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Prop Decouple Question...TAS vs IAS


flyaf05
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Question for all the smart herk drivers out there...can anyone explain why the -1 uses TAS vs IAS when we talk decoupling? I haven't been able to find anyone who can explain the reasoning behind using TAS. The book says 150TAS will result in a decouple. Assuming standard day/standard lapse rate 150TAS would give you around 147IAS at 1,000ft and 125IAS at 10,000ft...is the force at 10,000ft and 125IAS enough to force a decouple being that there is less air molecules available to cause the -6000pds negative torque required to decouple the prop? Wouldn't IAS be a better measure of force against the prop then TAS since TAS is only "the speed of the aircraft relative to the airmass in which it is flying." Any help would be greatly appreciated!

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TAS is the true measure of aircraft performance in cruise, thus listed in aircraft specs, manuals, performance comparisons, pilot reports, and every situation when actual performance needs to be measured. It is the speed normally listed on the flight plan, also used in flight planning, before considering the effects of wind.

For this reason, TAS cannot be measured directly. In flight, it can be calculated either by using an E6B flight calculator or its equivalent function on many GPSs. The data required are Outside air temperature (OAT), Pressure altitude and CAS (IAS corrected for installation and instrument errors). Modern aircraft instrumentation use an Air Data Computer to perform this calculation in real time and display the TAS reading directly on the EFIS.

Since temperature variations are of a smaller influence, the ASI error can be roughly estimated as indicating about 2% less than TAS per 1,000ft of altitude above sea level. Thus for a given IAS, the True Airspeed is about 2% higher than IAS per 1,000ft of altitude above sea level. An aircraft flying at 15,000ft with an IAS of 100kt, is actually flying at 130kt TAS, or 130kt through the air.

The IAS is an important value for the pilot because it directly indicates stall speeddensity altitude. and various airframe structurally limited speeds, regardless of density altitude.

Okay, you start with Basic Airspeed (BAS), corrected to Indicated Airspeed (IAS), corrected to Calibrated Airspeed (CAS), corrected to Equivalent Airspeed (EAS) corrected to True Airspeed (TAS).

So TAS is the "most correct" airspeed and the biggie, this is the first one corrected for Density Altitude, Density Altitude is what the prop knows and you get a more accurate airspeed (atmospherically) using TAS.

Now why we always fly off of IAS and not TAS.....I dont remember off hand????!!!!

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I appreciate all the thoughts but I'm still not sold...I want to understand why the prop at 10,000ft requires less force (i.e. lower IAS) to generate -6,000pds of torque in order to decouple vs. a higher force at 1,000ft. That doesn't make sense to me because there are less air molecules turning the prop towards -6,000pds the higher you go. Wouldn't a consistent force (i.e. IAS) instead of the speed through an airmass (i.e. TAS) be more relevant?

Also I'd agree that the prop knows density altitude, but what density altitude tells us is that the higher the density altitude the more TAS is required to achieve the same amount of power/lift...aka the prop is less effective. We fly off IAS because stall speeds directly correlate to IAS not TAS which is corrected for density altitude.

Keep the ideas coming...I really want to get a handle on this.

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"I want to understand why the prop at 10,000ft requires less force (i.e. lower IAS) to generate -6,000pds of torque in order to decouple vs. a higher force at 1,000ft."

150 IAS is a different "force" at different altitudes.

150IAS at 1K = 152TAS

150IAS at 10K = 174TAS

"TAS is only "the speed of the aircraft relative to the airmass in which it is flying." "

If the TAS is constant (150) the force sensed by the aircraft, prop and wing is constant and therefore a constant decouple setting.

The force “PRESSURE†(amount of resistance or maybe drag) the prop “feels†is based on TAS not IAS therefore the prop feels the same amount of "force" at 150TAS but the BLADE ANGLE is higher at higher altitudes.

"That doesn't make sense to me because there are less air molecules turning the prop towards -6,000pds the higher you go."

Agree, so to maintain the same TAS we must increase the Blade angle to “chop†more molecules.

For example 120,000 lb aircraft, standard day, at 30,000'

220TAS, 136IAS, 900TIT, 7300 TQ (high Blade angle)

And the SAME 120,000 lb aircraft, standard day, at 10,000'

220TAS, 190IAS, 740TIT, 7200 TQ (lower Blade angle)

TAS is the same, the aircraft feels the same pressure, the blade angle is the difference.

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150 IAS is a different "force" at different altitudes.

150IAS at 1K = 152TAS

150IAS at 10K = 174TAS

If the TAS is constant (150) the force sensed by the aircraft, prop and wing is constant and therefore a constant decouple setting.

The force “PRESSURE†(amount of resistance or maybe drag) the prop “feels†is based on TAS not IAS

Respectfully, you have this exactly backward. Aerodynamic forces are constant with a constant CAS (IAS without the measurement errors). In fact, IAS is nothing more than a measure of the dynamic pressure (force) acting into the pitot tube. For a constant IAS, the other aerodynamic forces (lift and drag) on the airplane will be constant regardless of altitude (assuming subsonic flow). One simple example of this is the fact that a given airplane at a given weight and configuration will stall (a function of the lift force produced) at the same IAS at 10,000 ft as it will at sea level (but at very different TAS)

As to the original question; I don't have a good answer, but I'll toss out a couple of things to consider. There are some things, (control surface flutter for example) which are more dependent on "the speed the air molecules are moving past" (TAS), than they are on "the force exerted by the air" (IAS). This apparently is one of those things. Perhaps it is related to how fast the prop can windmill, rather than the force exerted on the prop. They are not the same; intuitively, a prop windmilling at 250 knots in air will turn much faster than one windmilling in water at 10 knots, but the water at 10 knots will have more than twice the dynamic pressure (force).

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From my orginal post

150 IAS is a different "force" at different altitudes.

(this should have been "speed" not force)

150IAS at 1K = 152TAS

150IAS at 10K = 174TAS

"One simple example of this is the fact that a given airplane at a given weight and configuration will stall (a function of the lift force produced) at the same IAS at 10,000 ft as it will at sea level."

Agree, stall speed is a function of gross weight and not affected by altitude due to the fact you use IAS as the stall speed. "(but at very different TAS)"

Indicated airspeed is the number of air molecules flowing over the wing... In order to have the same number MOLECULES at various altitudes you must increse the TAS to push enough molecules into the pitot tube at higher altitudes.

The wing does not change its angle to expose itself to more air it is fixed and thus needs a certian ammout of airflow to maintain lift....

The prop changes angle, different angle for different altitude same TAS fixed decouple value.

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