Jay Carter, Jr. President, CarterCopters, L.L.C.

Presented at the American Helicopter Society Vertical Lift Aircraft Design Conference, San Francisco, CA.
Copyright © 2000 by the American Helicopter Society, International. All rights reserved.

INTRODUCTION

MACHINE DESIGN ® Magazine published an article about the CarterCopter (CC) on April 18, 1996, titled 'Greatest plane ever - or a pipe dream'? This title sums up the response the CarterCopter has drawn since its development was first announced in 1995.

The following paper provides the basic information needed to understand the design approach that makes the CarterCopter possible. Our initial tests using a 1/6 scale model in our 'poor boy' wind-tunnel (see below) showed that the CarterCopter can fly faster, more efficiently than any other rotorcraft - including the current tilt rotor designs and in many cases fixed wing aircraft. This conclusion continues to be supported by all flight test data collected since the prototype began flying in September 1998.

Figure 1 - Picture of CC flying w/ L.G. and nose boom removed

WHAT IS THE CARTER COPTER

The CarterCopter is a hybrid autogyro and fixed wing aircraft. The two technologies are combined in such a way that the aircraft can perform zero roll take-offs and landings yet have a true cruise speed as high as 500 mph with potentially less cruise drag than a certified single-engine propeller driven airplane. The rotor is not powered in flight by a drive shaft, but rather by the air flowing through the rotor like the wind powers a windmill. With no torque reaction on the fuselage, the tail rotor or twin rotor configuration is eliminated. Zero roll take-offs and landings are achieved with an ultra high inertia rotor. Low cruise speed drag is accomplished by unloading and slowing the rotor down to a tip speed as low as 100 mph and providing the lift from a small wing sized for high speed and the thrust from a propeller or jet. This concept for reducing drag is not new. What is new is to slow the rotor down and still provide rotor stability at a rotor advance ratio as high as 8.

While the CC design cannot hover for more than 5 seconds, its ability to take off and land vertically safely (no dead man zone) and its low initial and operating costs relative to a helicopter will allow it to take over as much as 80% of the present helicopter market. Its ability to fly as fast and efficiently as propeller-driven aircraft without the need of an airport runway and its ability to make a zero roll landing with a dead engine will allow it to compete with all the propeller airplane markets.

FLIGHT ACHIEVEMENTS TO DATE

When the 12th flight test series unexpectedly concluded December 16, 1999, with an accident that caused substantial damage - the prototype had logged more than 15 hours of flight time and 30 hrs. of high speed taxi and static tests - full throttle engine, prop and rotor tests. It had flown to 126 mph in patterns of 6-8 mile lengths and had climbed at 600 feet per minute to an altitude of 800 feet. During the forced emergency landing that led to the accident, the CC continued to fly at a pitched attitude of 30 degrees with aileron reversal and with the rotor at only 230 rpm and a forward airspeed of 74 mph.

CAUSE OF ACCIDENT

The accident occurred as a result of the pilot not lowering the collective soon enough as the aircraft speed increased. The speed increased much faster than normal, because the stored rotor energy was expended more quickly with full collective held for a longer period. The result was much less drag than usual. Initially, what accounted for the lower drag was that the rotor was able to provide the lift without the drag associated with tilting the rotor back. A few seconds later, drag was less due to the fact that the rotor rpm had dropped to its most efficient, best lift to drag condition. Eventually, the rotor flapping exceeded 9 degrees. When the pilot cut the throttle and slowed the aircraft down, the rotor did not speed up quickly due to the high blade pitch. Upon initiation of the flair, the pilot realized something was wrong and increased the throttle. The aircraft pitch increased to 30 degree with the wings now in a full stall with aileron reversal. Not knowing for sure what happened and because of a very sluggish responding cyclic control, the pilot felt the need to land as soon as he could level out the aircraft. Upon landing, the rotor plane of rotation was too far aft due to the low rpm and high flapping. As the aircraft pitched over, the pilot instinctively pulled back even further on the stick causing the rotor to hit the tail surfaces and prop. As the nose boom dug into the soft asphalt, it caused the aircraft to spin around and try to roll. The wing prevented the roll but broke in the process. Neither pilot realized until the data was reviewed later that their rpm had dropped to 230 rpm or that they had accelerated to nearly 100 mph. They did not recall that the excessive rotor flapping alarm was flashing in the cockpit.

Fortunately, no one was hurt. We plan to make a number of improvements at the same time repairs are made. Next flight will probably not occur until late spring of the year 2000.

NEAR-TERM GOALS

There are two remaining big questions, which the flight test program will answer.

