Negative Density Altitude

PTK · November 4, 2016

We as pilots tend to discuss and be mindful of high DA ops. But we really don't talk too much about low DA. It's that time of year when we NA folks get to enjoy really low or negative DA and all the performance gains that go with it! On a recent flight with DA well in the negative I was at WOT with CHT's well up in the 380's and climbing. I have my EDM set to alarm at 400. I actually had to throttle back a bit to bring them down. This was interesting because it was on takeoff and it got me thinking.

Given that:

our NA power plant can make 100% power under certain conditions: std day, max RPM, at sea level,
Barometric pressure for NA engine equates to MP,
MP is directly proportional to power,
a small deviation in baro pressure has a huge effect on DA. (<= 1 inch Hg can swing DA ~1000 feet),

Taking these into consideration can we actually overboost our NA engines at full throttle with really low or negative DA?

**carusoam** · November 4, 2016

Yes we can over boost an NA engine. Cold high pressure days are dangerous ICP makers. Broken cylinders are the usual result. The head can crack and separate.

There are limits and guidelines.... now, Where to find the details...

Best regards,

-a-

201er · November 4, 2016

Not in NJ

mpg · November 4, 2016

an example ive wondered about recently,,,

I check the atis at my 1000 ft msl airport, it says 29.92, i dial that in, altimeter says 1000 ft, its warm in the sun, temp is 15 C... my engine has 28.92 manifold pressure to work with..

I take off and fly for awhile and land at copalis beach, its a sand runway at the ocean. now at sea level my engine can have all of the 29.92 manifold pressure,,,, cool...

another day in the winter the atis says 30.92, altimeter again says 1000 ft msl. i fly for awhile and land at copalis again,,, i ponder that DA stuff i read about. i reset the colsman window to 29.92 and see my pressure alt at -1000 ft, now my engine has 30.92 manifold pressure to work with,,, and its cold, my OAT says 0 C,,, sooo,, i whip out my e6b and dial in my -1000 pressure alt and my OT of 0C and find a DA of -3350 ft msl,,,, wow! my engine now has 33.5 manifold pressure to make power out of

kortopates · November 4, 2016

Of course we have real below sea level airports here in the west:

Death Valley,CA L06 -212' msl

Thermal, CA KTRM -115' msl

but then our winter lows aren't very chilly

Andy95W · November 4, 2016

58 minutes ago, mpg said:

an example ive wondered about recently,,,

I check the atis at my 1000 ft msl airport, it says 29.92, i dial that in, altimeter says 1000 ft, its warm in the sun, temp is 15 C... my engine has 28.92 manifold pressure to work with..

I take off and fly for awhile and land at copalis beach, its a sand runway at the ocean. now at sea level my engine can have all of the 29.92 manifold pressure,,,, cool...

another day in the winter the atis says 30.92, altimeter again says 1000 ft msl. i fly for awhile and land at copalis again,,, i ponder that DA stuff i read about. i reset the colsman window to 29.92 and see my pressure alt at -1000 ft, now my engine has 30.92 manifold pressure to work with,,, and its cold, my OAT says 0 C,,, sooo,, i whip out my e6b and dial in my -1000 pressure alt and my OT of 0C and find a DA of -3350 ft msl,,,, wow! my engine now has 33.5 manifold pressure to make power out of

Nice analysis. Thanks for posting it.

M016576 · November 4, 2016

29 minutes ago, Hyett6420 said:

So how does Holland handle this. They have cold winters, most of the country is below sea level and yet from my knowledge they dont have a plethora of cracked cylinders.

Probably by setting standard day manifold pressure or thereabouts when it gets cold out. Or they just fly them standard and it's a non-issue.

Or maybe they have a government mandated top overhaul spread equally amongst all pilots every 200 hours, except the richest 45% of owners pay for the poorest 65% of owners top overhauls. That would mitigate cracked cylinder risk for the Dutch pilot population

**carusoam** · November 4, 2016

When Amsterdam gets 0°F weather, they get concerned as well... realistically they are only a few negative feet AGL. They have similar weather to the pacific North West. Westerly winds come in off the ocean. Often getting warmed and hydrated by the ocean. Their closeness to the arctic circle is masked by the warmer temps. But, not hidden by the early sundown in the winter.

The low altitude is not much, but Enough, that it shows up on the GPS altitude instruments at Schiphol. The GPS antenna is on the top of the plane's fuselage. it reads a negative number in tens of feet.

This does serve a good point about the difficulty of getting a grasp on density altitude. The weather changes the density of the air. And density altitude is a way to describe the serious nature of it.

Too high, we run out of power in some cases... a challenge around NJ.

