Indicated Speed vs. Real Speed

[Golfer]

I'm familiar with those rules of thumb and I'm sure there are some folks who will thank you for that tip.

I'm trying to help answer the question of how did the Analog TAS indicator work which is still an unknown to me.  Thanks for the tips. 

[Mace2004]

I'm familiar with those rules of thumb and I'm sure there are some folks who will thank you for that tip.

I'm trying to help answer the question of how did the Analog TAS indicator work which is still an unknown to me.  Thanks for the tips. 

There are several types of analog TAS indicators.  In some GA aircraft there is an additional dial around the IAS scale.  You manually rotate the dial to set your pressure altitude adjacent to an external temp scale and the pointer will give you TAS.  It's basically your standard IAS indicator with an E6B attached.  I've also seen automatic but pure mechanical TAS indicators which are essentially just a IAS indicator with an altimeter and external temperature gauge built-in.  It computes TAS mechanically.  I suspect the analog TAS indicator in the Phantom got it's information directly from the CADC.  Given the age of the F4 it was probably an analog electrical signal which drove the gauge.

[Traveler]

I'm familiar with those rules of thumb and I'm sure there are some folks who will thank you for that tip.

I'm trying to help answer the question of how did the Analog TAS indicator work which is still an unknown to me.  Thanks for the tips. 

Airspeed indicator
A device that computes and displays speed of an aircraft relative to the air mass in which the aircraft is flying. The commonest type is the indicated airspeed meter, which measures differential pressure between the total ram pressure from the Pitot system and the total static pressure; it then converts this difference into units of speed (mi/h or knots) under standard conditions. Although the indicated values are incorrect above zero altitude, the relationship to the aircraft handling remains essentially unchanged, thus providing a measure of the flyability of the aircraft.

True airspeed indicators are similar but include a more complex mechanism that also senses both the absolute pressure and temperature, and compensates for the change of density of the air mass, thus obtaining true airspeed. This indication is of value in computing course information.

Hope this helps

[Golfer]

Answers.com makes my eyes bleed.  Commonist?  Really? 


I know what they do which is all that that blurb said.  I don't know specifically how the one pictured works to do what it does.  I've not personally flown behind an analog TAS indicator since by in large they've gone the way of the Dodo Bird in modern airplanes.  I have an FMS that tells me everything I need to know using my favorite system of PFM 

[Traveler]

Answers.com makes my eyes bleed.  Commonist?  Really? 


I know what they do which is all that that blurb said.  I don't know specifically how the one pictured works to do what it does.  I've not personally flown behind an analog TAS indicator since by in large they've gone the way of the Dodo Bird in modern airplanes.  I have an FMS that tells me everything I need to know using my favorite system of PFM 

Does it really matter?  It's a calculation based on pressure altitude, temperature and indicated airspeed.  None of it matters in AH.  In AH you use indicated air speed.   

[Golfer]

That is interesting there. I thought I read somewhere that this is not something that is normally in an airplane. My appologies.

As a "how it works" kind of person I am, I have a question.

Can you guess?

How does an analog TAS indicator acquire it's measurement? The IAS I understand uses air pressure much like the way my boat does, but what reference would an analog gauge use?

This is what dffk49 wrote.  I'm trying to help him get the information.  I'm perfectly happy with "PFM" as an answer to how things work.

[MachNum]

...(and Mach Number but we won't go there).

Hey! I resemble that remark

[Golfer]

Here's the email I just received from the taker of the TAS photo.  Turns out the indicator didn't work mechanically as I expected:

The F-4 TAS indicator has an analog display but its input is an electrical signal from the Air Data Computer(ADC.) ADC inputs are dynamic pressure from the pitot tube on the vertical fin, static pressure from the 2 static ports on either side of the radome, total temperature from the Total Temperature Sensor(Rosemont probe) on the left air conditioning intake duct, angle of attack from the dual-slotted AOA probe on the left forward fuselage and 17th stage engine bleed air from the auxiliary equipment compartment.

The only pilot control over the system is a Central Air Data Control switch which sets, resets or turns off the static pressure compensation system which corrects outputs for static source error which varies considerably with aircraft maneuvers and speed and altitude changes. If the Static Pressure Compensator(SPC) fails or is switched off the pilot should use speed and altitude correction charts if accurate numbers are needed.

Here are some definitions as used by the military.
Definition: (DOD) The speed of an aircraft relative to its surrounding air mass. The unqualified term "airspeed" can mean any one of the following:

a. calibrated airspeed--Indicated airspeed corrected for instrument installation error.

b. equivalent airspeed--Calibrated airspeed corrected for compressibility error.

c. indicated airspeed--The airspeed shown by an airspeed indicator.
d. true airspeed--Equivalent airspeed corrected for error due to air density (altitude and temperature).

Note that slow movers don't usually worry about compressibility error so "equivalent" airspeed isn't normally in their lexicon.

I hope this answers your question. If not, let me know.