Arrow climb power - 25-squared?

nanjason

New Member
What's with the 25-squared recommendation for climb in an Arrow? (Lycoming IO-360C1C6). I don't see anything like that in the POH but my instructor insists on using it for climb. But this brings the climb rate so far back it's pathetic. Aside from being quieter with lower RPM, what is the reason for this? I have read John Deacon's columns on Avweb and he has collected data that even flies in the face of the old "keep-MAP-lower-than-RPM" rule of thumb. Thanks!
 

pilot602

If specified, this will replace the title that
You have two seperate issues here.

1) Flying square.

2) Lower power settings for climb out.

1) Flying square is a throw-back to the radial engine days. The radials (and only because of the materials used at the time ... not the fact they were radials) tended to blow cylinders when MP exceeded RPMs by very much. However, nowadays (since about WWII on up) MP over the RPM is generally accepted and a proven way to increase fuel efficiency. Lindbergh used the technique to prove the P-38 actually had a lot more range than it was getting. However, only use what's printed in the power chart. Excessive MP or arbitrarily setting MP above RPM can still cause damage if it's too extreme.

However, flying square is an easy way to transition into an unfamiliar aircraft because so long as you keep it squared you cant blow anything up.


2) Lower power settings on climbout help with engine wear and tear. In most rentals no one cares and every one just invokes "rental power" and redlines everything they can. But if you own an aircraft, or really want to operate an aircraft correctly, you don't want to just redline the thing. We generally carry redline till about 1,000 AGL and then bring it back to a 25/25 climb. This is on an Apache (O-320s).
 

E_Dawg

Moderator
OK I hope I get this right:

Really the only reason you'd have to keep the MP lower than RPM would be because of the increased temperatures associated with the high manifold pressures and low RPM. It raises the brake mean effective pressue (BMEP - average pressure in the cylinder during the power stroke) because it raises the volumetric efficiency (the ratio of the amount of air going into the cylinder in the intake stroke over the amount of air able to enter it) because the engine runs slower allowing more time for the cylinder to suck the air in.

Higher BMEPs mean more efficient operation, as long as the engine doesn't exceed the operating limits. That's why on descent its better for the engine to have lower RPMs and higher MP (as opposed to low MP and high RPM) - it keeps the temps up and increases fuel efficiency.

Along with the increase in pressure, the temperatures will increase. If the temperature gets too high, you'll get detonation.

After saying all that, it almost definately won't be a problem in the NA four banger Arrow, assuming you don't purposely try to kill it.
 

Alchemy

Well-Known Member
Don't know the reasons behind it, but my instructor has also always been adamant about having Manifold Pressure lower than or equal to prop RPM's. In the seneca he reccomends we use 25"/2500RPM for climb and 23"/2300RPM for cruise. However, if the POH provides cruise settings in its performance tables which include higher manifold pressure than rpm settings, I see nothing wrong with using those settings. The Cessna 182RG POH for example, provides cruise power settings for 2200 RPM and 23" MP, 2100 RPM and 23" MP, and 2100 RPM and 22" MP. I see no problem with operating at these settings since the POH never mentions anything discouraging it.

The one advantage I could see to always keeping the MP lower than RPM is to avoid stressing the engine if full power is suddenlly needed and you forget to advance the props before the throttles (downdraft or windshear, etc). Having the props set at 2000 RPM and the throttle suddenly full open is probably not a good idea. Doing this forces the prop into a very high pitch position, imposing stress on the prop blades and prop governor mechanism. It would be kind of like trying to ride a bicycle up a really steep hill then suddenly shifting into the highest gear.
 

Tired

New Member
MAP lower then RPM is a bunch of crap. Check your POH. For example in the Seminole it's perfectly acceptable to run at 26.7 MAP and 2200 RPM. I wouldn't pull the Arrow back after takeoff, it climbs so slow it'll be better for the thing to just climb at full power.
 

MikeD

Administrator
Staff member
[ QUOTE ]
MAP lower then RPM is a bunch of crap. Check your POH. For example in the Seminole it's perfectly acceptable to run at 26.7 MAP and 2200 RPM. I wouldn't pull the Arrow back after takeoff, it climbs so slow it'll be better for the thing to just climb at full power.

