O-320 idle RPM

[videobobk]

As I prepare for the First Flight, I wonder what idle rpm would be best. I am running an O-320 160 hp with 885 hrs TTSN and a three blade Catto. Idle is set at 610 but seems a little too slow to be smooth. I know it will be higher on glide but thought I'd check. What you think?

Bob Kelly, taxi testing

 

[Kyle Boatright]

A slow idle is probably best. At 600 rpm, your prop is still generating a fair amount of thrust. Add that to a clean airframe, long wings, and ground effect, and you'll be surprised how far your airplane will float on landing, particularly if you don't use full flaps, or if you carry a few extra knots into the flare.

 

[hngrflyr]

My RV-6 has a 0-320 turning a two blade Catto Propeller. My experience says keep the idle slow. Just a few extra RPMs and the airplane can float along in ground effect and you'll eat up a lot of runway getting it down. Even with full flaps it takes very little power to keep it flying. Sometimes my engine gets a little rough during the rollout. I just bump the power up a little bit till it smooths out. My engine has a harmonic balancer. I haven't tried it without it.
It has taken a little learning, but I can fly our RV-6 from any strip our Cessna 120 can use. I've used a 1400' grass strip without difficulty.

Bob Severns

 

[Mel]

700 rpm....

 

[Leland]

As I prepare for the First Flight, I wonder what idle rpm would be best. I am running an O-320 160 hp with 885 hrs TTSN and a three blade Catto. Idle is set at 610 but seems a little too slow to be smooth. I know it will be higher on glide but thought I'd check. What you think?
Bob Kelly, taxi testing

Bob, I also have a Catto 3-blade on an O320 in an RV9A. Originally, my idle was set to 450 RPM but that lead to a rough idling engine. Its now set to about 600 RPM and that is a good compromise. I hold 60 knots on short final with full flaps and it seems to land OK. As an aside, I have to start power reductions some 10 miles out to keep the CHT cooling rate below 50 degrees F/minute.
Leland

 

[gmcjetpilot]

Normal situation with light props

Vidobobk: "Idle is set at 610 but seems a little too slow to be smooth."

The nature of light weight props without a lot of rotational inertial, and you will have lots of residual thrust.

One advantage of metal fixed props is the smooth low RPM idle. A constant speed metal prop allows low pitch and makes a little less thrust at idle than a fixed prop. These are very small trade-offs, the Catto is a nice looking and flying prop I hear, and it's probably better in regards to idle than a wood two blade fixed prop. I know some guys with super light props used even higher RPM than 700 for a smooth idle. Some added a "harmonic damper", a weighted disk to help the low prop inertia. They where in favor years ago but have somewhat lost favor. Not sure they really helped with the higher idle issues. The did help with CG if you where tail heavy.

So with the residual idle thrust you can gain some speed on hard surface taxiways (good reason to fly off of grass ), so you'll have to change your brake pads a little earlier and may use an extra 30 feet on roll out? Just don't drag your brakes. Let the speed build and than check the speed to a slow taxi and release and let the brakes cool while the speed builds. If you taxi way to fast consider a drag-chute.

 

[videobobk]

Idle RPM

Think I will kick the rpm up to 700 to start with. I'm not too worried about float here with a 5000' runway (I'll use that to hone my flairing skills!) and 700 doesn't seem to make it need excessive braking on taxi. I will work it down from 700 gradually during Phase 1 if I feel it needs it. I would guess I will get back to about where it is now. Thanks for all the input!

Bob Kelly