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moving the CoG

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gc72 created the topic: moving the CoG

I have come across this practise Q twice now however can not understand the answer.
QUESTION 40

Which of the following effects will result from moving the centre of gravity?

A. Longitudinal stability will deteriorate.
B. Greater elevator deflection is required to achieve a given attitude change.
C. The stalling angle will increase.
D. The indicated stalling speed will increase.
A=A
I was under the impression that moving the CoG forward or back would effect the lateral axis (the pitch). Im so confused
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John.Heddles replied the topic: moving the CoG

... however can not understand the answer.

I'm not surprised. As you have stated the question, it is faulty. That is to say, either there was a typo in the question or you have, inadvertently, misquoted it ..

Which of the following effects will result from moving the centre of gravity?

I suggest that the following word was omitted at the end of the question stem - "aft". The question intended probably was Which of the following effects will result from moving the centre of gravity aft ?

A. Longitudinal stability will deteriorate. (a) now is correct. Longitudinal static stability (long stab or LSS) increases as the CG moves forward as the stick loads required by the pilot to hold off-trim airspeeds increase and the slope of the stick force gradient also increases.

B. Greater elevator deflection is required to achieve a given attitude change. (b) now is wrong. This relates to (a). As the CG moves forward, the pilot has to use heavier stick loads and more effort generally to get a result in regard to pitch manoeuvres. Indeed, one of the concerns with aftwards CG movement is what is referred to as "stick force per g", which reduces. As the CG continues to move aft, this parameter readily gets to the point where the aircraft is so twitchy in pitch that the pilot can damage the structure if he/she doesn't concentrate very particularly on pitching inputs. This often is one of the limits on aft CG ranges.

C. The stalling angle will increase. (c) is wrong anyway. For the certification stall (slow approach to the stall and used to determine stall speeds for the POH) we can presume that the stall angle is reasonably consistent. Not so for accelerated and dynamic stalls - the dynamic stall, in particular, is a different animal altogether.

D. The indicated stalling speed will increase. (d) now is wrong. Associated with varying tail down loads as the CG moves, we find that, with forward CG, the download is greatest. This means that the wing has to do more work in generating lift to keep the whole thing afloat. End result is that the certification stall speed increases as CG moves forward as the wing thinks it sees a heavier aircraft at forward CG due to the increased tail download. This is one of the reasons that you usually see the forward CG limit slope back aftwards at high weights: we really don't need that flexibility in loading and would prefer not to have to wear the increased stall speed penalties for such things as takeoff and landing speeds and associated distances.

I was under the impression that moving the CoG forward or back would effect the lateral axis (the pitch). Yes, but not terribly relevant unless you want to delve into equations and suchlike ....

I'm so confused Welcome to the world of aviation. Fortunately, as a pilot, the emphasis is on having a practical idea of what you do with the controls (and why) to make the aeroplane do this or that .. rather than needing an engineering understanding of the design detail.

The important thing is that you recognised that there was a problem of sorts with the question - keep on with it - I think you'll do just fine.

Engineering specialist in aircraft performance and weight control.
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gc72 replied the topic: moving the CoG

Hi John,
Thanks for taking the time mate I appreciate it. The Q was missing the "aft".
Ive got the flying down pat not so much the theory, it seems to be more trickery to understand the fancy way the Q's are written not necessarily your knowledge, anyway we all go through it. See how I go on Friday. Thanks again for your time
Kind regards
Greg
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John.Heddles replied the topic: moving the CoG

Greg,

One of the problems when you are learning the theory side of the art during initial licence training is that you only have time (and syllabus requirements) to skim the surface a bit ... so it can all seem to be a bit overwhelming and confusing.

So long as you view flying as an exercise in forever learning (and actually keep reading stuff) it gets easier as you go along.

