Owain

Thanks for your reply.

After a while the graph seems to have some sense. Lately I have lost my ability to interpret graphs. I was good at that some time ago, but I'm growing old... What is on the vertical axis, exactly? Anyway I think I understand what you mean. Comes to my mind a graph with those lobes of pressure in the upper and lower surfaces.

So symmetrical airfoils never create any couple, but cambered ones always do, Right?. I will accept that as an empirical fact, because a theoretical explanation of it can be extremely complicated, I deem.

Can we consider the effect of the airstream on the wing as producing a total reaction plus a pure moment (a couple) that we call Aerodynamic Moment?

In case this is affirmative, the point at which the Lift is considered to be acting (i.e. the CP)... Does it take into account that couple, too? or you have to consider the moment of Lift (acting on the CP) about the CG and then add "the couple" or aerodynamic moment to find the total pitching moment effect?

PD
I think that maths are a kind of language but it is is good to frequently translate it into normal words

Microburst

Know the feeling:ok:

Vertical axis is pressure difference relative to ambient pressure divided by freestream dynamic pressure - negative values are suctions, positive values are pressure above ambient. So the +1.0 is where the local pressure equals stagnation pressure. It is in fact one of those graphs with lobes of pressure on upper and lower surfaces. You just have to be aware that at negative AoA the pressure on the lower surface LE is the one that is negative.

Yes, and I am darned glad you don't want a theoretical explanation ;)

Yes

Maybe I've misread your intent, but aerodynamic moment is usually used for the combined effects of reaction (lift) and couple, referred to some chosen reference point e.g. 25% chord or aircraft CG rather than just the couple.

The point at which the lift is considered to act (cp) takes the couple into account, which is why the cp is calculated as Cmo/CL chords away from the 25% chord point (assuming that to be the chosen reference).

Agreed, but I think you need to retain the discipline of mathematical conventions in your "normal" wording.

I think I start to understand

The moment exists physically, which I was not sure. I mean, if the wing was made of paper or something frail it would be bent and maybe even folded or broken because of the couple. And its effect is all included in the cp.

Now, regarding the tail lift as being negative or positive, I had the notion that it had to be always negative because of the aerodynamic moment, and that a tailplane was necessary (an then the germans invented the flying wing)

But what I think is just that for the airplane to be in trim, the tailplane lift has to able to balance all the other pitching moments, whichever they are. These are basically those due to the wing plus fuselage lift at the wing-fuselage cp, the thrust and the drag.

The Drag-Thrust couple usually gives a nose up (positive?) moment, and the Lift gives a nose down (negative?). The tail lift has to balance the resultant of those moments. I guess that the Lift effect is much greater than the T-D couple? It seems so if Lift is many times bigger than Trust for Drag, but it depends on where the wing-fuselage cp lies. I deem it does lie well aft of the CG?. That would be why the tailplane has to produce a pitch up (positive?) moment with negative lift. But can the cp be ahead of the CG for "normal" angles of attack?

To answer your last point. Yes, but the a/c would not be naturally stable.

Hmmm ... but on a certain very fast tailless aircraft the wing/body cp and cruise CG (with fuel transfer) were arranged so that the elevons were deflected slightly TE down (positive lift) to keep the drag down.

Maybe if we put in some numbers? these are not specific to any one type, but they are realistic.

Modern airliner wings have the section (camber and thickness distributions) and twist (local AoA variations) carefully tailored to maximise performance (NOT to minimise drag since wing loads/weight come into the equation). In addition their planform is usually swept and has a compound taper. Consequently it is very difficult to pick a 'real' chord to represent things and it is usual to calculate a 'mean aerodynamic chord' (mac or amc) based on planform and use that as a reference 'axis'.

The wing body CP in cruise conditions varies between 30~40% mac depending on lift coefficient and Mach number, with the higher end corresponding to the maximum cruise Mach (Mmo). If you back off to best economic cruise conditions then the CP will be around 35% mac.

