I understand the rule of thumb regarding "flying by the squares".  I also understand it is mainly of convenience and is controversial since the relationship between manifold pressure and RPM doesn't really exist.  And yet,  I have read a number of posts by modern people, that still practice this.  So let me understand...the only time you guys ever violate that rule is when you are chopping the power on landing?
The reason I ask is actually more specific and has to do with the A2A Stratocruiser.  The manual provided by A2A specifies an RPM of 2350 on final approach.  Flying this ac down the glideslope at 23.5 inches of manifold pressure gives me a mighty problem reducing to the correct speeds. It does a lot better at about 19 or 20 inches. The aircraft is also equipped with Torque guages:  I know in real life, this would actually supercede the MAP.  I seriously doubt I know what I'm talking about, but am I correct in assuming that as long as there is positive torque on the propellers (being generated by the engine, and not the wind) that we are in safe territory?
As a side note, I don't suppose there are any old fellows on Simviation that have ever flown a C-97 or DC-6 or 7 that would have had these last-generation radial engines?

The shift-2 page provides takeoff, METO, and what appears to be maximum climb.  I don't have it running in front of me as I type, but I don't recall any advisories on landing, because that would be dependent on weight.  Actually, the figures I adhere to in practice, when flying this aircraft I got from here:

http://www.enginehistory.org/r-4360ops1.htm

The guy mentions that, concerning the descent power, you shouldn't let it drop below 1" for every 100 RPM...but I can't determine if it would be ok to indeed drop below square on short final.  To rephrase my question from above a little bit, as long as the engine is driving the propeller instead of the propeller driving the engine, this would be indicated on the torque guages as >0 torque, right...and if so, then there should not be a problem?

On the other hand, it could just be that A2A gives this plane a little bit too much power at 23 inches and in real life they flew the sqaures right up until the flare.  I probably should post this question on A2A.

So Brett, concerning no turbo GA aircraft with constant speed propellers, I haven't seen one yet where I didn't have to reduce the MP way way way below my RPM all through the descent.  (Speaking as a Sim Only pilot)  The Cessna 182 requires about 15 inches or less at 2400 RPM when flying in to land.  So am I to gather that every time one is descending to land, it represents a strain on the engines?

Yeah..  I can't vouch for the A2A engine/power accuracy, but I'm sure it's pretty close..

OK.. I see how you're looking at this now (RE: light GA). There is no concern about letting MP get too low for a given RPM.. it's the other way around. You can reduce power to idle (just do it gradually), no matter the RPM setting.

Unless you're descending rapidly, the constant-speed prop will see to it that there's ALWAYS a load on the engine. THAT is how a constant-speed prop controls RPMs. And even during a steep descent at idle, there's no harm done by having a prop actually applying tourque to the engine.. not unlike a steep descent at idle in a fixed-prop aircraft. All that really happens, is that the prop becomes a sort of speed brake.

What you want to avoid, is placing too much of a load on the engine. This is where a good understanding of constant-speed props is essential.. Ponder this scenario:

Cruising along at 23/2300 and then advance the throttle. As the torque increases; the constant-speed prop steepens the blade pitch in order to keep the RPMs constant.. and that of course increases the load on the internal engine components.. at some point, that stress becomes destructive. The same thing happens if you reduce RPMs. The relative load on the engine, increases, because the prop is trying to force the engine into a lower RPM. A common method used by pilots intimately familiar the airplane they're flying, is to reduce RPMs to a point just above where they KNOW the engines will start  "crying"  (bucking and vibrating).

As for the 377/AccuSim... I have it too (love it). I've never made a conscious effort to adhere to certain settings when descending and landing.. I just kinda follow instincts. Before initiating a descent; I set the mixture to "auto-rich", and increase prop-rpm to the top of the green-arc (I think that's ~2550). This gives me a wide marging for power application.. because an approach in that beast can make for a busy cockpit. A top of the green RPM lets me use relatively high MP (if needed) during the approach, without worrying about engine stress.

The only reason I can see for not reducing power too much on those big ole birds is drag.  Those old planes were not the most aerodynamic, and you throw in the drag from four monster propellers, they come out of the sky in a hurry.  I do know with radials, and all air-cooled engines, you don't want to reduce power quickly to avoid shock cooling the engine.  Turbo and superchargers are that much worse for that as well.

