Some of the following information is covered in part by Patent number 5,727,754 and 2 follow on pending patents. It explains how we control the CarterCopter, share the lift between the wing and rotor, slow the rotor down, and maintain both rotor lift equilibrium and rotor stability at high forward speeds.

Control Stick

The control stick operates the cyclic (rotor control), ailerons and horizontal stabilator at the same time. At low speeds the ailerons and horizontal stabilator are not very effective at controlling the aircraft movements, but the rotor is very effective. As the speed increases and the wing carries more of the load, the rotor becomes less effective while the ailerons and stabilator become more effective. The wing incidence and initial trim relationship between the rotor spindle angle and the horizontal stabilator is set so that as the aircraft speed increases, the lift on the wing also increases. To keep from climbing, the pilot naturally pushes forward on the stick, which moves the stabilator in the direction to pitch the aircraft over and reduce both the wing angle of attack and its lift. The stick also tilts the rotor spindle forward to reduce the rotor plane of rotation relative to the airstream. The rotor is driven by the air flowing through it, much in the same way as the wind drives a wind mill, so tilting the rotor forward reduces the air flow through the rotor, the rotor rpm, and the rotor lift. We use a simple tilting spindle like many of the small autogyros to control the rotor plane of rotation rather than a more complex helicopter swash plate. The rotor center of lift is placed on the spindle tilt axis and the blade CG is placed on the blade pitch axis, so cyclic stick forces are minimized and cyclic boost may not be necessary except on heavier aircraft. When the rotor is unloaded and the flapping is low, which should be for most of the flight, cyclic loads are further reduced.

Collective

We essentially use the collective to control rotor flapping, and the rotor plane of rotation relative to the airstream to control rotor rpm. At the minimum level flight speed, the collective pitch is at its steady state greatest and at a value which provides a safe controllable rotor rpm. As the speed increases, the pilot, in addition to pushing the stick forward, decreases the collective pitch to reduce / hold rotor flapping at approximately 3-5 degrees. We will have a better feel for the best flapping angle after we complete the flight test program. If the flapping is low, then the collective pitch should be increased to increase the flapping. If the flapping is high, then the collective pitch should be reduced to reduce the flapping. At lower air speeds a change in collective also changes the rotor rpm, but at some increasing forward speed a change in collective will not effect rotor rpm.

Trim between rotor and wing

The trim relationship between the rotor spindle and horizontal stabilator is initially set so that at the airspeed where the wing can provide all the lift, say 150 mph, the horizontal stabilator is holding the wing at the angle for best L/D. At this point the rotor spindle has the rotor at an angle relative to the airstream to hold approximately 150 rpm. Also as the aircraft approaches 150 mph, the rotor collective pitch has been reduced to between zero and 2 degrees (to be optimized based upon flight tests) to hold rotor flapping within desired limits. As the airspeed increases, the control stick is pushed forward to keep from climbing. This not only pitches the aircraft forward slightly, but also tilts the rotor forward even more. At some point the rotor will be tilted forward to the point that there is not enough air flow through the rotor to maintain the desired minimum rpm. When this happens the pilot (or automatic controller) operates a trim switch which engages a servo to change the angle relationship between the spindle and stabilator. This servo tilts only the spindle until the desired rotor rpm is obtained - rearward tilt increases both the airflow through the rotor and the rotor rpm and forward tilt decreases the airflow and rotor rpm. This trimming does not move the stabilator, however when the rotor rpm changes, its lift will also change and will require a slight stick movement to change the wing angle to maintain aircraft lift equilibrium.

Accelerate past Mu of 1

If the pilot wishes to accelerate rather than climb, then as the aircraft speed increases, the stick is pushed forward, which causes the aircraft to pitch over and reduce the wing angle of attack and keep the wing lift constant. This stick movement and the pitching over of the aircraft reduces the rotor plane of rotation relative to the airstream and would slow the rotor down if the pilot did not use the rotor trim to tilt the rotor back to keep its angle relative to the airstream nearly constant. The mechanism is also designed so the more the trim tilts the rotor back, the less the stick movement changes the spindle tilt so that at nearly maximum trim the control stick can move its entire travel and change the spindle angle only about 1 degree. This washout keeps the rotor pitch angle relative to the aircraft nearly constant at low wing CL's, so that if the pilot should rapidly pull back on the stick, the rotor pitch does not become the sum of both the aircraft pitch change and the spindle angle change. Because of the high rotor inertia and the anemometer effect (retreating blade has more airfoil drag than the advancing blade), we are hoping this rotor rpm control will not be too sensitive. The pitch washout does not effect the side to side movement of the spindle relative to the stick. Once the rotor has been sufficiently unloaded by providing the lift with a wing and the thrust with a propeller or jet engine, then the rotor blades can maintain lift moment equilibrium about the hub at mu's (rotor tip speed ratio) greater than 1 with only rotor flapping. Rotor flapping also maintains rotor blade lift moment equilibrium for helicopters and autogyros at mu's less than 0.5.

