Wing positioning to reduce drag

[Dupre]

I was taught that in cruising flight, there is normally a downward force acting on the tailplane. This is presumably because the centre of lift is behind the centre of gravity, and can be readily seen by looking at the elevator in the piston single I fly. Of course this downwards force on the tailplane opposes the wing lift, and any lift force creates induced drag.

My question is: Why not arrange things so that the tailplane HELPS the wings? This would reduce the lift required by the wings, hence reduce the induced drag. Moving the wings forwards would seem to do this.

In some aircraft (e.g. small high wing a/c) it’s not practical for visibility – but in low wing aircraft, big and small, I don’t see why the designer couldn’t design the wings forward of their normal position to reduce the induced drag.

Any thoughts?

[Intruder]

It has to do with stability and stall entry & recovery properties.

A canard in place of the rear-mounted horizontal stab can work, but designers are reluctant to let the lawyers second-guess them these days...

[SNS3Guppy]

The load on the horizontal stab depends on the center of gravity, configuration, etc. To suggest that the horizontal stab is consistently negatively loaded, or consistently acts under a download, is incorrect, In particular, it also depends on the specific airplane.

Think about the change in loading when operating with an aft CG.

[Lookforshooter]

Dupre...you got it right...see the Beech Starship, Piaggio Avanti, and some of Burt Rutan's designs...the 'tail' is in the front, (canard) creates lift upward just like the wing...enhances stability...generaly dynamicaly stable, aids in turbulance penetration, less drag equals less power needed, for a given speed.... Other advantages include being able to actualy see your tail...icing, problems, issues, disymetry... So why aren't there more? The consensus is that the aviation market isn't very keen on change, new designs, especialy when a billionare has to sit in the back and try it out for the first thousand hours. The Piaggio is a perfect example of designing a plane that all surfaces are considered a lifting device...you can put some smaller engines on them, it flies higher and faster then most turboprops, and gives some people thoughts that they get close to jets...now if they designed jets with Canards, I suspect you would see some really nice performance increases. The Piaggio with some jet engines...???

[Dupre]

Wow thanks for the replies!

Guppy - I wasn't trying to say the horizontal stab. is negatively loaded under ALL conditions, just referring to a normal aircraft in a normal cruise with a normal payload - when you want the aircraft to be at it's most efficient.

But now we've got a new question - why aren't airliners designed with canards? What are the negatives of a canard design?

Keep em coming!

[Karl Bamforth]

Yep Canards are more efficient........... but generally require longer runways and do not have short field TO ability.

If you got a long runway and want to fly fast then choose canard.

If you want to operate from short or grass strips then you will need the tail in the propwash.

[SNS3Guppy]

Yep Canards are more efficient........... but generally require longer runways and do not have short field TO ability.


You should see the difference the Peterson conversion does to a Cessna 182, with respect to slow flight and short field capability.

A canard, or forward lifting surface, doesn't of it's own accord increase runway distance or reduce short field capability. A given airplane might have poor short field performance, but that's not the fault nor function of a canard...just the design of that particular airplane.

I wasn't trying to say the horizontal stab. is negatively loaded under ALL conditions, just referring to a normal aircraft in a normal cruise with a normal payload - when you want the aircraft to be at it's most efficient.


Then operate the airplane efficiently. To start, put the CG in a location most favorable to an efficient flight, given the loading conditions.

[Intruder]

The load on the horizontal stab depends on the center of gravity, configuration, etc. To suggest that the horizontal stab is consistently negatively loaded, or consistently acts under a download, is incorrect, In particular, it also depends on the specific airplane.

Think about the change in loading when operating with an aft CG.
I believe that you will find that EVERY conventional wing/horizontal stab airplane in the [US FAA] normal, utility, or transport category has a horizontal stab that produces downforce at ALL times in level flight when within certified CG. At "aft CG," there is merely LESS downforce on the tail than with a more forward CG.

If you know of any exceptions, please show us.

[ahramin]

On the A380 the horizontal stab lifts up instead of down.

[SNS3Guppy]

Dupre,

This site may provide you with a straight forward, easy to follow presentation on your question: Angle of Attack Stability, Trim, and Spiral Dives [Ch. 6 of See How It Flies] (http://www.av8n.com/how/htm/aoastab.html#toc105)

[OutOfRunWay]

AFAIK, Canards inherently are more unstable, especially at transonic speeds. In modern aircraft viz Eurofighter Typhoon, this is easily dealt with with computerized flight controls, but there is understandable reluctance to design a passenger aircraft which will become almost unflyable, if the software ceases to function. (In the Eurofighter, you can at least eject)

[Intruder]

On the A380 the horizontal stab lifts up instead of down.
I cannot find ANY reference to confirm that. Please provide a reference/link.

