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Idle Speed

J

jpowell13

Well Known Member
I was inspired by Mark's "Liquid Plumber/Primer" confession to admit my own rookie mistake since it might be of help to some. After about four hours and seven landing on my 6A I was returning from a Phase 1 test flight and "ballooned" on landing. I chalked it up to a poorly stabilized straight-in approach, but the next landing was worse, and I realized my idle speed had crept gradually up to about 1,050 RPM (static). A light RV loves to keep flying and doesn't land well if the RPM's are too high. (I have a fixed pitch Sensenich.)

Turns out the jam nut on my throttle cable linkage had loosened and the cable had backed out of the rod end bearing a couple of turns, so, based on my mistake:

1. Be sure to verify and document in your builder's log that the throttle linkage jam nut is tight, and...

2. I am adding an item to my start-up and run-up checklists to "verify correct idle speed".

John
 
Hi John,

What have you got your idle speed set to, mine at the moment is around 600rpm, same setup fixed pitch prop and am considering trying to drop the idle another 50 rpm if I can

Cheers
 
Hi Eddie,

The operating manual for my engine says 750 to 850. I set mine for 750, but after warming up fully it is at 900. I'm headed to the airport now to adjust it down another 100 RPM. Below 700 my engine begins to "stumble". I think you need to be careful not to set it so low that your engine dies on approach to landing, but I'm not a Lycoming expert by any means, and your engine may be different.

John
 
jpowell13 said:
I was inspired by Mark's "Liquid Plumber/Primer" confession to admit my own rookie mistake since it might be of help to some. After about four hours and seven landing on my 6A I was returning from a Phase 1 test flight and "ballooned" on landing. I chalked it up to a poorly stabilized straight-in approach, but the next landing was worse, and I realized my idle speed had crept gradually up to about 1,050 RPM (static). A light RV loves to keep flying and doesn't land well if the RPM's are too high. (I have a fixed pitch Sensenich.)

Turns out the jam nut on my throttle cable linkage had loosened and the cable had backed out of the rod end bearing a couple of turns, so, based on my mistake:

1. Be sure to verify and document in your builder's log that the throttle linkage jam nut is tight, and...

2. I am adding an item to my start-up and run-up checklists to "verify correct idle speed".

John
Click to expand...

Loctite is your friend.
 
I think you need to be careful not to set it so low that your engine dies on approach to landing
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The IAS whilst flying will increase the minimum RPM. You could do some stalls whilst airborne and establish what the idle is there and make sure you are happy it will not stop.

You don't tend to hear of engines stopping on finals for this reason, but you do with engines stopping on landing rollout, or even in stall turns / spins (when IAS is low, and other factors may limit fuel supply).

Depending on Prop, the lower the idle may well help control landing distance from 50' (as OP points out). I would also suggest any checklist should have a pre-takeoff idle check - it covers a multitude of factors / settings.
 
Thanks guys, I always checkup idle speed after runups to ensure the engine does not want to stop, I'll see what effect dropping it a little more might have.

Cheers
 
Jrskygod,

Last night I re-check my engine operating manual again and found a statement that the idle speed should be set according to the airframe manufacturer's recommendation. I had set mine at 750 to 850 based on some of the RV6A Pilot Operating Handbooks on this website. You might find some good information for your RV9 there also.

John
 
Idle hands...

eddieseve said:
Hi John,

What have you got your idle speed set to, mine at the moment is around 600rpm, same setup fixed pitch prop and am considering trying to drop the idle another 50 rpm if I can

Cheers
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G'Day Steve,

I have found over the past 20 years that anything above 700 with a good FP prop and you will never land at my 1800' strip. You will also be tapping the brakes during taxi on pavement. With electronic ignition, clean plugs and good oil PSI I set mine at 550. Why? Speed control and S.T.O.L! I have found the slower I can idle, the steeper and slower I can make my approach. C/S is another story although my HR2 with a LyCon IO540 C4B5 C/S and E.I.was set at 550 as well. Realize that when cold it will quit if you pull it all the way back. Set it for normal operating temp.

