Manifold Pressure Question

[DC8 Flyer]

..NVM

 

[VNugget]

Almost. Remember when we lower the RPM we lower the rate at which the mass of air enters the cylinder (2700 RPM - 45 times a second 2200 RPM = 33 times a second). We dont change the mass we simply change how many times it enters the cylinder over a given time period. Since Horsepower is work over time, the lower the rate at which the fuel is burned lowers the work over time equation. So we are changing power, otherwise we could take off at 2200 RPM and have the same power as 2700 RPM.

Hmm... come to think of it, you're right. I forgot about that.

 

[Donsa320]

Almost. Remember when we lower the RPM we lower the rate at which the mass of air enters the cylinder (2700 RPM - 45 times a second 2200 RPM = 33 times a second). We dont change the mass we simply change how many times it enters the cylinder over a given time period. Since Horsepower is work over time, the lower the rate at which the fuel is burned lowers the work over time equation. So we are changing power, otherwise we could take off at 2200 RPM and have the same power as 2700 RPM.

Just keep in mind I am talking about the same airplane at the same point in space and time just one instance is 2700 RPM and the other is 2200 RPM, throttle and mixtures are the same only difference is RPM by prop control.

Whoops, sorry, you would get the same MAP change if you did it in flight with the engine not running, just windmilling in idle cutoff. Anytime you are at Part Throttle the throttle plate restriction is the reason the MAP changes with RPM changes. Avbug is totally correct. It has nothing to do with engine power. The engine is just an airpump in this situation.

~DC

 

[DC8 Flyer]

..NVM

 

[Donsa320]

Nope, sorry. All the MP gauge measures is absolute pressure at the intake valve (thats where the sensor is). If the engine is windmilling, ie no power, there is a very slow rate of "suction" thus the largely scaled MP gauge would read atmospheric pressure (absolute pressure in this case).

You guys need to get off this idea that just because the engine sucks air in, that it is creating a vacuum, if that where true the engine would be starving itself because in a vacuum there is no air.

You are stubborn, aren't you. I have done this exercise for over 50 years and whatever you are trying to say is WRONG! Man, wrong beyond my comprehension. Asquared, Avbug, the Marine guy...anybody... help. <bg>

Seriously, go get a constant speed equipped airplane and try this stuff out. Whatever you are reading in books is not cutting it!

~DC

 

[DC8 Flyer]

..NVM

 

[avbug]

Donsa,

He's quite wrong, but can't apparently see the forest through the trees.

First of all, the point at which manifold pressure is sampled varies with the engine installation. I've never seen a manifold pressure "sensor," though most all installations from small four cylinder horizontally opposed engines to large twenty eight cylinder radial installations use direct tube installations (bourdon tube). This may sample at any point in the intake manifold downstream of any boost device, and may sample at the cylinder, or at any other select point in the intake manifold. (That's why it's called manifold pressure, not intake-at-a-single-cylinder pressure...but you probably already knew that).

Second, the engine is indeed an "air pump." In a direct drive system, propeller RPM is crankshaft RPM, and it matters not in a normally aspirated engine weather the engine is driving the propeller or the propeller is driving the engine with respect to indicated manifold pressure. Indeed, shutting off the engine and letting the engine windmill with a closed throttle will indicate manifold pressure below barometric...how far below is entirely dependent upon engine RPM.

Shut off the engine and let it windmill with the throttle plate closed, you'll see the same low manifold pressure that you see at idle with the engine running. Pull the nose up, slow the prop, note manifold pressure climb as engine RPM decreases. Not because of a change in airflow velocity through the induction system but purely as a result of a decay in engine RPM...the air pump is slowing down...just like slowing down the proverbial vacum cleaner motor.

 

[DC8 Flyer]

..NVM

 

[gern_blanston]

You two are talking about two different phenomena, engine running, engine windmilling. The windmilling engine rise and fall in MP is due to expansion since the velocity of the air through the manifold is very slow 300 to 400 RPM.

The intake manifold doesn't care whether the engine's 'running' or 'windmilling', just that it's 'pumping'.

Now lets get back to the OP's original topic. Why does MP rise when he does a prop control check on runnup. Because with the throttle plate fixed as the air pump (piston) slows down the speed at which the volume of air is moving through the manifold decreases, raising its pressure.

Doesn't the volume of air entering the intake manifold decrease in proportion to the RPM change, minimizing any velocity change? It IS a positive-displacement pump, after all.

I'm gonna' have to jump in with Avbug and Donsa. I don't think there's enough of a velocity change to make that much difference. You're the physics whiz, so what's the formula for Pressure Delta -vs- Velocity Delta? I mean what sort of velocity change would be required to drop the pressure 60%, like going from 29" to 10" of manifold pressure when you go from full throttle to idle?
I love math, so let's see the numbers. Thanks.

 

[USMCmech]

You two are talking about two different phenomena, engine running, engine windmilling. The windmilling engine rise and fall in MP is due to expansion since the velocity of the air through the manifold is very slow 300 to 400 RPM.

Windmilling or running, the MAP will vary with trottle movement. A fact that many pilots don't realize. I routinely cover this when I do a BFR (what will happen if the engine quits, prop keeps spinning, ect.).

Now lets get back to the OP's original topic. Why does MP rise when he does a prop control check on runnup. Because with the throttle plate fixed as the air pump (piston) slows down the speed at which the volume of air is moving through the manifold decreases, raising its pressure.

AvBug, you really need to stop and think about what you are saying and read a book. Just because something sucks air in, doesnt mean it lowers the pressure entering or raises just because its a pump, there are Physics laws for the changes in pressure and you as a pilot should know them. I've given you multiple examples, formulas, Fluid Dynamics laws, its your turn to come with something other than "I said so". Anyone who flys an airplane with variable pitched props and a manifold pressure gauge know as you pull the props back the MP rises, there is only one way to describe it, and its not because the engine is a vacuum cleaner sucking air, therefore the pressure rises because the engine isnt sucking as hard. What does that explain? Look at a vacuum system and how it works, given suction to maintain an airstream velocity. Same as your precious vacuum cleaner analogy but pressure falls as engine speed is increased so it is vented overboard.

http://www.avweb.com/news/columns/182081-1.html

Deakin speaketh the truth.


Througout the intake system, from the trottle plate aft, the MAP is a measure of how much suction the "downward" moving piston is creating. There is very little change in total velocity in this system (the venturi is fwd of the throttle plate). The air pressure is trying to colapse the intake manifold (think about sucking a milkshake through a straw) because the piston is creating a vacum and the throttle is not allowing enough air to fill it.