 

1. Will the drag on the rotor really drop as much as calculated when the rotor rpm is reduced from 300 to 100 rpm?

2. Can the rpm be reduced to this level and the rotor still remain stable at tip speed ratios much greater than 1?

In addition to answering the above questions, our NASA SBIR Phase III contract contains the five goals listed below:

1. Perform a zero-roll takeoff.

2. Perform a zero-roll landing.

3. Fly non-stop 600 miles.

4. Fly above 10,000 feet.

5. Fly greater than 150 mph with the rotor at less than 160 rpm (Mu > 0.8).

GENERAL DESIGN

Figure 1 - Picture of aircraft from above w/ 3 people beside it

ROTOR:

1. At high aircraft forward speeds, the rotor has to be slowed down to prevent the advancing blade from seeing a velocity too near the speed of sound.
   
    

2. The only way to prevent the retreating blade form stalling at high forward speeds is to unload the rotor by providing most of the lift with a wing and the forward thrust from a propeller or jet.
   
    

3. The centrifugal force from 55-pounds of depleted uranium in each blade tip keeps the rotor rigid and stable at the reduced rotor rpm and high forward speeds.
   
   Even with the 110-pounds of depleted uranium, the rotor and rotor hub weigh only 240-pounds. This weight is 7.5% of the aircraft's gross weight - which is as good as any other rotor craft. This means that even with the 110-pound weights, and because a heavy continuous duty gearbox is eliminated the useful load of the CC is not compromised but can be improved.
   
    

4. Both the advancing and retreating blades have to produce essentially the same lift during flight. As long as the rotor is turning, basically unloaded, and free to flap, it is possible to obtain rotor lift equilibrium even when the retreating blade experiences a high reverse airflow over the trailing edge.
   
   Not only can this fact be proven analytically, but we have demonstrated in gusty, turbulent wind conditions that a 1/6 scale model rotor can maintain stability at a forward speed to rotor-tip speed ratio up to a Mu of 8. These tests were made in our, 'poor-boy wind tunnel' which comprised a model on an 8-ft boom extended from the front of a truck. A Mu of 8 means were we were going down the highway eight times faster than our rotor tip speed. This test proved to us it is possible for a rotary wing aircraft to attain a forward speed in excess of 500 mph, twice the existing certified world record for a helicopter. The limitation would be that at 500 mph the advancing rotor-tip, although slowed to 100 mph relative to the aircraft, would see 600 mph of relative wind. Higher speeds would creation additional problems as the rotor-tip speed approached mach 1.
   
   In the 8th flight test series ending June 3, 1999, the rotor was slowed sufficiently to transfer half of the aircraft's 3,000-pound weight onto the wings. As expected - the rotor rpm and rotor drag both decreased as the rotor was unloaded. Even with forward airspeed increasing and the rotor rpm decreasing as the weight was transferred, blade flapping did not increase - showing that lift equilibrium was being maintained between the advancing and retreating blades. This situation is the opposite of that which occurs on helicopters and autogyros and so far indicates the patented flapping control concept is working and will eventually allow a jet powered CarterCopter to fly as fast as 500 mph true airspeed.
   
    

5. The rotor profile horsepower required in flight is essentially a function of RPM³. Dropping the rotor rpm from 300 to 100, for example, reduces the rotational aspect of the rotor profile horsepower to approximately (1/3) ³ or 1/27 of the rotational profile hp @ 300 rpm. This is a major drag reduction.
   
   Because the surface area of the rotor can be small in comparison to a fixed wing aircraft for the same lift, the forward velocity drag component on a slow turning rotor can be small even relative to an airplane wing.
   
    

6. The rotor was designed for an efficient low disk loading and allows solidity factor. The resulting 28 sq. ft of blade area and 32 ft diameter would, if it were not turning, have essentially the same drag as 28 sq. ft of wing area. But since it slowly turns unloaded in cruise, it has a drag equivalent area of 33 sq. ft. The problems associated with stopping the rotor do not justify the equivalent 5 sq. feet reduction in wetted area.
   
   Note the rotor diameter is being increased to 43½ ft for the next flight tests with a corresponding increase in blade area, but this increased blade area is still small in comparison to the wing of a similar weight airplane.

WINGS:

The CC wings are 20-25% the size of a conventional fixed wing aircraft of the same gross weight. Since the wings are not used for takeoffs and landings, they are designed specifically for low profile drag and low induced drag - to be very efficient at high forward speeds and high altitudes.

