Aircraft cruise speed

[grtmk14]

Can someone please tell me why for example the Cessna CJ4 is shown to have a cruise speed of 451 ktas but I get an overspeed warning at 305 kts.

[plainsman]

You are confusing two different speed measurements. The 451 ktas is a true air speed value. The instruments display a measured air speed, which drops as you ascend in altitude. The measured value depends on the air particles moving through a pitot tube. Since air is less dense at high altitudes, fewer particles move through the tube a a given speed. If you examine the display a little more, the computer will give a TAS or GS value (I rarely fly the CJ4, I don't remember which). the first is true air speed the other is ground speed. That value should be correspond to 451 kts. if TAS or it may be off by the wind speed if GS. That is a computed value.

[lnuss]

The instruments display a measured air speed, which drops as you ascend in altitude.

I'd like to add that the "measured air speed" is normally termed Indicated AirSpeed, or IAS. While your description is right, it sometimes is easier to picture if you say that the airspeed indicator is a pressure-driven instrument, and the less dense air at higher altitudes has less pressure applied through the pitot tube at a given actual (True) airspeed than it has at lower altitudes.

 

As with so many things in aviation, it's not intuitive.

[Energizer23]

I believe you need to be under 300 until over 10,000 feet.

[Rupert]

Well said Larry! I'd also point out that is why high altitude, less density, is ground speed's best friend. The higher you fly the faster your true ground speed can be.

[plainsman]

Here is a screenshot of the screen with all three speed parameters. I also added a little wind which is shown as well. Note the wind isn't a pure tail wind, so it isn't additive.

BTW, this was a short flight of less than 190 miles, so my cruise altitude is only 29,000 feet. Normally this aircraft would cruise at over 40,000 feet on flights of 1500 miles. The speed difference would be more dramatic at that altitude.

Edited September 29, 2021 by plainsman

[lnuss]

Note the wind isn't a pure tail wind, so it isn't additive.

Not a straight addition, for sure, but it is proportional, and trigonometry can provide the amount added (thus the birth of early flight computers, that is, something like the E6B and earlier) by solving the wind triangle more easily than doing the actual math.

 

The speed difference would be more dramatic at that altitude.

It would indeed.

 

I'd also point out that is why high altitude, less density, is ground speed's best friend. The higher you fly the faster your true ground speed can be.

In calm winds the max attainable true airspeed (therefore groundspeed) is highest, indeed, at those higher altitudes. Or you could say that, for a given power setting (typically given as percent of max power) you will have a higher TAS because there is less drag in less dense air (same reason IAS is lower).

 

BUT, higher altitudes usually entail stronger winds, too, so eastbound you're likely to have a groundspeed that is higher than TAS, while westbound the GS is likely to be lower than TAS. This isn't ALWAYS the case, but winds (in the northern hemisphere) aren't called "prevailing westerlies" for nothing -- they are westerly more often than not. Certainly for those "down under" the converse is true.

 

One last thing, perhaps a tad more technical, is that the above references to altitude actually are for "density altitude," that is, altitude under "standard conditions," that is, at sea level, 29.92 Hg" barometer (1013 millibars), 59º F (15º C), and 0% humidity. Hotter air is thinner, and obviously the barometer has its effects on a pressure instrument (the altimeter, thus altimeter settings).

 

I see I'm already beyond how far I intended to take this, so I'll stop.

[grtmk14]

Many thanks to every ones input and help

[lnuss]

I believe you need to be under 300 until over 10,000 feet.

 

In the U.S. the speed limit is normally 250 kts indicated (KIAS) below 10,000 MSL. Other countries differ.

 

FAR 91.117 says:

 

(a) Unless otherwise authorized by the Administrator, no person may operate an aircraft below 10,000 feet MSL at an indicated airspeed of more than 250 knots (288 m.p.h.).

 

(b) Unless otherwise authorized or required by ATC, no person may operate an aircraft at or below 2,500 feet above the surface within 4 nautical miles of the primary airport of a Class C or Class D airspace area at an indicated airspeed of more than 200 knots (230 mph.). This paragraph (b) does not apply to any operations within a Class B airspace area. Such operations shall comply with paragraph (a) of this section.

 

© No person may operate an aircraft in the airspace underlying a Class B airspace area designated for an airport or in a VFR corridor designated through such a Class B airspace area, at an indicated airspeed of more than 200 knots (230 mph).

 

(d) If the minimum safe airspeed for any particular operation is greater than the maximum speed prescribed in this section, the aircraft may be operated at that minimum speed.

[plainsman]

Thanks for the reply Gray! There are a lot of people on the forum that want to be helpful. It is always appreciated when someone acknowledges that the response was helpful and that they understood the answer.

[plainsman]

One more speed designation appears on the screen that I failed to note. At the base of the IAS you see the number .684, which is usually written 0.684. This is the Mach speed or Mach Number. It is the speed relative to the speed of sound, with the speed of sound at Mach 1.000. Most jets report their speed in Mach above 15-20,000 feet. Most commercial aircraft operate at Mach 0.85 or less. As you get above Mach 0.85, pressure builds up in front of the aircraft. Consequently, more fuel is needed to maintain a speed above Mach 0.85. The Convair 990A was an airliner designed to operate at just over Mach 0.9. It had very poor fuel economy per passenger, and so was a commercial failure, even though it was the fastest airliner of its time.;