Aerodynamics...

[Jonzza]

Hi,

I have been trying to answer a couple of aerodynamic questions the last week but I have found it very hard to find the answers on some of them..
Hopefully some of you guys posses the knowledge that I don't.

1. If you hold a constant Mach during climb out what happens to TAS, IAS and AOA as you approach your level off altitude?

I understand that TAS will increase, I think the IAS will decrease (but not sure why) as far as the AOA I have no clue if it will increase or decrease.

2. Trying to maintain L/D Max what needs to happen with increase in altitude regarding AOA/ IAS?

I really appreciate all help I can get! Been reading of material but still can't find this answers.

[HazelNuts39]

You started your pilot training in january 2009 and four years later you don't know the answer to these questions????

[Jonzza]

obviously not since I am asking smart ass!! And from the look of it you don't know either.
One of the thing I cant stand in this world is monday morning quarterbacks.
You seem to fit right in at that category.

[john_tullamarine]

Sometimes a little humility is a useful quality. HN39 is extremely well qualified to answer your questions ... if you will but ask nicely and then listen.

[Jonzza]

That might be the case but I will not have him talk down to me just because I ask a question. I really dont see any reason for running around and talking down to other pilots just because you posses more knowledge in one specific subject.
I am Obviously new to jets and want to learn....

[IFixPlanes]

...I am Obviously new to jets and want to learn....

Maybe you first start to learn being polite.
It can open more doors than you think.

[Jonzza]

"You started your pilot training in january 2009 and four years later you don't know the answer to these questions????"

How can you answer polite to a statement like that?

I am really a humble guy but I just dont like being talked down to for no reason.

[BOAC]

Originally Posted by JT

if you will but ask nicely and then listen.

- asking was 'nice', but listen to what exactly, John?

[john_tullamarine]

HN39 sometimes presents his views more strongly than some may prefer. In this case he didn't appear to overstep the mark .. albeit that some might take offence ? Just a view from the sidelines ...

The occasional harsh comment is a small price to pay for the availability of his elder statesman background in performance work.

Jonzza may find that a somewhat more conciliatory response might elicit a more useful answer even if that might be uncomfortable.

[BOAC]

Jonzza Q1 - 4 part 'answer' - give us your answers?

1) What happens to the speed of sound as you increase altitude?
2) What then happens to your 'speed' at a fixed Mach number " ---"---?
3) What happens to IAS at a fixed TAS as you "---"---"?
3) What happens to AoA as IAS reduces?

Not sure I understand what Q2 is seeking.

[HazelNuts39]

Originally Posted by JONZZA

If you hold a constant Mach during climb out (...) I understand that TAS will increase, I think the IAS will decrease (but not sure why) as far as the AOA I have no clue if it will increase or decrease.

If these are serious questions I apologize. I'm probably not the right person to answer them.

[Jonzza]

Hi BOAC,

My understanding is that;

1) Speed of sound decrease as temperature decreases so answer on question one is decrease.
2) TAS will increase with a fixed Mach number in a climb.
3) IAS should decrease since TAS is increasing with altitude.
4) It will increase.

Thanks for getting back to the subject!

[Jonzza]

Q2, in order to maintain L/D Max as the air get less dense will I have to increase AoA and IAS, or will it be the same as at a altitude with higher pressure?

[keith williams]

For the first question try this previous thread.

http://www.pprune.org/professional-p...highlight=ECTM

[BOAC]

Well, Keith - you have lost a finger there - that link was about the 3-finger rule!

Jonzza - you seem to be pretty mixed up there. Make sure you understand what TAS is first. Now start with answer 1), assume you are climbing at Mach1, and what happens to TAS?

3) Again indicates some confusion in your mind over what TAS and IAS are. Likewise your 'explanation' of Q2 suggests you are not au fait with these factors.

What 'level' are you at in aviation training? It will help to know. I looked to see your posting history, but where/how HN39 'discovered' your 'pilot training from 2009' - "You started your pilot training in january 2009 and four years later you don't know the answer to these questions????" I cannot see - have you deleted some earlier threads?

[Jonzza]

BOAC,

Q,3 was a typo from me I understand that IAS will Increase wit a constant TAS climb.

TAS will increase with altitude and are the speed witch the airplane moves through the air.

My understanding of IAS is that it does not change with altitude since it measures the difference between static pressure and dynamic pressure.

The question about LD/MAX is a question Delta ask on there interview and I have been trying get a good enough understanding on the subject to answer that but I'm just not there yet...

I been an instructor for the last 2 years and are getting ready to go to the airlines. I have 3 interview coming up and realize that my high altitude knowledge together with my understanding of Mach is not where it should be.
I am not trying to find a easy way out by asking questions here, but lately I have been reading a lot of books and seem like I have just confused my self even more.

[Jonzza]

Hi Keith,

The 3 finger rule was cool! Never heard about that before. However I like to understand why? I have a hard time accepting things I don't understand..

[Jonzza]

BOAC,

Forgot to answer your first question.

At a constant Mach climb TAS decreases.

Here is where I get confessed, I know I should accept this but I want to understand why.
Both Mach and TAS increase with altitude, so the only thing that make sense to me is that Mach is increasing in a faster rate than TAS and thats why TAS is decreasing in a constant Mach climb.

Thank you for taking your time and helping me with this!

[BOAC]

Can you explain how HN39 'knew' your pilot training history since you appear to have no posts before this thread.?