On an aircraft without fuel tanks in the tail the CG limits will range from 15~20% mac at the forward end to about 30~35% mac aft. If the aircraft has a tail tank then the aft limit will stretch to 40% mac, maybe a touch more.

Just glancing at those values you can see that in most cases the CP will be behind the CG and you will need negative tail lift to trim, but if you have and use the tail fuel tank then the CG can be at or behind the wing/body CP and the tail lift will be either very small or positive. Factor in the nose-up pitch from the drag/thrust couple and you will definitely have positive tail lift to trim.

Just to round things off, the complete aircraft (tail on) aerodynamic centre will be around 50~55% mac so the aircraft would be stable even at the aft CG limit (as it must be of course to meet regulations)

Maybe also mention that on most low tail aircraft, there will be downwash from the wing
hitting the stabilizer creating a downforce. The higher airspeed, the stronger the downwash
and more downforce.

I used to fly competition RC model aircraft. The class rules specified a maximium wing loading limit so there was an incentive to build them light so they were smaller. On one occasion a competitor appeard to have taken this a bit too far when the tail boom broke on a straight and level but very high speed run. Due to the nature of the break it appeared the down load on the tail was the cause, probably compounded by damage that occured on a previous flight.

Not exactly true - the higher the airspeed the lower the wing AoA and the lower the downwash angle at the tail (which is usually about 40% of the wing AoA)
Plus of course the fact that downwash doesn't change the force on the tail needed to balance the airplane - it just changes the tail/body setting necessary to generate that force.

My understanding of tail (trim tank) fuel is to reduce the amount of trim and therefore drag, not to swap pitch up trim for pitch down trim. The a/c still needs to be stable. Especially air transport category, to comply with regulations. Think about failure modes. If engines are lost with the a/c in non normal trim, hand flying, whether it be fly by wire in direct law or traditional control systems, will be very tough for an average pilot. Military a/c performance is a completely different kettle of fish.

OG,

Pilot's Handbook of Aeronautical Knowledge Chapter 3 - American Flyers

Even though the horizontal stabilizer may be level when the airplane is in level flight, there is a downwash of air from the wings. This downwash strikes the top of the stabilizer and produces a downward pressure, which at a certain speed will be just enough to balance the “lever.” The faster the airplane is flying, the greater this downwash and the greater the downward force on the horizontal stabilizer (except “T” tails). [Figure 3-13] In air-planes with fixed position horizontal stabilizers, the airplane manufacturer sets the stabilizer at an angle that will provide the best stability (or balance) during flight at the design cruising speed and power setting. [Figure 3-14]

If the airplane’s speed decreases, the speed of the air-flow over the wing is decreased. As a result of this decreased flow of air over the wing, the downwash is reduced, causing a lesser downward force on the horizontal stabilizer. In turn, the characteristic nose heaviness is accentuated, causing the airplane’s nose to pitch down more. This places the airplane in a nose-low attitude, lessening the wing’s angle of attack and drag and allowing the airspeed to increase. As the airplane continues in the nose-low attitude and its speed increases, the downward force on the horizontal stabilizer is once again increased.

fig 3.13 and 3.14
http://www.americanflyers.net/aviati..._3-2_img_6.jpg

Sure, tail fuel is used to reduce trimmed drag, but do you really understand how it does so?
Negative lift (pitch up trim) means that to balance the overall lift/weight equation the wing must produce more lift than just the aircraft weight to offset the negative tail contribution and that means more drag due to lift. If you move the CG aft so that the amount of negative tail lift is reduced then you will reduce the wing lift requirement and get a lower trimmed drag. Going further to the point where the trim is pitch down (positive tail lift) is merely an extension of that process. So swapping pitch up trim by pitch down trim is exactly what you need to do to reduce trimmed drag.

