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Does this mean that wire services are allowed to listen to these frequencies? If they can, why can't ITN?

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JJC

Bik_116.80

No, I meant what I said.

I know most people tend to think in terms of a T/D couple and a L/W couple, the reason for this is probably that this is the way most basic PofF texts / notes tend to break it down, normally with the intent of discussing the balancing forces required on the tail.

But it is a little off the mark. You don't have to have a pair of opposed forces, applied to different lines of action, in order to produce a rotational effect.

All you need is one force that doesn't act through the C of G.

In our example we are imagining an aircraft in the cruise. The forces are all cancelling one another out, so the aircrafts flight path is steady. The rotational effects of all the forces are also cancelling one another out - we know this because the aircraft attitude is constant.

i.e. in Newtons laws terms there are no unbalanced forces (so no acceleration) and in angular terms there are no unbalanced moments (so no angular acceleration).

i.e. you can forget about them.

All we are saying is: if we add some more thrust, what is the effect of the extra. You might as well be looking at a space ship in deep space (so forget all the other forces).

If the thrust line doesn't pass through the C of G you'll get angular acceleration.

Like I said, its basic mechanics.

Imagine a glider. No thrust so there is no thrust drag couple. Now stick a pole 5 feet up from the canopy with an airbrake on it. Deploy the airbrake. Glider pitchs nose up, because the line of action of the force is above the C of G of the object.

Hope that helps.

Must dash. Mrs Pit Bull is announcing the readiness of dinner.

CPB.



[This message has been edited by Capt Pit Bull (edited 09 November 2000).]

Bally Heck,
Can't remember the name of the aircraft you saw, but I saw an article in Flight about 7-8 years ago about it. Its a Russian aircraft that they used for serious rough field work, IIRC the idea was the engines above the wing gave a lot more protection from FOD, also the take off roll was reduced as the jet efflux over the wing helped create more lift.

Of course I could be wrong ;) http://www.!!!!!!!!!!!!!!/open.file?id=119353
Edit to add link to picture

[This message has been edited by Pdub (edited 09 November 2000).]

Capt Pit Bull, that all sounds fair enough. And thanks for the explanation - much appreciated. I think that your view of reality is somewhat clearer than mine. Cheers mate!

Can anyone say where it is illegal to listen(not transmit) to aircraft frequencies?-as far as I know its not, or police frequecies for that matter.It is illegal to act on information gained thereof.

Bally Heck,
Did you mean the Fokker VFW 614??

here's a pic of it: http://www.!!!!!!!!!!!!!!/open.file?id=74490

Mach78--

It's illegal to listen to just about anything in the UK, unless you have a licence. According to the Radiocommunications Agency:

"Although it is not illegal to sell, buy or own a scanning or other receiver in the UK, it must only be used to listen to transmissions meant for GENERAL RECEPTION. The services that you can listen to include Amateur and Citizens' Band transmissions, licensed broadcast radio and weather and navigation broadcasts.

It is an offence to listen to any other radio services unless you are authorised by the Secretary of State to do so. "


The full document's at http://www.radio.gov.uk/document/ra_info/ra169.htm and gives chapter and verse.

Welcome to Edwardian Britain...

R

Oh Dear

Where do I start. Ok chaps I think it was the Antonov. It was parked up at EDI a couple of days ago….maybe still there.

Capt. Pit Bull. Not convinced with your C of G theory. The C of G is acting vertically down and not in the direction of thrust and drag. ie it is opposing lift. I go for the thrust drag couple myself. If any egghead out there has a convincing thesis. Happy to listen.

Latest. I wouldn’t worry to much about listening in without a licence. Consenting adults in private. If I was the telecoms agency and it bothered me, I could get thousands of punds of fines in a trawl of airport viewing galleries. Surprisingly they seem to have better things to do. Might get arrested listening to the ATIS in my house. Oh no…I have a licence…or is it the radio…Do http://www.pprune.org/ubb/NonCGI/confused.gif

Confuscious (?sp) he say cg is centre of mass and if you apply force not aligned through centre of mass to any body (even mine...) the body will surely rotate

good here init

In Outer space perhaps. But me thinks on planet Earth, what with gravity, drag, etc it's pretty negligible. Not convinced yet.