I couldn't find that discussion.. but here's an interesting quote:


Quote:

snippyfsxer wrote on Dec 30^th, 2009 at 11:59am:


I'm quoting myself just to bump this up as a new comment for anybody who was curious about it, instead of a modify

Brett_Henderson wrote on Jan 11^th, 2010 at 12:33pm:


I am curious for the reason as well, since all of my experience IRL has been in modern piston engine aircraft.  Perhaps it is the gear reduction, but wouldn't some turboprops (Garrett?) have similar issues?

These are great discussions.. 

Cushioning makes perfect sense.. and I always did bring RPMs back with power.. in real-life, small stuff.. and in big, sim radials.

Where it gets confusing, is the "squaring". I learned about it on small aircraft, with no gear reduction (prop RPM  =  engine RPM), but I suppose if all you're concerned about is engine RPM, they still have a linear relationship. A pilot has to play mechanically stupid, and just follow published numbers. Problem is.. I can't DO that..lol

I wondered many years ago, how we got so lucky that GA engine RPM ranges, and GA props  worked out so that it "squared". One of a prop's limiting factors (aside from aerodynamic harmonics), is that you don't want the prop-tips exceeding the speed of sound. I'll spare you the geometry, other than to say that 6 foot prop at 3000RPM has tips hovering around Mach 1.0  . Obviously the massive props on a R4360 would have super-sonic tips at much less than 3000RPM.

ANYway.. it seems that the negative torque thing, is mostly about the gear-reduction. Why there'd be wear-factor in one direction and not the other, is something I'll force myself to stupidly accept. The idea though, that a wind-milling prop can be more stressfull to rods and bearings; than an engine that's trying to keep an airplane airborne does not compute 


Quote:


Be careful letting your brain go down that path. Aside from deliberate feathering, or some sort of 'Beta' setting.. these are still constant-speed props.. meaning you do not control prop-pitch. You select an RPM and the constant-speed prop modulates pitch to maintain that RPM... based on airspeed and engine power.

Brett_Henderson wrote on Jan 12^th, 2010 at 8:57am:



That makes you more than just a pilot, a good pilot.  Any monkey can follow procedures, but by having that understanding of the mechanics and physics of flight, you will be more able to handle abnormal situations.  As I've always heard, and said, a pilot's license is only a license to learn. 

Now back to the original post.  I also don't see why negative torque would be any more harmful to the engine than the same amount of positive torque.  Those engines are putting out over 1000 horsepower to the props, and should be able to handle at least some of that coming back, much like semi trucks using the engine brake.  But I do admit, my knowledge of those large, high power radials is slim.  I do know a couple of warbird pilots around here, perhaps next time I see them I will pose the question.

specter177 wrote on Jan 12^th, 2010 at 4:06pm:


That's the other discussion..  do you keep it squared as you climb ? What's your cruise setting for something like 8,000msl  (and your climb setting(s) getting there) ? How soon after takeoff do you go to 25/25 ?

Full power is relative..    ..  You lose an inch or so of MP  per  1000 ft, anyway. .. so if you plan on cruising much above 5000msl, you'll firewall the power for takeoff, and just leave it there.

I was just wondering if you continuousy lowered RMP as MP went down.. but is sounds like you do it similar to the way I do.. relatively high RPM, even as MP drops, for a climb.

At altitude is where a constant-speed prop starts earing it's keep. You're already dealing with lowered MP .. and with a fixed-pitch prop, you have to reduce MP even further, to keep the RPMs low.. whereas the constant-speed prop allows wide open throttle, with economical RPMs 

One other reason why there are limitations in how fast you can change throttle on turbocharged engines is that most aircraft engines do NOT have water cooled turbos. You kill the engine oil and destroy oil flow when the turbo cools down too fast. The same reason why most tuned Imprezas have turbo timers, that will pump oil and water for a few minutes after shutdown to keep the turbo bearings in good condition.

What also makes these things a bit complicated I think is that most American and European GA pilots don't fly radials that much. For Russian GA pilots these are the only thing they see when getting airborne, so it is quite possible that the whole handling thing is not in the basic 'how to fly' lists for European and US pilots anymore