Constant Speed Prop.

[Sidious]

I thought the whole purpose of decreasing the RPM and increasing the pitch at cruise speed was to achieve the most efficiency of the propeller for a given forward velocity BUT the effect of decreasing RPM seems like it would cancel out the effect of an increase in pitch....

When we are talking about the Rotational Velocity, Forward Velocity and Resultant Velocity it seems to me it becomes counter intuitive when we decrease the RPM on a constant speed prop when we get to our cruise speed.


After climbing we usually decrease the RPM to about 2400 in an Arrow at training cruise. By decreasing the RPM aren't we decreasing the Rotational Velocity as well? By decreasing the Rotational Velocity we are also giving the forward velocity the ability to decrease the angle of attack. In my mind this cancels out the effect of increasing the pitch.

Here are some rough pictures of what I am talking about. In picture 1 we have a section of a blade with a given forward velocity (FV) at cruise.

We then increase the pitch by decreasing the RPM and I believe we will then have the same AOA because of the decrease in rotational velocity.

Anyone that has read this far, thank you and if you have any comments it would be appreciated.

 

[BrewMaster]

You're thinking too much.....

You decrease RPM which increases pitch, which gives you better fuel burn and greater angle of attack, which alltogether gives you better cruise performance.

Less fuel, longer engine life, greater angle of attack=better cruise performance.

The amount you decrease is actually not that much if you think about it, and the RPM is more than made up for with the increase in blade angle. Just listen to how the plane sounds when you decrease that RPM, it isn't about to shut off. You are just helping it to increase the blade angle and resultant thrust.

I've never had to get that deep into it, and you probably don't need to. I may not be 100% correct on my wording, but the idea is there.

 

[dc3flyer]

Not sure what you are looking for? It has been a while since I have thought about this, but here is how I think of it....

When you decrease the RPM, you are increasing pitch. This causes the prop to take a bigger "bite" of air, so it is pushing more air with less RPM's. So, theoretically, you don't have to run the engine as hard to achieve the same airspeed.

 

[BrewMaster]

The "bite" is what I was looking for.

Decrease RPM, increases pitch which takes a bigger "bite" of air. Which makes you go faster, and drink less fuel. (that is how I explain it to non-pilots)

Leave out all that rotational velocity stuff, it will just confuse you. You will probably never be tested over it anyway. They are more concerned about how the prop dome, governor, etc. works. Just understand how it affects flight in a nutshell.

 

[tgrayson]

We then increase the pitch by decreasing the RPM and I believe we will then have the same AOA because of the decrease in rotational velocity.

You have the cause/effect backwards.

The RPM decreases....due to...the increase in AOA. We know that the resulting decrease in rotational velocity and its effect on AOA can't exactly compensate for the AOA increase, because, if it did, the propeller would never have slowed in the first place. In other words, RPM would be neutrally stable, since there would be no tendency for it to return to or even establish an equilibrium condition.

Clearly, the AOA changes produced by the decreased rotational velocity are much smaller than that provided by the governor. A few back-of-the-envelope calculations could demonstrate that.

 

[Sidious]

Clearly, the AOA changes produced by the decreased rotational velocity are much smaller than that provided by the governor. A few back-of-the-envelope calculations could demonstrate that.

Okay, that is what I was trying to determine, if the effects of the reduced rotational velocity had enough of an change on the overall scheme to drastically alter the AOA.

What do you mean by back-of-the-envelope?

 

[tgrayson]

What do you mean by back-of-the-envelope?

Rough calculations. Figure out what the change in velocity of the prop tip is 2700 vs 2400 rpm and compare that figure to the forward airspeed. A little trig will tell you what the change in AOA is.

Let's see, 6 ft prop means that the propeller arc is pi * 6 = 19 ft. And the prop tips covers that distance in either 2700 or 2400 RPM. Converting all that to ft/s gives 954 ft/s for 2700 RPM and 848 ft/s for 2400 RPM. A cruise speed of 140 knots is only 236 ft/s.

Using tan(theta) = 236/954 vs tan(theta) 236/848 is 15.5 degrees vs 13.89 degrees. So the relative wind changes angle 1.6 degrees. I can't say exactly how many degrees the prop pitch changes to slow to 2400 RPM, but I think you'll agree that it's intuitively more than 1.6 degrees.

Check my math.

 

[dc3flyer]

Rough calculations. Figure out what the change in velocity of the prop tip is 2700 vs 2400 rpm and compare that figure to the forward airspeed. A little trig will tell you what the change in AOA is.

Let's see, 6 ft prop means that the propeller arc is pi * 6 = 19 ft. And the prop tips covers that distance in either 2700 or 2400 RPM. Converting all that to ft/s gives 954 ft/s for 2700 RPM and 848 ft/s for 2400 RPM. A cruise speed of 140 knots is only 236 ft/s.

Using tan(theta) = 236/954 vs tan(theta) 236/848 is 15.5 degrees vs 13.89 degrees. So the relative wind changes angle 1.6 degrees. I can't say exactly how many degrees the prop pitch changes to slow to 2400 RPM, but I think you'll agree that it's intuitively more than 1.6 degrees.

Check my math.

WAY TOO MUCH TIME ON YOUR HANDS!!!!!!!!!!!!:banghead:


Great explanation though!

 

[Sidious]

Rough calculations. Figure out what the change in velocity of the prop tip is 2700 vs 2400 rpm and compare that figure to the forward airspeed. A little trig will tell you what the change in AOA is.

Check my math.

Okay great! Thats exactly what I was looking for and your math is dead on. Thanks everyone

 

[BrewMaster]

WAY TOO MUCH TIME ON YOUR HANDS!!!!!!!!!!!!:banghead:


Great explanation though!

:yeahthat:........Kudos for learning that stuff, it sounds like you are working on your commercial, I would suggest learning oil movement, drag, aerodynamics, fuel system, hydraulic system, read all the FAR's applicable to Commercial pilot (if that is what you are doing) reading through emergency procedures and, understand airspace and sectionals, learn the new NOTAM contractions, performance tables, and W&B!!! as opposed to rotational velocity, and the like. It will take you a lot further down the road.

Just a little friendly advice!! Happy flying!!

 

[Sidious]

Just a little friendly advice!! Happy flying!!

Thanks, I appreciate it. Im working on my CFI right now and I figure it can't hurt to go a little more in depth than what I was taught. Plus this stuff is really fascinating to me.

 

[sundog]

it's really not much different that shifting gears in your car.

 

[BrewMaster]

Thanks, I appreciate it. Im working on my CFI right now and I figure it can't hurt to go a little more in depth than what I was taught. Plus this stuff is really fascinating to me.

more power to you, good luck!!:rawk: