Comparing x vs. v-tail [Archive]

Tuomo

Jan 03, 2007, 07:22 AM

No, not again :) I know the basic merits of both designs. I would like to see more dicussion like this (http://www.charlesriverrc.org/articles/design/donstackhouse_conventionalvsvtail.htm), only little more precise

There are a number of ways to size a V-tail. Most of them (except for the common-but-wrong "projected area method") will give you pretty much the same numbers, and pretty much the same results. I won't go into those methods here (I've done that enough already many times before), but if you want to get into the details, I've discussed some of them, including the simpler ones, in the "Design" section of the "Ask Joe and Don" department on our website. You can also use these methods for converting a V-tail to a conventional tail.

jojoen

Jan 03, 2007, 10:04 AM

Most F3J planes are available with an option for x or v-tail. Is there a simple formula to aerodynamically compare these desigs? I am mostly interrested in stability and possibly different CG locations. I know x-tails require more nose weight and v-tails are hard to transport :D
Opposite way around.
A Xtail (with all moving elevator) offers more stabillity and can in most cases fly with more different CG locations. This because you can change the EWD (elevator wing dihedral).
A Xtail with only flipper is almost the same as a V-tail but will offer more stabillity in more situations than a V-tail.
A V-tail with flipper is in most cases lighter and in most cases less drag than a X-tail (with the same EWD).

So the big issue is that we like X-tails (or T-tails) for the fact that we can change the EWD and CG for different tasks. But for less drag we tend to fly V-tails in for example F3B were the speed is more important than stabillity while thermalling.

These are only very simple answers and will probably be replied on....
Dont hang me for them... Written fast in a forum..

ThermalBuster

Jan 03, 2007, 10:11 AM

Tuomo,

There is an abundance of design info on the web for V-tails as well as cross-tails. Either one can be very effective in the design of an aircraft to meet the design goals. You have already stated the only real differences in your question - ie. nose weight and transportation problems.

Now we get into the perceived differences in terms of flight characteristics. In the 'magic zone' v-tails are attributed with a number of characteristics that seem to vary by user. Pitch stability, yaw effectiveness, CG locations are all argued to change with a v-tail design compared with a cross tail design. I personally don't know if any of that is true. Nor can I present an airtight arguement that its not true. From personal experience I cannot consistently attribute a specific characteristic to a tail type. I have found problems with both types when trimming out airplanes, but its usually not the tail's problem. Its a problem with CG location, control surface movements, radio mixes, lack of or too much aileron differential, excessive rudder mixing or warped wings. With both tail types it helps to have extra area if you fly very slowly, particularly when 'dragging' the airplane to the spot during landing. I can't say I've ever found an airplane with a tail that 'looks about right' that doesn't fly well when appropriately trimmed. (V-tails ARE more difficult to get trimmed correctly.)

My best opinion is - "Fly what you enjoy looking at". And have fun. Trying to find truth in v-tail versus cross tail is an academic exercise.

PS. It IS a lot of fun trying different tail sizes to see if you can feel the differences and if you can trim out those differences.

Ollie

Jan 03, 2007, 10:41 AM

Please read:
http://www.charlesriverrc.org/articles/design/vtailmsganthology.htm
http://www.charlesriverrc.org/articles/design/donstackhouse_conventionalvsvtail.htm
http://www.charlesriverrc.org/articles/design/markdrela_vtailsizing.htm
http://www.charlesriverrc.org/articles/design/donstackhouse_vtailadverseyaw.htm

markdrela

Jan 03, 2007, 10:47 AM

No, not again :) I know the basic merits of both designs. I would like to see more dicussion like this (http://www.charlesriverrc.org/articles/design/donstackhouse_conventionalvsvtail.htm), only little more precise

It depends onwhat you mean by "precise". There are a number of methods out there for evaluating or sizing tails. In order of increasing precision and complexity:

1. Tail/wing area ratios.
Sh/S
Sv/S
Too crude for sailplanes.

2. Tail volumes.
Vh = (Sh/S) * (lh/c)
Vv = (Sv/S) * (lv/b)
Much better than 1, and not much more difficult. Good for most model design work.
For computing the effective Sh and Sv of a Vtail, see
http://www.charlesriverrc.org/articles/design/markdrela_vtailsizing.htm

3. Vh and Vv are used as in 2. But these only measure pitch and yaw stability and control power. To also evaluate pitch and yaw damping, we must also look at alternative tail volumes based on the pitch and yaw radii of gyration rather than chord and span:
Vh' = (Sh/S) * (lh/rgy)^2
Vv' = (Sv/S) * (lv/rgz)^2
For most airplanes, pitch stability and yaw damping are most important, so Vh and Vv' are the most relevant quantities. But determining the radii of gyration (i.e. the moments of inertia/mass ratios) is relatively difficult, so Vv' may not be practical.

