Jet Transport performance-Please Help!

[DECU]

Hi People,

Just a few questions I dont really understand with regards to jet aircraft performace. Any help would be much appreciated, especially from the Performance Geniuses!

1)Why does VR decrease and V2 increase when the temperature decreses and at constant Pressure altitude?

2) Why does the VMCG decrease with an increase in ambient temperature for the same pressure altitude?


Thats all folks. Thanks in advance for any help offered.

[RadarContact]

As to 1) This very much depends on the limiting conditions of the takoff. The variable here is air density, which is higher in colder conditions.

2) Same reason, really. Higher air temp, lower air density, lower T/O power, lower VMCG

[Keith.Williams.]

DECU,

Among other things the minimum control speeds are related to the thrust available. The greater the thrust from the live engine, the greater will be the yawing moments caused by asymmetric thrust, following single engine failure. So anything that increases thrust will increase VMCG and VMCA. If air temperature decreases, the air will contract, causing its density to increase.

Thrust is proportional to mass airflow through the engine, so the increased air density will increase the thrust available at any given engine RPM. this will increase both VMCG and VMCA.

V2 must be at least 1.1 x VMC, so when decreasing temperature increases VMC, it will also increase the minimum acceptable value of V2 (V2 Min).

VR must be such that the aircraft can accelerate to V2 by screen height. The increased thrust at lower temperatures, increases the acceleration rate and so decreases the minimum value of VR.

[m&v]

The vr and V2 speeds are considered 'in the air 'speeds and must comply with TAS to give full value.Thats why when using flex one had to bug the speeds up to meet the TAS for a cold day.
Vmcg is lower because it's 'denser'

Wait for the gentleman from Melbourne:rolleyes:

[john_tullamarine]

DECU,

A number of boundary conditions to consider when looking at these sorts of questions.....

From where does the question arise ? (e.g. exam question, training course, idle review of a flight manual, your company's RTOW tables, etc., etc. .. are we looking at a specific aircraft, for instance ?) .. and for what conditions does the question relate ? (e.g. min speed schedule, BFL, overspeed schedule, runway parameters and condition, etc., etc.).

One of the problems with these questions is that they may presume quite specific conditions which, in turn, lead to the appropriate, but not necessarily general, answer ... and which might not be so appropriate under other conditions ... and, sometimes, one can find parts of the envelope in one AFM which might appear to contradict the conclusion for either other conditions or another aircraft.

I don't wish to be evasive, but it does help to have the whole story ....

[Alex Whittingham]

This is my understanding, although it only partly answers the question.

As stated above V2 and VR are based on functions of the stall speed and Vmca. On most aircraft the stall speed function is usually dominant and Vmca only controls the speeds at light weights and when the density altitude is low leading to high asymmetric thrust in the engine out case.

In the area where stall speed controls VR and V2 then the EAS of these speeds should remain unchanged for altitude. Because we read the speeds as indicated airspeeds (IAS) not EAS we should find a small rise in the IAS of VR and V2 as density altitude increases. To get specific, as JT suggests, the L1011-1 flap 10º graphs illustrate just this, at least with VR. The wierd thing is that V2 goes the other way, it decreases as density altitude increases. Now why would that happen?

[edited to get 'increases' & 'decreases' right. Doh!]

[Alex Whittingham]

JT, can you help?

[Keith.Williams.]

Thanks for the correction Alex, you had me puzzled there for a while!!

I do not have access to an L1011-1 manual but I (and most people studying for the JAR ATPL) do have a CAP 698. The take-off speed charts in this (figs 4.8 for 5 degree flap and 4.9 for 15 degree flap) comply with your amended observations. As density altitude increases, VR increases slightly while V2 decreases.

My own explanation of these phenomena are detailed below. I appreciate that you are probably already aware of most of what I am saying, but the ATPL students among our readers might appreciate a detailed explanation.

