Manifold Pressure vs Propeller

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LiebermanD

Filing Flight Plan
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Daniel L. Lieberman
Can anyone suggest an article that discusses throttle vs prop useage in more depth than throttle first. For example the advisability of a "square" setting. Also any rules of thumb without reference to the POH for max power vs max economy.

TIA
 
LiebermanD said:
Can anyone suggest an article that discusses throttle vs prop useage in more depth than throttle first. For example the advisability of a "square" setting. Also any rules of thumb without reference to the POH for max power vs max economy.

TIA
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Oyyyyy.... "square, over square, under square"..... even "throttle first"... there is NO rule or rule of thumb that covers all applications. First off, the whole "over square" thing (RPM in 1000s lower than MP in inches) ruining your engines is a crock of crap. Minimum RPM with maximum manifold pressure will give you greatest range and efficiency. I'll refer you to the Lindbergh Doctrine from pre WWII when Lindbergh was showing Chenaults men in Burma how to increase their combat patrol and attack mission range in the P-40s. You do need to watch out for some RPM limitations due to harmonic issues, but they are case specific and placarded for applicable engine/prop combinations.

Throttle First..... Can't think of a better rule to kill a GTSIO 520. All my power reductions from altitude are done props first, and depending on the engines and mission, I may never touch the throttles from the point where I advance them to full for takeoff until I'm about to land, doing all my power adjustments with props, mixture, and potentially wastegates.

The conclusion, don't look for rules of thumb, they will be wrong in some if not most applications, and occassionally are wrong in ALL applications. The lack of mechanical understanding amoungst the pilot community never ceases to amaze me.
 
CJones said:
Thanks for the link. I'll have to read up on it. I've always thought that the whole "reduce to X-squared" seemed arbitrary. It doesn't seem that the actual numbers have any direct correlation at all.
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Damn good of you to notice that..... Ask the guys flying behind the 1340s if they can even get the engine "square"....
 
interesting Henning, we always make power reductions with throttle on our GTSIO's. I set RPMs to 1700 in cruise and never touch them until landing. rarely have cracked cylinders and unless errant jackhammers on the ramp get in the way run the engines to TBO. different strokes i suppose.
 
CJones said:
Thanks for the link. I'll have to read up on it. I've always thought that the whole "reduce to X-squared" seemed arbitrary. It doesn't seem that the actual numbers have any direct correlation at all.
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About the only thing that's true about the "undersquare" concept is that it's easy to remember.
 
tonycondon said:
interesting Henning, we always make power reductions with throttle on our GTSIO's. I set RPMs to 1700 in cruise and never touch them until landing. rarely have cracked cylinders and unless errant jackhammers on the ramp get in the way run the engines to TBO. different strokes i suppose.
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It's not about the jugs, it's about the gear sets and bottom ends. Pull the throttles and put the nose down, and before you know it, the props are driving the engines, but if that's the way your boss wants you to operate the engines, you do as he requests.
 
i guess since I plan my descents, the MP rarely gets below 25 in. until short final. Ive never had the props drive the engines except in the flare.
 
Henning said:
Oyyyyy.... "square, over square, under square"..... even "throttle first"... there is NO rule or rule of thumb that covers all applications. First off, the whole "over square" thing (RPM in 1000s lower than MP in inches) ruining your engines is a crock of crap. Minimum RPM with maximum manifold pressure will give you greatest range and efficiency. I'll refer you to the Lindbergh Doctrine from pre WWII when Lindbergh was showing Chenaults men in Burma how to increase their combat patrol and attack mission range in the P-40s. You do need to watch out for some RPM limitations due to harmonic issues, but they are case specific and placarded for applicable engine/prop combinations.

Throttle First..... Can't think of a better rule to kill a GTSIO 520. All my power reductions from altitude are done props first, and depending on the engines and mission, I may never touch the throttles from the point where I advance them to full for takeoff until I'm about to land, doing all my power adjustments with props, mixture, and potentially wastegates.
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Been flying behind the gitso 520s for something like 25 years now, and have to agree that the old saw about flying over square is not applicable to these engines. I routinely fly my 411 at 1850 and 19". Started that after reading the story about Lindbergh, though the story I read had him in the South Pacific as a tech rep for Lockheed. He's credited (at least in this book) with having shown the P-38 pilots how to extend the range of their Lightnings enough that they were able to take out Yamamoto. Whichever version is right, the bottom line is he proved the efficiency of flying over square. I also agree most heartily that back-driving the engines is a sure-fire killer of 520s. That's one of the most difficult things pilots new to geared engines have to get used to.

