Introduction

The AeroComposites' propeller design makes use of the best of commercial, military and general aviation propeller technology, developed by all the major propeller manufacturers over the past 50 years. This technology has been evolved through thousands of hours of development testing and hundreds of thousands of hours of  in service flight operations. In addition, published general aviation propeller service data, as well as FAA REQUIRED upgrades, issued through Airworthiness Directives (AD's), were reviewed in depth by AeroComposites' engineers where "lessons learned" from these data have been incorporated in AeroComposites' designs. In addition, AeroComposites' patented design innovations such as its fail-proof blade retention have been incorporated in the design of its advanced technology, next generation composite blades.

Following completion of the design phase, pre-production components and propellers were produced for test evaluation. This involved component level testing as well as propeller ground engine-dyno tests (e.g. instrumented vibration tests), many ground aircraft tests, followed by flight testing. Following is a brief summary of this testing. AeroComposites propeller test evaluation is part of its on-going program to verify its designs in preparation for FAA Certification.

Component Testing

Following is a brief description of some of the propeller component testing conducted to date.

Blade Retention-As part of blade retention system testing, a blade was cut at its mid-point and the outboard blade area was built up with multiple layers of fiberglass. This area was then machined flat and fitted with a bolted multiple steel plate mechanism, allowing the blade to be pulled in the centrifugal direction. The blade root, specifically the two corrosion-resistant, stainless steel nested retention rings, were then mated with ball bearing retention components identical to those used in an actual aluminum hub installation, except these were set in a 2-inch thick steel plate simulating the hub. The blade was tested to 50,000 pounds of outward, radial pull load (limit of the test rig used), which is 2.5 times normal operating centrifugal load at 2,700 rpm. No delamination, separation or movement of the composite structure was observed and no dimensional distortion of the open internal retention diameter was measured (0.001-inch sensitivity) during and after load application. Analysis predicts that the blade retention system can withstand radial loads in excess of 200,000 pounds without failure.

Coupon Strength-Multiple composite coupon test samples, of characteristic blade structure, thickness, and width, were pulled to failure to statistically measure the strength of representative blade composite samples under tensil load. Test data showed that properties of all samples tested were within 5of calculated theoretical values used in the design. AeroComposites' composite blades are designed to have more than 10 times the centrifugal load capacity required in normal service.

Resin Ductility-The resin used in AeroComposites' blades is a toughened epoxy with tested impact parameters up to three times greater than resins used in many commercial composite propeller blades flying today. This toughness provides greater resistance to foreign object damage such as from stones and/or ice, etc. striking the blades. Impact resistance tests were conducted with a 16-ounce carpenter's hammer on a 1/8-inch thick by 12-inch square flat panel, representative of the outer blade's composite laminate. After many repeated heavy blows to the surface with the hammer, no damage was sustained other than minor cosmetic blemishes on the toughened epoxy surfaces.  Pure resin samples, unsupported by fiberglass or carbon fiber, can be bent a substantial amount without breaking demonstrating the resin's excellent ductile property.

The combination of this tough resin, together with braided fiberglass tubular socks (wrap-around un-cut edges) in AeroComposites' blades, translates into greater blade fracture toughness, providing an extra measure of safety in the unlikely event of a bird strike, or even impact with snow banks and/or the ground. While no blade can resist all of these events without some damage, AeroComposites' blades are much less likely to shed large brittle pieces that can result in heavy rotary unbalance loads, damaging engines and mounts. 

In a propeller ground strike incident involving an AeroComposites 3-blade propeller (aircraft nose gear collapsed on landing), the 3 composite propeller blades flexed (did not fracture) and the leading edge sheathes remained bonded and attached throughout multiple propeller rotations and blade ground strikes.  As a result, the high strength AeroComposites composite blades absorbed/carried most of the ground impact load with minimal damage sustained to the underside of the aircraft. 

The composite resin used is also ductile. Samples of cured resin, not reinforced with fiber, can be bent a substantial amount without breaking. This ductile property makes for a blade that is not brittle under vibratory bending stress and one that can readily transfer loads between adjacent plies within the molded composite blade without developing internal macroscopic fractures.  The AeroComposites blade design can accurately be characterised as having unlimited fatigue life.

Foam /Resin Bond-The bond joint between the cured toughened epoxy resin and the high density foam core internal to the blade is important. A good bond joint assures that the foam core will move as an integral part of the blade under bending stresses and will not degrade under mechanical stress. One of the functions of the foam core, integral to the blade, is to resist blade buckling under bending loads. Bonding tests conducted have verified an extremely strong and uniform bond joint between the resin and foam core material. Material failure would occur before bond joint failure.