Since the rotor provides lift at slow and intermediate speeds, the wing can be sized very small and yet provide all the lift needed for cruise conditions. This combination of a very small high aspect ratio wing with a slow turning rotor results in significantly less net lifting surface drag than a comparable fixed wing propeller-driven aircraft.
 

1. To achieve a 400 mph cruise speed efficiently, you must reduce drag. Profile drag reduction is obtained by reducing the lifting surface drag to the minimum square footage necessary to support the aircraft; at the airfoil CL (coefficient of lift) for best L/D (lift-drag ratio), at gross weight, at 400 mph, and at 45,000 ft where drag, because of the thin air, is less than 1/5 the drag at sea level for the same speed.
   
    

2. To keep induced drag as low as practical, the wing span, which affects induced drag by the inverse of span squared, is set at 32 ft. for the CC prototype. The long, narrow wing provides an almost sailplane like aspect ratio.
   
   The wing area needed to support a 3000-pound gross weight CC at 400 mph and 45,000 ft altitude with a wing CL (coefficient of lift) of 0.9 is 42 sq. ft.
   
   With this wing loading, the CC at sea level must fly at 176 mph to fully unload the rotor.
   
   The wing area required to support a 3000-pound conventional fixed wing aircraft at sea level and at 70 mph with an average flapped wing CL of 1.6 is 150 sq. ft.
   
   The equivalent lifting surface wetted area of the CC is 75 sq. ft (33 rotor + 42 wing), which is 0.5 (75/150 = 0.5) of the above fixed wing aircraft and represents a significant reduction in lifting surface profile drag.
   
   Note: Since the prototype has been designed to break some altitude, distance, and lifting records, its wing area was compromised and increased to 76 sq. ft.
   
    

3. In cruise the CC flies like a conventional fixed wing aircraft. If one loses an engine, the aircraft is nosed down slightly to maintain the best glide speed. As the aircraft speed is further slowed upon landing approach, the rotor automatically provides more of the lift and flight control. The control is very simple from both a mechanical and pilot standpoint. The pilot is not aware of the transition between rotary and fixed wing flight.
   
    

4. Because of the drag reducing design of the small high aspect ratio wing, slowed rotor, streamlined fuselage and rotor head, the glide ratio at best L/D can be better than 12.

POWER PLANT

To obtain the horsepower needed in the thin air at high altitudes, the power plant selected is a compound turbo-charged water-cooled piston engine.

NOTES:

1. A piston engine is more efficient relative to a gas turbine.

2. It is more difficult for gas turbines to maintain horsepower ratings at high altitudes.

3. Two turbos in series - a large low pressure and a smaller high pressure turbo with an intercooler and aftercooler - are used to efficiently obtain the needed manifold pressure for 300 hp at 45,000 ft altitude.

4. A water-cooled engine is generally more efficient and easier to cool than an air cooled engine at high altitude, high horsepower outputs.

5. Our Corvette LS-1, V-8 engine with prop reduction drive weighs 450 lbs. not including turbos, turbo coolers, radiator and coolant.

PRESSURED CABIN:

For practical operations between 15,000 and 50,000 ft, the cabin must be pressurized. The 60-inch wide, 5 place CC prototype with its ¼ inch thick stretched acrylic windshield and special door whereby the loads are transferred across the windshield and door rather than around it was filled with 6000 lbs. of water and pressure tested to 25 psi. This is twice the proof pressure required of a pressurized aircraft certified to 45,000 ft.

PROPELLER:

In order to have good static thrust for jump take-offs and good cruise efficiency at true airspeeds up to 400 mph at 50,000 ft, a propeller and controller were developed. The resulting 30-lb. propeller is 8 ft in diameter and uses a twistable carbon spar with a 50^o pitch travel. The blade chord increases ideally from the tip to the root in order to accelerate the air uniformly over the full diameter. The resulting large blade cuff with a close fitting spinner reduces root losses. The tip has a swept shark fin profile to reduce compression drag and noise at high tip speeds. In tests the prop tip was run above the speed of sound for several minutes. Best static thrust is over 1400 lbs. at 300 HP. Propeller cruise efficiency is expected to reach 94%.

To optimize cruise efficiency over a wide range of HPs and air speeds, a computer program was developed which monitors true airspeed, HP and air temperature, and then controls the prop pitch to obtain the rpm required to achieve the best efficiency for the given conditions. This controller does for prop efficiency what computers have done for engine efficiency and emissions and makes conventional mechanical prop governors as antiquated as the Model T.

