Theory Q: Cold Air, Lower Fuel Consumption?

On Feb 5, 7:00 am, Mxsmanic wrote:
Stubby writes:
First, lift is proportional to mass-air flow over a curved surface.

A curved surface has nothing to do with lift.


Riiiight.
NACA sections are just to make the wing look pretty

george writes:

Riiiight.
NACA sections are just to make the wing look pretty

No, they reduce drag and increase the range of angles of attack
through which lift is produced. The curves are not necessary for
lift.

--
Transpose mxsmanic and gmail to reach me by e-mail.

john smith wrote:

Here is a question to challenge your understanding of engine operation...

During Winter in the colder climes, do you use less fuel when you fly
because the air is more dense?
It takes less energy to compress air in the cylinder (or in any other
centrifugal or axial compressor) at lower temperature so I would say that
the thermodynamic cycle (in a reciprocating engine or gas turbine) is more
efficient.

The higher the density altitude, you get better gas mileage for a give
TAS. So colder means lower density altitude and WORSE gas mileage
(everything else being equal, which it probably isn't). There may be
some other things going on, I am not sure about them.

You will be able to climb better and go faster at full throttle when
it is cold.

"Doug" wrote in message
ups.com...
The higher the density altitude, you get better gas mileage for a give
TAS. So colder means lower density altitude and WORSE gas mileage
(everything else being equal, which it probably isn't). There may be
some other things going on, I am not sure about them.

You will be able to climb better and go faster at full throttle when
it is cold.

This last part is more correct/to the point.

In colder weather, your engine CAN develop more power. Power = Fuel.

In cold weather you can go FASTER because your engine can develop more
POWER. For a given engine, faster means less MPG.

"Matt Barrow" wrote in message
...

"Doug" wrote in message
ups.com...
The higher the density altitude, you get better gas mileage for a give
TAS. So colder means lower density altitude and WORSE gas mileage
(everything else being equal, which it probably isn't). There may be
some other things going on, I am not sure about them.

You will be able to climb better and go faster at full throttle when
it is cold.

This last part is more correct/to the point.

In colder weather, your engine CAN develop more power. Power = Fuel.

In cold weather you can go FASTER because your engine can develop more
POWER. For a given engine, faster means less MPG.
Let's beat this dog some more. Going faster also means less time
required to get to the destination. Considering that as pilots we are more
interested in fuel consumption, gallons per hour, than MPG is it more
economical to fly faster/higher?
Let's presume a 250NM trip at 10Kft [for the nitpickers, the wind is
nil both ways]. According to the POH for the C-172M that I fly the trip will
take 2.8 hours and burn 15.4 gallons at 89 KTAS. Shove the throttle to the
firewall and the trip will take 2.1 hours and burn 15.9 gallons at 119 KTAS.
To steal a line from Paul Harvey; "And now, the rest of the story."
At first glance, it is going to cost more to fly faster. But the
analysis isn't quite over, yet. Let's pay $4/Gal for the fuel, that means I
paid $2 more for fuel to go the distance. But the airplane is also costing
me an hourly rate of about $85. By flying faster, I shaved off 0.7 hours. or
$59.5. We could extend this into other cost advantages, such as extending
the miles flown between overhauls, but I think I've made my point.
My point is, you may get more miles per gallon at the penalty of other
expenses. When you are cruising, push the throttle to the firewall and screw
the friction lock down -- but by all means keep the engine properly leaned.

On Mon, 05 Feb 2007 17:30:43 GMT, "Casey Wilson"
wrote:


"Matt Barrow" wrote in message
...

"Doug" wrote in message
ups.com...
The higher the density altitude, you get better gas mileage for a give
TAS. So colder means lower density altitude and WORSE gas mileage
(everything else being equal, which it probably isn't). There may be
some other things going on, I am not sure about them.

You will be able to climb better and go faster at full throttle when
it is cold.

This last part is more correct/to the point.

In colder weather, your engine CAN develop more power. Power = Fuel.

