MP gauge
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MP gauge
I got a question about my MP gauge. Its a reverse reading vacuum gauge connected across the carburettor butterfly right? So it reads 30" (atmospheric pressure) till I start the engine when it winds back to the idle reading of 12" or so.
As I feed in throttle & the butterfly opens the vacuum decreases & the gauge climbs again. If I test carby heat the warm air is less dense so the gauge climbs another inch or two. So far so good.
But when I descend from altitude in flight the MP gauge also climbs. But a descent should be into denser air which means the gauge should fall shouldn't it?
As I feed in throttle & the butterfly opens the vacuum decreases & the gauge climbs again. If I test carby heat the warm air is less dense so the gauge climbs another inch or two. So far so good.
But when I descend from altitude in flight the MP gauge also climbs. But a descent should be into denser air which means the gauge should fall shouldn't it?
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From: Oz
If you test carby heat, the warm air is less dense so it is harder to "suck" into the engine, so the gauge will indicate less MP.
When you descend from altitude the MP gauge climbs because the denser air with its higher pressure pushes more air into the carby.
When you descend from altitude the MP gauge climbs because the denser air with its higher pressure pushes more air into the carby.
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22clipper - I'm guessing from your name that you're talking about a Robbie with the governor on. In that case I think the primary impact of the carb heat is on the mixture rather than the air pressure, and it's the governor that's making the changes to the MAP.
As you say the warm air is less dense (not necessarily lower pressure) so the mixture is richer, less of the fuel is burnt, the power output reduces and the rpm drops. The governor then opens the throttle to compensate and that is the MAP rise you're seeing.
As you say the warm air is less dense (not necessarily lower pressure) so the mixture is richer, less of the fuel is burnt, the power output reduces and the rpm drops. The governor then opens the throttle to compensate and that is the MAP rise you're seeing.
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From: England
Ok First and formost a MAP Gauge measures Manifold Air Pressure (or the pressure of the air entering the engine) hence its name. Not the pressure accross the butterfly.
So when the engine is not running the map gauge reads atmospheric pressure which is about 30".
The butterfly controls the amount of air entering the manifold.
So when you start the engine the pressure drops.
When you put in the carb heat as you said the air is less dense so less enters the engine. This causes the engine rpm to drop which means that it is sucking less air but the butterfly has not moved so the MAP rises.
Now when you are in flight the govenor is controling the butterfly and hence the air pressure in the manifold.
When you raise the colective you increase the blade angle which makes more drag so you need more power. Which means the butterfly opens further so you have more pressure and therefore a higher MAP.
When you lower the colective the oposite happens so less blade angle....... less drag.......... less butterfly opening........... less MAP.
The only thing altitude does is alter the maximum pressure obtainable in the Manifold at the time.
Hope that is clear
So when the engine is not running the map gauge reads atmospheric pressure which is about 30".
The butterfly controls the amount of air entering the manifold.
So when you start the engine the pressure drops.
When you put in the carb heat as you said the air is less dense so less enters the engine. This causes the engine rpm to drop which means that it is sucking less air but the butterfly has not moved so the MAP rises.
Now when you are in flight the govenor is controling the butterfly and hence the air pressure in the manifold.
When you raise the colective you increase the blade angle which makes more drag so you need more power. Which means the butterfly opens further so you have more pressure and therefore a higher MAP.
When you lower the colective the oposite happens so less blade angle....... less drag.......... less butterfly opening........... less MAP.
The only thing altitude does is alter the maximum pressure obtainable in the Manifold at the time.
Hope that is clear
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From: England
Yes but don't forget that during flight the butterfly does not remain starionary because it is being controled by the govenor. So the butterfly will change position to keep the MAP constant and therfore the engive revs constant.
IF the butterfly remained constant you are correct and the MAP would change. But so then would your engine revs.
IF the butterfly remained constant you are correct and the MAP would change. But so then would your engine revs.
