Mixture of questions.

[Mohit_C]

Came across the following questions which I have a few doubts on:

Human Performance and Limitations

Q159) [DONE] Once we have constructed a mental model we tend:
A) To give undue weight to information that confirms the model (mark scheme answer)
B) To give undue weight to information that contradicts the model
C) To give equal weight to contradicting and confirming information
D) To alter that model unnecessarily frequently
-> I would think the answer is B.

Instrumentation

Q182) [DONE] Given:
Pt: total pressure
Ps: static pressure
Pd: dynamic pressure
The altimeter is fed by:
A) Pd. (mark scheme answer)
B) Ps-Pt.
C) Pt-Pd.
D) Pd-Ps.
-> The altimeter is fed with static pressure, which is the total pressure minus the dynamic pressure. I would go for C.

Q236) In a steep turn, the northerly turning error on a magnetic compass on the northern hemisphere is:
A) Equal to 180° on a 090° heading in a right turn. (mark scheme answer)
B) None on a 270° heading in a left turn.
C) None on a 090° heading in a right turn.
D) Equal to 180° on a 270° heading in a right turn.
-> I don’t really understand those answers.

Q262) [DONE] In order to measure temperature the cylinder head temperature (CHT) gauge utilises a:
A) Thermocouple consisting of two dissimilar metals. (mark scheme answer)
B) Wheatstone bridge circuit.
C) Ratiometer circuit.
D) Bourdon tube.
-> My Instrumentation book gives an example of a Wheatstone bridge circuit as that used to measure CHT. So I would think it does have two possibilities, right?

Performances

Q41) [DONE] The lowest point of the thrust required curve of a jet aeroplane is the point for:
A) Minimum drag. (mark scheme answer)
B) Maximum specific range.
C) Maximum endurance.
D) Minimum specific range.
-> I would think that the lowest point on the thrust required curve of a jet aeroplane would essentially be the minimum power required, which would actually be the maximum endurance of a jet aeroplane (minimum fuel flow) but as it’s in the unstable range, or “backside of curve” the speed for maximum endurance is taken to be the minimum drag speed. Not too sure on this one.

Q58) [DONE] How does the specific range change when the altitude increases for jet aeroplane flying with the speed for maximum range?
A) First increases than decreases. (mark scheme answer)
B) Decreases.
C) Does not change.
D) Increases only if there is no wind.

Q156) "Maximum endurance"
A) Is achieved in unaccelerated level flight with minimum fuel flow. (mark scheme answer)
B) Is the same as maximum specific range with wind correction.
C) Can be flown in a steady climb only.
D) Can be reached with the 'best rate of climb' speed in level flight.
-> Well without doubt I went for A, but isn’t it true that this speed is actually equal to the speed Vy (maximum rate of climb).

Q175) Which of the equations below defines specific range (SR)?
A) SR = True Airspeed/Total Fuel Flow (mark scheme answer)
B) SR = Indicated Airspeed/Total Fuel Flow
C) SR = Mach Number/Total Fuel Flow
D) SR = Groundspeed/Total Fuel Flow

Q176) At a constant Mach number the thrust and the fuel flow of a jet engine
A) decrease in proportion to the ambient pressure at constant temperature. (mark scheme answer)
B) increase with increasing altitude.
C) are independent of outside air temperature (OAT).
D) increase in proportion to the ambient pressure at constant temperature.

Q204) [DONE] For a piston engine aeroplane, the speed for maximum range is:
A) that which gives the maximum lift to drag ratio. (mark scheme answer)
B) that which gives the minimum value of drag.
C) that which gives the maximum value of lift
D) 1.4 times the stall speed in clean configuration.
-> Isn’t B true also?

Q274) Which one of the following statements concerning drift-down is correct?
A) When determining the obstacle clearance during drift-down, fuel dumping may be taken into account. (mark scheme answer)
B) The drift-down procedure requires a minimum descent angle after an engine failure at cruising altitude.
C) The drift-down procedure requires a minimum obstacle clearance of 35 ft.
D) An engine failure at high cruising altitude will always result in a drift-down, because it is not permitted to fly the same altitude with one engine inoperative as with all engines operating.
-> I understand A but B is also true; you first decelerate to minimum descent angle and then descend to the altitude which you can maintain with one engine out.