[TURIN]

But now we've got a new question - why aren't airliners designed with canards? What are the negatives of a canard design?

Airport design?

Most airbridge (Jetways) would have to be repositioned for a start.

[transfer jack]

Hi,

please check this picture.....
http://www.pilotsreference.com/image/preview_image_cd/PilotsReference_Preview46.jpg

any, naturally stable airplane has to fulfill this criteria. Stability cost performance, thus fuel. This applies for ALL stabilities around ALL axis.

When the airplane is pitched up by a disturbance the AOA of the wing and tailplane is increased. This increases the lift of both surfaces (Possibly the lift of the tailplane is negative, but after, less negative through the AOA increase). When the combined forces (multiplied by the arms) are higher than the original disturbance, the airplane is stable.
All the civil airplanes we are flying are naturally stable, those certified for IFR more than those for VFR only. Therefore we are experiencing the bumps as up and down movements, although they are originating from small pitch changes.
Same applies to directional stabiliy. You will not experience this much yawing in turbulent air in a multi as compared to a single engine airplane. Multies have to provide more natural directional stability to pass single engine flight criteria...and this cost performance.

[PB4]

Quote:
On the A380 the horizontal stab lifts up instead of down.
I cannot find ANY reference to confirm that. Please provide a reference/link.

during my atpls I was also told Airbuses have lifting (up) horizontal stabs, and they could do so because stability is improved by fly by wire.. now if anyone has docs on this I would be interested in reading

[transfer jack]

During the testing of the X-planes the Mach tuck or tuck-under was first experienced in the transsonic speed range (btwn mach crit and approx 1.2) because the center of lift moves backwards when the lift distribution changes above mach crit. The conventional elevator could not provide enough elevator down force to counteract the pitch down tendency (mach tuck). The "flying tail" was invented - the angle of the entire tailplane can be changed (like in a Piper 28). The term "flying tail" is misleading towards the conclusion that the tailplane produces upward lift. Look at the shape of tailplanes and trim ranges of airliner's "flying tail" (circular cut in the rear fuselage)! And, if you should ever fly supersonic in your airliner, trim nose-up (pitch-down the flying tail) to get out!

[ChristiaanJ]

transfer jack,
Your last post is interesting, but it has absolutely nothing to do with pitch stability on a conventional aircraft.

On the other hand, the picture you linked to in your earlier post says it all, unfortunately it doesn't clearly explain it's a matter of moment, not just delta L.

CJ

[john_tullamarine]

those certified for IFR more than those for VFR only

Now, that's interesting, could you perhaps point to where the standards require this ?

[Intruder]

during my atpls I was also told Airbuses have lifting (up) horizontal stabs, and they could do so because stability is improved by fly by wire.. now if anyone has docs on this I would be interested in reading
There's an inherent problem there, though: If the airplane is inherently unstable (like the F-16) or marginally stable (like the F/A-18), loss of FBW means loss of airplane (F-18) or a VERY limited manual backup capability (F/A-18A, but not even that in the E/F, IIRC).

AFAIK, at least the smaller airbusses have reasonable manual reversion capability, as demonstrated in the DHL "shootdown" in Bahgdad. Therefore, they have to have inherent stability that is better than that demonstrated by an upward-lifting tail.

Go back to whoever told you the story during your atpls, and have him show you the references.

[mcgoo]

Heres a quote from the A380 book by Guy Norris and Mark Wagner.

By June 1999, flight tests had also begun of an A340 fitted with a reduced stability flight-control configuration, as part of efforts to validate the technology for the A380. Natural stability was reduced by transferring fuel between tanks so the aircraft's centre of gravity was moved aft. This meant that the aircraft was more finely balanced and required less downward force to be applied by the horizontal tailplane for stability. By using a high-fidelity FBW flight control system, Airbus believed it could take maximum benefit from the move by reducing the size of the 2,580 square-foot tailplane by up to 10% to approximately 2,153 square feet. This could reduce trim drag by about 0.5% and save 1500 pounds in weight, directly contributing to overall operating cost reduction.

[SNS3Guppy]

The quote provided above doesn't indicate that the horizontal stab is producing positive lift, just that the downward force is reduced, along with trim drag.

