Constant speed propeller

[Panther91]

I don't understand how to control the constant speed propeller.
-Does the tachometer show the RPM of the propeller or the engine crankshaft?
-Why is this called a constant speed propeller?
-When I was in flight with Cessna 182 which has this kind of propeller, I first set the throttle and propeller control (the blue one) to full. The tachometer was showing a RPM of 2500. I then reduced the throttle to half, but I didn't touched the blue control. The RPM remained 2500 rpm, it didn't change but the aircraft started to slow down. Why? If the RPM is the same why is he slowing down? And why does my aircraft always bump somehow when I set a new propeller (blue control) setting?
-Does this propeller mean that I can save some fuel or not? In the FS2004 manual they said that he allows something like the gears do in a car, that after the plane is on speed I can reduce the power, but which power, throttle or propeller? I don't understand anything regarding to this propeller.

[DaveSims]

There was a good thread in the Real Aviation forum not too long ago about constant speed propellers.  But I will try to answer your questions here.  First, the tach needle indicates the propeller speed and the crankshaft, as they are still bolted together.  It is called a constant speed propeller because the propeller will change its pitch to maintain the rpm you set with the prop control (blue lever).  When you reduce the engine power, the propeller reduces the pitch of the propeller to maintain the rpm.  Reducing the pitch means the prop takes a smaller bite of air, which slows you down.  Normally in flight, once you get to altitude, you maintain wide open throttle, which gives reduced power due to the elevation and air density.  On normally aspirated engines, it is practice to run what is called 23 square or 24 square.  That means for 23 inches of manifold pressure, you run the prop at 2300 rpm.  That gives you the best balance of prop pitch for engine power.  

Clear as mud right?

[DaveSims]

Found the thread, so I now bow down to the wisdom that is Brett.  

http://www.simviation.com/cgi-bin/yabb2 ... 1212879811

[Panther91]

I think I understand how this constant speed propeller works now. But I don't understand something else with it. I know that the rule is that for example an 2500 RPM setting 25 inches of manifold pressure is required. And I know that increasing the throttle first and and not balancing the RPM with it can damage the engine.

It can damage it because the prop will adjust it's angle to remain the set RPM and the engine wants to run faster due to increase of MAP. But the prop doesn't let it run faster. It can damage the engine. It's like applying the brake in a car and releasing the clutch while the car is in gear, right? The brake will acctually turn of the engine probably.

But what when we talk about the opposite? If I first increase the RPM and not balancing the MAP with it, the prop will adjust it's pitch to spin faster and the propeller will force the engine to spin faster altough the MAP is not increased (balanced). Won't this also damage the engine? The rule is 2300 RPM/23 inches of manifold pressure, but that's not what we did when we were landing the Baron in Shortfield takeoff and landing in Flying lessons. The prop was all the way in, and the throttle after the flare was all out. And also when preparing to take off, landing and taxiing? How come this doesn't damage the engine?

[DaveSims]

As long as the prop rpm is set high, you can add throttle.  If the engine is not creating enough energy to turn the prop at the set speed, then the prop just won't reach the set rpm, like when on the ground at low power settings.  What you want to avoid is a high throttle setting with low rpm because it puts too much stress on the engine, crankshaft, and propeller hub trying to stop the propeller from spinning fast.

[Panther91]

This is also confusing. I flown in a Cessna 182 which has a constant speed propeller on 1000 ft. First I set up my RPM to 2300 and MAP to 23 inches. The aircraft accelerated to 115 kts. After it stopped accelerating I reduced the MAP to 20 inches and I didn't touched the prop control. I looked at the tachometer and it still showed 2300 RPM which means that the propeller changed it's pitch now to mantain 2300 RPM, right? But the aircraft slowed down to 105 kts. If the RPM was 2300 RPM all the time, during 23 inches and during 20 inches of manifold pressure, why did the aircraft slow down on 20 inches if the RPM stayed 2300? I was in level flight, the aircraft didn't pitch up or something. This is what confuses me, can seomeone explain this to me?

