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Question on forces acting on an aircraft in climb

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Question on forces acting on an aircraft in climb

Old 2nd May 2009, 13:09
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Question on forces acting on an aircraft in climb

Hi Guys,

I was reading through Gary Bristow's book,Ace the Pilot Technical Interview and in the first chapter,there is a question asking about the forces acting on the airplane in flight and the answer is as follows:

"Drag,thrust,lift and weight.When thrust and drag are in equilibrium, an aircraft will maintain a steady speed.For an aircraft to accelerate,thrust must exceed the value of drag.When lift and weight are in equilibrium,an aircraft will maintain a steady,level attitude.For an aircraft to climb,lift must exceed weight of the aricraft. In a banked turn,weight is a constant,but lift is lost due to the effective reduction in wingspan.Therefore,to maintain altitude in a banked turn,the lift value needs to be restored by increasing speed and/or the angle of attack."

What I find peculiar is the part when he mentioned about the aircraft climbing.He states that lift must exceed the weight of the aircraft.But I supposed he failed to mentioned that thrust also plays a part when an aircraft climbs cause without thrust,your airspeed will decay and lift won't be enough to sustain the climb.That's what I feel.

However,I would like the opinion of fellow PPRUNERS on this topic as I feel that you guys are in a better position to answer.Thank you.
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Old 2nd May 2009, 13:21
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If I recall correctly from PPL lesson 2, in a climb, lift is less than weight and thrust is greater than drag. I remember this because it was somewhat bizarre as you'd expect lift to be greater than weight because you're going up. But not so. What gets you up is excess thrust.
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Old 2nd May 2009, 13:29
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If I recall correctly from PPL lesson 2, in a climb, lift is less than weight and thrust is greater than drag. I remember this because it was somewhat bizarre as you'd expect lift to be greater than weight because you're going up. But not so. What gets you up is excess thrust.
Correct - The only time left is (much) more than weight is during a turn or descending. It's a little more in stable level flight, but not a great deal - The wing has to generate a little more lift than weight to counteract the downforce from the tail.
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Old 2nd May 2009, 17:16
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Vibes,
The text you quote is so full of mistaeks and ambiguity that it's not worth commenting on.

There is a long thread on this subject elsewhere in 'Tech Log'... not sure I can still find it.

What JulieFlyGal says is correct. It was also the conclusion of the other thread.

CJ
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Old 2nd May 2009, 19:13
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Hi All,

First and foremost,thank you very much for answering to my queries especially JulieFlyGal,I really appreciate the comment given and I tend to agree with you.

As for CJ,I don't see why I am not able to make any clarifications on this great forum when the fundamental purpose of this forum is for us to come and share our ideas and thoughts.If you feel that my queries are not worth your time to answer,then I feel that you shouldn't be here in the first place.

And CJ,why don't you look up the book itself before accusing what I've quoted as "so full of mistaeks and ambiguity that it's not worth commenting on".And if making a mistake is wrong,why don't you start correcting yourself first by learning how to spell?
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Old 2nd May 2009, 19:51
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Vibes

CJ said the text you quoted was full of ambiguities.

I have to say, I agree. There is almost nothing in that paragraph that is correct.

"Drag,thrust,lift and weight.When thrust and drag are in equilibrium, an aircraft will maintain a steady speed.For an aircraft to accelerate,thrust must exceed the value of drag.When lift and weight are in equilibrium,an aircraft will maintain a steady,level attitude.For an aircraft to climb,lift must exceed weight of the aricraft. In a banked turn,weight is a constant,but lift is lost due to the effective reduction in wingspan.Therefore,to maintain altitude in a banked turn,the lift value needs to be restored by increasing speed and/or the angle of attack."

The stuff in red is wrong, or only true in particular circumstances (that are not specified). The stuff in green is correct.

IF its quoted correctly, then the book seems (to me) to be of very dubious quality.