FBW means that more options exist for creating stability beyond neutral point issues and the positive stability created by the use of a downloaded horizontal stab. An airplane requiring extreme maneuverability can operate with an unloaded horizontal stab and use computer input to create an artificial stability of any form desired, for use by the pilot.

In the case of the A380 described in the quoted paragraph above, the fly by wire technology has been employed to reduce drag and the download by replacing traditional stability with artificial compensation...but it still uses a download...it's just reduced.

[ahramin]

My apologies. I read it in an aviation magazine a few years ago but from the information posted here I am obviously wrong. :O

[transfer jack]

@ ChristiaanJ, I am meaning airliners with conventional airplanes and they have all flying tails because of their large CG and speed envelope.
Sure, every aiplane manufactorer tries to increase aft CG position up to a point where it is very close to the max. forward position of the aerody. center. This means that the pitch behavior of the airplane may tend to neutral at some points within its envelope. Computer systems artificially increase stability to pass Part25 certification but the natural stability of the aiplane (without any computers) cannot be negative, but this would be the case if the tailplane produced upwards lift.
They had this on early 340ies, but I think it was suspended later. MD11 is another example. Very likely that AB tries it again on the 380 (fuel is not getting cheaper)

Millitary fighters are different! Many of them are not naturally stable! But they do not have to pass civil certification and transport people but have ejection seats instead.
Natural unstability + artificial stability system not properly working = YouTube - F-22 Crash(YF-22 prototype) (http://www.youtube.com/watch?v=Ieef0tLrv9c)

[john_tullamarine]

FBW means that more options exist for creating stability beyond neutral point issues and the positive stability created by the use of a downloaded horizontal stab.

Just so long as we typical mug pilots don't think that we can play with CGs outside the aft limit. For a statically unstable aircraft, one can fly it if one knows what one is doing (tiring and a bit like hitting your head against a brick wall) but, for average folk, it is a hull loss. Dynamic instability is not on for the human pilot and is a hull loss. Computers, having much higher feedback rates can do a better job.

Please do respect the AFM aft CG limit with great reverence.

up to a point where it is very close to the max. forward position of the aerody. center

I'm not too sure that that statement is reasonable. The aft limit for a conventional aircraft is a consequence of design calculations validated by the TP's in-flight measurements.

pitch behavior of the airplane may tend to neutral at some points within its envelope

likewise ?

[SNS3Guppy]

Just so long as we typical mug pilots don't think that we can play with CGs outside the aft limit.


John, I didn't mean to imply such a thing. What flight by wire and computer control does enable is a different envelope than might be achievable without the automation and control. This is entirely irrelevant to the pilot, but does achieve greater performance and more possibilities with a given airframe, than would be achievable without it.

Clearly a pilot should never operate the airplane beyond the prescribed limits, which certainly includes those established for the center of gravity.

[chornedsnorkack]

I do not think that the tailplane needs to create downforce to provide stability in normal flight. After all, the main wing is not unloaded in normal flight.

Consider for simplification a plane where the stall AoA of main wing and tailplane are precisely equal. However, they are located at different angles to fuselage and airflow.

How does tailplane stabilize the plane? Well, suppose that the nose pitches up so that the main wing AoA exceeds stall AoA and main wing stalls. It does still produce some lift, but less than before.

If the AoA of tailplane is smaller than the main wing AoA then the tailplane has not yet reached its stall AoA and generates lots of lift. The main wing drops but the unstalled, lift-generating tailplane turns tail up so that main wing AoA decreases below stall AoA and the wing resumes flying.

If tailplane had same AoA as main wing, they would stall simultaneously. And if tailplane AoA were bigger, the tail would stall first, turning the main wing to stall and causing deep stall.

Note: you do not need the tailplane to generate downforce when the main wing stalls. You could perfectly well use a tailplane that stalls slightly after main wing and which at the main wing stall is creating a lot of lift. All you need is that the tailplane AoA should be smaller.

The other limit is at forward CoG limit. If the tailplane reaches inverted stall negative AoA, the plane falls over.

Some planes are deliberately unstable in pitch. Such as Wright Flyer, and this is pilot flown like several other unstable planes. There are also unstable planes flown by computers.

