Engine / Constant speed prop operation

[CerroTorre]

So this question actually comes from studying for the Commercial. it just made me scratch my head a little…

Study/exam question and answer as graded correct:

“Which statement best describes the operating principle of a
constant-speed propeller?
A) The propeller control regulates the engine RPM, and in turn, the
propeller RPM.”

Doesn’t the prop control regulate the prop governor and oil pressure which then controls engine RPM as a function of prop pitch? So in other words the exact opposite of the answer graded correct?

Or am I confusing myself?

I realize his is a bit academic but I don’t have my High Performance or Complex yet so I thought I’d clarify. Thanks.

 

[CerroTorre]

Just answering my own question (and since I don’t see a delete button for the post)…

since the flywheels are connected to the engine directly, I guess I can see how the answer above could be graded correctly. Sort of. But it seems more correct to say you’re adjusting oil pressure to the propeller which mandates its change in pitch and RPM … which then slows down (or speeds up) the engine RPM at the same power setting.

All happening basically simultaneously and yes, still mostly academic. And still a little confusing to describe the way the FAA seems to prefer to.

I’ll crawl back into my study cave now. Unless someone has some wisdom to share. =)

 

[mandm]

Kind of a bit tricky, but what I recall and unsure if I’m 100% correct is:

This was a question on the commercial checkride oral portion for me, basically the Prop Governor does the work of adjusting the propeller pitch angle (or oil pressure there) to meet the set RPM by the blue lever. The prop governor is also supposed to make sure you don’t set an improper setting by overriding your selection if necessary.

Also I believe the propeller still receives the same RPM from the engine but the angle of the prop creates drag (it’s blade angle) to slow or speed up the propeller RPMs.

 

[CerroTorre]

Kind of a bit tricky, but what I recall and unsure if I’m 100% correct is:

This was a question on the commercial checkride oral portion for me, basically the Prop Governor does the work of adjusting the propeller pitch angle (or oil pressure there) to meet the set RPM by the blue lever. The prop governor is also supposed to make sure you don’t set an improper setting by overriding your selection if necessary.

Also I believe the propeller still receives the same RPM from the engine but the angle of the prop creates drag (it’s blade angle) to slow or speed up the propeller RPMs.

Thanks mandm. sorry I edited my second post while you were posting. But yeah I think I follow your points. And I think I generally have a fair understanding of it … but then I get a question like the above. =)

It gets sort of strange when you start being very specific about the actual, chronological chain of events when the system is designed as a self regulating control.

 

[edo2000]

The answer is poorly written IMO, but not incorrect. Since the propeller and engine crankshaft are bolted together they always turn at the same rpm.

 

[CerroTorre]

The answer is poorly written IMO, but not incorrect. Since the propeller and engine crankshaft are bolted together they always turn at the same rpm.

=) Not the first time we’ve seen that problem on FAA exams I guess.

Thanks.

 

[midwestpa24]

These types of questions and answers are why I say the FAA written tests are garbage, and why I have no problem with using test preps like Shepard just to get past the test.

 

[Capt. Geoffrey Thorpe]

The answer is poorly written IMO, but not incorrect. Since the propeller and engine crankshaft are bolted together they always turn at the same rpm.

Think geared engines.
Since you push the knob to get a desired engine RPM, in theory, the governor is controlling the engine.
In practice, ain't no stinken difference. Geared or not.
POA gots nothing on the FAA when it comes to picking nits.

 

[Rgbeard]

The answer is poorly written IMO, but not incorrect. Since the propeller and engine crankshaft are bolted together they always turn at the same rpm.

But, as was hinted above, the propeller’s not always bolted to the crankshaft. But in everything you have flown, it probably is.