Question (from a noob non-pilot): What's up with all the ...

Discussion in 'Soaring' started by copec, Feb 22, 2019.

  1. Feb 22, 2019 #1

    copec

    copec

    copec

    Active Member

    Joined:
    Aug 3, 2015
    Messages:
    33
    Likes Received:
    6
    Location:
    Santa Clara, UT
    What's up with all the appearance and disappearance of V-tails when looking at the history of sailplanes?

    Do V-tails pose more drag in actual use than T-tail, Cruciform-tail, or Conventional-tail sailplanes? I would think in many circumstances you would be causing more drag to create force vectors with opposing components to cancel - through a necessarily stronger and heavier tail assembly.

    Although, there should be less total area to get the same set of forces, and with modern construction the tail isn't going to be substantially heavier.
     
  2. Feb 22, 2019 #2

    Dana

    Dana

    Dana

    Moderator

    Joined:
    Apr 4, 2007
    Messages:
    8,400
    Likes Received:
    2,802
    Location:
    CT, USA
    The theoretical advantages of V-tails don't really pan out in the real world, not enough to be worth the added complexity of the control mixer and the structural disadvantage of not being able to have a straight stabilizer spar right across the fuselage.
     
    BoKu, copec and Victor Bravo like this.
  3. Feb 23, 2019 #3

    BBerson

    BBerson

    BBerson

    Well-Known Member

    Joined:
    Dec 16, 2007
    Messages:
    10,986
    Likes Received:
    1,966
    Location:
    Port Townsend WA
    Gliders tip over to the side after stopping. The V tail avoids ground strikes on the tail tips. Same with T-tail.
     
    BoKu and copec like this.
  4. Feb 23, 2019 #4

    Victor Bravo

    Victor Bravo

    Victor Bravo

    Well-Known Member

    Joined:
    Jul 30, 2014
    Messages:
    5,585
    Likes Received:
    4,480
    Location:
    KWHP, Los Angeles CA, USA
    With respect to BBerson, Dana's answer above is the more correct. There are plenty of "regular" low cruciform tailed gliders that tip over on a wing and don't damage the tail. Everything from training gliders (SGS 2-33, LET L-13 Blanik ) to club and sport gliders (SGS 1-26, 1-34, Schleicher Ka-6) to Open Class competition sailplanes (Schleicher AS-W17, SZD-42 Jantar)
     
    BoKu and copec like this.
  5. Feb 23, 2019 #5

    BBerson

    BBerson

    BBerson

    Well-Known Member

    Joined:
    Dec 16, 2007
    Messages:
    10,986
    Likes Received:
    1,966
    Location:
    Port Townsend WA
    copec likes this.
  6. Feb 23, 2019 #6

    Riggerrob

    Riggerrob

    Riggerrob

    Well-Known Member

    Joined:
    Sep 9, 2014
    Messages:
    1,105
    Likes Received:
    297
    Location:
    Canada
    V tails have 2/3 the interference drag where they join the fuselage. They also only need 2/3 as many parts. Please note that V tails still need the same surface area (tail volume) as conventional tails.The disadvantage is they stability and control are not quite as good as conventional tails.
     
    copec likes this.
  7. Feb 24, 2019 #7

    Dan Thomas

    Dan Thomas

    Dan Thomas

    Well-Known Member

    Joined:
    Sep 18, 2008
    Messages:
    4,677
    Likes Received:
    1,912
    On powered airplanes, V-tails came and went away. T-tails came and went away. On sailplanes, I've seen V's and T's and dihedraled stabilizers and straight stabilizers, and to me the ground-clearance thing is a minor reason. If a straight stab is atop the fuselage, with that long wing preventing any significant bank angle when the wingtip hits the ground, the stab tips are well clear.

    I used to tow gliders in the 1970s.

    The tail configuration is, IMHO, nothing more than style. Fashion. Plenty of reasons are put forth at the time of introduction to support it, but the marketers are careful to keep quiet about the drawbacks of their designs. V-tails, with their larger surfaces and heavy structure within the fuselage to take the cantilevered loads, can weigh more, they're more complex to build, and they do tend to wag around in turbulence. Their control systems are somewhat more complicated. Take a look at the inside of a Bonanza V-35 sometime when the tailcone fairing is off. More parts and weight. The T-tail (or cruciform, as on the Rockwell Commanders) requires a considerably heavier vertical fin and mounting structure because it has to take the horizontal stabilizer's loads, and those loads are through attach points that are very close together so they have to be heavier. The rotating propeller slipstream also tries to rotate that stab,as it does in most airplanes, and the rotation has to be resisted thorugh that narrow mounting. The Commanders have cracking issues at the vertical stab rib at the HS forward attachment. The Lake Amphibian has had horizontal stab failures at the aft spar attach at the fin and there's an expensive AD against it.

    Fashion is more important than function when it comes to selling airplanes, exactly as it does with cars. The best vehicles often tend to be the plainest, strongest, simplest and lightest. And homely. Buyers are motivated more by style and fancy paint than by any practicalities. I see this when I get to work on some old airplane that's had a nice new paint job and maybe new seat upholstery. The buyer sees the paint but misses the corrosion and cracks and worn-out expensive stuff. When I was young, secondhand car dealers were famous for taking rusting and tired cars, painting them, and selling for massive profit. It works, because fashion sells.
     
