Best Idle speed

Knowing the 9 is hard to slow down with a fixed pitch prop I am wondering what idle speed is best used on my fixed pitch O320 9A? I have a rebuilt stock O320-E3D with dual mags / carb and it seems happy to idle all the way down to 500-600 RPM but I am concerend that might be too slow after it warms up on a longer run. I don't want to get it set too high and complicate the slowing process for landing.

I have mine set to about 650 but rarely run it there. It helps alot with landing.
Most ground ops are around 800 but it is good to know I can pull back further.

Don't get it too slow - if you have an impulse coupling on the magneto, at the low RPM the coupling could engage and it would likely cause some unwanted wear and possibly a failure of the coupling (read - parts drop into the accessory housing and you could shortly be looking for a place to land).

My suggestion is to go the the Lycoming Type Certificate data sheet and see what the RPM range is for that engine - don't go any lower than the bottom end of that range.

Dan

Thanks for that warning Dan.
My engine was a bit fast from the factory and I was hard to taxi without heating the brakes. I turned it down to resolve that problem (which it did) but I will certainly check and turn it back up a bit if I am in a danger zone.
I did a quick search for the manual on the web since I am away from home and I don't see a value jump out but I will dig further.
Thanks

Textron Lycoming Operator's Manual

I happen to have the operator's manual on my desk for series O-320 engines. Purchased new from Lycoming several years ago. Idle speed is addressed in section 5, sub section 2, FUEL SYSTEMS, sub sub section e, (Idle Speed and Mixture Adjustment), QUOTE "Set throttle stop screw so that the engine idles at the airframe manufacturer's recommended idling RPM." Mike Seager told me to set it around 400-450 rpm for light weight fixed pitch prop, or the airplane won't slow down enough for a proper landing flare. He further said the engine will not idle smoothly at that RPM range, and will shake and bounce on the engine mounts. During taxi and ground maneuvers set the idle just outside of the rough range. I have an RV-4 with stock O-320D1A, MT 2 blade f/p prop, so my carb adjustment questions to him revolved around this model and equipment.

Brian Vickers said:

I happen to have the operator's manual on my desk for series O-320 engines. Purchased new from Lycoming several years ago. Idle speed is addressed in section 5, sub section 2, FUEL SYSTEMS, sub sub section e, (Idle Speed and Mixture Adjustment), QUOTE "Set throttle stop screw so that the engine idles at the airframe manufacturer's recommended idling RPM." Mike Seager told me to set it around 400-450 rpm for light weight fixed pitch prop, or the airplane won't slow down enough for a proper landing flare. He further said the engine will not idle smoothly at that RPM range, and will shake and bounce on the engine mounts. During taxi and ground maneuvers set the idle just outside of the rough range. I have an RV-4 with stock O-320D1A, MT 2 blade f/p prop, so my carb adjustment questions to him revolved around this model and equipment.

Click to expand...

Ditto this. Mike Seager was right. Set at about 650 or 700 rpm, the -9 was hard to land. Now it's around 500 rpm and though it won't run smooth at that rpm, I just add a bit of throttle for ground ops. BTW, that's static rpm. Even with throttle all the way back, I never see below 900 rpm or so on the approach.

Impulse & idle speed

A Quote from the Slick Overhaul Manual. "the impulse coupling will not engage above 475 rpm".


Don Broussard

RV 9. Rebuild in Progress

Wonder how long it takes a coupling to come apart if it gets engaged on ground idles? I know they get a few extra clacks on shutdown, buts thats different than clacking away every time you pull the throttle to the stop.

I had a similar issue with my RV-8 and a Catto prop. My initial idle speed was 750 RPM and I couldn't get it to flare and land very well. I set the idle at 600 RPM and now it lands fine, however, there are a few issues that cropped up at 600 RPM. The light weight prop lacks enough inertia to run smoothly and my engine, when hot, surges a bit at 600 RPM if I let idle there. I have F/I and when hot, the low fuel flow at idle tends to cause the engine to run a bit uneven. I usually increase the idle after landing to 700-800 RPM and then the issue goes away. Also, for very brief moments while rolling out after landing and the idle is at 600 RPM, I occasionally get low oil pressure warnings, which are set at 45 psi. This is mostly due to the oil being hot, low viscosity and the engine idle wavering at 550 to 600 RPM, if I add a few RPMs, the oil pressure goes up immediately and its non issue. I plan to bump my oil pressure up an bit to avoid this issue, but it never happens at any RPM above 700 RPM.
As previously stated, Slick Mag impulse couplers should not engage above 475 RPM, however, if the idle was set at 500 RPM and the idle waivers down a bit, it might get close to that threshold. I have seen an impulse coupler on a Bendix Mag come apart. The parts go down in the accessory drive gears and churn around in the accessory case or migrate to the sump. Moderately expensive.

Just a note about my surprise. While the engine roughness is there at 650 RPM and below with Mags, the lower idle does help slow the plane down. After replacing with dual Pmags the idle is at 580 and very smooth. It does indeed make a difference during the landing roll out. Not saying you should buy Pmags just was surprised at the difference in smoothness at low idle and the shorter roll out that 75 RPM makes. Larry

Ditto on the 600-650.

When I flew with Mike Seager in the "Old Blue" RV-6 the idle was set so low that the engine wouldn't run with the throttle all the way back at taxi speed. I'm sure he did that to make the airplane easier to land but it caused me problems. The checkout went well except I let the engine die on all the stop and goes except maybe one. Just couldn't remember to bring the throttle up a little as the airplane was slowing down.

I have my Sensenich fixed pitch 0-360 set for about 550 and it idles okay there and lands fine without excessive float.