Principles of Flight questions.

[Mohit_C]

Hi all,

I have a few questions from the Volare question bank which I have problems with and would appreciate an explanation:

93) If flaps are deployed at constant IAS in straight and level
flight, the magnitude of tip vortices will eventually : (flap
span less than wing span)
A) increase.
B) remain the same.
C) increase or decrease, depending on the initial angle of attack.
D) decrease. (according to the mark scheme)

101) If the altitude is increased and the TAS remains constant
in the standard troposphere the Mach Number will:
A) decrease.
B) not change.
C) increase or decrease, depends of the type of aeroplane.
D) increase. (according to the mark scheme)

According to the diagrams relating IAS, MN and TAS with altitude for an isothermal layer, if TAS stays constant and altitude is increased, MN also stays constand and IAS decreases.

225) What is the effect on induced drag of weight and speed
changes ?
A) induced drag increases with decreasing speed and induced
drag increases with increasing weight
B) induced drag decreases with increasing speed and induced
drag decreases with decreasing weight (according to the mark scheme)
C) induced drag decreases with decreasing speed and induced
drag decreases with increasing weight
D) induced drag increases with increasing speed and induced
drag increases with decreasing weight

I would say that A and B say the exact same thing:
-if induced drag increases with decreasing speed, it obviously decreases with increasing speed.
-if induced drag decreases with decreasing weight, it obviously increases with increasing weight.

330) Which statement is correct about the gust load on an
aeroplane (IAS and all other factors of importance
remaining constant) ?
1. the gust load increases, when the weight decreases.
2. the gust load increases, when the altitude increases.
A 1 and 2 are incorrect.
B) 1 is incorrect and 2 is correct.
C) 1 is correct and 2 is incorrect. (according to the mark scheme)
D) 1 and 2 are correct.

What happens to the IAS of maximum rate of climb when the temperature decreases at low altitudes?
A) Increases
B) Decreases
C) Doesn't vary
D) First a) and then b)

Thanks.

[Flying Squid]

Well as I'm the first to reply I'll take 93 although PoF seems like a long ass time ago, but here goes.....

If I remember rightly the wing tip vortices resulting from the formation of lift are counteracted by the vortices created by the flaps which flow in the opposite direction. So as flaps are extended the induced drag will eventually decrease.

[RTN11]

101) the troposphere has a standard lapse rate of 2C/1000ft, therefore with an increased altitude at contstant TAS, mach will increase. It is the tropopause which is isothermal.

[Pugilistic Animus]

93.True-a thread in tech log.... ,

101. true , but why ?

225 true, why

330.--but the stall speed decreases--therefore so does the design limit load--not quite so simple

last. C,...why?


PA

[Subhuman]

For Mohit_C

Firstly the Volare database is a) old. and b) poorly translated from the original italian and c) some questions in it are total rubbish.

Q 101 is simple "ECTM" , it states "standard troposphere" therfore it is isa and 2C/1000'. so as altitude increases, speed of sound is decreasing, meaning mach is increasing.

for 225 - that is a "volare" question - probably translated wrong.

Q 330 - now Im not 100% since its been a while - but gust loading always decreases with altitude, so 2 is definately incorrect, and 1 - well logic it out - a heavier aircraft has more intertia and therefore is harder to be pushed by a gust? my pof is rusty on that one :P.

And the last one - well im not sure what its really asking, D) is clearly meaning a Vmo/Mmo climb, if you go back to your Thrust Required/Thrust Available graph, for Vy your looking for max available thrust, now that is a fixed value i think, again i think this is a "volarism" . C?





For Pugilistic Animus as well

[Pugilistic Animus]

Q 330 - now Im not 100% since its been a while - but gust loading always decreases with altitude, so 2 is definately incorrect, and 1

the TAS values for the 'standard gust' decrease with higher altitudes because in the lower densities the 'rho' in the dynamic pressure equation is less therefore the forces are less and hence a reduction in the required gust loading

- well logic it out - a heavier aircraft has more intertia and therefore is harder to be pushed by a gust? my pof is rusty on that one :P.