4. Traditional handbook methods or a vortex-lattice method is used to directly evaluate all the stability, damping, and control derivatives, neutral point location, etc. These derivatives are used instead of the tail volumes for tail sizing. This approach eliminates all ambiguities with volume definitions, Vtail/Xtail conversions, etc. It also accounts for aspect ratio and sweep effects, which the volume coefficients in methods 1,2,3 ignore.

5. The derivatives from method 4, and the mass and inertias, are also input into a flight dynamics analysis to directly examine the flight behavior (oscillation frequencies, damping rates, etc.) over a range of flight speeds and maneuvers.

AVL is set up for methods 4 and/or 5.

Tuomo

Jan 04, 2007, 02:15 AM

Thanks for the comments. I guess I should have included also the other links from CRRC to the opening message :o I try to elaborate my questions and ideas here.

I feel that I am one of the few flyers who do not have clear peference to x or v-tail. I fly both types. However, most of my friends seem to leaning strongly for x-tail. Also recent WCs have been won with x-tail models. In fact, when selling my used equipment, I have heard comments like "nice plane but it is v-tail"...

Jojo's point to F3B and EWD is interesting. To my understanding, the main benefit of a all-moving x-tail elevator is that incidence can be trimmed easily from transmitter. This should a huge benefit in F3B where we want our planes to perform well in broad flight envelope!

I feel the incidence is not a huge problem in F3J. Sometimes I have used tape under v-tail to trim it, but the effect seems to be very small. How much does it matter if you have v-tail elevators trimmed +/-0.5mm out of line. Not much, I think.

However, my experience that flying with a v-tail is definately little different from x-tail. V-tail rudder feels "softer". Some say this is bad, I think it makes flying also little less critical.

On the other hand, most v-tail planes seem to be directionally less stable than x-tails. This can be a slight problem in tow, controlling the direction of zoom is not that accurate. The other side of the coin is that directionally les stable stable planes indicate thermals better. At least I think so....

I hope you undestand now better what I was after, when opening this thread. Most manufacturers (maybe except Samba with their Perfect) seem to offer both v and x-tails version of their planes. One has to make the decision when buying a plane - not easy concidering thet according to popular knowled x-tail is somehow "better" :confused:

Personally, I maybe I am leaning towards v-tail - if I can be sure that it is properly designed! V-tail is light, simple and somehow more "elegant".

jojoen

Jan 04, 2007, 02:35 AM

...snipp
I feel the incidence is not a huge problem in F3J. Sometimes I have used tape under v-tail to trim it, but the effect seems to be very small. How much does it matter if you have v-tail elevators trimmed +/-0.5mm out of line. Not much, I think.

Key issue. Yes incidence changing does matter. Well maybe not calculating it but in real life.
You see we do not fly in the stable inverioment of the computer or mind. The plane is pushed around by forces we need to correct. But we are not always able to correct them properly and what you feel was a wingdrop might have been that your tail stopped flying. A clean tail is most important for the plane to fly...
So everytime you move that flipper on your V-tail or X-tail with flipper you make a lot of drag. That drag can make your tail stop flying. But an all moving elevator will tend to fly longer.

Another issue you also mentioned is the directional stabillity. We dont see the wool string in the cockpit glass that the full size gliders doo. If we did we would have flown so much better. So a model that is not direction stable will yaw slightly (maybe not noticable) and cause a whole lot of drag that again will have influence on the energy you want to keep. That energy is worth a lot of seconds in the air.

