Also just a quick question regarding overboosting.

It is my understanding that the reason overboosting occurs is a combination of [b]high manifold pressure[/b] and [b]low RPM[/b] 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 [b][i]too high[/i][/b] and the rpm is [b][i]too low[/i][/b] 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 [i][b]recommended published highest manifold pressure and lowest rpm combination[/b][/i], 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:
[center][attachment=339]Propellers_2.jpg[/attachment][/center]

Effect of RPM and TAS on Propeller Angle of Attack:
[center][attachment=338]Propellers_1.jpg[/attachment][/center]

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.

Hi Basketball,

When the pilot wants to increase the RPM they move the prop lever forward in the cockpit. This then adjusts the tension on the speeder spring. The governor now wants equilibrium at a higher RPM. The old RPM is now considered to be an underspeed condition and the governor corrects accordingly.

[center][attachment=341]Governor.jpg[/attachment][/center]

The new tension in the speeder spring fights against the forces generated by the spinning fly weights at the old RPM. This new speeder spring tension wins out and moves the pilot valve into a position which lets more oil into the hub, forcing the blades to finer pitch. This results in less prop torque and the RPM increases.

Another case is if the pilot does not change the position of the prop lever but the RPM starts to increase because, for example, the pilot has started a descent. In this case the flyweights spin faster and overwhelm the tension of the speeder spring. The pilot valve then moves into position to allow oil to drain out of the hub and back into the sump. The drop in pressure in the hub allows the blade angle to coarsen and the increasing prop torque works against the increasing RPM.

If we increase RPM and then power with a CSU this is what happens:

Step 1) Prop lever set for higher RPM - blade angle decreases to allow for the higher RPM.
Step 2) Power increased - the added engine torque now starts to overwhelm the prop torque generated with the finer pitch so the governor coarsens the blade angle again to compensate.

The eventual TAS you get will depend on the thrust/torque ratio you are getting out of the prop for the RPM airspeed combination you have.

You normally apply finer pitch for take-off, when climbing and as a precaution just before landing. The thrust/torque ratio is closer to optimal with finer blade angles when operating at lower speeds e.g. take-off, climb, landing (in case of go-around).

If you were in cruise at the manufacturer's recommended cruise settings and you applied fine pitch you would get a drop in TAS. This is because at cruise speeds the finer pitch and higher RPM means the prop is no longer working as efficiently as it was before and thrust drops away.

Cheers,

Rich

Thanks for the great explanation Richard 🙂

Just wanted to clarify one thing.

If we changed RPM only and left the power constant, then regardless of if we increase or decrease rpm, the resulting true airpseed would be lower, assuming we were operating at the optimum rpm/power combination before? This is because the propellor is not operating as efficiently for the given rpm/power combo? The blade would still be operating at the best angle of attack, just that the fine pitch/rpm/power combo does not give the best overall efficiency?

Thanks again.

The thing to remember is it is the RPM and the TAS which determine the angle of attack the blade is making with the airflow and there is only one angle of attack which gives the optimum thrust/torque ratio and this occurs at different RPMs for different airspeeds. You can prove that using the diagram 2.6 on page 2.3 in the textbook.

When you set an RPM, you are telling the governor to maintain that RPM. It has no idea if that is an efficient RPM for the TAS or not. That's your job. You are supposed to use the recommended figures provided by the manufacturer.

By the way those figures don't just take into consideration the prop but also the engine efficiency too. They should give you the overall best bang for your buck so to speak.

So, back to your question: if you are flying at the optimal RPM/power setting and you change the RPM setting you will most likely get a drop in TAS. As you said, the propeller is not operating as efficiently as it could because the thrust/torque ratio is now wrong.

The blade would still be operating at the best angle of attack, just that the fine pitch/rpm/power combo does not give the best overall efficiency?

To summarise:

RPM and TAS determine the blade's actual angle of attack.

Best prop performance is determined by blade angle of attack so therefore prop performance is dependant on RPM and TAS.

The governor controls the RPM whereas Power affects the eventual TAS the system stabilises to.

More power increases engine torque and the RPMs will increase. The governor makes the blade angle greater to maintain the RPM set by the prop control lever.

Less power reduces engine torque and the RPMs will decrease. The governor makes the blade angle finer to maintain the RPM set by the prop control lever.

Optimal settings are provided by the manufacturer. These will give expected fuel flows and TAS for various RPM/Power combinations at various altitudes and temperature ranges for various profiles (e.g. maximum cruise power, recommended cruise power).

Cheers,

Rich

Thanks again Richard, Makes sense now! 😀

No worries, glad to help B)