JAR 25 criteria are as follows:

V2 Min must be not less than;

1.2 VS for 2 and 3 engine turboprops and jets.
1.15 Vs for 4 engine turboprops and jets.
1.1VMC.

As explained in a number of the earlier posts in this string, the magnitudes of the minimum control speeds are related to the thrust available. Increasing altitude causes air density, mass flow through the engines and hence thrust to decrease. This decreases the magnitude of the asymmetric thrust problem following an engine failure. This in turn reduces the control authority required to maintain control following engine failure, so the minimum control speeds decrease. Decreasing VMC permits V2 to be reduced, provided this does not violate any of the other criteria specified above.

If we look at the CAP 698 figures referred to above, we see that V2 does indeed decrease as pressure altitude increases, but only at masses of 50000 Kg and below at flap 5 and at masses of 55000 and below at flap 15. At higher masses, the 1.15 VS or 1.2 VS criteria become the lower limit for V2. So V2 increases very slightly with increasing density altitude.


JAR 25 criteria for VR are as follows:

VR must be not less than:
V1.
1.05 VMC.
That required to attain V2 by screen height.
That required to ensure VLOF is at least 1.1 VMU with all engine operating and 1.05 VMU with one engine failed.

As altitude increases, the decreasing thrust also reduces the acceleration rate during the take-off. This reduces the allowable gap between VR and V2. So although V2 is decreasing slightly, VR must also increase to ensure that V2 is attained by screen height. The CAP figures illustrate this effect for all masses and flap settings.

V1 is the decision speed. It is worth noting that this is the only speed at which a decision needs to be made to complete or abort the take-off in the event of an engine failure. For engine failure at all other speeds, the decision has already been made by the regulations. At lower speeds the take-off must be aborted and at higher speeds it must be continued.

V1 is the speed at which it is possible to complete or abort within the ASDA and TODA. This means that at V1 it must be possible to:

Decelerate to a halt within the remaining ASDA.
Accelerate to V2 by screen height.

Acceleration rates decrease with increasing mass and/or increasing density altitude, so the allowable gap between V1 and V2 also decreases. The minimum value of V1 therefore increases with mass and altitude. This means that the minimum value of V1 at any given mass decreases as density altitude decreases.

But the aircraft must remain controlable at all times, so the absolute minimum value for V1 is VMCG. But at low mass and low density altitudes the increased VMCG becomes the limiting factor. This is illustrated by the shaded areas in the lower regions of the CAP tables, where reference must be made to the VMCG tables.

[john_tullamarine]

Alex,

Re the L1011 V2, a manufacturer doesn't have to use a min V2 schedule and, if the commercial considerations are appropriate, a modest overspeed might have been scheduled to provide a better WAT limit at the lower values of Hp .. particularly if the specific aircraft/model has high performance brake units so that a higher V1 can be tolerated in parallel .. there is not much point in going overspeed on other than a long runway unless the V1 can be pushed up as well. In the absence of an AFM I am only speculating .. but Mr Lockheed might be reducing the V2 from a slight overspeed schedule as Hp increases the better to get an optimal weight ?

I presume that CAP698 is a study guide document of some sort using a "typical" aircraft as an example. Not having that document I am not able to offer any additional comment. However I am never comfortable with these generalist questions as one has to put too many qualifications to any answer ... or else the answer becomes a rather lengthy and boring dissertation ... additionally VR and V2 considerations correlate better with T/W than a simple consideration of thrust.

.... and ... if only the real world decision to abort were as simple as it might be presented in the ATPL lecture classroom ....

[Alex Whittingham]

Thanks Keith, I hadn't checked it in CAP 698, I guess that makes it a question more specific to the TriStar.

Thank you also JT. CAP 698 is a UK CAA publication used for the ATP exams, its a B737. The TriStar had/has separate methods to calculate increased V2 TOM for both WAT and obstacle limits but it might have been what you suggest. The only thing possibly against that arguement is that the aircraft brakes were not brilliant, you could get VMBE limits on take-off at high weights and temperatures.

