Using Manifold Pressure Gauge with FP Prop

[Blue Doughnut]

I was hoping someone here might be able to share some best practices for using manifold pressure to optimize performance with a FP prop.

I've always wondered why I have a manifold pressure gauge in my aircraft, and a little digging around suggests some people are using it to optimize their aircraft performance. I haven't been able to find a definitive quite to how exactly it can be employed so I was hoping someone here might be able to break it down for me.

Thanks!

 

[1600vw]

It's used to know how hard one is working the engine. I like to fly with MP between 21-23. Anything over 23 and I am working the engine hard. Anything under 21 and I need to pay very close attention to carb ice. The MP gauge is the only gauge to tell you how hard you are pushing your engine. The RPM gauge tells you how fast it's turning not how hard the engine is working. That is the job of the MP gauge. Many engine builders will recommend this gauge for the reason I state. The company who built my engine told me to keep her under 24 MP and she will last a very long time. Push her up at 25 and above, I was told my engine will be back in their hands in a few years.

Tony

 

[1600vw]

When flying an experimental or EAB, this gauge " MP " can also be used to see if you have the engine propped correctly. If you are running very low MP but RPM's are very high, you are under propped. On the other hand if you are running low RPM's and high MP you are over propped. This is a very important gauge in an EAB or homebuilt for this reason. IMHO

Tony

 

[MAKG1]

It's used to know how hard one is working the engine. I like to fly with MP between 21-23. Anything over 23 and I am working the engine hard. Anything under 21 and I need to pay very close attention to carb ice. The MP gauge is the only gauge to tell you how hard you are pushing your engine. The RPM gauge tells you how fast it's turning not how hard the engine is working. That is the job of the MP gauge. Many engine builders will recommend this gauge for the reason I state. The company who built my engine told me to keep her under 24 MP and she will last a very long time. Push her up at 25 and above, I was told my engine will be back in their hands in a few years.

Tony

For FIXED PITCH, the RPM gauge gives just as much information as the MP gauge. Actually a bit more, since the mixture plays a role, too.

The RPM is a direct measurement of the load on the engine at a given airspeed. The MP gauge is a measure of the air flowing through the engine for each stroke (not total airflow unless you multiply RPM). Engine power is not a function of that alone; you won't get much if the mixture is on ICO for instance. It is also quite sensitive to spark timing and the camshaft. If correctly leaned and all that, it approximates engine torque, but translating that to thrust is problematic at best.

There is a reason MP gauges are not standard on fixed pitch aircraft. They are simple gauges, so they could be.

A carb temp gauges is substantially more effective for carb ice than an MP gauge.

Now, if you have a constant speed prop, MP is all you have.

And things are different if you're on a dyno measuring the power curve. But the power curve is known for an aircraft in service.

 

[kayoh190]

Interesting. Don't think I've ever flown an airplane with a fixed pitch prop and a MP gauge. What are some examples?

 

[flyingron]

What it does do is give you a good idea of efficiency. As Tony says, this can be helpful if you're playing around with prop pitch (even if it was fixed) or other tweaks.

 

[Pilawt]

Interesting. Don't think I've ever flown an airplane with a fixed pitch prop and a MP gauge. What are some examples?

Piper was putting them in Tri-Pacers and some early Cherokees (all fixed-pitch) in the late 1950s and early 1960s.

 

[pmanton]

My PA22 came with one from the factory. I pulled it out and used the hole for something else.

 

[kayoh190]

Piper was putting them in Tri-Pacers and some early Cherokees (all fixed-pitch) in the late 1950s and early 1960s.

Wow - that's cool. I had no idea.

 

[coloradobluesky]

I flew with one a Super Cub (of all planes) and saw no benefit in it. It goes up and down when you move the throttle. So does the tachometer. It doesn't move when you pull the mixture (and the tach does, a bit), but so what? I just used the tachometer for throttle setting like any plane that has only that. With a constant speed prop, you need it for throttle setting because the tachometer doesnt move (much) when you adjust the throttle.... No need for one on a fixed pitch plane. Thats why they usually dont have them.

 

[Blue Doughnut]

Interesting. Don't think I've ever flown an airplane with a fixed pitch prop and a MP gauge. What are some examples?

I've got one in my Tiger. I guess I figured I ought to stop ignoring it since they went to the time and expense to include it on the panel.

Thanks all for the feedback!