Thanks.

[DarkSoldier]

Human Performance and Limitations

Q159) Once we have constructed a mental model we tend:
A) To give undue weight to information that confirms the model (mark scheme answer)
B) To give undue weight to information that contradicts the model
C) To give equal weight to contradicting and confirming information
D) To alter that model unnecessarily frequently
-> I would think the answer is B.


Marked answer is correct. If you have constructed a mental model then you would give preference to information than confirms it. It's just our natural habit...

[bajadj]

Is this volare again??? loads of the volare questions have been removed from the cqb, but I'm pretty sure your first question 159 was in my JAA paper and the mark scheme answer is correct. We tend to see a situation as we expect it to happen, not necessarily as it does happen so a is the correct answer.

[Mohit_C]

Yes this is Volare but a lot of questions have been removed from the original Volare question bank.

For the first question, I guess I got confused with the word "undue".

[ldyypd]

Q182) Given:
Pt: total pressure
Ps: static pressure
Pd: dynamic pressure
The altimeter is fed by:
A) Pd. (mark scheme answer)
B) Ps-Pt.
C) Pt-Pd.
D) Pd-Ps.
-> The altimeter is fed with static pressure, which is the total pressure minus the dynamic pressure. I would go for C.


This is a mistake with the question. A) should be Ps. Same thing as Pt - Pd

[EK4457]

Hi Mohit. The firsrt two are sorted, so I'll have a crack at the rest:

236) Errm, I think somthing is lost in translation here. Accelleration and turning errors are fairly simple (remember UNOS) but I don't have a scoobys what is going on here. Whith you.....

262) Just one you gotta remember. Thermocouple temp gauges are used to measure high temperatures. They are simple, cheap and easy to fit/replace- things that bean counters and engineers like. Don't know why you would spend extra on a wheatstone bridge.

41) This is somthing you need to get straight from the off and I think it's a bit muddled with you.

Basically, there are 4 graphs you need to know inside out: thrust/drag prop, thrust/drag jet, power prop, power jet. Also, remember that piston engines produce POWER (the prop converts it into thrust) and jet engines produce THRUST.

This is pretty much all you need to know to answer all of these type of Q's.

With this Q, you just need to know that thrust required is really drag. Answer A. Simple.

This really is max endurance speed, not a pretend one (as you think).This is a jet which produces thrust. It will used fuel the least at least thrust required, min drag or Vmd. Vmp is max endurance for a piston as they produce power. I think you are a bit mixed up with this.

Also remember that max range for a piston is Vmd but for a jet it is 1.32 Vmd.

Go and dig your books out!

58) Jet engines are designed to run at their highest efficiency at around 95% rpm. From above we know that max range is 1.32Vmd.

At low altitude, 95% rpm will give you well above 1.32Vmd, so not the best range. You will need to lower the thrust to get the correct speed making the engines less efficeint. As you climb, density decreases, thust decreases and so you have to incrase engine rpm to maintain 1.32Vmd. Eventually, you will get to an altitude where 95% rpm (most efficient rpm) will give you 1.32 Vmd (max range speed). Perfecto!

As you climb past this height, the engines go past their best rpm and you loose out again. So, initially you increase SFC, then it decreases. Answer A.

I'll let somone else have a pop if they want. I need a break!

EK

[Mohit_C]

I think I'm lacking one main concept. This is what I think, and always thought about the curves for propeller and jets.

PROPELLER AIRCRAFT
When we represent drag versus TAS, the lowest point is (obviously) minimum drag. Now the drag versus TAS curve isn't a symmetrical curve (not an x^2 function graph); parasite drag increases as TAS increases and induced drag decreases as TAS increases, however parasite drag increases at a much larger rate than induced drag decreases. From this you lead to the power required versus TAS graph.

When we represent power required versus TAS the minimum point, yet again, would be the minimum power required but, as explained in the paragraph before, it occurs at a TAS little lower than minimum drag. The minimum drag occurs at a point that is tangent to the curve power required versus TAS. Flying at the minimum drag we get maximum range (in still air) and maximum L/D ratio. Flying at the minimum power required we get the maximum endurance. The speed for maximum endurance is about 75 % that of the speed for minimum drag.

