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Thread: Vertical tail volume co efficient --empirical or not ?

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    Vertical tail volume co efficient --empirical or not ?

    When doing preliminary layouts and configuration evaluations we apply a set of 'filters' that determine first orders of feasibility and one that occurs often is assessing the directional stability and controllability of some novel design.

    The formula for horizontal tail volume co efficient is about as simple and intuitive as it gets (it is the 'eyeball' test put into measureable terms) --in fact it looks more like a rule of thumb that might not have a rigorous derivation --does anyone have a source for it's origin ? The damping factor for dynamic stability and the other extraneous factors that impact on pitch plane stability should in theory make for a much more complex equation but don't seem to .

    The picture with VERTICAL tail design is much less clear as is the derivation -- for example we can have a stable and controllable aircraft with NO vertical tail or rudder (eg flying wings,birds ) --and sometimes very short coupled aircraft with seemingly inadequate vertical tail seem to work OK ( eg Genesis sailplane, Marske tailess (that's horizontal tail -less ) the SG1, FS 26 and others --or just tip fins of the winglet type (SB 13, Mitchell wing etc ) or "Weltensegler" type --- some (most) of these are long span aircraft which should require massive vertical tails 'by the book' (and rearswept wings should supply some directional stability by some of "fin less" or small fin types named are in fact forward swept or unswept (Fauvel etc )

    Do any of you aero gurus have any comments on how to assess the amount of vertical tail or other yaw control on some more exact or even empirical basis ? Or an explanation for how these aircraft 'get away' with so little Vtv ?
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    Re: Vertical tail volume co efficient --empirical or not ?

    Quote Originally Posted by Aircar View Post
    Do any of you aero gurus have any comments on how to assess the amount of vertical tail or other yaw control on some more exact or even empirical basis ? Or an explanation for how these aircraft 'get away' with so little Vtv ?
    "Guru" certainly doesn't apply to me but I snarfed something from a forum a while back that fits that request.
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    Re: Vertical tail volume co efficient --empirical or not ?

    This one goes way, way back and as I understand it, was empirically derived from successful designs. There is a lot behind your overall question, most of it highly specific to each design, but you raise an interesting point that I often comment on. Horizontal and vertical tail volume coefficients differ fundamentally, even though they can be set up to represent a similar 'mechanical' premise. A horizontal tail tends to be more predictably stable than a vertical tail. I think "preload condition" is behind the difference.

    A horizontal tail is typically pre-loaded, by angle of attack and/or camber, to overcome pitching moment and provide positive static stability. A vertical tail, less so, since it usually isn't being pushed hard in a single direction all the time. What little 'vertical tail preload' exists on many planes is due to propwash and torque.

    However, in both horizontal and vertical cases, the effect of a directionally pre-loaded surface is to create a stronger static stability condition, which is generally appreciated by the pilot and passengers. Sometimes when the tail loading gets near-neutral, a self-limiting, tail wagging instability (like a flag waving) can begin to occur.

    I like how a twin tail design can create and exploit a solid, stable feel in yaw. The drag cost of this feature doesn't always appear, however, since the action can be used to reduce induced drag and for other constructive purposes, such as filling the aftbody wake. An example of the former is the VariEze and the latter, the HondaJet... if you consider its pylons tails, for the sake of making the point.

    One more thing about the way Vertical Cvt is figured: It doesn't consider anything other than wingspan, wing area and tail areas. Things like thrust line, prop placement, and fuselage profile can have a huge impact on the numbers that work. I dislike powerful simplifications like this because they have a way of excluding good, creative thinking too early in the design process.

    As to the influence of swept wings, twist, and lift distribution on the elimination of vertical tails, Al Bowers has been carrying the biggest megaphone. This link offers a better summary than most.

    http://www.youtube.com/watch?v=223OmaQ9uLY

    Last edited by Synergy; December 14th, 2011 at 03:00 AM. Reason: link added
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    Re: Vertical tail volume co efficient --empirical or not ?

    Al has spoken well ! Me likes Al !

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    Re: Vertical tail volume co efficient --empirical or not ?

