Theory Q: Cold Air, Lower Fuel Consumption?

[john smith]

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?

[Jim Macklin]

more if you produce more power, the same at same power.



"john smith" > wrote in message
...
| 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?

[pgbnh]

I would say more fuel for a given power output. Cold fuel in cold air does
not vaporize and burn as well. Therefore it takes more fuel to produce the
same power output. I suspect this effect is more than offset by the
aerodynamic improvements of wing and propellor in the thick air.
"john smith" > wrote in message
...
> 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?

[Aluckyguess]

Cold cool air means more O2 the engine runs better. Better mileage.
"pgbnh" > wrote in message
...
>I would say more fuel for a given power output. Cold fuel in cold air does
>not vaporize and burn as well. Therefore it takes more fuel to produce the
>same power output. I suspect this effect is more than offset by the
>aerodynamic improvements of wing and propellor in the thick air.
> "john smith" > wrote in message
> ...
>> 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?
>
>

[Andrew Sarangan]

All engines have an optimal operating temperature. If you are above or
below it, the efficiency is likely to be lower. It's probably not a
big effect.

On Feb 3, 10:32 pm, "Jim Macklin"
> wrote:
> more if you produce more power, the same at same power.
>
> "john smith" > wrote in message
>
> ...
> | 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?

[Danny Deger]

"john smith" > wrote in message
...
> 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?

The engine produces more power and uses more fuel.

Danny Deger

[Stubby]

john smith wrote:

> During Winter in the colder climes, do you use less fuel when you fly
> because the air is more dense?

First, lift is proportional to mass-air flow over a curved surface.
Cold air is denser so at a give speed, you'll get more lift from cold
air. Now, if you choose to, you can throttle back a bit to get better
fuel economy.

Then, the engine power is controlled by the fuel/air mixture. To burn
X fuel molecules you need Y air molecules. Cold air will supply the X
molecules of air easily.
--Bill

[Mxsmanic]

Stubby writes:

> First, lift is proportional to mass-air flow over a curved surface.

A curved surface has nothing to do with lift.

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

[[email protected]]

On Feb 4, 12:00 pm, 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.
>
> --
> Transpose mxsmanic and gmail to reach me by e-mail.

Of course it does, the propeller produces more thrust when the air is
colder ( A propeller is a curved surface )

Now, back to the orginal question, if the engine is maintained at a
constant rpm at a some cold and hot temperatures, it will burn more
fuel at the colder temperature, but it will also produce more thrust.
So to get the same thrust, you would throttle back and almost the same
fuel burn with less rpm.

[george]

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

[Mxsmanic]

writes:

> Of course it does, the propeller produces more thrust when the air is
> colder ( A propeller is a curved surface )

A propeller is an airfoil. That's why it produces lift. A flat board
presented at a slight angle to a stream of air also produces lift and
is also an airfoil, even though it is not curved.

The curved cross-section of most airfoils increases the critical angle
of attack for stalls and reduces drag, but it does not improve lift
and is not necessary to produce it.

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

[Mxsmanic]

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.

[Stefano]

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.

[Doug[_1_]]

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.

[Matt Barrow]

"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.

[Casey Wilson]

"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.

[Peter R.]

On 2/3/2007 10:23:39 PM, john smith wrote:

> During Winter in the colder climes, do you use less fuel when you fly
> because the air is more dense?

My turbo Bo uses 15.5 gph in the warm summer air and 17.5 gph in the very
cold winter air.

--
Peter

[Roger[_4_]]

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

[Doug[_1_]]

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.

[Casey Wilson]

"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

[Matt Barrow]

"Peter R." > wrote in message
...
>
> My turbo Bo uses 15.5 gph in the warm summer air and 17.5 gph in the very
> cold winter air.

Your Turbobo?

[Matt Barrow]

"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.

[Peter R.]

On 2/6/2007 9:38:35 AM, "Matt Barrow" wrote:

>
> "Peter R." > wrote in message
> ...
> >
> > My turbo Bo uses 15.5 gph in the warm summer air and 17.5 gph in the very
> > cold winter air.
>
> Your Turbobo?

That be the one.

--
Peter

[Newps]

> "Peter R." > wrote in message
> ...
>
>>My turbo Bo uses 15.5 gph in the warm summer air and 17.5 gph in the very
>>cold winter air.



Temp 36 today at 4500. 190 MPH indicated on 14.5 gph in my S35 at
23/2500. Trues out to 200 mph even. Gotta love six new Milleniums.
Don't need no stinkin' turbo.

[Peter R.]

On 2/6/2007 2:33:12 PM, Newps wrote:

> Temp 36 today at 4500. 190 MPH indicated on 14.5 gph in my S35 at
> 23/2500. Trues out to 200 mph even. Gotta love six new Milleniums.
> Don't need no stinkin' turbo.

