PDA

View Full Version : Some things about speeds!



GH_Klingstroem
08-09-2005, 04:06 PM
Too many times I have seen people complaining about how they can not reach the top speeds of their aircraft so I will try to clarify in this post in a simple way how it works.

Indicated airspeed(IAS): This is simply the speed that is indicated on the instrument inside of the aircraft. On the leading edge of most (small) aircraft there is a tube sticking out about 5cm. The purpose of this tube is to give a speed reading to the instrument inside the aircraft. As the aircraft moves forward the air flows over the wings, fuselage and also into this tube. The more air (the faster you go), the more air will flow into the tube and give you a faster indicated speed inside the cockpit. We all know that the higher you go the less dense the air becomes and the lower you go, the more dense the air becomes. It becomes more compressed lower down because the whole atmosphere above is heavy and compresses the air below. Think of the air as amount of air molecules. This means that when you are flying around at low altitude there will be lots of air molecules going into the tube and give u a high indicated airspeed which is great. But say now that you start climbing, things will become different. Up high the amount of air molecules is much less, so there is less air molecules going over the wing and into the tube in the wing and so you will get a slightly lower INDICATED reading(there is also less power for the engine)!! Quite simple actually! We all know what happens when the indicated speed becomes too low! We lose lift and stall! Pretty much the only thing you use indicated speeds are to know when you will stall! You can not really use it for proper navigation at all!

True airspeed (TAS): Most of us have seen this in the game but a lot seem not to understand it. The TAS you cannot read in the cockpit in these planes on full real settings but is very important for navigation. Now, as I wrote above the air gets thinner and thinner the higher you go. That¿s the way things are and we cannot do anything about it, however the formula of lift states that in order to maintain the same amount of air molecules over the wing, when there are less of them around you, is to fly faster. This happens automaticly. We as pilots cannot control it! Even if you read 200knots on the indicated speed gauge at sea level and you now read 200 knots at 20 000 feet, you will go much faster trough the air(actually 273knots) and therefore also over the ground! Basically the aircraft has automatically compensated for the thinner air and is now flying faster through the air in order to keep the same amount of air molecules over the wing, and in the tube! Great isn¿t it?! Now this is all well and through tables you can read get your TAS, if the IAS is known and the altitude and the temperature (since the temp also changes the density of the air. Cold air = higher density=heavy. Warm air=lower density=light).
What you see in Cockpit OFF mode in this game is the TAS. You will see as you go higher your TAS will be higher and higher but your IAS will get lower and lower for the above reasons. At sea level TAS and IAS will be the same as the aircraft doesn¿t have to compensate for the difference in density. Try it in the game and you will see! The top speeds of aircraft are given in TAS and usually at what alt that top speed is achieved and NOT in IAS!! If there is no wind your TAS will be the speed that you are flying over the ground! Hence you will reach your destination faster if you go high! If there is wind the speed over the ground is different and I will explain why below.

Ground speed (GS): Ok to continue with what I wrote above. Say now that you are cruising around in your P-51 at 20 000 feet and the speed you want is 200knots IAS. Knowing that at this altitude there are less air molecules per area so that your P-51 is automatically compensating for this by flying faster to get the same IAS. Actually you and your P-51 are flying at 273 knots, through the air to give you enough air molecules to give you a reading of 200 knots on the gauge!! This is in NO WIND condition!! I will explain this now with an example



Example 1: NO WIND!!

We are flying from west to east (90 degrees on the compass) at 200 knots IAS at 20 000 feet (temp -20C) in these conditions we are going through the air as stated above at 273 knots TAS and since there is no wind to push us around this day, this is also our speed over the ground! Easy!

Example 2: now with 20 knots tailwind.

This means that we have wind helping us to get to destination. Ok we are still flying from west to east but now we also have 20 knots of wind from the west pushing us towards destination. What this does is to decrease our TAS. This happen because TAS is the speed of the air over the wing from ahead. Now there is 20 knots of wind from behind working in the opposite direction to the airflow over the wing so you TAS will decrease by 20 knots but your ground speed will increase by 20 knots. So if you did 273 knots TAS and 273 knots over the ground in NO WIND conditions, you will now fly 293 knots over the ground but your TAS is only 253 knots. Imagine a windy day, when you run with the wind there is almost no wind hitting your face but you run faster,(low TAS but high groundspeed in this case), but if you turn around and running towards the wind much more wind will hit your face(High TAS but you run slower over the ground ). Same principle! You can control nothing of this as a pilot and usually you are not even aware of it. On the GPS today you can see the speed over the ground and therefore work out the TAS if you know the wind speed and direction, IAS and the temp.

Example 3: now with 20knots headwind.

We are still flying our P-51 east at 200 knots IAS at 20 00 feet and on the thermometer we can see its still -20C outside. On todays forecast they anticipated 20knots of wind from east to west. This is a headwind working against us, pushing us back preventing us to get to destination in time! Now our TAS up here was 273 knots but know we have another 20 knots of air flowing over the wing so the true speed of the air going over the wind is actually 293 knots, but our speed over the ground has slowed down to 253 k nots!