Read again what I said a couple of posts ago:
QUOTE]Just to round things off, the complete aircraft (tail on) aerodynamic centre will be around 50~55% mac so the aircraft would be stable even at the aft CG limit (as it must be of course to meet regulations) [/QUOTE]

Not much to be doing with failure modes either - if the CG is ahead of the aerodynamic centre the aircraft will be statically stable whatever the FBW is doing.

This has nothing to do with engine failure (except if you are fool enough to take off with full aft CG and get an engine failure near Vmca, in which case the lateral control might be a problem, but even then the aircraft has to be certificated for engine failure at aft CG). And if the aircraft is certificated for flight with an aft CG then that is NOT non normal trim. Moreover it has to be flown without difficulty in that state by an average pilot - that is part of the airworthiness code.

XPM

Hmm - different definition of 'downwash' is causing a problem. In this FAA document they seem to be using downwash to describe a force where I am using it to describe an angle of flow.

Consider for example this extract from Richard Shevell's "Fundamentals of Flight"
In that expression CL is the lift coefficient and AR is the wing aspect ratio. I hope we can agree that if airspeed is increased the lift coefficient is reduced, so that downwash, to my definition, would be decreased. This is just the opposite of what the FAA are saying, but Shevell (and I) are talking about the incident flow angle onto the tail not the force generated on the tail by the downwash, which varies with speed squared as well as flow angle.

I think that is the reason for our apparent difference of opinion :ok:

Cl does decrease, but total lift force is constant regardless of airspeed (assuming level flight)
This means that to maintain lift at higher airspeeds (lower aoa and Cl), more air mass gets accelerated faster (vertically) giving
an increased downforce on the tail. F=m*a

An interesting discussion is developing.

For those who might think that tailplanes are not associated with downforce, a simple Google search for "friendly fire Caribou accident" will lead to a very well-known photo of a Caribou's last seconds after having the empennage shot off by friendly fire on short final into a forward strip in Vietnam during the mid-60s.

Once you have figured out the perspective, you will see that the aircraft has pitched nose down and through the vertical .. in the space of a second or two.

Agreed

Except that it doesn't work like that. Force is equal to the rate of change of (vertical) momentum; that is it equals the mass flow over the wing times the vertical velocity imparted to that air by its passage over the wing. [Just rewrite your F=m*a (or F=m*(dV/dt) to read F= (dm/dt)*deltaV - still the same equation but different emphasis]
As you point out, at a higher airspeed that is more mass flow and since the lift is constant that means that the vertical velocity when the air leaves the wing must be less. But that vertical velocity is the downwash ...

If we are just talking about the changes in force on the tail that result in a stable airplane, then yes, lower AoA goes with a bigger tail downforce because that is what pitches the airplane back towards its stable condition - I don't have a problem with that, but it it the change in tail AoA that comes from the reduction in aircraft AoA combined with the change in downwash angle that produces the additional downforce (at constant airspeed that is).

If we are talking about changes to the original trim coming from an increase in speed, then both wing and tail forces are going to be increased and unless the AoA is reduced the aircraft will develop some normal acceleration.

Owain, thanks a lot

Now I undestand it very well. Some nubers help very much indeed.

Regarding FBW, with these airplanes it is difficult to talk about stability. I mean, they cannot be left either stick fixed nor stick free, to begin with. The system is always making inputs!

Yeah, it is difficult to see exactly what is meant by stick fixed/free stability these days - but then stick free stability hasn't meant much since aircraft have been fitted with hydraulically driven controls, since changes in control hinge moments due to aerodynamic changes don't get reflected back into stick position and thence control movements.;)

The way I look at it, the most important requirement is controlability (pilot capability to control)

If the airplane was unstable it would not have controlability. If it was too stablebit would neither have controlability because of lack of manoeuvrability.

But if the airplane had a FBW flight control system such that it can always meet pilot demand no matter how unstable the basic airplane might be, what sets the lower limit of stability?

