TODAYS Performance exam

[spitfire747]

Hello all....

Todays performance exam seemed to go well, with only a few questions foxing my brain.....

- what happens to the speeds Vx and Vy with increasing altitude for a given TAS?

a- both increase
b- both decrease
c-Vy remains constant, Vx either increases or decreases
d- ????????

I went for a as everything I have seen said Vx is constant and Vy decreases, this wasn't here and as TAS decreases when climbing I took a guess and went for this...


and this one



- At what speed is the most induced drag produced for straight and level flight ata given alt and mass

a- Vmo
b- Vs1
c- speed for slowest flight at that altitude
d- ?????

I went for c- and not b as VS1 is the stall speed and hence stalled hence no lift and no induced drag, and at slow flight the angle of attack would need to be highest to provide as much lift as possible hence more induced drag..



Anyone KNOW the correct answers


Cheers, Spitty
:-0

[pugzi]

You were right on both accounts, except your answers for the second were a tad different than what was in the JAA exam. The correct answer was worded, "slowest possible speed for any given configuration".
A point to note, VS1 is not a stalled condition by the way, the aircraft is still flyable, admittedly "just flyable"
Vs1 - stalling speed or minimum steady flight speed in a specific configuration
For example in Stall Demonstartion in JAR 25, Vs1 corresponds to the stall speed with flaps in the approach position, the landing gear retracted, and maximum landing weight). (See ACJ 25.201(a)(2).)



Well done

Steve

[spitfire747]

well thats a relief, thanks steve for setting my mind straight

Cheers, Spitty
:)

[Alex Whittingham]

That second question worries me. If VS1 is quite legitimately interpreted to mean 'minimum steady flight speed' both answers (B) and (C) seem to be the same. Pugzi?

[CAT3C AUTOLAND]

Spitty,

Looks as if you may have 100%, or maybe 96% if second question does not come home :ok:

Good Luck mate.

CAT3C

[spitfire747]

Alex

Thankyou, i am not 100% certain that those were the exactly worded answers, I did however raise a point of writing to the examiner about this and a few other questions.. (not word for word

...... why are step climbs required.......?

a- when the optimum altitude is exceeded
b- because atc require you to use high alts
c- something to do with cruise bffet boundaries
d- something to do with 1g stall speeds

i went for B, because A if rubbish, you climb when the optimum altitude exceeds you not when you exceed it

B is not strictly correct either becuase ATC do not require you to use a higher alt, infact it would easy to manage the system if everyone wanted one height for their duration, it is the pilots who require a higher altitude, for better economy etc.., again i made a note to the examiner, but who knows what they want

CAT III C

I know for a fact i got a fuel consumption question wrong !! so roll on 92%

[Dick Whittingham]

Being retired and away from the data I can only offer my interpretation of Vs1 as being the speed at which you are in trouble. You may be in trouble because you have stalled and cannot maintain steady flight or because, although not fully stalled, for some other reason you cannot maintain steady flight. Lateral stability problems come to mind.

So Vs1 is a point where you are in a changing rather than steady situation, and the lowest steady speed flight condition is just above Vs1, at max useable CL. At Max Cl you will have max induced drag.


Dick W

[Keith.Williams.]

I think that you have picked the correct answers Spitty but for different reasons. I am also more concerned about the first question than I am about the second.

QUOTE
- what happens to the speeds Vx and Vy with increasing altitude for a given TAS?

a- both increase
b- both decrease
c-Vy remains constant, Vx either increases or decreases
d- ????????

My problem with this question is that the speed you have chosen to climb at (whether constant CAS, EAS or TAS) has nothing whatever to do with the variations in the values of VX and VY.

As discussed in previous strings on this subject, option (a) is true if we are talking about the TAS value of Vx and Vy. But option (b) is true if we are talking about the CAS values. I suspect that they are looking for the TAS values (option a), but there is nothing in the question to makes this explicitly true.

You could be climbing at constant TAS watching a Vxometer (indicating the value of Vx) and Vyometer (indicating the value of Vy) (yes I know such meters don't really exist). If these meters were indicating in terms of TAS, both readings would increase as you climbed, with Vx increasing faster than Vy. If you repeated the process having switched your vxometer and vyometer to CAS you would see the readings go down, with Vy decreasing faster than Vx. The question of what type of speed you were holding would not affect the readings on these meters

The most difficult question here is why do the examiners not ask something like " How do the TAS values of Vx and Vy vary with increasing altitude?" They could make a set of three questions covering CAS, EAS and TAS and we (the FTOs) would be happy that we were able to teach the subjects properly and expect the students to get the marks in the exams. Having expended a good deal of effort in ensuring that my students have a thorough understanding of this subject, I find it very annoying to see the hair brained questions they are then asked in the JAR exams.