With decreasing weight the Vs decreases and since the limit load factor assigned to an airframe can be determined---- as the wing carries it maximum weight at the stall

by reasoning back to the Va speed given by Va =Vs *sq rt[the limit load] so it can be seen that as Vs is decreased so does Va which has a rough correlation to VTP/Vra which my be altered either way at higher altitude to maintain sufficient margins

101) If the altitude is increased and the TAS remains constant
in the standard troposphere the Mach Number will:
A) decrease.
B) not change.
C) increase or decrease, depends of the type of aeroplane.
D) increase. (according to the mark scheme)

let me lie and forge some easy numbers

SoS decreases with decreasing T [actually as the sq rt of the temp ratio]

let's say TAS = 600mph and remains constant--as the question constrains


and at alt 1 and height 1 SoS =300mph
M= 600/300=2



even higher SoS= 200mph 600/200 = 3

at the highest before tropopause height 100

600/100 =6

I believe that the trend is increasing.

or 225 - that is a "volare" question - probably translated wrong.

no, it isn't ----why?--- it is simple reasoning

[Mohit_C]

I have one more question that I'm stuck on:

Q289) When flaps are extended in a straight and level flight at
constant IAS, the lift coefficient will eventually :
A) remain the same. (mark scheme answer)
B )increase.
C) decrease.
D) first increase and then decrease.

[Keith.Williams.]

Q289) When flaps are extended in a straight and level flight at
constant IAS, the lift coefficient will eventually :
A) remain the same. (mark scheme answer)
B )increase.
C) decrease.
D) first increase and then decrease.

The key parts of this question are "level flight" and "constant IAS".

If the aeroplane is to remain in level flight then the lift force must remain unchanged.

And to have unchanged lift force with constant IAS we must have constant CL.

So as the flaps go down we must reduce the pitch angle such that CL remains unchanged.

[Mohit_C]

Okay, I agree on that point but going back to the lift formula: L = 0.5 x density x TAS^2 x Coefficient of lift x surface. Yes, in straight and level fight at constant IAS the dynamic pressure in the formula of lift will stay constant but one thing makes me doubt. The question says "flaps". Now as it doesn't specify which type of flaps you could argue saying that in the case of Fowler flaps the surface increases and therefore the lift coefficient would eventually decrease.

[LH2]

as it doesn't specify which type of flaps you could argue saying that in the case of Fowler flaps the surface increases and therefore the lift coefficient would eventually decrease.

That's a good point, actually. Perhaps the question meant to ask the "lift force", and not the "lift coefficient" would stay the same? Talk to your groundschool and see what they think, it might be an appealable question.

[Keith.Williams.]

MohitC

You have just made the classical mistake of answering the question that you wanted to be asked instead of answering the question that you were actually being asked. Many students fail JAR exams by doing this. In some cases they start to pass the JAR exams only after they have recognised their error and started to take more care to answer the question that is written on the exam paper.

You are of course correct in saying that Fowler flaps increase wing area. So to maintain constant lift at constant IAS as Fowler flaps are deing deployed you must reduce pitch angle even more, so that the CL actually decreases enough to negate the increase in camber and the increase in wing area.

But the question said nothing about Fowler flaps. And out of five or so types of flaps that are included in the JAR syllabus, only Fowler Flaps increase wing area. So you have picked an answer that is true for only 1/5th of the possible interpretations fo the question. In doing so you have given yourself less than a 20% probability of success.

A more rational approach would be to say "the examiners have not stated the flap type, but only Folwer Flaps increase area. So it is more probable that they are thinking of a system which does not". This approach will bring you to the correct answer and get you the mark.

You should of course always appeal any question which appears to have more than one correct answer.

The various posts that you have made in this forum recently, have had one unifying theme. They have all been intended to give you a much greater understanding of these subjects than can be gained by simply memorising questions and answers in a database. This approach is commendable.

Unfortunately the vast mojority of such databases contain few if any detailed explanations. In this sense they are more suited to the process of rote learning than they are to the development of any meaningful understanding of the subjet matter.

It is of course true that many of the questions in the JAR CQB cover material that has no intrinsic long-term value. But this cannot be said of all of the questions.

[Mohit_C]

Thanks for the replies. I guess I'll have to change my way of interpreting questions from now on. A large number of questions I have asked from the different subjects I believe were just because I was trying to relate some knowledge I had regarding the topic with the question. Of course in some of them, like this one, I was trying to relate all the knowledge I had about flaps, which I take is not the best thing to do.

On another note I must say that I have got great help and a much better knowledge from this forum on the ATPL subjects, which sometimes not all ground school instructors are able to deliver to students in the most appropriate manner, all which I without doubt appreciate a lot.

