Propeller Indexing

[GraemeSmith]

Reading the installation instructions for the Prince P-Tip carbon propeller - it states:

" Alignment of propeller to engine firing sequence can be at any position"

Really? As I understand it - wood or wood composite props are more likely to damp vibrations caused by blade position and a cylinder firing being less than optimal - and that traditionally a wood blade position was more likely to be set to aid hand propping.

But even if the vibrations tend to get absorbed and we are not hand propping the AeroVee - The engine/propeller wouldn't benefit from indexing to reduce vibration?

And if so - where to index?

On a right hand turning engine - you bring No 1 to TDC and - looking aft - mount the prop blades at 2/8 O'clock.

So on the left turning AeroVee - would that be No 1 to TDC and mount the blades at 4/10 O'Clock?

[SNX1508]

From Sensenich tech support, September 2017, in regards to installing my VW-W54JV5L-44C prop (I had inquired whether the 4 O'clock - 10 O'clock positions were as viewed from the fwd or aft side of the prop):

"The #1 cylinder @ TDC is for hand propping only. If you are not hand propping then there is no concern where you install the propeller."

[GraemeSmith]

"The #1 cylinder @ TDC is for hand propping only. If you are not hand propping then there is no concern where you install the propeller."

Interesting - another reason for asking is because our local guru who dynamically balances prop and engine combos - first thing they do is index the prop.

As I understand it (and this paraphrasing may be extremely glib) - if the prop is at its worst case "Inertia" position as a combustion impulse is applied from a firing cylinder - it attempts to throw the prop even harder. The extra acceleration applied has a tendency to push the main mass of the prop onwards and leaves the tip behind. The prop flexes (and in a metal prop this adds significant fatigue) and sets up a vibration - which if you are unlucky can also be at resonant frequency. Result - more vibration. If you index the two bladed prop in one of its three possible positions - on a 4 cylinder engine - one of those is a sweet spot where the flexing and vibration are reduced.

In fact there is a prop/engine combination out there on Cessnas that had a restricted RPM range (I believe it is on the C17x planes with a Lycoming (I)O-360 and a stock McCauley prop)- because even with indexing - the prop vibrates too much.

So even with a wood or wood/carbon prop to absorb vibration - would it not still be better overall for the life of everything if the prop was indexed to minimize vibration?

[Onex107]

The experts have it right. The prop is nothing but a flywheel and the position doesn't make a difference in engine vibration. In a four cylinder engine all four pistons stop at the top or bottom of the stroke at the same time. The inertia of changing the travel direction of four pistons every revolution makes the engine rough. Automotive engines have a counter balance shaft that smooths out the transition. With a six or eight cyl. the strokes, piston positions, overlap so they are not all stopping and starting at the same time. Props don't vibrate unless they are out of balance. This opinion isn't worth the paper it's written on.

[GraemeSmith]

The experts have it right. This opinion isn't worth the paper it's written on.

Looking for the "grin" emoji.....

This just in:

With a propeller that absorbs harmonic vibration usually very little vibration is dected, however there have been occurrences almost always on pusher aircraft if there is a vibration moving the propeller one bolt hole will reduce this but this mostly is due to the exhaust timing striking the propeller. It has been never reported to my over the 41 years that vibration has been reduced with a Woo/wood core propeller being clocked.

You are most likely correct, I do not have data or specifications to prove this.

www.princeaircraft.com
Lonnie Prince
CEO/President

I have a friend with a dynamic balancing rig - I think we will go do some experiments.....

[Onex107]

There was a notice for my Continental O-200 to avoid running, I think it was in the range of 2200 rpm. Evidently engine vibration reaches a harmonic at that speed that makes it worse.

[Rynoth]

Automotive engines have a counter balance shaft that smooths out the transition.

This is beside the point, but automotive horizontally opposed engines (like the Aerovee) also don't use/need a balance shaft since they are inherently balanced, such as Subaru engines. Effectively all the balancing needed is contained on the crankshaft itself.

[markschaible]

Here is a practical consideration: at the factory we like to install props on the Xenos so that, with the engine at TDC, the blades are parked horizontally. This covers the cooling inlets when soaring and leaves the minimum amount of blade area outside of the cowling’s frontal area to minimize drag. Although we don’t typically worry about this with our other models, even if you don’t have a Xenos, reduction of drag when the engine isn’t running is a good thing. Most of the time, the engine wants to naturally stop turning at TDC when we shut it down. If not, we just blip the starter to set it in that position.