    Two excellent responses --thanks Norman and John -- the more I dug around the usual references the more I got the feeling that it was a rule of thumb that had been passed from father to son... Mark Drela's modified formula and reasoning makes sense --the Dutch roll case is indicative of directional stability being too high and dihedral (equivalent) also too high --the Russian propensity to nil dihedral swept wings and really long undercarriages shows their approach to it while Boeing uses a split rudder with active yaw dampers and the spiral dive problem is especially bad for gliders or other long spans unless something is done to avoid it (and you get enormous dihedral just from wing bending --check out the Eta ) With cross wind take off the excessive weathercocking has cost many a broken tailboom and I believe at least some gliders are too stiff in yaw (too directionally stable ) --the all flying vertical stabilizer always came out better in that respect in NACA papers -the SR 71 for an example --or really old gliders pre T tail and V tails --you can even effectively 'cancel' the fin with a tab flown all flying vertical --the Rutan Rhino rudder is a case in point for a floating fin.

    John's thesis on loaded surfaces is also intuitively right --it is like toe in on car wheels --and why you test wind tunnel models upside down --vague variations around the null point are possible and with laminar sections as tails I would conceive of some non linearity from this (the break out force thing where the trailing edge is within the wake and does nothing much until it is outside --although the main effect is felt upstream .

    IN design for safety David Thurston reccomends very thin tail surfaces as being more effective (the rag and tube tailfeathers on just about every Cub type aircraft --no airfoil section at all ) --several older gliders with fat fins had poor rudder authority (eg ASW 15 and Std Jantar ) --most noticeable . I cannot think of a fabric type aircraft with a thick tail profile either -- the analogy to loaded rudders is to fly an aircraft nose heavy with lots of nose up trim -- much more deadbeat .

    The B Genesis sailplane is still an anomaly in my estimation though --highly swept and thick fin with almost no tail arm and a heavy forward swept wing --yet gets full marks from pilots as to handling. There was a thread on the 'bad effects' of polyhedral --as on Jodels and T 18 --in the opinion of I think it was Raymer .

    I must run some figures with Drela's criterion on one of my latest brainstorms ...

    any more contributions to the subject will be appreciated ( I knew that you would have the erudite understanding on this one Norman -- the Mitchell wing would be interesting to analyze from the sweep.dihedral and winglet combination --was it known to be spirally stable or overbank etc ? If I recall correctly the fins are symmettrical and all moving .

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    Re: Vertical tail volume co efficient --empirical or not ?

    If you have the arm then the thin foils are ok ...when they don't have to respond to anything else like oscillations of the separation of the flow in after fuse etc.

    Ever checked the FW-190As rudder...it is like a log ( timber ); http://fi.wikipedia.org/wiki/Focke-Wulf_Fw_190

    Yeah and JOHN and NORMAN are right on the ball park.
    Last edited by topspeed100; December 14th, 2011 at 05:54 AM.

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    Re: Vertical tail volume co efficient --empirical or not ?

    I lost something in translation there (??) -- one strange rudder was that on the Douglas skyhawk --an "inverted" structure with the ribs sort of on the outside, "Heineman's Hotrod" as it was called --no idea why it was so different .

    Getting back to directional stability --it is not at all apparent why a longer wing should require any more rudder (ie that it should be more unstable somehow yet span is the prime denominator in Vv) --think of a meter rule as a wing say and then yaw it --does the drag on either wing change unsymmetricaly somehow ? ( A rear swept wing will create a stable yawing moment but we still see huge fins --apart from the engine out case for podded airliners it would seem unlikely that the fin Cl gets much away from zero
    in normal flight-- sideslip and spin recovery should have a separate criteria related to aircraft mass density etc in some rational manner rather than a rule of thumb )

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    Re: Vertical tail volume co efficient --empirical or not ?

    Quote Originally Posted by Aircar View Post
    for example we can have a stable and controllable aircraft with NO vertical tail or rudder (eg flying wings,birds )
    Birds and flying wings without vertical surfaces do not have static yaw stability. A professor of mine made analogy to a Dorito chip...throw it, and no corner has any tendency to lead the others. A flying wing is no different.

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    Re: Vertical tail volume co efficient --empirical or not ?

    Quote Originally Posted by Aircar View Post
    I lost something in translation there (??) -- one strange rudder was that on the Douglas skyhawk --an "inverted" structure with the ribs sort of on the outside, "Heineman's Hotrod" as it was called --no idea why it was so different .

