Performance - difference of opinion

[going round]

I came across an Oxford feedback question which goes like this:

With which conditions would the aircraft need to be flown, in order to achieve max speed?

a) Thrust set for min drag
b) Best lift drag ratio
c) Max thrust & max drag
d) Max thrust & min drag

Oxfords answer is (c). I have spoken to an oxford student who said the reasoning for this relates to the graph with power available and power required curves. The vertical axis being power, the horizontal being TAS. At the point where both curves meet and the aircraft is going as fast as it can, both values are at their maximum, therefore answer (c) applies.

However, I have just returned from mod 2 at Bristol where the same question is answered with (d). I raised the point with Alex who said words to the effect that no pilot would decide to configure the aircraft for max drag in order to achieve max speed and that this was a question that was looking for a common sense answer. (Appologies if I miss-understood or miss-quoted you Alex).

So, two respected schools, a differences of opinion. I'm sitting in August. All contributions considered and I'll go with the majority!

[mad_jock]

There is nothing in the question to say that the airframe is dirty.
So we have to presume it isn't.

When in stable steady flight thrust equals drag.

And we are looking at the fast end of the drag curve not the dirty side.

So when the thrust is at a max the drag will be at its max for level flight and that configuration.

In fact even when the airframe is dirty as long as your level the max speed for that configuration will occur at max thrust which will also equate to max drag because of said graph and everything needing to be in balance.

Even in a descent discounting any of the limits airframe put on air speed, for any rate of descent the max speed will be at max thrust, which will be at max drag because otherwise the plane would accelerate. The only difference between this and level is the airspeed would be greater.

So looking at it we don't even need to presume the airframe is clean or that we are flying S & L.

Max thrust and min drag would give you the best excess energy which would be best acceleration or best climb.

MJ

I have just looked up swatton

section 13.3.3 page 98

"The maximum speed is achieved when maximum thrust is equal to the total drag"


Sorry Mr Swatton if this breaches copyright.

[Keith.Williams.]

This is another wonderful example of the ambiguous nature of many JAR exam questions.

Alex has interpreted the question as asking for the configuration that should be used. Using this interpretation he is quite correct. To maximise speed you must maximise thrust and minimise (coefficient of) drag.

Oxford and Mad Jock have taken the question more literally and considered the conditions that will exist when an aircraft is flying at its maximum speed. To do this they have considered the drag and thrust curves and examined the values of each at maximum speed. At this point the curves cross, so drag and thrust are equal and are both at their maximum values. Using this method minimum drag occurs at Vmd which is most certainly not maximum speed.

If we wanted we could look a little deeper and find even more problems. Thrust from a propeller is greatest before brake release and reduces as forward speed increases. So maximum thrust occurs when the aircraft is stationary on the ground. And thrust (for a given throttle setting) is minimum when forward speed is maximum. so for a prop aircraft we might conclude that at maximum forward speed the thrust is minimum and the drag maximum.

Personally I believe that Oxford and Mad Jock are correct. Unfortunately none of us have any way of knowing what the examiners think the correct answer is. Unless of course we could find a student who scored 100% in an exam in which this question apeared. (I'm not holding my breath here).

So if all of the students at OATS and all of the students at BGS (and any other school we might choose to name) remember exactly what they have been taught, some will get the marks and some will not!!!!!

This problem will be resolved only if all students finding this question in their exam challenge it on the basis of the above argument. They should then report the problem to their CGI, so that he can also callenge it.

A similar problem exists with questions relating to changing pitch angle in a glide. If the nose is already pointing down and we push it further down have we increased or decreased pitch attitude. The (conventional) answer to that question depends on which school you go to. But once again, only the examiners know what they accept as the correct answer.

[Alex Whittingham]

You've been in the books too long.

The premise you are putting forward is that the aircraft would need to be flown at max thrust and max drag in order to achieve maximum speed.

Getting picky, I agree that at any steady flight speed thrust equals drag and power available equals power required. It follows that at the maximum steady flight speed maximum thrust equals drag, but not maximum drag.

[mad_jock]

So why wouldn't it be maximum drag for that configuration?

[Alex Whittingham]

The question doesn't say maximum drag for that configuration any more than it says maximum thrust for a particular throttle setting.

You have assumed that the parameters of the question allow you to maximise thrust with the throttle setting. Both configuration and airspeed change drag, why assume one and not the other? My point is that you hit max speed when max thrust equals drag, not maximum drag. You can't increase the thrust in that condition, but you can increase the drag. You will actually get a higher maximum speed if you can reduce the drag.

