RCU Forums - prop position

RCU Forums (https://www.rcuniverse.com/forum/)

- Beginners (https://www.rcuniverse.com/forum/beginners-85/)

- - prop position (https://www.rcuniverse.com/forum/beginners-85/2493-prop-position.html)

What is the proper position of a prop (2-blade) to the piston? I have heard 'it doesn't matter' to 'heavy blade opposite piston' to 'parrallel to ground if the engine dies'.

Hand starting tip... if the prop is parallel to the ground as the engine enters compression, flip backwards pressing downward on the right hand blade. As the engine kicks, snap your hand away. (easy if you just use fingertips on the blade... when it kicks your fingers slip off and automatically snap out of the way. ;) )

This has you handling the less sharp leading edge of the prop, instead of the sometimes knifelike trailing edge. (I sometimes use APC's as knives... to demonstrate just how sharp the are. :eek: )

The tradition of having the prop parallel to the ground when it enters compression comes from the desire to reduce the number of props you need to replace, and easier hand starting, by having the blades alighned where you can get the best flip action. This alignment is never wrong.

The idea of even having a heavy blade is bad. Ballance the prop by either lightly sanding the front of the heavy blade, or applying paint to the light tip. The heavy blade may partially act as a "harmonic ballancer", but only at one rpm, and usually thats down near idle. An unballanced prop at high rpm will shake things up in the aircraft much worse than failing to "compensate" for the piston. (which has already been compensated for in the construction of the modern engines...) The high frequency vibrations can lead to cracking loose glue joints... especially CA joints which are brittle.

Quote:

Originally posted by Cyclic Hardover
If your engine quits in flight, the prop will rotate backwards to the point of compression. This is where i set mine so that if i go into the weeds or whatever when the engine quits, my prop will stop at a horizontal position, thus saving the prop.

To get your prop to stop at this desired position how would you install it on the crankshaft?

The prop is not going to rotate backwards when the engine dies... unless it stops just short of TDC, and compression pushes it backward.

Put a prop on a dowel or screwdriver that allows it to turn easilly. Aim the prop into the wind. It will NEVER turn backward without you pushing the blades and forcing it. (but it might spin fast enough to develop enough "blade lift" to pull itself forward...)

The reason some hand cranked engines in the 1920's and 1930's could be restarted in air was the prop "windmilled" forward.

I'm with Gene Chernosky. 2 oclock IS the PROPER position for any prop blade, on compression. Ballance the prop. not to do so will have you here with many more more troubles.
david

I have noticed with my TT Pro .46 that is likes to run backwards a lot. I sometimes need to start and stop it 2-3 times before it will finall go in the proper direction. I am wondering if the prop position makes a difference.

Its not prop position that's making the engine run backward. Its not spinning the prop forward fast enough that first revolution for it to carry past TDC as the fuel ignites, and then, the engine reverses.

That's one reson I use the reverse flip. I'm purposefully using the effect, to have the "backfire" flip the prop forward for me. ;)

Prop position can effect how effectively you can hand flip the engine... and the most efficient forward flip comes from the prop being parallel to the ground (3:00/9:00) to just a bit past (2:00/8:00) as the engine enters compression. If you tend to have your hand flips go backward, move more toward 3:00/9:00.