In cold weather you can go FASTER because your engine can develop more
POWER. For a given engine, faster means less MPG.
Let's beat this dog some more. Going faster also means less time
required to get to the destination. Considering that as pilots we are more
interested in fuel consumption, gallons per hour, than MPG is it more
economical to fly faster/higher?
Let's presume a 250NM trip at 10Kft [for the nitpickers, the wind is
nil both ways]. According to the POH for the C-172M that I fly the trip will
take 2.8 hours and burn 15.4 gallons at 89 KTAS. Shove the throttle to the

but at 10,000 feet you probably will not be able to get more than 55
to 60% power full throttle in a normally aspirated engine. In the Deb
I find maximum true speed comes some where between 6000 and 8000 feet
depending on temperature.


firewall and the trip will take 2.1 hours and burn 15.9 gallons at 119 KTAS.

almost 16 in a 172 just doesn't sound right even at maximum crusise at
lower altitudes let alone 10,000
I only burn 14 at 75% in the Deb witha 260 HP, 6-cylinder IO-470N
engine. At 10,000 the fule burn is down to about 12.5.

To steal a line from Paul Harvey; "And now, the rest of the story."
At first glance, it is going to cost more to fly faster. But the
analysis isn't quite over, yet. Let's pay $4/Gal for the fuel, that means I
paid $2 more for fuel to go the distance. But the airplane is also costing
me an hourly rate of about $85. By flying faster, I shaved off 0.7 hours. or
$59.5. We could extend this into other cost advantages, such as extending
the miles flown between overhauls, but I think I've made my point.
My point is, you may get more miles per gallon at the penalty of other
expenses. When you are cruising, push the throttle to the firewall and screw
the friction lock down -- but by all means keep the engine properly leaned.

Roger Halstead (K8RI & ARRL life member)
(N833R, S# CD-2 Worlds oldest Debonair)
www.rogerhalstead.com

Let's say you keep your TAS the same. Now, up high, you can't go as
fast as down low, full throttle. But at full throtte at 10,000'
DENSITY ALTITUDE say you have a TAS of 120 knots, and a manifold
pressure of 20". You burn, 8 gph, getting 15 miles/gallon.

Now at 5000' DENSITY ALTITUDE, to run at the same 120 Knots TAS, you
need 22" of manifold pressure, and burn 9 gph, getting 13.3 miles/
gallon.

Now, when it gets colder, the same true altitude is a lower density
altitude. So running at the same TAS, when it is cold, you get worse
fuel mileage than when it is warm.

So if you keep the TAS the same, the true altitude the same, the
barometric pressure the same, you will get WORSE gas mileage when it
gets colder.

"Roger" wrote in message
...
On Mon, 05 Feb 2007 17:30:43 GMT, "Casey Wilson"
wrote:

firewall and the trip will take 2.1 hours and burn 15.9 gallons at 119
KTAS.


almost 16 in a 172 just doesn't sound right even at maximum crusise at
lower altitudes let alone 10,000
I only burn 14 at 75% in the Deb witha 260 HP, 6-cylinder IO-470N
engine. At 10,000 the fule burn is down to about 12.5.
Roger Halstead (K8RI & ARRL life member)
(N833R, S# CD-2 Worlds oldest Debonair)
www.rogerhalstead.com

Hi Roger,

That was 15.9 gallons start to finish, not GPH

Casey

"Casey Wilson" wrote in message
news:7RJxh.1999$5U4.453@trnddc07...

"Matt Barrow" wrote in message


In colder weather, your engine CAN develop more power. Power = Fuel.

In cold weather you can go FASTER because your engine can develop more
POWER. For a given engine, faster means less MPG.

Let's beat this dog some more. Going faster also means less time
required to get to the destination. Considering that as pilots we are more
interested in fuel consumption, gallons per hour, than MPG is it more
economical to fly faster/higher?

Let's presume a 250NM trip at 10Kft [for the nitpickers, the wind is
nil both ways]. According to the POH for the C-172M that I fly the trip
will take 2.8 hours and burn 15.4 gallons at 89 KTAS. Shove the throttle
to the firewall and the trip will take 2.1 hours and burn 15.9 gallons at
119 KTAS.

Your TAS increases 33%, but your Fuel Flow increases only 4%?
My fuel flow doubles between Econ Cruise and Max Cruise (9.4 gph/172kts vs
18.4gph/205kts).

Check your data.