Frequent observation about landing and idle RPM (CFI perspective)

[Parker_Woodruff]

I have not been doing much flight instruction work over the past few years, but I have noticed a couple common occurrences recently, both on flights and on various social media posts (Facebook, in particular).

It seems a pilot will complain about the airplane not settling down, yet their approach speed is correct (such as 70 KIAS for an M20J).  Here are the things that come to mind when I hear this:

Observation 1

- Idle RPM set too high.  I was flying with a client and he couldn't get the plane to settle.  I said "let me demonstrate one".  So I take it around the pattern and have the same problem he had.  We taxied off the runway...I think his idle RPM was 800-900.

It's quite possible you may know exactly what to do and, all things considered, be using correct pilot technique.  But in reality, a high idle RPM could be forcing bad results, or worse, forcing you to use to bad pilot technique to get a desired result.

Observation 2

- Not closing the throttle all the way.  This one is often recognizable to me by sound.  A pilot sets up a very good final approach attitude, airspeed, glidepath, etc.  Everything is great...until the end  But in the flare, the pilot hasn't completely closed the throttle control.  The pilot may not be able to experience a full flare unless he spends an exceptional amount of time floating down the runway trying to bleed off airspeed.  "Ballooning" is often the end result leading to either a go around or wasted runway as corrections are made.

Note on your next flight

- Check your engine idle RPM after the engine is fully warmed up.  You may find your idle speed is too high and spoiling your otherwise great flying technique. 

- Make it a point to feel the throttle control reach its "closed" limit before you touch down.

 

Happy Saturday!  Insurance is fine and all, but it's nice to talk about flying.  

[rbp]

This is the way 

[N201MKTurbo]

Couldn't agree more. I set my idle as low as I can get it without the engine dying on the runway. I can usually get it down to 350 - 400 or so.

It makes a huge difference on how short you can land. Some say that is too slow and bad for the engine. I think they are nuts, besides it almost never runs there, not even on rollout. The plane slows so much I have to give it throttle to taxi off the runway  

Besides, your tires and brakes will last a lot longer.

[Tim-37419]

Like others have already said, couldn't agree more based on my last plane. I'm going to add a twist for any other new owners.

 

I bought my J a few weeks ago from NM and had it ferried to TN. My most outstanding ferry pilot / transition CFI told me we have to adjust the idle or I would have a very hard time landing.

 

I assume the idle shot up due to the change in field altitude. Adjusting the idle was one of the easiest things I've done on an aircraft. We also checked idle mixture while we were at it. He knew what to do but I watched a YouTube video.

 

[Bartman]

My idle is very low as well.  Less than 500 when fully closed, and it sounds like she is about to quit sometimes on rollout. 

[PT20J]

Some of us know our airplane very well. But for the average pilot, there is such a thing as too much of a good thing.

For the IO-360, the RSA fuel injection system doesn’t meter fuel according to airflow at idle speeds because the airflow is too low to create enough pressure differential through the venturi and impact tubes to operate the metering diaphragms. Instead, a “constant head idle spring” internal to the fuel metering unit holds the ball valve slightly off its seat to provide idle fuel flow.

This is why the idle mixture is so sensitive to idle speeds. If the idle speed is set very low at a standard day at sea level, it can be too low at higher density altitudes. Mooney recommends 600 rpm (700 rpm for hot temps). Personally, I’ve found my engine to be very stable at idle over a wide range of density altitudes when set to 650 rpm at sea level. But, each engine is a little different - this in 90 year old technology 

[LANCECASPER]

This is another good reason for fine wire plugs. The engine runs a lot smoother at low rpm idle on fire wires.

[Guest]

17 minutes ago, LANCECASPER said:

This is another good reason for fine wire plugs. The engine runs a lot smoother at low rpm idle on fire wires.

Except for when your fine wire plug sheds its porcelain insulator into your cylinder.  I have sixteen of them to worry about.

[LANCECASPER]

51 minutes ago, M20Doc said:

Except for when your fine wire plug sheds its porcelain insulator into your cylinder.  I have sixteen of them to worry about.

Ouch, were they part of the Tempest 2018 bad batch or Champion?

[Will.iam]

1 hour ago, M20Doc said:

Except for when your fine wire plug sheds its porcelain insulator into your cylinder.  I have sixteen of them to worry about.

Maybe that comanche engine is just too hot for them. 

[Guest]

11 hours ago, LANCECASPER said:

Ouch, were they part of the Tempest 2018 bad batch or Champion?

I scrapped the Champion ones when I saw that he side electrodes were cracking and replaced them with Tempest.  We’ve seen Tempest issues with cracked centre insulators.

[Tim-37419]

On 4/22/2023 at 10:42 PM, PT20J said:

This is why the idle mixture is so sensitive to idle speeds

Thanks @PT20J, did you mean to say idle speed vs idle mixture? I didn't touch the mixture but idle speed was definitely more sensitive than I anticipated.

[T. Peterson]

16 hours ago, M20Doc said:

Except for when your fine wire plug sheds its porcelain insulator into your cylinder.  I have sixteen of them to worry about.

Show-off!!

[Pinecone]

Pfft, 16 is just getting started.

