Single versus Twin Tail

Discussion in 'Aircraft Design / Aerodynamics / New Technology' started by GESchwarz, Dec 17, 2007.

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  1. Dec 17, 2007 #1

    GESchwarz

    GESchwarz

    GESchwarz

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    In designing my folding wing airplane I have come up with a horizontal stabilizer span constraint of 6’5”, which is a bit on the short side. I figure that I can get more pitch authority by capping off the tips with a twin tail, one fin on either tip of the horizontal stabilizer.

    By question is…Are 2 six square foot fins equivalent to 1 twelve square foot fin, or is there some ratio between one and two fins for obtaining the same yaw authority?
     
  2. Dec 17, 2007 #2

    orion

    orion

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    Yes, that sounds very short and will most likely result in a very low aspect ratio. The twin verticals will be of some help but their benefit will most likely be minimal. Additionally, when that close together, the conservative design area will be the net projected area - not the gross planform. The reason for this is the effect one fin will have on the other and the "tunnel effect" that is formed between them. The detail of sizing is however a function of the overall configuration so it's somewhat difficult to give you a more precise answer.

    The "old timer" trick to sizing the vertical is to draw the total side view of your airplane on a piece of cardboard and then cut out the silhouette. Then determine where the wing is (draw the wing's MAC airfoil on the profile) and mark the aft-most position of the allowable CG range, or the position of the neutral point. Balance the silhouette on a straight edge at that point.

    When you let go, if the silhouette falls off on its nose then you have insufficient vertical area. If it falls off on its tail, then you have enough.
     
  3. Dec 18, 2007 #3

    DaveK

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    I think I remember you were also suppose to add in the area of the prop and some portion of the wing too in that old school method.
     
  4. Dec 18, 2007 #4

    orion

    orion

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    I've heard that too but the practical examples I've seen don't really require it. One reason is that this simplified approach assumes that the profile surfaces all have the same ability and/or flow characteristics. This of course isn't the case - for instance, the cowl is certainly not able to generate the lift curve that the vertical tail area can and as such, the area of the vertical fin is more effective that this simplified approach assumes.

    The issue of wing effect comes in if you're dealing with a swept wing, especially if it's swept forward. Otherwise where this quick and dirty method is concerned, the wing can be ignored. The same holds true for the prop.

    If however you're going for a particular look or area distribution, where the tail volume may be a bit on the marginal side, then yes, incorporating the miscellaneous effects becomes a bit more critical. The gentleman that showed me this method initially (Walt Mooney) advocated a somewhat larger vertical for increased yaw stability and control effectiveness. But then he was a glider and tail dragger pilot and felt that many airplanes would benefit with an additional amount of area back there.
     
  5. Dec 18, 2007 #5

    GESchwarz

    GESchwarz

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    Yes, the horzontal stabilizer will have a small aspect ratio. In what way will this hurt me? Just how important is aspect ratio for a stabilizer. Do you suppose that the twin fins add perhaps a half a chord or so in aspect ratio? Many of the WWI fighters had small AR's and seemed to do okay.

    Orion, you spoke of a "tunnel effect"...If the MAC of my verticals is roughly 2' and they are spaced 6.5' appart, to what degree do they have a degrading influence on each other?

    I've just learned of the AR-5 and AR-6. What phenominal aerodynamics! The area ruling and careful shaping of fillets really proved effective. It goes to show how far away air streams and pressures are affected. The biggest thing I learned is how vortexs are generated at intersections such as at the wing root.
     
  6. Dec 18, 2007 #6

    orion

    orion

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    The real question in cases like this is more the issue of application than anything else. There's a substantial difference in flying qualities and characteristics between let's say an ultralight and a Glasair. Whether a low aspect ratio tail will significantly hurt you or not will be a function of the type of airplane you're after, the performance envelope you're shooting for and of course, the compromises you're willing to make.

    Comparing performance and application is of course a viable means of verifying a design but when you do so, you must really make sure to select the proper category and configuration or the comparison is meaningless. The WW1 airplanes (like many military aircraft) you refer to were what's called a point design - in other words, they were designed to do only one thing. They flew with only a single pilot and outside of fuel and ammo, carried virtually no other payload; they had only simple flight control devices; and they did not use flaps or other high lift systems. What this means is that they flew at pretty much one CG point and as such, the tail did not have to provide much trim authority outside of the control effectiveness that was necessary to achieve the airplane's mission. They had very low wing loading and achieved only relatively low speeds so even though they used under-cambered sections most of the time, the pitching moment was within reasonable limits so as not to diminish the tail authority due to trim needs. This also applies to the subject of flaps - without the high lift systems you don't need too much elevator authority to overcome the dramatically increased pitching moment on landing. In this manner you can get away with a less than optimal tail since you only need it to be fully functional during air combat, which usually occurs at high speed.

    So, taking this into consideration, if your airplane is no more complex than a single seat ultralight then I'd say the tail efficiency may not be a driving issue for you. If on the other hand you want to carry a payload, have a wider allowable CG range, or use flaps, then you might want to pay a bit more heed to the specifics of the design.