Rotor Lift Equilibrium

Rotor flapping is the mechanism by which the advancing blade and retreating blades can produce the same lift moments. In order to work, the blades must be free to move up and down. This free flapping allows the advancing blade, which has more lift due to a higher velocity across it than the retreating blade, to rise or flap up. As the advancing blade rises, the resultant (vector sum of horizontal and vertical air velocities) flow angle across the blade (angle of attack) drops and reduces its lift. The faster the advancing blade rises, the more the resultant angle of attack is reduced and the more its lift drops. The opposite occurs on the retreating blade. As the advancing blade goes up, the retreating blade drops because the blades are tied together and because the retreating blade is not producing as much lift as the advancing blade. As the retreating blade drops (flaps), the resulting angle of airflow across the blade goes up and increases its lift. The faster the retreating blade drops, the more its angle of attack is increased and the more its lift increases. This characteristic whereby the lift on the retreating blade increases as the blade drops, and works whether the air flows from leading edge to trailing edge or from the trailing edge to the leading edge, allows the rotor to operate at a mu greater than 1. The flapping automatically increases until the vertical velocity component changes the angle of attack on both the advancing and retreating blades until they both have the same lift.

Rotor Centrifugal Force

As the rotor rpm slows down, the centrifugal force decreases until at some point there is not enough centrifugal force to keep the relatively soft and flexible rotor blades stable. To allow the rotor to be slowed down as much as practical, weight is added to the blade tips. For the CarterCopter prototype with a 43 ft. dia. rotor, 55# of depleted uranium (weighs 1.7 times lead) is added to each blade tip.

Slowing Down

As the throttle is reduced and the aircraft slows down, the control works basically the same in reverse. To maintain lift as the speed decreases, the pilot pulls back on the stick, which causes more air to flow through the rotor and the rpm to increase. As the speed continues to slow down the rotor RPM will eventually speed up higher than desired. To slow the rotor down and increase flapping, the pilot operates the trim. If the trim should fail at this point, the aircraft should still be able to land safely. The rpm and drag will go up faster than normal, but the aircraft will continue to hang from the rotor. With the horizontal stabilator, spindle side to side movement, collective, throttle and rudder still working, the pilot should be able to control the aircraft. Obviously, this is something we will want to check out gradually in the test program.

Flight Analysis Program

So that we could confirm that this control system would work prior to first flight and to have a better understanding of how all the flight control surfaces would work together we developed a computer model of the CarterCopter. Shown below is a spreadsheet predicting how the aircraft performs at speeds from 30 to 225 mph. Our inputs for each calculation are altitude, weight, CG location, airspeed, and collective. Based on these inputs and the model specs, everything else can be calculated. Basically the program has to solve several unknowns simultaneously until the sum of all the moments and forces acting on the aircraft are balanced. The program requires many loops and a fast computer. We generally have to make some fairly close guesses as to what some of the values will be in order for the program to solve without being stuck in a loop. If there is not a solution, it will not solve. We worked on the program for 3 years prior to the first flight. To determine the rotor rpm, the theoretical blade was broken into 10 sections from tip to root and analyzed at 0, 90, 180, and 270 degrees. The rpm was then changed until the sum of the driving force moments equaled the sum of the drag force moments. We determined flapping by also breaking the rotor blade into 10 sections. The vertical flapping velocity was then changed until the sum of the lift moments on the 10 sections of the advancing blade equaled the sum of the lift moments on the retreating blade. As a check we compared our 10 element flapping analysis results to the Bailey method (commonly used by helicopter manufacturers). As the enclosed spreadsheet shows, the results are very close at low mu's. However, the Bailey method is not accurate at mu's greater than 0.5, which was the reason we had to develop an analysis that would work over a wide range of mu's. Eventually the program will be modified so that calculated data agreed with actual data. Thus far actual rotor rpm for a given collective and forward speed is 50 to 100 rpm greater than the calculated rpm. Actual flapping based on collective and airspeed is close, but based on rpm and airspeed flapping is about 3 degrees higher. Actual aircraft pitch for a given airspeed is greater than that calculated because we did not account for the rotor downwash effect on the slipstream, but the wing angle of attack is very close to the calculated value. The program also shows that collective has less effect on rotor rpm as the airspeed increases, indicating collective should not be used as a rpm control. This supports our control design.

Performance

The better people understand how the CarterCopter works, how much the drag can be lowered by reducing the rotor rpm and how the low rpm can be achieved and still keep the rotor stable, the more believable the performance numbers become. For most people however, the proof will be when we finally achieve our goals.

Aircraft Flight Analysis Table

-Jay Carter