QUOTE
- At what speed is the most induced drag produced for straight and level flight ata given alt and mass

a- Vmo
b- Vs1
c- speed for slowest flight at that altitude
d- ?????

If we take option (c) to include the possibility that we are using landing flap, we can go down to Vso without stalling. I suspect that this is what the examiner is after.

[spitfire747]

Dick, Keith

Thankyou for your replies, i remember this Vx Vy TAS question in a previous paper and i am sure there was uproar over it then.. but it is sometimes difficult not knowing the EXACT wording to make a complaint.

I raised some points on the paper, maybe someone else who sat the exam can update the questions above more precisely

Cheers all
Spitty :-)

[pugzi]

Alex,

You're right, it seem a little unclear and in my honest opinion this question is a unfair and there are grounds to send it back to CAA for a re-word. A tanacious battle would be on their hands if the refused. VS1 is also defined as a stall speed in a specified take off configuration. This then means it is restricted to either 5° or 15° flap setting for specimen aircraft 737. Making it a little clearer that the "slowest possible speed for any given config" would be the better answer because it would include all possible range of flap.
Alex, there was also one question which begs to be clarified from yesterdays exams and I would very grateful if students who saw this question could correct the wordings.
"Flying at Vmd at a constant angle of attack, what is the effect on ROD, Descent angle and the L/D ratio if you maintain a constant forward speed at a lower mass"
a) increase increase increase
b) increase increase decrease
c) derease constant decrease
d) no idea.

This is tricky. The question hinges around this point. Is the lower weight aircraft at the old Vmd or the new Vmd (lower weights have lower Vmd).
The suggestion is that speed is constant and angle of attack is constant and therefore the aircraft is still at the old Vmd. If this were the case then angle of descent would increase (because you are no longer at best descent range speed or best angle of descent speed) With the same descent speed as before but now with a higher descent angle then ROD would increase and since you are not at the new Vmd, then the L/D ratio would decrease (Vmd being max L/D ratio)
Answer A.
However if the question is just poorly worded and the JAA mean the aircraft is at the lighter weight Vmd then the answer changes wildly. ROD would decrease, descent angle would remain the same and the L/D ratio is constant.

Please clarify my thinking and or make suggestions whether the wording is correct.

TA

[Alex Whittingham]

Vx and Vy are only meaningful as indicated airspeeds. What's the point of knowing the TAS of your best rate of climb speed, you want to know which speed to fly at, the IAS.

Uses of Vy that I can immediately think of are

1. as a target IAS for fighter pilots trying to gain height as quickly as possible for an interception.

2. as a target IAS on light twins, single engine. Vyse, otherwise known as blue line speed.

There is still an arguement, though. One can prove the EAS of Vx stays the same and the EAS of Vy decreases but, if you're talking CAS, then compressibility slightly increases the CAS of Vx as altitude increases.

In summary, I'm fairly convinced the examiners are assuming Vx and Vy are expressed as an IAS. If they assume IAS = CAS = EAS then the correct answer is that Vx stays the same and Vy decreases, if you assume IAS = CAS but that CAS and EAS are not the same then the answer is that Vx increases slightly and Vy decreases. I have to say I don't understand what is meant by 'for a given TAS' and I'm afraid I don't agree with Keith's option (b) for CAS values.

[Send Clowns]

"what happens to the speeds Vx and Vy with increasing altitude for a given TAS?"

I am with Keith and Alex, curious as this question makes no sense at all. Vx and Vy are both CASs. Therefore they are not dependent upon TAS. Also Vy reduces and Vx increases with increasing density altitude, until the absolute ceiling where they are identical.

Was the question about rates and angles of climb, in which case both decrease?

-----

Not concerning the main topic, but Spitfire something you said would have my FI(R) ground instructor spinning in his grave were he not still alive, as it was something he is always trying to irradicate as a misconception. You should not have been taught this at any stage, let alone ATPL!

"... stalled hence no lift and no induced drag ..."

