Propeller Damage, Direction of Engine and Propeller Rotation

Propeller Damage

It is an unfortunate fact of life in aviation that accidents occur. There are many forms of aircraft accidents which can inflict potentially serious damage to the propeller and, by extension, to all the internal components of the engine system.

The term "engine system" includes all the rotating and reciprocating parts of the engine proper, AND the PSRU (if applicable), AND all driven accessories (prop governor, distributors, magnetos, pumps, etc.)

The picture below shows an obvious example of severe propeller damage. However, you should be aware that serious damage to the entire engine system can occur during events far less serious than what is shown in this picture.

In the world of certificated aircraft, there are required procedures to be followed in the event of a prop strike. These procedures are well-established, and have been developed mainly from the hard school of experience.

In the experimental community, there appears to be a somewhat cavalier attitude about such events, apparently based on denial and wishful thinking. In fact, after the accident pictured above, there was a strong opinion among the people directly involved that no inspection to any engine system parts should be needed because the prop involved wasn't a metal prop!

The discovery and correction of potentially fatal damage to engine systems resulting from a prop strike incident is the subject of a recent EPI Service Bulletin (hyperlinked below).

A portion of this Service Bulletin is presented here in order to give the mose exposure possible to the seriousness with which ANY damage to a propeller should be treated. These definitions and procedures have evolved from the experience gained from post-accident investigations over the course of many, many years. Consider them to have been written in blood.

The following definition of PROPELLER STRIKE is taken from a blending of the requirements stated in Lycoming Service Bulletin SB 533-A, Continental Service Bulletin SB96-11, and AD-2004-10-14.

A propeller strike is defined as follows:

1. Any incident, whether or not the engine is operating, that requires repair to the propeller other than minor dressing of the blades;
2. Any incident during engine operation in which the propeller impacts a solid object which causes a drop in RPM and also requires structural repair of the propeller (incidents requiring only paint touch-up are not included). This is not restricted to propeller strikes against the ground, and although the propeller may continue to rotate, damage to the engine may result, possibly progressing to engine failure.
3. A sudden RPM drop while impacting water, tall grass, or similar non-solid medium, where propeller damage is not normally incurred.
4. Any propeller strike occurring at taxi speeds and during touch-and-go operations, involving any propeller tip-to-ground contact.
5. Any situation where an aircraft is stationary and the landing gear collapses causing one or more blades to be bent or substantially damaged, or where a hangar door (or other object) strikes the propeller blade. These cases should be handled as a sudden engine stoppage because of potentially severe side loadings on the propshaft flange, front bearing, and seal.

Whereas the definitions stated above are intended to be unambiguous, the reality of aircraft ownership is that those definitions are likely to form the genesis of endless quibbling about such arcane trivia as:

1. How sudden is "sudden",
2. How tall is tall grass?
3. How deep does the water have to be?
4. How much of an RPM drop? (50 RPM, 500 RPM) ?
   {Were you really watching the tach when the prop hit?}

These arguments are specious and merely constitute an attempt to rationalize out of doing the required inspections. And, even though a large number of such inspections find no damage, the consequences of undetected damage from a "prop strike" can be grave.

(If you doubt the susceptibility to such damage, take a look sometime at the tiny dowel that locates the accessory drive gear on the end of a Lycoming crankshaft.)

The information contained in the complete EPI Prop Strike SERVICE BULLETIN should be of interest to ALL operators of propeller-driven aircraft.

ONE EXAMPLE

The picture at the top of this page will, undoubtedly, bring some form of gut-wrenching to all builders. It was taken just after an accident which resulted from an unfortunate power loss in a brand-new, V8-powered Lancair 4P, on its way to OSHKOSH-2001. According to many people who saw this aircraft (prior to this event), it had the potential to capture the Grand Champion award.

The builder completed this aircraft in July, 2001. It was subsequently inspected and signed off by the FAA. The initial test flight program was conducted by Lancair Factory test pilot Don Goetz. During the initial flight hours, the builder received transition training from Don, and continued on to fly off the remaining test hours. The FAA then removed the restrictions, and the aircraft was prepped for the flight to Oshkosh.

In preparation for the trip to Oshkosh, the crew performed a final inspection of the aircraft, which included removal of the cowling and a thorough inspection of the engine system (EngineAir turbocharged V8 with EPI Mark-9 PSRU).

Early the next morning, the builder and a friend boarded the aircraft and departed for Oshkosh (on a flight plan). While climbing through 8,000 ft. on the way to the assigned cruising altitude, the engine began to gradually lose power, and ultimately stopped producing any power at all. The only indication of malfunction was an extremely low MAP, suggesting at first, a broken throttle linkage.

The pilot (and builder) handled the situation by the book. First and foremost, while initially analyzing the situation, he FLEW THE PLANE. Maintaining best-glide airspeed, he contacted ATC to declare an emergency, located potential landing sites, selected the best, and flew toward it.

Note that, during this adventure, the prop is windmilling and driving the engine through the PSRU, creating the effect of a large vacuum pump (MAP = 5" HG) and a very effective speed brake. In a normal power-off descent, this aircraft, with roughly 30 PSF wing loading, will manifest a descent rate unfamiliar to pilots accustomed to small GA certificated aircraft. In this situation (with the prop driving the engine at very low MAP), the descent rate necessary to maintain safe airspeed would be daunting to all but the most experienced pilots.

