Doubt about speed at sea level vs. high altitude

[crime]

Hi,

I have just bought lock on gold at extremely low price at a big store in my city. I could not believe it!, but now I have my copy so many time after the release.

Well, I directly upgraded to patch 1.12 and I've been doing some testing flying the su-25t. Now I have a doubt about the speed. At sea level, I can fly 500 km/h and everything is stable and soft. But at 8000 meters, with the same speed it is like an earthquake. Why is it that diferent behaviour if the indicated speed remains the same?

Thx!

[aimmaverick]

Because you are looking at the hud which displays IAS-indicated airspeed. Your speed is actually much higher up there thats way tremors. Check external view.

;)

[GGTharos]

Hi,

I have just bought lock on gold at extremely low price at a big store in my city. I could not believe it!, but now I have my copy so many time after the release.

Well, I directly upgraded to patch 1.12 and I've been doing some testing flying the su-25t. Now I have a doubt about the speed. At sea level, I can fly 500 km/h and everything is stable and soft. But at 8000 meters, with the same speed it is like an earthquake. Why is it that diferent behaviour if the indicated speed remains the same?

Thx!

 

At that altitude, your actual speed is much closer to 800km/h or even more, much like aimmaverick said :)

Use your mach needle at high altidudes instead of the indicated airspeed on the HUD :)

[Gripes]

Guys, crime is talking about indicated a/s which "indicates" how your a/c

will perform when flying through different densities of air...

[Corsair7662]

the reason why it trembles is because it is shaking due to the maximum airflow supposily allowed to flow over the wings. normally every plane would prolly do it at different speeds but they dont because only the 25's have the full out realistic flight dynamics.

[Ironhand]

...At sea level, I can fly 500 km/h and everything is stable and soft. But at 8000 meters, with the same speed it is like an earthquake. Why is it that diferent behaviour if the indicated speed remains the same?

 

As the others have noted, airspeed on the HUD and, also on the outer ring of the airspeed dial, is Indicated Air Speed (IAS). Here's what's happening in very simple terms:

 

There's a tube in the aircraft's nose. As you fly, air enters the tube and hits a pressure plate. The amount of pressure with which the air presses against that plate is translated into Indicated Air Speed. And you read it on the HUD and the outer ring of the airspeed indicator. So far, so good.

 

At sea level, the air is dense and, so, at a given speed the air is hitting that plate with a certain amount of pressure. However, at high altitude, the air is less dense. In order to have the same amount of pressure on the plate, you have to fly faster in order to collect the same amount of air to press with the same force. So flying with an IAS of 500 km/hr at sea level and flying with an IAS of 500 at 8 km altitude mean two very different things in terms of how fast you are actually moving (True Air Speed or TAS). On the Su-25T, TAS is actually the number you see on the inner dial of the airspeed indicator.

 

Hope this makes sense.

 

Rich

[crime]

Guys, crime is talking about indicated a/s which "indicates" how your a/c

will perform when flying through different densities of air...

 

I was thinking how to express in english a reply and Gripes has done it for me. I thought that the flying performance of a plane depends on its indicated airspeed. Now I'm not sure what is wrong, lock on or me. So, if someone with aeronautical knowledges could tell...

[Colt40Five]

I suggest you reread Ironhand's explanation because it's dead on. Think about it like this at sea level when the HUD(IAS) says 500 you are really going 500 (or pretty dam close) and IAS(speed indicated by the pressure in the pitot tube) and TAS(speed over the ground) will be about equal. If you were to maintain the same TAS, as you get higher into thinner air the pressure in the pitot tube decreases and the IAS goes down while the TAS would stay the same.

 

Short answer (as stated above) is that at high altitude the speed indicated in the HUD(IAS) does not give you an accurate account of your speed over the ground(TAS). But that's why there is a gauge which shows both.

 

Does that make any more sense?

[hitman]

Plain ol' english: your plane is shaking because your going to fast.