Where's all the rocket scientists??

Captain Pit Bull, Following this with interest. I agree with your example of the glider and the airbrake, but I have one question. I am flying along in my Tiger Moth, engine working, straight and level, thrust/drag, lift/weight all happily balanced. The engine quits, so I no longer have "thrust" from that source. I am now in a biplane glider

I remember the mantra "convert speed to height". I do that. Naturally, as per instruction's warning voice, I always had a forced landing field selected and now I want to land on it. So now glider, I reconvert height to speed, or potential energy to (I hope I have this right) kinetic energy. I establish a new, albeit now descending, flight stability. Gosh, I am doing this well, I think I am going to make it, and here at my lap top my pulse rate is also steady.

Everything nicely balanced. Constant speed. I still have weight, and I am not dropping like a stone, but at steady speed, so I must have lift, and it must balance the weight. I still have drag, and I am not decelerating, so the drag must be balanced by ...................? Would we not in the normal case call it "thrust" or "thrust equivalent"?

Davaar,
I have flown Tigers ,I think pulling back on the stick after an engine failure would only cause you to slow down rather than climb :)
If in your example lift still equalled weight you wouldn't be descending :)
Speed is constant in a descending glide because the weight vector always points toward the ground therefore giving you a 'forward' component to weight which balances drag.
Draw an aeroplane with a nose down attitude,draw in a weight vector pointing straight down,draw in a lift vector the same way you would if the a/c is S&L .There are also going to be vectors for vertical component of lift and drag but to keep it simple and if you don't understand vector addition simply think of the two vectors as pieces of string and 'pull' them .Which way does the aeroplane move?
I have flown a jet with the same configuration as the 145 (F28)and increasing /decreasing power just causes acceleration or deceleration,after a while of course the aircraft will climb/descend but more as a function of IAS variance and therefore lift variance than a 'pitch couple'.

MM.

Thanks, mountain man. Perhaps example was ill-chosen. I always thought myself that the convert speed to height routine had more to it so piety than practical value, given the speeds and drag of a Tiger. Let me put that aside. Although of course in other types one could well convert speed to height, it is not really material here.

I recognise right away your point about the weight, the lift amd the descent, because at one time I looked at it your way, and then later changed my mind. Maybe I should change it back again. My current reasoning is that there are four forces in balance, two opposing two. If any falls out of balance there is an acceleration in that pair. If it is a thrust/drag imbalance there is a speed acceleration/deceleration until balance is again reached. If it is a weight/lift imbalance there is a vertical accelaration/deceleration until balance is again reached.

The basic proposition is that an aircraft in any stable flight condition is in balance.

I see what you say about the weight vector in a steady descending glide. As you say,this element gives a forward component to weight, and balances drag. I agree. But is that not tantamount to saying that weight provides part of the thrust? The aircraft is not accelerating, and drag is not compensated by any balancing force from any source other than the descent. In a glider the weight element is the only source, in a powered descent it is a partial source.

Perhaps I did not express it well, but that really was what I was addressing to Captain Pit Bull. I think he had said that a glider does not have a thrust/drag pair, but it seems to me that it does, and the source of the "forward" component that creates the balance, commonly called thrust, is exactly the forward weight vector that you describe.

Thank you for your patience.

Davaar,
descending is a verticle acceleration ,ie it has direction and magnitude.
Lift is a function of IAS,wing area,wing shape (camber etc),Angle of Attack among other things (Cl=1/2RhoVsquared.S).In a gliding a/c such as your Tiger IAS is less than crz,wing area hasn't changed,neither has air density(in a measureable sense) so it can be seen from the Lift Formula that lift must be less,unless of course you want to increase AOA which is a VERY short term view followed closely by stall/spin.Therefore in a gliding descent Lift<Weight
There is no thrust from the engine but drag still exists but probably less due to less Form Drag from lower IAS.If you are at the optimum glide speed this equates to the maximum coefficient of lift so ideal AOA for min induced drag.
Overall in a glide Lift<Weight and the 'forward' component of weight = Drag.

By the way congrats to the 145 crew ,well done !