Tuomo

Jan 04, 2007, 04:19 AM

Yes incidence changing does matter. Well maybe not calculating it but in real life.
You see we do not fly in the stable inverioment of the computer or mind. The plane is pushed around by forces we need to correct. But we are not always able to correct them properly and what you feel was a wingdrop might have been that your tail stopped flying. A clean tail is most important for the plane to fly...

But who said tail airfoil has to be symmetrical?

Trimming my v-tail plane elevators 0.5mm up (say in floating settigs), migh actually be more effective than chanching the incidence of a summetrically airfoile x-tail elevator? (Just trying to create an argment...)

Another point. If you think moving control surfaces creates so much drag, why not fly with a more forward CG for (at least partial) "hands-off stability"?

So many flyers truly believe a rearward CG being more efficient --> x-tail is better because it geberally allows more extreme CG placemet. It might be so, but the difference is marginal and masked by other issues, one of them being that a forward CG generally allows the plane to fly longer times without active control movements.

(Opposite to the conventional knowledge, I often add some lead to nose in floating and weak lift condition. In wind Cg is not so much of issue, a neural setting with rearward CG migh actually work better. Am I doing something wrong here?)

So everytime you move that flipper on your V-tail or X-tail with flipper you make a lot of drag. That drag can make your tail stop flying. But an all moving elevator will tend to fly longer.

I am quite sure that also an allmoving elevator tends to stall (at least partially) due to control movements. With low Re, stall occures at less than 10 deg incidence(?).

However, I definately do not like the way how the other elevator of a v-tail is deflected when one is turning with plenty of rudder and some elevator! A very typical situation, when working a small thermal.

Another issue you also mentioned is the directional stabillity. We dont see the wool string in the cockpit glass that the full size gliders doo. If we did we would have flown so much better. So a model that is not direction stable will yaw slightly (maybe not noticable) and cause a whole lot of drag that again will have influence on the energy you want to keep. That energy is worth a lot of seconds in the air.

Cannot argue against that one :) Or maybe I can? What is the right compromise between directional stability and instability? We do not like planes that fly like an arrow through the thermals. In F3J one anyway uses most of the flying time circling, except in perfectly still air - when directional stability is not an issue.

Just trying to see the complexity of this issue... Maybe, to get more practical knowledge, I should buy both v and x-tail fuselages for my next plane :)

Jurgen

Jan 04, 2007, 06:50 AM

So we swim in a sea of doubts, question marks and believes.
For empirical refining, should we not put a whole model into a wind tunnel where we can induce plane oscillations and stuff to study, having the mathematical optimised model as a start point. And having there the opportunity to tune and play plane geometries to see and measure its results. Wouldn't that be nice?
Even then real life differ from wind tunnel, but we cannot get any closer. Only the wind tunnel include the gremlins overlooked by math, and neutralises the subjective feeling. There will still be room for personal preference but backed up with measuring graphs, bey bey doubts.
I guess tunnel testing is too expensive for our purposes but if its possible for airfoil testing (where do polars come from anyway) we should do the whole model too right?
Jurgen.

PS: i like the Toumo - Jojoen saga about CG which is going on for some time now, very educative :)

Ollie

Jan 04, 2007, 08:33 AM

First know all aspects of tail design. How much boom bend stiffness and torque stiffness with kinds of tails? How much boom weight for tails? How much for interfence drag and gap drag? What Dr. Drela said! Etc.

Compare good v-tail design to good x-tail design. Please don't compare bad V-tail design with good X-tail design or good V-tail design with bad X-tail design.

Wing-span

Jan 07, 2007, 06:24 AM

Another issue you also mentioned is the directional stabillity. We dont see the wool string in the cockpit glass that the full size gliders doo. If we did we would have flown so much better. So a model that is not direction stable will yaw slightly (maybe not noticable) and cause a whole lot of drag that again will have influence on the energy you want to keep. That energy is worth a lot of seconds in the air.

Jojo, you have so much to learn!

You can though temporarily attach a long piece of wool string or audio cassette tape, say 2m or so to the nose of the model. Let it stream underneath the model in flight. When turning you will be able to see any side slip of the airframe in relation to the airflow. I have done this for years. Seems to work quite well. All you have to do now is tune the rudder/aileron inputs and there you have it, a clean turning plane!

