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Guest

Temperature And Airspeed

Wed Sep 26, 2001 1:16 am

Hi guys,

I got a quick question. Just now I was playing with FS, and I put the ground temperature to -50 Degrees Farenheight. I then took off, and did a short trip. However, at FL350 M.84 I noticed my GS was at 380kts. Normally it is at around 490knts ...

So my question is that does temperature have an effect on Ground Speed and why?

Thanks

-Regards, VA
 
cdfmxtech
Posts: 1319
Joined: Wed Jul 05, 2000 11:37 am

RE: Temperature And Airspeed

Wed Sep 26, 2001 5:08 am

Temperature would not affect groundspeed. Groundspeed is something that is calculated from your IRUs.

Now since temperature and the density of air decreases with altitude, and u already started off with an extremely low temp and air density...it going to be much lower than normal at altitude.

 
aeroguy
Posts: 66
Joined: Wed Aug 15, 2001 1:33 am

RE: Temperature And Airspeed

Wed Sep 26, 2001 7:30 am

It has to do with how ground speed is calculated. Your ground speed is calculated using the pressure measured at altitude but ground level density. Since you put in a lower temperature for the ground, the ground density has now gone up. The velocity calculated by the instruments is inversely proportional to density. In your case with higher ground density, the calculated velocity will be lower than what you would have on a warmer (i.e. standard) day.
 
cdfmxtech
Posts: 1319
Joined: Wed Jul 05, 2000 11:37 am

RE: Temperature And Airspeed

Wed Sep 26, 2001 10:12 am

First
Any modern commercial aircraft that you come across today that has an Inertial reference system, uses it to calculate groundspeed.
Straight from a Physics book...just for u:
The speed of an airplane with respect to the Earth is called its groundspeed. With no wind blowing, they would both be equal.

Sure as you get higher in the air, the air is not as dense, and that is why air data computers begin to use a mach number. But as far as ground speed is concerned, IT IS NOT calculated using a pressure measurement or anything from air data....at least not on any aircraft that I have ever worked on.

Second
Give me an example of what you're talking about.
 
EssentialPowr
Posts: 1656
Joined: Wed Sep 06, 2000 10:30 pm

RE: Temperature And Airspeed

Wed Sep 26, 2001 11:07 am

CDfmxtech's explanations are rather confusing, and the def of groundspeed he offerred doesn't answer the question. Aeroguy's explanation sounds right, but as always, the proof is in the requisite equation.

Assume still air conditions.
From Aerodynamics for NAval Aviators,

TAS = EAS x SQRT (std sea level air density/ actual air density)


Holding everything else equal, As temp decreases, actual air density increases. Holding compressibility and pressure constant, the SQRT () value decreases, therefore the TAS decreases. For the no wind condition, Ground speed would decrease as well.

I think the origina question asked, if no wind was blowing, and the temp was severly decreases (sea level and alt), then the TAS would decrease, as would the groundspeed. Indicated - calibrated - equivalent - true.
For zero wind condition, TAS = Groundspeed.

cheers

 
aeroguy
Posts: 66
Joined: Wed Aug 15, 2001 1:33 am

RE: Temperature And Airspeed

Wed Sep 26, 2001 12:52 pm

It seems that EssentialPowr and I are thinking along the same lines. I don't doubt that an airliner would use its INS to determine ground speed. That's all well and good but I don't see how that explains why ground speed would change with temperature. That's why I interpreted ground speed to be equivalent airspeed. Thanks to the equation posted by EssentialPowr we can see why the change in speed could be the result of the change in temperature. Now this raises a question in my mind. In order for the equivalent airspeed to change due to temperature, the game software must have calculated (and used) a new density based on that new temperature. So does this really happen with a real airspeed indicator? Can you actually recalibrate it to a different ground level density? I'm not sure I understand the logic of doing that since equivalent airspeed is essentially based on an arbitrary reference state anyway (sea level standard day), right? Does anyone have any insight into this?
CDfmxtech, I'm not sure I understand how an air data computer uses mach number. How does it obtain it and what does it use it for? Could you explain further?
 
cdfmxtech
Posts: 1319
Joined: Wed Jul 05, 2000 11:37 am

RE: Temperature And Airspeed

Wed Sep 26, 2001 1:36 pm

Victor Alpha's question was does temperature have an effect on groundspeed...I said no, and I mean temperature does not have any effect on an aircrafts determination of groundspeed.

