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Flying Instructors & Examiners A place for instructors to communicate with one another because some of them get a bit tired of the attitude that instructing is the lowest form of aviation, as seems to prevail on some of the other forums!


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Old 5th Apr 2012, 19:10   #1 (permalink)
 
Join Date: Jul 2010
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constant speed propeller

hi folks

a couple of questions about reciprocating engines and constant speed propellers:

is it a correct statement that the reason my MAP is always less than ambient pressure due to the vacuum induced in the intake manifold as the piston moves away from the cylinder head?

why when you set the cruise power as you reduce the RPM's does the map increases?

how is the MAP affected by the propeller pitch?

many thanks

baobab72
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Old 5th Apr 2012, 19:27   #2 (permalink)
 
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Try this to get an understanding - JDs other articles are as good

Pelican's Perch #15:<br>Manifold Pressure Sucks!
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Old 5th Apr 2012, 19:30   #3 (permalink)
 
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This one is the follow on and is worth reading with #15

Pelican's Perch #16:<br>Those Marvelous Props
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Old 5th Apr 2012, 19:48   #4 (permalink)

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Baobab, It helps to consider the engine as an air pump, for that is what it is, powered by liquid fuel.

There is an ongoing battle between the engine trying to pump air out of the inlet manifold (causing low MAP, measured after the throttle plate) and the atmosphere trying to rush in past the throttle plate to replace the air pumped out (resulting in increased MAP).

If the engine rpm increases at a constant throttle setting, there are more pumping strokes / minute so the engine wins a little and successfully reduces the MAP.

If the engine rpm decreases at the same throttle postion, the atmosphere regains a little, so the MAP increases again.
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Old 5th Apr 2012, 21:35   #5 (permalink)
 
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constant speed propeller

many thanks for those links!! i really learnt something today!! that guy really knows his stuff!!!

another maybe silly question: based on what do you normally select the rpm setting for the cruise when given more options? noise? or performance?

i am getting checked out in the pa28 arrow and i am trying to gather as many info as i can since i will be flying with an instructor who barely speaks english and i dont wanna bury him under a heap of technical questions!!
so any info related to that plane - pa28 r - will be really appreciated.

baobab72
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Old 5th Apr 2012, 21:49   #6 (permalink)
 
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If it's Arrow II with Hartzell prop, the only useful cruise RPM is 2400 rpm, which is too loud anyway. Other option would be 2000 rpm, but it's usually way too rough and the power reduction with that low RPM is very significant.

If one was to simplify cruise operations with CSP: for best economy (lowest BSFC) use maximum possible manifold pressure with minimum RPM. Just be sure to read John Deakin's articles first, as suggested before - and don't forget to check the maximum continous manifold pressure chart for your engine (even normal aspirated engine have these) - I'll just spoil by telling you that it's not 25"/2500rpm, 24"/2400rpm - contrary to popular belief.
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Old 6th Apr 2012, 01:04   #7 (permalink)
 
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Quote:
Originally Posted by FlyingStone View Post
If it's Arrow II with Hartzell prop, the only useful cruise RPM is 2400 rpm, which is too loud anyway. Other option would be 2000 rpm, but it's usually way too rough and the power reduction with that low RPM is very significant.
.
The Lycoming powered Pa 28R have a restriction prohibiting continuous operation between 2000 and 2200 RPM. To imply that the only RPM choices is 2000 or 2400 is simply incorrect. When I flew the Arrow I usually used 22 in and 2300 RPM which at lower altitudes gave about 65% power and was smooth and reasonably quiet
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Old 6th Apr 2012, 01:44   #8 (permalink)
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Though I agree that the Lycoming 360 series, and perhaps other engine types, do permit, and provide information for, "over square" operation (MP in inches higher than the RPM in hundreds), there are other engines for which this is certainly not a good idea. When not approved for the engine, you are treading into detonation territory running over square, and the bad outcome of that gone wrong is really bad, and quite quick. A friend melted two pistons in his C 180 doing that on a hot day.

