General Knowledge Question - Get yer brains working!

[joker]

This being a flight training forum, I thought I'd pose a couple of general knowledge questions on subjects which are common difficulties in checkrides, which some of you might like to think about! Any answers welcome...let's see where this goes.

 

 

Question 1: When you do a magneto check, why does the manifold pressure always go up?

 

Question 2: Suppose you're cruising at 4,000 feet, full throttle, RPM normal, and you raise and lower the collective so the RPM varies between the limits of the operating range. What would you expect to see the MP do during this huge variation in RPM?

[flingwing206]

OK Joker, here's a couple more for the turbine folks:

 

3. If you turn on the bleed-air cabin heat, what will the result be (besides a warmer cabin)?

 

4. Will it take less torque to hover OGE with the bleed air heat off?

Edited February 11, 2006 by flingwing206

[Eric Hunt]

Q1. Can't say i ever noticed it. But pistons were a long time ago...

 

Q2. If at full throttle, there ain't no more MAP? So it stays constant?

 

Q3. T5 goes up, N1 goes up, N2 stays same

 

Q4. TQ should be the same, but at a higher N1 and T5 - you might top out under some circumstances.

 

Ah, it's nice to live in ignorance of such things...

[West Coaster]

Question 2 (R-22);

 

In a Robbie at least, the MAP is the equivilant to Torque in a turbine. Raise the collective and MAP will rise, lower the collective and it will drop. Should be a dead give-away from the power checks that should be done in the hover before departing on the flight.

 

Question 5: You're flying a Robbie and raising/lowering collective produces the opposite effect in regards to MAP. Why?

[Guest pokey]

#1. Not really sure IF i ever noticed it going up ! ( too busy looking at the tach i guess)? But getting back to the basics, the manifold pressure gage is quite a bit more complicated than one would 1st think. It's main function is to moniter air pressure on the engine side of the throttle plate. With the engine NOT running, the pressure is atmospheric. A RUNNING engine however is below atmospheric due to the position of the throttle plate/verses the piston sucking air into the cylinder. With this in mind, my only conclusion is that since you get an RPM drop during mag check, the airflow into the cylinders must be less too. (less air=less suction=more manifold pressure) When looking at the problem this way, & always being taught that manifold pressure is related to power output/availability, the manifold pressure gage could be thought of as an "airflow gage", since the engine is nothing more than an airpump anyhow, so?,, NO ( or less) airflow would tend to get back to atmospheric pressure.

 

If anyone else can explain it? be my guest.

[Guest pokey]

question #2, dis-regarding correlation? and any "ram effect" i would say should be along the same lines as my reply to question #1, any loss of RPM due to increased pitch=higher manifold pressure. Remember that (dis-regarding ram) an engine will never "pull" atmospheric pressure ( due to friction losses in the intake syatem,, ie air filter, bends,,,etc)

 

WestCoaster? i agree w/ increasing collective increases manifold pressure----ALTHO? i think what you are seeing is due to the "correlater" ( the mechanical device that adds throttle as you pull pitch) WIth the engine off, standing outside the ship, & throttle locked at say 1/2 open,, pull on the collective as if you were bringing it into a hover,,, does the throttle open @ the carburetor? ( i have very limited experience w/ the R22, but i know the 300 will & has fairly good correlation)

 

AGAIN, i must say,,,, i find this to be an interesting topic & is alot more to it than one would 1st think.

 

 

let me add a hypothetical question to this thread ? "question 2A"

 

Suppose we have an idling engine, say 15 inches of manifold pressure on a standard day (29.92 inches) & we have the ability to continously add pitch (but no throttle) till the engine cant produce the power to sustain RPM & ( the engine) stalls. What would you see on the manifold pressure gage? compared to the tach? AND? would it matter if it was at wide open throttle, rather than idling?

[West Coaster]

Totally agree, in flight the throttle governor and correlator is what increases MAP with pitch as the engine has to work harder to sustain RPM with the increased drag on the blades. (Pretty much answers what could be wrong if the opposite happens in flight.... governor failure, though not the only possibility)

 

Just a thought though on your comment about the MAP being an airflow gauge. If a heli is flying downwind you can expect a higher MAP reading as more power is needed to maintain altitude. At the same time if flying into wind the MAP will also read slightly higher to maintain ground speed. Airflow is different in both cases but the result to MAP is the same. It can be used though to gauge air density. If on one day you hover with MAP of 20, and today with 23 (assume all other factors equal) then you can bet that air density has dropped quite a bit and plan the rest of the flight accordingly. Although also consider a mechanical problem if all factors are equal both days.

