To idle or not to idle
akarmy said:
So this is one area that I still take issue with. I have heard some opinions that "shock cooling" really is only an issue with the larger engines that the 0-320 / 0-360's that we run.
Also, Mike Seager teaches pull to idle on downwind to slow. He has many thousands of hours on his trainers doing just that every day and his engines are not "killed"
In the Rv9 especially people tend to have a hard time slowing it down if they are not willing to close the throttle.
Just some more thoughts on the issue...
Fair enough, Mike has no doubt lots of experience and if he thinks pulling the throttle to idle, OK. Mike Seager is teaching low time pilots, often with no high-performance experience, so it is easier teaching the throttle than planning 5 miles ahead method of speed control. However I don't think it is the only way or ideal or even preferred. I am fully aware that most first time RV pilots are flying the hottest thing they have ever flown, performance wise. I am not criticizing low time pilots, Mike?s technique or his teaching method, but I would not teach it this way. No right, wrong, just different. Fly the plane; don't let the plane fly you.
I try to teach pilots transitioning to their fast RV to think way a head, 5-15 minute ahead. At RV speeds you can be doing 3 to 3.5 miles a minute. It is more headwork than hand and feet skill, which is a challenge to teach sometimes.
For me I teach a gradual stepped reduction in power from cruise to initial approach speed, final approach power and than idle at touchdown, with out oscillating power back-N-forth, which is a little more elegant. Yes you will slow quicker with idle power, but if you ask Mike, I think he would not disagree with a slow, continuous, gradual reduction in power.
With a fixed pitch prop, slowing can be a little bit of an issue in any RV (apparently a little more in the RV-9), but again with proper planning there is no reason to go to idle until you are in the flair. With a constant speed prop going to high RPM slows you down without pulling the throttle (MAP) back. With a C/S prop it is more reducing the residual thrust not adding drag, but it does feel like you are adding drag. A fixed prop has quite a bit of residual thrust even at idle. Also with a wood prop, guys run higher idle to overcome the lack of prop flywheel inertia, which compounds the problem. So may be going to idle is all they have to get the speed in some cases.
To be honest 99% of my RV time is with a constant speed prop. I do have friends with fixed props and doing formation flying with them is a pain because they can't slow as quick, even with idle power. In formation flying you are jamming the throttle all the time. Really not good for the engine, but you have to do what you need. It is like the skydiver-jump planes. They go thru several cylinders every few 100 hours.
Engine cooling:
From Lycoming and other sources going to idle in flight is not a great thing to do for many reasons, albeit doing it for a few seconds on down wind the damage may be small but not insignificant. One issue is with out heat in the cylinder, lead scavenging additives in the fuel don't work. You heard the term "loading up"? Hot plugs are happy (lead free) plugs. Granted a momentary idle and than back to power will not load the plugs. Also I think pilots should practice idle power approaches time to time to keep current on forced landings, regardless of engine longevity.
Second, read Lycoming's recommendation for operating their engine. Throttle changes and quick cooling can cause ring and valve issues. Just from common sense an engine that is held at a constant RPM will last longer than one with a throttle jockey. A gradual reduction in of power in the pattern to touch down is only one-way to fly, but it should reduce or minimize engine wear.
Bottom line is "you got to do, what you got to do." How much engine damage are you doing? I don't know, probably little, but what is the affect of not changing your oil often? If you understand internal engine stress in the reciprocating parts, especially valve train of an engine, any change in RPM causes "jerk" or shock loads (jerk = acceleration of acceleration). Maintenance, activity and operation is key to long life of your engien, so throttle to idle is only a small part of the picture. From Lycoming they only address long idle descents from altitude, not pattern work. However in another article they talk of slow throttle movment. SO if you must go to idle, move the throttle slowly.
http://www.lycoming.textron.com/mai...keyReprints/operation/avoidSuddenCooling.html
http://www.lycoming.textron.com/mai.../operation/proceduresAvoidStickingValves.html
RV-9:
I don't know about the Magic of the RV-9, it is clean but is not a glider. May be it has a better glide ratio than other RV's and is optimized for slower (lower stall) speed, but
a little pre-planning and airmanship can overcome a low drag airframe. Have you ever been in a commercial airliner and felt the thrust coming up-down-up-down-up-down, than you know what I mean about an elegant approach and an abortion.
Sometimes you have to chop-N-push to get it down, but that should not be the normal procedure, in my opinion. (It pisses the flight attendants off when the airsick bags get filled.)
TECHNIQUE:
Either come screaming in the pattern, chop the power to idle, slip, do what ever you can to slow down to get the flaps out, OR you can plan ahead, pull the power back to an appropriate partial MAP/RPM and slow gradually over a longer period (distance). With out jockeying the throttle, use a gradual continuous reduction in power and not power oscillations to control speed. Control speed by planning ahead and aircraft configuration (flaps).
It is like driving to a Red light with your foot on the Gas and at the last minute stomping the brakes, vs. seeing the light will change to red and coasting and gradually braking. One style is smoother and burns less gas.
Cheers George
ATC told the airline pilot: "I need you to get down faster."
Pilot: "uhaa ...this is as fast as I can get down."
ATC: ...got crossing traffic, I kept you up too long,....uhaaa don't U got speed brakes on that thing?
Pilot: uhaa, Roger I do have speed brakes... but they are for my mistakes, not yours...
