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Manifold pressure for FP

LettersFromFlyoverCountry

LettersFromFlyoverCountry

Well Known Member
It's interesting to be tooling along and everything in the plane is fat and happy and starting thinking, "I wonder....."

Today's flight "I wonder" question:

"I wonder what the value is of the manifold pressure indication is for a fixed pitch propeller? What is the significance of, say 22" at 2400 vs., say, 24" at 2400?
 
LettersFromFlyoverCountry said:
. . . What is the significance of, say 22" at 2400 vs., say, 24" at 2400?
Click to expand...
Significance is less power (i.e. less torque to spin the crankshaft/prop) which equates to less pull through the air, which equates to less speed in cruise or less climb capability or, well, less anything that requires the ability to pull or push (if the case may be) the aircraft through the air.
 
I installed a MP gauge in my O-360 FP RV-4. I used it for the 48 rule of thumb. That is, if the MP in inches and the RPM in thousands totals 48 or less than the engine is running at no more than 75% power, therefore leaning can be safely performed.

Or so I am told....
 
I remember when I was working on a certification project with the FAA. They wanted a backup manifold pressure indicator along with the electronic one.
At some point, one of the FAA engineers, exasperated by his fellow regulators... said: " if you can hear engine noise.... it is still running!!"
Having said that, most of us have MP on a readout somewhere. But, we all flew FP for many years in trainers without one. You really won't miss it at all I suspect.
 
So I assuming 22 at 2400 isn't as efficient as 24 at 2400, what is the process to increase to 24?

If the prop is swinging through the air at 2400 in either case, does it matter? It must be swinging through at the same speed, right?
 
I've always used MP to adjust power. MP is somewhat instantaneous while it takes several seconds for rpm to settle.
 
I've played with the idea the, hey its just a barometer, so I should be able to use it to set my altimeter, before starting the engine. However, mine doesn't have the resolution to be very accurate.

I have also heard it can be an indicator of carb icing, I believe it will start falling slightly if ice builds up.
 
LettersFromFlyoverCountry said:
So I assuming 22 at 2400 isn't as efficient as 24 at 2400, what is the process to increase to 24?

If the prop is swinging through the air at 2400 in either case, does it matter? It must be swinging through at the same speed, right?
Click to expand...
This question depends on what your definition of efficient is. If you are referring to efficiency of burning fuel, 22 inches of MP will burn less fuel than 24 inches, due to the decrease in power produced by the engine which will take less fuel to produce. However, in terms of efficient movement of the airplane through the air, 24 inches of MP is going to move the airplane more efficiently than 22 inches of MP will. If your definition of efficiency is related to the speed of the aircraft through the air then more MP will be more efficient than less MP.

As you can see, the idea of efficient is a very relative term.

As for your second question concerning swinging the prop at 2400 RPM and getting 24 in MP at the same time. A fixed pitched prop airplane is going to be limited on its ability to run at "24 squared" (24 in. MP/2400 RPM). You will be able to do so perhaps at a specific range of altitudes while in cruise but will not be able to do so during high power climb or at high altitudes. The engine [Upower[/U], whatever it may be, is not going to be able to overcome the physical resistance of the weight of the airplane, air pulling against the prop, decreased amount of air flowing through the engine, etc. that is restricting the ability of the engine pistons to push the crankshaft.
 
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LettersFromFlyoverCountry said:
So I assuming 22 at 2400 isn't as efficient as 24 at 2400, what is the process to increase to 24?

If the prop is swinging through the air at 2400 in either case, does it matter? It must be swinging through at the same speed, right?
Click to expand...

Bob, if you have more MP then your engine is producing more power. You can have a steeper pitch on the prop and go faster for the same RPM. It generally takes more throttle and more fuel to do that.

Efficiency is a different thing. I'd think of that as fuel consumption for a given speed, and while that's dependent upon the overall power output, it also depends on many engine design characteristics. The compression ratio is a major factor, for example.

Dave
 
MP is one of parameters (along with RPM, TEMP, and DA) that allow you to go to the Lycoming engine performance charts to find the engine output HP.

The value for fixed pitched installations is to allow the pilot to determine engine performance should an alternate prop pitch is being considered. Obviously aircraft performance can indirectly give you that info too...

