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Thread: Vertical tail sweep optimization

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    Moderator addaon's Avatar
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    Vertical tail sweep optimization

    Roskam describes an approach for optimizing the sweep angle of a vertical tail with fixed aspect ratio and mounting point. Basically, increasing the sweep decreased dCLa, and increases moment arm; increase sweep until these factors are balanced.

    His accompanying image, though, shows this at a reasonable cruise angle of attack. Shouldn't this be performed at the most critical flight condition for tail authority, and isn't this usually Vs0 for for a single engine? If so, this leads to a pretty substantially different result, as much as 10 different.

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    Moderator Topaz's Avatar
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    Re: Vertical tail sweep optimization

    Raymer says flat out that tail sweep on piston GA aircraft is purely for aesthetics, although yes, you can extend the effective arm a bit that way. If you choose to sweep the vertical, then yeah, I would think the critical case would be at Vs0, with cross-wind ops defining the tail power required. Gotta watch Roskam sometimes. He's not always completely clear about what he's saying.
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    Moderator addaon's Avatar
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    Re: Vertical tail sweep optimization

    Raymer's just wrong on this. On a Cessna 172, it's close enough to true not to matter; but if you look at, say, a Questair Venture and run the numbers Roskam style, you'll see that, because it's so short-coupled, adding a bit of sweep moves the AC of the tail back far enough to more than outweigh the decreased effectiveness due to sweep.

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    Moderator Topaz's Avatar
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    Re: Vertical tail sweep optimization

    Well, he advocates a simply longer fuselage, which can be lighter in some cases, especially when taking the possible reduction in horizontal tail area into account as well. You also get better damping, too. Not saying you shouldn't sweep, but rather that there's no clear benefit other than aesthetics.

    The Venture would've benefited from a longer fuselage on a couple of counts: damping and aesthetics, and from the latter, sales. I "get" that it was not "really" short-coupled, but it looked that way to pilots, and developed a (probably undeserved) reputation for being so, even if the air didn't see it that way. It might have lost a couple of knots of cruise with a longer tail, but that's a small price to pay for the objective and subjective benefits, IMHO.
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    Moderator addaon's Avatar
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    Re: Vertical tail sweep optimization

    Eh, I'm not fully convinced. The kitplane market seems an awful lot about the "long tail." There's so many designs out there that you're probably trying to capture a small, well defined subset of the market (unless you're RV); and "unique looks," even if most people don't like them, seem to help more than hurt in that respect.

    But back on topic, yes, the Roskam approach makes the assumption of a fixed mount point, which is somewhat bizarre early in the design process; but, given a mounting point, it is interesting to answer what the optimal sweep is, and there's enough designs out there with the length limited by something or other (tipback, aesthetics, W&B) that the results seem meaningful.

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    Re: Vertical tail sweep optimization

    Something that I find interesting is that just about all the (specifically designed for the consumer market) piston aircraft that I can think of seem to have some level of rake(sweep) to the hingeline of the vertical tail post. But in direct contrast, every custom designed/built piston powered race plane/war plane that I can think of has a vertical tailpost(hingeline). Formula 1: Nemesis, Endeavour, Invictus, Mariah, Madness, Outrageous, One Sly Dog, all the Cassutts and shoestrings, etc, etc. Sport Plane Class: Nemesis NXT and the newly finished GP-5. All the WW fighters/bombers that come to mind, vertical. All the Red Bull air racer designs, vertical. All the aerobatic planes that come to mind, vertical.

    These race/military/aerobatic planes were designed with nearly zero concern for aesthetics, just the most efficient layout for speed/efficiency/performance. Jets are a totally different story, of course, but when talking about sub 500mph piston powered planes, the story seems to be pretty consistent. It seems that every time aesthetics take a back seat to performance, just about without fail, when whetted area, fuselage length, tail area, structural loads, etc. are taken into account, vertical tailposts have been the standard for, well, from the very beginning.

    To be very clear though, I am not at all qualified to judge what is ideal or not regarding rudder sweep. But I couldn't help but notice the the stark contrasts between sweep/no sweep ruddered planes. This could certainly leave one with the conclusion that rudder sweep in the end rudder sweep carries more weight in the aesthetics department than the optimum design department.

    That said, I simply could not imagine planes such as those of the Lancair designs without that swept rudder shape.


    George

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    Moderator addaon's Avatar
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    Re: Vertical tail sweep optimization

    It is interesting to keep in mind that rudder sweep and tail sweep are surprisingly independent... but yes, your point is well taken. The counter-example that comes to mind is, again, the short-fuselage Questair Venture, with a swept tail and swept rudder; recall that this was a design who held several speed records that stood up until the Nemesis NXT hit the scene.

    I do agree that sweep tends to be overdone for aesthetics; I was just noting that Roskam seems to overestimate it from a performance point of view as well.

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    Re: Vertical tail sweep optimization

    Isn't the vertical hinge line a structural thing? If the hinge is vertical, you're not inducing any vertical forces by using straight cables or a direct push-pull tube, while, using a swept hingeline, you either have to resolve those forces, or design the hinges/structure to withstand them.