Fair Dinkum :)

V/R
Smokey
 
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As I mentioned here..... http://www.vansairforce.com/community/showpost.php?p=762002&postcount=3 .... an idle speed of 550 rpm will have the prop speed lower than the stall speed.

There are two idle speeds. First, the lowest idle that the motor will sustain with reasonable smoothness. Second, the speed at which the motor feels good at.

For my O320 Sinsinich FP metal, my low idle at the stop is 550-. I increase the idle to 700 when I am sitting at an idle. I taxi at about 700. The Lightspeed does help with a smooth low idle.

And very aggressive leaning is important for this low rpm. Right after start up, set rpm to 1000. lean till you see an rpm increase. Mine is about 100. leave the mixture there and reduce rpm to 1000 for warm up. Lower rpm to 700 for taxi. This will give you enough rpm range to taxi, but not enough to take off. The engine will quit at 1200 rpm.

When you are ready to do your mag check, you will need to go to full rich.... or the motor will let you know if you forget.

With the "A" model having the main gear located farther back, has the advantage of a deeper flair and an increased AOA to induce the stall just as the mains touch..... The rpm's must be low to do this and not float.
 
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Good stuff Guys,

I'll try for 550rpm and see how it goes, at present I am still floating down the runway for a fair while, so I imagine this should help lots.

Cheers
 
For the first 70 hours I've been living with about 750 rpm idle when warm that I'm pretty sure has contributed to longer than necessary landings, especially on my -9.

Can anyone give me an idea about how much turn on the idle adjustment screw is required to drop the idle about 100 rpm? Also, which direction will I turn the screw?
 
Turn it in to increase. Go a 1/4 turn with the motor off... then check. You should hit it right on the next time.
 
This is really helpful. I set my idle at about 750 yesterday. I'll try it there for a few landings and then move the idle down 100 at a time to 550 if it will idle OK there. My rings have about 7 hours on them including the break-in process. Does this affect the way the engine idles? (I have an eMag on one side and Bendix on the other. The prop is a Sensenich.)

John
 
Couple comments regarding posts and questions in this thread (Most already answered, just acknowledging that the answers are right on) .

- RV's equipped with fixed pitch props need the idle RPM adjusted as low as possible (constant speed props, it is not as critical, but the lower it is the more drag at idle you get, which can help steepen decents when needed)

- Idle RPM can't be accurately set unless the engine is fully warmed to normal operating temp.

- Idle speed will change (some times a lot) during engine break in. Particularly since when it was originally adjusted before first flight, the engine probably wasn't at full operating temp. (See previous tip).

- Idle RPM will be higher in flight than static on the ground.

- Desired static idle RPM will be lower than the lowest smooth idle RPM (when operating on the ground it quickly becomes second nature to keep the throttle set just slightly open of being on the idle stop).

- If you need to make an adjustment... turn it a measured small amount and note the effect. Then you will no how much change a given amount of turn causes on YOUR airplane (they are not all the same) Also, there is a difference between the different models of carbs, and or fuel injection servos.
 
After 70 hours, I figured it was time to take care of the floating, floating, and floating down the runway issue.

So, today, I went around the pattern a couple times today to warm things up. After taxiing back to the hangar I noted my idle rpm to be around 800 rpm at full rich. Took the cowl off, adjusted the idle adjustment screw a couple times in quarter-turn increments and also moved the cable aft a bit using the throttle bracket nuts to ensure that I was getting the carb's throttle lever all the way to the idle stop. The end result was that a total of 1/2 turn counterclockwise on my idle adjustment screw dropped the idle rpm from 800 rpm to about 560 rpm. Didn't take much at all.

The engine now runs a bit rough at idle but keeps running. From from what I understand (based on this thread), this is what I want, correct? Any forward speed either in the air on approach or on the ground during rollout is only going to tend to make my idle rpm higher and therefore smoother, correct?

Just want to make sure I approached this correctly before doing a test flight.

Thanks.
 