JET AIRCRAFT
When we represent drag versus TAS, the minimum point would be minimum drag, which jet aircraft fly at when the flight condition required is for maximum endurance, maximum glide range with all engines out, maximum climb angle and the drift down speed (minimum descent angle); they all give maximum L/D ratio.

Now when we represent the thrust required versus TAS curve, for the reasoning given in the first paragraph, the tangent to the curve gives minimum drag (max. end.) but as for jet aircraft maximum range in still air is achieved at maximum TAS/drag ratio and approximately 95% RPM (engine efficiency), the speed for maximum still air range occurs at 1.32 times the speed of minimum drag.


Is some of this incorrect or does not make sense?

[EK4457]

As far as I can see, you have the prop a/c sorted.

I think the confusion for jets can be sorted out by this:

Thrust required = Drag

In other words, the two graphs you are talking about in the last 2 paragraphs of your last post are one and the same!

So, V Min thrust required = Vmd = Max endurance (Jet).

Does this help you answer Q41?

[Mohit_C]

If I get this right, for jets the drag and thrust required versus TAS are the same. Therefore, trying to use a similar explanation as in my previous post, the lowest point on both these curves would be the speed for minimum drag (maximum endurance, maximum glide with all engines out, etc...) and the tangent to both these curves gives a speed which is 1.32 times the speed for minimum drag. This speed gives the maximum range in still air.

If this is correct, then that's a few questions less from my doubts.

[EK4457]

Thats pretty much it. Simple really.

If you go back to basics, the 4 forces in balanced flight are:

weight = lift and (importantly for these curves) thrust = drag.

Now, looking at things like that, we can see that drag is, by definition, the amount of thrust required in order for them to cancel out.

They are just two names for the same thing. Q41 is just really seeing if you understand this. Another way of writing the same Q is to say:

on a drag curve, what is the speed for minimum drag. Answer, Vmd.

Yes?

[Mohit_C]

Ah, right. I understand this now. Thanks!:ok:

5 questions to go.

[Dick Whittingham]

Back to basics with the drag curves.

First, the curves are for steady level flight where thrust equals drag. Second, it is normal to start with curves for EAS v. drag and EAS v. power required because it is EAS that determines drag. A shift to TAS is ok, but you then have to stretch the V axis to take account of the EAS/TAS ratio, which varies with density and therefore height and temperature. Keep it simple and use EAS. (Or IAS, since both are almost exactly the same at low altitude and at the speeds we are dealing with). Note that the type of propulsion is not relevant, these are aircraft drag curves

The two starting curves are EAS v. Profile drag and EAS v. Induced drag. Total drag is the sum of these two curves. Profile drag is Cdp times Vsq, Induced drag is Cdi times 1/Vsq so Total drag is CdpVsq + Cdi/Vsq. The values we choose for Cdp and Cdi will affect the shape of the curves and the numerical values we get for Vmd etc, but the curves remain straightforward examples of exponential curves of the form y = xsq, y = 1/xsq and y = xsq + 1/xsq. If you solve the Total drag curve for (A) the point where Total drag over V is a mimimum, which is where the tangent from the origin touches, and for (B) the point where total drag is at a minimum you will find that A is 1.316 times B.

Note that you don't know the actual speeds. These will depend on what values you use for Cdp and Cdi. Note also that we have assumed both Cdp and Cdi are constants. Because this and other hidden assumptions are not always true for the real world it is best to say "about 1.32"

Because in a jet, roughly, fuel flow is related to thrust, if you power this aircraft with a jet you will stay up longer at Vmd and fly furthest at 1.32Vmd. If you use a piston/prop engine, where fuel flow is related to EHP, engine horsepower delivered, you fly at Vmp for endurance and Vmd for range. If you look at the maths, you will se that flying at Vmd gives the best ratio of V over V x Drag, which is V over Power required. Also of interest is that on the graph of power required against V the tangent to the curve touches at minimum drag speed

All of this is for low level where you will have massively more thrust or power available that you actually need to fly for range or endurance. The next two stages are to get to a height here your engine can operate at max efficiency and the TAS/EAS ratio kicks in to extend your range - but that is another story

Dick