Fri Nov 14, 2008 2:38 pm
retroaviation wrote:Okay, so now it's ME that's showing my ignorance here, but........
If the boost gauge shows "0" at ambient pressure, how does it show a difference (with the engine not running) at different altitudes. For example, if it shows "0" at sea level, will it show "-2" or so, when here at Midland, which is just shy of 3,000 ft. above sea level? Our manifold pressure gauges only show roughly 27" (give or take, depending on barometric conditions) here.
Am I making any sense at all? Heck, now I've gone and confused myself.
Gary
I understand what you're saying. I think maybe because each unit in boost covers more than inches in MP, that ambient will be pretty close to the '0' on the gage. Also, the gage has an adjustable ring with an arrow and a mark, so maybe you adjust for difference. Here's a picture:
http://www.warmkessel.com/jr/flying/td/ ... ometer.jpg
Where's Bill when you need him
Steve G
Fri Nov 14, 2008 2:40 pm
We use the guage to obtain power within the limitations of the engine.
With a controllable pitch prop and a supercharger or turbocharger we can overboost an engine.
And GARY AIN"T GONNA LIKE THAT.
Sully
With a controllable pitch prop and a supercharger or turbocharger we can overboost an engine.
And GARY AIN"T GONNA LIKE THAT.
Sully
Fri Nov 14, 2008 3:25 pm
TriangleP wrote: Btw, saw you at the PoF airshow last year, maybe the hottest on record I think!
Who, Mudge? ????? : )
Tulio
Fri Nov 14, 2008 3:29 pm
Sully wrote:We use the guage to obtain power within the limitations of the engine.
With a controllable pitch prop and a supercharger or turbocharger we can overboost an engine.
And GARY AIN"T GONNA LIKE THAT.
Sully
If its one of mine I ain't gonna like it.
Rich
Fri Nov 14, 2008 3:30 pm
In my warbird check out of the P-39 the instructed made it clear to:
"Increase RPM first, then manifold pressure"
Is that right?
He also explains if you don't do this the added pressure will build up in the engine and it could cause it to explode!8)
"Increase RPM first, then manifold pressure"
Is that right?
He also explains if you don't do this the added pressure will build up in the engine and it could cause it to explode!8)
Fri Nov 14, 2008 3:32 pm
Tulio wrote:TriangleP wrote: Btw, saw you at the PoF airshow last year, maybe the hottest on record I think!
Who, Mudge? ????? : )
Tulio
I was gonna say something about Mrs. Mudge...but I already got knocked around for that...
Fri Nov 14, 2008 3:39 pm
nice thing about having the gauge read in inches of mercury is that if you are at somewhere without a weather brief, you can set your altimeter to the manifold gauge reading before you start the engine. ie, if it reads 29.50 on the manifold gauge, set 29.50 in the kollsman window of the altimeter .
And you are correct, lead with the prop advancing power, and manifold when reducing power.
And you are correct, lead with the prop advancing power, and manifold when reducing power.
Fri Nov 14, 2008 3:49 pm
You need to make sure that your power setting, which is a combination of Manifold Pressure and RPM, are within specs. This keeps you from damaging the engine and results in a known fuel consumption for planning purposes. Most aircraft with a constant speed prop will have a Manifold Pressure gauge.Mudge wrote:OK, there's a MP/Boost gauge on the panel. Why? If I'm at idle or full throttle or cruise, why do I care what the MP is?
With a fixed pitch prop, throttle setting (Manifold Pressure) results in an RPM based upon the (fixed) pitch of the prop, so there is one less variable. That is why you typically don't need a Manifold Pressure reading on an aircraft with a fixed pitch prop.
Fri Nov 14, 2008 4:02 pm
bipe215 wrote: I understand what you're saying. I think maybe because each unit in boost covers more than inches in MP, that ambient will be pretty close to the '0' on the gage. Also, the gage has an adjustable ring with an arrow and a mark, so maybe you adjust for difference. Here's a picture:
http://www.warmkessel.com/jr/flying/td/ ... ometer.jpg
Where's Bill when you need him
Steve G
Ahhh, I see. Thanks for the info. See? We didn't need Bill after all.
