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PostPosted: Sun Sep 24, 2017 8:04 pm 
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Hey everyone! Love this bird!

I have some technical questions regarding the landing gear selector and engine power settings.

After raising / lowering the gear is it recommended to move the gear position switch from Up/Down to the neutral position? Or should the switch remain in the corresponding up/down position instead of neutral?


For power settings I simply can't remember surprisingly what I used to do when flying a real world 182 8+ Years ago.

Do we adjust the prop before engine power? Or pull the engine back first, then the prop?

For example after takeoff, do we bring the prop back to the green and then the main throttle? Or throttle then prop?

Thank you! :)

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PostPosted: Sun Sep 24, 2017 11:48 pm 
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Gear operation on the Comanche is by electric motor, so the switch should always be returned to neutral once the gear is locked up or down,

To reduce power, set manifold pressure then prop. To increase power, set prop, then manifold pressure. So after take off, reduce power to 25"MAP, then 2500RPM for initial climb. Watch CHT and keep below 400 degrees or in the green by varying airspeed and therefore airflow over the cylinder heads. Maintain 24"MAP during the climb, then set cruise power once you've arrived at your desired altitude. There is some marked vibration at 2100rpm, particularly with the MacCauley prop, so you may want to avoid that. In his demo video, Scott advises a cruise setting of 23"MAP and 1900rpm. In the descent, avoid power reduction to the point where your cylinder heads cool too much. Try to descend keeping the CHT in the green arc.

Like you, I love this plane, and I try hard to get it right. Hope this has been of some help.


Cheers,
Mike

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PostPosted: Mon Sep 25, 2017 10:45 am 
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Dogsbody55 wrote:
Gear operation on the Comanche is by electric motor, so the switch should always be returned to neutral once the gear is locked up or down,
I'd do this as well, but it is not absolutely mandatory: there are of course limit switches, that cut the power from the motor when the desired position is reached. I'd advise like you: I don't like having "hot" systems only isolated by a position-detecting microswitch :).

And as mentioned, throttle first, then prop (unless having a really good scenario and reason to do otherwise). In A2A Comanche 250, you can climb with full throttle, full RPM pretty comfortably, but watch out for the CHTs: keep them well under 400, around 380 or so.

-Esa


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PostPosted: Mon Sep 25, 2017 11:04 am 
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Thank you all very much! I've been in the habit of returning the gear switch to neutral, but just wanted to be sure.

I am glad I asked about the power sequence, I may have been doing it wrong for a little while. Shame on me since I once flew a similar high performance airplane.

Just out of curiosity sake, does anyone know why we reduce power then prop? and reverse on power increases? Does it have to do with the prop governor?

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PostPosted: Mon Sep 25, 2017 11:07 am 
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Oh and of course, for power addition, it is the opposite: prop up first, then the throttle. As a general rule. :)

Think about driving a car: when you want to overtake some guy, you'd first switch down, bringing up the revs, then slam on the pedal. And when back on the lane, you'd ease on the throttle and then switch back to the high gear, bringing down the engine revs. It is the same logic here.

-Esa


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PostPosted: Mon Sep 25, 2017 11:16 am 
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crippy wrote:
Just out of curiosity sake, does anyone know why we reduce power then prop? and reverse on power increases? Does it have to do with the prop governor?
This is actually fairly complicated in details, but a simple answer would be to note that the perfect ignition timing varies as a function of the RPM: faster the engine turns, earlier you'd want to bring in the flame. GA engines are relatively slow-turning, and have quite large ignition advantages. And their timings are fixed. They have been, for a big part, optimized to run at high power settings at extremely rich mixtures at their rated RPM, and for relatively low power cruise. At reduced RPMs, but at high power settings, their ignition advance easily gets excessive, "punching against a rising piston". Therefore you'd want to relieve the load on the engine before bringing down the RPM.

There is also an automotive analogy of attempting to accelerate at too low a gear: you'd note how unhappy your engine sounds - even if the car engine negotiates this to a big part by retarding the ignition timing, which your airplane cannot do!