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alpinelakespilot2000 said:
The engine now runs a bit rough at idle but keeps running. From from what I understand (based on this thread), this is what I want, correct? Any forward speed either in the air on approach or on the ground during rollout is only going to tend to make my idle rpm higher and therefore smoother, correct?

Just want to make sure I approached this correctly before doing a test flight.

Thanks.
Click to expand...

Did you adjust your idle mixture? You may then need to readjust your idle speed....repeat until you achieve the result you are after. You should see a 50rpm or so increase on cut-off when set correctly. Otherwise it sounds like your getting close.
 
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My experience ...

The comments about idle speed 600 or under to not float on final are spot-on for me.
With my Silverhawk FI, a quarter turn on the idle speed screw changes the idle speed by 120 to 150 rpm. I also agree with the comments about idle speed affecting brake life. When my idle was at 750 rpm I had to constantly ride the brakes to keep taxi speeds under a flat-out sprint. That light and slippery RV REALLY wants to roll with just the smallest amount of encouragement from a few rpm and an FP prop.
 
JonJay said:
Did you adjust your idle mixture? You may then need to readjust your idle speed....repeat until you achieve the result you are after.
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No, I didn't touch the idle mixture. I just adjusted the idle speed using the screw that sets my closed-throttle position. Should I have done something with my idle mixture? (I don't know enough to know why you asked that question.)
Thanks.
 
alpinelakespilot2000 said:
No, I didn't touch the idle mixture. I just adjusted the idle speed using the screw that sets my closed-throttle position. Should I have done something with my idle mixture? (I don't know enough to know why you asked that question.)
Thanks.
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If it was adjusted properly before, (carb) there should be no problem reducing the RPM with the mixture left alone.

But, test it by seeing if you have an increase in rpm just before the motor dies when you pull the mixture to cutoff.
 
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In my experience with the Bendix/Silver hawk injection, adjusting the idle will effect idle mixture. When I adjust one I almost inevitably end up adjusting the other.
 
Walt said:
In my experience with the Bendix/Silver hawk injection, adjusting the idle will effect idle mixture. When I adjust one I almost inevitably end up adjusting the other.
Click to expand...

That was my experience, but I can not say in confidence that it was adjusted properly before.
 
Now that my warm idle is (intentionally) a bit rough, should I do my mixture idle cutoff on shutdown at idle or up where it smooths out a bit, say 650-700 rpm? And, would I be looking for the same 50 rpm rise regardless of whether I shut down at 550 rpm or 700 rpm?
Thanks to everyone for the educating.
 
I idle my carb O320 at 1000 rpm for 15 to 30 seconds leaned from taxi in. Then pull to mixture cut off and see a rise just before it shuts down. Mags off....Master off.

The only time you use that (to the stop) idle is for landing. Any other time, it should be set at 700 or so.......... metal prop.
 
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alpinelakespilot2000 said:
... would I be looking for the same 50 rpm rise regardless of whether I shut down at 550 rpm or 700 rpm?
Click to expand...

From what I was told by the Precision folks (maker of the Silverhawk units), the answer to your question is yes. Prior to idle mixture adjustment, I saw almost 200 RPM rise just before leaned shutdown. I also had visibly black exhaust at idle and taxi. Once I leaned the idle mixture to a 40-50 RPM rise, the black smoke went away.
 
Update: Success!

Went out and did my post-adjustment test flight. As I noted, before adjusting, my warm idle was up around 800 rpm. After adjusting yesterday down to 550, I chickened out and added about 1/8 of a turn. (It ends up that my new warm idle is about 600-620 rpm.) Went out and did a couple stalls and simulated approaches at altitude to make sure my prop would keep turning at idle. No problems there. Then went back to the pattern and, all I can say, even at about 600, is WOW. Night/day difference.

The effects I observed:
1. At the same approach speeds on a no-wind day, I cut my landing distance down probably about 500-600 feet.
2. Just as important, the tendency to balloon (if I'm at all hamfisted in the flare) is much lower. The elevator seems much less sensitive without the extra airflow.
3. I need to be ready to touch down much sooner in the flare and, therefore, be careful about making sure I flare low. The bottom drops out a little quicker than before.