Gary
Fri Nov 14, 2008 4:34 pm
As mentioned earlier the manifold pressure instrument is typically used when you have a controllable propeller.
In a fixed pitch prop you control the engine RPM with the throttle and manifold pressure indications aren't really necessary. In a controllable pitch prop you control the engine RPM with the propeller and the power of the engine with the throttle or manifold pressure (MP).
Since you can vary the RPM of the engine regardless of the throttle (sort of) you can have too much power (MP) or throttle for a given RPM. This can be deadly to an engine as it can put too much stress on the engines internal parts. It can lead to the internal parts becoming external parts.
The pressure in the cylinders has also been referred to as Brake Mean Effective Pressure or BMEP. If you have too much manifold pressure for a given RPM the BMEP, or pressure created in the cylinders, can overstress the cylinders, connecting rods, bearing and other internals.
The other side of this is to have too little MP for a given RPM and the propeller will drive the engine. This isn’t as big a deal with little engines but when you get into bigger engines, especially ones with geared nose cases, It can be deadly to the engine because of reverse loading… another topic all by itself…
In a controllable pitch prop there is a certain sequence that you use to change power settings. To increase power you first increase the RPM and then you increase the throttle or MP. To reduce power you first reduce the throttle/MP and then you reduce the RPM. This procedure helps to make certain that you do not create a situation where you have too much BMEP or stress on the engine.
One other very important thing about the manifold pressure is it can be used to help determine the health of your engine. Before you start your engine take note of the field barometric pressure or ambient pressure as indicated on the MP instrument. During your run up it is recommended to check mags at this setting and there is a specific RPM that your engine should reach at field barometric pressure. The beauty of this is that no matter what your field baro reading is you should attain the same RPM when the throttle reached that setting. Whether you are at sea level (30 inches MP), 3000 feet above sea level at Midland (27 inches MP) or in Denver (25” MP) the engine should reach the specific RPM. If it doesn’t then it’s likely there is a problem because your engine is not reaching full power.
Very few folks do field barometric pressure checks but it is a very important tool to determine the health of your engine. I have seen way too many engines do a normal mag check at 1500 or 1700 RPM only to develop a problem at field baro. This check can also tell you if your prop has been set correctly to its low pitch stops.
One of the troubles is exactly what should the reference RPM for a specific engine. In the military that RPM was determined at overhaul on the test cell. I have seen where the reference RPM was stenciled or placarded on the instrument panel for the engines so the pilot had a ready reference during the run up. The typical Reference RPM for a Pratt R-985 is a range of between 1950 and 2050 at Field Baro pressure. Any less and you should find out why before you fly.
Another helpful indication from the MP instrument is for carb icing. In cruise you should set and monitor your manifold pressure closely. If you see it drop slowly at a constant altitude then you are likely building ice in the carburetor.
Manifold pressure is a great tool to keep track of your engines health.
In a fixed pitch prop you control the engine RPM with the throttle and manifold pressure indications aren't really necessary. In a controllable pitch prop you control the engine RPM with the propeller and the power of the engine with the throttle or manifold pressure (MP).
Since you can vary the RPM of the engine regardless of the throttle (sort of) you can have too much power (MP) or throttle for a given RPM. This can be deadly to an engine as it can put too much stress on the engines internal parts. It can lead to the internal parts becoming external parts.
The pressure in the cylinders has also been referred to as Brake Mean Effective Pressure or BMEP. If you have too much manifold pressure for a given RPM the BMEP, or pressure created in the cylinders, can overstress the cylinders, connecting rods, bearing and other internals.