[Send Clowns]

One thought (performance is not in my speciality, so I may have the wrong end of things) is that for level, unaccelerated flight thrust and drag are equal, so max drag in level flight = max thrust. Max drag for level flight is therefore assumed constant (as thrust available is assumed constant), so at max level speed drag is a maximum, as the aircraft will accelerate until this maximum is reached. Considering how the graphs of thrust available and thrust required/drag against EAS are drawn, i.e. for level flight, this may be the consideration the examiners are making.

I see the question as reported here to be ambiguous (it does not say level flight). The level of debate, even among instructors, suggests it is a poor question.

[mad_jock]

It dosn't matter if it is level flight or not sends clown.

You can be climbing or decending the only thing that will change is the max speed obtained.


Alex

Since you say configuration isn't mentioned so we can discount it. In that case we can discount the dirty side of the drag curve and presume S & L

But how can you decrease the drag of a clean aircraft flying at its max thrust at constant speed in straight and level flight?

MJ

[Send Clowns]

Not true, mad jock.

For example in a steady descent

drag = thrust + W sin (angle of descent)

So max drag will be greater than the max drag the aircraft can achieve in speed-steady, level flight.

At speeds above minimum drag speed drag increases with speed. An aircraft at maximum thrust kept level will eventually reach a steady speed, its maximum level speed at that density altitude. Drag is increasing with speed so this must also be the maximum drag the aircraft is capable of sustaining and remaining in level flight. If the nose of the aircraft is then lowered without changing the thrust, the speed will increase. This will increase drag.

If instead of lowering the nose the drag was increased by some other method, for example deployment of speed brakes, the thrust would not be sufficient to keep the aircraft at that speed in level flight. If the aircraft were kept level it would slow down until drag was again equal to thrust.

Thus the drag can only be increased beyond maximum thrust, in level flight for a transient period, this cannot be sustained.

The thrust and drag curves are drawn for sustained, level flight, so that may be the assumption for the question. Knowing JAA they may have forgotten to specify this.

[FlyingForFun]

Surely max speed for a given aircraft is defined as Vne. And to get to Vne, I would cut the power (depending on engine/propellor configuration) and point the nose down. That's not given as an option though

Has to be one of the most badly-worded questions I've ever seen come out of JAR-land. And that's saying something! Unless the feedback is wrong, and the actual question is worded better, of course...

FFF
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[mad_jock]

yes but it dosn't change the fact that max drag occurs at max speed at max thrust which is what is was trying to get at.


Think of it in energy terms.

Energy in always has to equal energy out Energy in:

Energy from the engine
Any potential energy converstions.

Energy out:

Drag which obeys the speed squared law


So for a steady state flight, climbing, decending or S and L the potential energy converstions are going to be constant.

The only 2 things left are energy from the engine and Drag

so E engine = constant + Edrag

So the Energy of the engine is directly proportional to the energy lost to drag.

So when the energy input of the engine is at its max the energy lost to drag must be at its max for whatever steady state the aircraft is in.

So as the questions, answers have no reference to changing the aircraft away from steady state. And it also states max thrust so throttle setting dosn't come into it, it just means max power output.

Sorry again for Swatton but fig 14.6 on page 111 demonstrates this quite well.

If for any reason you can decrease the drag (which i am not saying you can) the plane will then have excess energy going into the system so it will accelerate until the drag again equals the energy into the system so we are back to max thrust (energy in) equals max drag (energy out)
MJ

[Alex Whittingham]

You can't reduce the drag on a clean aircraft but you can increase it, that's why I'm saying its not maximum drag. If you are going for a maximum speed, the drag is actually as small as you can get it at that speed, minimum drag.

Your quote from Pete Swatton, who is usually careful with his words:

"The maximum speed is achieved when maximum thrust is equal to the total drag"

He does not say 'maximum drag', he says 'total drag', which is exactly correct.

[FlyingForFun]

So for a steady state flight, climbing, decending or S and L the potential energy converstions are going to be constant

Sorry MJ, I don't think that's right... For a steady descent, gravitational potential energy is being lost. That's why the equation which Send Clowns gave works:

drag = thrust + W sin (angle of descent)

That's also why it's possible to have a steady descent with no power.

FFF
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[mad_jock]

In that case they are one in the same thing.

max drag equals min drag.