Assuming you want some definite amount of positive pitch stability, there are several ways to reach it. Such as having a small tailplane which produces downforce most time and is close to inverted stall at forward CoG, low AoA condition. In which case you are going to have a large induced drag at the tailplane, plus the main wing must compensate the downforce with extra lift and suffer its own share of extra induced drag. Alternatively, you might have a big tailplane which is safely far away from inverted stall, have a AoA which is definitely smaller than that of main wing but close to it, and which produces little downforce or actually upforce, so that the induced drag at the tailplane is small, and the induced drag at main wing may be decreased. There would still be some trim drag - trim drag only vanishes when tailplane flies at exact same best L/D AoA as main wing and at that condition stability also vanishes. But when do you minimize the total trim drag for a given amount of stability?

[Troy McClure]

Trimmed aeroplane:

Pitch nose down, increased (negative) AofA on tailplane = more downforce on tail, nose pitches up.

Pitch nose up, decreased (negative) AofA on tailplane = less downforce on tail (or increased positive AofA on tailplane = upforce on tail), nose pitches down.

C of G forward of centre of lift.

Unless you've got computers to do it for you (Eurofighter etc), it has to be so.

Get 3 pieces of balsa wood (fuselage, wing and tail) and a lump of blu-tac to move C of G. Get it to fly other than with blu-tac on the nose and tailplane with negative AofA and you'll be a millionaire.

Or an easier one to visualise: hold a cane by the top end and let it hang from your fingers. Easy. Now try balancing it on the end of your finger. Yes it's possible, but just a little bit harder.... First case is a Cessna 152 (or Boeing 737, etc. etc.), 2nd case is a Eurofighter.

[ChristiaanJ]

I do not think that the tailplane needs to create downforce to provide stability in normal flight....Just as well that engineers don't care about what you "think".
Read a few engineering textbooks before filling a page with waffle, please?

[bjornhall]

Just as well that engineers don't care about what you "think".
Read a few engineering textbooks before filling a page with waffle, please?

What nonsense.

See the link to See how it flies earlier in the thread, it explains nicely:
a) that a downforce from the tail is not necessary for stability,
b) why that is so (decalage!),
c) why, a) and b) notwithstanding, most aircraft need a downforce from the tail most of the time to remain stable (prompting the myth that a downforce is necessary, and the various hand waving arguments to "prove" it),
d) what simple experiment you can make to find out whether your light GA aircraft has a downforce or an upforce from the tail in different flight regimes,
e) why canards typically, but not necessarily, need longer runways than more ordinary designs (exception: delta + canard vs pure delta).

[ChristiaanJ]

bjornhall,
Maybe you should go and read the same engineering textbooks as "chorned".

[Wizofoz]

Re canard configuration, one other problem is "Deep Stall".

In the normal run of things, the fore-plane is designed to stall first, thus creating a pitch-down moment, reducing the main-plane AOA and stopping it from stalling at all. This is why Canards are sometimes claimed to be "Un-stallable".

If, however, an external influence (Severe Turbulence, wake or rotor for example) does force the main-plane past it's critical AOA, the resultant loss of lift causes a nose UP moment, exacerbating the stall.

This has been experienced in aircraft such as the Long-ezy and Cozy, and is unrecoverable.

[Lookforshooter]

Personaly I want to try a canard plane...I heard the Starship guys say they never flew a better plane in turbulance.

[ChristiaanJ]

Wizofoz,
Thanks... food for thought!
Never flown a canard, although I see one flying around here regularly (a Long-Eze, I think).

"Unrecoverable"... not purely in theory... because unlike the T-tail deep stall you still could have proper flow over the forward plane, not being blanketed by the main plane. But in practice I suppose that on an "Eze" you simply do not have the control deflection necessary to get a downforce on the canard once the mainplane has stalled and you're mushing down at what ... 40° to 50° AoA?

CJ

[SNS3Guppy]

Re canard configuration, one other problem is "Deep Stall".

In the normal run of things, the fore-plane is designed to stall first, thus creating a pitch-down moment, reducing the main-plane AOA and stopping it from stalling at all. This is why Canards are sometimes claimed to be "Un-stallable".

If, however, an external influence (Severe Turbulence, wake or rotor for example) does force the main-plane past it's critical AOA, the resultant loss of lift causes a nose UP moment, exacerbating the stall.

This has been experienced in aircraft such as the Long-ezy and Cozy, and is unrecoverable.


That's a function of the specific design, not of the use of the canard or forward lifting surface.

