CarterCopter Propeller System

Edited by Rod Anderson

Patent # 5,997,250 and a second patent (recently approved but whose number is still forthcoming) covers most of the following information. The prop can be designed for either pusher or tractor applications of any diameter, engine HP or cruise speed. The prop and controller can (and should) be matched to the engine so that the package will optimize the propulsion efficiency for the operating envelope of the target aircraft. This simple prop design should cost much less to manufacture and maintain than all other comparable size props because it does not have the weight, complexity and cost of a spindle, spindle housing, and bearings. We have about 60 hours of testing on the first prop configuration, which was 2 inches short of 8 ft in diameter. The current prop's diameter is 8 ft.

Ultra-High Performance & Increased Safety

The use of carbon composites is a key to our success. Our current 2-blade 8 ft diameter prop weighs less that 30 lbs and produces a static thrust of over 1400 lbs at 300 HP. Our 4-blade 8 ft diameter prop will weigh less than 60 lbs and produce approximately 2500 lbs of static thrust from 700 HP.

Since this low-inertia propeller design is inherently light, it can be designed for a higher safety factor and still be very light. The blades of the current prop are made using prepreg instead of a wet layup. This makes the prop even lighter and stronger than the first configuration. Loads and weights are further reduced because our blade uses a spar that can bend so it does not have to carry both the bending loads associated with thrust and the gyroscopic precession caused when the aircraft changes direction. The simple 1-piece blade tip to blade tip spar also reduces weight and complexity. The prop has been proof-tested at a tip speed over Mach 1.0 for several minutes of continuous running to insure structural integrity.

Prop Description

The two carbon composite blades are hollow shells at the root section. A carbon composite spar with an "I" beam shaped cross section extends from blade tip to blade tip inside the two shells. This continuous "I" beam spar is connected at its center to the prop drive shaft then extends outward 30.5 inches in both directions (through the blade shells) before it attaches to the blades. The further from the center the "I" beam spar extends, the narrower it becomes until the top & bottom caps of the "I" beam finally come together at the attachment point for each blade. The spar then continues from the attachment points to the blade tips. The front half of the blades are solid from the attachment point outwards. The "I" beam spar is stiff in the edgewise direction and soft in the flatwise direction, allowing the blades to bend when they develop thrust or to flap as needed to reduce gyroscopic loads when the aircraft direction changes.

To handle rain, a stainless steel abrasion strip is bonded to the leading edge of the blades in a molded-in grove. To achieve a low noise profile, the tip of the blades is shaped like a shark's fin to increase the critical Mach number. The fact that the prop controller limits blade tip speeds to Mach 0.85 at full horsepower regardless of forward speed also helps produce a very quiet-running prop.

Pitch Change Process

The blade shells are torsionally very stiff, permitting them to be rotated (without deforming) about the spar centerline to adjust pitch. When this happens, the torsionally soft spar inside the blade shells is twisted between the spar hub and the point where it attaches to the blade at the 30.5 inch radius. The cross-shaped piece seen in the photo is used only to support the spinner. The rectangular bar seen behind the cross piece is used to rotate the blades by way of links and ball joints at the bar and the blade pitch horn attachment. The bar is mounted on a 1-3/8 dia tube that extends through the prop drive shaft and slides in and out on Teflon bearings. This tube is attached to a hydraulic cylinder on the end of the prop shaft that is pressurized by engine oil pressure and controlled by a spool valve operated by a computerized controller or by manual override.

A weight arm extends from the round root cuff of each blade to the otherwise empty space inside the spinner (see photo), and is located 90° to the blade's dynamic center-of-mass. This weight arm balances the pitch moment caused by the centrifugal force trying to force the blade's dynamic center of mass to the prop plane of rotation. The addition of this arm greatly reduces the moment required to change the prop pitch and makes the moment nearly constant throughout the entire pitch travel.

The spar can be twisted ±25° (50° total; it was proof tested at ±40° at 3 times its max centrifugal force in a special pull fixture). We plan to fly the CarterCopter at 400 mph at 45,000 ft. and for this we need a ¾ radius blade pitch angle of 57°. We set this pitch at one end of the travel. Therefore the minimum pitch we can achieve is presently +7° (57-50). If we were to accept a slower cruise speed, say 300 mph at 30,000 ft, then the max pitch would be approximately 43°. This would allow a negative pitch of -7° and provide reverse thrust capabilities.

Max Efficiency at Cruise Speed & Altitude

The blades use a 25% thick airfoil at the root (4.25" thick) which allows the root to operate at very high angles-of-attack at slow forward speeds without stalling. At the tip the thickness drops to 10%. Everything is optimized for the cruise condition. The root fits very close to the spinner to reduce root losses due to the air spilling over the edge. The blade chord increases from the tip to the root to accelerate the air uniformly over the full diameter. The optimized chord distribution does not vary with airspeed but the optimized blade twist distribution does. The blade twist distribution is therefore optimized for the predetermined max cruise speed where the aircraft will spend most of its time. At speeds lower than max cruise, the root must operate by necessity at a higher than ideal angle-of-attack, hence the reason for a larger percent thickness airfoil at the root to keep the airflow from stalling.

For max efficiency, it is important to match the prop design to the engine and predetermined cruise speed and altitude. If the aircraft is designed to cruise at 300-mph at 30,000 feet then the prop's blade twist needs to be less severe than the CarterCopter's 400-mph design. This less severe twist slightly improves prop efficiency for all speeds lower than the 300-mph cruise speed (when compared to the 400-mph prop design). This improved efficiency will manifest itself by slightly improved static thrust for the same HP. Prop efficiency will suffer if the aircraft is flown faster than the cruise speed for which the prop was designed.

Electronic Control System

The electronic control system measures rpm & torque (HP), air temperature and thrust. Based on this information, the controller then calculates the rpm needed for optimum efficiency and changes the prop pitch to obtain this rpm. Propeller efficiency is calculated and displayed in the cockpit to allow optimization of the rpm and pitch setting. This computerized controller does more for prop efficiency than solid state ignition and fuel injection did for I.C. engine efficiency.

In the event one of the controller input sensors should fail, the controller will signal an alarm, go to the backup sensor and continue to do its job. In the unlikely event both sensors should fail, then the controller will hold a rpm based on certain assumptions. The pilot can at any time go to manual control and use the prop efficiency display to fine tune the rpm for maximum efficiency.

4-Blade Prop

Two of the 2-blade props can be combined to make a 4-blade prop. The system was purposely designed to provide this flexibility. To make a 4-blade prop, we install the second set of blades behind the first set. We then change the control bar to a cross configuration so we have 4 points at which to connect the control rods that go to the blades.

FAA Certification

We have not talked with the FAA regarding their requirements to certify the prop. The prop system is patented and available to companies wishing to license and manufacture the prop for their own use and/or outside sale. Current plans are to have the licensees handle the certification process while we provide technical support.