The other side of this is to have too little MP for a given RPM and the propeller will drive the engine. This isn’t as big a deal with little engines but when you get into bigger engines, especially ones with geared nose cases, It can be deadly to the engine because of reverse loading… another topic all by itself…
In a controllable pitch prop there is a certain sequence that you use to change power settings. To increase power you first increase the RPM and then you increase the throttle or MP. To reduce power you first reduce the throttle/MP and then you reduce the RPM. This procedure helps to make certain that you do not create a situation where you have too much BMEP or stress on the engine.
One other very important thing about the manifold pressure is it can be used to help determine the health of your engine. Before you start your engine take note of the field barometric pressure or ambient pressure as indicated on the MP instrument. During your run up it is recommended to check mags at this setting and there is a specific RPM that your engine should reach at field barometric pressure. The beauty of this is that no matter what your field baro reading is you should attain the same RPM when the throttle reached that setting. Whether you are at sea level (30 inches MP), 3000 feet above sea level at Midland (27 inches MP) or in Denver (25” MP) the engine should reach the specific RPM. If it doesn’t then it’s likely there is a problem because your engine is not reaching full power.
Very few folks do field barometric pressure checks but it is a very important tool to determine the health of your engine. I have seen way too many engines do a normal mag check at 1500 or 1700 RPM only to develop a problem at field baro. This check can also tell you if your prop has been set correctly to its low pitch stops.
One of the troubles is exactly what should the reference RPM for a specific engine. In the military that RPM was determined at overhaul on the test cell. I have seen where the reference RPM was stenciled or placarded on the instrument panel for the engines so the pilot had a ready reference during the run up. The typical Reference RPM for a Pratt R-985 is a range of between 1950 and 2050 at Field Baro pressure. Any less and you should find out why before you fly.
Another helpful indication from the MP instrument is for carb icing. In cruise you should set and monitor your manifold pressure closely. If you see it drop slowly at a constant altitude then you are likely building ice in the carburetor.
Manifold pressure is a great tool to keep track of your engines health.
Fri Nov 14, 2008 4:41 pm
Taigh Ramey wrote:
Manifold pressure is a great tool to keep track of your engines health.
Sure is. I even have one in my turbo-diesel truck. Will also have one in my tow-vehicle for my little air racer, which is supercharged.
Gary
Fri Nov 14, 2008 5:46 pm
OK...now youse troops have made me cross-eyed with all the technical explanations. It interesting BUT...my problem is, I'm a visual learner. If I can see it, I can understand it.
All I need to do now is find a ride in an a/c that's equipped with the MP gauge. Not bloody likely, however.
Mudge the overwhelmed
All I need to do now is find a ride in an a/c that's equipped with the MP gauge. Not bloody likely, however.
Mudge the overwhelmed
Fri Nov 14, 2008 5:51 pm
Mudge wrote:OK...now youse troops have made me cross-eyed with all the technical explanations. It interesting BUT...my problem is, I'm a visual learner. If I can see it, I can understand it.
All I need to do now is find a ride in an a/c that's equipped with the MP gauge. Not bloody likely, however.
Mudge the overwhelmed
I'm sure some ride in an aircraft with MP could be arranged... Just find somewhere that'll rent you a "complex" aircraft like a Cessna 172RG or a Piper Arrow and an instructor to go along.
Ryan
Fri Nov 14, 2008 5:54 pm
Mudge,
Ever seen one of those cars with a light that comes on if you give it a lot of gas? "Economy" is what some legends are.
That is a poor man's MP guage.
Sully
Ever seen one of those cars with a light that comes on if you give it a lot of gas? "Economy" is what some legends are.
That is a poor man's MP guage.
Sully
Sat Nov 15, 2008 7:38 am
TriangleP wrote:Ztex wrote:Tulio wrote:TriangleP wrote: Btw, saw you at the PoF airshow last year, maybe the hottest on record I think!
Who, Mudge? ????? : )
Tulio
I was gonna say something about Mrs. Mudge...but I already got knocked around for that...
Doh! I was talkin' about the weather!
Noooo!!!! You did not write anything like a Bushism; just having a little fun, that's all.
Saludos,
Tulio