    Getting back to directional stability --it is not at all apparent why a longer wing should require any more rudder (ie that it should be more unstable somehow yet span is the prime denominator in Vv) --think of a meter rule as a wing say and then yaw it --does the drag on either wing change unsymmetricaly somehow ? ( A rear swept wing will create a stable yawing moment but we still see huge fins --apart from the engine out case for podded airliners it would seem unlikely that the fin Cl gets much away from zero
    in normal flight-- sideslip and spin recovery should have a separate criteria related to aircraft mass density etc in some rational manner rather than a rule of thumb )
    That is awesome I have 1/72 model of the A4 at home; Douglas A-4 Skyhawk - Wikipedia, the free encyclopedia

    I have sorta A4 and FW190 rudder done in Colomban MC 30 style !! No I am not going to explain it here yet.

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    Re: Vertical tail volume co efficient --empirical or not ?

    Quote Originally Posted by Aircar View Post
    I lost something in translation there (??) -- one strange rudder was that on the Douglas skyhawk --an "inverted" structure with the ribs sort of on the outside, "Heineman's Hotrod" as it was called --no idea why it was so different .

    Getting back to directional stability --it is not at all apparent why a longer wing should require any more rudder (ie that it should be more unstable somehow yet span is the prime denominator in Vv) --think of a meter rule as a wing say and then yaw it --does the drag on either wing change unsymmetricaly somehow ? ( A rear swept wing will create a stable yawing moment but we still see huge fins --apart from the engine out case for podded airliners it would seem unlikely that the fin Cl gets much away from zero
    in normal flight-- sideslip and spin recovery should have a separate criteria related to aircraft mass density etc in some rational manner rather than a rule of thumb )
    Coordinated turns. The Nimbus 3D for example is waaaaaaaaaaaaay under-ruddered. Half aileron at low speeds and it's impossible to make a coordinated turn. Full aileron and things are horrific. It's successor has much more fin and rudder and flies an awful lot better in slow turns.
    Aude somniare

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    Re: Vertical tail volume co efficient --empirical or not ?

    I thought moment of inertia is a contributor? Big span, big moment. Same with twins.

    Rob

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    Re: Vertical tail volume co efficient --empirical or not ?

    Quote Originally Posted by topspeed100 View Post
    Al has spoken well ! Me likes Al !
    That's just the short version. You can get the long version on video tape from TWITT. It's an old tape so he may have added some new material to the live presentation. He's also one of the organizers of the ESA Western workshop some years and is a frequent guest speaker there. Guess what he talks about
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    Re: Vertical tail volume co efficient --empirical or not ?

    I think birds can morph their tail feathers into something like a vertical rudder as needed briefly (or V tail). For quick maneuvering or in turbulence.
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    Re: Vertical tail volume co efficient --empirical or not ?

    Birds do braking like B-2 wing does..needs no rudder.

    Bird wings work like a propeller but in sync..whole fuse is a big rudder stab for it.

    I once put a engine + prop flying with a fuel tank and a small mounting..it was like a broken wing bird..all over the place...cool to watch actually !
    It was just a .02 cox baby bee abd the mount was for a glider.
    Last edited by topspeed100; December 15th, 2011 at 06:15 AM.

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    Re: Vertical tail volume co efficient --empirical or not ?

    Quote Originally Posted by Aircar View Post
    I lost something in translation there (??) -- one strange rudder was that on the Douglas skyhawk --an "inverted" structure with the ribs sort of on the outside, "Heineman's Hotrod" as it was called --no idea why it was so different .
    Maybe he thought there was some weight advantage to be gained by puting stiffeners on the outside of a really thin fin instead of beefing up the internal structure

    sideslip and spin recovery should have a separate criteria related to aircraft mass density etc in some rational manner rather than a rule of thumb )
    It's my understanding that spinning about an axis is an inertial coupling problem first and an aerodynamic problem second. If you remove the tail feathers from a conventional design the resultant monstrosity will tumble and/or spin at the drop of a hat but if you shorten the fuselage the tendency to rotate also is reduced. Searching this site for "skid/yaw" and "inertial coupling" will turn up several other threads that could be useful to you. I also posted youtube links to vertical spin tunnel tests that go along with the NASA TM mentioned in the link above
    Last edited by Norman; December 14th, 2011 at 05:21 PM.
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