MM.
PS While I've not done a lot of gliding,gliders climb because the air they are descending in is going up at a greater rate. :)

While you are all discussing Theory of Flight (It must be true "Get 3 Pilots together and you'll get 4 opinions") no one has commented on TripleIRS ' question
"Whilst I don't doubt that the crew did a good job, I was wondering whether their decision to continue all the way to MAN was a sound one. With a control difficulty such as that, I would have thought that Land ASAP would have been the better decision. They seemed to have regained sufficient control whilst still over N. Ireland. Comments??"

And as for Growlers statement that they had identified the problem etc - well the Alaska Air lads had the problem identified but they still ended up dead. As far as I remember they pulled (and reset) the appropriate CB. With some prodding, unfortunately, from their Maintenance.
No point in being a Dead superhero!

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The Stamp is mightier than the Toolbox!!

34DD’s little book of odds:

Davaar 10 000 to 1 ON

MM 1000 to 1 AGAINST

But I will still fly with MM ‘cos life proves everyday that (we) pilots don’t have to know how an aeroplane works in order to use one


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Forward CGs are better than aft ones

WideBodiedEng:

Yes, I was thinking what a pity this had turned into an ego-massaging exercise by some!!

Still, I guess that's much more important than discussing what I thought was an important flight safety issue.

It seems that, understandably, the crew were seeking advice from the Isle of Man over their Company Frequency. Still in an Emergency situation, with evident control difficulties, it seems very strange that they should have continued to MAN.

34DD,
Whilst I agree it's way off the thread topic and it's been nearly twenty years since I've given Principles of Flight any thought care to enlighten about my ignorance?

Everyone:

34DD is spot on when he says that we don't have to understand every aspect of an aircraft inorder to use it.

In science you basically have a model of how a system behaves. By necessity, that model is usually a gross oversimplification of what is going on. But as long as the model holds good, for the operating regime that we are interested in, who cares?

Well, inadequacies in your model only become a problem when you try and extrapolate the behaviour of the system outside of the regime the model was designed to cover. At which point, one of two things happens:

1) Either the system still behaves as predicted by the model (you slap yourself on the back!).
2) The system doesn't behave as expected, the model has been shown to be imperfect.

When 2 occurs, you try a more complex model. Your simpler model either gets binned completely , or, since it is simple, you remember it as a way of introducing the system to students that are trying to get to grips with it.

The problem most pilots have with principles of flight is that they do not have a solid scientific education. Therefore most PofF books, by necessity, provide very simple explanations for what is going on.
view of a starting point for studying PofF.

This doesn't really represent a problem. The models are good enough to give everyone a rough idea of whats going on so that they can go and start poling the aeroplanes.

But don't be to hopeful in expecting the models to acurately predict more complex situations. You may even find that the pure behaviour of the system is the exact opposite of what you have been told, because secondary effects may be predominant.

The case in point:

You can say with absolute certainty (I'm not going to prove it here, because its lengthy) that when the line of action of thrust is above the line of action of drag a nose down tendancy will result (and vice versa), but you can only say this for sure when in the cruise with T = D).

It does not hold true when T <> D.

Lets extend my glider example. Having opened the airbrake on the top of the pole, you push forward on the control column to maintain the attitude. I.E. you have provided a pitching moment to counteract the pitching moment caused by the airbrake.

Now attach a JATO pod halfway up the pole. Its below the drag line (because the airbrake is providing the majority of the drag). Its still above the C of G. Fire the JATO pod. Aircraft will pitch nose down because the thrust is applied above the C of G. The fact that the drag line is above it is irrelevant.

CPB

Davaar,

Your arguement is flawed. I suspect this is because your basic definitions of Lift is wrong.

Its often worth remembering that the behaviour of aerodynamics will provide us with an overall aerodynamic force on the aircraft.

We call this the Total Reaction. Most text book introduce it on page 3 and then never mention it again.

Lift and Drag are one way of splitting the total reaction into separate components. We do this in order to simplify our models.

But there are times when it is convenient to merge lift and drag back together again and just think about the total reaction.

Gliding is the case in point. With Thrust taken out of the equation, and lift + drag merged into one force, that leave only 2 forces to look at:

Weight (acting down)
Total Reaction.

