cycling the prop
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Maurus
The Great Gazoo
This is a simplistic way of thinking about it. Manifold pressure measures "suction".
The piston fights against the throttle as the throttle's job is the allow (open) or restrict (close) the path the air needs to take to get to the cylinder. Close the throttle and the MP goes down because air can not enter the system fast enough to compensate for the increase in volume caused by the piston's intake stroke. In a fixed pitch prop system the engine would slow down as a result. Just like a syringe. Cover the end of the syringe and the plunger becomes harder to pull out.
Now there is a trade off between the prop and the MP. The engine needs to overcome some aeronautical effects such as drag on the prop. This takes away some of the engine RPM as well and this is where a constant speed prop comes in.
A prop is just like a wing. Decrease the AoA and you get less induced drag. Increase the AoA and you get a high amount of induced drag. The constant speed prop system allows the engine to run at the set RPM even if you reduce MP. When you reduce MP the prop gov. goes into an under-speed state because the engine wants to slow down do to the lower pressure in the manifold (think back to the syringe). Because the prop gov. is in an under-speed state it will cause the prop to go into a lower pitch setting to reduce the prop's drag load on the engine which will allow to engine to maintain the RPM despite the reduction in MP.
As far as the MP rising during your prop test. Say your run up calls for you to increase to throttle till the engine is running at 2000RPM. At this point the prop is still at its low pitch mechanical stops and the MP is still low. When you bring the prop handle back far enough, the prop will go to a higher pitch and slow down the engine RPM. As a result of the lower RPM there is more time between each piston intake stroke for more air molecules to enter the manifold system which will cause an increase in pressure within the system. The increase you will see will be roughly 1 inch of MP.
I know this may be confusing, but I am not as good at writing than I am drawing pictures and explaining the systems in person.
The piston fights against the throttle as the throttle's job is the allow (open) or restrict (close) the path the air needs to take to get to the cylinder. Close the throttle and the MP goes down because air can not enter the system fast enough to compensate for the increase in volume caused by the piston's intake stroke. In a fixed pitch prop system the engine would slow down as a result. Just like a syringe. Cover the end of the syringe and the plunger becomes harder to pull out.
Now there is a trade off between the prop and the MP. The engine needs to overcome some aeronautical effects such as drag on the prop. This takes away some of the engine RPM as well and this is where a constant speed prop comes in.
A prop is just like a wing. Decrease the AoA and you get less induced drag. Increase the AoA and you get a high amount of induced drag. The constant speed prop system allows the engine to run at the set RPM even if you reduce MP. When you reduce MP the prop gov. goes into an under-speed state because the engine wants to slow down do to the lower pressure in the manifold (think back to the syringe). Because the prop gov. is in an under-speed state it will cause the prop to go into a lower pitch setting to reduce the prop's drag load on the engine which will allow to engine to maintain the RPM despite the reduction in MP.
As far as the MP rising during your prop test. Say your run up calls for you to increase to throttle till the engine is running at 2000RPM. At this point the prop is still at its low pitch mechanical stops and the MP is still low. When you bring the prop handle back far enough, the prop will go to a higher pitch and slow down the engine RPM. As a result of the lower RPM there is more time between each piston intake stroke for more air molecules to enter the manifold system which will cause an increase in pressure within the system. The increase you will see will be roughly 1 inch of MP.
I know this may be confusing, but I am not as good at writing than I am drawing pictures and explaining the systems in person.
This article will tell yo why and a lot more about MP
http://www.avweb.com/news/pelican/182081-1.html
You have got to love the title
That's it right there. That one sentence clears it up for me. Good explanation. Thank you.
gotWXdagain
Polished Member
What's been said above is the explanation I usually go with, but also, Bernoulli can help us out here (dang that guy, he keeps popping up everywhere in aviation!). Think of an engine running at a high RPM, its moving alot of air through it. You decrease the RPMs, less air gets pulled through the system... you're slowing down the air velocity, as velocity decreases, pressure increases.
z987k
Well-Known Member
Inside the engine.....