    The tunnel effect I mentioned is the phenomenon where the verticals' direct the air flow between them, preventing one of the surfaces from reading the full characteristics of the free-stream flow. Let's take the extreme case and assume that you're yawed to one side. The result of the airplane being turned is that one of the surfaces will be slightly forward of the other WRT the prevailing flow. As the airstream approaches the leading surface, the flow will turn and align itself with the fin. The stream will complete the turn around that fin's leading edge and then flow parallel to its surface. But, because of the physics of the flow, that turning of the stream will be accomplished even a substantial distance away (entrained flow) from that leading surface.

    So, if we now look at the second fin - since it's somewhat behind the first one, much of the flow it sees is already turned in the direction of the first fin. As a result it does not see as much yaw as the first fin sees and as a result its centering (or correcting) force vector is significantly smaller than that of the first fin (wind tunnel testing has shown that this is also true, although to a lesser extent, for even small yaw angles). For this reason and for the purpose of staying a bit conservative, when designing twin tailed aircraft it is good practice to ignore the second fin and just use the net projected area for the design and for all the stability calculations.

    And this is especially important for "V" tailed aircraft where the situation is a bit more complex.

    If you want to save a bit of weight, yes, provided you do the math you probably can get away with making the fins smaller, but to be on the safe side I'd probably recommend making that reduction relatively small.

    Regarding the AR5, it is really proves three things. First, it proves that good aerodynamic performance is really an exercise in paying attention to detail. Aerodynamics isn't magic - there's no secret formula, there's no supernatural shape or arcane technique to achieving excellent performance. It is just the correct application of the science, getting a few percent improvement here or a fraction of a percent there, it's getting rid of a bit of leakage at the cockpit or a small bit of interference at the tail wheel, it is the optimization of the cooling system - in other words it is working with and optimizing all the little things that adds up to good performance.

    Second, it proves what many in the homebuilding industry already know - when you make a tiny airplane it'll simply go like stink, even on a little power. The majority of performance improvement that we see in many homebuilts over that which we see in the certified counterparts can simply be attributed to physical size and of course, more power per pound. Yes, we do tend to utilize better building and finishing techniques but the bottom line in homebuilt performance numbers is simply that the airplanes are smaller, but they use pretty much the same engines that the larger production airplanes use. So, simply, smaller airplane, more power, more speed.

    The third issue then is a corollary to the above - if you have a small airplane, and thus are willing to compromise volume, comfort and payload, then do all the right aerodynamic things, then you too can make a record setter. This is the unfortunate thing about physics - the AR5 is a great example of what can be done but in reality, beyond setting a couple of records, the airplane has no utility or range. And this is why we generally say the any airplane design is an exercise in compromise. It's too bad - I'd love to say that I could develop a 300 mph four place utility airplane that I could power with only a couple of hundred horses, but I've been doing this long enough to know that it's highly unlikely. Oh well, we can all dream.
     
  7. Dec 26, 2007 #7

    mstull

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    Gary,

    If your plane is a tail dragger, the dual rudders could be a problem. Neither is directly in the prop wash. So they'll have little authority in some critical phases of take off and taxi. It is possible to compensate for this problem with technique, differential brakes, and a high quality, steerable, tail wheel. But there will probably be times when you'll wish your rudder was in the prop wash.

    If your plane is tricycle... no problem.
     
  8. Nov 13, 2008 #8

    LBarron

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    Flipping through the 2009 Kitplane Buyer's Guide I noticed the Comp Air 10 with twin vertical stabilizers. The fact that they're awfully close together has me wondering about the reason for the design. Given Orion's comment about the tunnel effect of the air flow around the stabilizers, does anyone know of the potential benefit of this configuration? Could it be just to provide adequate stabilizer surface area (volume) for what appears to be a large, heavy airplane?
     

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  9. Nov 13, 2008 #9

    orion

    orion

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    I actually did call around at the time this thing came out - could not find anyone who had any idea why this was done or what purpose it served. From an engineering standpoint it certainly was (an is) an oddity.

    The only possible explanation I could come up with for the configuration is that the twin surfaces were used in order to provide sufficient maneuvering and control at high angles of attack and into deep stall, and to provide a level of spin resistance if the airplane ever exceeded a deep stall.

    If you look at the airplane closely, one of the features it clearly displays is a very wide and squarish tail-cone. One guess we've been able to come up with for this bizarre configuration is that at high angles off attack that fuselage will most likely generate sufficient separated flow to blanket much of the back end, submerging a more centrally mounted fin in this turbulent flow - the result would be a loss of yaw stability or control. This would especially be the case when you also consider the positioning of the large horizontal surface in relation to the verticals.

    By placing the two fins at the extreme edges of the fuselage frame the airplane increases its chances of having enough fin or rudder in an effective flow region to provide the airplane with the necessary level of control.
     
  10. Nov 13, 2008 #10

    Dana

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    Or they just wanted it to look like an F-15...

    -Dana

    When Marriage is Outlawed, Only Outlaws will have Inlaws.
     
  11. Nov 13, 2008 #11

    LBarron

    LBarron

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    Thanks Orion - I figured you'd have the answer.

    Although I kind of like Dana's too...

    LB
     

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