[Keith.Williams.]

Alex,

One of the aims of my last post was to point out the fact that your tristar observations (as amended) are exactly as illustrtaed in the CAP 698.

JT

I am sorry if you found my last post a "length and boring dissertaion", but it was not intended to be addressed to experts such as yourself. The initial question clearly required a fairly fundamental approach and that is what I attempted to give in my first post. I unfortunatley made the mistake of not emphasising the fact that real-world situations are often very different.

The CAP 698 is indeed a study guide. It is supplied to JAR ATPL students during their training and is used in the examinations. I believe it is based on a specific aircraft type, rather than a generalised aircraft. This view is based on the fact that it is clearly easier and cheaper to use existing data rather than generate new material. (Rumour has it that the 737 books were used.)

The old CAA performance examination questions were almost all practical exercises using the CAP tables. Since the introduction of JAR, the emphasis has shifted to questions requiring an understanding of why the curves have particular shapes. My earlier posts in this string were intended to address the matter at that level.

Finaly I think it is worth considering why people do or do not post questions in this forum. Some are looking for detailed real-world information about specific aircarft types and situations. Simplistic answers based on ATPL exam-level material are clearly inapropriate here. Adressing such questions should clearly be left to those with the required experiance. But even in these circumstances, we should rememeber the wider readership. Far too many answers are totally incomprehensible to many readers.

For questions with titles such as "Jet transport performance please help", a more fundamental approach is required. Answers such as "there are too many boundary conditions to answer such generalised questions" are unlilkely to be of any value to the originator of the question. A depressingly large number of students tell me that they are not prepared to post quetions on pprune, because they know they will not get an intelligible answer. Conversely, the many who ask me questions by e-mail often comment on how surprised they are at what a simple e-mail can do.

[john_tullamarine]

Keith,

Oh dear ... have I been careless again with my words ? .... the debilitating effects of the recent OzBash obviously haven't worn off yet ..

May I hasten to clarify the situation .. as a longtime classroom teacher, I find your posts both interesting and illuminating .. in fact, if my recollection is sound, I made the statement some time ago that I would love to sit in on a class or two sometime .. and I stand by that assessment.

If I may try again ..

(a) the reference to "long and boring" was in no way directed at your post .. but related only to the situation where one attempts to give an answer to a generalist question and tries to cover all the angles. For each apparently simple component statement in that answer, one must include a list of exceptions, boundary conditions, etc., etc., .. by the end of the paper, the qualifications end up being the substantive part of the answer. As you observe in that case, the cure may be ineffective in fixing the problem.

(b) in regard to abort considerations, the normal sorts of presentations which we might give in the classroom are, to a large extent, idealised and need to be tempered in real world operations by somewhat greyer decision making.

Peace, colleague, there was neither real nor intended slight directed at you or your post and, should anyone think otherwise, then I apologise without reservation.


On the question of whether the ATPL student ought to trawl here for answers, I have to disagree with you. I think that it is entirely appropriate. Furthermore, I would be horrified if any of us with dirty hands experience did not seek to help the new guys and gals coming up through the ranks. This industry is, as much as any and more than most, an apprenticeship system. And, for anyone who has trouble with an answer, the matter ought to be probed a little more ... rephrase the question or whatever ... it is just as important to keep in mind that the reader has to interpret the question without benefit of body language, gesture, discussion, and so on.

At the end of the day, if the person with the question ends up no better informed, then the exercise has been a little pointless.

[Keith.Williams.]

John,

I suspect we are misinterpreting each other a little. As you say, it is easy to do so without the benefit of body language.

I did not mean to suggest that students should not trawl pprune for information. I think it is essential that they do so. But for them to benefit from this exercise, all contributors, particularly those with greatest experience, must remember their wider audience. Too many seem to assume that the reader knows as much as they do. As I said in my previous post, this has resulted in many students simply deciding not to bother.

[john_tullamarine]

Keith,

I concur.