Now, you can take that opinion or leave it; neither I (nor CJ) are obligated to go out and buy it to conduct a full review for your benefit. Likewise, I am not required to provide you with a complete lesson covering these topics; (although I am available for consultancy for a very reasonable rate!).

pb
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Old 2nd May 2009, 20:54
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Originally Posted by Vibes
As for CJ, I don't see why I am not able to make any clarifications on this great forum when the fundamental purpose of this forum is for us to come and share our ideas and thoughts.If you feel that my queries are not worth your time to answer,then I feel that you shouldn't be here in the first place.
Sorry if you misunderstood me.
It was simply the text you quoted that got my hackles up, because it contained so much rubbish (as Capt Pit Bull also points out), that I really saw no point in refuting every single ambiguity.
But I agree I didn't reply correctly to your final question, and as you say, that's what PPRuNe is about.
See below.

And CJ,why don't you look up the book itself before accusing what I've quoted as "so full of mistaeks and ambiguity that it's not worth commenting on". And if making a mistake is wrong,why don't you start correcting yourself first by learning how to spell?
If your quote is anything to go by, I certainly see no reason to look up the book itself.... as an aeronautical engineer I do have better literature on my bookshelves....
And apologies if you didn't get the deliberate "speling mistaek".

What I find peculiar is the part when he mentioned about the aircraft climbing.He states that lift must exceed the weight of the aircraft.But I supposed he failed to mentioned that thrust also plays a part when an aircraft climbs cause without thrust,your airspeed will decay and lift won't be enough to sustain the climb.That's what I feel.
What I find interesting that your "feel" was right from the start - in the other thread I mentioned it took a fair amount of kicking the idea around, and drawing diagrams, before everybody agreed.

Much of the confusion seems to come from the differences between what I would call 'flight dynamics' and steady-state flight.
You're exactly right. If you start climbing by pulling back on the stick, momentarily the lift is greater than the weight, but if then you don't increase the thrust, the airspeed will decay.

Another thing that seemed to cause confusion is the coordinate system....
Lift, thrust and drag are all defined relative to the flightpath, while weight/gravity always acts along the vertical.

In a steady-state climb, lift, thrust and drag on the one hand, and gravity on the otherhand, are no longer at right angles. Hence, lift is less than in horizontal flight. Thrust is no longer equal to drag, but more, because it now has to compensate for the component of the weight no longer 'supported' by the lift.

The extreme case, quoted in the other thread, is of course a steady-state vertical climb, where the lift (force at right angles to the flightpath) is now zero, and thrust minus drag is equal to the weight.

When the question was first asked in the other thread, most of us had some trouble fully getting our head around it.
So my compliments to you for getting it right first time!

Cheers,
Christian
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Old 3rd May 2009, 06:56
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Bristow Book

Guys, having read the bristow book at length some 2 years ago, not having interviewed in over 12 years prior, I was very disappointed to say the least! It is simply full of holes! There is some good stuff but you would have to really know your stuff to sort the wheat from the chaff here....
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Old 3rd May 2009, 07:56
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I totally disagree with captainsmiffy....

I find the information contained in this thread to be quite enlightening. In my almost 40 years of flying, I never knew some of these things...and
I take Bristow as 'gospel'.


NOT!!!!!!!!!

(Only joking with you, captainsmiffy!!!!)

Fly safe,

PantLoad

By the way, the U.S. Navy has great pubs explaining how an airplane flies.
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Old 3rd May 2009, 08:45
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Bristow's book is a known quantity, and is well understood to be full of errors.
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Old 3rd May 2009, 10:59
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The U.S. Navy book is: "Aerodynamics for Naval Aviators".

I haven't read it yet, but everybody here has been unequivocally recommending it, so I'll be ordering it too.

Just Google the title. It can be had for about $10, and even less second-hand.