Maybe I'm misunderstanding something a little bit. Are you guys telling me (And I'm not tryin to be funny), that groundspeed actually means ground airspeed??
I'm not buying it. That's a great formula, but that is not, to the very best of my knowledge, the way modern commercial aircraft compute groundspeed.
Temperature might affect how your engine can produce, which in turn would affect how fast the aircraft is moving thus the slower airspeed. A slower airspeed means a slower groundspeed...but temperature does not have any direct impact on the determination of groundspeed.

VA said he changed the OAT temperature. He took off from a airport where the temperature was -50F. He did not alter any aircraft settings, and the only settings you change in most aircraft is the OAT input to the FMC which is for adjustment of the N1 limit. So the temperature that he entered is irrelevant as far as groundspeed is determined. Besides, let's assume he could enter a dummy temp into the aircrafts computer. As he's flying along, how is the aircraft going to determine the temperature on teh ground as he moves away from that cold area into say a warmer one. It can't.

Guys just do me favor an answer these questions for me. let's assume that b4 VA put in a temp of -50 deg F that teh temperature was 0 deg F. Does that mean that when he put in that -50 deg temp, the temp at altitude is 50 deg colder??
No it doesn't. When you are 35 ft in the air, the temperature on the ground means very little. In fact, the difference in teh temp at 35,000 during the winter and the summer changes very little.

 
Guest

RE: Temperature And Airspeed

Wed Sep 26, 2001 7:12 pm

OK actually I dont understand what you guys are trying to say and I see lots of confusion so I'll rephrase my scenario.

This is on FS2k using the default 777-300.
In the weather, I set Temperature to be -50F, all else remains the same. I go to Chicago O'hare and perform a departure and climb to FL350, and went for a normal M.84 (under Mach control) cruise at this level.

Using the GPS map view finder (Alt-3), I see my GS (GroundSpeed) to be around 390 knots. In my PFD, my IAS shows 294 knots and M.84.

When I go back to the weather settings, and perform a "Clear All Weather", the IAS speed suddenly drops to 220 knots, and only at M.65. The engine then powers up to bring me back to M.84. I then checked the GS again and it shows 496 knots at M.84.

My question is why is there a difference when I put the temperature to -50 and then afterwards back to the default temp (think its 29).

Thanks
-Regards, VA
 
Guest

RE: Mistake IN Topic "Temperature And Airspeed"

Wed Sep 26, 2001 9:16 pm

Hi guys

I just noticed I made a mistake in the topic, it should read "Temperature and GROUNDSPEED" instead of Airspeed.

Sorry for the inconvinence.

-Regards, VA
 
aeroguy
Posts: 66
Joined: Wed Aug 15, 2001 1:33 am

RE: Temperature And Airspeed

Thu Sep 27, 2001 5:04 am

OK, Victor Alpha, we may be on to something here. Can you give us a little more info?

1) When you cleared the weather and accelerated to M.84, and 496 knots GS, do you know what your IAS was? (It might help us figure this out)

2) When you talk about a mach number displayed on your instrument panel, do you know what this is based on? Is that an indicated mach number or a true mach number? (If you know.)
 
Guest

RE: Temperature And Airspeed

Thu Sep 27, 2001 11:53 am

Aeroguy,

1. Both times the IAS was around 294-6 knots.

2. It should be an Indicated Mach Number, similar to that of Indicated Air Speed, cos the IAS showed both to be around 294-6knots for M.84

-Regards, VA
 
DC-10Tech
Posts: 291
Joined: Fri Jun 15, 2001 6:40 pm

RE: Temperature And Airspeed

Thu Sep 27, 2001 3:40 pm

I've never heard of a 'true mach number' since mach is always calculated based on temperature and density of the air.

To answer the question on how the CADC uses 'mach' at altitude:

The IAS can still be used, but what you see as altitude increases is that mach speed actually increases for a given airspeed. While you could fly at 300 KIAS near the ground and not come anywhere near Mach I, 300KIAS at cruise is very near Mach I. (as the air density decreases, the IAS appears to decrease because there is not as much ram air for the pitot tubes to read. Your TAS is compensating for atmospheric changes and will read much higher than your IAS at altitude.

You can try this in your flight sim. Fly at Mach .82 at FL 50 and look at your KIAS, then fly at mach .82 at FL 350, then you'll see what I'm talking about.

Hope I have not confused anyone!

Forums.AMTCentral.com
 
EssentialPowr
Posts: 1656
Joined: Wed Sep 06, 2000 10:30 pm

RE: Temperature And Airspeed

Fri Oct 05, 2001 11:13 am

A/c (that are capable of) compute groundspeed the same way John Paul Jones did (GO Navy)...Distance per unit time. The Air data computer uses INS (or GPS) data to divide distance by time. The equation is that simple.