This situation has all the elements for a tragic outcome:

1. a total loss of power,
2. a significant increase in drag,
3. a pilot with very little experience in high-wing-loading aircraft,
4. the added emotional burden of a pilot who has just invested a significant fraction of his life in the construction of this potentially-prize-winning aircraft (not to mention the MONEY involved).

Fortunately, the weather was good.

To his great credit, this pilot overcame all these problems, maintained control of the aircraft and delayed deployment of flaps or gear until he was assured he could make his targeted runway.

Unfortunately, there was not quite enough energy in the system for him to reach the approach-end of the targeted runway. The pilot realized he would be a bit short, kept the aircraft configuration clean and fought off the strong impulse to try and milk a few more feet of glide. He maintained control through a premature touchdown, a few hundred feet short of the runway. The aircraft came to a controlled stop a few feet after sliding onto the pavement, and both occupants emerged unharmed (picture above).

The cause of the power loss was interesting. During the post-accident engine system was disassembly, the intercooler was found to be completely blocked by a large mass of ground paper particles, having the same color as the paper towels used during the cowl-off inspection. Further searching revealed a fault in the alternate-air system which, apparently, allowed a paper towel (probably left in the engine bay during the inspection) to be ingested by the turbocharger, pulverized by the compressor, and discharged into the intercooler. The sharp edges and small passages in the intercooler quickly collected most of these particles, and ultimately blocked it completely. There was no damage to the engine system.

The EPI Mark-9 PSRU and Woodward prop governor were inspected in accordance with EPI Service Bulletin 260002. After this complete disassembly and thorough inspection, both the PSRU and the governor were found to have suffered no detectable damage. The PSRU bearings and seals were replaced during the rebuild simply as a matter of good practice.

Direction of Engine and Propeller Rotation

This page presents the factors to consider when deciding on the direction of engine and propeller rotation. One of the main decisions is whether or not the aircraft designer is willing to build an aircraft with a propeller which rotates in the "wrong" (for American aircraft, but "right" for European aircraft) direction.

The aircraft in which most of us learned to fly have propellers which turn clockwise as viewed from the cockpit. Most pilots are unwilling to reverse all those hard-learned synapses which cause automatic application of the correct rudder pedal in climb, stall, and high-gyroscopic-moment maneuvers. (This reluctance, of course, does not apply to pilots who use the pedals only during taxi.)

Let’s assume that a counter-clockwise rotating propeller (on a single-engine airplane) is not acceptable. Most engines considered suitable for conversion turn clockwise as viewed from the cockpit. If one wishes to retain that direction of rotation on the propeller, one could select a two-mesh PSRU, or a PSRU using belts, chains, or certain planetary arrangements, in which the output shaft rotates in the same direction as the input.

The output of a single-mesh externally-toothed gear reduction is opposite of the input. Therefore, if one decides to use an externally-toothed offset gear-PSRU, one can: (1) reverse the engine rotation and use a single-mesh reduction attached to the flywheel-end of the engine, (2) retain the original direction of engine rotation and use a PSRU with an idler gear, or (3) retain the original engine rotation and drive a single-mesh PSRU off the wrong end of the crankshaft (a VERY bad choice). Here is an expanded discussion of each option.

1. Reversing the engine (covered on another page of this site) so that it turns in the opposite direction of the propeller has some interesting advantages. An obvious one is that, all other things being equal, a single mesh gearbox will weigh less than one with an idler gear. EPI has designed propeller reduction gearboxes in both configurations (with and without an idler). One model for an engine producing up to 600 lb.-ft. of torque weighs 73 pounds in the two-mesh (idler configuration), including an integrated gear-drive for the prop governor. The single mesh version of that same EPI PSRU weighs just 59 pounds including the same integrated prop-governor drive. A fourteen pound weight reduction can be hard to find on a 500 pound powerplant.
   
   A less obvious, but significant advantage is that the gyroscopic moment of the engine can reduce or cancel the gyroscopic moment of the propeller with respect to the loads the engine mount applies to the airframe. In fact, when EPI designed the engine mount system for the Orenda conversion of a popular high-performance twin, the calculations showed that the gyro-moment of the counterclockwise-rotating engine significantly reduced the gyro-moment of the large metal propeller.
2. A gearbox which contains a single idler gear requires special attention with respect to gear fatigue. The teeth on the PSRU input and output gears are subjected to severe fatigue loading: zero to maximum and back to zero, every revolution. However, the teeth of a single idler which meshes with both the input and output gears are subjected to the most severe form of fatigue loading: fully-reversing load. They are fully loaded in one direction when they mesh with the input gear and fully loaded in the opposite direction when they mesh with the output gear. In the single-idler case, the allowable working stress level is about 2/3 of the unidirectional-load value, and tooth load limits are usually established by the idler at that reduced working stress.
   
   Some gearboxes use an idler having two different gears on a common shaft, one meshing with the driver and one meshing with the driven gear. That arrangement has two advantages: (a) because of the compound reduction, it can implement a large reduction ratio with relatively compact gears, and (b) it eliminates the additional fatigue load experienced by a single idler gear. However, that arrangement adds weight and cost.
3. There are several good reasons not to drive off the wrong end of the crankshaft, including (a) lack of crankshaft structural strength to support the torque output stresses, and (b) torsional vibration issues potentially destructive to the crankshaft. The folly of this practice has been proven time and agiain in several classes of boat racing, but still it happens.... A further discussion of this particular example ofWishful Thinking Engineering is beyond the intended scope of this presentation, but be assured the arguments are both valid and persuading.