Sheath Bond-The nickel alloy leading edge sheath is bonded in place during blade molding. The requirement from analysis is that a 40 pound per square inch bond shear force is required for the sheath (tip region) to remain attached at 2700 rpm (7000 g's centrifugal load). Square samples (1-inch square bond area each side) of the sheath and blade laminate were cut from a blade and pull tested to 1250 pounds (rig limit) with no sheath bond degradation occurring.  This represents more than a 25 times safety margin for sheath bond adhesion.  

Blade Flexure-Severe flexural tests have been conducted to verify that the composite structure of the blade can sustain heavy bending loads, similar to a bird strike, without structural damage or failure. Cyclical flexural impulse loads of 250 pounds were applied repeatedly near the blade tip. No damage was observed, demonstrating the outstanding strength of the blades under severe bending conditions.

Propeller Vibration-Propeller vibration testing has been conducted on 4, 6, and 8 cylinder aircraft engines. Initial ground testing was conducted on engine test stands with both thrust and vibration data collected. Testing was conducted with fully instrumented (strain gauges) propeller blades with a microprocessor based system mounted on the nose of the propeller, capable of storing large amounts of vibratory strain gauge data. This was followed by propeller ground testing using a high speed strobe light system (view propeller blades from the side) to look for unsteady vibration phenomenon and verify blade tracking.  The strobe light has the ability to indicate gross vibrations at any rotational speed.  Vibration free propeller operation with near perfect blade tracking was recorded in all tests conducted. 

Ground vibration testing was then followed by flight testing, again using instrumented propellers. The recorded data and strobe light testing has shown the AeroComposites propellers to be essentially vibration free for the engines tested and over the entire range of propeller rotational speeds (idle to 2850 rpm). Testing was conducted on both counter weighted and non-counter weighted engines. Every customer flying AeroComposites propellers to date has reported extremely smooth and quiet (cockpit environment) flight operation.

Tear Down Inspection-As part of an on-going AeroComposites program to verify its designs, select propellers in the field have been disassembled and inspected at designated points in the accumulation of flight time to verify as-designed operation. Initial tear downs at 20, 50, and 150 hours have shown "as new" condition of all parts. Additional teardowns are planned at 500 and 2000 hours.

Lightning Strike-AeroComposites propellers include lightning protection. The protection is provided by an "Astrostrike" aluminum mesh that is molded into both blade surfaces (full chord width) from blade tip down to the metal retention ring at the root. This is a proven lightning protection technology used in commercial propellers.

Additional Tests Planned-In preparation for FAA Propeller Certification, AeroComposites is planning full scale blade fatigue tests at elevated stress levels, both with and without centrifugal load applied. Laboratory test rigs have been designed based on the use of test blades mounted in 2-blade hubs, complete with bearing assemblies, to properly simulate attachment load paths.

Engine Dyno Testing

AeroComposites propeller testing was conducted on an engine dyno test stand to evaluate propeller performance (e.g. static thrust, vibration, pitch change). In one of the tests, an 80-inch diameter 3-blade constant speed propeller was tested on a 300-hp 6-cylinder engine. The thrust measured was 1000 pounds of static thrust where the propeller did not fully utilize the thrust producing capacity of its blades due to surrounding structural components and lack of aerodynamic streamlining of the test bed.

Performance Flight Testing

A number of flight tests have now been conducted with AeroComposites propellers with data compared with competitor's propellers (same number of blades), of aluminum, wood-core composite, and other composite blade constructions. In each of these propeller test programs aimed at comparing the performance of the different propellers, care was taken to insure that tests were conducted under comparable conditions. In most cases, tests were conducted on the same day within a 2 hour time frame. In the tests, aircraft weight, pressure-density altitude, engine manifold pressure, propeller rpm, air speed during climb, were parameters that were carefully monitored and controlled to insure valid propeller test results. Tests were conducted with 2-bladed propellers and 3-bladed propellers on single engine aircraft with engines that include the Lycoming O-320, Lycoming IO-360, Continental IO-520, Continental TSIO-550, and EngineAir V-8.

Propeller tests conducted have involved aircraft with cruise speeds ranging from 200 to 350-mph. In many flight tests, pilots reported increases in cruise speed ranging from 5 mph to 10 mph.  In one test, it was reported that the two propellers tested had about the same cruise speeds but that the AeroComposites propeller resulted in better aircraft acceleration, especially at the higher power settings.  Cruise speed improvements reported are attributable to the thin, low drag AeroComposites blade designs where the blade design is biased toward cruise rather than takeoff/climb performance. In all tests conducted, quieter operation, fast speed control response, and smooth operation (little to no vibration) were reported. Increases in climb rates have been reported in a number of applications on the order of 300 feet/minute. Significant weight savings were also realized in installations where AeroComposites propellers were replacing aluminum propellers (savings ranged from 17 to more than 25 lbs).