[Hagar]

This is also confusing. I flown in a Cessna 182 which has a constant speed propeller on 1000 ft. First I set up my RPM to 2300 and MAP to 23 inches. The aircraft accelerated to 115 kts. After it stopped accelerating I reduced the MAP to 20 inches and I didn't touched the prop control. I looked at the tachometer and it still showed 2300 RPM which means that the propeller changed it's pitch now to mantain 2300 RPM, right? But the aircraft slowed down to 105 kts. If the RPM was 2300 RPM all the time, during 23 inches and during 20 inches of manifold pressure, why did the aircraft slow down on 20 inches if the RPM stayed 2300? I was in level flight, the aircraft didn't pitch up or something. This is what confuses me, can seomeone explain this to me?

The CSU reduced the pitch of the prop to maintain the RPM. This would reduce the airspeed.

It's like changing to a lower gear in a car.

[Panther91]

The CSU reduced the pitch of the prop to maintain the RPM. This would reduce the airspeed.

It's like changing to a lower gear in a car.

Now I understand. I thought that the propeller at 2300 RPM in higher pitch and 2300 RPM at lower pitch will give the same airspeed. But the airspeed is the combination of both the RPM and manifold pressure. Propeller with higher pitches are better for cruising and higher speeds, right? It's like a higher gear in a car.

One more thing. About the rule 2300 RPM/23 inches of manifold pressure. I know that if we increase manifold pressure to for example 29 inches and decrease the RPM to 2000 RPM we can damage the engine, right? But what about when the prop control is all the way in, lets say 2000-2300 RPM, and the manifold pressure is about 15-17 inches, like when landing. Can this damage the engine?

Brett_Henderson said in the thread about constant speed propellers (link above) that we can fly with the prop control all the way in and only change the manifold pressure but that can wear the engine more. But what about when landing? Then usually the prop control is all the way in and we only touch the throttle and then there is no rule 1700 RPM/17 inches manifold pressure. The pressure is always much lower than RPM/100. Does this wear out the engine?

[Brett_Henderson]

I love these discussions..

I'll re-state that most of the confusion is because you always have it in the back of your mind that the prop-control is the pilot's way to control blade pitch. Just utterly erase that concept. The prop-control is like a governor control. You use it to to dial in the maximum RPM. Since nearly every stage of the flight has the engine producing enough power to surpass that RPM, the prop is always holding the engine back (governing it).

The more power the engine produces, the more the prop blades increase pitch to govern it. The increased power shows up as thrust.

The throttle has more to say about blade pitch than the prop-control.

The only rule, is to not let the engine produce more power than can be easily bleed off, in the form of thrust. There is no harm for the opposite. Letting the MP fall way below RPMs is more inefficient than harmful.. though obvioulsy using more RPMs than the job calls for, is excess wear along the line of every revolution is one more closer to the last, before something wears out. Too much power for a given RPM is literally, physically abusive.. on combustion chambers, rods, bearings and even the prop-hub..

[Panther91]

OK, now just tell me is it true that propeller spinning with 2300 RPM and high pitch won't give the same airspeed as a propeller spinning at 2300 RPM but a low pitch? The propeller with 2300 RPM and high pitch will give higher airspeed than the one with lower pitch. Is this correct?

[Brett_Henderson]

OK, now just tell me is it true that propeller spinning with 2300 RPM and high pitch won't give the same airspeed as a propeller spinning at 2300 RPM but a low pitch? The propeller with 2300 RPM and high pitch will give higher airspeed than the one with lower pitch. Is this correct?

Correct..

[Brett_Henderson]

And.... if you wanted to impress the person who asked you to increase prop-pitch while maintaining 2300RPM;

[Panther91]

[quote]But you're STILL thinking in terms of prop pitch. Forget that notion. You don't know what it is.. don't care and can't find out. All you know about, or care about ,