Faster in Winter

[201er]

I have definitely observed that my plane is faster in the cold of winter but can't explain why. By simple logic and reason, true airspeed should be a factor of density altitude. So in winter you might get 6000ft summer performance at 8000ft. But in reality, I notice faster speeds across the board and faster maximum speed overall.

 

What other factors are at play that are less obvious then just temperature and density altitude? Humidity? Constant speed prop taking a different bite of air? Better engine cooling make it faster? ASI affected by temperature and less accurate at summer or winter temps. Different density of fuel when colder? Plane feels cold so it gets there in a hurry?

 

I surmise that density altitude has a predictable/linear effect on the wing. But something is different and favors engine or prop performance in winter. Who can explain?

[Andy95W]

Greater air density = more oxygen molecules = capable to burn more fuel and make more power from your engine.  Plus the bigger bite of air thing.

[ArtVandelay]

Less humidity also means more power, water molecules take up space, less oxygen, testing has shown up to a 10% decrease in power when humidity is high.

[AndyFromCB]

Smooth air maybe, little summer microbumps continuously slow you down by a few knots? That's my theory.

Edited December 11, 2016 by AndyFromCB

[201er]

11 minutes ago, N1395W said:

Greater air density = more oxygen molecules = capable to burn more fuel and make more power from your engine.  Plus the bigger bite of air thing.

But more molecules to drag the wing through so you get a lower TAS. That's just density altitude. Based on density altitude alone, you should have same speed at same density altitude year round but it doesn't appear to be the case.

[aaronk25]

Way faster in winter by 5kts if really cold. And yes your right there is more drag but the increase in power outweighs the drag penalty. Now I'd agree if someone said what temp yields the highest true MPG or NMPG and the hotter the better but the true airspeed will be lower.


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[Hank]

On December 10, 2016 at 10:37 PM, 201er said:

But more molecules to drag the wing through so you get a lower TAS. That's just density altitude. Based on density altitude alone, you should have same speed at same density altitude year round but it doesn't appear to be the case.

Cold air is more dense, so should create a smidgen more drag. BUT:

• Denser air has more oxygen in each cylinder full, so the engine makes more power, which becomes more speed. (Chemistry)
• Denser air lets the prop create more thrust, which becomes more speed with each revolution. (Aerodynamics)
• Lift is proportional to speed, so you can reduce the lift vector, adding it to thrust, creating more speed. (Aerodynamics)
• Denser air also creates more lift, so you can again reduce the angle of attack and give yourself more speed. (Physics)

Flying is truly a multidisciplinary effort!

Apparently we agree that:

More Power + More Thrust + More Lift > More Drag

with the result that our planes fly faster. It is simply proven using a simple Time to Climb chart, or look at your VSI on takeoff (one hit summer day,mine cold winter day). The most spectacular climb to altitude I ever had, it was 8°F at takeoff. The deck angle was unreal! Wish I had thought to look at the VSI . . .

Edited December 14, 2016 by Hank

[carusoam]

It is clearly an increased HP issue.  More power is good!  Better T/O performance, better climb rates...

the costs are there too.  More fuel burned, more stress (ICP) and wear on the engine.

 

Air density has a few other things that go with it.  

1) I assume the prop gets a better bite in dense air, but the CS prop probably adjusts it un-noticeably.  

2) Stall speed is affected by DA.  DA is going lower in the cold.

3) Bernoulli gives us more lift as the medium gets denser... prop and wings.

4) drag increases with dense air

5) On a percentage basis, MP is what we use to measure power improvements.  Expect colder air to come with higher MP.  From experience, Moonies always like an extra inch of MP.  This why some of us have plumbing to circumvent the air filter...

 

follow-up question.

1) Is the AOA sensitive enough to show any differences related to DA?  The beauty of the AOA, weight and temperature independent, no?

 

PP thoughts only, not a CFI...

Best regards,

-a-

[Andy95W]

3 minutes ago, carusoam said:

follow-up question.

1) Is the AOA sensitive enough to show any differences related to DA?  The beauty of the AOA, weight and temperature independent, no?

Here we go again...

[carusoam]

2,000+FPM as recorded by Byron in an old Long Body Mooney...  real photographic evidence was obtained...

Best regards,

-a-

 

[carusoam]

Sorry about the dp...  it occurred while we were typing simultaneously....

Something has gone amiss around here.  I sent a message with details to Craig...

Best regards,

-a-

[PTK]

Cold air contracts and becomes denser. The engine is taking in dense air and therefore more O₂ for each combustion event. 

But as the Second law of Thermodynamics says...there's no free lunch. Entropy must increase.

The corollary to the increased O2 and more speed is that the engine compensates by consuming more fuel. 

 

[Yetti]

Dead sea effect.  The molecules are denser so they hold the plane up better leaving more HP to move forward.

[Hank]

16 minutes ago, PTK said:

Cold air contracts and becomes denser. The engine is taking in dense air and therefore more O₂ for each combustion event. 

But as the Second law of Thermodynamics says...there's no free lunch. Entropy must increase.