Your premise that there are no unbalanced forces is spot on.

Your premise that Weight is balanced (solely)by Lift is false.

What is happening is that Weight must be equal to Total reaction.

If you want to see what is happening with lift and drag your definition of them must be sound.

People get so used to the picture on page 1 of their text book showing lift up, weight down, thrust to the left and drag to the right that they get it fixed in their subconscious that:

Lift is defined as 'Up' relative to the ground, and drag is defined as being horizontal.

This is cobblers and confuses no end of people.

Lift and drag are more correctly defined with reference to the aircrafts flight path:

Try this as a model:

Drag is the component of the total reaction that acts backwards along the flight path.

Lift is the component of the total reaction that acts perpendicular to the flight path.

Ergo they are mutually at right angles.

(a picture is worth a thousand words!)

So our glider has its weight opposed by total reaction. To see whats happening with lift and drag, split the total reaction up based on the flight path.

Lets say the glider is pretty poor, and has a glide angle of 10 degrees. Sketch this. The flight path is inclined at 10 degrees to the horizontal. Our drag line is in the same orientation, tilted up 10 degrees relative to the horizontal. The lift line in at right angles (i.e. 10 degrees from the vertical). If you vector add the lift and drag together, you should end up with a right angle triangle with lift, drag, and total reaction on the sides. Lift<Weight, the shortfall is made up by a vertical component of drag.

Try it again with something that has a very poor L/D ratio. Say a House Brick, falling at terminal velocity. In this case the total reaction is made entirely of drag, so its just (weight of house brick) down, opposed by (drag = weight of house brick) up.

With reference to your earlier post, weight (or any component there of) will never cause a pitching moment. Why? Because weight acts through the CofG. Therefore the distance between its line of action and the CofG is always zero. Therefore it never really causes rotation about any axis, although some explanations for some kinds of stability related design features would suggest that it does - remember my earlier post - they are just simple models to give you a rough idea.

Hope that helps.

Maybe we should move this to Tech Log.

CPB

Pit Bull

"Maybe we should move this to Tech Log"

Agreed

How do we do that?

JF

No problem JF, it's on it's way ;)

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PPRuNe Radar
ATC Forum Moderator
[email protected]

Mountain Man

Thank goodness I found you!

I owe you an apology. I should not have made fun of a serious topic.

Where I agreed with Davaar and not you was over your 07.52 post. For me an aircraft (in this case a gliding Tiger Moth descending at a fixed rate) has lift equal to its weight otherwise it would accelerate downwards.

Then I also had trouble over your explanation that following a speed reduction from the cruise there was no way to keep the lift the same at a lower speed. For me all I do is increase the back pressure which increases the AoA and keeps your lift formula satisfied. Providing we do not go above the straight line bit of the Cl versus AoA curve we are not going to stall.

Indeed re that last point surely the only way pilots ever vary their lift in the normal course of flying is to run up and down that Cl versus AoA line. We do not normally change V to control lift surely?

Even better here on the tech log eh? – sort of more grown up.

Excuse my female ramblings earlier (34DD gedit?)

Capt Pit Bull

I think we need to be pretty pedantic about describing how weight and lift are orientated.

For my money weight is the easy one as it only ever acts downward by definition. But this does mean that for an aircraft not flying level there will be an element of weight acting in the fore and aft axis – this has to be added to any thrust in a descent and added to the drag in a climb.

Where we need to be careful is over the orientation of the lift arrow. The lower pressure above a wing compared with the higher pressure below the wing produces a force that can only act at 90 deg to the chord line which we must remember is tipped leading edge up by any AOA. Surely it is this slightly rearwards (with respect to the flight path) inclination of the lift arrow that is what gives us our induced drag term? (ie the lift dependent drag)

Cheers

JF

TripleIRS-

Landing ASAP doesn't seem to be warrented in this case. That is necessary, of course, if you can't put the fire out or if you're running out of fuel, etc. Once you've got the problem stabilized, now take your time and think things out throughly.

In this case, perhaps burning some fuel would move the CG to a more favorable position. In any case, I would consider doing one or more "practice" approaches at 10,000 or so to see how the a/c would handle when I finally committed to the approach. I don't need any surprises at low altitude. Perhaps they did that while diverting. I'm not familiar with the airports in question, but perhaps one had a more favorable runway or winds, etc, than the other.