Only joking, ;) Your past results speak for themselves.

jojoen

Jan 07, 2007, 07:22 AM

Jojo, you have so much to learn!

You can though temporarily attach a long piece of wool string or audio cassette tape, say 2m or so to the nose of the model. Let it stream underneath the model in flight. When turning you will be able to see any side slip of the airframe in relation to the airflow. I have done this for years. Seems to work quite well. All you have to do now is tune the rudder/aileron inputs and there you have it, a clean turning plane!

Only joking, ;) Your past results speak for themselves.
That is a good idea! It will be an eye opener for many. It will for sure make many pilots understand more on what they are doing. I am sure I would benefit on a session with it also.

The only thing I would say is that I think it is the input by the sticks that should make your turning good. To adjust differential and using combiswitch (ail-rud mix) will make your plane turn in just that speed but the next day with differrent air and speed it will be wrong. I adjust the plane to roll straight and use the rudder and not differential to keep that virtual string straight. The reason is that differential and combi switch will in so many cases take away energy to make the plane turn and especially straighten up again. Correct input is so much better. Offcourse I do have a little bit more differential in low speeds. But not as much as many do.
Aileron is for rolling and rudder is for yawing. Almost as simple as that!

Wing-span

Jan 07, 2007, 09:27 AM

Without want to drift away from the subject. Sorry Tuomo. Jojo, how much diff do you use in your thermal setting then? and could setting of diff be different on the same model but V or X configuration, to get similar turning performance. What are your thoughts?

Differential settings are a very personal thing, I know. Some say not to use it at all. I was just wandering how V and X tail gliders respond or react?

Tuomo

Jan 07, 2007, 02:24 PM

PS: i like the Toumo - Jojoen saga about CG which is going on for some time now, very educative :)

I agree, good insights pop out of this discussion. Definiteve answers, maybe not so many? Everyone has to draw his own conclusions and create his style. I do it in national level, Jojoen on WC level. I admire his expertise but I am sure, copying his setup is not the way how i can improve my flying ;)

Whatever the tail, v or x, I think stable and well cordinated flying is the key. A 2m casset tape attached to nose might teach us something about how to use aileros and rudder in cordinated thermal turns, must try that sometimes. Another issue is speed managament. Flying too slowly is common mistake that spoils airfoil efficiency, causes small oscillations that add drag -- and create the need to use coarse control movements (tah yet again add drag).

But even the best flyers do mistakes all the time. Maybe planes that have large tail surfaces, say Pike Perfect, are more tolerable to the "bad flying" that happens to everybody in the heat of a contest. But is this at all connected to issue of v vs. x-tail? Maybe not?

Looking at many of the older designs, say from pre 2000 era, I see slightly small tail surfaces. No doubt, plane designes have also noticed this fault. During the same time period when x-tail has become more common in F3J, tail size (tail volume) has also grown up, making planes more easy and to fly and more tolerable to all kind of mistakes.

psoff3x

Jan 08, 2007, 05:12 AM

Here are some thoughts on aileron differential posted earlier in another thread. The comments by Dr Drela make interesting reading.
http://www.rcgroups.com/forums/showthread.php?t=552660

aeajr

Jan 12, 2007, 10:25 PM

V-tails are less likely to get hung in the grass when you land. ;)

chetosmachine

Jan 13, 2007, 04:32 AM

All i can add is that 2-piece V-tail is lighter than x tail and easier to transport than 1 piece v-tail......
Those who sat that a x-tail turns better, or that a x-tail plane is better than a v-tail plane, may suffer from severe placebo effects, from what the've have been told!
regards,
Chets

Wing-span

Jan 13, 2007, 05:50 AM

Not that many planes are produced any more with separate V-tail halves though, are they?

OVSS Boss

Jan 13, 2007, 08:08 AM

Zenith, or Corrado, isthe only one I know of. But they are nice, too bad more are not produced.

Levi Jordan

Jan 13, 2007, 11:20 AM

Manufacturers were pressured by US pilots to produce X tail configs because of our landing tasks. The Cross tail is heavier, and more drag, but is much more powerful at correcting at low speeds on the final landing approach. When half of your f3J contest score is from a precision landing (AKA on the 6mm line, or in a 100cm circle) the cross tail is superior.

