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Wing positioning to reduce drag

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Wing positioning to reduce drag

Old 8th Oct 2008, 22:17
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Wing positioning to reduce drag

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?
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Old 8th Oct 2008, 22:23
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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...
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Old 8th Oct 2008, 22:34
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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.
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Old 9th Oct 2008, 00:28
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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...???
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Old 9th Oct 2008, 01:25
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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!
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Old 9th Oct 2008, 01:59
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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.
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Old 9th Oct 2008, 02:45
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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.
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Old 9th Oct 2008, 03:56
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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.
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Old 9th Oct 2008, 05:10
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On the A380 the horizontal stab lifts up instead of down.
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Old 9th Oct 2008, 05:54
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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]
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Old 9th Oct 2008, 10:53
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Instability, perhaps?

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)
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Old 9th Oct 2008, 18:57
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On the A380 the horizontal stab lifts up instead of down.
I cannot find ANY reference to confirm that. Please provide a reference/link.
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Old 9th Oct 2008, 19:39
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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.
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Old 9th Oct 2008, 20:02
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Hi,

please check this picture.....
http://www.pilotsreference.com/image..._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.
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Old 9th Oct 2008, 20:12
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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
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Old 9th Oct 2008, 21:01
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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!
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Old 9th Oct 2008, 22:00
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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
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Old 9th Oct 2008, 22:30
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those certified for IFR more than those for VFR only

Now, that's interesting, could you perhaps point to where the standards require this ?
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Old 9th Oct 2008, 23:32
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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.
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Old 9th Oct 2008, 23:56
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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.
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