The a/c that had a jammed elevator (flying tailplane) and had to land by modulating power was a Delta L1011 at LAX about 20 years ago.

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34DD,
No appologies required.I should point out that there was an entire second para that I deleted due to the fact that it was off thread topic and I was aiming at as simple an explanation as possible for Davaar.
My aerodynamics text books are packed due to moving to Singapore this afternoon so I'm trusting to memory however,
If you resolve the individual lift and drag vectors into one resultant then that resultant is equal to weight so you have a balance however you must tilt the lift vector forward to achieve that balance otherwise the a/c slows down and lift decreases. Increasing AOA is not going to work as it will also increase Induced Drag.So by lowering the nose and tilting the lift vector forward you maintain IAS thereby maintaining lift , all be it reduced compared to that which the a/c wing produced when traveling 20 or 30 knots faster,and when that reduced lift is added to the drag acting back along the flight path(remember the drag vector now has a vertical component)it equals weight. :)
All said and done I prefer the thinking of primary vectors as strings on which you pull to discover the resultant,it's simple.

MM.

MM

You have a good trip

Talk some other time I'm sure

34DD

JF,

The bottom line is that a system can be analysed by applying any coordinate system you like, as long as the same system is applied rigourously throughout the entire analysis.

When looking at ressolving forces, far and away the easiest way of doing this is to use a coordinate system where the axes are at right angles (or mutually at right angles if you are doing a 3-d thing).

I'm guessing you are the JF who is a TP of some reknown. In which case I don't doubt that you will have been taught Aerodynamics at some depth, certainly by guys who know more than me. I wouldn't hazard a guess as to what coordinate system is used by Aerodynamics experts.

What I do know is that 99% of pilots are pretty vague about it, and the coordinate system I have described works pretty well for deriving most of the commonly accepted basic formulae of flight.

It is intuitive to visualise that drag is the force that slows you down. Therefore, where ever the aircraft is going is the best axis to define drag on - defining it back along the (tangent of the) flight path.

Having fixed the drag axis, the best thing to do (as mentioned above) is define the other axis at right angles.

If I'm not mistaken, I recognise your discusion point as the standard way of explaining induced drag.

I would say that it would be better to say that the total reaction has a bigger rearwards component than it used to have, so the drag has gone up, rather than talking about tilting lift.

May I point out a few possible weaknesses in defining lift as being at right angles to the chord line (and presumably drag along it?).

Imagine S+L flight. We typically have a small alpha in the cruise. Therefore our lift would not be defined 'up' but tilted a few degrees backwards.

(Assumption - no thrust vectoring)

Instead of L=W and T=D, you get:

(L Cos AOA) = W + (D Sin AOA)
and
T = (L Sin AOA) + (D Cos AOA)

Now, if you were to plug some numerical values (measured against those axes) into these equations you would get exactly the same results (i.e. that all the forces balance).

(like I said, use any coordinate system you like as long as you stick to it). Clearly though, the first way is a lot simpler.

Second observation. What do you do if you've got more than one aerofoil on the aircraft? Say a wing and a tailplane. Now you have two different mean chord lines, one for each aerofoil, usually bolted onto the fuslage at different riggers angles.

If you define lift as being at right angles to the chord you are opening a horendous can of worms because lift will defined in different directions on different bits of the aeroplane.

Much better to keep it simple and define lift at right angles to the flight path.

Hope that all makes sense. Its got late somehow and I wouldn't be surprised if I put a plus instead of a minus somewhere above.

Cheers.

CPB

34DD.

Quote:

"For me an aircraft (in this case a gliding Tiger Moth descending at a fixed rate) has lift equal to its weight otherwise it would accelerate downwards."

You are missing something fairly fundamental:

Lift is less than weight during a descent.

To keep your aircraft on the same flight path you don't need to show that there enough lift to balance your weight, you just have to show that when you add up *all* the forces that they *all* balance.

If you email me I'll send you some JPEGs of some sketchs showing whats going on.

As an aside, lift is also less than weight during a climb as well. Counter-intuitive, but a great way of winning beer once you can prove it.