BTW, the "square rule" and others are simplifications or approximations of these charts for pilot/operational convenience.
 
Thank you for starting this thread.

When I was purchasing my RV I asked the owner (not the builder) why there was a Manifold Pressure Gauge on fixed pitch plane. He said he didn't know.

I hadn't thought about it much since then.

So in theory, I can stay WOT and lean on the MP to see what the RPM results? or should I stay with WOT and lean on the EGT/CHT?

I'll do some web surfing but if you guys have some guidance I'm happy learn.

Thanks!
 
LettersFromFlyoverCountry said:
If the prop is swinging through the air at 2400 in either case, does it matter? It must be swinging through at the same speed, right?
Click to expand...

For a fixed pitch prop, 2400 is 2400; the only way to get more power and speed is to push in the throttle and get higher RPM. Not the case for a constant speed prop, where the rpm can be held constant while the pitch of the propeller changes to take a bigger or smaller bite of air. Apart from the previously mentioned "what if" questions concerning power at various rpm, I don't see that MP has much value for a fixed pitch prop.

erich
 
rolivi said:
Thank you for starting this thread.

When I was purchasing my RV I asked the owner (not the builder) why there was a Manifold Pressure Gauge on fixed pitch plane. He said he didn't know.

I hadn't thought about it much since then.

So in theory, I can stay WOT and lean on the MP to see what the RPM results? or should I stay with WOT and lean on the EGT/CHT?

I'll do some web surfing but if you guys have some guidance I'm happy learn.

Thanks!
Click to expand...
Lean using the EGT/CHT information. Those temperatures will give you information much quicker than will the MP information. EGTs move quickly which is why references for leaning are given in terms of leaning "rich" or "lean" of PEAK EGT. Once LOP operation is established pay attention to the CHT information. CHT is the critical temperature for the health of your engine (a cool engine is a happy engine). EGT is the more dynamic temperature that can be used to monitor immediate changes to the performance of the engine.
 
FP with MAP gauge

I didn't use the MAP gauge much until I heard the folks at Airflow Performance say, "Lycomings do not like to be run lean above 24" MP." After that, I used the MP gauge to judge if I was safe for LOP, almost regardless of altitude. The last two years of bug smashing have shown that my indicated % power is virtually always less than 76% anytime I have MP at 24" or less.
At 4000' or less, 24" often results in 2450 - 2500 RPM; anywhere above that it's very close to 2400.
 
I?m hoping this does not turn into another C/S vs. F/P conversation, but the ability to run the engine with the throttle wide open (WOT) (not to be confused with 100% power, or max RPM) is one of the big advantages of a C/S. Anytime you ?throttle? an engine you reduce its volumetric efficiency (VE). It is an air pump, after all. Lindbergh showed many P-38 pilots the virtues of large throttle opening/low RPM ops back in WW-II, and that?s the concept of highly overdriven transmissions and computer aided LOP ops on cars today.

Obviously it?s not practical to run around with the throttle wide open on fixed pitch airplanes like many of us do with C/S, but the basic concept of WOT = VE should be in the back of all our heads somewhere.
 
MP and FF are very important for running a FP properly. Do some searched on LOP operation and learn all you can about where you should run your engine. In most cases, the old advice of 100F ROP is the worst possible place to run unless you are at very low MP settings - which depend on altitude, Temp, RPM, ect.
 
EGT

pa38112 said:
MP and FF are very important for running a FP properly. Do some searched on LOP operation and learn all you can about where you should run your engine. In most cases, the old advice of 100F ROP is the worst possible place to run unless you are at very low MP settings - which depend on altitude, Temp, RPM, ect.
Click to expand...

Lycoming position years ago was you can't hurt a normally aspirated engine with the mixture as long as the power is below 75%.
TSIO Continental works just fine at 100 rich of peak.
 
You got me curious.

I have a 72" long by 74" pitch FP Catto prop on my O-360 and a MAP gauge, which I never look at so because of this thread, I pulled the data from my SkyView to see what kind of numbers I was getting.