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From: Southern UK
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From: England
The only problem with this artical as the same thing you will find with a lot of aviation. It is all very true. Howerer. it is aimed at a bunch of planks. And has lots of releivence to us rotory boys right up untill the point when we put the govenor on and raise the colective.
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From: Denver, CO and the GOM
Actually a rotor and a constant-speed prop act the same (as far as MP is concerned).
But back to the R22. The MP gauge is measuring the absolute pressure inside the intake manifold. When you do your carb heat check, you are at 70% RPM (if memory serves) and flat pitch. In this instance, the engine is trying to draw air across a partially closed throttle plate, pulling down the MP. A small reduction in RPM (due to full carb heat) will cause a noticable rise in MP. The same thing happens during the mag checks. The rise is not an indication of changed air density, it is an indication of reduced engine RPM
Now about that pesky MP rise when you go from a high to low altitude. Here's something that's initially hard to wrap your mind around - for a given MP setting, the engine makes more power the higher you go. Remember, the engine not only has to pull the air in, it has to push it back out too. The lower the air density, the less back pressure, so you get more power for the same MP. That's why you have decreasing MP limits right up to the "Full Throttle" point. You can reverse-engineer this - at 6,000', the engine will make 121 HP with a lower MP than when making 121 HP at sea level.
But back to the R22. The MP gauge is measuring the absolute pressure inside the intake manifold. When you do your carb heat check, you are at 70% RPM (if memory serves) and flat pitch. In this instance, the engine is trying to draw air across a partially closed throttle plate, pulling down the MP. A small reduction in RPM (due to full carb heat) will cause a noticable rise in MP. The same thing happens during the mag checks. The rise is not an indication of changed air density, it is an indication of reduced engine RPM
Now about that pesky MP rise when you go from a high to low altitude. Here's something that's initially hard to wrap your mind around - for a given MP setting, the engine makes more power the higher you go. Remember, the engine not only has to pull the air in, it has to push it back out too. The lower the air density, the less back pressure, so you get more power for the same MP. That's why you have decreasing MP limits right up to the "Full Throttle" point. You can reverse-engineer this - at 6,000', the engine will make 121 HP with a lower MP than when making 121 HP at sea level.
Last edited by Flingwing207; 25th March 2006 at 03:54.
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From: Sydney
Worms for lunch anybody, I've just opened a can. I've read the Deakin article, some complex issues eh?
OK First point. The MP gauge is a diaphragm sealed on one side with the other side connected to the intake manifold. So its an absolute reading gauge, reading manifold pressure relative to the fixed reference on the sealed side of the diaphragm NOT to atmospheric pressure. That's why, with the engine not running, its rest reading (atmospheric pressure in the manifold) varies with altitude or air pressure changes with the weather.
Second point. When I pull carby heat with the govenor off @ 80% the warmer, & therefore less dense, air should make the MP pressure drop. The fact that it rises must be because the engine slows a bit generating less vacuum. If I pull carby heat with the govenor on in flight the MP still rises. Here the govenor maintains a fixed RPM by opening the butterfly to compensate for the less dense air. So the net result of thinner air & a bigger hole must be the bigger hole wins & the MP rises.
Third point. When I descend the air density in the manifold increases causing a rise in MP. Having the govenor on, (tries to maintain engine RPM by closing the butterfly slightly) probaby tends to counter the effect by reducing MP. Descents with the govenor off would have even migger rises in MP I guess.
Fourth point. When you climb MP drops. Pull carby heat MP rises. So why don't we just pull carby heat on climb to maintain MP??
OK First point. The MP gauge is a diaphragm sealed on one side with the other side connected to the intake manifold. So its an absolute reading gauge, reading manifold pressure relative to the fixed reference on the sealed side of the diaphragm NOT to atmospheric pressure. That's why, with the engine not running, its rest reading (atmospheric pressure in the manifold) varies with altitude or air pressure changes with the weather.
