Composite Rotorblades for Rotorway Helicopters
Technical Information
Background | Airfoil | Spar | Skins | Root End | Tip | Drag Reduction
| Velocity/Height Curves | Hovering
Autos/Maximum Performance Climbs
Set-Up | Balance | Autorotations
Waitman Helicopters, Inc. composite rotorblades were designed and built to replace the aluminum skinned blades made for the Rotorway Aircraft and Rotorway International helicopters. These include all models of Scorpion and Exec's, and the current Exec 162F.
The blades are manufactured at the facilities of Canadian Hme Rotors, Inc. in Marianna, FL. The helicopter blades are a result of the joint venture between Waitman Helicopters, Inc. and McCutchen Sky Wheels, who jointly own the process for these rotor blades, and are solely designed for helicopter use. Canadian Home Rotors Inc. are now producing these baldes for us under a licensing agreement.
The blades are designed to operate at 495 to 525 RPM for normal range. This was to accommodate the 500 RPM of the earlier Scorpions and the later 520 RPM of the Execs. Due to the construction of the blades, lower RPM limits of 430 RPM have been encountered with no damage. The blades have been oversped to 580 RPM with no damage to the blades or retention bolts.
The composite blades provide an option to the original aluminum blades which will provide the pilot a margin of safety not normally found in the amateur built helicopters. The best part is that these blade do not have a limited life as normally associated with aluminum blades. The only maintenance required is to keep the surface clean and free from bug splatters. The white composite epoxy gel-coat finish is easily resurfaced if it becomes worn from use. Thin blade tape used on conventional blade does not present a problem.
The blade uses the NACA 12 airfoil (25% chord balance) with an 8" chord. The airfoil is a true CAD/CAM engineered airfoil. The cutter was designed to conform to the exact profile and then used to cut the mold out of heavy aluminum. The end result is a mold that is a true airfoil and produces a blade with an extremely accurate airfoil. Since the blades skins and surface are of composite material and formed in a precise mold, each blade is identical in size and shape.
The spar is made from extruded aircraft aluminum that is CAD/CAM designed specifically for these blades. The dies were designed to produce a seamless D-channel spar. A notch was designed in the spar to accommodate the brass balance weight that is later bonded into the leading edge. Brass was chosen over steel for corrosion resistance. The spar is sandblasted to provide a superior bonding surface between the spar and composite skins. The bonding strength of 25,000 lbs. per square inch was tested. The failure was of the composite material itself and not the result of delamination of the material from the spar.
The skins are made of layers of Carbon Fiber and Graphite materials which provides a very rigid blade in the twist mode. The dampening is 43 cycle as compared to the 44 cycles of the aluminum factory blade from Rotorway. The skins are laid up in a S-glass and E-glass resin and cloth. The span-wise rigidity is stiffer than the aluminum thus a lower coning angle is seen in both the static and rotating mode. High frequency vibration is reduced by absorption in the composite material, producing a smoother ride and reduced fatigue on the metal airframe and components.
The root ends of the blades were designed to eliminate the need for composite doublers and wood blocks used with the standard aluminum blades provided by Rotorway. The extra layers of material are laid into the mold which is designed to provide the 2-1/4 inch at the root for the blade straps. This layering for the internal strength is carried out onto the blade approximately three feet. This serves as the means to spread the retention strength over a wider area. The airfoil is increased at the root end on the leading edge to reduce the drag in this area.
TIPS (Swept tips are currently not available)
The original swept tips of the blades where filled the last six inches and an equalizing air hold is provided. The swept tip reduces the drag at the tip and aerodynamically increases the blade length. The vortices are forced outward thus reducing the effect of the vortex on the advancing blade. This reduces the vibration transmitted to the advancing blade and stabilizes the blade in fast forward flight. The tip alone is responsible for a 15% reduction in fuel consumption and a reduction in drag. This along with a true airfoil and not having to include a reflex trailing edge, also reduces the fuel consumption by another 15%. Thus the overall savings due to drag reduction and other treatments equates total reduction in fuel consumption by 30% and a 1 to 1-1/2 inch reduction in manifold pressure required for both hover and forward flight. * Swept tips my be available in the future as an add on to the square tip blades
The treatments so far discussed also go along with items to produce a blade of outstanding design. Since the airfoil is symmetrical, the center of pressure does not move along the chord line and thus change the balance of the blade with changes in power. This eliminates the need of the reflex trailing edge to adjust the collective pressure as is required with the asymmetrical airfoil of the present Rotorway blades. The trailing edge of the Rotorway blades causes additional drag as a result of bending this edge to adjust collective pressure. Due to the pressure changes required in hover versus flight, the standard aluminum blades cannot compensate for the changes. The collective pressures change from hover to forward flight with the asymmetrical aluminum blades. The composite blades being symmetrical, do not change collective pressure from hover to forward flight. In fact, the blades become more stable and collective pressure is virtually eliminated. Each blade is made in the same mold, with the same amount of material. This virtually eliminates the requirement for a static teeter balance. Each blade becomes a perfect match of the other in both size and weight. Contact us before you add any weight.
The most dramatic difference between the composite blades and the standard aluminum blades is in the autorotations. The composite blades weigh approximately 1-1/2 lbs. more per blade. This allows for more stored energy for autorotations. The entry into an autorotation is less eventful as the blade, by design, does not want to increase angle of attack with reduction of power. The standard asymmetrical blade wants to reduce angle of attack with increase in power (pitch down). This characteristic causes unwanted changes with reduction of power or an engine failure. Composite blades with the symmetrical airfoil do not want to change angle of attack with power changes. Thus the helicopter enters the autorotation with very little altitude change. Since more energy is stored in the blades, entry into an autorotation is easier and less blade RPM is lost at the beginning. The autorotation is stabilized faster and with less loss of altitude. Then, during the decent, the blades actually tend to increase RPM thus storing more energy. With the ability to withstand overspeed, flares and quick stops at the bottom are less of a problem. More energy is available during the final stage of the autorotation. During the decent stage of the autorotation, the decent rate is reduced by approximately 40%. This equates to a less severe altitude loss and more time to execute a better approach and landing.
No tests have been done at this point to lower the envelope of the height/velocity curves. The helicopter should be flown with no change in the operations manual. The changes referenced are to provide a margin of safety, not a change in procedures.
HOVERING AUTOROTATIONS/MAXIMUM PERFORMANCE CLIMBS
Due to the performance of the aluminum blades purchased with the helicopters, blade inertia is not easily maintained under the conditions encountered when attempting these maneuvers. To successfully execute these maneuvers a blade must have sufficient available power, and the engine must have reserve manifold pressure. The composite blades provide an extra benefit in these areas and thus the change of successfully completing these maneuvers is higher. It is not the intent to say that use of these blades will guarantee the outcome of such maneuvers, but it is to say the benefits of these blades could be the difference if you find yourself inadvertently left with no options.
The blades are easier to install and rig as there are not wood blocks or glass doublers to worry with. No elaborate equipment is required to position the straps on the blades as the balance line is at 2 inches from the leading edge. Do not use the root end in finding these points as it is wider on the leading edge for the first several feet of the blade. The existing straps and aligner block may be used with no changes. Detailed information is provided to assist you in mounting retention straps, tracking, and lead-lag balancing.
Normal tracking and lead-lag adjustments are used according to the instructions supplied with the helicopter. We recommend using the flag stick method for tracking and simple bubble levels for lead-lag. If you do not understand these methods, we will be glad to discuss them with you prior to your beginning dynamic balance and run-up.
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