Wait, Now thinking I guess that certification requirements must account for failure of the FCS. Is that so? The aft CG limit of an A320 or a B777 is determined because of the direct law possibility?

Hi Owain Glyndwr,
That's odd because, from B707s ({#1} no hydraulic power controls) to 747s, the control yoke has always caused the same deflection in the flight control surface position despite any aerodynamic changes. The effort to move the yoke may have changed due to the artificial "feel" (Q pot on the 707; - Air Data Computer adjustment to spring stiffness on others), but we always knew how much the control surface was displaced by simply looking at the yoke (similar to the power steering on your car).

Stick free has simply been the same since Cherokee days. Let go and see what happens after the aircraft has been displaced from trimmed speed.

I agree it is completely different on Airbus FBW. The side stick gives no clue as to what the control surface is doing, and the only time "stick free" makes sense is in Direct Law.

{#1} We had "rudder boost" (hydraulic assistance to rudder for take off and landing only) to reduce VMCA / VMCG

Not much to be doing with failure modes either - if the CG is ahead of the aerodynamic centre the aircraft will be statically stable whatever the FBW is doing.

[/QUOTE]

I think we are in agreement Owain.

However, reading some of the other posts here (and elsewhere) the suggestion by some is that the CofG maybe AFT of MAC.

It can be so for certain high performance a/c but not air transport catagory.
Failure modes have to be taken into account. Unfortunately, things go wrong.
In an ideal world all a/c would be designed with all surfaces lifting (Canard?) but we are not in that world.

Good thread this. :ok:

rrr

That's because you are going one way down the control path, I'm going the other!

Let me quote again from my "Bible" - Dick Shevell's book (p.315 in Iss 2)

"Stick fixed stability means that the pilot maintains the control column in a fixed position so that the control surfaces, specifically the elevators, are not permitted to move as the airplane changes its angle of attack. Stick-free stability describes the value of dCm/dCL with the elevators assuming the position, at each airplane angle of attack, for which the elevator aerodynamic hinge moment is zero. This will be the angle at which the elevators will float with no control force. Stick free stability is important for airplanes with direct cable control of the surface and/or its control tabs mounted on the surface. Aircraft with full irreversible powered controls will have stick fixed stability even when the pilot is flying "hands off" provided the pilot has trimmed the column to zero force at the initial flying speed"

In other words stick free stability depends on the feedback from control aerodynamics, whereas you, I think, are talking about the direct yoke/control surface path? Your B707 would have had stick free stability, but not the B747. And on my definition, stick free stability is meaningless on the A320 even in direct law, because it still uses irreversible powered controls.

On aircraft without any artificial feel (any left? - well maybe your Cherokee and similar) the control forces felt at the column would be proportional to the aerodynamic hinge moment on the control, but otherwise you get what the designer thinks you want (which I agree may not be the same as you actually do want ;))

Correct :ok:

Yeah, aircraft designed for high manoeuvrability might be designed to be unstable and rely on artificial stability.

Don't start me off on canards - they have their disadvantages :D

Hi Owain Glyndw,
You are correct.
Many thanks for all your lucid explanations.

Owain

Is this Dick Shevell book apt for non engineerrs?

What's the name of the book? I hope I can get a pdf copy

Microburst,

As you probably guessed I am (was) an engineer, so perhaps not the best person to judge whether Shevell's book is apt for non-engineers. Dick (I met him once) was Chief Aerodynamicist at Douglas and then went to Stanford, so his book is full of realistic data. Looking through it it seems to be about 95% good, understandable text and explanations, but some maths is inevitable - only you can judge if it is too much, but I think you could just take the maths for granted and read the text.

Anyway, it is : Fundamentals of Flight, by Richard S Shevell, published by Prentice Hall, ISBN 0-13-339060-8. I don't know whether it is still in print though, and I doubt you will get a .pdf version - it is over 400 pages.

