MAP Gauge does not show "power"

Available pilot literature says that the MAP gauge reads out power being used, and can therefore be used to determine how much more "power can be pulled" (i.e. up collective.)

This is a common misconception that will be covered shortly. The reason for this document is covered in the last section.

Caveats

Roadmap

A simplified internal-combustion refresher

The engine is actually one big sucking machine. As the starter engages, the pistons move, and in their intake phase they suck air from the intake manifold. The throttle controls how much air can go through.

It's not necessary to discuss the other parts of the engine as they really don't relate to the MAP gauge here.

Just remember, the engine is one big air-sucking machine, and the intake manifold is where the air comes from, and the throttle butterfly-valve is what restricts that airflow.

The intake manifold venturi

Depending on design, either before the throttle butterfly valve or after exists a venturi that takes the air from the "outside" and expands the pipe diameter thereby decreasing the ambient pressure (but of course not changing the volume).

Intake Pessure

The intake pressure is external ambient air pressure. This is 29.92 in hg at 0ft and 15C. Ambient air pressure decreases by "about" 1in hg per 1000ft but of course since the atmosphere doesn't end at 29,920ft it's a nonlinear relationship. For heli pilots flying at or below 10,000ft this is accurate to within 0.5in hg (at 15C).

Map Gauge and Intake Pressure

The MAP gauge has a sensor that measures this air pressure. This is why when the engine is not turning, the MAP gauge displays the ambient air pressure. Yes, that's right, it's a barometer. Yes, you can check it against your altimeter.

Checking Your Altimeter Against the MAP Gauge

  1. Using the altimeter adjustment knob, set the pressure to 29.92in hg (pressure altitude).
  2. Read the altitude in thousands of feet
  3. Subtract that from 29.92 (or 30 to make it simpler)
  4. That is what your MAP gauge should read.

Verifying Discrepancies

If your MAP gauge reads as expected skip this section.

If your MAP gauge does not read as 30-(alt/1000) then one of the gauges is in error.

Get your altitude. Possible sources are field signs, ATIS, sectional charts, your memory from the last time you flew, etc.

GPS receivers can make this easy to figure out.

  1. Get your "exact" altitude. Note that since we only care about 1,000ft units, if you are using a GPS so long as the GPS reading is within 500ft its reading is "exact enough for this.
  2. Subtract your exact altitude in 1,000ft units from 29.92.
  3. If your MAP gauge matches the number, your altimeter is off.
  4. If your MAP gauge does not match the number, note by how far it is off, and remember to make that correction mentally every time you look at the MAP gauge until you can get it fixed.

Post-Venturi

Past the venturi is an area of lower pressure. The simplest reason for this is that this lower intake manifold pressure provides a "suction source" which allows the carb to identify how much air is being sucked in, and feed fuel accordingly.

There is also another MAP sensor that reads this pressure. The MAP gauge then displays the difference between the two as a positive number.

The MAP gauge shows a pressure DIFFERENCE

The higher the number, that means there is a huge pressure difference, meaning there is a lot less than ambient pressure in the intake manifold, so the throttle is either closed to some degree, or the engine is working so hard it's sucking so much air that not enough air can make it to satisfy the engine.

The lower the number, that means pressure after the venturi is exactly the same as ambient pressure. This will never occur at helicopter operating alt ;)

What do various reading ranges mean?

The throttle is NEVER fully closed. If it was, the engine could not idle. However, the reason MAP gauges don't show large numbers at idle is because the engine isn't rotating quickly and there is no power demand on it, so it is not sucking in a whole lot of air, so there's not a big suction past the venturi. Hence the pressure diff is not large.

When seeing smaller numbers, the engine is not air-deprived.

When seeing larger numbers, the engine is not getting the air it wants.

If the throttle could ever be fully closed, MAP Gauge would show barometric pressure.

NOTE: It's clear that if all the MAP Gauge did is show difference between two sensors, then with engine off it should show zero. However, when there is no difference, it just shows the reading. When there is a difference it shows suction (negative air pressure) as a positive number in units of in hg.

What is MAP anyway?

Manifold Absolute Pressure

How do the controls affect it?

Before we get to that...

Why did you bother writing this document?

The current published literature makes mention of the MAP Gauge only in terms of function. It also indicates the controls in terms of function (e.g. "more prop pitch increases MAP" and "higher throttle increases the MAP". The other literature not supplied to pilots and even Internet resources (ask.yahoo.com, wikipedia, etc.) do not explain the real relationship.

Further, since rotary-wing pilots don't vary RPM, the only control is the blade pitch (collective) and understanding how the same collective setting can provide totally different MAP readings is crucial.

What do the controls do?

At idle, there is no demand for the engine, and RPM will sit in the idle range. (75% on a Robinson R22). The MAP will be somewhere around 11-15. The throttle is not wide open, but the demand on engine power is slight (keep the rotor turning) so the amount of suction (air-starvation) is slight.

Remember, even full atmospheric pressure at sea level (29.92in hg) is only 14.7 PSI, or half that found in your car tire.

Once the helicopter is about to hover IGE the increase in collective pitch causes a greater resistance on the rotor, and more resistance to the engine rotating, and more torque requirement, and therefore to keep the RPM in the range, the engine wants more air and more fuel and the suction increases (MAP gets higher).

Once IGE hover is established, MAP is high because of the inefficiency of a rotor working in its own downwash.

Once forward flight is established, MAP immediately reduces even though not only is the same altitude being maintained, but forward motion is ALSO being exerted, (and consequently ALSO TR resistance increased to compensate for MORE torque). You'd think that with all this "requirement for more power" MAP would go up. Nope, it goes down. Why? The actual energy being exerted is less because the rotor efficiency is much greater.

Now you start climbing, again MAP goes up. This is because the demands of climb are more than that of forward flight so the engine is air-deprived. If you COULD give it more throttle it would be happy (fixed-wing guys CAN. We heli guys can't.) So we stay within our operating parameters, and maybe get some more forward speed to make our rotor more efficient.

OGE Hover until settling (VRS). Lots of "demand for throttle" but satisfied by throttle so no high MAP.

Rules of Thumb

I hope that was helpful. Any additions or questions, drop me a line at the following in reverse: net dot wetwork at gavron.

Initial Version: 2009-01-23

Ehud Gavron