Simviation Forums

I understand the rule of thumb regarding "flying by the squares".

You're really thinking this stuff through

The shift-2 page provides takeoff, METO, and what appears to be maximum climb.

My rule of thumb, with a constant speed prop, is keep the RPM at or more than the inches.  This is for your common opposed cylinder engine - other engine types require different settings.

Yeah..

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.

I understand what you are saying too about not stressing the engine with TOO MUCH manifold pressure for a given RPM and as such I always adjust RPM/MP in the correct order.

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.

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.

I understand what you are saying too about not stressing the engine with TOO MUCH manifold pressure for a given RPM and as such I always adjust RPM/MP in the correct order.

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

"The Strat was a peculiar beast. Its pilots never knew for certain what might happen next, but we never had to ditch one, as Pan-Am had to do on two occasions. An engine once caught fire, burnt itself out, then conveniently fell off, and there were a few landing accidents (not too surprisingly). That great first gentleman of the air, Capt O.P. Jones (we were on the same Strat course) later landed one short at Goose Bay, fortunately without hurting anyone. Next day he went out to examine where his wheels' marks were. He resigned and never flew for BOAC again. He had done enough for his airline and country."

An update to this old thread:

The answer is-- DO NOT let the torque go negative.

[quote]An update to this old thread:

The answer is-- DO NOT let the torque go negative.

Interesting..  I don't doubt the advice, but I can't see where negative torque would be a bad thing.. except for some sort of aerodynamic thing on approach.. or maybe the gear-reduction doesn't like it  :-?

This will make for good research 

[quote]Interesting..

This is from a real C-97 (boldface my emphasis):

[i]

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 

I guess if you really need to bleed off that speed, you could obviously just increase the prop pitch

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.