In coming down from altitude I leave the throttles and props alone. Just trim for 600 fpm descent and relax. Engines stay nice and warm so shock cooling is not an issue. Requires a little planning on the pilot's part so he starts his descent soon enough, but that's no biggy. I haven't completely thrown the Continental owner's manual away, though. You seem to have had good luck flying with a much higher than recommended MP. Never have I left the throttles all the way against the stops above roughly 1200 feet AGL. Throttles back to climb MP, then props. Setting up for the approach, props back then throttle. So far (knock on wood) I've managed to take several pairs of the engines to TBO without exception.

Take the tin off and the 520s look like massive tough guys. Misleading. They are tough, great engines, but they have to be treated with the respect they deserve. A ham-handed pilot is their worst enemy.
 
2256Q said:
In coming down from altitude I leave the throttles and props alone. Just trim for 600 fpm descent and relax. Engines stay nice and warm so shock cooling is not an issue. Requires a little planning on the pilot's part so he starts his descent soon enough, but that's no biggy. I haven't completely thrown the Continental owner's manual away, though. You seem to have had good luck flying with a much higher than recommended MP. Never have I left the throttles all the way against the stops above roughly 1200 feet AGL. Throttles back to climb MP, then props. Setting up for the approach, props back then throttle. So far (knock on wood) I've managed to take several pairs of the engines to TBO without exception.

Take the tin off and the 520s look like massive tough guys. Misleading. They are tough, great engines, but they have to be treated with the respect they deserve. A ham-handed pilot is their worst enemy.
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sounds like about the same way we run. cruise at 32" and 1700 and i usually pull back to 28 or 25 inches within about 5 minutes of destination. Mostly need that just to get down under 165 mph for the gear. ive managed to hold the power at 25 and adjust flaps/gear/speed to make the entire approach at constant power and the power comes out slowly on short short final and in the flare.

expecting more than 50 hp out of any one cylinder on an aircraft engine is generally asking a lot. the GTSIO seems to push it to the max.
 
tonycondon said:
sounds like about the same way we run. cruise at 32" and 1700 and i usually pull back to 28 or 25 inches within about 5 minutes of destination. Mostly need that just to get down under 165 mph for the gear. ive managed to hold the power at 25 and adjust flaps/gear/speed to make the entire approach at constant power and the power comes out slowly on short short final and in the flare.
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I always have the same issue. The 411 loves to fly and hates to slow down. I have the cowl flaps that I use to help get her down to gear and flap speed. Gives that speed up unwillingly, for sure. Dang, I love those airplanes! :yes:
 
no cowl flaps on the B model but we can do 15 flaps below 200 mph which helps a little. mostly we just have to plan on a mile or so of level off at some point on the way down to get the gear down.
 
tonycondon said:
no cowl flaps on the B model but we can do 15 flaps below 200 mph which helps a little. mostly we just have to plan on a mile or so of level off at some point on the way down to get the gear down.
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Yeah, I had a B several years ago. Great flying airplane, but not as nimble as the 411. Extra 4 feet of wing, if I remember right. Not to mention the extra weight of the pressure vessel. My wife liked it better, though. Said it was smoother flying. Ooookay, said I. :dunno: She just hated the oxygen mask above 13 that the 411 had to have.
 
2256Q said:
I routinely fly my 411 at 1850 and 19".
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Tony Condon said:
cruise at 32" and 1700
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Ray, aren't those RPM numbers for the props not the engines? While I'm in violent agreement that "oversquare" concerns are generally baseless I suspect that WRT engine RPM you aren't oversquare with 1850 RPM and 19" MAP. And isn't 19" awfully low for cruise? Tony's 32 sounds a lot more plausible unless you're going for absolute max range.
 
tonycondon said:
sounds like about the same way we run. cruise at 32" and 1700 and i usually pull back to 28 or 25 inches within about 5 minutes of destination. Mostly need that just to get down under 165 mph for the gear. ive managed to hold the power at 25 and adjust flaps/gear/speed to make the entire approach at constant power and the power comes out slowly on short short final and in the flare.

expecting more than 50 hp out of any one cylinder on an aircraft engine is generally asking a lot. the GTSIO seems to push it to the max.
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I think what Henning was saying that you can do the same thing with the mixture control and save a little fuel. Just lean it out until you have 25". When I ran the Six that is t he way I ran it. WOT then adjusted with prop and mixture.