[A purist would argue that lift is less than weight even in level flight, but that is a level of nit-picking beyond the scope of this thread!]

CPB

Capt PB

I agree with all you say.

I guess at the heart of several of the posts here (including mine) is that the use of words like “lift” can get quite sloppy, making it quite difficult to know what the user means and therefore opens the possibility of misunderstanding or constructing an argument against what has been said. I think a lot of people just think of it as the upwards force that opposes weight without getting into too much detail about some of the angles involved!

Personally, as I said earlier, I always tell students that the physical force on a wing caused by the pressure difference between top and bottom (alone) can only act at right angles to the chord line (which is a matter of physics/mechanics rather than aerodynamics). As you point out, this then allows for a nice explanation of induced drag, but does ignore all other types of drag which exist and have to be scooped up into another arrow (parallel to the direction of travel) and talked about on the lines of:

“the natural resistance of the air to the passage of the aeroplane that depends for its magnitude on how ‘streamlined’ the aeroplane is” (ie lump the wings non lift dependent drag in with the rest of the airframe drag)

Certainly, if I may presume to but into your conversation with 34DD, I agree that lift as you use the word is less than weight in a descent and a climb. And for my money what she should have said was that the sum of the lift plus the vertical component of drag equals the weight. But that I guess is where we came in – definitions of words used. I suspect in her head 34DD was thinking that the Tiger was not accelerating downwards therefore "lift" had to equal weight. A sound enough "model" for the mechanics eh?

Oh yes, your point about the axes used to define things is of course vital. My recollection is that wind axes are the conventional ones used by the pros when discussing aero stuff. (Since using wind axes means drag will act in opppostion to the dirction of travel of the aircaft your explanation of your coordinates shows you are thinking about wind axes – so that’s neat!) Body axes come in to the act when considering inertia effects and with today’s very high alpha manoeuvring the weirdness of what results from keeping (say) sideslip zero when rolling about the long body axes (inevitable due to inertia effects) is very apparent to the external observer.

Good taking to you.

PS It may have been late for you but its a bit early for me - so sorry if I’ve screwed up somewhere!

JF

Capt Pit Bull,
yup Lift<weight in a climb and I used to thorougly enjoy winning beers with that one. :)

34DD :)
Davaar, understand now ? It's an interesting topic all said and done.

MM. :)

Some assorted points from this thread.

I thoroughly agree with Capt PB's comments on physical models as applied to aerodynamics or any other bits of physics. So please take what follows in the spirit of "if you want to extend the model and are prepared to accept the complexity that goes with it, this might help", rather than just nit-picking on my part.

The definition of lift and drag as used in standard aerodynamics texts is always with respect to the direction of incident airflow. Lift is the component perpendicular to the incident freestream airflow, drag is parallel to it. That's substantially the same as using the 'flight path' as Capt PB suggests.

However, the difference between flight path and local airflow is vital in the classic explanation of induced drag, and I hope JF will forgive me for saying that his model of induced drag is oft quoted but misleading.

When a wing of finite length generates lift, it creates a vortex which leaves the wingtips in what we all know as wake vortices. Just as the winds around a Low pressure area can be felt hundreds or thousands of miles away, so the effect of the vortices is wide ranging. Not only does it affect the air flow behind the wing, but also in front of it.

If you follow through the direction of the vortices, you'll see that there's a general downward movement within the wingspan, and an upward movement beyond the tips. Thus the incident flow approaches the wing not along the flight path, but from a direction slightly above the flight path by an angle known as the 'induced angle of attack'. It actually varies along the span -- it's greater closer to the tip and therefore the vortex, but it's convenient to picture it as an average downflow. This induced angle of attack is not the same as the true angle of attack of the aerofoil. For a start, it varies with the aspect ratio of the wing. It's typically much smaller, by a factor of about half the aspect ratio. So for an aspect ratio of 10, an AOA of 10 degrees gives an induced AOA of about 2 degrees.

Because lift is conventionally defined as being perpendicular to the bulk incident airflow, the effect of the induced AOA and the tilted airflow as it approaches the wing is like tilting the lift vector of an infinite aerofoil back by the induced AOA, as well as reducing the effective AOA. It's the picture of that tilted lift vector that often leads to the misconception that as the AOA increases so the lift vector tilts back perpendicular to the chord line. But the effect is nowhere near as great as that.