The original question as I interpreted it asked how groundspeed (there is no "ground airspeed") varies with a change in temp. Since there is no direct relationship due to the above equation (what is "ground", anyway???) I modified the question to "How will TAS vary with a change in temp, assuming other variables are held constant?"

The equation I ref'd answers that question.

Cheers-



 
KU104
Posts: 21
Joined: Tue Dec 12, 2000 10:15 pm

RE: Temperature And Airspeed

Fri Oct 05, 2001 11:24 am

*Still pondering at Essential Powers Answer*
VA, what you need to do is also correct temperatures other than ground. As I recall you set the ground temp to -50F. When you climbed didn't you notice the temperature rise? FS sets temperatures on 3-4 layers. If you change one you must change the others or else you'd be getting warmer as you climb. Try doing that and give us some feedback on what your results are  Smile
Regards,
Kuwaiti-104 (EGLL-OKBK)
 
EssentialPowr
Posts: 1656
Joined: Wed Sep 06, 2000 10:30 pm

RE: Temperature And Airspeed

Fri Oct 05, 2001 11:31 am

Okay, I'll try again.

Ground speed = distance/time. That's it...
Does temp appear in that equation? No; So the question was invalid, mathematically, unless some assumptions are made.

Does that make sense?
Cheers-
 
Minuteman
Posts: 260
Joined: Sat Aug 05, 2000 1:01 am

RE: Temperature And Airspeed

Fri Oct 05, 2001 11:43 am

FOR F*CKS SAKE PEOPLE!!!

When you drop the temperature down to -50, the *SPEED OF SOUND WILL DECREASE*, causing Mach .84 to be slower than it would be on a hot day.

I dunno if FS assumes the temperature stagnation in the stratosphere begins at a certain temperature or an altitude after having the temp drop by a stantard lapse rate. It sounds like they do the latter.
 
EssentialPowr
Posts: 1656
Joined: Wed Sep 06, 2000 10:30 pm

RE: Temperature And Airspeed

Fri Oct 05, 2001 11:57 am

Roger that...

The IsofTexasRuponu. The Boz was 4-0 against ye...
 
Guest

RE: Temperature And Airspeed

Fri Oct 05, 2001 9:34 pm

Sorry if I seem ignorant, but why does the Speed of Sound decrease with a lower temperature?

Assuming it is so, any aircraft flying into the Antartica airspace would perhaps be faced with a GS of 350kts at M.84?

DC10-Tech - Err... I actually meant how ground speed varies with the temperature as what EssentialPower has stated. It was a typo error on my part, and I apologise.

-Regards, VA
 
Skystar
Posts: 1339
Joined: Thu Jan 13, 2000 3:58 pm

RE: Temperature And Airspeed

Fri Oct 05, 2001 9:49 pm

No not really,

Because Outside Air Temperature is reasonably constant.

I'm not good with °F, but if you clear weather in FS, you'll get 15°C at ground level, and at FL350 around -20°C. At FL350, you should have -55°C in theory (ISA).

You should insert a second temperature layer at FL350 at -55°C. I took off from PHX in a LH343 at 45°C, and I was wondering why the cruise performance was strange at FL350 - then I inserted a second temperature layer.

Cheers,

Justin
 
Minuteman
Posts: 260
Joined: Sat Aug 05, 2000 1:01 am

RE: Temperature And Airspeed

Sat Oct 06, 2001 12:56 am

First, sorry for the abrasiveness of my post. It had been a long day.

Now, a method for calculating the speed of sound involves the ratio of the specific heats of a fluid (cp/cv, which we'll call gamma and is roughly 1.4 for air), the specific gas constant for the fluid, R (~287 J/(kg*K) for air), and the average temperature of the fluid expressed in an absolute scale (usually Kelvin), T. This gives us:

Speed of Sound = sqrt(gamma*R*T)

Working the example of -50°F (-45.5°C), the speed of sound will be:

sqrt(1.4*287*(-45.5+273.15)) = 302.43 m/s = 587.9 knots

Mach 0.84 under these conditions is 493.8 knots, which is pretty damn close to what Victor Alpha was seeing.

Now, lets back out the ambient temperature from a 380 knot groundspeed (really true airspeed for this)

(380 knots)/(Mach 0.84) = 232.72 m/s (this is the speed of sound)

(232.72^2)/(1.4*287) = 134.8 K = -138.35°C ambient

This would give you a lapse rate of about -2.5°C per 1000 feet with -50°C at the surface, which is a little high, but hey, its MSFS!

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