Now, bonus points for anyone who cares to tell me why there is a small pool of oil in the bottom of the gauge!! (I have a couple ideas, but I'm always interested to get other perspectives).

 

[kgruber]

Neither MP or tachometer is a good indication of power being produced. A stopped engine still has both in flight.

Fuel flow is a better indication of power.

 

[Pilawt]

My C-172N - 180 hp (fixed pitch) came to me with a manifold pressure gauge installed, and I love it. Now I wouldn't want an airplane, CS or FP, without it.

The tach only tells part of the story. On a fixed-pitch installation rpm is sensitive to subtle changes in airspeed, even if power setting is constant. On a recent long trip with some minor mountain wave action, the rpm was +/- 75 rpm all day long as I maintained altitude in the up-and-downdraft cycle, while the mp gauge remained constant.

When I just go up to stooge around at low altitude in the local area, I generally use a reduced power setting of about 2350 rpm. That works out to about 21" on the manifold pressure gauge. Now when I get to altitude, I just pull the power back to 21", and the tach will eventually stabilize close to where I want it.

If carb icing conditions are present, a slight drop in rpm could be carb ice, or it could just be a fluctuation in airspeed. If MP is dropping too, then there's a better chance it's ice.

The MP gauge also helps in calculating % power. My airplane came to me with this little card stuck to the panel next to the MP and tach. Unlike rpm-only power setting charts, this works at any altitude:



It's not exact, but for my airplane it's close enough for operational purposes -- and a whole lot easier than getting out a ruler and plotting points 'A', 'B', 'C' and 'D' on the two-page power setting graph in Lycoming's manual. I don't know the source of this, or whether it would work with any other engine/prop application.

If you're one of those who believes you should never operate a piston engine "oversquare", by all means do not put a manifold pressure gauge in your fixed-pitch airplane. You might be freaked out seeing this:

 

[Tantalum]

I've often wondered the same thing as the OP.. if it is so critical to the CS folks and engine health why not include one for the FP folks.. even if we cannot change the pitch it would still be a good indicator?

It's used to know how hard one is working the engine. I like to fly with MP between 21-23. Anything over 23 and I am working the engine hard. Anything under 21 and I need to pay very close attention to carb ice. The MP gauge is the only gauge to tell you how hard you are pushing your engine. The RPM gauge tells you how fast it's turning not how hard the engine is working. That is the job of the MP gauge. Many engine builders will recommend this gauge for the reason I state. The company who built my engine told me to keep her under 24 MP and she will last a very long time. Push her up at 25 and above, I was told my engine will be back in their hands in a few years.

Thanks for this comprehensive response! I always thought RPM was a dubious way to show how hard the engine is working, or how much power it is making. For example... sure, in a static sense the RPM may be a good indicator, but if I'm descending at cruise power I have to monitor the RPM to keep it around 2,600, I might have the throttle itself halfway out. While the engine may be spinning just as fast, and having similar overall friction, the temps and pressures in the engine are undoubtedly going to be different than if I have the throttle wide open

For FIXED PITCH, the RPM gauge gives just as much information as the MP gauge.

How does that jive with what Tony said above? I guess it depends on how pedantic we want to be with how we define "information." Frankly, being a data sciences and OCD person, I would love to have MP

**Incidentally, it always surprised me how little engine instrumentation comes (came) with new planes. There is a relatively new Archer III at the club and it has simply an RPM gauge and some basic temp info. Same with the stock Skyhawks, etc. The plane I fly has a JPI and I find it to be incredibly valuable. The engine is arguably the most critical feature on an airplane and we're given an RPM gauge and the bare minimum to keep it somewhat healthy

 

[Sac Arrow]

Neither MP or tachometer is a good indication of power being produced. A stopped engine still has both in flight.

Fuel flow is a better indication of power.

Depending on how you are leaned. For LOP conditions it is a reliable indicator. On a full rich climbout, however, not so much.

 

[Tantalum]

Neither MP or tachometer is a good indication of power being produced. A stopped engine still has both in flight.

Thought provoking point. Does anyone know how the Cirrus avidyne shows "percent power"?

 

[Blue Doughnut]

My C-172N - 180 hp (fixed pitch) came to me with a manifold pressure gauge installed, and I love it. Now I wouldn't want an airplane, CS or FP, without it.