But as all the texts refer to min drag Vimd as the tangent intercept of the power speed curve that is changing the whole concept of the term minimum drag for one question.

FFF you can't loose energy you can only convert it.

So by exchanging potential energy of a big lump going up or down for kinetic energy of said big lump going through the air. The energy input still needs to equal energy out.

So by decending you will add kinetic energy in which means the amount of energy through drag will increase, the only way it can do that is if the air speed increases. The drag will increase as will the airspeed but at max thrust you will still have max drag.

MJ

just read your question FFF.

In a glide you are using the potential energy of your height, by decsending you are converting that energy to kinetic which is your airspeed which is balanced by your drag for steady state flight.

[Alex Whittingham]

Why not? You've changed the concept of maxium thrust to read 'maximum thrust at that speed'. Maximum thrust occurs at zero airspeed unless you're talking low bypass jets at high subsonic Mach numbers and beyond.

[mad_jock]

No I haven't, thrust is an energy input. Which is dependent on multiple variables which the question hasn't mentioned so we can presume are fixed.

And this is for a steady state system. So for a fixed energy input which we have agreed because both answers include Max thrust the plane must be converting the same amount of energy as drag as the energy input of the thrust.

If we change any variables so that the max thrust changes the drag will alter to that energy output And so the max drag would also vary.

At zero airspeed is the system steady state or will it accelerate?

[Alex Whittingham]

You're only refining your arguement to a much more correct statement which I would paraphrase as:

"As speed increases on a turbojet the power available increases and the power required increases. As long as power available exceeds power required in level flight the aircraft will accelerate. Once power required increases to equal power available the aircraft will not accelerate."

If this is what you're saying I agree with you entirely. We only differ on the use of the word 'maximum'. I'm saying that the power required on a clean aircraft at max level flight speed is not the maximum power required that can be achieved, I can increase it by popping the speedbrakes out and that will reduce the speed. I reckon that it is the minimum power required that can be achieved in that steady state. The lowest value achieved in a steady state gives the highest speed.

[Keith.Williams.]

My goodness isn't this fun????

We can argue as much as we want, but even if we manage to beat each other into agreement on a single answer we will have achieved nothing useful.

We will still have no way of knowing what the examiner believes the correct answer to be. This is the fundamental point of my original post.

Although apparently simple, this is a very badly worded question. The fact that each of us believes that we have the (obviously) correct interpretation and cannot understand each others (irational) viewpoint, illustrates the ambiguity of the question.

As I said earlier, we can resolve the matter only by challenging the question whenever it appears in an exam. But we are likely to challenge it only if we can recognise the problem.

[mad_jock]

It uses the word thrust so its talking about a force pure and simple, nothing at all to do with engines and there various effects
(swatton 13.2 page 96)

You can go from fully dirty spoilers out the whole lot at max thrust and say right well we have max speed now for this configuration so therefore is we are at max drag. Because thrust must equal drag for a steady state system.

But we are dirty so spoilers down.

Speed increases until max drag again equals max thrust and the beast stops acc.

But we are still dirty so up with the gear.

Speed increases again until max drag again equals max thrust and the beast stops acc.

But we are still dirty so away with the flaps

Speed increases again until max drag again equals max thrust and the beast stops acc.

Hang on we are clean now. Can't reduce drag anymore

So we are sitting at max thrust and max drag again.

Through all these cases the value of the max drag and the max thrust hasn't changed only the the speed at which it occurs. Which is the maximum for each of those configurations.


In fact looking at fig 14.6 page 111 in swatton it even has on the graph "Max Speed in Level flight". Where the drag and power curves cross.



Reply to Kieth

To be honest Kieth I don't think it is that badly worded.

My gut feeling is that it was written by an Engineer.

The word "conditions" tells me we are dealing with an ideal system so not to worry about in this case, flaps, spoilers, engine effects etc. So basic principles only.

"in order to achieve" says what in a simplistic system do you need to get x to be y in this case max speed.

And that question wouldn't be out of place on a first year Eng degree paper, but we wouldn't have given them the multiple choice and would have expected them to reproduce the power speed graph along with the prose with the reasoning, proberly worth about 5% of the marks, 10 minutes to answer it.

But as you say who knows what the world of JAR thinks is the answer. But as its written in a very typical engineering format for a question I would be pretty confident with my answer.

[Send Clowns]

Keith - excellent, that is very succinct. You have reduced this to the very essence of the problem with the JAA exams. That's why we love this job.