After taxiing back to the hangar, I did the rpm rise test. I idled at about 900 rpm, expecting to see about 950 when I pulled the mixture. However, pulling the mixture all at once did not result in any observable rise in rpm. Within 1.5-2 seconds the engine was stopped with no discernible rise. Is this because the Dynon is not sensitive enough or just because the stop occurred too fast? Does this suggest that maybe my idle mixture is set too lean? (To be quite honest, I never paid much attention to the rpm rise, if any, before I did the adjustment yesterday, so I don't know what is normal for my plane.)

Anyway, mostly success. Thanks again for the ongoing help.
 
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Idle speed

1st landing on my -6 was dead stick, power to idle on short final, prop stopped on rollout. Learned cold idle and warm idle are not in the same zip code. Being a quick study, I repeated the process on the first flight of my newly overhauled engine....
 
Steve, it varies a little from engine to engine, but at 900 you're near the top of the idle circuit and the main jet is taking over from idle bleed. Try your cutoff a little slower (not molasses) and listen to the sound as well as watching the tach. Try it down a hunnerd RPM too. The greater the throttle plate opening, the less fuel gets pulled through the idle holes. At some throttle position and airflow the idle circuit is negated.
 
High idle settings might be part of the flip over problem.................

With the wing of the 9, the rpm must be set low or it will float and float and float etc.
 
aerhed said:
Try your cutoff a little slower (not molasses) and listen to the sound as well as watching the tach.
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Thanks. How long is "a little slower"? 2-3 seconds to pull fully out? Longer? I can get the rise in rpm if I just screw out the mixture, but I'm thinking that's not what is meant by checking for rpm rise on cutoff.
 
5 seconds isn't too slow. Screwing it to cutoff isn't bad either. It's just that you get a choppy progression sorta. You don't have to go to quit either. You can sweep for the rise and push back in to re-catch. That way you can check several times without restarting. Your desired rise is whatever RPM rise your book says (like 25) and add about ten RPM per thousand feet up to around 80 or a hundred (Leadville).
 
I set the idle on my fixed-pitch 9A to about 550 and it certainly helped with landing. I don't actually run it at 550 but with a P-mag EI it's happy at 600. Make sure that your idle mixture is correct. My engine stopped twice during stall testing and it turned out the mixture was way too rich. I wouldn't want to get a little slow on final, open the throttle and find the engine quits! According to Precision documentation you lean it at 750 rpm and should see a 10-50 rpm rise with a warm engine.
 
gasman said:
With the wing of the 9, the rpm must be set low or it will float and float and float etc.
Click to expand...

...and float and float and float ... :D

With my Silverhawk, I have to check lean RPM by using the Vernier adjuster. When I just pull the mixture back, it quits right now . Not at all like the FI C-172s I used to fly. They would sputter and shudder before shutting down.
 
Rupester said:
With my Silverhawk, I have to check lean RPM by using the Vernier adjuster. When I just pull the mixture back, it quits right now . Not at all like the FI C-172s I used to fly. They would sputter and shudder before shutting down.
Click to expand...

Same is true with my carb.

So, what I'm gathering is that I don't have to pull all the way until it quits necessarily, just slowly screw out the vernier checking to see that I get the rpm rise.

Thanks for the interesting reading Gil. I'll have compare what I have against that.
 
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Gil, that's backwards then. You want correct mixture at sea level with a little cushion (RPM rise). When you go to altitude your mixture will be richer, whichs gives a larger RPM rise as you sweep towards cutoff.
 
My idle (warmed up) is now at about 590RPM. Seems to run fine at that speed without quiting, and landings are better and shorter. I added Locktite to help secure the throttle linkage jam nut on my carburated 0-320, and noticed that I can get a pretty good look at the linkage through the exhaust opening of my lower cowl with a flashlight. I plan to take a look whenever I add oil. That, along with verifying idle speed prior to takeoff, should cover it for me. Thanks to everyone for sharing experience and knowledge.