Does that mean the V-tail is inferior? Not at all. In calm conditions I would prefer a V-tail... there is little need for last minute fixes and major control inputs. You then get lighter weight and better drag CF.

but if the conditions are windy, or constantly changing, then the added pitch and yaw authority of a full flying stab and large vertical rudder make getting your points an easier task.

Europeans have much easier landing tasks in the normal contest, and so the V tail is better in that arena.

The manunfatcurers today all provide exceptional airframes.... and the best thing you can do to get the edge is to simply fly and know your airplane inside-out. X or V--- if you know it, and fly it you will excell. The winner is usally the one who practiced and trained more... not necessarily with the best airplane.

OVSS Boss

Jan 13, 2007, 12:01 PM

Levi, I agree that a well designed V is not really at any real disadvatage in most cases. That being said, a lousy one sucks! I flew a Phoenix F3B model, there might have been guys that made that work, but boy could that bite you in the hind side. I was actually shot down with the ship (only damage was the V dies) so got a bigger one from another model and it did help. Now, my Zenith was a champ, great rudder forces and the split side were nice to travel with, it really spoiled me.

Marc

Joe W

Jan 13, 2007, 12:50 PM

Dr. Drela captured the essential methods of quantitative evaluation of V-tail vs X-tail. Most V-tail aircraft on the market have significantly lower damping than the alternative X-tail configurations, which is quite evident in the differing handling qualities between the two versions of most designs.

As to the US driving the market, I think not. The market that drives most design criteria is in Europe. Note how many planes were X-tail at the last WC (F3b or F3j), and also note the change in the amount of X-tails being used as compared to prior WCs. I remember a few cycles ago when it was only the US team using x-tails, now many teams are using x-tails. But, until Europe started using x-tails, the market was flooded with v-tail designs.

To some degree, it is a matter of what is trendy. When I first got into F3B, t-tails were the hot ticket, and there were many t-tail designs being campaigned. When was the last time you saw a competitive t-tail design?

Back on subject, yes there is a difference in most mfrs implementations of x-tail vs v-tail. One thing that scares me, is that many designers haven't gotten beyond the concept of equivalent projected area as the method of sizing one vs the other. All it takes is to do this once to understand the basic fallacy in the aero assumption for projected are equivalence equalling aero equivalence. I learned this lesson two decades ago... I built a v-tail version of a x-tail slope racer with equivalent projected area, first flight had high spectator entertainment value, as the cg was the same as the x-tail version. The plane was highly unstable in pitch, and needed about 6 oz in the nose to get back to the same static margin as the x-tail.

Joe

PS One interesting result from appropriate v-tail sizing, is that for equivalent v-tail and x-tail aero characteristics, the pitch control authority on the v-tail is much higher than the x-tail.

jojoen

Jan 13, 2007, 01:33 PM

---snipp---

PS One interesting result from appropriate v-tail sizing, is that for equivalent v-tail and x-tail aero characteristics, the pitch control authority on the v-tail is much higher than the x-tail.
Hi Joe!
Yes this is a cool point. Yes around zero a V-tail is MUCH better. Then again for landing it starts the down movement nice but not always enough for my landings...

Levi, I agree that a well designed V is not really at any real disadvatage in most cases. That being said, a lousy one sucks! I flew a Phoenix F3B model, there might have been guys that made that work, but boy could that bite you in the hind side. I was actually shot down with the ship (only damage was the V dies) so got a bigger one from another model and it did help. Now, my Zenith was a champ, great rudder forces and the split side were nice to travel with, it really spoiled me.

Marc
Not fair Marc !)
In this case the dihedral on the wing makes a large impact on rudder control. So no wonder you like the Zenith... Lots of dihedral makes rudder happy ;)

jojoen

Jan 13, 2007, 01:42 PM

--snipp---

Europeans have much easier landing tasks in the normal contest, and so the V tail is better in that arena.

NO ;)
Our landing tasks is kind of much harder. Our goal is so much on the time instead of landing that you will see hovering over the spot or/and coming in hard with lots of down elevator and butterfly.
US / TD landings are a good method of NOT having dork landings. This because the pilots use more energy on the landing spot coming in low and slow instead of dorking.

