Constant Speed Propellers

[barit1]

The old wives wisdom of always touching the propeller before moving manifold pressure is a ridiculous idea which is taught in ignorance...it has no valid reason for being taught, and is a false concept.

I'm sure there are engines (probably smaller ones) that will tolerate max throttle at low RPM at sea level - but not the big radials, especially those with older carburetors. Ask the oldtimer engine guys what they think of pilots that abuse the hardware this way.

[SNS3Guppy]

I'm sure there are engines (probably smaller ones) that will tolerate max throttle at low RPM at sea level - but not the big radials, especially those with older carburetors. Ask the oldtimer engine guys what they think of pilots that abuse the hardware this way.

Why don't you ask me...I'm one of those that spent years flying large radial engines...and here's news for you...none of them came with, or come with newer carburetors.

The carburetor has nothing to do with the manifold pressure setting, nor the RPM, save for carb air temperatures (icing in particular). I'm not sure what you think you're saying here, but you're saying it wrong.

I proposed no abuse of the engine. I quite specifically stated that as long as the engine isn't operated outside the published parameters for that particular engine, and is kept within the power tables published for it, then it doesn't matter a whit whether you move propeller RPM nor manifold pressure (throttle) first or last. It never has made a difference.

What you have (and you appear to be a product of) is bad information being passed from inexperienced and uneducated instructor to student, who then becomes an instructor and passes it along because that's what he or she was taught...a heritage of inexperience and ignorance. The concept that the propeller must always be pushed up in advance of the throttle is an idiotic and ignorant concept which has no grounding in reality nor basis of fact.

If an engine may be run at 2500 RPM at 32 inches, and it's being operated at 28 inches at 2500 RPM, then you may open the throttle without moving the RPM at all. Will you see an RPM increase? No. Will you hurt the engine? No. Do you need to move the RPM first? No. Is there any earthly need to do so, other than ignorant tradition? NO.

Further, if that engine may be operated at 32" at 2400 RPM, and you're presently operating at 28" and 2500 RPM, then you may retard the RPM all by it's little lonesome, and not bother with touching the throttle. Is there any need to retard the throttle first? No. Will it hurt the engine? No. Is there any earthly reason to move the throttle first, other than ignorant tradition? NO.

I said nothing of carburetors, "max throttle," nor sea level, nor the small vs. large engines...you've read into my comments something that was never there, then argued an arguement which is both non-existant, and wrong. But never mind that. What you do need is an education on your powerplant, because clearly you don't understand it.

You do understand what manifold pressure is, correct? You understand why your manifold pressure drops when you close the throttle? You understand that your engine is an air pump, and that closing the throttle sucks the manifold pressure down? You understand that by opening the throttle you're doing nothing more than removing a restriction from the air induction, and allowing the manifold pressure to rise every bit like blocking a vacum cleaner hose and slowly removing your hand from the opening to the hose? You understand that your manifold pressure rises all by itself to barometric when the engine is shut down...and miraculously doesn't hurt a thing...even though nothing is done to increase the speed of the propeller?

Do you understand why manifold pressure drops when RPM is increased? For a given throttle setting, increasing RPM increases engine speed, resulting in more suction from the air pump...the engine, which draws down the manifold pressure slightly. Do you understand why manifold pressure increases when RPM is decreased? As engine speed is decreased by decreasing propeller RPM, the engine produces less suction, and manifold pressure rises. Do you suppose that some how you're increasing the pressure in the engine or harming the carburetor by increasing this manifold pressure? If so, you suppose incorrectly. Do you suppose the engine is somehow being abused when the manifold pressure rises? You suppose incorrectly.

Nothing I described was in any way abusive to an engine. If you believe so, point out exactly what it is, and why, and we can educate you as to how this is not so. Aviation is full of myths. This is one of them. It's been passed along to you by those who didn't understand the concept, but just taught what they'd been taught...much like ground effect being a cushion of air beneath the wing.

What part of the concept do you not understand?

[411A]

Gotta agree with SNS3Guppy, and I too have operated big radials....from the R4360 on the Stratocruiser, to the R2800 on the DC-6B, and even the 1649 Constellation, with it's complicated turbocompound CurtisWright engines.

Big radials need care and attention to detail in their operation, and positively do NOT like rapid/abrupt throttle movements, but otherwise are quite rugged.
In addition, some of the geared engines on GA airplanes have quite specific instructions in their operating procedures regarding rapid/abrupt throttle movement, and two types, the TCM GTSIO series as well as the Lycoming IGSO series...have service bulletins advising against advancing the propellor control to high RPM just prior to touchdown.

[WHBM]

Manifold pressure :

Old "high-performance" sports cars in the 1930s had a manifold pressure gauge. I suspect few knew what part it should play in handling the throttle/gears, but they then disappeared in more recent times. I wonder why.

Actually my current car still has a manifold pressure probe, it is just not displayed but used by the automatic gearbox and all its electronic wizardry to determine when to change gear. If you think about it, the concept is somewhat similar to a constant-speed prop, except in five steps rather than truly constant.

And before you ask, yes I CAN do a manual gearbox on a car

[barit1]

Brian Abraham in the "Ring Chatter" thread refers us to some historical lessons.

[lugs4744]

i know this an old thread but i'm currently studying this topic and stumbled across it.My understanding of the subjest is when increasing power you advance the rpm lever first then the throttle for the following reasons, the advancing of the RPM lever will reduce manifold pressure, then opening the throttle will increase it again this keeps the manifold pressure down, if you advance the throttle first you are increasing the manifold pressure straight away and this could exceed the critical pressure of the mixture leading to detonation. Likewise when decreasing power it is advisable to close the throttle first(decreasing manifold pressure) then retard rpm (increasing manifold pressure) otherwise you are again running the risk of increasing manifold pressure to much in the first instance resulting in possible detonation.If i am way off the mark please help me out as I am pretty confident i have this understood(maybe misguided confidence i admit)but some posts on this thread point to this as poor information.