A stalled aircraft is producing lift, and is producing induced drag. Draw a lift coefficient curve. The highest point is the stalling point. What happens after this? A stall is not a sudden, immediate loss of all lift. What happens is a reducing lift coefficient with AoA and a rapid increase in drag, that is a stall!

[spitfire747]

Send Clowns

the question was worded EXACTLY the same as the questions in everyones feedback about altitude and Vx and Vy with the addition of the word TAS

also i take your point about my answer, no lift after the stall, and i was not taugt this but emphasised it in this way... less lift is less induced drag, hence the worng answer, i hope....

does anybody else who did the exam on monday have a more exact wording for the question ?

[Keith.Williams.]

Sorry Alex you are of course correct about CAS. Now why did I write that (my senile dementia must be getting worse. I'd take more fish oil tablets but cannot remember hwere I've put them).
The rest of your post illustrates admirably that none of the listed options are correct. So just how many bites of this cherry do the CAA need to finally get it right? Or is option d (???????) the answer they are looking for?

[spitfire747]

if i remember correctly answer D?????? had nothing to do with the question, so in hindsight i guess it could have been the CAA answer..

[Send Clowns]

Well I'm perfectly willing to believe that Spit747, having sat the blasted things 3 years ago. I seem to recall joining discussion of a question remarkably like this between some students and our PofF instructor some time ago. Should have been removed by now. Would be comic if it were not so important to the students!

[spitfire747]

i thinnk my group had a chat about every subject after the exams, always seemed to be at least one dodgy question per sitting.... oh well thanks everyone and only a week to go :)

[Keith.Williams.]

The first two questions in this string were appealed by a number of schools. The appeals were rejected on the basis that the feedback was inaccurate and the questions as written in the exam paper were safe.

[Send Clowns]

I believe the question is about TAS that relates to Vx and Vy, not constant TAS. Therefore they both increase with increasing altitude.

The moral is: RTFQ, read the question!

[spitfire747]

Keith

The CAA didn't hint at the right answers did they ?
Spitty
:bored:

[Send Clowns]

Spitfire, TAS of Vx and Vy always increases with height, because Vx increases and Vy reduces slowly. The TAS of a given CAS increases more quickly than either effect, so is dominant.

Hence the answer is "both increase".

[spitfire747]

SC

Thanks buddy, got that one wrong then

what about the second question about induced drag..
what the opinion the correct answer is for that one

Cheers
Spitty

[Dick Whittingham]

Cutting right back to the key point on the second question, and ignoring the side issue of stall definitions, the answer is C.

Cdi is proportional to Cl squared, but induced drag itself is a function of Cdi and V squared. In level flight -note the qualification - you can reach Cl max for your configuration at various speeds, depending on what Cl max your configuration gives you. As you fly slower V is coming down, and as lift is constant Cl required for level flight is going up proportional to the square of the speed reduction. Cdi is thus going up as Cl to the power 4. So with actual induced drag being Cdi times V squared, induced drag is going up as the square of the speed reduction. Got that?

This means that, however you adjust Cl with flap etc, you will still need a big Cl for slow flight, and this means a massively bigger Cdi and a bigger induced drag. This is a key practical point in handling rotate and lift-off. Do not rotate to extreme attitudes and do not lift off early or induced drag will bite you.

Dick W

[spitfire747]

Thanks Dick

I will leave it and call the chapter closed.. unless of course I need to re-open it once the white envelope arrives from the campaign against... which i do hope i never need to..

Thanks for all you input... roll on the weekend :ok:

[Send Clowns]

Dick is right that C is correct. However, according to the fine people in the Belgrano, it was not one of the answers on offer! This is an RTFA, i.e. read the answers. The answer is therefore stall speed.

On the plus side, this is a useful, technical disussion of PofF for our students sitting next month :D

[MorningGlory]

Well I'm awfully confused now, maybe the atpl's were too long ago, because I remember one of my instructors telling me that in prop as altitude increases vx increases and vy decreases, and in a jet as alt' increases, vx remains constant and vy also decreases.



:= :confused: :=

[Send Clowns]

MG - V speeds are all CAS. This was asking about the effect of altitude on the equivalent TAS. Since The TAS equivalent to a given CAS is increasing with altitude much faster than any change in Vx or Vy, this is the dominant effect. Therefore the TASs always increase with altitude.

[MorningGlory]

Yep gotcha, after sitting here scratchin' me head trying to figure this one out all over again for the last 20mins, I now remember and yes you are quite correct :ok: