Hey Bob/Rich,

I'm having a little trouble understanding the first section in the propeller section. Especially the stuff regarding the behaviour of the relative airflow about the blade (on the first propellers page).

Have you maybe got an alternate way of explaining it, or a couple more diagrams or anything?

Cheers!

Daniel

Also just a quick question regarding overboosting.

It is my understanding that the reason overboosting occurs is a combination of high manifold pressure and low RPM which causes the gas to be pushed strongly towards the cylinder, which when roating at low RPM, means the inlet & exhaust valves are open for a longer period. Thus allowing too much charge to enter the cylinder.

This is why you increase RPM before manifold pressure and decrease manifold pressure before RPM.

However, its said that the best volumetric efficiency occurs with the very same combination of high manifold pressure and low RPM that causes overboosting...

Doesn't this mean that if you want the best volumetric efficiency out of your engine you're going to overboost it?

That's a good question. It's all a matter of degree. It is true that if the manifold pressure is too high and the rpm is too low overboosting can occur.

However the manufacturer publishes a table of values for manifold pressure and rpm combinations that he knows the engine can handle. As long as you use the recommended published highest manifold pressure and lowest rpm combination, you can be sure you will not overboost the engine.

I suggest that you re-read the book at page 2.2. Maybe if you try drawing the diagram on that page on a blank piece of paper and use a piece of string or wool to allow you to actually watch what happens to angle of attack on the propeller blade as RPM and True Airspeed change it might help.

Ahh, that makes sense! I was like "well, that's not a very good design, if you're going to overboost in trying to make your engine run more efficiently."

I will do that, for sure. I think I'm just having trouble figuring out which way the prop is spinning and I'm still a trad unsure about what the line A-B represents and such...

Thanks! =]

In figure 2.2, the propeller blade is moving down the page. The line AB represents the distance the propeller moves in its own plane of rotation at a given RPM value in a brief period of time. [Imagine the aircraft was not moving forward].

The line BC represents the distance the whole aircraft moves forward at a given TAS in the same brief period. Because both these movements occur simultaneously, the actual path taken by the propeller blade is along the line AC. So the relative airflow is moving up the line CA,

Breaking the propeller's motion up into two separate modes like that, allows you to consider the effect of changing RPM [different lengths for the line AB], and TAS [different lengths for the line BC].

Lines AB and BC:
Effect of RPM and TAS on Propeller Angle of Attack:

Thanks heaps, Bob! That little bit extra explanation was all I needed.

I've got the exam (hopefully, assuming positions are available) Tuesday week. I feel pretty confident seeing as I get high 90% with all the exercises in the book. All I've got left is the last chapter and the final exam questions and then the real thing. Fingers crossed!

Good luck with it mate!!

i was too stumped by the whole line AB, i read it over and over again and couldnt make heads and tails of it, finally i understand it cheers for your valuable help Bob

Hi Madox,

Glad to see the forums are helping other students and not just the ones who originally posted. That's what we hoped would happen here.

By the way, I just corrected those image links in Bob's post. There was something screwy going on there I think.

Cheers,

Rich

Just wondering, if MP is kept constant, and rpm is increased, does the blade angle temporarily adopt a higher angle of attack? This then leads to propellor torque being greater than engine torque, leading to the engine wanting to slow down. The governor senses this and reduces the blade angle? Does this then also result in a fall in TAS?


I was thinking about what temporarily happens to TAS and Blade angle as we increase power and rpm on CSU's, by first increasing RPM, and then MP.

Thanks.