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    Re: Vertical tail sweep optimization

    [QUOTE=addaon;85596]The counter-example that comes to mind is, again, the short-fuselage Questair Venture, with a swept tail and swept rudder; recall that this was a design who held several speed records that stood up until the Nemesis NXT hit the scene.QUOTE]

    Certainly the Venture performs quite well. There were many reasons why it performed so well, but it appears that many of those unique design elements that helped it fo faster also gave it a unique look that seemed to turn many potential buyers away. I am fascinated by the Venture personally. It was/is brilliant in many, many ways. But sadly, it appeared to be too far removed from the mainstream easthically(among a few other reasons) to sell like the snarkier Glasairs a Lancairs. The Venture had a noticably lower total whetted area compared to the Glasairs and Lancairs, and the rudder sweep presumably was there in an effort to keep the fuse as short as possible. There were so many unique features on the Venture that it would be difficult of course to guess how much the sweep of it's rudder effected it overall performance.


    Good point regadring the independence of rudder sweep and tail sweep. The GP-5 and the GP-4 is a fairly extreme set of examples of vertical posts with severe leading edge sweep.


    George

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    Registered User Norman's Avatar
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    Re: Vertical tail sweep optimization

    I'd always heard that sweep on the leading edge of the tail was just aesthetic flair. I don't think the extra few inches that it gives to the tail arm is going to have much impact on directional stability. But what is my opinion worth in this context. My favorite airplane doesn't even have vertical surfaces and if I were to choose a favorite from among the tailed monstrosities it might well look like this:
    Attached Thumbnails Attached Thumbnails Vertical tail sweep optimization-mooney-n383a.jpg  
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    Re: Vertical tail sweep optimization

    Internal topics for this discussion:

    Swept tail reduces tail effectiveness and rudder authority when 1/4 chord at tail MAC is kept in the same place;
    Swept tail moves the 1/4 chord at tail MAC only a small amount;
    The tail arm would have to be pretty small for a swept tail to result in an increase in stability and control authority - In conventional looking airplanes, the vertical tail is best unswept.

    Swept tail increases moments in the tail without changing moments in the fuselage;
    Since moments in the tail are greater, the fin will have to be stiffer and stronger than if it were straight, although most are overbuilt enough that this is probably a very small weight change.

    Swept tails can greatly decrease rudder authority in a spin, which could drive additional area or other fixes.

    Summarizing: Swept tails are less effective in most birds, may drive additional weight, and works more poorly in spins. So, why would you do it?

    Now if you have a short fuselage (Venture) and high aspect ratio foils, or a short fuselage plus you put a horizontal tail on top (John Roncz designed a sailplane that way), it might have pay value.

    Billski
    Last edited by wsimpso1; December 19th, 2010 at 09:36 AM.

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    Re: Vertical tail sweep optimization

    Question is when does it start to be negative for flying quality or drag (need bigger area) most vertical tail have the hinge line at 55 to 65% of the chord, (45 to 75% is seen) the most practical is to make the hinge line vertical (90 deg) and rudder a constant % of the local chord, then the 25% line will be slightly tilted back.
    If say bottom chord is 1,0 and tip is 0,6 and span (height) is 1,2, with a vertical hinge line at 40%, the 25% chord line will be tilted about 6 degree.
    and 0,25 MAC is moved back 6/82=7+ % of VT MAC
    say that wing MAC chord is 1.0 and tail length is 3 X MAC, then it moves back 2% of tail length, not a big improvement.

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    Re: Vertical tail sweep optimization

    From a purely pilot perspective, I have recently spent a lot of time in an older C150J model with a straight tail. I have also flown a newer model with the swept tail quite a bit. The old straight tail model is approved for spins, and the new swept model is not. I think it has more to do with the control horn design on the rudder than the swept tail..but I do know that the straight tail seems to behave a lot better when performing stalls. Straight tail is faster too. I have experienced spinning the old straight tail model...exciting..1000 feet to recover for 1 full turn, fully developed pro spin entry..very steep and spun up.

    I also like the older straight tail 172 and 182 models...they can keep the *aesthetics* of the swept tail, I don't really like the way it looks on the cessnas...it looks like the fake jets and tail fins on a car...just plain stupid. IMHO

    I have also developed an affinity for manual flaps...and a dislike of electric flaps.

    YMMV

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    Re: Vertical tail sweep optimization

    Quote Originally Posted by wsimpso1 View Post
    Summarizing: Swept tails are less effective in most birds, may drive additional weight, and works more poorly in spins. So, why would you do it?
    Crosswind is the only general reason I can think of. Sweep makes the fin/rudder more effective in a crosswind because the stall aoa is higher.

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    Re: Vertical tail sweep optimization

    Quote Originally Posted by autoreply View Post
    Crosswind is the only general reason I can think of. Sweep makes the fin/rudder more effective in a crosswind because the stall aoa is higher.
    Lower AR wings/fins also stall at higher AOA don't they?

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