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The Evolution of the Nose Gear Retraction System for Experimental Canard Pusher Aircraft PDF Print E-mail

By: Jack Wilhelmson

The canard pusher experimental aircraft as designed by Burt Rutan was both revolutionary and evolutionary. It has been said that Burt really never expected to sell more than fifty sets of plans for his Varieze.  Probably because he knew that some of the creature comfort (or lack thereof) features of the design would appeal only to the real aerodynamic efficiency proponents and individuals who fully appreciate engineering excellence.

The fact is that as innovative as his designs were they all suffered from one drawback (depending on your viewpoint) the need to park them with the nose on the ground. This also required that the nose be lifted manually before boarding the aircraft, making the aircraft unusable by anyone with back problems and rather difficult to board. All subsequent derivations of the design also had this requirement. One derivation of the design (Velocity) escaped from this requirement, but only by extreme modification and compromise of the very efficient aerodynamics and lightweight construction.

In recent years the application of general purpose, 12 volt, linear actuators came into use for electrical actuation of the nose gear with the airplane loaded. The addition of the standard spring to absorb shock from bumps and hard landings completed this evolutionary step.

With the linear actuator the airplane can be parked nose down for stability, boarded (in the nose down position) and raised by the push of a button. In addition a method of emergency extension, if the electrical power fails, is available. The drawbacks to this innovation are added weight, space, installation modifications (in finished aircraft), and of course cost.

The linear actuator using a 90% efficient ball screw is the best feature of the available system. from this the present day system evolved. The length of the linear actuator after addition of the 4” shock spring was what caused most of the problem. It interfered with the radio trays and the trim system and required cutting away and reinforcing the F22 bulkhead. An engineering analyzes of the forces during the various positions of the gear under static load revealed that the system was at a severe leverage disadvantage during the first 18”of lift from the full nose down position. These forces range as high as 2500 lbs. axial force in the ball screw during the initial lifting of a fully loaded 2000 lb. gross wt. Canard aircraft.

The leverage disadvantage was made worse by the need to keep the actuator unit low under the instrument panel so as not to interfere with the elevator torque tube. Also there is considerable dynamic shock loading from hard landings and the inevitable bounces that occur with botched landings etc. This reaffirmed the need for a shock control system and consideration of the dynamic forces created when the stored spring energy is suddenly released. The present shock control system (consisting of a preloaded spring retained by high strength steel center rod with a .25 dia. bolt in double shear at each end has served well in most cases. So this system was analyzed to determine it’s ultimate strength and use that as the design criteria (plus 50% to allow for the heavier aircraft being built today) for the new system. In addition a recoil shock absorber consisting of five O-rings was added so that the 1700 lb spring could not damage the actuator after repeated hard landings and many rough fields.

Another goal of the new design was to eliminate all welded joints in the main load carrying part of the system. The desire to eliminate welding came primarily from the quality control problems (stress relieving, x-ray etc.) with welding.

Analyzing the force path through the linear actuator revealed that; if the forces can be transferred from the bolt in NG3 to the spring, then directly to the ball nut, through the ball screw to the thrust bearings, and then to the mounting pins, the outside housings only act as braces to keep everything in line and mount the components. Therefore the outside housing can be made much lighter than the stock linear actuator system especially if they are made of high strength 4130 steel.

It was apparent that the unit had to be shortened so that it could be contained ahead of F22,  It was apparent that if the spring center hole could be utilized for the ball screw to pass through the over all unit length could be shortened by at least four inches accomplish the goal of keeping the installation ahead of the F22 bulkhead. In addition, we were able to reduce the travel of the unit to only the required amount and by using high strength steel and aluminum we were able to reduce the overall weight of the completed assembly to ten pounds. This brings the added weight penalty to only four pounds over the standard manual retraction system.

The completed system requires no modification of the existing structure (except to drill one 5/8” hole in F22 for the manual extension shaft). It is a “drop in installation”. The illustration and pictures included are self explanatory. In addition the system is more capable of lifting the load due to the better leverage angle that it uses to apply the force to the nose gear. The system can be completely removed by removing three accessible bolts and lifting it upward through the nose access door.

The electrical control system used with the new actuator has an electronic system that extends the gear if the pilot forgets. The system uses an airspeed sensing feature that extends the gear automatically at 90mph (adjustable) airspeed (no delay). If the airspeed increase above 90 mph a 20 sec delay is provided in the automatic retraction of the gear with gear switch in the up position. This is designed to allow time for the gear to be fully extended even if the airspeed is increased above the set point.

A momentary switch is used so that the pilot can defeat the automatic feature instantly at any time. (for: slow flight, high pucker factor takeoffs over an obstacle and parking on the ground with the nose down.)

As many years passed, and many actuators were installed a problem showed up due to some changes in the drive motor from a sleeve bearing motor to a ball bearing motor. While this was an improvement in efficiency it created a "back drive"

problem in that the unit would no longer hold a significant load with no power applied(a desirable feature of the older units). This problem was solved by designing a electrical circuit that created dynamic passive braking to the unit with power removed. However, the manual emergency extension feature was no longer possible because the dynamic braking made the manual extension torque too high. This was solved by installing emergency braking in the electrical harness using a small sealed back up battery. The emergency back up is controlled by a panel switch position. In emergency mode the complete micro switch system that controls the actuator position is by passed. Overload auto resetting is provided so that the mechanical down stop is used for position.



Last Updated on Tuesday, 05 February 2019 18:31
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