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Flight Model Verification and Data Analysis 101


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#441 Crumpp

Crumpp
  • Posts: 51

Posted 17 June 2016 - 15:16

 Understood.

 

Thus the tachometer is the best indicator of best climb-angle (and best climb-rate) in a sustained climb.

 

Cheers.

 

Correct but not the entire picture....In a fixed pitch propeller aircraft, the tachometer is your primary engine instrument for establishing climb power.

 

Airspeed is your primary for establishing the L/D ratio you want to be at to achieve best climb rate.  It represents the ratio of dynamic pressure to lifting/drag pressure that is fixed by the physical shape of the aircraft to achieve the L/D ratio where combined with the engine power settings is the point that Power Available to Power required is maximized.

 

Because those ratios occur at a specific angle of attack in a fixed relationship.....

 

In a World War I aircraft without an airspeed indicator, this would be the Angle of Attack based on the angle the little string tied to the strut shows us!

 

:)


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#442 Chill31

Chill31
  • Posts: 1891

Posted 17 June 2016 - 22:51

Crump,

How did you come to this conclusion?

"Using standard aircraft performance math shows the relationship...

If the Albatross achieves 116mph at 3250 feet then it will achieve 116 * .865 = 100KTAS/1.04932 SMOE = 95.6 KTAS at sea level"

Engine power and parasite drag decrease at the same rate as air density decreases with increasing altitude (assuming normally aspirated engines).

Full throttle, maximum speed should yield the same TAS regardless of altitude (for the first few thousand feet until induced drag causes airspeed to drop)

The albatross mixture setting was (supposedly) adjusted to give best power at about 1000m. Is that what you are referring to?
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#443 Crumpp

Crumpp
  • Posts: 51

Posted 17 June 2016 - 23:17

I come to that conclusion because that is how standard aircraft performance math is done with a normally aspirated engine. As you correctly note, It all changes IAW atmospheric density. Vmax is the point drag = thrust at maximum power in level flight. That equilibrium point will then change based on the density.

Specifically, the inverse of the square root of the density ratio. The other name for the inverse of the square root of the density is the Standard Means of Evaluation (SMOE).
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#444 Crumpp

Crumpp
  • Posts: 51

Posted 17 June 2016 - 23:39

Chill,

You are a piliot, correct?

Are there any power curves on the Albatross engine? I have a very hard time believing it produces significantly more power at altitude than it does at sea level. It was certainly theorized that it would but was proven to be not correct or at least not significant.

I very easily see that a high compression engine of World War I vintage would require two fuel circuits to achieve optimum power. The NACA paper talks about this fact and the additional complexity of such a design. A rich throttle for low altitude and a lean throttle circuit for high altitude.

That is of course independent of mixture control, understand? You still need to lean the circuits for actual conditions.

If you read the NACA paper, high compression ratio changes the slope of the curve but it does not produce significantly more power at a higher altitude than it does a lower altitude. It still is a normally aspirated engine and decreases power IAW density. If higher compression did have that effect, superchargers would not be required.

The advantage of high compression is that it can sustain a leaner fuel to air ratio which means it can achieve a higher absolute ceiling than a lower compression engine. To get that gain in performance at the high compression required, it needs a second leaner fuel metering circuit.

Although done for different reasons, I think of it like the auto-rich and the auto-lean circuits of the more advanced World War II era engines.

That is what I get out of reading the description of the Albatross engine.
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#445 JoeCrow

JoeCrow
  • Posts: 4145

Posted 19 June 2016 - 05:19

Correct but not the entire picture....In a fixed pitch propeller aircraft, the tachometer is your primary engine instrument for establishing climb power.

 

Airspeed is your primary for establishing the L/D ratio you want to be at to achieve best climb rate.  It represents the ratio of dynamic pressure to lifting/drag pressure that is fixed by the physical shape of the aircraft to achieve the L/D ratio where combined with the engine power settings is the point that Power Available to Power required is maximized.

 

Because those ratios occur at a specific angle of attack in a fixed relationship.....

 

In a World War I aircraft without an airspeed indicator, this would be the Angle of Attack based on the angle the little string tied to the strut shows us!

 

:)

Thanks.

Overall it explains clearly why each aircraft has its own, unique, best climb-angle. It doesn't take much thought to realise that this has serious implications in combat situations.

Cheers.


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#446 Crumpp

Crumpp
  • Posts: 51

Posted 20 June 2016 - 15:04

Thanks.

Overall it explains clearly why each aircraft has its own, unique, best climb-angle. It doesn't take much thought to realise that this has serious implications in combat situations.

Cheers.

 

You are welcome and Thank you for your contributions!


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