The corollary to the increased O2 and more speed is that the engine compensates by consuming more fuel. 

 

Close, but no. Higher entropy does not mean higher fuel burn. Slow flight with flaps and gear down is also a higher entropy state.

In the case of colder air, slightly more fuel is burned because there is more oxygen in the cylinder, so there is a more complete combustion process, making Peak EGT a little higher. Gases at higher temperature have more internal motion, which is a state of higher entropy. Higher temperature means higher internal pressure, more push on the downstroke of the piston, or more power. Even when leaned to the same amount ROP / LOP, starting at a higher peak you have a higher temp exhaust stream, also a higher entropy state. And if course, as your plane moves through the atmosphere, you leave a roiled wake of disturbed air. At higher speed, the wake is larger, another increase in local entropy.

i just love it when dentists try to use Engineer Speak . . . At least I've picked up some Medical Speak through two decades of medical device development and manufacturing.  

[PTK]

21 minutes ago, Hank said:

Close, but no. Higher entropy does not mean higher fuel burn. Slow flight with flaps and gear down is also a higher entropy state.

In the case of colder air, slightly more fuel is burned because there is more oxygen in the cylinder, so there is a more complete combustion process, making Peak EGT a little higher. Gases at higher temperature have more internal motion, which is a state of higher entropy. Higher temperature means higher internal pressure, more push on the downstroke of the piston, or more power. Even when leaned to the same amount ROP / LOP, starting at a higher peak you have a higher temp exhaust stream, also a higher entropy state. And if course, as your plane moves through the atmosphere, you leave a roiled wake of disturbed air. At higher speed, the wake is larger, another increase in local entropy.

i just love it when dentists try to use Engineer Speak . . . At least I've picked up some Medical Speak through two decades of medical device development and manufacturing.  

What I said was that higher fuel burn means higher entropy. Energy in the fuel is converted to unusable energy. The increased speed comes at a cost. 

I was simply referring to the increased O2 requiring increased fuel flow.

 

 

[cbarry]

I think it's just that aircraft is cold and in a hurry just like the rest of us this time of year!  

[Marauder]

Or it could just be an airplane thing -- your ASI is about to fail.


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[201er]

2 minutes ago, Marauder said:

Or it could just be an airplane thing -- your ASI is about to fail.


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From going too far off the fast side of the scale?

[peevee]

I notice in the winter I'm really fast east bound

West bound sucks a fat one. 

[Guest]

14 hours ago, N1395W said:

Here we go again...

In cooler more dense air, parrot wings create more lift and thrust equalling less airframe drag for any given AoA, resulting in higher TAS.  Applies only to Mooney airframes.

Clarence

 

[Hank]

It never ceases to amaze me, given how much better our planes perform in cold weather, how many people ask when am I going to put the plane away for the winter..

[klystron]

21 hours ago, Hank said:

It never ceases to amaze me, given how much better our planes perform in cold weather, how many people ask when am I going to put the plane away for the winter..

I get the same thing: "Can you still fly in winter?"

My reply is always "Winter is the best time of year to fly!"

 

One reason we might see higher speeds during cold weather is that the efficiency at which thermal energy is converted to mechanical energy increases as the outside temperature decreases. This is primarily due to changes in the ratios of specific heat capacity of air. 

 

Also, now that I think about it.... Anyone know of an STC to put 10:1 compression ratio cylinders in an O-360???

[aviatoreb]

Besides that the engine produces more horsepower total because there are more O2 molecules in each cylinder-full shot of air (lower density altitude), is it true that the engine will be more efficient at converting this chemical energy to mechanical energy because the engine will be more efficient as it is more naturally cool - not as much effort to cool itself?  I am uncomfortable with this so I am just throwing it out there as an idea.

Not to mention at least we can leave our cowlings closed.

[Hank]

1 hour ago, aviatoreb said:

Besides that the engine produces more horsepower total because there are more O2 molecules in each cylinder-full shot of air (lower density altitude), is it true that the engine will be more efficient at converting this chemical energy to mechanical energy because the engine will be more efficient as it is more naturally cool - not as much effort to cool itself?  I am uncomfortable with this so I am just throwing it out there as an idea.

Engine efficiency should be reduced at low outside temps, as the air at the intake requires a larger ΔT  to reach combustion temperature. One if my Thermo profs in college worked as a consultant and helped companies increase their boiler efficiency, sometimes as simple as running the air intake up near the ceiling to get warmer air.

So in cold weather, our combustion efficiency is reduced but power output is increased.

[PMcClure]

14 minutes ago, Hank said:

Engine efficiency should be reduced at low outside temps, as the air at the intake requires a larger ΔT  to reach combustion temperature. One if my Thermo profs in college worked as a consultant and helped companies increase their boiler efficiency, sometimes as simple as running the air intake up near the ceiling to get warmer air.

So in cold weather, our combustion efficiency is reduced but power output is increased.

Sounds like Dr. Dyer! But, I think the physics is different for IC engines. Higher delta T should mean higher efficiency. Also, fuel is more dense in addition to the denser air = more horsepower, in theory.