 

Try 56 - https://en.wikipedia.org/wiki/Pratt_%26_Whitney_R-4360_Wasp_Major

 

[PT20J]

5 hours ago, Tim-37419 said:

Thanks @PT20J, did you mean to say idle speed vs idle mixture? I didn't touch the mixture but idle speed was definitely more sensitive than I anticipated.

No, the idle mixture generally must be reset whenever the idle speed is changed and vice versa since the injector is not actually metering according to airflow at idle. Any change in idle speed affects idle mixture and changing the idle mixture affects idle speed. So, usually you have to go back and forth a couple of times to get both right. I usually make quarter to half turn adjustments of the speed stop screw and one click at a time on the mixture thumbwheel.

[N201MKTurbo]

I typically need to run the engine 5 - 10 times to get the idle settings where I like them.

[Guest]

5 hours ago, Pinecone said:

Pfft, 16 is just getting started.

 

Try 56 - https://en.wikipedia.org/wiki/Pratt_%26_Whitney_R-4360_Wasp_Major

 

That’s the starter motor for one of these.

https://en.wikipedia.org/wiki/Lycoming_XR-7755

 

[Hank]

5 hours ago, Pinecone said:

Pfft, 16 is just getting started.

 

Try 56 - https://en.wikipedia.org/wiki/Pratt_%26_Whitney_R-4360_Wasp_Major

 

 

14 minutes ago, M20Doc said:

That’s the starter motor for one of these.

https://en.wikipedia.org/wiki/Lycoming_XR-7755

 

Hope crap, ya'll!!!

I'm getting dizzy just thinking about watching the fuel flow for either of these!!

[N201MKTurbo]

6 minutes ago, Hank said:

 

Hope crap, ya'll!!!

I'm getting dizzy just thinking about watching the fuel flow for either of these!!

580 GPH at takeoff. They say it was one of the most fuel efficient engines ever made.

[Pinecone]

Interesting.  Basically 8 inline 4 cylinder water cooled engines that shared a crankshaft.

Too bad it never got to fly in anything, unlike the R-4360

[N201MKTurbo]

6 hours ago, Pinecone said:

Interesting.  Basically 8 inline 4 cylinder water cooled engines that shared a crankshaft.

Too bad it never got to fly in anything, unlike the R-4360

4, 9 cylinder radials stuck end to end.

We get 50 HP/cylinder out of our Lycomings, they were getting 150 HP/cylinder. There must be a supercharger in there somewhere.

[exM20K]

Back to @Parker_Woodruff’s topic here… Every plane I’ve flown has an idle check as part of the pre-takeoff check list(except the Diamond diesels IIRC).  IMHO, this check is as important as the run up mag test.  Will the plane idle at or below the specified maximum? If not, you may find yourself consuming a ton of runway when you want to land.

I had my OEM vernier throttle replaced with a McFarlaine vernier-ish unit two years ago because the original had a lot of slop in it. No matter how we tried to adjust that throttle cable, it hung up on something when I wanted it closed, and it took a really firm tug to get to idle.  My landings, especially at my short home field frankly sucked. And I’m not a big fan of yanking on controls to get them to do what I want.

Replaced with an OEM unit (true vernier control also manufactured by McFarlaine), and life is good.

also, my TSIO550 is very sensitive to fuel system setup and induction leaks.  They should idle nicely at 700 RPM.  If not, it’s wise to figure out why.

-dan

[Pinecone]

6 hours ago, N201MKTurbo said:

4, 9 cylinder radials stuck end to end.

We get 50 HP/cylinder out of our Lycomings, they were getting 150 HP/cylinder. There must be a supercharger in there somewhere.

Not exactly, as if you read the info, the cylinders were in line and water cooled.  So like a 4 cyl in line engine.  But 9 of them around a common crankcase.

[PT20J]

6 hours ago, N201MKTurbo said:

4, 9 cylinder radials stuck end to end.

We get 50 HP/cylinder out of our Lycomings, they were getting 150 HP/cylinder. There must be a supercharger in there somewhere.

All the radials had superchargers. Some were 2 stage and some were 2 speed and they had to shift gears at a certain altitude to get more boost to continue climbing.

The most complex (and troublesome) radial was the Curtiss-Wright R-3350 turbo compound that also had three power recovery turbines driven by exhaust and fluid coupled to the crankshaft. It was used on the Connie and the DC-7. I think that it generated the comment that the DC-6 was a four engine airplane with three blade propellers and the DC-7 was a three engine airplane with four blade propellers. 

[N201MKTurbo]

31 minutes ago, PT20J said:

All the radials had superchargers. Some were 2 stage and some were 2 speed and they had to shift gears at a certain altitude to get more boost to continue climbing.

The most complex (and troublesome) radial was the Curtiss-Wright R-3350 turbo compound that also had three power recovery turbines driven by exhaust and fluid coupled to the crankshaft. It was used on the Connie and the DC-7. I think that it generated the comment that the DC-6 was a four engine airplane with three blade propellers and the DC-7 was a three engine airplane with four blade propellers. 

The local A&P school has an R-3350 that is partially apart, you can see how it all goes together. One of the highest strung piston engine ever made.