  8. Feb 24, 2019 #8

    copec

    copec

    copec

    Active Member

    Joined:
    Aug 3, 2015
    Messages:
    33
    Likes Received:
    6
    Location:
    Santa Clara, UT
    ^I kind of figured as much. Thanks for the perspective!
     
  9. Feb 24, 2019 #9

    Victor Bravo

    Victor Bravo

    Victor Bravo

    Well-Known Member

    Joined:
    Jul 30, 2014
    Messages:
    5,585
    Likes Received:
    4,480
    Location:
    KWHP, Los Angeles CA, USA
    If there was not a measurable advantage of a T tail, I can guarantee you that the European sailplane designers and manufacturers would not use them. They weigh more, cost more, and take longer to build. The people whose financial and personal success depends entirely on performance and drag are all using the T tail.
     
    copec and BoKu like this.
  10. Feb 25, 2019 #10

    BoKu

    BoKu

    BoKu

    Well-Known Member

    Joined:
    Aug 16, 2013
    Messages:
    2,132
    Likes Received:
    2,627
    Location:
    Douglas Flat, CA
    Apologies if this sermon is a repeat.

    As others have observed, V-tails and conventional orthogonal tails are pretty much neck and neck in terms of stabilizing effectiveness per unit wetted area and per unit mass. However, there is definitely a real and measurable advantage to the T-tail in sailplanes: Packaging.

    The European sailplane designers behind the composite revolution of the 1960s (Waibel, Wilks, Lemke, Holighaus, et al) understood that to improve soaring flight performance, the thing to optimize is not the absolute performance of the machine, but rather the overall performance of the pilot, and the machine, and its support infrastructure, as a system. And the easiest metric by which to do that is the enjoyment of the soaring experience. And the easiest way to assess that metric is whether or not they write you a check. Because unlike many other velocity sports, the person in the pilot's seat of a sailplane is almost always the one writing the checks, not the one cashing them.

    Anyhow, back to tails. The designers listed above all understood that a T-tail with a removable one-piece horizontal tailplane offered, at only a modest increase in mass and complexity, a substantially superior packaging solution over any other tail configuration. Because the stabilizer is one piece, the moment reaction between the right and left halves is built in, so the mounting system only has to accommodate relatively light loads. And when you remove the tailplane, it is essentially planar and easy to handle and find stowage space for. And its location at the top of the stabilizer makes it easy to position and easy to inspect; it is right there at eye level, no kneeling or stooping required.

    With the V-tail, however, one-piece tails are much more problematic. They occupy substantial volume, and are hard to handle and hard to make space for. So far as I know, Spud Kohler is the only glider guy to try it. Instead, most designers adopt a two-piece tail with a moment reaction joint at or near the middle. You have to design those joints and their related mechanisms carefully.

    Dick Schreder designed his V-tails to fold by pivoting the stabilizer around the forward attach and the upper aft attach, with a spring-loaded taper pin at the lower aft attach. Schreder's approach was a marvel of industrial engineering and effective compromises therein. Primarily it had low unique parts count and was easy to fabricate and assemble, which was critical to his model in which the parts were made by apprentices and high school kids and assembled in garages by kit builders. Trivia point: The skins for the Schreder ruddervators take exactly one third of a 4'x12' sheet of aluminum, and when properly cut exactly to the template scribe lines the wastage from that sheet is a strip of aluminum 3/8" x 48"-- 0.8% waste. However, Schreder tails joints tend to get loose with age, and you have to be careful when folding and deploying them.

    The Austria takes a different approach to the V-tail; the fuselage has two tubular steel stubs that stick up out of the aft fuselage, and you slide the stabilizers down over the stubs. With this arrangement, you have two stabilizers to handle and stow, and you also have bruises on your shins and calves from walking into the steel stubs.

    Going off-topic, one of the the best testaments to the effectiveness of the composite revolution is the movie The Sun Ship Game, filmed at the 1969 US Nationals in Marfa Texas. The film features many V-tailed gliders, but all as backmarkers or in supporting roles. Three years prior, Dick Schreder had stood on the podium at the Reno nationals and was recognized as the US national champion soaring pilot, having won that contest in an aluminum V-tailed sailplane that he designed himself and built in his own workshop. But a mere three years later, film coverage of the Marfa nationals concentrated on Moffatt, Scott, and Derujinsky, and their molded composite sailplanes. Schreder received only about a dozen frames worth of coverage, and most of that coverage was because he happened to be sitting behind Suzanne Moffatt at the awards ceremony. In those intervening three years, it became fully recognized that the complementary technologies of composite construction and highly laminar shapes pretty much made all other sailplanes obsolete.

    --Bob K.
     