[Mohit_C]

I was doing the ATPL Feedback database and came across this question:

Q163) Which statement about minimum control speed is correct?
A) VMCA may not be lower than VMCL
B) The nose wheel steering control may used to determine VMCG.
C) Crosswind is taken into account to determine VMCG.
D) VMCA depends on the airport density altitude, and the location of the engine on the aeroplane (aft fuselage or wing). (mark scheme answer)
->I do agree with D but in my book it also states that VMCG is affected by cross-wind, making VMCG increase when the cross-wind comes from the side of the inoperative engine during the take-off run (C).

Secondly I still have Q93) and the last question from my first post in doubt.

[Keith.Williams.]

Q163) Which statement about minimum control speed is correct?
A) VMCA may not be lower than VMCL
B) The nose wheel steering control may used to determine VMCG.
C) Crosswind is taken into account to determine VMCG.
D) VMCA depends on the airport density altitude, and the location of the engine on the aeroplane (aft fuselage or wing). (mark scheme answer)
->I do agree with D but in my book it also states that VMCG is affected by cross-wind, making VMCG increase when the cross-wind comes from the side of the inoperative engine during the take-off run (C).

JAA Regulations give specific definitions for terms such as VMCG, VMCA and VMCL. None of these inlcude any crosswind. Qustions in the exams relate to these specific definitions. So although a crosswind may make it more difficult to stay on the centre line following a critical engine failure, it will not change the value of VMCG as defined in the regulations.

A similar problem arises with questions relating CofG position and VMCA. Forward movement of the CofG will reduce the minimum speed at whcih it is possible to maintain control in the air following a critical engine failure. But the legal definition of VMCA includes the words "with the CofG at the most unfavourable position". So the actual CofG position does not affect the value of VMCA as defined in the regulations.

93) If flaps are deployed at constant IAS in straight and level
flight, the magnitude of tip vortices will eventually : (flap
span less than wing span)
A) increase.
B) remain the same.
C) increase or decrease, depending on the initial angle of attack.
D) decrease. (according to the mark scheme)

Most aeroplane flaps do not extend over the full span of the wings.

If flaps are extended and the aeroplane remains in level flight then the lift force must be unchanged in magnitude. But because the flaps do not extend all the way to the wingtips, the lift force must have moved towards the wing roots. This means that the pressure differences above and below the area of wing close to the tips must have reduced.

It is this pressure difference above and below the wingtips that drives the air into the wingtip vortices. So will less pressure differences close to the wing tips, the strength of the wingtip vortices will be reduced.

101) If the altitude is increased and the TAS remains constant
in the standard troposphere the Mach Number will:
A) decrease.
B) not change.
C) increase or decrease, depends of the type of aeroplane.
D) increase. (according to the mark scheme)

According to the diagrams relating IAS, MN and TAS with altitude for an isothermal layer, if TAS stays constant and altitude is increased, MN also stays constand and IAS decreases.

Mach number is the TAS expressed as a fraction of the Local Speed of Sound (or LSS). Mach = TAS/LSS.

LSS is proportional to the air temperature. So if air temperature decreases then the LSS also decreases. As altitude increases in the ISA Troposphere the air temperature decreases. This causes the LSS to decrease.

So if an aeroplane climbs at constant TAS in the iSA Troposphere, the decreasing LSS causes the Mach Number to increase.

As an example

661 knots TAS at ISA msl (where LSS = 661 knots) = Mach 1
661 knots at 36000 feet (where LSS = 573.25 knots) = Mach 1.153

You should be aware of the ECTM lines, which describe the variations of EAS, CAS, TAS and Mach Number with increasing altitude. If not then please review your course notes. If your notes do not cover the subject please send me a PM for details.

225) What is the effect on induced drag of weight and speed
changes ?
A) induced drag increases with decreasing speed and induced
drag increases with increasing weight
B) induced drag decreases with increasing speed and induced
drag decreases with decreasing weight (according to the mark scheme)
C) induced drag decreases with decreasing speed and induced
drag decreases with increasing weight
D) induced drag increases with increasing speed and induced
drag increases with decreasing weight

I would say that A and B say the exact same thing:
-if induced drag increases with decreasing speed, it obviously decreases with increasing speed.
-if induced drag decreases with decreasing weight, it obviously increases with increasing weight.

