[briand]

I thought I read in a thread where someone claimed to be using MAP to adjust the rpm of their FP prop. I have never heard of this. Can someone explain this in detail? Or is this just chain pulling.

 

[n5lp]

I thought I read in a thread where someone claimed to be using MAP to adjust the rpm of their FP prop. I have never heard of this. Can someone explain this in detail? Or is this just chain pulling.

MAP does control the RPM of a FP prop assuming all else is equal. In level flight increase MP by increasing the throttle setting and RPM goes up. You can also increase RPM by leaving MP the same and lowering the nose.

 

[Mel]

I always use MP to adjust my engine. Once you are familiar with you setup, it is much faster to adjust using MP as it responds immediately. It take several seconds for the rpm to respond. Also during decent, you can use MP to reduce power an inch or two at a time to avoid shock cooling.

 

[briand]

I always use MP to adjust my engine. Once you are familiar with you setup, it is much faster to adjust using MP as it responds immediately. It take several seconds for the rpm to respond. Also during decent, you can use MP to reduce power an inch or two at a time to avoid shock cooling.

Would I need a three lever quadrant and 3 cables to do this?
Is it basically set up the same as if you had a cs prop only with no prop. gov. or hose going to the prop?

 

[Mel]

Manifold pressure is controlled solely with the throttle. Power is controlled with the throttle. Manifold pressure is simply a measurement.

 

[RVbySDI]

Manifold pressure is controlled solely with the throttle. Power is controlled with the throttle. Manifold pressure is simply a measurement.

This post is intended to clarify the idea being discussed in the above posts about Manifold Pressure posted by Mel and Larry. Either or both please correct my thinking if I am incorrect.

So to fly by the manifold pressure I will use my throttle lever to adjust the speed at which the engine is receiving fuel/air. This I will monitor not by observing the tachometer, which will read out the revolutions per minute (RPM's) that the engine is turning, but rather, I will monitor the MP gauge, which will read out the pressure of the air/fuel in the combustion chamber of the engine as it is processed? By monitoring the MP gauge I can more closely control the throttle setting for the engine and better manage the fuel burn, temperatures, etc.? In so doing the MP gauge will allow me to make more minute adjustments to the throttle setting?

Is this an accurate explanation of what you guys are saying?

 

[Mel]

Exactly! You may still monitor the tach for rpm, it's just that the MP responds immediately so you don't have you head "in the cockpit" waiting for the rpm to stabilize. If the rpm stabilizes lower than you want, crank in another inch of MP, etc.

 

[Radomir]

Can someone explain what the difference is whether you fly by RPM or MP.. why would you do MP over RPM on a FP setup (when you can't change 'em separate from each other like CS guys can)? Sounds like you're choosing a different arbitrary number to fly by...

 

[airguy]

Sounds like you're choosing a different arbitrary number to fly by...

Exactly correct. The only big difference is that upon throttle change, MP difference is immediate, RPM difference may take several seconds.

 

[hevansrv7a]

What is MP - and why use it

Just my 2 cents, but MAP has a longer scale which lends precision and it is more linearly related to actual power. You can see the same RPM going, for example, up hill or down hill, but with different power. MP combined with RPM is power (yes, I know there are altitude corrections). If your prop is a Prince, then MAP is better because the prop flexes under different load conditions. Elsewhere on this site Walter Atkinson and others have discussed at length the relationship between MAP, RPM and power. It's just as true for FP as for CS except with FP you have less control of the variables independently. Walter says:
HP =100-((((2700/100)-(RPM/100))*2.5)+((29.92-MAP)*3.5)).
I think I have this right and the parens are for making it work in Excel.

 

[RVbySDI]

Just my 2 cents, but MAP has a longer scale which lends precision and it is more linearly related to actual power. You can see the same RPM going, for example, up hill or down hill, but with different power. MP combined with RPM is power (yes, I know there are altitude corrections). If your prop is a Prince, then MAP is better because the prop flexes under different load conditions. Elsewhere on this site Walter Atkinson and others have discussed at length the relationship between MAP, RPM and power. It's just as true for FP as for CS except with FP you have less control of the variables independently. Walter says:
HP =100-((((2700/100)-(RPM/100))*2.5)+((29.92-MAP)*3.5)).
I think I have this right and the parens are for making it work in Excel.

If I plug in numbers to this formula I will get what? The horse power output of my engine? I don't think I fully understand this or am missing something.

When I plug in 2700 for RPM in the formula and 27 for MAP the formula spits out a HP number of 89.78. Does this mean that no matter how powerful an engine I am using that setting of 2700 rpm and 27 MAP will yield 89.78 hp? It does not matter if I am running with a 118 HP O-235 or a 200 HP O-360 at this setting I am still only going to produce 89.78 HP? Surely I am missing something in my analysis of this formula.

Perhaps this formula is for a 100 HP engine. So if I replace the "100" at the beginning of the formula with the appropriate HP rating for whatever engine I am using perhaps I can see a more useful number displayed in the HP. In doing this for a 180 HP rated engine I get the following formula:

HP = 180-((((2700/100)-(2700/100))*2.5)+((29.92-27)*3.5))

This formula yields 169.78 HP.