There is plenty in the way of engineering books on the internet which can help out - run some searches on "aerodynamics for pilots", "mechanics of flight for pilots", and similar for starters ... Davies (Handling the Big Jets), Kermode (Mechanics of Flight), and Hurt (Aerodynamics for Naval Aviators) are as good as anywhere to start without the angst of too much in the way of mathematics. Many useful books are available for nil-cost download.

If you want to run the gauntlet of some mathematics, there are numerous undergrad level engineering texts which will provide more in the way of details ... but it is not really necessary to go beyond the three cited above for pilot use.

Don't forget to let us know how the exam goes ....

Engineering specialist in aircraft performance and weight control.
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Pete Wu replied the topic: moving the CoG

I think that dynamic stall is just a term for accelerated stall. You are saying that they are different stalls. Have I missed ?
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bobtait replied the topic: moving the CoG

Yes, I'm sure the word 'aft' has been omitted. Was that question in the book or in the practice exams? I'll have to track it down and change it. All the best with the exam.

Bob
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John.Heddles replied the topic: moving the CoG

I think that dynamic stall is just a term for accelerated stall. You are saying that they are different stalls.

Yes, I am suggesting that the two are quite different.

Probably better if I waffle on a bit about stalls rather than try and pick out specifics. Some of the following stuff will be familiar, some not so familiar.

There is a variety of “different” sorts of stalls. The terminology can be a bit loose at times and that doesn’t aid understanding.

Regardless of whatever terms you may read in various books, the main types of stalls with which pilots should be familiar are described below.

Stalls occur when the airflow runs up against a pronounced adverse pressure gradient. This just means that, as the airflow goes on its merry way, if the local downstream static air pressure gets less (we refer to a “favourable” pressure gradient), it’s a bit like getting water to run downhill; the airflow accelerates and has a wonderful time. If, however, the local downstream static air pressure increases (an “adverse” pressure gradient) then it’s a bit like trying to get water to run uphill; the airflow doesn’t like it and slows down. If the airflow slows right down, it eventually gets to the point where it goes off and does its own thing and leaves the contours of the structure about which we would like it to move (we say that the flow “separates”). This process, in the extreme, results in significant regions of separated flow, lots of turbulence and we talk about the flow “stalling”. The different types of stalls just reflect different ways we can set up an adverse pressure gradient leading to a stall. The varying characteristics we can see with stalls just reflect the greater or lesser extent to which the gross flow tends to be relatively steady or unsteady during the process.

Certification Stall This is the very nice and sedate stall where the speed reduction to the stall is slow (at a rate originally not more than 1 mph/sec and, subsequently, not more than 1 kt/sec reduction in speed). While the regulatory certification specifics have varied a little over the years, the general idea is that the recovery occurs once there is a positive sign of stall (especially a pitch break, or nose drop) – unstall the wing by reducing the pitch attitude, roll wings level if there was a wing drop and, once positively unstalled (usually speed increasing through 1.2Vs), increase power and recover from the diving attitude.

This type of stall is used to determine the certification (or AFM/POH) stall speed and, subsequently, takeoff and approach speeds. While it is often referred to as a 1g stall, the stall occurs, necessarily as the power is low/idle, during a slightly less than 1g descent. Generally, this sort of stall in a certificated aircraft is fairly well-behaved. As such, it is not of much value for training beyond an initial exposure.

Turning Stall This is usually approached during a steady turn so that there is a load somewhat in excess of 1g. The wing sees this as a heavier aircraft and the stall speed will increase proportional to the square root of the load factor.

Accelerated Stall If the approach to the stall is a little more aggressive than for the 1g stall (the rate of speed reduction is somewhat in excess of 1 kt/sec), either wings level or in a banked turn (you may see the term “wind up turn” in some books), then the stall and post stall manoeuvres might get a little more interesting. We might see significant wing drop, spin entry (especially if the pilot doesn’t prevent slip/skid during the stall manoeuvre) and departure from controlled flight. Especially at low level, this is not a good idea in the normal course of things ...