-Esa


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PostPosted: Mon Sep 25, 2017 11:27 am 
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AKar wrote:
Oh and of course, for power addition, it is the opposite: prop up first, then the throttle. As a general rule. :)

Think about driving a car: when you want to overtake some guy, you'd first switch down, bringing up the revs, then slam on the pedal. And when back on the lane, you'd ease on the throttle and then switch back to the high gear, bringing down the engine revs. It is the same logic here.

-Esa

Excellent analogy.

An instructor once taught me, "You want to match gearing to your speed. If you're doing 50 mph and still in 2nd the engine is laboring...if you're in 5th the engine is lugging. Match your speed."
(Motorcycles)

It parallels in flying.

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PostPosted: Mon Sep 25, 2017 1:01 pm 
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Dogsbody55 wrote:
Gear operation on the Comanche is by electric motor, so the switch should always be returned to neutral once the gear is locked up or down


No. You are correct that the Comanche uses an electric motor to raise and lower the gear, but the gear selector switch is never placed into a "neutral" position - it is always placed in the up or down position and left that way. The same is true for every other retractable gear airplane.

(I've never heard of an aircraft that requires an extra movement of the landing gear selector into some position other than "up" or "down." In fact, I would guess that if a manufacturer attempted to certify a plane that required such a step, the FAA's aircraft certification branch might refuse; introducing an extra step into the crucial process of raising and lowering the gear seems like a very bad idea.)


Dogsbody55 wrote:
To reduce power, set manifold pressure then prop. To increase power, set prop, then manifold pressure. So after take off, reduce power to 25"MAP, then 2500RPM for initial climb. Watch CHT and keep below 400 degrees or in the green by varying airspeed and therefore airflow over the cylinder heads. Maintain 24"MAP during the climb, then set cruise power once you've arrived at your desired altitude.


This part is all perfectly correct - I would just add "increase throttle setting as needed during the climb to maintain 25"MAP. This requires regular attention since atmospheric changes cause manifold pressure will decline by 1" for each 1000 feet you climb."

And for the section on power reduction, it would be more correct to say "reduce MP to desired setting, then set RPM, and then return to the throttle and reset the MP (because MP rises whenever you reduce RPM)."

This is the way we teach it at the Comanche Pilot Proficiency Program (CPPP) which are put on several times a year by the Comanche Flyer Foundation ( http://www.cffpilot.com )

Incidentally, any sim pilot who falls in love with the Comanche is welcome to come attend a CPPP seminar - they are open to all, not just members of the International Comanche Society ( http://www.comancheflyers.com ). The classroom portion of the course is 8 hours on Saturday plus 2 hours Sunday morning, and includes over an hour in the hangar with a Comanche on jacks, performing emergency gear extensions.

Malcolm Dickinson
CFI; CPPP instructor
N9284P


Last edited by SaxTeacher on Tue Sep 26, 2017 1:47 pm, edited 1 time in total.

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PostPosted: Tue Sep 26, 2017 7:52 am 
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SaxTeacher wrote:
No. You are correct that the Comanche uses an electric motor to raise and lower the gear, but the gear selector switch is never placed into a "neutral" position - it is always placed in the up or down position and left that way. The same is true for every other retractable gear airplane.
Huh. :) Care to explain that to all the Boeing pilots doing up & off on every takeoff? :mrgreen:

In case of Comanche, I don't know how manufacturer intended it, for it is not discussed in the manuals but, if the up limit switch fails open, it is likely that the gear motor burns its fuse after driving the gear up. I'd bring it to neutral right after gear retraction, but as mentioned, not doing so has no ill consequences except in case of limit switch failure.

-Esa


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PostPosted: Tue Sep 26, 2017 11:58 am 
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AKar wrote:
SaxTeacher wrote:
No. You are correct that the Comanche uses an electric motor to raise and lower the gear, but the gear selector switch is never placed into a "neutral" position - it is always placed in the up or down position and left that way. The same is true for every other retractable gear airplane.
Huh. :) Care to explain that to all the Boeing pilots doing up & off on every takeoff? :mrgreen:


I confess I have no knowledge of Boeings - only Piper, Cessna, Mooney, Beech, and Lake. On these aircraft, the only positions ever used are UP or DOWN.