From this we can see that TAS and groundspeed are only the same if there is no wind that day! There is almost always wind up there so very seldom is the TAS and the GS the same!
In this game however there is no wind modelled as far as I know so the speed you read in NO cockpit mode is also the speed you are flying over the ground!

As you can see IAS is only used as a way of knowing how many air molecules that are flowing over the wing producing lift. Its good to know for take off and climb out and for landing. But for actual navigation it has no purpose!

One more for you. At sea level with no wind at all, your IAS will the same as your TAS and since there is no wind it will also be your groundspeed! Hope this helps some of us!
Cheers!

GH_Klingstroem
08-09-2005, 04:06 PM
Too many times I have seen people complaining about how they can not reach the top speeds of their aircraft so I will try to clarify in this post in a simple way how it works.

Indicated airspeed(IAS): This is simply the speed that is indicated on the instrument inside of the aircraft. On the leading edge of most (small) aircraft there is a tube sticking out about 5cm. The purpose of this tube is to give a speed reading to the instrument inside the aircraft. As the aircraft moves forward the air flows over the wings, fuselage and also into this tube. The more air (the faster you go), the more air will flow into the tube and give you a faster indicated speed inside the cockpit. We all know that the higher you go the less dense the air becomes and the lower you go, the more dense the air becomes. It becomes more compressed lower down because the whole atmosphere above is heavy and compresses the air below. Think of the air as amount of air molecules. This means that when you are flying around at low altitude there will be lots of air molecules going into the tube and give u a high indicated airspeed which is great. But say now that you start climbing, things will become different. Up high the amount of air molecules is much less, so there is less air molecules going over the wing and into the tube in the wing and so you will get a slightly lower INDICATED reading(there is also less power for the engine)!! Quite simple actually! We all know what happens when the indicated speed becomes too low! We lose lift and stall! Pretty much the only thing you use indicated speeds are to know when you will stall! You can not really use it for proper navigation at all!

True airspeed (TAS): Most of us have seen this in the game but a lot seem not to understand it. The TAS you cannot read in the cockpit in these planes on full real settings but is very important for navigation. Now, as I wrote above the air gets thinner and thinner the higher you go. That¿s the way things are and we cannot do anything about it, however the formula of lift states that in order to maintain the same amount of air molecules over the wing, when there are less of them around you, is to fly faster. This happens automaticly. We as pilots cannot control it! Even if you read 200knots on the indicated speed gauge at sea level and you now read 200 knots at 20 000 feet, you will go much faster trough the air(actually 273knots) and therefore also over the ground! Basically the aircraft has automatically compensated for the thinner air and is now flying faster through the air in order to keep the same amount of air molecules over the wing, and in the tube! Great isn¿t it?! Now this is all well and through tables you can read get your TAS, if the IAS is known and the altitude and the temperature (since the temp also changes the density of the air. Cold air = higher density=heavy. Warm air=lower density=light).
What you see in Cockpit OFF mode in this game is the TAS. You will see as you go higher your TAS will be higher and higher but your IAS will get lower and lower for the above reasons. At sea level TAS and IAS will be the same as the aircraft doesn¿t have to compensate for the difference in density. Try it in the game and you will see! The top speeds of aircraft are given in TAS and usually at what alt that top speed is achieved and NOT in IAS!! If there is no wind your TAS will be the speed that you are flying over the ground! Hence you will reach your destination faster if you go high! If there is wind the speed over the ground is different and I will explain why below.

Ground speed (GS): Ok to continue with what I wrote above. Say now that you are cruising around in your P-51 at 20 000 feet and the speed you want is 200knots IAS. Knowing that at this altitude there are less air molecules per area so that your P-51 is automatically compensating for this by flying faster to get the same IAS. Actually you and your P-51 are flying at 273 knots, through the air to give you enough air molecules to give you a reading of 200 knots on the gauge!! This is in NO WIND condition!! I will explain this now with an example



Example 1: NO WIND!!

We are flying from west to east (90 degrees on the compass) at 200 knots IAS at 20 000 feet (temp -20C) in these conditions we are going through the air as stated above at 273 knots TAS and since there is no wind to push us around this day, this is also our speed over the ground! Easy!

Example 2: now with 20 knots tailwind.

This means that we have wind helping us to get to destination. Ok we are still flying from west to east but now we also have 20 knots of wind from the west pushing us towards destination. What this does is to decrease our TAS. This happen because TAS is the speed of the air over the wing from ahead. Now there is 20 knots of wind from behind working in the opposite direction to the airflow over the wing so you TAS will decrease by 20 knots but your ground speed will increase by 20 knots. So if you did 273 knots TAS and 273 knots over the ground in NO WIND conditions, you will now fly 293 knots over the ground but your TAS is only 253 knots. Imagine a windy day, when you run with the wind there is almost no wind hitting your face but you run faster,(low TAS but high groundspeed in this case), but if you turn around and running towards the wind much more wind will hit your face(High TAS but you run slower over the ground ). Same principle! You can control nothing of this as a pilot and usually you are not even aware of it. On the GPS today you can see the speed over the ground and therefore work out the TAS if you know the wind speed and direction, IAS and the temp.