As the main training concern with stalling is the very dangerous stall during the turn onto final, where we need to minimise height loss and get the aircraft back under control quickly and effectively, we have seen a traditional emphasis on training using an accelerated wings level or turning (or turning/descending) approach to the stall. In this, very dynamic, situation we can see all sorts of aircraft antics during the post stall recovery.

One problem was that folks in the training side of the house tended to lose sight of the main thrust in recovery which was to get the wings unstalled. After some accidents some years ago, the Industry started to lift its game and the emphasis, progressively, is changing to emphasise the need to get unstalled before things get needlessly exciting. As an aside, for routine, normal category operations, a far better plan is to stay away from the stall other than on training occasions when you may have a specific interest in playing with stalls.

Dynamic Stall If the approach to the stall involves quite high accelerations and very high pitch rates, the nature of the stall aerodynamics changes quite considerably. As the wing transits the normal stall angle, the wing sees a short-lived spanwise vortex formed which then sheds rearwards. During this process, the flow reattaches and the lift force can increase quite significantly until the vortex effect ceases.

This phenomenon rarely is seen in fixed wing aircraft (the pitch rates are too low) but is very relevant to rotary wing aerodynamics (retreating blade stall) where it often limits the maximum forward flight speed for a helicopter.

Also it is relevant to flapping wing aerodynamics (insects and birds) and wind turbine power generation. For man-made structures, the dynamic load variations can cause concerns with structural strength and fatigue lives.

Mach Stall (or Shock Stall) When the flow over the aircraft surfaces gets toward the speed of sound, we see the formation of weak shock regions which strengthen as the speed increases. Associated with the shock regions, there are rapid flow discontinuities and it is very easy for the airflow to separate from the aircraft surfaces in a manner similar to that seen during a normal, low speed stall. Separation at higher transonic Mach numbers results in aircraft movements similar to what we see in low speed stalls and we refer to Mach or Shock stalls. Obviously, this is not of much importance to the PPL/CPL level of flying activity but, if you end up playing with fast aircraft, transonic aerodynamics becomes a subject of considerable interest.

A couple of thoughts to add, if I may. There are two specific concerns with shock stalls (these caused much angst in the early days of high speed flight) which have largely been designed out of contention in more modern high speed aircraft.

Due to shock downstream separation, we can see both a significant reduction in control effectiveness due to disrupted airflow over control surfaces (especially the elevators - one of the reasons higher speed aircraft generally have all flying tailplanes or stabilators) and, due to both separation and a transonic aft movement of the CP (in subsonic flight, CP - strictly we refer to the aerodynamic centre but you can think of CP for this discussion - typically is around 25% chord while, for supersonic flight, 50% chord), we can see a quite significant and (for a conventional subsonic design, difficult to control) nose down pitching moment (often referred to as "Mach tuck") and very high control stick loads should the flow over the elevators remain attached.

Both of these effects were responsible for numerous aircraft losses during early days of high speed flight and it took quite some effort for the design and flight test community to sort out just what was going on in the transonic flight region. It was due to these sorts of problems, and the significant drag rise in the region of Mach 1 which led to the coining of the term "sound barrier".

If you have any specific questions, post them and I will endeavour to answer them without too much confusing technobabble.

Engineering specialist in aircraft performance and weight control.
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gc72 replied the topic: moving the CoG

No good on the exam today, however the silver lining is ive seen how the questions are written. Back to the books and give it another go in a month or so.
Thank you to everyone who has responded ti my Q.
Cheers
Greg
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John.Heddles replied the topic: moving the CoG

Never mind, it's all character building. The aim,now, is to knock the exam off next time around.

How about you post what you can recall of the questions and we can speak to them ? Helps other folk in their preparation as well.

Engineering specialist in aircraft performance and weight control.
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gc72 replied the topic: moving the CoG

Hi John,
Can do, I will start another thread with a more relevant name so it may be easier to find.
Cheers
Greg
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