Manifold Pressure Question

[DC8 Flyer]

OK... it took a long time to read through this thread over a couple days due to a lot of ahhh...let call it disagreement. But God help us...lets get back to the original question and better yet lets expand on it a wee bit. (If Av-bug and DC-8 could both provide a short simple explanation...and hopefully they'll be in agreement)...I'll copy and paste it into my CFI notes.

#1. During a run-up, why does manifold pressure increases when RPM is dropped...
and (let see if yall agree )

When you do a run-up, you typically dont have the throttle at full power, so the throttle plate is restricting the airflow. So as the ambient pressure is trying to push air into the manifold as the piston goes down it can't get the entire cylinder volume past the throttle plate and you end up with a lower MP reading.

Now when you pull the prop lever back for a prop check you slow the piston down and the ambient pressure has more "time" to push air into the manifold and the MP reading rises.

#2 Why at cruise does manifold pressure rise when you increase the throttle but don't touch the prop lever and...

Same reason as above as to why you have a lower than ambient MP reading while doing a run-up. When you open the throttle you reduce the restriction caused by the throttle plate and ambient pressure can push into the manifold more easily and pressure goes up. Notice that (in non turboed) engines MP is never higher than ambient other wise the air would be flowing out of the manifold. Also try it with the throttle full open and then pull the props back 200 or 300 RPM you wont see any rise in MP, save a small amount that is just recovery from the other restriction in the manifold (filter, friction, bends, etc.)

#3. At crusie power (not low power like in a run-up)... if you reduce rpm by pulling back on the prop lever does manifold pressure still rise?

Yes, as long as the throttle is not full open. If the throttle is full open you are already getting as much "air pressure" from ambient pressure as you can into the manifold.

#4. And I can't help it but I'd like to know if you agree on why during low rpms on the ground does the throttle control RPM and manifold pressure. With props full forward we are able to add the throttle and increase the RPM to approx 17-18 hundred RPM... but at higher throttle settings RPM is controlled by the prop / governor.

Again the governor has high and low pitch stops. So with the prop lever full forward the governor is trying to maintain 2700 RPM (depending on a/c model). When the prop hits the low pitch stops and you keep pulling the throttle back, restricting airflow, reducing fuel flow and thus reducing power, the airload on the prop starts to slow the engine down since the blade can't "flatten out" anymore.

Just the opposite for why you dont go over 2700 RPM (usually) with full throttle, the high pitch stops of the prop are set to maintain 2700 RPM at full throttle at sea level on a standard day plus or minus some fudge
factor.
Chec the above statement, now I cant remember how it is actually set up, since I recall on more than one occaision takeing off with an OAT of negative 20 and Altimeter of 30.??. So that doesnt quiet hold water.

If you could theoretically turbocharge that engine you could get the prop over 2700 RPM because the high pitch stops would not let the prop twist anymore to a higher angle of attack and with the increase in air mass into the engine, it would have enough power to "overspeed" the prop.

Don't confuse that with the slight overspeed(s) you get when you go from low power settings to high power settings, or low RPM settings to high RPM settings, rapidly in flight. That is just the governonr not being able to increase blade angle quick enough to maintain the "set" RPM.

MUCOS GRACIAS

Gern pretty much answered it all, I'll just add mine so you have more than one way to describe it.

Just to clarify, my "strong" stance on this came from the fact I had actually done an experiment in college to prove my theory. We took an engine that was used as a display model and hooked an electric motor to the crankshaft and varied the voltage to the motor to vary the speed. We sealed up the cylinder with the MP gauge on it and getting about 400 to 600 RPM out of the motor we could show the rise and fall of MP with engine speed without touching the throttle plate. The problem was, and no one else picked up on it either, is that with the changing mass of air due to throttle plate restriction, that through the whole velocity pressure rise and fall theory out the window, since bernoullis law depends on constant mass and constant energy. All it is, is an equation to show the conservation of energy laws, but since the mass was being changed that equation doesnt work.