Read it. Great article. Here is something else I've often wondered. In the 172RG I'm flying, the checklist for engine starting states that the throttle should be pumped once or twice and then left open 1/4 inch for engine start. What I don't understand is: what is pumping the throttle actually accomplishing? If the engine is not yet turning and there is zero ram air pressure going through the induction system, no fuel is going to be sucked into the venturi, correct? So what does pumping the throttle accomplish? My only guess would be that it allows air to enter the intake manifold. But then again, the throttle plate never actually closes all the way, so with the engine not running, the intake manifold should already be at ambient air pressure, correct? So again, I'm without an explanantion for the throttle pumping?
z987k
Well-Known Member
accelerator pump
Sorry. Not following. Can you elaborate?
z987k
Well-Known Member
that makes sense. thanks
Oh really? so how come as you increase the throttle from 1000rpm to redline RPM the MP gauge indicates the highest pressure when the air is travelling the fastest trough the manifold?. According to your theory it should be reading "low".
Bernoulli is not the answer for this one.
MP gage is a suction gauge. 0 MP means Vacuum between the throttle plate and the intake valves. When the engine is stopped its reading no suction (the ambient pressure).
Bernoulli theory applies for a fixed mass flow going trough a sort of constriction. Think of this how come our static port does not read "LOW PRESSURE" --- HIGH ALTITUDE on the altimeter as the airplane accelerates faster and faster on level flight?. Air is moving faster relative to the aircraft yet there is no static pressure drop. If you were to place the static port on a wing shaped part (that would act as a constriction) it would read lower pressure as you increase the airspeed.
z987k
Well-Known Member
:yeahthat:
Wx, Read the article LV posted, it will help clear that up.
Also, the only place Bernoulli applies in aviation is in the Carburetor.... unless some system is escaping me at the moment.
gotWXdagain
Polished Member
I understand what you're saying here, and I do not disagree with the premise that the greater time between strokes = more air molecules = raised MP, though I contend that Bernoulli still applies, as was explained to me by a +1000 hour MEI when I was going through my MEI course.
Bernoulli's principle states: As the velocity of a fluid increases, the pressure exherted by that fluid decreases. The principle itself mentions nothing of a pipe. The amount of mass moving through the system doesn't change, but its velocity slows down, therefore the pressure has to increase, this assuming the throttle remains in the same position while the rpm is changed. You point out that a throttle revved up from 1,000 RPM to Redline, according to my "theory," should cause the MP to read zero, which is entirely untrue, because changing the throttle at a constant RPM will cause a change in the total mass allowed to pass through the system, therefore opening the throttle from 1,000 rpm to red line will result in an increase in total mass allowed to enter the system, resulting in a decreased suction, an increase in MP.
Maurus
The Great Gazoo
Just because the guy has over 1000 hours teaching as an MEI means he knows everything.
The scientific rules you use are incorrect.
Keep it simple and use Boyle's law. An increase in volume will cause a decrease in pressure. The speed of the air coming into the system is insignificant and really only increases MP as a result of Ram air pushing more molecules into the system.
Once you realize that the manifold system is just a constantly changing volume caused by the intake stroke of the piston, Bernoulli's principle will go out the tailpipe.
The scientific rules you use are incorrect.
Keep it simple and use Boyle's law. An increase in volume will cause a decrease in pressure. The speed of the air coming into the system is insignificant and really only increases MP as a result of Ram air pushing more molecules into the system.
Once you realize that the manifold system is just a constantly changing volume caused by the intake stroke of the piston, Bernoulli's principle will go out the tailpipe.
Thats my point. Bringing back the RPM is also changing the speed of the fluid as well as the total amount of mass that goes by.
Imagine having 100 air molecules going by at one point in the manifold per second. You increase the RPM, now 200 air molecules are going by per second at twice the speed they did before. That does not cause a decrease in static pressure because you changed the mass flow.