 

I'll take a stab too at your 2A:

If at idle and you pull full pitch without addition of throttle I doubt the drag would be enough to stall the engine, though I could be wrong. You might see a drop in MAP though as well as a drop in RRPM. Never thought about it at idle before. I think the effects would be more pronounced at full throttle as drag on the blades at flight idle is huge compared to ground idle.

 

Good thread, good questions!

Edited February 11, 2006 by West Coaster

[flingwing206]

Piston engine:

 

With constant RPM, opening throttle increases MAP (and more HP is produced).

 

With fixed throttle, reducing RPM increases MAP (and less HP is produced).

 

Wide-open throttle and maximum RPM will produce most power, but not highest MP.

 

Wide-open throttle and minimum RPM will produce less power but higher MP.

[joker]

My quick answer to question 2 is as Eric Hunts...

 

As you are already at full throttle (butterfly is fully open) the pressure behind the butterfly, where the MP gague sender is, is as close to ambient pressure as its ever going to get.

 

Thus, changing the RRPM is not going to make any (noticable) effect on the MP which will already be about 1.5-2" less than ambient.

 

When I get back home, I'll add some more...

 

Joker

[delorean]

Here's another one:

 

On preflight, at what altitude would you set the altimeter to check the accuracy of the Manifold Pressure Gauge's static reading?

 

HINT: Field/current elevation is dead wrong.

[flingwing206]

As you are already at full throttle (butterfly is fully open) the pressure behind the butterfly, where the MP gague sender is, is as close to ambient pressure as its ever going to get.

 

Thus, changing the RRPM is not going to make any (noticable) effect on the MP

I seem to remember this topic being bandied about a certain coffee table a while ago! What if the RRPM goes to zero? I realize that you wouldn't be flying for long, but it is the logical extension of the argument.

 

I'll go along with what you say if RRPM is running in the green - there will be a negligable MP change.

[PhotoFlyer]

Here's another one:

 

On preflight, at what altitude would you set the altimeter to check the accuracy of the Manifold Pressure Gauge's static reading?

 

HINT: Field/current elevation is dead wrong.

 

I believe it's density altitude.

[delorean]

I believe it's density altitude.

 

Nope, not pressure altitude either.......

[67november]

now I'm ratteling the brain cells here, but the test should be run at standard barametric pressure of 29.92 inches when checking the calibrated setting of said piece of equipment.

[Guest pokey]

Delorean, i remember this question from many many years ago in A&P school--(UNFORTUNATLY, i forget the answer !) son of a ! Atho i believe it was along the lines of local pressure corrected for sea level pressure,, OR something like that

[joker]

What if the RRPM goes to zero? I realize that you wouldn't be flying for long, but it is the logical extension of the argument.

 

Fling...I think what you are getting at here, is that if you kept (theoretically) slowing the blades down somehow, until the RRPM was at zero, you would also reduce the sucking power of the pistons. This is a classic example of why it is not accurate to say that MP is a measure of 'power' unless you take all the other factors into account and certain conditions exist. In this example, now the pistons are not sucking, so there is no 'airflow' through the venturi. So this means no fuel flow either....MP is still high, but power is zero!! Is that what you were thinking?

 

 

On preflight, at what altitude would you set the altimeter to check the accuracy of the Manifold Pressure Gauge's static reading?

 

Delorean, good question...!,

 

The MP should show the ambient pressure (or near enough).

 

Thus I think you must have to set the field elevation. Then you must (as Pokey says) adjust it 1" per 1000' of elevation. That will give you the figure pressure in the Kollsman window which should correspond to the MP guage before start up.

 

Joker

 

In fact, having a general idea of what the 'pre-start', 'idle' and 'operating' MPs are is an important factor which is often overlooked by helicopter pilots. (Be honest, how many could say right now what the MP is at those moments?!)

 

By knowing these approximate values, it is easier to spot the signs of a Manifold Intake Leak.