John
 
So today at shutdown I let it run full rich at about 900 rpm and then slowly screwed out the mixture knob until it stopped (taking about 15-20 seconds). I really got no rise, maybe +10 rpm max, although I'm not even sure about that. Does this mean I'm too rich, too lean, or neither? If either of the first two, which way do I turn the mixture adjustment thumbscrew on the carb? (I'm at about 1750 MSL, and it was about 85F if that matters.) Thanks.
 
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If you really did a good check, it means you are too lean. The screw needs to go counter clockwise. Try half a turn. You should be able to reach it through the bottom exit without uncowling. I'd try it once more to confirm and then crawl under, adjust, and do it again.
 
aerhed said:
If you really did a good check, it means you are too lean. The screw needs to go counter clockwise. Try half a turn. You should be able to reach it through the bottom exit without uncowling. I'd try it once more to confirm and then crawl under, adjust, and do it again.
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Thanks. My earlier post a couple weeks ago suggested I could get the rise by slowly screwing it out.. I must have been incorrect because today slowly screwing out mixture vernier (about a 1/2 turn at a time) got me nothing for the rpm rise. I'll try to very slowly pull it out next time rather than screw it out and see if that makes a difference.
 
Steve, IMHO, 900 RPM is far to high for an idle RPM. Idle (closed throttle) should be as close to 500 as you can get. My O-320 carb with a lightweight FP prop idles at about 450. It doesn't idle smoothly, but it doesn't quit, either. I don't let it sit at this low idle when doing ground ops, I elect to open the throttle just a bit to get a smooth idle at about 700-750. But I want a low idle so that during landing, the engine isn't producing much thrust to lengthen my landing distance.

At 900 RPM you may not be using the idle circuit of the carb exclusively.

I would recommend you adjust the idle stop screw to allow a slower idle, then repeat the test.
 
aerhed said:
Gil, that's backwards then. You want correct mixture at sea level with a little cushion (RPM rise). When you go to altitude your mixture will be richer, whichs gives a larger RPM rise as you sweep towards cutoff.
Click to expand...

Not if you are setting the idle at your home base, and that happens to be at 3000 MSL.
 
PCHunt said:
Steve, IMHO, 900 RPM is far to high for an idle RPM. Idle (closed throttle) should be as close to 500 as you can get. My O-320 carb with a lightweight FP prop idles at about 450. It doesn't idle smoothly, but it doesn't quit, either. I don't let it sit at this low idle when doing ground ops, I elect to open the throttle just a bit to get a smooth idle at about 700-750. But I want a low idle so that during landing, the engine isn't producing much thrust to lengthen my landing distance.

At 900 RPM you may not be using the idle circuit of the carb exclusively.

I would recommend you adjust the idle stop screw to allow a slower idle, then repeat the test.
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Thanks Pete. If you read up earlier in the thread you'll see where I moved the idle from about 800 down to below 600 where it is now. I just mentioned 900 rpm above for testing my idle mixture adjustment. I agree that there is a huge difference in landing with my idle set lower... a whole different airplane in that respect.
 
Gasman, the objective is to make the engine run acceptably at sea level and the pilot does the regulation as you go uphill. If you set up full rich idle for 3000 MSL you are a bit too lean for Sea level operation. Not a big deal at 3000', but say you set up for 25 RPM bump at 5000'. It may not even run down at sea level. Or, it may be really balky when you punch the go knob. A Marvel carb should always be increasingly leaned manually with a altitude for idle ops. That's just the way it is. They don't have all the dingle levers and dashpots our Holley's had as kids.
 
aerhed said:
Gasman, the objective is to make the engine run acceptably at sea level and the pilot does the regulation as you go uphill. If you set up full rich idle for 3000 MSL you are a bit too lean for Sea level operation. Not a big deal at 3000', but say you set up for 25 RPM bump at 5000'. It may not even run down at sea level. Or, it may be really balky when you punch the go knob. A Marvel carb should always be increasingly leaned manually with a altitude for idle ops. That's just the way it is. They don't have all the dingle levers and dashpots our Holley's had as kids.
Click to expand...

That's what I get for living at sea level all of my life..............:rolleyes:
 
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