Perhaps this is more in line with what Walter was saying about this formula. In analysis of this formula it appears to me that it is simply looking at the % of max hp plus the difference from standard pressure multiplied by defined constants for both. Then that number is subtracted from the max HP rating of the engine.

 

[Mel]

I think the formula gives you %hp. Therefore your sample would be 89.8% of available hp.
You guys are making this way too complicated. Using MP does not change any power parameters. It simply reacts faster than RPM.

 

[RVbySDI]

I think the formula gives you %hp. Therefore your sample would be 89.8% of available hp.
You guys are making this way too complicated. Using MP does not change any power parameters. It simply reacts faster than RPM.

I understand Mel. It is a more accurate way to monitor your power setting.

 

[hevansrv7a]

Oops

Yes, it gives pct hp. Sorry. I was just trying to show how the two measurements taken together produce a power measurement. But that's what you want anyhow. BTW, GRT and other engine monitors do this for you and I find % PWR a very nice piece of information for managing the engine during flight - better than RPM or MAP alone. Just for accuracy, even this is only an approximation of power. To get it right, you'd have to measure BMEP or Torque. This is pretty good, though and the answers are really close.

 

[Kevin Horton]

Just my 2 cents, but MAP has a longer scale which lends precision and it is more linearly related to actual power. You can see the same RPM going, for example, up hill or down hill, but with different power. MP combined with RPM is power (yes, I know there are altitude corrections). If your prop is a Prince, then MAP is better because the prop flexes under different load conditions. Elsewhere on this site Walter Atkinson and others have discussed at length the relationship between MAP, RPM and power. It's just as true for FP as for CS except with FP you have less control of the variables independently. Walter says:
HP =100-((((2700/100)-(RPM/100))*2.5)+((29.92-MAP)*3.5)).
I think I have this right and the parens are for making it work in Excel.

According to the Lycoming power charts, the power produced for a given rpm and MP varies with altitude. I don't see altitude in that equation. I wonder which altitude it is optimized for.

Also, the torque curve for the angle-valve IO-360s differs from that of the parallel valve engines. At lower rpm, for a given rpm and MP, the 180 hp parallel valve O-360s and IO-360s produce about the same amount as the 200 hp angle valve IO-360s. Below 2300 rpm, the 200 hp engines make less power than the 180 hp ones. Above 2300 rpm, the angle valve engines start to breath better, and they make more power than the parallel valve ones. I wonder which series of engines that equation is optimized for.

The following data is from a Python script that replicates the 
Lycoming power charts.  It is fairly close to the Lycoming charts,
but it is not 100% accurate.

altitude  rpm       MP        O-360-A   IO-360-A
0         2000      22        101       97
0         2100      22        106       104
0         2200      22        112       110
0         2300      22        116       117
0         2400      22        120       124
0         2500      22        123       130
0         2600      22        126       137
0         2700      22        129       143


altitude  rpm       MP        O-360-A   IO-360-A
7000      2000      22        115       107
7000      2100      22        120       116
7000      2200      22        127       122
7000      2300      22        131       130
7000      2400      22        135       136
7000      2500      22        137       144
7000      2600      22        140       151
7000      2700      22        143       157

 

[hevansrv7a]

Good questions, I have no answer

According to the Lycoming power charts, the power produced for a given rpm and MP varies with altitude. I don't see altitude in that equation. I wonder which altitude it is optimized for.

Also, the torque curve for the angle-valve IO-360s differs from that of the parallel valve engines. At lower rpm, for a given rpm and MP, the 180 hp parallel valve O-360s and IO-360s produce about the same amount as the 200 hp angle valve IO-360s. Below 2300 rpm, the 200 hp engines make less power than the 180 hp ones. Above 2300 rpm, the angle valve engines start to breath better, and they make more power than the parallel valve ones. I wonder which series of engines that equation is optimized for.

 The following data is from a Python script that replicates the 
 Lycoming power charts.  It is fairly close to the Lycoming charts,
 but it is not 100% accurate.
 
 altitude  rpm       MP        O-360-A   IO-360-A
 0         2000      22        101       97
 0         2100      22        106       104
 0         2200      22        112       110
 0         2300      22        116       117
 0         2400      22        120       124
 0         2500      22        123       130
 0         2600      22        126       137
 0         2700      22        129       143
 
 
 altitude  rpm       MP        O-360-A   IO-360-A
 7000      2000      22        115       107
 7000      2100      22        120       116
 7000      2200      22        127       122
 7000      2300      22        131       130
 7000      2400      22        135       136
 7000      2500      22        137       144
 7000      2600      22        140       151
 7000      2700      22        143       157

Kevin, I can always count on you to help me get it right. And I thank you for it; you have always been generous with your expertise. However, the formula came from a prior thread and it's Walter's not mine. I think it illustrates the basic point about RPM and MAP combining to give a power measurement, but, of course, it's not precise when you add in those variables that you mention.