As long as you familiarize yourself with the permissible power settings, you'll be fine. If in doubt, don't operate a normally aspirated engine over square.

Avoid rapid movement of the propeller control, particularly when moving from course to fine pitch (RPM increase). Rapid movement of the propeller control into higher RPM, with any amount of power being developed can result in the engine momentarily overspeeding. Overspeed is possible during takeoff if you slam the throttle up - be gentle. I once had a Cessna 340 engine overspeed just at rotation on takeoff, I had to move the propeller well into the Low RPM setting to keep the engine within it's limiting RPM range. It worked, and I flew it home that way, as no maintenance was available at the departure airport.

On the other hand, on a warm day, and with due consideration for the possiblity of shock cooling, or other engine temperature excursions, set the plane up in a safe glide. With the engine at idle, slowly move the propeller control toward Low RPM. You'll see the change you can expect, should you have an engine failure, and you choose optimize your glide that way.

Also be aware that at low power settings and fine pitch, the RPM might sit right in the yellow "avoid" range. Coarse the prop a bit, and it will drop below that engine speed.

When flying the Arrow, familiarize yourself with the possible installation of the automatic gear extension system. Many Arrows had it disabled, though some still have it. If it is there, understand it, and how and when to lock it out. Be aware that if it is there, it will prevent you from retracting the gear at a lower speed, which can surprise you in a bad way on rare occasions.

If the Arrow is a turbo, be very certain to not overboost it when applying takeoff power, and be certain to idle the engine for two minutes after you're finished taxiing in after the flight, to cool the turbo, before you shut it down. Be wary of enthusistic people walking toward the plane as you do this.

If it is a "T" tail Arrow, they are very nice to fly, but get some training on landing and takeoff if you've not flown "T" tail Pipers before - they are different.

When you move on to the next phase of constant speed props, and start to fly twins whose props can be feathered, remind yourself that if you have an engine failure, and you let the engine stop turning, you probably will not get the prop feathered after that. Flying a twin with one stopped in fine pitch is miserable, and you will not get the performance stated by the flight manual (it might not climb at all).
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Old 6th Apr 2012, 10:51   #9 (permalink)
 
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Quote:
Originally Posted by Big Pistons Forever
The Lycoming powered Pa 28R have a restriction prohibiting continuous operation between 2000 and 2200 RPM. To imply that the only RPM choices is 2000 or 2400 is simply incorrect.
I beg to disagree. Type certificate for Arrow II (page 15) shows:

"Avoid continuous operation between 2000 - 2350 r.p.m." for Hartzell prop
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Old 6th Apr 2012, 14:28   #10 (permalink)
 
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Quote:
I beg to disagree. Type certificate for Arrow II (page 15) shows:

"Avoid continuous operation between 2000 - 2350 r.p.m." for Hartzell prop
Comes back down to reading the manual for the aircraft you are operating and know the limitations for that aircraft.
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Old 6th Apr 2012, 14:47   #11 (permalink)
 
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Quote:
Originally Posted by foxmoth View Post
Comes back down to reading the manual for the aircraft you are operating and know the limitations for that aircraft.
Excellent advice. I should have specified that the Arrow I was flying was a the 180hp version which indeed has a 2000-2200 RPM limit. As was noted in an earlier post the 200 hp variant has a broader prohibited range of 2000 -2350 RPM. Since I have only flown the Arrow 180 and the Turbo Arrow 4 I should have looked at the TCDS before making such a general statement.
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Old 7th Apr 2012, 07:19   #12 (permalink)
 
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Quote:
another maybe silly question: based on what do you normally select the rpm setting for the cruise when given more options? noise? or performance?
Cruise - performance. But then, even in GA I was paid to keep to a schedule.
Holding - noise (rpm) & economy (power). Especially over a town.
A scenic / fun round trip - economy.
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