyester materials designed with different weaves, thickness, and orientation to fit the different shapes and laid down at different angles to provide the stiffness and flexibility where needed for the specific wing design. Automated machines typically cut the fabric to precision tolerances, and the panels are sewn together with high-strength thread.
What are washout struts?
A. They’re the tubes near the tips that keep the wingtip trailing edge up during very low or negative angles of attack. They can be inside or outside the double surface of a wing. The reflex cables may not go to the wingtip, so washout struts are used to hold up the trailing edge at the tip at very low and negative angles of attack.
Q. Without a tail like an airplane, how does the trike maintain yaw stability?
A. There is no significant turning about the vertical axis because the weight-shift-control (WSC) wing is designed to fly directly into the relative wind. Any sideways skidding or yaw is automatically corrected to fly straight with the swept wing design. An airplane uses the vertical tail to stabilize it to fly directly into the relative wind like a dart. The unique design of the WSC aircraft performs the same function through the swept wing design, but also the wing twist and airfoil shape from root to tip assists in the correction about the vertical axis.
There is a slight amount of adverse yaw similar to an airplane that can be noticed when a roll is first initiated. The amount varies with the specific manufacturer’s design and make/model. In addition, the wing can yaw side to side to some degree, with some different manufacturer’s make/model more than others. The higher performance wings with less twist and a greater nose angle are noted for less yaw stability to gain performance. These wings also require more pilot input and skill to minimize yaw instability through pitch input. In addition to the wing planform, twist, and airfoil shapes to minimize yaw, some wings utilize horizontal stabilizers similar to those in airplanes, and others use tip fins. Generally, the WSC wing is yaw stable with minor variations that are different for each wing and can be controlled by pilot input, if needed.
Q. Is there a special technique for a crosswind takeoff?
A. The technique used during the initial takeoff roll in a crosswind is generally the same as used in a normal takeoff, except that the pilot must control the wings’ tendency to weathervane into the wind during the takeoff roll. Additionally, the pilot should keep the weight-shift-control (WSC) aircraft on the ground and accelerate to a higher speed before rotation. As the aircraft is taxied into takeoff position, it’s essential that the wind sock and other wind direction indicators be checked so that the presence of a crosswind may be recognized and anticipated. During taxi and takeoff, the windward side of the wing needs to be slightly lowered so as to not let the wind get under it and lift it off; but not too low or additional pilot effort is required and unnecessary stress is placed on the carriage.
The crosswind takeoff is performed similarly to the normal takeoff except two different techniques are utilized. First, as the WSC aircraft accelerates and the pilot steers the carriage straight down the runway, the wing will want to weathervane into the wind. This creates stress on the wing attachment to the carriage, the carriage mast, and the keel of the carriage. Therefore, the pilot must hold the wing control bar straight to the carriage, which requires significant force and muscle. Second, the pilot must accelerate to a higher speed before rotating to account for the crosswind component. This requires the nose to be held down to prevent the WSC aircraft from popping off the ground before the higher airspeed is obtained. Since this technique requires the pilot to muscle the wing rather than using a light touch, a mastery of the normal takeoff is needed before crosswind takeoffs should be attempted. As the WSC aircraft accelerates down the runway, the forces of the wing try to weathervane it into the wind, and the nose raises up to trim. The wing should be held straight with the nose down until rotation..
When a rotation speed faster than normal takeoff is used, a smooth but quicker push out to rotate is desired to get the front and rear wheels into the air quickly, avoiding any tendency to remain on the rear wheels. After liftoff, the WSC aircraft automatically rotates into the relative wind since momentum is straight down the runway, and the characteristics of the wing point it directly into the relative wind. The WSC sets up the wind correction angle (or crab angle as it’s also called) as it lifts off.
Q. What are the advantages of a strut braced or topless wing?
A. The primary advantage is the elimination of the king post and ground wires on top of the wing. No king post is needed because the struts can take a compression load and hold the wings up on the ground and also take the negative loads during flight. With struts, a weight-shift control aircraft is much shorter in height, allowing it to fit into hangars with lower doors and ceilings. This can make a big difference in finding suitable storage for the aircraft if leaving it set up.
Some strutted designs allow the wings to be folded back while still on the carriage. This can also be helpful when using a smaller space for storage by folding the wing up without taking it off the carriage. It is also convenient for sea trikes since the aircraft does not have to be taken out of the water to fold up the wing.
Strutted wings have a clean upper surface with no holes required for the king post or wires to go through the top of the sail. This reduces interference drag on the top of the wing. Increasing overall efficiency, no holes in the sail also eliminates any high pressure leakage underneath the wing from getting sucked up to the lower pressure on top of the wing.
Q. What are the unique aerodynamic features of a weight shift wing?
- The Weight Shift Control wing is pitch stable and does not require a tail because of the combination of airfoil design from root to tip, wing sweep, twist, and planform.
- WSC wing flexibility allows the wing to twist from side to side by shifting the weight providing the control to roll the aircraft without control surfaces.
- The WSC wing only has two axes of control, pitch and roll, while no yaw control is needed because it is yaw stable.
- The WSC wing is stall resistant because under normal flight conditions the tip chord is still flying while the rest of the wing is stalled - similar to the airplane canard system.
Q. What is trim speed as it relates to a trike?
A. Trim speed is the speed the wing will fly with hands off, or with no control bar pressure fore or aft.
If the pilot wishes to increase the trim speed, the center of gravity (CG) is moved forward. This is done by moving the hang point forward on the wing. Similarly, to reduce the trim speed, the hang point/CG is moved rearward on the wing. Additionally, some manufacturers have developed various types of systems to adjust trim speed in order to fly a faster or slower trim speed.