[/ QUOTE ]

Which is one of the reasons I personally, when on final approach, won't push the props full forward for a potential go around, but advance them to their climb setting, hence one less lever to have to move on the go. On go around, I just advance the throttles full, and the props are set. Personal technique.
 

Tired

New Member
One thing I find annoying is when people shove the props forward on their approach to landing causing the engines to spin back up to 2700. Wait until the prop comes off the govener and then bring them up. It's much better technique if done this way.
 

MikeD

Administrator
Staff member
[ QUOTE ]
One thing I find annoying is when people shove the props forward on their approach to landing causing the engines to spin back up to 2700. Wait until the prop comes off the govener and then bring them up. It's much better technique if done this way.

[/ QUOTE ]

True enough. And it just sounds terrible too when they do that (the deep WAAA, waaa, waaa noise), not to mention that the engine probably doesn't appreciate that treatment anyway. Agree, poor technique.
 

ananoman

New Member
I am not sure where this comes from. I guess if you are too lazy to read the POH, you can be reasonably sure you will not hurt anything flying an opposed engine in this manner. Ask your CFI why you can't. Make him justify his answer. "Because I said so", is not acceptable. If you can't get an answer, find some one else. It usually works better when the CFI knows more than the student.

Most people don't realize that they operate their car in this manner every day. Everytime you get on the highway and shift into high gear, you are cruising at a higher manifold pressure than RPM. As others have said, it is the preferred method of operation. It is quieter, more efficient and better in every way.

On most of the big radials it was mandatory to carry more MAP than RPM except when pulling the power to idle when landing. They were all supercharged and had no shortage of manifold pressure. 55" on takeoff was not uncommon. These engines could be damaged by 'torque reversal' where the prop drives the engine. If you pull the power to idle and descend, the prop is driving the engine. The oil holes in the crankshaft are in the wrong place for this and the master rod bearing can fail, causing the loss of the engine.

It is really not that good for opposed engines either. Descending at high speed and low manifold pressure causes excessive cooling, It also causes excessive stress. The air in the cylinders has a cushoning effect. Remember that the pistons are moving up and down at a tremendous speed. If you take the engine in the Seminole at 2700 rpm, the pistons are moving almost 2000 fpm. They are reversing direction 90 times per second. The air in the cylinders provides cushoning to help the outward moving pistons decelerate before being yanked back down. It also keeps the piston rings pushed down into the bottoms of their grooves. Carrying too little MAP can cause ring float and breakage.
 

Alchemy

Well-Known Member
So I suppose the old rule about reducing manifold before RMP and increasing RPM before manifold is baseless as well? Surely there has to be some basis for these things. My Instructor really jumps my case if I leave the manifold higher than the RPM for any reason, even if it's less than a 1" difference for a second while I'm synchronizing the props or something. The only verbal reason I have wrangled out of him is "it puts too much stress on the engine".

Like I said earlier, to me personally it doesn't make much sense to make a big deal out of having MP higher than RPM. Nothing I have read or otherwise encountered so far in my meager study of aviation has convincingly substantiated this "unwritten rule". However, I really dislike the idea of simply telling my CFI what he's teaching me is wrong unless I have some hard evidence to back me up ("some guy on the internet said so" usually doesn't hold much weight). Does anyone have an article I could print out and give to my CFI to "get the ball rolling".
 

Acadia

Well-Known Member
[ QUOTE ]
One thing I find annoying is when people shove the props forward on their approach to landing causing the engines to spin back up to 2700. Wait until the prop comes off the govener and then bring them up. It's much better technique if done this way.


[/ QUOTE ]

Depends a little on the aircraft and the conditions. Im flying a Saratoga (PA32R-301) and on good gusty crosswind days when Im full of passengers I keep it at 100kts right to the numbers. On a normal glide path this requires a fair amount of MP and I always go prop forward (smoothly!) about 1/4 mile out. Now and then I am forced to go around due to bird activity and I 100% want my props already at max 2700rpm before I start the go around. You just have to do it smoothly and not 2 miles out.