But, surely, there is a responsibility on the part of whoever should ask a question - if the answer does not sit comfortably, then the question ought to be pursued a bit more.

Any concern about the possibility of "looking silly", or whatever, needs to be put aside, even if that might be difficult for some .... we all started off knowing nothing .. and we all only get to where we are by listening, reading, asking, doing, mentoring, and probing.

The student ought not to accept an answer which "doesn't make sense" as, in almost all cases, that answer can be put in a different way which, for that particular student, better fits the need.

Have we not all had a student or two for whom a "standard" explanation made little sense ? .. subsequent reflection on the part of the instructor ... and a variation on the explanation ... miraculously works a treat ....

[Keith.Williams.]

John,

Your of course correct in saying that people receiving unsatisfactory answers should persist. But we must remember that many will inevitably be a bit reluctant to do so. Looking at all of the (what to many at that stage are ) unintelligible answers, it is easy for them to feel that the fault is theirs.

Sending the first post is always the most difficult. Far more students would get over this hurdle if they felt they were could expect to understand the answers they were likely to receive. I think this would best be achieved by a general improvement in the quality of answers, rather than expecting individual questioners to repeat their questions. If readers find they can understand and learn from the answers to questions that have been posted by others, they are far more likley to ask their own questions.

I also think that there is a wider problem which originates in some of the FTOs. During a consolidation course last week, I was discussing the question of whether lift is created by low presure over the wing, or by the downward acceleration of air. A student suddenly said " That makes perfect sense, but we could never have held that conversation with our ground school instructor. He would have simply accused us of being stupid." Students experiencing this type of treatment in the classroom are unlikely to be prepared to post a first question in pprune, let alone persist in repeating or rephrasing it when unsatisfied with the answers.


Finally, I can see that I have now allowed myself to make what I consider to be another very common pprune mistake. This string
started with a question about V1, VR and VMCG. We have moved on to a totally different subject without first ensuring that the originator is satisfied with the answers he has received. Only DECU can answer that question...........

[john_tullamarine]

Keith,

I promise to try and make my posts simpler ... do feel free to chastise me in a kindly fashion as required when I fail ...

Those instructors who resort to anything other than being helpful, especially those who are screamers, sarcastic or demeaning, are extremely counterproductive and can put a student back significantly and, on occasions, destroy the student's ability to learn in the short to medium term .... if the student is funding the exercise, then this is not a very satisfactory situation at all ...

Perhaps DECU can tell us if he/she is happy so far, or would prefer some more discussion .....

... and it is a laudable, if not often realised, hope for a thread not to wander off course a little ....

[miss magenta]

Thanks Keith,
I dont know if DECU has had his queries answered to his satisfaction, but you have certainly made things clear to me. I often read this forum but have never posted for the reasons you have alredy said. Maybe now I will have a go and ask when I am stuck.

[Alex Whittingham]

Have you two finished?

Keith, the TriStar observations are not the same as the B737. There V2 decreases only because of the VMCA limit. The TriStar V2 decreases as density altitude increases AND is weight related. It shows a line which appears to be a limit of 1.25VMCA, a figure I cannot reference, separately. Get an old copy of the CAA Perf A specimen graphs, figure 16, this may be where DECU's question came from.

[Keith.Williams.]

JT,

Sorry if I'm being a bit agressive (or pedantic). All I'm really saying is that if we all try to be a bit more clear, the wider readership will benefit. If the (currently) passive readers feel they can understand what is being said, they are more likley to take part and benefit from the process.

Alex,

Your first post stated that the L1011 V2 increased with increasing density altitude. This did indeed appear to be strange. You then amended this in your second post. I interpreted this amendment as meaning that V2 decreased with increasing altitude, so I commented that this is what the CAP illustrates. Your most recent post is the first time you have mentioned a red line at 1.25 VMCA. I can only suggest that this might (and I stress might) be an extra little safety margin to avoid deep stall.