 

Anyone who has done any 'internet' research in the last few days will have come across this article written by John Deakin.

 

Manifold Pressure Sucks by John Deakin

 

In fact, I have come across a different version of that same article (adapted by a friend of mine with John Deakin's permission), which is more specific to helicopters. I'll try to find a way to post it up here.

[joker]

Is this going to work??...Nope...attachment too big! I'll have to think of some other way.

[delorean]

Here's another one:

 

On preflight, at what altitude would you set the altimeter to check the accuracy of the Manifold Pressure Gauge's static reading?

 

HINT: Field/current elevation is dead wrong.

 

Pokey, yep, we had this one in A&P school as well. Stumped us all.

 

THE ANSWER: Set it to ZERO ft elevation. The pressure in the Kollsman window will be ambient pressure, which what the MAP gauge should show if the engine is static.

 

The stumper is that barometric pressure (or "altimeter" by ATC terms) is the local ambient pressure ADJUSTED for sea level. It doesn't matter what airport you're at in the country in terms of elevation, a standard day is ALWAYS 29.92" Hg.

 

If you're sitting on the ground at an airport with a 5000' MSL elevation, and it's a standard day, ATC will give an altimeter setting of 29.92", right? But that's not the AMBIENT pressure--the ambient pressure is 5,000 divided by 1,000 ft per 1"Hg.....so......5" off 29.92 = 24.92" ambient pressure.

 

Your altimeter only has a range of 28.00" - 31.00" so you have to do the math for this this check if you're at an airport above 2000 ft MSL. If you're below that, just set it to ZERO and voila!

 

I forgot to post this, but it led into what was going to be my next question......Is it illegal to fly in reported barometric pressures above 31.00" Hg?? Yep, FAR 91.144 (see below). It's because altimeters can't be set that high. Don't worry, this only rarely happens during winter months in the northern Alaska.

 

This thread is fun!

 

-Jonathan

 

 

-------------------------------

 

§ 91.144 Temporary restriction on flight operations during abnormally high barometric pressure conditions.

top

(a) Special flight restrictions. When any information indicates that barometric pressure on the route of flight currently exceeds or will exceed 31 inches of mercury, no person may operate an aircraft or initiate a flight contrary to the requirements established by the Administrator and published in a Notice to Airmen issued under this section.

 

( Waivers. The Administrator is authorized to waive any restriction issued under paragraph (a) of this section to permit emergency supply, transport, or medical services to be delivered to isolated communities, where the operation can be conducted with an acceptable level of safety.

[joker]

Thanks Delorean (and everyone else who has chipped in),

 

Doesn't my method get you too the same number....I'm having a brain fart!

 

That 91.144 is one of those often forgotten FAR entries...

 

This thread is fun!

 

I love the excuse to get my head back into the books.

 

If people want, I will post a semi-closed question on a semi-regular basis....sort of, pose the question, let the thread run its course, then pose another! Try to keep one active at a time, so its not 'death-by-questions' but enough to give people something to think about when they're bored! Questions on different subjects and aimed at different levels. What do you all think?

 

It could be fun.

 

Joker

[flingwing206]

Fling...I think what you are getting at here, is that if you kept (theoretically) slowing the blades down somehow, until the RRPM was at zero, you would also reduce the sucking power of the pistons. This is a classic example of why it is not accurate to say that MP is a measure of 'power' unless you take all the other factors into account and certain conditions exist. In this example, now the pistons are not sucking, so there is no 'airflow' through the venturi. So this means no fuel flow either....MP is still high, but power is zero!! Is that what you were thinking?

Yep. Since there is always resistance to the intake airflow, even at WOT, the higher the intake airflow rate (higher engine RPM), the lower the manifold pressure. The opposite is true for lower RPM (less airflow, higher MP).

[Guest pokey]

I love the excuse to get my head back into the books.

 

If people want, I will post a semi-closed question on a semi-regular basis....sort of, pose the question, let the thread run its course, then pose another! Try to keep one active at a time, so its not 'death-by-questions' but enough to give people something to think about when they're bored! Questions on different subjects and aimed at different levels. What do you all think?

 

It could be fun.

 

Joker

 

 

Sounds like a plan Joker.