You are correct. A and B are effectively saying the same thing and they are both correct.

330) Which statement is correct about the gust load on an
aeroplane (IAS and all other factors of importance
remaining constant) ?

1. the gust load increases, when the weight decreases.
2. the gust load increases, when the altitude increases.

A 1 and 2 are incorrect.
B) 1 is incorrect and 2 is correct.
C) 1 is correct and 2 is incorrect. (according to the mark scheme)
D) 1 and 2 are correct.

In this type of question the term “gust response” means the degree to which the load factor is changed by the gust. You should be aware of a number of calculations in which the load factor in a gust can be estimated using the equation

Load Factor in gust = Load factor before gust x (New CL / Old CL)

For straight and level flight prior to the gust initial load factor would be 1 so the equation can be simplified to

Load Factor in gust = (New CL / Old CL)
We can use this equation to see how changes in weight would affect gust response. Let’s assume that we have two identical aeroplanes. Aeroplane B is twice as heavy as aeroplane A.

If they are both flying at the same EAS then aeroplane B must be generating twice as much lift as much lift as aeroplane A. But with the same EAS they both have the same dynamic pressure, so the CL and hence angle of attack of B must be twice that of A.

If they both hit the same gust they will both experience (approximately) the same increase in angle of attack and the same increase in CL.

Let’s suppose that

Initial conditions are CL A = 1.0 and CL B = 2.0
Increase in CL is 0.5 in both cases.

For aeroplane A

New load factor = New CL / Old CL) = (1.5/1.0) = 1.5

For aeroplane B

New load factor = New CL / Old CL) = (2.5/2.0) = 1.25

So the increase in load factor (gust response) of the heavier aeroplane was only half that of the lighter aeroplane.

This gives us the general statement that gust response decreases with increasing weight. If you think back you may find that you experienced this when doing your PPL flying. If the ride with the instructor on board was much smother than flying solo, it wasn’t (just) your bad flying. The weight of the instructor reduced the gust response.

So statement 1 in your question is true.

The second part of the question is more problematic because it does not include enough information. But if an aeroplane at constant weight and flying at constant speed hit a gust of a constant speed at two different altitudes, the change in angle of attack and CL would be the same in both cases. So the gust response would be the same in both cases. But all of this assumes that we are using the same type of speed (EAS, CAS TAS) for both the aeroplane speed and the gust speed.

So statement 2 in your question is incorrect.

As with all JAR questions there are a number of (often un-stated) simplifying assumptions at work. As indicated by some of the previous post in this thread, it is also possible to use different interpretations of terms such as “gust response”. But those that I have used above are the ones that will get you the correct answer in the JAR POF exam.

If we were to interpret the question as meaning “the maximum gust response” or “maximum load factor attainable in a gust” then increasing weight and increasing altitude would both decrease the maximum gust response. So statement 1 would still be true and statement 2 would still be untrue.

What happens to the IAS of maximum rate of climb when the temperature decreases at low altitudes?
A) Increases
B) Decreases
C) Doesn't vary
D) First a) and then b)

The first point to note is that the questions is not asking about the “maximum rate of climb. It is asking about the “IAS for maximum rate of climb”. This speed is Vy.

The qualifying term “at low altitudes” has probably been included in this question to eliminate the possibility of shock induced separation and shock stall.

Vy, the speed for maximum rate of climb is the speed at which excess power is maximum.

This is because Rate of climb = Excess Power / Weight

Decreasing temperature will increase air density. This is the equivalent of a decrease in pressure altitude (it is actually a decrease in density altitude).

This will cause the following

1. The Power Available curve will rotate anti-clockwise about the origin.
2. The Power Required curve will move down and to the left, sliding
along a tangent drawn from the origin.

The overall effect of these changes will be a slight increase in the IAS equating to Vy.

With an increase in altitude we would see Vy gradually decrease to equal Vx at the absolute ceiling. The situation in this question is the opposite. Density altitude is decreasing so Vy increases, causing it to move away from Vx.

[Mohit_C]

Regarding the last question, if instead of asking "How does the IAS for the maximum rate of climb change as temperature decreases at low altitudes" the question asked "How does the maximum rate of climb change as temperature decreases at low altitudes" is this explanation valid:
-as temperature decreases, the air density increases which increases the power available
-an increase in air density increases the drag too
-therefore the net result would be that the maximum rate of climb wouldn't change?