Now you could hook up a MP gauge in front of the throttle plate and you would get small changes in MP due to velocity but not nearly the changes you see downstream of the throttle plate. A two inch increase in MP equates to descending 2000 feet, why that obvious error never donned on me. I dont know, but if all you have to do to get more power out of your engine is to slow the prop down and voila you got 2000 feet of altitude back, we would all be running around at 900 RPM, just doesnt add up, and why it took me 6 years to figure it out, well Im just blonde I guess.

 

[VNugget]

The problem was, and no one else picked up on it either, is that with the changing mass of air due to throttle plate restriction, that through the whole velocity pressure rise and fall theory out the window, since bernoullis law depends on constant mass and constant energy. All it is, is an equation to show the conservation of energy laws, but since the mass was being changed that equation doesnt work.

That, plus Bernoulli's law relates fluid velocity to static pressure, while the MP we deal with is total pressure (or absolute pressure, or stagnation pressure, whatever you wanna call it), which also stays constant

 

[avbug]

I may be wrong, but I could have sworn that I saw a B-17 and a B-24 wiht PRTs. Thunderbird and Dimond Lil to be precise.

The B-29 and Connie definately did.

The R-1820 and R-1830 didn't use or have in the design a power recovery turbine. Each did use a supercharger with a two stage blower, and a turbocharger was used as part of the exhaust system.

The B-29 used PRT's for some installations, but not all.

Technically "Diamond Lil" isn't a B-24, but a converted LB-30.

 

[Donsa320]

I may be wrong, but I could have sworn that I saw a B-17 and a B-24 wiht PRTs. Thunderbird and Dimond Lil to be precise.

The B-29 and Connie definately did.

Hey, I'll check with Randy Sohn later today about the B-29. He is the guy that brought "Fi-Fi" out of China Lake originally and is the final authority on B-29s. The Constellations started out with R-2600s in the smallest model, the C-69 and I think 049s. As they stretched the airplane to the 749, 1049 and 1649 they went to the 3350s and finally the 3350 turbo-compond in the biggest Constellations.

The B-24 and B-17 surely did not have PRTs. Are you sure you are not confusing them with the turbo-superchargers. They both had them. PRTs are not even visible unless the cowling is off, they are very integral with the engine crankcase at the accessory section. Your remark that they are removed or wired off is consistent with the turbosuperchargers. "Fi-Fi" still has them but they are not normally used. They keep the grossweight low and avoid maintenance.

Later
~DC

~DC

 

[USMCmech]

The B-24 and B-17 surely did not have PRTs. Are you sure you are not confusing them with the turbo-superchargers. They both had them. PRTs are not even visible unless the cowling is off, they are very integral with the engine crankcase at the accessory section. Your remark that they are removed or wired off is consistent with the turbosuperchargers. "Fi-Fi" still has them but they are not normally used. They keep the grossweight low and avoid maintenance.

Ah, I was mistaken. I saw the turbo on the bottom of the cowling and thought that was a PRT.

You learn something new every day.

 

[Donsa320]

This from Randy Sohn re; PRTs on a B-29


no, never, they jsu tahd the plain ole 3350 with eitehr carbs and later fuel inecjections, latest figure for ovrhaul a 3350 is now 150M
best, randy

Some times Randy types like Stephen Hawkins speaks but I believe him.
~DC

 

[erj-145mech]

I may be wrong, but I could have sworn that I saw a B-17 and a B-24 wiht PRTs. Thunderbird and Dimond Lil to be precise.

The B-29 and Connie definately did.

I've worked on Thunderbird, N900RW, for five years, and I can assure you that the R-1820-97 does not have power recovery turbines. They are Aireasearch turbo chargers, and they are functional. The Connie did not, but the Super Connie did.

 

[avbug]

The R-2600 never had provision nor design for a PRT, either.