On thrust/drag couples I have to disagree slightly with Capt PB, although it may be a question of interpretation. There's a difference between the instantaneous and transient effect of a force, causing an angular acceleration about the pitch axis, and the steady state change in equilibrium position.

From the point of view of 'pitch up' or 'pitch down' tendency, I think it's usually the latter that concerns us, because the time scales for AOA pitch stability are pretty short.

If you apply an unbalanced force to a body that's in equilibrium (linear and rotational) the resultant motion does indeed depend on the position of the centre of mass (CM). The force will cause an acceleration. If you don't mind using non-inertial frames of reference and you're in to double-entry book-keeping, you can add an 'inertial force' at the CM in the opposite direction and look at the effect of that couple.

However, forces rarely stay unbalanced for long. As the airspeed increases, the drag increases about the appropriate point as determined by the aerodynamics of the body, not by the position of the CM. As soon as the acceleration stops, the drag has increased to equal the thrust, and the thrust-drag couple is balanced in the usual way by the pitch-stability mechanism of the aeroplane.

In the case of your glider and JATCO pod example, the immediate angular acceleration might be pitch down, but the 'glider' will rapidly accelerate to a point where the increased drag at the end of the pole makes a bigger impact than inertial force, and the natural stability from the decalage of the mainplane and tailplane is left to counter the now pitch-up couple. The steady state condition will be a higher AOA than before the JATCO pod was fired, and a much higher pitch attitude of course if the aircraft is now climbing. The details of the forces and geometry will determine the relative sizes of the effects and whether the pilot perceives this as a pitch down followed by a pitch up, or just a pitch up.

The example might seem trivial, but it's for these reasons that the pitch-up/down effects of flap extension get quite complicated.

Another good example is the mythical dependence of wings-level engine-out controllability of a twin on CM position. The effect is purely about aerodynamic couples and thrust balancing and has nothing to do with the position of the CM, contrary to what we're told in most standard texts.

By the way, is the UserName 'Mrs Pit Bull' taken? Bookwormess is announcing the readiness of dinner and I can't help but think that 'Mrs Pit Bull' might be more fitting. :)

Bookworm

Cor! I take my hat off to you for trying to explain Lanchester’s model of a horseshoe vortex (once the starting vortex has been shed) using a text based medium!

However, I came over a bit dizzy when I tried to tie those core principles of the rotational flow notions round a wing with your comment copied here:

Quote “Thus the incident flow approaches the wing not along the flight path, but from a direction slightly ABOVE (my caps – don’t know how to do the bold thing) the flight path by an angle known as the 'induced angle of attack'. It actually varies along the span -- it's greater closer to the tip and therefore ….” Unquote

I must say that every flow visualisation that I have ever seen in tunnels various as well as in flight showed an upflow (relative to the undisturbed true free flow direction well ahead of the aircraft) To my simple mind that represents an increase in alpha not a reduction. And indeed is linked to why the stagnation point moves steadily rearwards along the undersurface as alpha is increased.

Perhaps it was a typo though due to culinary pressures.

I am sorry you don’t think the notion of the total aerodynamic vector produced by the wing having an increasingly rearwards component, as alpha is increased, is a useful one when discussing induced drag for pilots.

For me it has the advantage that when applied to a flat plate it is correct at both zero and ninety alpha and in between these extremes it changes in magnitude in the correct sense. Which can’t be all bad.

We have come quite a way from that Mayday call…..

JF

Capt Pit Bull

Sorry to be so long getting back to you, one has only to go away for a few days to find everything has changed/moved.

You are right and my new found knight in shinning armour (JF thank you) said what was in my mind.

This is all well above my head now (Lanchester – sounds like WWII) so I shall just lurk. I’m in a mood anyhow because I believe I may have to change my call sign to 36E.

Still your 34DD (in my head)

I don't think I can add anything to the aerodynamic argument, but JAR-25 doesn't state what direction pitch changes with power, flaps, etc. should be.

