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WannaBeBiggles
23rd Mar 2011, 18:15
Hi All

I came across this question and it got me thinking.

Climbing at X which is Best L/D at a constant EAS
At a higher altitude EAS will be

a. Higher than X
b. Lower than X
c. The same as X

There are a number of variables which could be thrown in here.

i.e. Through and Isothermal layer or through an inversion layer.

I've always had trouble visualising what each speed is going to be doing in each condition.
I found a handy graph which showed CAS, TAS and MACH, but did not include EAS in the equation.

So if anyone can shed some light on the above, it'd be greatly appreciated.

eckhard
24th Mar 2011, 17:19
If EAS is RAS(CAS) corrected for compressibility, then at higher altitudes compressibility will increase (due to the higher TAS at higher altitudes).

So if compressibility increases, the RAS(CAS) will over-read when compared to the EAS.

Therefore EAS will be less than the RAS(CAS).

Therefore answer (b) is correct.

I'm assuming that 'X' is an ASIR/IAS, which for the purpose of this discussion can be assumed to be equal to RAS(CAS).

Here's a repeat of my answer to a previous post in a different forum:

This is my understanding of the various different speeds that can be used and their corrections:

ASIR - Air Speed Indicator Reading; the uncorrected reading of the instrument.

IE - Instrument Error; a correction made for errors in the construction and calibration of the instrument.

IAS - Indicated airspeed; ASIR corrected for IE.

PEC - Pressure Error Correction; a correction made for erroneous static port measurements caused by the position of the port and the local airflow disturbance cuased by airspeed, altitude and configuration.

RAS - Rectified Air Speed; IAS corrected for PEC.
CAS - Calibrated Air Speed; US term for RAS.

CE - Compressibility Error; the error induced by the increase in apparent air density at high speeds due to the compressibility of air

EAS - Equivalent Air Speed; the RAS or CAS corrected for CE. This is the speed that the aircraft 'thinks' it is flying at.

DE - Density Error; an error due to the fact that pitot pressure at a given speed varies with air density. The instrument is calibrated to read correctly with an asssumed air density of 1.225kg per cubic metre (i.e. the air density at sea level in the ISA)

TAS - True Air Speed; EAS corrected for DE.

So, the full sequence is:

ASIR
IE
IAS
PEC
RAS(CAS)
CE
EAS
DE
TAS

Hope this helps!

Eck

WannaBeBiggles
24th Mar 2011, 20:17
Thank you so much! :ok:

Microburst2002
27th Mar 2011, 17:14
I hate that erroneus way of referring to the relation between EAS, TAS and density! there is no such thing as density error.

If it was an error, pilots would wish for TAS indicator, instead of a normal ASI giving IAS. But we don't. We wouldn't mind to have an accurate EAS indicator, though. But we don't really need it, anyway.

Both IAS and EAS are "suffering" from that error, but we don't care. What we really want to know is what is the dynamic pressure around us. EAS would give exactly that. TAS would not.

IAS is subject to position, instrument and compressibility error.


The question makes no sense. Climbing at constant EAS? EAS will be the same, of course! (that's why it is constant)

Jetpipe.
28th Mar 2011, 16:55
Lets begin with the basics..

We get our IAS (dynamic pressure) from the difference between Total pressure and Static pressure. This has to be corrected because of the position of the pitot tubes and static vents, aircraft configuration, aircraft attitude to the airflow, together with some almost negligible instrument error, to CAS. Next step is correcting CAS for temperature, pressure and density to get TAS. Then from TAS we can calculate our Mach.

At higher airspeeds IAS must be corrected for compressibility in our tubes and vents. This gives us EAS. So EAS=IAS at low airspeeds and EAS<IAS at high airspeeds. With increasing altitude we have to increase our IAS in order to prevent going into a low speed stall. This means that theoretically we can keep a constant EAS while climbing and IAS will increase.. As you already have found a graph including the rest (IAS, TAS and Mach) i wont bother more. You will just have to draw a straight line on the left side of IAS begining from the same point as the other speeds and then spread it upwards.

So with increasing altitude,

EAS<IAS<TAS<Mach

Now for the Best L/D speed. L/D max is obtained at a specific angle of attack which corresponds to a specific IAS for a given weight. That is for level flight. If we increase altitude we have to increase IAS and this ofcourse will increase Drag too. This means that at higher altitude L/D max is found att a higher IAS (supposing weight is being kept constant). But as described above, EAS will always be lower than IAS and therefore the answer they would like you to give them would be EAS is lower than best L/D speed with increasing alt.

In my opinion, this is a wrong question because compressibility is something we get in our instruments, not outside in the free airflow so the relation between L/D and speed is probably more right for EAS than it is for IAS. Secondly, the same graph-drag curve could be plotted were instead of IAS is EAS, giving us the best L/D speed in relation to EAS.The same applies for TAS, etc! A good example for you to understand what i mean is, if we want to measure something, say a wooden stick, we could use m,ft,in,nm,st.m..etc but it still would have the same lenght..

The sum is, EAS, IAS, CAS, Mach are mathematically linked to each other, and L/D max is a just a measure of aerodynamic efficiency.

Jetpipe.:}