The tach only tells part of the story. On a fixed-pitch installation rpm is sensitive to subtle changes in airspeed, even if power setting is constant. On a recent long trip with some minor mountain wave action, the rpm was +/- 75 rpm all day long as I maintained altitude in the up-and-downdraft cycle, while the mp gauge remained constant.

When I just go up to stooge around at low altitude in the local area, I generally use a reduced power setting of about 2350 rpm. That works out to about 21" on the manifold pressure gauge. Now when I get to altitude, I just pull the power back to 21", and the tach will eventually stabilize close to where I want it.

If carb icing conditions are present, a slight drop in rpm could be carb ice, or it could just be a fluctuation in airspeed. If MP is dropping too, then there's a better chance it's ice.

The MP gauge also helps in calculating % power. My airplane came to me with this little card stuck to the panel next to the MP and tach. Unlike rpm-only power setting charts, this works at any altitude:

View attachment 53726

It's not exact, but for my airplane it's close enough for operational purposes -- and a whole lot easier than getting out a ruler and plotting points 'A', 'B', 'C' and 'D' on the two-page power setting graph in Lycoming's manual. I don't know the source of this, or whether it would work with any other engine/prop application.

If you're one of those who believes you should never operate a piston engine "oversquare", by all means do not put a manifold pressure gauge in your fixed-pitch airplane. You might be freaked out seeing this:

View attachment 53727

A lot of interesting points here, thank you. I've often wondered if there was a better way to determine percent power as I always suspected RPM was really only indicating part of the complete picture.

 

[JC150]

I thought you're causing more turbulent airflow and stress on the engine by creating more suction (low MP) than high MP? I know we're talking about FP here, but with a constant speed prop, isn't it better to have a higher MP and lower RPM?

 

[Sac Arrow]

I thought you're causing more turbulent airflow and stress on the engine by creating more suction (low MP) than high MP? I know we're talking about FP here, but with a constant speed prop, isn't it better to have a higher MP and lower RPM?

Manifold pressure is absolute pressure, the higher the MP, the less "vacuum" you have, and the more air is available to power the engine. There are some people that will argue that high MP/ low RPM combinations will place more stress on the engine, analogous to flooring the accelerator on your car in high gear at a low speed. In reality, as long as the MP and RPM's are within the limitations stated for the aircraft, your engine is perfectly fine.

In any case, a wide open throttle (maximum MP) is the most efficient way to run the engine.

 

[ktup-flyer]

Interesting. Don't think I've ever flown an airplane with a fixed pitch prop and a MP gauge. What are some examples?

I've flown a 172 with one

 

[mondtster]

Piper was putting them in Tri-Pacers and some early Cherokees (all fixed-pitch) in the late 1950s and early 1960s.

View attachment 53722

Interesting. I've never seen a Cherokee with those control wheels in it. Was this a photo of a production airplane or something that was possibly pre-production?

 

[Pilawt]

Interesting. I've never seen a Cherokee with those control wheels in it. Was this a photo of a production airplane or something that was possibly pre-production?

According to the N-number placard on the panel, that's N5001W, the first production Cherokee (c/n 28-2). It's still registered to a private owner in Pennsylvania. I don't know when they would have switched to the "bow-tie" yoke, but it had to be very soon after.

 

[JAWS]

I find that interesting that Cherokees initially came with MP gauges. My friend has a 1961 28-160 and he asked about it and I told him that a previous owner must have wanted more info to calculate power. He looked confused at my response. So I googled it and came up with this fairly simple explanation. http://www.eaa691.net/images/technotes/Tech_Note_5_Aircraft_Performance.pdf
I re-learned some stuff. My friend just ignores MP and is happy!

 

[Blue Doughnut]

In any case, a wide open throttle (maximum MP) is the most efficient way to run the engine.

Can you expand on this a little?

I know diesels tend to perform better near(ish) WOT, and hard use benefits marine outboards as well, but I always thought some setting below max was better for long term engine health, but as I type this I can't for the life of me recall where I picked that up.

 

[Sac Arrow]

Can you expand on this a little?

I know diesels tend to perform better near(ish) WOT, and hard use benefits marine outboards as well, but I always thought some setting below max was better for long term engine health, but as I type this I can't for the life of me recall where I picked that up.

Thermodynamically, all IC engines perform most efficiently in a non-throttled condition. It's excessive power that reduces engine life, not the throttle condition. For example, if you run lean of peak, running full throttle, regulating power by fuel flow and not by air flow, you will get better efficiency than if you ran rich, controlling the power by air flow (throttling.) And actually, for the most part, your engine will last longer as well, for the same power level.