Thank you, sir

TURIN....

I have to completely disagree with your stance. Not to be personal but your attitude contributes to the lack of scientific literacy throughout North America and many other places. I can't stand people who justify using ambiguous and/or incorrect terminology or explanations, because in reality they don't understand the topic themselves. Frankly, if you don't understand it, you shouldn't be the one telling everyone how it should be described! You should look into Richard Feynman - he was a physicist who is known for his excellent explanations yet he also shares the same point of view as I do on this matter. If you don't want to listen to me, at least listen to a man who definitely had a good idea of what he was talking about. [/rant]

To add onto what John Tulla posted, for those of you who still don't quite understand/believe that a tail almost always will produce downlift, look at this video at 0:40.

Oh dear! Italia 458, As I'm not from North America I can't say either way if your rant is accurate our not.
However, one thing is certain. You need to settle down. My quoted post was meant to be light hearted. Life is too short. If you really can't stand me because I don't use high level maths to understand/explain a simple opening post such as the one above that is your problem. I'll stick to normal, conversational English if you don't mind. I seem to hang on to more friends that way. Be well.:)

TURIN...

What exactly do you mean by 'light hearted'? Is that supposed to mean that you actually didn't mean a word of what you said?

I never asked you to use "high level maths". But what I would ask is that you use "accurate" descriptions and examples to explain something. Math is a good way to do that - I suggest you learn a little bit sometime! :ok:

Hmm... that attitude seems to go hand-in-hand with the attitude that you should explain everything as simply as possible, even if it's not correct. So if your friends are wrong about something you just disregard it? My friends are generally happy to have an engaging discussion and so I don't have a problem with pointing out a potential error in our discussion.

Like I said. Life's too short to get into pointless discussion such as this. I enter into pprune to learn and hopefully pass on some of my experience that others may also learn. If you want to get into a fight I suggest you join a gymnasium. I'm not interested.
Good night.

Microburst2002, Owain Glyndwr;

Fascinating thread - thank you for this interesting exchange between the two of you. Microburst2002, I can highly recommend Owain's contributions on the AF447 threads. You can use mm43's PPRuNe search tool to search/find anything in PPRuNe.

I searched for a pdf copy of Shevell's work and while doing so I found the following related pdf on Aircraft Design - Synthesis and Analysis. One of the authors is Ilan Kroo, Professor of Aerodynamics and Astronautics at Stanford but the introduction states, interestingly:

Microburst,

PJ2 seems to have found the .pdf source I recommended to you via a PM ;)

It is very good.

I don't know what you guys are talking about, honestly.

Processing data....

:}

Microburst,

If "you guys" are PJ2 and myself, I was referring to the alternative source of material similar to that contained in Shevell's book (which you were interested in obtaining) and which can be found at the URL PJ2 gave.

Owain

Thankyou very much for your message, i received it well.

I just wanted to humoristically thankyou two for it without "incriminating" myself for breaching some rule :)

But it didnt go well... :(

Microburst,

Guess I'm taking things too seriously ;)

I understand the tailplane lift downwards for stability purposes, but for manoeuvring, or for any other reason, would tailplane lift ever be upwards in flight on your typical FAR 23 or 25 aircraft - Airbus/Boeing computerised relaxed stability aside. I say in flight, because I know about lifting the tail of a tail dragger on take off.

Thanks Gals/Guys.

megan,

Is short, no. It's a design requirement that full elevator in available up till Vmo without overstressing the airframe, and that would not be sufficient to generate positive lift on the tail.

BTW, it isn't the negative lift that is needed for stability- just that the lift component of the tail changes sufficiently with changes of AofA to ensure a corrective moment, while not stalling.

It is the last bit that means that most tails create negative lift, but it is not an actual requirement.

I realise it's not a requirement, but as wondering if any aircraft actually produced positive (upward) lift on the tailplane in order to reach its negative "g" limit as per its V-n diagram.