[ QUOTE ]
Which is one of the reasons I personally, when on final approach, won't push the props full forward for a potential go around, but advance them to their climb setting, hence one less lever to have to move on the go. On go around, I just advance the throttles full, and the props are set. Personal technique.


[/ QUOTE ]

Help me with this one. In a go around you want max performace which will require takeoff power and prop settings until you reach a safe altitude to reduce it again. This may also be a piper product specific setting, but four of my complex POH's (the only 4 complex aircraft I have flown) - Commanche, Arrow III, Saratoga SP, & Seminole all specifiy prop full forward for a go around. The Arrow III and Seminole specify props full forward as part of the final approach check list as well.
 

MikeD

Administrator
Staff member
[ QUOTE ]
[ QUOTE ]
Which is one of the reasons I personally, when on final approach, won't push the props full forward for a potential go around, but advance them to their climb setting, hence one less lever to have to move on the go. On go around, I just advance the throttles full, and the props are set. Personal technique.


[/ QUOTE ]

Help me with this one. In a go around you want max performace which will require takeoff power and prop settings until you reach a safe altitude to reduce it again. This may also be a piper product specific setting, but four of my complex POH's (the only 4 complex aircraft I have flown) - Commanche, Arrow III, Saratoga SP, & Seminole all specifiy prop full forward for a go around. The Arrow III and Seminole specify props full forward as part of the final approach check list as well.

[/ QUOTE ]

My reasoning behind this one is that, to me, there's not that much extra power gained behind the extra 100 or so RPM of having the props full forward. And an average go-around is simply a maneuver to get you climbing again, nothing too overly exciting, unless it's maybe Telluride, CO in the middle of summer with a full load of pax and fuel. But that's generally my idea behind my technique.
 

ananoman

New Member
Think of the rules to increase RPM before MAP and to reduce MAP before RPM as a general guidelines and not absolutes. It is usually best practice to do it this way, just use common sense. If you were climbing at 25 squared and level out, you might find a cruise setting of full throttle and 2300 RPM. In this case I would just go full throttle first, then set the props to 2300. No limits are exceeded. The same thing goes if you are cruising at 18"/2300. If you want more power you can just use the throttle, within reason. When making big changes, use the rules. At other times, use your head. Most power charts are fairly consistent. In the Seminole you can usually go about 2" oversquare, so power changes can be made, then you can double check the setting as time allows. Unless you go way overboard it is pretty hard to hurt a normally aspirated engine, until you start to play with the red knobs.
 

EatSleepFly

Well-Known Member
My boss freaked the other day when I was flying him someplace in the 182RG because I had the MP at 24", and pulled the prop back to around 2250. Oh how I would've loved to tell him to sit down, shut up, and go to sleep. I reached back and pulled out the POH for him to peruse instead.
 

Baronman

Well-Known Member
I climb the arrow with full power.

Alot of people forget that most engines cool better using full power (as the article explains), that's why less than full power take-offs really don't help "preserve" piston engines.

On final I usually push the props forward SLOWLY and usually when I'm slowed to my approach speed when the prop is already at it's goverened limit. Pax in the King Air don't like to hear those props speeding up and getting out of synch.
 

Baronman

Well-Known Member
Oh yeah, and don't most piston engines have a higher manifold pressure (compared to RPM) during take-off).

Typicall take off - 28-30'' MP (Depends on ambient pressure) with 2700RPM. So...for all you "AVOID HIGHER MP with Low RPM" guys...take that and smoke it!
 

Alchemy

Well-Known Member
Yes, MP is above RPM on takeoff, but the argument of the "low MP" crowd would be "That's why we do a power reduction as soon as we reach 500 AGL". My CFI has told me repeatedly that I was going to burn up the engine if I didn't reduce power at 500 AGL even though the seneca POH states multiple times "THERE IS NO TIME LIMIT FOR MAXIMUM POWER".

In fixed pitch-prop airplanes I've always used full throttle for the duration of the climb.
 
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