Now then:

TAS will increase with altitude and are the speed witch the airplane moves through the air.

Second part correct. Let's keep this really simple. Say you are climbing at Mach1 all the way (NB that is a 'CONSTANT MACH NUMBER' - yes?). At sea level (and any level) TAS = speed of sound. Agreed? You say (correctly) that the speed of sound DECREASES with altitude. So, is your TAS at Mach1 higher or lower at 30,000ft than at sea level?

My understanding of IAS is that it does not change with altitude since it measures the difference between static pressure and dynamic pressure. I think we will ignore that as it complicates things. What you should understand as an instructor is the way IAS changes as you climb at a constant TAS. Google it if you need to. It applies to Cessna 152s as well as jets!

Let's sort TAS/IAS first then look at L/D.

PS Amongst your 'library', do NOT put too much store by 'Ace the technical Interview' - it has had very bad reviews on PPRune.

Both Mach and TAS increase with altitude,

- this is mind-bogglingly confused! How can Mach 'increase' if you are climbing at a constant Mach number?

[keith williams]

Jonzza,

Hopefully this will enable you to understand the basis for the four finger rule.


INDICATED AIRSPEED (IAS)
IAS is the speed shown on the Airspeed Indicator (ASI). It includes instrument errors.

CALIBRATED AIRSPEED (CAS)
CAS is the IAS corrected for position (pressure sensing errors) errors which are caused by the location of the pitot static probes and changes in the aircraft attitude.

EQUIVALENT AIRSPEED (EAS)
As true airspeed increases, the increasing dynamic pressure compresses the air in the pitot probe. This increases the air density in the pitot tube, thereby causing theASI to over read at high speeds. EAS is the CAS corrected for this compressibility error.

TRUE AIRSPEED (TAS)
TAS is the true speed of the aircraft relative to the air around it. TAS is CAS corrected for density errors. ASIs are calibrated such that IAS is equal to TAS at mean sea level inthe ISA. As altitude increases, the decreasing air density causes the TAS at any given IAS to increase.

MACH NUMBER
Mach Number is theTAS expressed as a fraction of the local speed of sound.

Mach Number = TAS/ Local speed of Sound.



EFFECTS OF CHANGESIN ALTUDE
The difference between IAS and CAS is generally not great and is not affected significantly by changes in altitude. For the purposes of the explanation below the differences between IAS and CAS have been assumed tobe zero.

The airspeed indicator produces an Indicated Airspeed output (IAS) that is proportional to ½rVsquared. Where r is air density and V is TAS.

Any given valueof ½ rVsquared will always produce the same IAS, regardless of altitude. Climbing at constant IAS therefore means climbing at constant ½ rVsquared. But r decreases with increasing altitude, so the TAS equating to any given IAS must increase, such that the rate of decrease in ρ is equal to the rate of increase in (TAS)squared . At 40000 feet in the standard atmosphere, ρ is approximately ¼ of its sea level value, so TAS is approximately twice IAS.

As altitude increases, the reducing air density makes the air easier to compress. This enables the air in the pitot probe to become compressed, making its density increase. This increased air density in the pitot probe increases the impact pressure that is sensed by the ASI. This in turn causes the ASI to over-indicate, such that the IAS becomes slightly greater than the EAS.

The Airspeed Indicator is calibrated such that at mean sea level in the ISA, the EAS, IAS and TAS are equal. But as described above,as altitude increases, the EAS becomes slightly less than the IAS and the TAS becomes greater than the IAS. If the three speeds were plotted against altitude they would take the form of three lines. Using speed increasing from left to right on the X axis and altitude increasing from ISA MSL at the bottom of the Y axis, these lines would start at a single point on the X axis (at mean sea level) and would fan out as altitude increases. The order of these lines reading from left toright would be EAS, IAS, TAS. Although the shapes of the lines are quite complex, straight lines are probably sufficient for the purposes of the questions posed in this thread.

As altitude increases in the ISA up to the tropopause at 36000 feet, air temperature gradually decreases. Local Speed Of Sound (LSS) isproportional to the square root of the absolute temperature, so as altitude increases, the reduction in temperature causes the LSS to decrease. Mach number = TAS / LSS so for any given Mach number, the TAS decreases as altitude increases up to the tropopause. The deacreasing TAS causes the IAS and EAS to decrease, so when climbing at constant mach number the EAS, IAS and TAS all decrease up to 36000 feet.

Combining all of the above factors permits the relationships between the various speeds to be represented by four lines on a graph. If the purpose of the graph is to illustrate the qualitative bahaviour of the four speed,rather than their absolute values, the four lines can originate at a single pointon the X axis and fan out as they move up the altitude scale. To test the effect of climbing or descending with any one of the speeds held constant simply rotate the fan until the line representing the constant speed is vertical. The slopes of the lines will then indicate whether the other speeds are increasing (sloping to the right) or decreasing (sloping to the left).

Above 36000 feet the temperature remains constant, so the Mach number at any given TAS also remains constant. But the EAS, IAS and TAS will continue to diverge as described above. This effect can be illustrated by removing the Mach line from the graph and using the TAS line to represent both MACH and TAS.

In an inversion the temperature increases with increasing altitude. This causes the relationship between changes in TAS and Mach to bereversed. This can be illustrated by using the four-line graph but with the order or the TAS and Mach lines reversed.