Second point. When I pull carby heat with the govenor off @ 80% the warmer, & therefore less dense, air should make the MP pressure drop. The fact that it rises must be because the engine slows a bit generating less vacuum. If I pull carby heat with the govenor on in flight the MP still rises. Here the govenor maintains a fixed RPM by opening the butterfly to compensate for the less dense air. So the net result of thinner air & a bigger hole must be the bigger hole wins & the MP rises.
Third point. When I descend the air density in the manifold increases causing a rise in MP. Having the govenor on, (tries to maintain engine RPM by closing the butterfly slightly) probaby tends to counter the effect by reducing MP. Descents with the govenor off would have even migger rises in MP I guess.
Fourth point. When you climb MP drops. Pull carby heat MP rises. So why don't we just pull carby heat on climb to maintain MP??
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From: Denver, CO and the GOM
Originally Posted by 22clipper
OK First point. The MP gauge is a diaphragm sealed on one side with the other side connected to the intake manifold. So its an absolute reading gauge, reading manifold pressure relative to the fixed reference on the sealed side of the diaphragm NOT to atmospheric pressure. That's why, with the engine not running, its rest reading (atmospheric pressure in the manifold) varies with altitude or air pressure changes with the weather.
Second point. When I pull carby heat with the govenor off @ 80% the warmer, & therefore less dense, air should make the MP pressure drop. The fact that it rises must be because the engine slows a bit generating less vacuum.
If I pull carby heat with the govenor on in flight the MP still rises. Here the govenor maintains a fixed RPM by opening the butterfly to compensate for the less dense air.
Third point. When I descend the air density in the manifold increases causing a rise in MP.
Fourth point. When you climb MP drops.
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Originally Posted by Flingwing207
Because the engine makes more power for a given MP, the rotor RPM would rise for a fixed collective setting, so the governer closes the throttle to maintain RPM.
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From: Denver, CO and the GOM
You are correct - the motor does run out of "grunt" at 12,000' - because the MP isn't available.
Think of it this way. Take an engine to 6,000' MSL, read the MP gauge before you start it. Say it reads 24". Now we start it, and run it at wide-open throttle. It shows 24" (we will ignore losses through air filters, ect). We measure the horsepower - say we get 140 HP at 2700 RPM.
Now we take the same engine to sea level. Now the MP gauge reads 30" before we start it. So we start it and run it at 24" MP and 2700 RPM. We will get less than 140 HP. Why? because the engine is working harder to expel the burned gases against the greater ambient pressure at sea level. On the intake side, the engine doesn't know or care why there is only 24" MP available (throttle plate or ambient pressure), but on the exhaust side, the engine at sea level feels like there is a "throttle" on the exhaust manifold.
Of course, if we open the throttle fully on the sea-level engine, we will get more than 140 HP, because there is 30" MP available at sea level, whereas there is only 24" at 6,000' MSL.
Think of it this way. Take an engine to 6,000' MSL, read the MP gauge before you start it. Say it reads 24". Now we start it, and run it at wide-open throttle. It shows 24" (we will ignore losses through air filters, ect). We measure the horsepower - say we get 140 HP at 2700 RPM.
Now we take the same engine to sea level. Now the MP gauge reads 30" before we start it. So we start it and run it at 24" MP and 2700 RPM. We will get less than 140 HP. Why? because the engine is working harder to expel the burned gases against the greater ambient pressure at sea level. On the intake side, the engine doesn't know or care why there is only 24" MP available (throttle plate or ambient pressure), but on the exhaust side, the engine at sea level feels like there is a "throttle" on the exhaust manifold.
Of course, if we open the throttle fully on the sea-level engine, we will get more than 140 HP, because there is 30" MP available at sea level, whereas there is only 24" at 6,000' MSL.
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From: England
Flingwing207
"Actually a rotor and a constant-speed prop act the same (as far as MP is concerned)."
"Actually a rotor and a constant-speed prop act the same (as far as MP is concerned)."
). Where as a helicopter controls the amount of power the engine produces to keep the engine speed constant.
22clipper
Fourth point. When you climb MP drops. Pull carby heat MP rises. So why don't we just pull carby heat on climb to maintain MP??