KTAS__ROC_______RPM___MAP__% Pwr
_42___-0-________2.140__28.2"__83% (Takeoff roll)
_62___-0-________2,177__28.2"__84%
_82__1,607 FPM___2,228__28.1"__85%
132____979 FPM___2,380__24.4"__69%
141__1,045 FPM___2,440__24.2"__74%
151___-0-________2,458__21.1"__66%

I couldn't find any data where I was running at 75% power as I never have the need to run that fast and typically run at between 55 and 65% power.
 
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My T-18 is Fixed Pitch and has a manifold pressure gauge. I liked it so much I built in a manifold gauge on my Catto equipped RV-8.
I use the Tachometer for take offs, landings and pattern work.
High Speed Cruise is where the MP gauge is useful. Small throttle changes don't change RPM very much, especially when you are descending for arrival after a high altitude flight. You notice power changes more on the MP gauge in those flight regimes.
I find it very helpful for arriving at pattern altitude also for slowly cooling the engine to avoid shock cooling at the end of a long descent.
It's handy, and when I upgrade to an EMS, MP will be included.
 
Leading indicator

Like Mel said, near instantaneous indicator of power changes. MP gives real time indication of power changes. It's especially useful in the airport environment to keep your eyes outside of the cockpit. Just glance at the gauge when you make power setting changes instead of waiting to see where RPM lands. As you get familiar with your RV, you'll find which MP settings work for each phase of flight, it's just quicker, easier and more consistent.
 
I must be doing something wrong because I never look at the power settings when entering or in the pattern. I make power adjustments to obtain certain airspeeds but keep my eyes outside, other than a quick glance at the AS indicator.
 
MAP in the pattern

I don't fly many rectangular patterns anymore, vastly preferring the overhead break to stay current for the engine-out, off-field scenario, but when I do:

it has long been my habit to pull 15" on downwind, 12" on base, and full idle on final with blips as needed to make the first 100 feet of runway for touchdown. Takes but a second to set up each leg, and the performance with appropriate flaps is very predictable (sinks like a manhole cover at 40 flaps and idle power, 70mph IAS - be ready, you only get one chance to flare right, 65 over the fence --PLOP!-- you're planted, first taxiway turnoff is always guaranteed if you want it.) I find without using the MAP, I have less consistent arrivals and more trouble nailing my desired TD spot.

6A, FP Sensenich, 160hp

YM(P)MV ;)
 
I'm going to have to confess I've never considered using MP in the pattern, don't even know what the numbers are at those speeds (FP prop).

But I watch the AOA like a hawk after the turn to base if needing short field landing performance.
 
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In the pattern, I keep eyes outside the cockpit and use power as needed to control airspeed and glide path, but outside the pattern I find it easier and faster to use MP to adjust power settings.
 
yeah, but...

Mel said:
In the pattern, I keep eyes outside the cockpit and use power as needed to control airspeed and glide path, but outside the pattern I find it easier and faster to use MP to adjust power settings.
Click to expand...

I think if you're watching airspeed, you have eyes inside the cockpit just as much as I do when I pull throttle to a specific MP. It is possible to fly a familiar airplane "by ear" and sight picture with no instrumentation, but I think a scan in and out of the cockpit is entirely appropriate in all phases of flight. A flight instructor once told me, "Pitch plus power equals performance." Took me years of flying to fully fathom the depths of this proverb, but it's spot-on. With a FP prop, MP is the only real power control you have, and pitch attitude plus MP will give predictable performance down low in the pattern where it matters most. My $.02, of course.

-Stormy
 
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Didn't say I was "watching" airspeed.

Bill Boyd said:
I think if you're watching airspeed, you have eyes inside the cockpit just as much as I do when I pull throttle to a specific MP.
Click to expand...

I think you misunderstood. I judge my airspeed by feel of the airplane, not by the indicator. If you were to take all the instruments our of my airplane, I can still land it safely and short.
When most all of your flying is off a 1500' strip, you get used to flying by feel.
 
Shock Cooling

skylor said:
Very recent AvWeb article:

Engine Operating Superstitions
Click to expand...

2500 hours plus in the Pitts, full throttle climb, throttle to idle, spin, full throttle climb and repeat. Flat spins require two additional power cycles per spin. Never a cracked cylinder, in fact the only engine issue was mag problems.
I do believe that "stage cooling" can't hurt and probably has some minor benefit on the turbocharged engines, but the stage cooling essentially ends at 24-25" when the turbocharger is essentially out of the picture.
 
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