The same may be said of some conventional designs for a variety of reasons. A LR35, for example, may be put into a deep stall which can become irrecoverable, or may take 12,000' or more to recover. The design, loading, use, and construction of the specific airplane are all critical to the issue. The lear may be flown to a place from which it cannot return. The solution? Don't fly it there. The experimental homebuilt airplane may have any number of unique features as a function of individual construction, and have further been released in plan form with differing airfoils, different modifications, etc. To suggest that the design itself is unrecoverable is in error, and far too simplistic.

With respect to flying in turbulence, the ability to ride out the bumps in comfort is a function of wing loading, like any aircraft. The Piaggio Avanti is a conventional airplane with a forward wing (Piaggio is loathe to call it a Canard because it isn't a control surface). It has about the same wing area as a Cessna 182 but weighs more, with a high enough wing loading it flies very well through turbulence. Depending on the CG, the horizontal stab can and does produce positive lift in that design. The Avanti is a conventional airplane with somewhat unconventional loading. It flies into and out of a stall quite beautifully, incidentally, exhibiting similiar characteristics to many "canard" aircraft...it sits at a high angle of attack with aileron control, buffeting with some rudder shake and rear vibration. It stays stable in that condition in a stable 2,000 fpm descent, with control authority available. It may be flown out with conventional technique by decreasing angle of attack, or may be flown out on power.

Other Canard aircraft I've flown such as the Long EZ, Cozy, etc, behave in a similiar manner. Certainly incidents have occured with some experimentals in which the aircraft became unrecoverable, and that may be a function of the angle of incidence of either wing, the airfoil, construction technique, loading, simply having flown beyond the design limits into an unrecoverable arena for that specific airplane, or other related or non-related factors. To suggest that it's a function of having a canard would be wrong.

Cases have occured in certain conventionally configured airplanes in which the airflow over the horizontal stab has been blanked or disburbed such that the surface was no longer capable of providing recovery, or the angle of attack reached such that the horizontal stab provides no response, or a negative response (or has been modified to produce such a result, such as tailplane icing). The same has occured with canard configured aircraft in which the forward wing disturbs or alters airflow over the main wing, producing similiar results. The general idea on the Canard is to make the forward wing stall first, but that can backfire, too. For a true canard, one providing pitch control, the "elevators" move backward to those found on a traditional tail-mounted horizontal stab. At high alpha or high AoA, the potential exists for a forward wing with a small elevator to respond to nose-up commands, but to lose authority in down commands as the elevator is deflected upward into the stalled region of the canard.

Variances in the actual construction of the airplane make a big difference. Most experimental canard configured airplanes are made of shaped foam, covered in fiberglass, Any variance in the foam or finish can produce differences in the airfoil, and every single difference varying from the design has an impact in some way. Additionally, some airfoil ventures such as the Glascow University airfoils produced unusual and unexpected results in the early years, such as significant loss of lift or pitching-down in the presence of contaminants such as raindrops. to further complicate that, many of the canard aircraft fly with airfoils that operate closer to laminar, experiencing airflow separation farther back on the airfoil than typical production airplanes. Any distrubance of the airflow causing separation also produces a correspondingly larger performance or handling penalty. In the past for some airfoils, this has included bugs, raindrops, nicked paint, etc...producing big performance losses. (I've seen 10-15 knot performance loss in some airplanes when entering visible moisture, for example, or changes in controls requiring repositioning the stick by 2" or more to compensate, upon entering visible moisture, or with a change as seemingly insignificant as a paint stripe).

There are numerous factors to consider, far beyond simple employment of a canard (or otherwise) when planning for performance or behavior.

[Lookforshooter]

While the canards typicaly are designed to stall before the wing, it's common knowledge that you can always figure out a way to screw up the airflow over a wing...the big advantage seems to be having non opposing lifting forces...all lifting forces on canard plane are typicaly going up..this leads to less power needed, less drag, better range for given speed and horsepower...throw the props on the back aircraft, like the Starship, Piaggio, EZs, then you don't have a disruption of thrust flow over wings...all adds up to more performance efficiency.

[St. Ex]

Quote:
Yep Canards are more efficient........... but generally require longer runways and do not have short field TO ability.
You should see the difference the Peterson conversion does to a Cessna 182, with respect to slow flight and short field capability.

Yes the Petersen has 'canards' out in front and yes the STOL performance is phenomenal but remember, the 'canards' are well inside the propwash. As a matter of fact, they are right in there behind the prop! Also, the Petersen 182 is not a true canard because the basic configuration with the negative lifting tailplane is retained.