Dan
 
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The obvious solution would to put in some metric manifold pressure gauges - then you will never run anywhere near "square" - so the under / over question will never come up.
 
lancefisher said:
Ray, aren't those RPM numbers for the props not the engines? While I'm in violent agreement that "oversquare" concerns are generally baseless I suspect that WRT engine RPM you aren't oversquare with 1850 RPM and 19" MAP. And isn't 19" awfully low for cruise? Tony's 32 sounds a lot more plausible unless you're going for absolute max range.
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My tachs are set up for prop rpm, yes. I'd have to convert to get the engine rpm. Remember, too, that Tony flies the 421. My 520s are rated at 340 hp, his at 375. I think my engines turn up to 3200 rpm and his to 3600, but I'm not certain. It's been too many years since I looked.

I don't have any memory of the MP running as high as Tony said his runs. Something must have changed. The 411 is 2450 and 24 and a half at top of the green. It's quite a while, but I didn't remember my B being that different from the 411 I owned at the same time. Oh, well. Getting old. :dunno:
 
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32 inches is the top of the green arc on the GTSIO-520-H's of the 421B. Redline is 39.5 inches. max prop RPM is 2275 and its a 3:2 ratio so engine RPM at max is a bit over 3400.
 
tonycondon said:
32 inches is the top of the green arc on the GTSIO-520-H's of the 421B. Redline is 39.5 inches. max prop RPM is 2275 and its a 3:2 ratio so engine RPM at max is a bit over 3400.
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Thanks, Tony. Like I said, it has been a long time and I ain't exactly a spring chicken anymore.
 
ha i just hope that i recalled all the numbers correctly! :)
 
2256Q said:
Your memories have to be better than mine. I sold my B in '87. Good grief, that's 21 years ago! No wonder I screwed up. :redface:
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I can help you, Ray. Just give me that 411 of yours (I'll gladly take it off your hands, free of charge no less) and go buy another 421. That way, you'll remember. :D

Ain't I a nice, selfless guy? :yes:
 
Ted DuPuis said:
I can help you, Ray. Just give me that 411 of yours (I'll gladly take it off your hands, free of charge no less) and go buy another 421. That way, you'll remember. :D

Ain't I a nice, selfless guy? :yes:
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By golly, a man doesn't get an offer like that every day. Tell ya what, with fuel prices the way they are, I'm almost tempted. 35 to 40 gph, 6 bucks a gallon - OUCH! You sure you want this thing? :confused:
 
dmccormack said:
I'll even fly down and pick it up!

And I'm a fellow vet!

:)
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Appreciate the offer, Dan, but think I'll hang on to the old girl. We've been together a long time and covered a lot of ground together. Like a good wife, she puts up with me where another plane might turn around and bite me. If nothing else, I can go out to the hangar and stroke her props. :goofy:
 
2256Q said:
By golly, a man doesn't get an offer like that every day. Tell ya what, with fuel prices the way they are, I'm almost tempted. 35 to 40 gph, 6 bucks a gallon - OUCH! You sure you want this thing? :confused:
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Like I said, Ray, I'm just that selfless! :yes:

Although Dan is a fellow vet, so I suppose he's more deserving than I am. I'm just some kid engineer with long hair. :)

On those fuel prices in high consumption aircraft, though, I've funded a couple trips in the Navajo for various reasons. We flight plan it for 40 gph. It does about 36 in cruise, but with takeoff and climb you flight plan for 40 and it comes out right on the nose. Once we had to make a flight in it because it was known ice in the clouds, forecasted IFR to minimums, so nothing less would do. 1.8 hours total flight time, 72 gallons. $400 flight. And that's paying just for the fuel, doesn't include the maintenance and wear and tear on the plane! :eek:

That fuel cost is part of why those Lancair 360s seem mightly attractive, but the all-weather capability and cargo capacity makes me frequently lean towards an Aztec/310/Baron.
 
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