 

[Blue Doughnut]

Thermodynamically, all IC engines perform most efficiently in a non-throttled condition. It's excessive power that reduces engine life, not the throttle condition. For example, if you run lean of peak, running full throttle, regulating power by fuel flow and not by air flow, you will get better efficiency than if you ran rich, controlling the power by air flow (throttling.) And actually, for the most part, your engine will last longer as well, for the same power level.

Great description. Best part about flying, you can learn something new every day.

Thanks!!

 

[RussR]

I think it's important to add to @Sac Arrow 's great post that wide open throttle, while more efficient from an engine performance standpoint, is only good for engine longevity if you're at an altitude where wide open throttle is a "normal" power setting. So, running around wide open throttle at 1000 MSL might have you generating 90% power. Not great for engine longevity. But at anything above 6000 feet or so, wide open throttle is like 23" of MP or less, and therefore is something like 75% power or less (normally aspirated, of course), and is therefore fine for engine health.

When I fly the 182, it's full throttle for takeoff, climb and cruise (assuming above about 6000 MSL). I won't even touch the throttle between takeoff and the beginning of descent.

 

[Blue Doughnut]

I think it's important to add to @Sac Arrow 's great post that wide open throttle, while more efficient from an engine performance standpoint, is only good for engine longevity if you're at an altitude where wide open throttle is a "normal" power setting. So, running around wide open throttle at 1000 MSL might have you generating 90% power. Not great for engine longevity. But at anything above 6000 feet or so, wide open throttle is like 23" of MP or less, and therefore is something like 75% power or less (normally aspirated, of course), and is therefore fine for engine health.

When I fly the 182, it's full throttle for takeoff, climb and cruise (assuming above about 6000 MSL). I won't even touch the throttle between takeoff and the beginning of descent.

I'm glad I started this thread. I've flown countless cross countries from Florida to Maine, the Midwest, etc. I've always just done what felt right for the aircraft, but never really bothered trying to wrap my head around why the performance improved when I did one thing over the other. I just tried to optimize performance, and ended up running d@nmed close to WOT when over 6000' appropriately leaned. It sounds ridiculous, but I just had some intangible sense that I was optimally configured. I'd be lying if I said I didn't feel bad about running with the throttle full forward, but this discussion helps reconcile that.

 

[wrbix]

Piper was putting them in Tri-Pacers and some early Cherokees (all fixed-pitch) in the late 1950s and early 1960s.

I'd be very interested to see what these planes' AFMs had to say referrable to the MP gauge

 

[Blue Doughnut]

I'd be very interested to see what these planes' AFMs had to say referrable to the MP gauge

This!!

Exactly why I brought this up. Mine makes absolutely no mention of it.

 

[wrbix]

This!!

Exactly why I brought this up. Mine makes absolutely no mention of it.

AFAIK, never standard in an AA5 - so would not be in POH.

 

[Blue Doughnut]

In that case I guess I can call it a "feature" when I list it for sale and I'll overcharge for it.

 

[wrbix]

LOL....sure you paid for it too.
Did your plane, by any chance, once have an MP prop?

 

[Blue Doughnut]

I'm sure I did, and no it didn't. It had the original McCauley prop (if memory serves). Had the seinsenech installed some years ago. Both FP which contributed to the mystery.

I get that there's information to be had from it, and I've gotten a lot of great feedback on how that information can be used, but I still wonder why a previous owner decided that the MP gauge needed to be installed on the panel over, say, a glide slope...

 

[wrbix]

.....or a blue donut

 

[Blue Doughnut]

Touché

 

[Ravioli]

While the POH may not mention it, the engine manufacturer probably has some recommendations you can rely on.

I know the parameters I use for the O-320 I fly behind are not in the POH of the RV (since there isn't one)

 

[Pilawt]

I'd be very interested to see what these planes' AFMs had to say referrable to the MP gauge

This from the 1960 Tri-Pacer owners manual:



In the performance section there is the usual "Altitude vs. True Airspeed & RPM at 75% Power Cruise" table. Then the owner's manual also contains the complete two-page horsepower computation graph, reproduced directly from the Lycoming engine manual. You have to have manifold pressure data in order to calculate horsepower using Lycoming's charts.