[Keith.Williams.]

No, your line of argument is not correct.

Drag = CD 1/2RhoVsquared S

Where 1/2RhoVsquared is the dynamic pressure.

The ASI gives us the indicated airspeed IAS, by measuring dynamic pressure.

The ASI is simply a pressure gauge that happens to be marked in knots instead of psi.

Every time the ASI measures the same dynamic pressure it will give the same airspeed indication.

This means that if we climb or descend at constant IAS, we are climbing or descending at constant dynamic pressure.

This in turn means that if we keep CD and S constant we will always get the same drag. This is why the drag against IAS curve does not vary with altitude. (Strictly speaking I should say the drag against EAS curve, but IAS and EAS are pretty close at reasonably low altitudes)

So changes in temperature or altitude will not change the drag at any given EAS (and will not change drag very much at any given IAS).


The TAS at any given IAS or EAS depends on air density (we discussed this subject in this forum several months ago). This means that as temperature or altitude increase, the TAS at any given IAS or EAS increases (this is why the ECT lines behave the way they do). But the drag remains constant(ish).


Power required = Drag x TAS.

So as temperature or altitrude increase the drag remains constant while the Power Required increases. (because the TAS in the "Power Required = Drag x TAS" equation is increasing)


So as temperature or altitude increase at any given IAS

Drag remains constant
Power Required increases.


You are correct in saying that decreasing temperature, increases air density and this increases both thrust and power available.

So the overall effect of increasing temperature or increasing altitude at constant IAS is

Drag remains constant(ish)
Thrust Increases

These effects cause excess thrust and angle of climb to decrease.

And

Power Required increases.
Power Available decreases.

These effects cause excess power and rate of climb to decrease.

When the aeroplane reaches its absolute ceiling

Thrust = drag so excess thrust and best angle of climb = zero
Power available = power required, so excess power and best rate of climb = zero.

If temperature or altitude decrease, the effects are reversed, so that best angle of climb and best rate of climb both increase.

[Mohit_C]

That makes sense. Thanks Keith!

[Pugilistic Animus]

Hello, Keith Williams, in the last question it was the mention of low altitude that caused me to choose answer C. because Vx increases and Vy decreases until in actuality until they converge at the absolute altitude, however many [as you know] OEM's

choose a stepwise and therefore discontinuous function in dealing with those performance speeds --or simply denote a 'blanket IAS chosen from a series of drag polars

Excellent Pedagogy--I have to say I was a bit lazy here--but 15 week of Power required power available---induced power required---blah balah blah I'm on my winter break

edit: PS
I see that you Wash'em out in ground school
Break Them Up,... Break them UP!!

PA


Mohit C.

I was worried that you were on a quest at memorization in lieu of understanding hence my initial Obtuseness---your subsequent post have proven those worries unfounded---That's the way to adapt and learn---

PS -you'll find few actual absolutes in aviation--never say always or never ---never ever---Jus' get crackin' and focus like a LASER beam on that Flight Handbook/ Pilot's Operating Instructions, Airplane Flight Manual, or Operations and Training Manual

LH2 -Pay attention!


Slow mouth, Fast mind, slow hands---

PS --the mountains and the ocean and the storms don't have any special theories-

PA

[flyer696]

Hi!

I have question which I don't understand. Can somebody explain it to me? The question is:

You're flying at a speed 2Vs. You expirience a rapid gust which slow you down to 1.8Vs. How much and in which direction the gust load factor will change?

I hope that somebody will give me explanation soon.

Kind Regards,
Konrad

[keith williams]

The gust reduces airspeed from 2Vs to 1.8Vs

So the new speed is 1.8 / 2 or 0.9 of the initial speed.

Looking at the lift equation Lift = Cl 1/2Rho Vsquared S we can see that at any given Cl, the lift is proportional to the square of the airspeed.

Assuming that the pilot has not yet reacted to the gust by changing angle of attack, the angle of attack will be unchanged so we will have.

New lift = 0.9 squared x initial lift.

0.9 squared = 0.81 so the new lift will be 0.81 of the initial lift. That is 81% of the initial lift.

Load factor = Lift / Weight, so with only 81% of the initial lift we now have 81% of the initial load factor.

Assuming the initial condition was straight and level flight (with a load factor of 1), the new load factor will be 0.81

So the gust caused the load factor to decrease by 19% from 1 to 0.81.