Now on the same 100 molecules per second manifold place a venturi tube and you will get the same 100 air molecules going by per second (trough the venturi) at a greater speed but trough a smaller section. The mass flow remains the same but the speed increases and that causes a decrease in static pressure. It could be wrong, but this is how I always though Bernoulli's principle applied.
I apologise if I'm wrong about this one. I'm here to keep learning.
gotWXdagain
Polished Member
Ok, lets look at it from an airfoil standpoint. If we have a wing moving through the air at 100 kias, there are X air molecules passing over the wings surfaces per second- we'll call this a mass flow of X kg/s. At X kg/s, there is a suction force exerted on the upper wing surface to the tune of, well say, S psi, because the static pressure is less than ambient static pressure (which we can assume given Bernoulli's principle), which is, we'll say, Apsi. We'll say the static pressure above the wing surface is (A-S)psi, or Ambient minus Suction, measured in psi. Now, we're going to decrease our airspeed to 70 KIAS. At 70 KIAS, there are Y fewer air molecules pass over the wing surfaces per second, so mass flow now equals (X-Y) kg/s. Apsi is the same, its the ambient air pressure. The suction force, because of bernoulli's principle, must decrease, so we'll say our suction force is now (S-T)psi, where T is the amount of suction decrease. The static pressure force acting above the wing surface has now increased to {A-(S-T)}psi, the resultant increase in static pressure causes a net loss of lift.
Now, we'll take it back to the RPM scenario. Our mass flow, X kg/s, is moving through the induction system at a velocity Vm/s when RPM is, we'll say, 2700 (We'll use this scenario: The moment after takeoff in a seminole when transitioning to cruise climb, the throttle has already been retarded to the cruise climb setting, the prop hasn't yet). At X kg/s, the MP gauge is indicating, we'll say, 24". Now, we will decrease the RPM to 2500. The airflow moving through the system must slow down, because there's less intake strokes happening per second than before, air is being drawn in at a slower rate, so our new fluid velocity will be (V-W)m/s. This decreased fluid velocity will contribute to a decrease in overall mass flow, which will now be (X-Y) kg/s. The same thing is happening here as is happening when you slow a wing down, and that is that as the velocity slows down, some of the suction force goes away, as will be evidenced by the manifold pressure gauge...if there is less suction, the manifold pressure will be closer to ambient, and in this case, should read about 25 inches now.
Maurus
The Great Gazoo
All the manifold system is is a syringe with an open tip. How open the tip is depends on the throttle. Increase the volume faster than the molecules can enter the newly created space and you get a pressure drop. Bernoulli has nothing to do with MP, it is all about Boyle's law. Bernoulli's principle doesn't involve a change in volume. Notice how all the equations don't include volume? The way you explain what happens in the engine simply does not occur. This "suction force" is just an increase in volume cause by the intake stroke of the piston. As volume increases pressure decreases. That is Boyle's law.
One note, when you start with "my CFI said" it typically shows you are running off primacy and not what is necessarily the truth. It doesn't matter how much time or experience the guy has, he will not be right 100% of the time. I am a CFI of over 1000 hours CFI, does this mean I have the same experience as your MEI or because he typically flies with two engines means he gets double credit?
Read books that explain these systems. I have. One example is Flying High Performance Singles and Twins by John C. Eckalbar.
I know you may not fly HP aircraft, but the engines run with the same principles and this guy explains the systems very well.
Also read that link above from avweb. The guy is spot on with the manifold system and has been instructing even more than your 1000 hr MEI.
One note, when you start with "my CFI said" it typically shows you are running off primacy and not what is necessarily the truth. It doesn't matter how much time or experience the guy has, he will not be right 100% of the time. I am a CFI of over 1000 hours CFI, does this mean I have the same experience as your MEI or because he typically flies with two engines means he gets double credit?
Read books that explain these systems. I have. One example is Flying High Performance Singles and Twins by John C. Eckalbar.
I know you may not fly HP aircraft, but the engines run with the same principles and this guy explains the systems very well.
Also read that link above from avweb. The guy is spot on with the manifold system and has been instructing even more than your 1000 hr MEI.