[golfsierra2]

Hope this thread does not end up in discussions about the relationship between true airspeed, indicated airspeed and ground speed....

 

This is very dry physics and math and I never liked it much...

[Ironhand]

Hope this thread does not end up in discussions about the relationship between true airspeed, indicated airspeed and ground speed....

 

This is very dry physics and math and I never liked it much...

Oh....but the relationships are fun. :)

 

 

Calculate your True Airspeed:

 

1) Take half of your altitude and add it to your indicated air speed.

Example: 250 KIAS @ FL240 => 240/2 = 120 -> 250+120 = 370 kts (TAS)

 

 

2) Divide your altitude by 1,000 then multiply by 5. Add the outcome to

your indicated airspeed.

Example: 35,000 ft / 1000 = 35 -> 35*5 = 175 -> 175+280 = 455 kts (TAS)

 

 

3) Increase your indicated airspeed by 2% per thousand

feet of altitude.

Example: IAS 180 kts @ 10,000 ft => TAS = 180 + 20% (2*10) = 216 kts (TAS)

 

 

4) Subtract TAT (total air temperature) from 444 = TAS at M.72. For every .01 Mach above/below .72 add/subtract 5 kts.

Example:Mach .75 > TAT=10 °C => 444 - 10 = 434, then add 3 (

75-72

) * 5 kts => TAS = 449 kts

 

 

5) Take altitude in thousands of feet and muliply by three, add this to indicated airspeed, and add seven knots for true airspeed. This will lose accuracy with very low temperature or high pressure. Normally accurate to within five knots.

Example: 24,000 ft > IAS=200 kts => 24*3 = 72 -> 72+200+7 = 279 kts (TAS).

 

:)

 

BTW, this isn't mine. Here's the source: Calculate your True Airspeed (TAS)

 

Rich

[Guest IguanaKing]

What the others have said is true. One thing to add though is that the ADC (Air Data Computer) is fed with pressures from both the Pitot (Dynamic) and Static systems. The Static system is connected to ports on the sides of the aircraft, and it measures outside air pressure to give you altitude. Static system pressure is also important in IAS. Pitot pressure goes into a balloon-like diapragm, which is mechanically linked to the indicator needle. Static pressure goes into an area surrounding the diaphragm, so it compensates just a tiny bit for the decrease in air pressure entering the pitot system. But...this compensation doesn't quite keep up with the decrease in pitot pressure, so your IAS falls further and further below TAS the higher you fly. Many aircraft, such as the A-10, have a "barber pole" on the ASI which will actually, slowly move counterclockwise as you increase altitude. The barber pole is your overspeed warning, and it moves to a slower and slower IAS as you increase altitude. So, the behavior you are seeing in LOMAC is accurate.

 

OK...whoops...sorry if I made anybody's head spin. :D

[crime]

Ok, now I see it.

It is not intuitive to me at first glance. The same pressure that acts on the pitot tube to make it mark a certain speed is acting on the rest of the front surface of the aircraft. So it seems that it should perform the same at the same IAS no matter the true airspeed is.

The shake of the cockpit and the lack of estability indicates being near transonic speeds, not the structural limit as I thought. The su-25 has low sweep wings and can't fly transonic speeds smoothly.

 

I see it so clear now that I feel ashamed of my question...

Thanks a lot for the replies.

[bogusheadbox]

So here is a question for you.

We know the relationship between IAS and TAS and the part that altitude plays on it..

But lets have a look at the shaking and modelling behind the shaking.

As the airpressure is less and therefore less dense at altitude. Then as a hypothesis (without surpassing the sound barrier) shouldn't a plane flying at IAS 400 at ground level have the same flight characteristics as a plane flying IAS 400 at FL300 ? (withrespects to buffeting, roll, pitch and yaw). For surely as the aircraft sensors are detecting the same force of wind acting on the airframe. Should it then have the same performance characteristics?