Too low, we can get excess power, even in NJ... but you have to have real drive to come from NYC to want to get in a plane that has been stored outside over night in 0°F temps. Real pre-heat and cabin heat recommended... cabin heat for instrument bearings, of course...

Part of the hard to measure challenge is the coldness that comes with that. The engine and it's systems have to come up to temperature from a really low number. All the gaps and clearances are at their minimums and the viscosity of the oil is at it's maximum.

Somebody posted a photo of a cylinder separation around here years ago. The head came off the cylinder...

The excess power is noticeable in a C as it accelerates down the runway, and the associated climb is fantastic!

For anyone keeping track, I don't go outside on those days much...

doing some arbitrary math...

If I had 280hp @ 28" of mp, I would get 310hp available at 31" of MP. A 10% increase... at no extra charge...

Generally speaking if your MP gauge rises out of the normal range, it is OK to throttle back into the normal max power range for T/O. This can be done during the T/O run as if you were running a TC'd engine with no MP controller on it...

So, if 1" of MP is equal to 1,000’ of altitude... getting three extra inches of MP (my sample above) would equate to -3k' of DA, or so...

Overboosting can have some extra cylinder costs to consider...

looking back on that experience, I was really dedicated to flying... even with a low flying budget.

Best regards,

-a-

M016576 · November 4, 2016

Just now, carusoam said:

When Amsterdam gets 0°F weather, they get concerned as well... realistically they are only a few negative feet AGL.

Enough that it shows up on the GPS altitude instruments at Schiphol. The GPS antenna is on the top of the plane it reads a negative number in tens of feet.

This does serve a good point about the difficulty of getting a grasp on density altitude. The weather changes the density of the air. And density altitude is a way to describe the serious nature of it.

Too high, we run out of power in some cases... a challenge around NJ.

Too low, we can get excess power, even in NJ... but you have to have real drive to come from NYC to want to get in a plane that has been stored outside over night in 0°F temps. Real pre-heat and cabin heat recommended... cabin heat for instrument bearings, of course...

Part of the hard to measure challenge is the coldness that comes with that. The engine and it's systems have to come up to temperature from a really low number. All the gaps and clearances are at their minimums and the viscosity of the oil is at it's maximum.

Somebody posted a photo of a cylinder separation around here years ago. The head came off the cylinder...

The excess power is noticeable in a C as it accelerates down the runway, and the associated climb is fantastic!

For anyone keeping track, I don't go outside on those days much...

doing some arbitrary math...

If I had 280hp @ 28" of mp, I would get 310hp available at 31" of MP. A 10% increase... at no extra charge...

Generally speaking if your MP gauge rises out of the normal range, it is OK to throttle back into the normal max power range for T/O. This can be done during the T/O run as if you were running a TC'd engine with no MP controller on it...

Overboosting can have some extra cylinder costs to consider...

looking back on that experience, I was really dedicated to flying... even with a low flying budget.

Best regards,

-a-

Exactly. I think some pilots just forget that throttle might not be supposed to be wide open at takeoff if the DA is significantly -0. These tractor engines rely on a fairly unpredictable computer to schedule fuel, air and spark....., us!

thinwing · November 4, 2016

Since the over boost is mainly temp related I would think the low temp and thicker air would aide cylinder cooling...I am surprised PTK saw 380 temps on a cold NJ day even pulling full throttle.380 is what I see pulling 36 in on a hot summer Central Valley takeoff.

**carusoam** · November 4, 2016

Heat generation increases with the additional hp.

The cooling surface area doesn't increase.

the transfer of energy is pretty low during the take-off run until the airspeeds really get up.

FF needs to be maintained for the usual 2-300 ROP in the climb...

The lower air temperature and higher density are helpful up to a point. But they are going to be balanced by these other engineering related challenges...

Now, if the engine were designed around Lower DA power settings, NA power would be really cool...

Best regards,

-a-

M016576 · November 4, 2016

3 hours ago, thinwing said:

Since the over boost is mainly temp related I would think the low temp and thicker air would aide cylinder cooling...I am surprised PTK saw 380 temps on a cold NJ day even pulling full throttle.380 is what I see pulling 36 in on a hot summer Central Valley takeoff.

Hard to say what his baffling looks like, or his climb attitude- all we really know is that he was at full power and below sea level DA. Those first two things (climb angle/speed and baffle condition) can easily account for 50 degrees F in CHT difference.

Edited November 4, 2016 by M016576

**Hank** · November 5, 2016

I wish I had thought to check the VSI that cold morning in WV when the nearby ATIS said 8°F and my wife stayed in bed instead of going to breakfast. Climb was brisk, to say the least!

November 5, 2016

Time to dust off the time to climb thread?

Clarence

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