CJ
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Old 3rd May 2009, 13:37
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Nobody please ever spend any money on ACE the Pilot Technical Interview! IMO, I think it is complete and utter confusing erroneous garbage. If you walk past a bookshop table with it remaindered out very cheap (or free), stillwalk on by! This rubbish does not merit discussion here.
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Old 4th May 2009, 14:48
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Hi Guys,

First and foremost,thank you for all the responses you've made to this thread.I really do appreciate it especially the clarification made by CJ.My apologies to CJ for the misunderstanding in the earlier posts.With regards to the book by Bristow,as far as I understand,I have heard about it containing misleading info.Hence,I decided,on my own accord,that the best way to learn is by taking his questions and clarifying whatever answers that were given by him.Though its a long method of learning,I feel that it is good as I get the opinions of many professionals in this field.Hence,its been great to be in the company of you guys as IMO the experience you guys have in this forum beats any books that's been written.And aviation,with a huge range of subjects from meteorlogy to aerodynamics,could be quite misleading in a sense that you have so many books and so many different authors,each expressing their own opinion.Thus,that's why I've asked time and again,many silly questions with regards to aviation cause books can only tell you so much.And I supposed in aviation,experience is the best lesson.And this forum has lotsa of them
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Old 4th May 2009, 15:05
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I was so concerned at the inconsistencies I found in this book, I started folding over the corner of the pages I wanted to check out and study further. Letīs say it quickly became apparent that something was amiss with the book! It looks to me like an author overreached themselves, and proof readers were used who didnīt know a whole lot about aviation, and if the spelling seemed OK, that was just dandy by them!
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Old 4th May 2009, 15:40
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Originally Posted by Rainboe
... proof readers were used who didnīt know a whole lot about aviation, and if the spelling seemed OK, that was just dandy by them!
Well, isn't that what proof readers are supposed to do?

In this case it sounds as if the book could have done with a couple of expert copy editors,before being handed to the proof readers.

Originally Posted by Vives
...I've asked time and again,many silly questions with regards to aviation...
Questions are rarely silly.
The problem is usually with the answers: how much does the person asking the question already know, and how much needs explaining?

CJ
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Old 4th May 2009, 16:22
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In a banked turn,weight is a constant,
Even this bit is arguably wrong. Weight is a force. F=M*A. In a turn the aircraft is subject to the sum of acceleration due to gravity AND the acceleration of the turn, so it's effective weight is increased. The wing needs to produce additional lift to support this, which is why it requires either a higher AofA or extra speed to maintain altitude.
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Old 4th May 2009, 20:07
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It would probably be of great asistance to the instigator of this post if someone could find and post a series of simple explanatory vector diagrams. I seem to remember that such diagrams were used to great effect during "theory of flight " briefings when I was learning to fly.
Such a diagram would show, for example, how the resultant of the lift vector
(lift and thrust) would be balanced by the resultant of the weight vector (weight and drag) in a constant climb, etc., etc.
I am unable to recall where I have seen the diagrams published, but it could have been "Flight Briefing for Pilots" by Allan Bramson.
Does anyone know what I am talking about?
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Old 4th May 2009, 20:56
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pineridge, Vibes,





Here's my earlier scribble (call it a vector diagram if you like) I did for the thread I mentioned earlier.
This one is strictly for the force balance in a steady-state climb, drawn for the horizontal and the vertical balance.
Re the sin(ac) and cos(ac) ... the "ac" is "angle of climb", happened to be the term used in the other thread for the flight path angle or climb angle.

Turns is another story.
Wizofoz, I've seen so many badly worded explanations I won't start on that one yet...

CJ
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Old 4th May 2009, 21:48
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Wizofoz, I've seen so many badly worded explanations I won't start on that one yet...
Mine or his?
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Old 4th May 2009, 22:38
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In a banked turn,weight is a constant,
I think that the statement above and other similar ones are often the result of a lot of people (strangely) assuming MASS to be synonymous with WEIGHT.
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