LANDING GEAR:

For safety and to make landings much more pilot friendly, a landing gear capable of absorbing a 20 ft/sec vertical impact without damaging the aircraft or hurting the occupants was developed. The landing gear has a "smart" mechanical valve which measures the vertical velocity in the first inch of landing gear travel and then regulates the air-over-hydraulic cylinder pressure over the remaining 17 inches of travel to provide an almost constant deceleration for the given landing condition. Cylinder pressure is approximately 1000 psi at 20 ft/sec, 250 psi at 10 ft/sec and so on for a "greased" landing nearly every time. In the event of a crash landing over 25 ft/sec, the smart valve holds the cylinder pressure at the maximum landing gear yield load for the full stroke so the landing gear does not fail until it bottoms out and the maximum energy has been absorbed.

CENTER LINE OF THRUST AND PITCH STABILITY:

To prevent forward snap roll (fatal condition which occurred in numerous autogyro accidents) which generally occurs at a slow speed, high thrust, zero "g" condition, the center line of thrust was placed through the aircraft's average vertical center of gravity. This forward snap roll condition generally occurs when the pilot pushes the nose over too quickly after the aircraft's speed has decayed following a full throttle rapid climb out. The moment caused when the center of thrust is not through the aircraft's C.G. and the aircraft is no longer hanging from the rotor results in a fatal pitch snap roll.

To prevent porpoising (another condition which has caused numerous fatal accidents in autogyros) a large horizontal stabilitor is used. Autogyros can fly without a horizontal stabilizer in slow speed flight, but as the speeds increases, the pitch change angle of the rotor for a given change in lift becomes less, resulting in smaller control stick movements (more sensitive). As a result without the stabilizing force from a large horizontal stabilizer, the pilot over controls the control stick. The situation is made worse because the aircraft acts like a free-swing essentially undamped pendulum beneath the rotor.

OTHER FEATURES OF THE CARTERCOPTER

The ultra high inertia rotor has over twice the available inertia per pound of gross weight than any other rotor. This inertia was achieved by placing 55 pounds of depleted uranium in each rotor-tip. Utilizing only inertia, the rotor should be capable of lifting the 3000-pound CC in a zero-roll take-off to a height of 50 ft and a speed of 50 mph in less than 4 seconds.

Note: In order to achieve this takeoff performance, the rotor diameter of the prototype is being increased by 10 ft and the spar redesigned so the collective force required by the pilot is significantly reduced. Flight test data indicate the pilot was able to pull only 6^o of collective at the maximum rotor overspeed rpm without boost due to the tennis racket effect and still fly the aircraft. Also at the higher blade angle of attack required for vertical liftoff from the smaller diameter rotor, the high compression drag due to the high angle of attack wastes too much of the stored rotor energy.

TAKEOFFS: No power goes to the rotor through the drive shaft during flight, hence there is no torque reaction on the fuselage which would have to be countered by some means such as a tail rotor. During rotor pre-rotation while the aircraft is on the ground, there is a torque reaction on the fuselage, but with the wheels locked the fuselage cannot turn. Since the pre-rotator drive is only used intermittently while on the ground and does not have to provide lifting HP, a simple lightweight belt drive and clutch system is used. Actual takeoff G forces can be varied from a little over the normal of 1 to 2 at the pilot's discretion depending upon the performance desired.

LANDINGS: Rotor pitch is set so the auto rotating rotor will automatically overspeed during landing. If needed, the pilot can pull collective and increase lift to control touchdown using the energy stored in the rotor. Normally, pilots will land and flair with some initial forward speed. Upon flair, the rotor acts like a aerodynamic brake, allowing the aircraft to touch down softly with a zero roll and without pulling collective. A vertical descent, however, would require pulling collective to stop the sink rate and provide a soft landing.

The aircraft rotor has the ability to store so much energy, the pilot could with a light load and a dead engine make a zero roll landing, then pull collective, jump 40 ft in the air and accelerate to 40 mph, re-establish a glide slope and make a second dead stick landing some 100 yards away.

 

SUMMARY

While the CC design cannot hover for more than 5 seconds, its ability to take off and land vertically safely (no dead man zone) and its lower initial and operating costs relative to a helicopter will allow it to take over as much as 80% of the present helicopter market. Approximately 20% of the present helicopter markets require continuous hover capability. Its ability to fly as fast and efficiently as propeller driven aircraft without the need of an airport runway and its ability to make a zero roll landing with a dead engine will allow it to compete with all the propeller-driven airplane markets. The CarterCopter design could reasonably be scaled up to carry 100 passengers. A jet powered CarterCopter capable of flying up to 500 mph true airspeed with the ability to take off from the top of buildings can beat the fastest jets door-to-door except for extremely long flights. Therefore, the business jet markets are also available to the CarterCopter.