    Last edited: Feb 25, 2019
    copec, Topaz, Victor Bravo and 2 others like this.
  11. Feb 25, 2019 #11

    Victor Bravo

    Victor Bravo

    Victor Bravo

    Well-Known Member

    Joined:
    Jul 30, 2014
    Messages:
    5,585
    Likes Received:
    4,480
    Location:
    KWHP, Los Angeles CA, USA
    Well, I have it on eyewitness account from one of the major contestants in that era that Suzanne was considered "quite a honey" in the soaring world, and the cameras liked to look at her as much as everyone else did.
     
    copec and BoKu like this.
  12. Feb 27, 2019 #12

    lr27

    lr27

    lr27

    Well-Known Member

    Joined:
    Nov 3, 2007
    Messages:
    3,216
    Likes Received:
    461
    Anything wrong with the way they do it on the Ka-8?
     
    copec likes this.
  13. Feb 27, 2019 #13

    Victor Bravo

    Victor Bravo

    Victor Bravo

    Well-Known Member

    Joined:
    Jul 30, 2014
    Messages:
    5,585
    Likes Received:
    4,480
    Location:
    KWHP, Los Angeles CA, USA
    Slightly higher drag due to more intersection drag.
    Stabilizer partly "blanked" or made less effective by being in the fuselage boundary layer... needs to be slightly larger than it would be if mounted in "clean" airflow.
    Stabilizer flow, drag, and efficiency affected by wing root and fuselage flow separation or turbulence at higher angle of attack.

    These small negative factors were considered acceptable for a medium performance glider, when balanced against the advantages of lower cost, weight and structural requirements for the low tail, especially in the era of the Ka-6 and Ka-8.
     
    copec likes this.
  14. Mar 4, 2019 #14

    copec

    copec

    copec

    Active Member

    Joined:
    Aug 3, 2015
    Messages:
    33
    Likes Received:
    6
    Location:
    Santa Clara, UT
    I really like V-tails and it is probably because it is pleasing how they look to me.

    However, I enjoy hearing the actual reasoned engineering. If a T-tail is well understood and seems to offer the least drag with the best control - as well as breakdown, storage and transportability - it makes sense why it became the predominant tail.
     
    BoKu likes this.
  15. Mar 4, 2019 #15

    Victor Bravo

    Victor Bravo

    Victor Bravo

    Well-Known Member

    Joined:
    Jul 30, 2014
    Messages:
    5,585
    Likes Received:
    4,480
    Location:
    KWHP, Los Angeles CA, USA
    Thread drift off into the margins, but BTW powered aircraft are a different story in many cases. Powered aircraft manufacturers (primarily Piper) experimented with T-tails in the 1980's, and after a few years the tails moved back down to the fuselage where they were before. The balance between advantage and disadvantage was simply not favoring the T-tail for the type of flying that these airplanes do.

    A friend of mine owns a Piper Lance (the biggest member of the single engine Cherokee family) and he explained that having the tail down inside the propeller blast allows him to do much more with the aircraft in terms of low speed control during takeoff and landing. Much more "control power" available. This is particularly important on aircraft with a wide range of CG, because many powerplanes are hopelessly nose-heavy when flown solo. Having the ability to hold the nose up using prop blast makes a significant difference in operating these aircraft.

    All of this is irrelevant of course to sailplanes.
     
    Pops, copec and BoKu like this.
  16. Mar 7, 2019 #16

    copec

    copec

    copec

    Active Member

    Joined:
    Aug 3, 2015
    Messages:
    33
    Likes Received:
    6
    Location:
    Santa Clara, UT
  17. Mar 7, 2019 #17

    copec

    copec

    copec

    Active Member

    Joined:
    Aug 3, 2015
    Messages:
    33
    Likes Received:
    6
    Location:
    Santa Clara, UT
  18. Mar 7, 2019 #18

    BJC

    BJC

    BJC

    Well-Known Member

    Joined:
    Oct 7, 2013
    Messages:
    8,530
    Likes Received:
    5,471
    Location:
    97FL, Florida, USA
  19. Mar 7, 2019 #19

    copec

    copec

    copec

    Active Member

    Joined:
    Aug 3, 2015
    Messages:
    33
    Likes Received:
    6
    Location:
    Santa Clara, UT
    To eventually build something like the Sunseeker Duo myself so that my wife and I can tour over southern Utah - Athough, that is somewhat in the future.

    Right now I am loosing weight so that I can even learn how to fly a sailplane (I'm down to 240lbs ~109kg from 255lbs ~116kg), and also studying all sorts of various materials about airplane design (which is part of my hobby of studying anyways).
     
  20. Mar 7, 2019 #20

    BoKu

    BoKu

    BoKu

    Well-Known Member

    Joined:
    Aug 16, 2013
    Messages:
    2,132
    Likes Received:
    2,627
    Location:
    Douglas Flat, CA
    Those books all seem good. I have the Thomas, and it is great on the theory, though a little shallow on practical matters of detail and execution.

    You might consider coming to one of our Akaflieg build sessions. The next one is 1-5 April, with tech talk by aircraft designer and retired NASA engineer Dr. Stephen Smith. We'll be making carbon fiber sandwich wing skins for an HP-24 and also some high-temperature racing jet parts. We're down to the last couple of spots, so let me know soon.

    --Bob K.
     
    copec and Topaz like this.

Share This Page

arrow_white