Fourth point. When you climb MP drops. Pull carby heat MP rises. So why don't we just pull carby heat on climb to maintain MP??
Flingwing207 You are correct - the motor does run out of "grunt" at 12,000' - because the MP isn't available.
Think of it this way. Take an engine to 6,000' MSL, read the MP gauge before you start it. Say it reads 24". Now we start it, and run it at wide-open throttle. It shows 24" (we will ignore losses through air filters, ect). We measure the horsepower - say we get 140 HP at 2700 RPM.
Think of it this way. Take an engine to 6,000' MSL, read the MP gauge before you start it. Say it reads 24". Now we start it, and run it at wide-open throttle. It shows 24" (we will ignore losses through air filters, ect). We measure the horsepower - say we get 140 HP at 2700 RPM.
At sea level you have 30" of map avaliable.
6000' above MSL you have 24" (Say)
12000' above MSL you have 20" (Say)
So this is the maximum the maximum MP you can get with the butterfly fully open. So there comes a point when there is just enough MP avaliable to maintain level flight.
Just had another thought the other thing that will affect things is the rotor. As you climb the air beromes thinner (less pressure) so there is less drag on the rotor. Therefore the power required to keep it at the same rpm is less. Therefore MP will drop as you climb (assuming you keep the colective in the same place). The reverse is also true. As you decend the air gets thicker (more resistance) and so more power required so more MP required.
As you may have noticed these things are not simple and are affected by many things.
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Originally Posted by Keef256
Just had another thought the other thing that will affect things is the rotor. As you climb the air beromes thinner (less pressure) so there is less drag on the rotor. Therefore the power required to keep it at the same rpm is less. Therefore MP will drop as you climb (assuming you keep the colective in the same place). The reverse is also true. As you decend the air gets thicker (more resistance) and so more power required so more MP required.
As you may have noticed these things are not simple and are affected by many things.
As you may have noticed these things are not simple and are affected by many things.
Originally Posted by Flingwing207
Originally Posted by R22 Clipper
Third point. When I descend the air density in the manifold increases causing a rise in MP.
Originally Posted by R22 Clipper
Fourth point. When you climb MP drops.
Last edited by puntosaurus; 26th March 2006 at 09:04.
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From: Denver, CO and the GOM
Originally Posted by Keef256
Just had another thought the other thing that will affect things is the rotor. As you climb the air beromes thinner (less pressure) so there is less drag on the rotor. Therefore the power required to keep it at the same rpm is less. Therefore MP will drop as you climb (assuming you keep the colective in the same place). The reverse is also true. As you decend the air gets thicker (more resistance) and so more power required so more MP required.
Avoid imitations
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It might help to think of the opposite situation - when the heli climbs the ambient pressure decreases.
If it didn't, aviation turbocharger manufacturers would be making a lot less money because their products wouldn't be required.
If it didn't, aviation turbocharger manufacturers would be making a lot less money because their products wouldn't be required.
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From: Sydney
I think I'm on top of it now;
When I pull carby heat the intake air is thinner so the engine slows (or the govenor opens the throttle) causing a rise in MP.
When I climb the intake air is also thinner which should tend to raise the MP. But other factors (less atmospheric pressure on the exhaust, blades easier to turn) overcome this trend leading to a net rise in MP.
Its interesting isn't it when there are competing trends how one wins out. Like when you climb the air gets thinner, its gets colder too which should make it more dense, but the loss of molecules is greater than the decline in their speeds which means ultimately less density.
When I pull carby heat the intake air is thinner so the engine slows (or the govenor opens the throttle) causing a rise in MP.
When I climb the intake air is also thinner which should tend to raise the MP. But other factors (less atmospheric pressure on the exhaust, blades easier to turn) overcome this trend leading to a net rise in MP.
Its interesting isn't it when there are competing trends how one wins out. Like when you climb the air gets thinner, its gets colder too which should make it more dense, but the loss of molecules is greater than the decline in their speeds which means ultimately less density.