AKar wrote:
I'd bring it to neutral right after gear retraction, but as mentioned, not doing so has no ill consequences except in case of limit switch failure.


Most Comanches (including mine, a 1968 model) do not have a middle switch position on the gear selector.
On those that do, it is used only when performing an emergency gear extension.


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PostPosted: Tue Sep 26, 2017 12:24 pm 
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Making more sense now. :) Typically, an added position tends to have some interest on to it, the neutral should not be there without an actual use. I should check if there was any SB status of interest regarding of this.

The usual reason to bring the lever to neutral or off, in the big plane world at least, can be, for instance, to reduce a chance of any minuscule leak bringing the system empty via a slightly leaking line or connection or whatever. In case of electrically driven gear, the issue would be that of over-currenting the motor if the limit switch fails, thereby blowing the fuse which in turn, if not easily detectable, could cause an unnecessary hair rise during the recovery steps.

-Esa


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PostPosted: Wed Sep 27, 2017 10:18 am 
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I leave it in neutral.

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PostPosted: Mon Oct 02, 2017 11:27 am 
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I also used to forget everytime if I should first lower MP and the prop or the other way around. Somehow I just couldn't get it to stick into my head. Until I came up with a mnemonic:

Prop on top.

A simple rhyming sentence like that is easy to remember! Whatever you do, the prop has to stay above, on top of, the throttle! So if you want to increase: first prop, then mp: prop on top! And if you want to decrease, first mp, then prop: prop on top!

Once you've got a mnemonic like that you will never ever forget it anymore. :wink:

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PostPosted: Sat Jan 26, 2019 4:05 pm 
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J van E wrote:
I also used to forget everytime if I should first lower MP and the prop or the other way around.
[...]
Whatever you do, the prop has to stay above, on top of, the throttle


Not actually true. In the real world, well-trained pilots of Comanches and other similar aircraft do not do it this way. When we make a power change, we always set the RPM first, then the manifold pressure.

The reason we set the RPM first is that manifold pressure changes whenever you change RPM.
So if you were to set the manifold pressure first, and then the RPM, you would find that when you moved the RPM control, the manifold pressure changes, necessitating that you then return to the MP knob and re-set it. Now you have made three power settings instead of just two.

The reason that always setting RPM then MP is not problematic is that it is not a problem to have the manifold pressure at a higher setting than the RPM. (Back in the days of radial engines, it used to be thought at having MP higher than RPM was bad for the engine. But it turned out not to be true... and in any case with modern piston engines it is not a problem.) For examples of how higher MP and lower RPM is ok for the engine, check the power setting chart for the Lycoming O-540. You will see plenty of approved power settings such as 26"MP with 2200RPM, or 24"MP with 2100RPM.

I do still run into a few pilots who don't understand this, and who react in horror that we teach always to set RPM first and to run with the lowest possible RPM in cruise. But it is not just me teaching these techniques. Look up some of the articles Mike Busch has written over the years in AOPA Pilot and other respected aviation periodicals. He (and I and many other flight instructors) have been trying to debunk the old wives' tale about "you must increase RPM before increasing MP and you must reduce MP before reducing RPM." Still, these old tales can linger around.


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PostPosted: Sat Jan 26, 2019 5:37 pm 
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Changing the prop second back in they wasn't for any concern about hurting the engine. It was to ensure that you didn't slam the sleeve into the governor stop when changing power. With some of the older design systems, especially the counter-weight type where you could get an extremely sudden change in prop pitch if you were outside the governed range and then brought the power up without first putting the prop at a higher RPM setting. The bigger concern was backloading the engine (reducing MP below RPM in the descent), which could cause harm with a boosted engine. You wanted to make sure that in the air the MP was always above the prop RPM, not the other way. Squaring power on non-boosted engines was simply an easy way to remember power settings. There's been several discussions in the past (mostly in the real world flying forum) talking about these things in depth.

Personally, I usually don't change prop first when coming back to cruise power is simply because you want to change the power (MP) to arrest the acceleration as you level off. Then you have a minute to get the prop right, set cruise power, and then start leaning without worrying about getting faster than you want because you were fiddling with the prop. That's a personal preference though, not a hard-and-fast rule.

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