So why is it that in game my plane flying at high altitude will suffer buffeting when its reading the same airpressure / forces as flying at IAS 400 at ground level. When at ground level buffeting won't occur until a much higher IAS is reached?

[crime]

So here is a question for you.

We know the relationship between IAS and TAS and the part that altitude plays on it..

But lets have a look at the shaking and modelling behind the shaking.

As the airpressure is less and therefore less dense at altitude. Then as a hypothesis (without surpassing the sound barrier) shouldn't a plane flying at IAS 400 at ground level have the same flight characteristics as a plane flying IAS 400 at FL300 ? (withrespects to buffeting, roll, pitch and yaw). For surely as the aircraft sensors are detecting the same force of wind acting on the airframe. Should it then have the same performance characteristics?

So why is it that in game my plane flying at high altitude will suffer buffeting when its reading the same airpressure / forces as flying at IAS 400 at ground level. When at ground level buffeting won't occur until a much higher IAS is reached?

 

The pressure at the atack border is same, but the speed at wich the wing is moving through the air is different. The aircraft must take apart air molecules. It is very diferent to take apart them at low speed than at transonic speeds. I'm not sure, maybe I'm mixing terms, but I think that it's called the compresibility problem at transonic speeds. This makes the air behave so diferent that the computer models need to be diferent at low and high speeds.

 

Last night I was having a dejà vu feeling about all this and a recall from the deep of my sick mind came out: I did a similar question to eagle dynamics when 1.1 demo was released! I ask about the su-25's speed limit at very different altitudes being determined by TAS and not IAS. But in a jet aircraft things are more complicated that in a cessna 150. This is what they told me:

----

Concerning the max speed I have to recall that there are several types of a

speed limit:

 

1. Physical or energy limit: the plane flies horisontally at the constant

altitude and the thrust is equal to the drag.

2. Structural restriction or IAS restriction. It's a never-exceed speed that

is defind by structural limits and/or controllability and stability reasons.

It is actual mostly at low altitudes. Modern fighters at low altitude

generally has this restiction lower than energy limit.

3. Mach restrictions or TAS limit. The second cause for a never-exceed speed

mostly for the planes with the non-swept wing such as Su-25 or A-10. If the

speed get closer to this limit there are some stability issues such as nose

tucking, the loss or reducing of pitch controllability, buffeting, etc. I

should note that IAS at high altitude is less than max structural IAS though

Mach number is high.

 

 

All these limits can be actual when the altitude varies.

 

 

Planes in 1.02 has the first limit regardless of Mach or IAS separately

because the thrust is a complicated function of Mach number, altitude

(temperature and pressure) and the drag is a function of IAS where CL and CD

coefficients are functions of Mach number.

Restriction 2 is very hard to implement because in Lomac very hard to

imagine how it can be modelled reducing of airframe lifetime because of

ovaerspeed. Nevertheless in 1.1 the dramatic reducing and even the negative

roll responce for Su-27 and MiG-29 is modelled. So there is no rreason now

to overspeed planes at low alt.

 

Su-25 and 25T as non-swept wing planes have the third speed limit basically.

Su-25 without payload is very overpowered plane so the energy limit is much

higher than the Mach limit and the pilots should control the thrust very

carefully not to overspeed the plane.

 

There are numbers of Mach resriction depending on various payload so be

ready to have a buffeting while you exceed 550-600 kph IAS with Vikhr or MBD

under the wings.

 

I'll be glad if I answer all your questions,

Regards,

Dmitry

[Gripes]

Crime... if you have any more of Dmitri`s responses with regards to

flight modeling... don`t hasitate to post them:) Nice explanation.

 

comressibility, ICE-T... time to hit the books again:smartass:

[ARM505]

It's not an IAS/TAS issue, it's hitting it's critical Mach number (the airflow over parts of the aircraft, esp. the wings, is becoming supersonic). As mentioned above, in a more verbose post than this one!