This month’s Ask a Tech Q&A come from the owner of a Lycoming O-320-D2A engine that had a noticeable vibration when it came out of a paint shop. And, dangerously erroneous heading errors with an old King electric HSI requires logical troubleshooting. First, the vibration trouble.
“After coming up empty during random troubleshooting attempts, my mechanic suggested that we try several different prop positions to see what impact it might have on the vibration problem.
“Apparently, my fixed-pitch propeller can be installed in any orientation on the mounting bushings of my O-320-D2A (Lycoming Service Instruction number 1098G notwithstanding).
“We moved it one more time and I am quite sure it is now back in the position it was in when I bought the airplane. There still seems to be a bit of what I consider ‘harshness,’ real or imagined, in the engine, but the vibration is significantly reduced. This is what puzzles me. Can prop position make such a large difference?
“Since being moved, I have gotten two differing views from two mechanics as to whether the prop is currently in the correct position on the flange or not. Fact is, this is how all the prop reorientation started because my mechanic was convinced the propeller wasn’t installed correctly at the paint shop.
“How should the prop be installed and what possible difference does it make, anyway?” IA Mike Berry weighed in:
The maintenance manual indicates that the prop should be installed on the crankshaft flange with the blade tips positioned at the 2 o’clock and 8 o’clock position. This is viewed from the front of the airplane with the engine at top-dead-center on the compression stroke of the number-one cylinder. Also, make sure that the ring gear (starter ring gear) is positioned on the crank correctly.
There is an “O” stamped on the crank flange right next to a bolt bushing. here will be a similar “O” on the ring gear. They must be installed with those index marks in mind. It makes a difference because the position of the propeller blade in relation to when any spark plug wires is very important—especially in the climb. It has to do with the relative wind and climb deck angle as related to impact air on the descending blade and when that prop gets another power pulse from one of the four cylinders.
It’s all very complex and sounds like so much double talk, but it’s true. It will affect how much vibration you have—especially when you consider that you have a “non-dampered” crankshaft. All the power pulses are felt by the prop and the airframe.
Some propellers can be clocked in any number of positions—others are indexed by flange bushing placement. Continental engines are generally equipped with two holes in the crank flange that accept the two prop dowel pins. Most of these installations allow the propeller to be installed in one of two ways. Lycoming engines use a shouldered bushing in the crank flange to determine ring gear and prop position but they are not always used. The aircraft maintenance manual and Lycoming SI#1098G (way back to April 2001) should be referenced for proper prop placement.
Lycoming also states in this SI that improper indexing may also result in excessive stresses on the propeller blades themselves. It should be noted that virtually all references to front, back, left and right are made from the pilot’s seat.
In the case of your Piper, the blade tips are referenced from the front of the airplane. You’ll find that when the blade tip located at the two o’clock position is pulled through by hand—such as in hand-propping—the impulse coupling snaps just as the blade pulls through the compression stroke.
To index the prop from the pilot’s seat will position the two-o’clock blade too low for safe hand-propping.
“My Cessna P210 has a King KCS55A HSI system that intermittently shows sizable heading error—as much as 40 degrees or more. The last time it happened the system was accurate during the takeoff roll (showing proper runway heading that agreed with the wet compass), but after takeoff (while IFR) the controller asked me what heading I was on. Luckily I have a plain-vanilla DG on the copilot’s panel and I was able to continue to VFR conditions, but this is pretty scary. Every time I bring it to the shop they can never duplicate the problem.”
Larry Anglisano responds with some simple troubleshooting tips:
The KCS55A compass system with slaved HSI has multiple components, and the problem could be with any of them. The first thing you want to try is putting the system in “free” mode, which takes the slaving circuitry out of the picture. The KA51B slaving accessory on the instrument panel has a toggle switch for manually slewing the heading in either direction. When it fails, take it out of slaved mode and slew it to the right heading and then look for excessive amounts of gyro precession. There should be the usual amount of minimal precession after making a few turns after some time. If it works fine in free mode, it could help your shop better troubleshoot the system in a logical manner.
The next step is to remove the KG102A remote electric gyro and bench test it, looking for heading error. The heading card on the geared HSI could be hanging up, so make sure it moves smoothly while manually slewing the heading. There’s also a KMT112 magnetic flux mounted in one of the wings, but it’s pretty rare to see a failure with it. A broken wire is more common.
