Understanding the Ground Adjustable Prop
Keeping it simple and effective for the common setup.
Let's discuss prop pitch and how it affects flight characteristics. It can help flight characteristics or it can hinder. I'm often asked; What's the correct prop pitch for a specific plane? There is no single answer as many props are available to us today for Rotax engines and for different fuselages. There are, however, some commonalities and that is where we are headed in this article.
Certain principles do apply in either the 2 stroke or 4 stroke engines, although the numbers will be different as with most ground adjustable props. This article won't get into all the designs, blade twists, angles, thrusts, shaft powers, etc...etc. Whoa, just thinking about it puts my brain in a tail-spin. We are going to keep it simple and easy to follow. I am going to use the Rotax 912ULS as an example.
First let's pick a few numbers to keep in the back of our minds for later. We are going to shoot for certain idle rpm, so let's pick 1700 +/- rpm and 5600-5650 rpm for wide open throttle (WOT), flat and level at your average cruise altitude. It wouldn't make sense to set a prop for sea level when you are at 9,000' msl all the time. Why 5600- 5650 rpm as a target? Through a lot of testing this was found to be the BEST balance point for climb, cruise, engine temps and fuel consumption. The "continuous run" rpm Rotax recommends for the Rotax 912ULS is 5500 and that rpm can be flown all the time if you chose to do so. Another good reason would be if you were to break a cable or had a throttle control failure. One carb would probably go wide open, as it's supposed to do and then you could advance your throttle and have the other carb go wide open. You could then fly to wherever you needed for a safe landing area; shut down and land. Anything over 5500 rpm (i.e. 5600-5800 rpm) would limit you to a 5 minute run time under normal flight situations for longevity of the engine, but in an emergency the engine can truly run for much longer times without fear of damage. A prop manufacturer will usually have some instructions for their prop and sometimes a suggested starting point for pitch depending on the engine. Another often asked question is “What should my static rpm be”? There is no specific or accurate answer for everyone's engine and prop. The static won't mean much if you only want to fine tune your existing setup. Static is more important for the first run owners or for new prop installation. The static rpm setting is just to get you in the ballpark and then you will need to fine tune it for your specific aircraft and needs while flying WOT at your average altitude. So in keeping it simple, you will want to set the pitch on most props to achieve a target with a beginning static (ground run) WOT rpm of around 4800-5000 rpm, but your static rpm may be slightly different depending on what you wanted for a final in flight WOT rpm outcome and different props can run different static rpms from one to another. The factors here are long vs shorter blades, two vs three blades and stiff vs flexible blades.
(Note: These next figures are general and yours may vary slightly) To do this properly, you will need to go fly at your average cruise altitude and fly flat and level at WOT for at least 1 minute. Now if your WOT rpm at this time is 5500 rpm and up to 5650 rpm you're probably set up fairly well for your engine, temperatures and fuel economy. If you are up at 5700+ rpm then you may want to land and add a little pitch (about .25-.75 degrees) back into the prop pitch, which will make it more coarse.
If you already have your prop setup is only turning 5200 rpm WOT flat and level you need to flatten or reduce the pitch approximately 1.5-2 degrees to achieve 5600-5650 rpm. Now you may have some special circumstance like a float equipped aircraft (heavy aircraft) that needs a little better climb, lots of high DA takeoffs or lots of tight short fields where better climb is more important so a climb pitch of 5650-5725 rpm WOT might be warranted. We need to tune our props for the type of flying that we do.
What else does my prop pitch do for me?
Setting the prop pitch excessively coarse (i.e. 5000-5300 rpm WOT) causes excessive stress on engine components and gearbox which may necessitate early maintenance and has been known to crack crankcases. Having the pitch too coarse will cause higher engine (CHT, EGT) and oil temperatures, excessive fuel consumption, poor climb and decreased cruise speed. Your engine doesn't have the horse power and torque to turn an excessively pitched prop. All piston engines have their limits and the props all have limits, too. So if your engine temps are up and your WOT engine rpm is below 5500 rpm try unloading the engine by reducing the prop pitch. If you have a prop that is too flat then it may climb well, but have a loss in cruise speed and of course engine temps and fuel are affected again.
Your exact numbers may vary some, but you now have a general idea on what to look for and how it may affect your flying and engine. We'll keep this discussion on the root topic of ground adjustable props. Special circumstance rpm settings and in flight adjustable props will warrant discussion in a future article.
One last parting comment: If adjusting prop pitch sounds complicated, it isn't; it usually will only take 30-40 minutes, a couple of wrenches, a prop protractor and/or a level. So take the time to fine tune, your engine will say thank you in improved performance.