Example 3: now with 20knots headwind.

We are still flying our P-51 east at 200 knots IAS at 20 00 feet and on the thermometer we can see its still -20C outside. On todays forecast they anticipated 20knots of wind from east to west. This is a headwind working against us, pushing us back preventing us to get to destination in time! Now our TAS up here was 273 knots but know we have another 20 knots of air flowing over the wing so the true speed of the air going over the wind is actually 293 knots, but our speed over the ground has slowed down to 253 k nots!

From this we can see that TAS and groundspeed are only the same if there is no wind that day! There is almost always wind up there so very seldom is the TAS and the GS the same!
In this game however there is no wind modelled as far as I know so the speed you read in NO cockpit mode is also the speed you are flying over the ground!

As you can see IAS is only used as a way of knowing how many air molecules that are flowing over the wing producing lift. Its good to know for take off and climb out and for landing. But for actual navigation it has no purpose!

One more for you. At sea level with no wind at all, your IAS will the same as your TAS and since there is no wind it will also be your groundspeed! Hope this helps some of us!
Cheers!

Bearcat99
08-09-2005, 04:20 PM
Well this was a pretty well done post..... http://forums.ubi.com/images/smilies/25.gif

GH_Klingstroem
08-09-2005, 06:02 PM
thx Bearcat99
bump http://forums.ubi.com/groupee_common/emoticons/icon_wink.gif

NonWonderDog
08-09-2005, 06:53 PM
Yep, but lets complicate it even more! http://forums.ubi.com/images/smilies/16x16_smiley-very-happy.gif

The airspeed indicator compares static pressure (vented with a little hole on the side of the plane) and dynamic pressure (from air rammed down the pitot tube). There's a little balloon inside, actually. As speed increases, the dynamic pressure from the pitot tube increases, the balloon expands, and the airspeed needle moves.

But there's a problem. The "static" pressure isn't really static. Since there's a vent port on the side of a moving plane, the pressure inside depends on your angle of attack, airspeed, and even sideslip! It's usually calibrated for cruise speed or so, but the indicator can show either high or low at different speeds (it often shows too high at high speeds and too low at low speeds, but it really depends on the plane). This effect is known as "installation error" or "calibration error." You can only really correct for this by memorizing the airplane's conversion chart. When you do so, you get CAS (calibrated airspeed).

We're not ready to convert to TAS yet, though. In addition to changing density, air also changes in compressibility (fractional change in density per unit pressure) as you gain altitude. Since the air being rammed down the pitot tube will naturally become compressed, this is important. Luckily, the compressibility of air changes very little below 30,000 feet, so this has no effect on the indicator at low altitudes. It also has very little effect at low speeds because the air won't compress much until you start going fast. At high speed and high altitude, though, the airspeed indicator will read too high. Since the formula is just insane, the only real way to correct for this is to memorize another chart. This chart applies for all aircraft, however. When you have corrected for the "compressibility error," you get EAS (equivalent airspeed).

The final step is to convert to TAS. The difference between EAS and TAS is due to the change in air density (and mach number, but that's complicated) with temperature and altitude, so you just need to divide by the sqare root of the density ratio (and some mach stuff, but that's complicated). Since you (or at least I) can't do that mentally, the easiest way is just to add 2% to your airspeed EAS for each 1,000 ft of altitude. Actually, when you do that, you might as well just go straight from IAS to TAS. http://forums.ubi.com/images/smilies/16x16_smiley-happy.gif

Groundspeed, of course, depends on wind. It depends on pitch angle, too, if you want to get silly.


Equivalent airspeed (EAS) is the one that's important to all the formulas. A plane will always (tm) stall at the same airspeed EAS with a given weight and wing configuration, for example. True airspeed (TAS) is the real measure of how fast you're moving relative to the air.

I'll just add that I have *no* idea whether or not calibration error is present in the sim. I would be inclined to say yes because of all the other little details that have been modelled, but I really don't know.

GH_Klingstroem
08-10-2005, 05:17 AM
I would say that the only speeds we have in this game are IAS and TAS. TAS and groundspeed will be the same in the game since there is no wind present as far as I know in the game...

NonWonderDog
08-10-2005, 07:49 AM
There actually is wind. The direction is coded into each map and never changes and the wind speeds are really low in all but "thunderstorm" weather, but there's wind nonetheless.

effte
08-10-2005, 09:25 AM
The wind was only present at low altitude and during the most adverse weather conditions.

Unless this has changed over the last few patches, which I haven't bothered with yet as I've been spending the summer on two wheels rather than in front of the computer, that is still the case. A shame.

Cheers,
Fred