With six new Milleniums (well, OK, they now have about 300 hours on them) and
a turbo I see speeds of 220 mph at 15.5 gph (2500/WOT and LOP by 70 degrees)
in the summer months at a cruise of 15,000 feet.

Don't need no stinkin' multi. :)



--
Peter

[Newps]

I'll assume 220 mph is a true airspeed as that would be well into the
yellow. I could lean to 14 gph but my indicated goes down by 8-10 mph.
At typical summer temps I'll lose about 5-7 mph true.



Peter R. wrote:

> On 2/6/2007 2:33:12 PM, Newps wrote:
>
>
>>Temp 36 today at 4500. 190 MPH indicated on 14.5 gph in my S35 at
>>23/2500. Trues out to 200 mph even. Gotta love six new Milleniums.
>>Don't need no stinkin' turbo.
>
>
> With six new Milleniums (well, OK, they now have about 300 hours on them) and
> a turbo I see speeds of 220 mph at 15.5 gph (2500/WOT and LOP by 70 degrees)
> in the summer months at a cruise of 15,000 feet.
>
> Don't need no stinkin' multi. :)
>
>
>

[Peter R.]

On 2/6/2007 3:24:46 PM, Newps wrote:

> I'll assume 220 mph is a true airspeed as that would be well into the
> yellow.

Sorry, yes - that is true airspeed. Indicated is usually down around 145-150
or so at that density altitude.

--
Peter

[Casey Wilson]

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

>>
>> 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.

Copied right out of the POH for the 172M, 1976 Skyhawk. Figure 5-7
Cruise Performance, pg 5-16. Got a fax number?



>
>
>
>

[Matt Whiting]

Peter R. wrote:

> On 2/6/2007 2:33:12 PM, Newps wrote:
>
>
>>Temp 36 today at 4500. 190 MPH indicated on 14.5 gph in my S35 at
>>23/2500. Trues out to 200 mph even. Gotta love six new Milleniums.
>>Don't need no stinkin' turbo.
>
>
> With six new Milleniums (well, OK, they now have about 300 hours on them) and
> a turbo I see speeds of 220 mph at 15.5 gph (2500/WOT and LOP by 70 degrees)
> in the summer months at a cruise of 15,000 feet.
>
> Don't need no stinkin' multi. :)

Until the single quits. :-)

Matt

[Matt Barrow]

"Casey Wilson" > wrote in message
news:pN7yh.113034$h75.97806@trnddc01...
>
>>> 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.
>
> Copied right out of the POH for the 172M, 1976 Skyhawk. Figure 5-7
> Cruise Performance, pg 5-16. Got a fax number?

Yeah, but I'm not going to post it on the internet. Can you scan it and send
a JPG file?

Also, I misread your numbers (in terms of measures) - you go from 5.5 GPH,
to 7.6 GPH. That's the way I'm used to calculating fuel loads.

I'd still like to see the numbers, but my example (2X fuel flow for 18%
better TAS is not too far from your 33% increase in TAS for 40% increase in
FF. Getting upwards, then over, 200kts makes a big dent in fuel use.

Another thing to look at it is this - it often doesn't pay to divert a long
way to save a few buck on gas at a cheaper FBO. AirNav has this option on
their fuel planner.

[Casey Wilson]

>
> Yeah, but I'm not going to post it on the internet. Can you scan it and
> send a JPG file?
>
> Also, I misread your numbers (in terms of measures) - you go from 5.5 GPH,
> to 7.6 GPH. That's the way I'm used to calculating fuel loads.
>
> I'd still like to see the numbers, but my example (2X fuel flow for 18%
> better TAS is not too far from your 33% increase in TAS for 40% increase
> in FF. Getting upwards, then over, 200kts makes a big dent in fuel use.
>
> Another thing to look at it is this - it often doesn't pay to divert a
> long way to save a few buck on gas at a cheaper FBO. AirNav has this
> option on their fuel planner.
I can't figure out your email address. How about sending me one at
to start. I'll be glad to share the JPEG with you. If I'm
wrong, I'd like to know it.

I probably should have emphasized that I was calculating bottom line
costs for block-to-block when I did the arithmetic. When I'm in the air, or
doing a flight plan, I strictly use GPH to calculate fuel consuption -- or
more important, fuel remaining.

Roger that about AirNav's fuel planner. Saving a $ on fuel at the cost
of spending $$ on the Hobbs is counterproductive. On the other hand, if the
diversion has something to offer AND I'm just on a fun cruise, I might go
there. Along that same line, I absolutely will not stretch a leg to save a
buck if it starts creeping anywhere near my personal reserve point (or
expected bladder capacity, which is diminishing with my age).

Cheers,

Casey