Written by Dave Lockhart
Sport & Aerobatics
As seen in the Fall 2021 issue of Park Pilot
Traditional aerobatic maneuvers include a variety of loops, rolls, point rolls, snap rolls, inverted flight, stall turns, and spins. Many trainer and sport airplanes (even some scale airplanes) can complete most or all of these maneuvers. In some cases, the control throws need to be increased to be able to complete the aerobatic maneuvers. Newer aerobatic airplanes are also capable of 3D maneuvers (post-stall) and extreme aerobatics. These aircraft have much larger control surfaces and utilize a lot of control surface deflection to perform 3D and extreme aerobatic maneuvers. Except for the most basic entry-level radios, all modern radios have some number of settings for dual (or triple or more) rates, exponential, and flight modes/conditions. The use of these functions can make an airplane easier to fly for different maneuvers. Aerobatic airplanes are generally quite responsive and don’t need a lot of control throw to take off, land, and fly simple aerobatic maneuvers. However, performing more aggressive maneuvers, such as snaps and spins, and flying 3D and extreme aerobatics, requires more control throw. The increased control throw can make basic flying, flying at high speed, and simpler maneuvers more difficult because the airplane can become “twitchy” or very reactive to small stick inputs on the transmitter. This is where the use of dual rates can be helpful.
The transmitter graph shows a linear response with 100% servo output at 100% control stick deflection. No dual rates or exponential are being used.
Dual rates are programmable electronic limits that, when activated, reduce control without making changes to the transmitter sticks or airplane linkages. Dual-rate settings are specific to each primary control axis: aileron, elevator, and rudder. Dual rates can be activated by switches for each control axis and, on some radios, multiple axes can be assigned to a single switch or assigned to flight modes and conditions. Additionally, flight modes and conditions allow mixing, trims, and other features to be assigned to a single switch. For example, an airplane set up for 3D might have 50° of up-elevator. When a dual rate at 50% is activated by a switch, the elevator throw is reduced to approximately 25°. When programmed in this way, 3D maneuvers are performed on high rates, and traditional aerobatics and the takeoff and landing are performed on low rates. Traditional maneuvers are much easier to fly smoothly on low rates. An additional, useful feature that many pilots use is exponential. As with rates, it can be programmed separately for aileron, elevator, and rudder. Normal control response on low or high rate is linear; 10% of stick movement results in 10% of available servo movement. Exponential response is not linear. Each increment that the stick is moved does not result in an equal amount of servo movement. The exponential is typically used to reduce the amount of servo movement when the control stick is close to center. The full control throw is not changed, so reducing servo movement close to the stick center means that servo movement is increased toward the end of the stick movement. This has the effect of low rate (dual rate) at the center for smooth flying while retaining high rate (full rate) at full stick deflections for aggressive maneuvers. Exponential, in some respects, can be thought of as an automatic dual rate.
This transmitter graph shows that the servo output is maintained at 100%, but the response close to center has been reduced, making the control feel less sensitive for small stick inputs. Also visible is a slightly increased amount of exponential for up-elevator to make up-elevator and down-elevator response feel more symmetrical.
The amount of exponential is adjusted by increasing the percentage of exponential from 0 to 100. Higher values increasingly reduce sensitivity at stick center with “steeper” curves to catch up and achieve full throw at full stick deflections. Please note that some radios use positive exponential numbers to denote a reduced control response at center, while other radios use negative numbers to denote a reduced control response at center. The control response for left rudder and right rudder is rarely equal. This is because of asymmetric forces associated with the motor and spinning propeller: torque, gyroscopics, spiral airflow, and P-factor. Some radios allow independent exponential values to be programmed in each direction (left/right, up/down). Using a higher exponential value for the right rudder compared with the left rudder can make the control feel more symmetrical. The same approach can be used with the elevator, where down-elevator is generally slightly less effective, especially when airplanes are nose-heavy. Using a slightly higher exponential value for up-elevator can result in a more symmetrical control feel.
The graph shows that the servo output is limited to 50%. A small amount of exponential is used to decrease the sensitivity close to center.
Some radios also have the exponential function available for use on the throttle. It is not uncommon to find combinations of motors and ESCs that do not “feel” linear in their power output. Exponential can be used to increase or decrease responsiveness on the low or high side of the throttle stick. A word of caution about using dual rates, exponential, and flight modes. Do not set up these functions in the radio if you will not use them. Always check all switches on the radio before flying to avoid being surprised by the control response. Some maneuvers are only possible with the correct rates, so it is important to remember what rates are activated to avoid unplanned landings ... which usually don’t fare well for the airplane.
Exponential can be used to alter the throttle response. In this case, the sensitivity is increased on the low side of the throttle stick to give a quicker response from idle.
Despite its large size (64-inch wingspan and length), this lightly built, 3-pound Slow Ride uses extreme control throws for slow-speed 3D flying and only needs minimal throws for moderate-speed aerobatics.
By Dave Lockhart | [email protected]
