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Enlighten me please!

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Dec 4, 2001
I've got two questions for you guys:

1. One of my students asked me a very "simple" question today."What is manifold pressure?"
My understanding is that there is a probe in cylinder nr 3 (in our case a 4 cyl. horizontally opposed reciprocating engine;180 hp) that measures the MAP. Is this a measurement of the intake manifold?

2. I've heard rumors that this question has been asked on interviews: How does icing affect Vmc?
Is this really the question they asked? It seems to me that you need a little more info about the icing to answer the question. Am I wrong?

Any input would be greatly appreciated.
Manifold Pressure equals inches of mercury.

When the engine is not running, the gauge reads around 30", roughly sea level pressure. Standard pressure is 29.92 at sea level.

When the engine is at idle, it reads around 12 or so.

What does this mean. The basics of it means that the manifold press guage reads like a barometer, in inches of mercury.

When you add full throttle, say on a C-182, you will get around 30", sea level pressure if you are at sea level, as you climb higher you notice that the manifold press drops off. Higher you go, less pressure.

The air pressure is rich with air and inturn requires a rich mixture setting at takeoff. As you climb higher the atmospheric pressure decreases inturn requiring a leaner fuel mixture, this is why you lean the engine when you go higher.

If you ever have flown a turbo charged airplane, you will notice that you can maintain a sea level (30") pressure or higher up at high altitudes, the turbo is sending compressed air (like sea level pressures) to the cylinders so it can maintain power .

You are correct, the MAP gauge is measuring the air in the intake to the cylinders.

Hope it helps,
2. Vmc v ice

My thoughts are such...

If you ad ice to an aircraft you are adding weight. When you add weight you must increase the angle of attack to maintain altitude. When you increase the angle of attack you are creating more lift. If you create more lift and you are using proper technique (3-5 deg bank into the good engine) then you will increase the horizontal and vertical components of lift. If the horizontal component of lift is increased then you counter the yaw created by the good engine by that much more. However when you increase the AOA you also increase P-factor. My understanding is that P-factor increases more than the horizontal component of lift and thereby increases the yawing moment. If the yawing moment increases then more airspeed is need to create enough rudder effectiveness. That means your Vmc is going to increase as the ice builds up.

That advice was free and I think you might have got your money's worth.
Heaven help me - I'm weak.

I know I shouldn't answer questions like this when asked supposedly by a 1000 hr CFI/II/MEI.

1.) Manifold Pressure is indeed a measurement of the intake manifold pressure. Let's answer your question by requiring you to look up another related topic:

You, a 1000 hr CFI, are flying along in instrument conditions in a bugsmasher. All of a sudden you notice a dummy light that states that your Vacuum pump failed. Thankfully, the owner of the plane has installed a standby vacuum system. It also is based on the intake manifold of your airplane. Question: Do you run the airplane at full throttle or at minimum throttle to stay at altitude? When you look up the answer by contacting the nice people at Precise Flight or one of the other standby manufacturers, you will understand so much about MP that you will become the resident field expert!

2.) When discussing Vmc a simple rule applies - anything that increases the power of the one working engine or decreases the effectiveness of the airfoils is bad for the pilot (raises the Vmc). Anything which weakens the strength of the engine or increases the effectiveness of the airfoils is good for the pilot (lowers the Vmc). When you are in a Vmc situation, you are limited by the rudder effectiveness being able to counteract the good engine which is trying to pull the airplane into a spin.

In an interview, your knowledge of this is being tested so you must explain your answer. If you are thinking of engines, the icing hitting the air intake/filter/need for carb heat would probably decrease the effectiveness of a recip. In a turbine, anti-ice may rob 1-2% of power. If you are thinking of the airfoils, without some sort of anti-ice, the airfoils get loaded with ice and hence you need a higher airspeed to create an effective lifting force.

I would discuss these matters and then state that the best course of action would be to keep an adequate margin of safety in airspeed and exit the icing conditions as quickly as possible.

My .02 - back to ironing shirts for a 4-day trip!
# 1. The manifold pressure is the pressure mesured in the intake manifold of the engine. when the engine is running at idle the butterfly valve is almost closed and the air has to pass it at a fast rate to get through there by causing a pressure drop through the carb and the intake manifold, thats why the manifold pressure is low at idle. when you put full throttle the butterfly valve is fully open the air is going in without any obstruction so the pressure almost equals to the ambient pressure but it can never be more than that unless you have a super charged engine.

#2. Icing on the wings will increse subtantial amount of drag and to over come that u'll have to use the power and as u know more the power higher the Vmc. ;)
I hope that this helps....

Check that its three greens.......
Manifold pressure is the pressure inside your intake manifold, pure and simple. Nothing more, nothing less.

Your manifold operates like a vacum cleaner hose. Your engine is the vacum cleaner. Close the throttle, and it's like putting your hand over the vacum cleaner hose opening. Pressure drops; the suction created by the engine continues, and there's no air passing the throttle butterfly to feed it. Pressure drops.

Open the throttle all the way on a normally aspirated engine, and the best you'll get is ambient air pressure. At sea level on a standard day, you'll get 29.92". It drops approximately an inch per thousand feet, so at five thousand feet, expect about 24 inches, max. (This is the primary reason that engines won't produce more than 75% power above approximately 3,000' density altitude, and consequently manufacturers recommend against leaning below that altitude, and recommend leaning above it).

The velocity of air through the carburetor or intake doesn't affect manifold pressure, except for a slight increase at higher airspeeds and propeller RPM's due to ram air rise.

As for Vmc and icing, it all depends on the icing. If you're talking engine icing, then this affects power output, and consequently Vmc. If you're talking airframe icing, it depends on the specific circumstances. Vmc is largely predicated on the moment arm of the rudder, against the moment arm of the critical engine. Any change in the moment of the thrust produced on an engine, or the CG arm for the rudder, will result in a change in Vmc.

Increase the CG arm, and Vmc goes down. Increase power on the critical engine, and Vmc goes up.

However, Vmc is the airspeed at which directional control can no longer be maintained with the critical engine windmilling, the good engine at takeoff power, gross weight, aft CG, banked 5 degrees into the good engine, and decreasing airspeed at the rate of one knot per second until directional control is lost. Any change from this produces a speed at which directional control may be lost, but it not actually Vmc. Therefore, ice may increase or decrease the speed at which directional control may no longer be maintained, but may or may not affect Vmc.

Generally speaking, ice will affect stall speed more than Vmc. Vmc by the definition in FAR 1.1 only stipulates that it is the minimum control speed with the critical engine inoperative. Under that more general definition, the Vmc speed will vary with ice, but stall speed will vary more. Other factors that can come into play are the uneven buildup of ice, tailplane icing and stalls, etc. These all affect controllability.

Ice decreases rudder effectiveness, and also decreases propeller efficiency. The rate at which these primary affecting factors occur can play a big part in weather or not Vmc appreciably changes with ice acceration.

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