What is does state is the maximum transient and continuous control forces the pilot should have to provide in such a case. In pitch the figure is 25daN (about 54 lb) transient and 2daN after trimming (about 5lb) for an aircraft with a wheel (slightly less with a stick).

I have to say that nobody has ever asked me to certify an aircraft with 50lb control force due to power, gear, etc. and I'd be very reluctant to approve it if they did.

G

Greetings all.

Haven't looked in for a few days as Mrs Pit Bull and I have been scouring the countryside looking for a new Kennel.

JF. With you. Once you start looking at how the aeroplane rotates (as opposed to where it goes), then I can see how the aeroplanes axes would be a better reference rather then its flight path. Presumably you now need to track its angular inertia (i forget the correct term) about each axis, and since the mass is distributed with reference to the a/c not the flight path, the a/c becomes a better reference?

Bookworm. Agreed. I think it is most important to distinguish between short and long term tendancies. With reference to the case in point, I recall being taught Effects of Controls on the old JP. Open the taps up, and the a/c pitchs down every so slightly for a few seconds before the increased airspeed overwhelms the effect and pitchs you back up again.

For All. With reference to a handy explanation for induced drag. When teaching P of F, I found most of the 'tilting vector' type arguements quite tricky to put across, so I preferred to track energy instead. Basic arguement goes like this:

- The generation of a vortex is reasonably easy to visualise, even if how it causes drag is less so.

- Although we often visualise the airflow around the wing, it is sometimes handy to remember that in reality the air is sitting there minding its own business when along comes a wing and churns it up. (Model: stationary aerofoil. Reality: stationary air)

- A formless aircraft with a perfectly frictionless skin would still form the vortices, due lift production.

- So, before the aircraft, the air was still. After the aircraft, it is rotating. Therefore it has gained energy.

- The energy the air has gained must have come from the aircraft. Anything that removes energy from the aircraft is drag.

Regards, everyone.

CPB

Hi Bookworm

I have asked around my elders and betters and all agree you are right about induced drag producing a reduction in AOA. Sorry I suggested a typo. All also agree that there is an upflow ahead of wings (as seen with flow visualisation) but the effect of which you speak is a downflow on top of the wing (which occurs aft of the leading edge) which can of course be represented as an apparent reduction in alpha.

With you now!

Regards

JF

PS Its never too late to learn I guess!

Capt PB

I like your "its stirred up the air and that took energy" approach. Its clearly good to have a variety of ways to give students a feel for lift dependent drag and I'm sure Mr Lanchester would approve of that one!

Cheers

JF

PS Sorry, I forgot to respond to your body axes point. Absolutely. The early days of long and heavy fuselages and smallish wings (say the F104, but it applies to plenty of others) produced the odd departure with the mechanism being termed "inertia cross coupling" If you roll such a beast quickly it wants to rotate about ITS long axes (naturally) SO, if you had a stack of AOA on at the time you started the roll, that AOA became sideslip after 90 deg of roll. 90 deg further on it was negative AOA - you get the picture I'm sure. So inertias were cross coupling with aerodynamics and produced pretty severe results.

If an aircaft has highish aero forces and lightish inertia forces (like any GA aircraft say) then as you do the start of the roll the directional stability works normally (overpowers the inertia effects) and reduces the sideslip that would otherwise result after 90 deg.

Sorry if I'm teaching granny to suck eggs

JF



[This message has been edited by John Farley (edited 18 November 2000).]

JF

I've learned a great deal from this topic, and I appreciate the spirited debate.

I prepared a response to your original a couple of days ago but my machine decided not to send it! For what it's worth, here it is...

---

I think it should be me to take off my hat to you JF. It's one thing to write a textual description of a Lanchester horseshoe vortex -- shurely Prandtl's Pferdshue? :) -- but it's quite another to read it and pick out the inaccuracies!

Nevertheless, I'm going to stick to my guns and claim that mine was a poor description of correct physics. You are of course correct to say that the local direction of airflow in front of the leading edge is upwards, just as behind the trailing edge it is downwards. That's the effect of the vortex bound within the wing, across the airflow. That applies to an infinitely long wing too, where the flow is the same all the way along. Your point about stagnation, is of course, well made.

Cheers

Bookworm