And with the new airfoils and thoughts about rudder design we have a whole new world in control with a X-tail :p

Look at Drela's airfoils AND look at transition. But also design of rudder. We have had so many complaints on the hingeline of the Perfect. Wonder why :rolleyes:

Ok I will not keep you thinking all night... The point is that the transition of airfoils and geometry of the rudder makes it impossible to have a normal hingeline all the way. The rudder will get stuck.

And just maybe doing some thinking and work on V-tail design will make an impact also :rolleyes:

Joe W

Jan 13, 2007, 02:16 PM

Hi Joe!
Yes this is a cool point. Yes around zero a V-tail is MUCH better. Then again for landing it starts the down movement nice but not always enough for my landings...

I slipped in a quiet inference in my quick comment on v-tail control authority. Note the specificity in my comment about control authority, and that I did not extend it to the yaw axis! The annoying thing about v-tails, as that the yaw control authority gets mucked up due to mutual interference between the v-tails. Also, the v-tail pitch authority is typically higher for TE up than for TE down. Yes, v-tails have non-linear control response...

OVSS Boss

Jan 13, 2007, 03:44 PM

JW, very correct comment about V's having more pitch authority vs, the X tails, I saw a gent here in IN that had an X tail Sharon that busted the fuse and got a V tail to replace. Took the stock program from the X and went to fly the V tail, first throw, sailed on across the field with some speed, hit the flaps and stuffed it nearly to the LE because the Ele to Fla cmpensation was through the roof on the new fuse. Actually broke it again cause of it. Also, articlulated surfaces have a higher authority than flying stabs per degree of deflection, correct??

Marc

aeajr

Jan 13, 2007, 04:38 PM

Why would an articulaed surface, meaning an elevator on a fixed stab, have greater authority than a full fllying stab which has much greater surface?

jojoen

Jan 13, 2007, 04:55 PM

Why would an articulaed surface, meaning an elevator on a fixed stab, have greater authority than a full fllying stab which has much greater surface?
With a flipper elevator (I choose to call it that) you create a lifting airfoil (up untill the point it starts making interference/drag instead of lift).
With a full flying elevator you enforce an angle of attack to create the same lift.

So on small corrections this is more effective. But mostly on authority.
In the perfect world we would change the airfoil and the EWD (ElevatorWingDihedral) like the birds.

In other words both would be good ;-)

Jurgen

Jan 13, 2007, 05:52 PM

Zenith, or Corrado, is the only one I know of. But they are nice, too bad more are not produced.I just stumbled on this Trinity V-tail picture, only F3B version is this way, F3J and F3F is one peace.
http://www.baudismodel.com/sportflying/trinityf3b/7b.jpg

jojoen

Jan 13, 2007, 06:05 PM

Lots of two piece V-tail models. But not so many that are built light on purpose. Pike V, WR, Plus, Superior, Brio... Can all be delivered with 2 piece tail. If you ask extremely nice and change the carbon joiners to hollow ones it can probably be built light too ;)

Oh and my new Crossfire.. ;)

jojoen

Jan 13, 2007, 06:09 PM

Forgot one... The coolest V-tail model on the planet.
Plug in tail!!!! Goes into a slot in the fuse. Very cool. And made in Norway!!

Tried to get one for a long time... He wont give me any :( Wont let me invest either...
http://www.seaxx.net (not updated for a while but cool)

Wing-span

Jan 13, 2007, 07:10 PM

I think you are right Jo.

This is a seriously nice model.

OVSS Boss

Jan 13, 2007, 08:52 PM

Jo,
That guy is out writing his own story isn't he. I will give him a great deal of credit for all the work he has put out in the model and winch.

Marc

Levi Jordan

Jan 13, 2007, 11:17 PM

Joe,

Thanks for the info... The yaw is the biggest factor for me with the Cross tail. Mostly because our landing lines always seem to get cross winded by the end of the day.

About the market... I just saw that the US team started the X tail thing, then the rest followed suit there after. True, the airplanes are all made over there, and there are more pilots... so obviously the demand.

So do you see the X tail as a trend? or superior for our F3J and TD competition tasks?

Thanks for the info, you're my hero, and look forward to seeing you at the AMA convention tomarrow.

Levi

jojoen

Jan 14, 2007, 04:32 AM

The story of the X-tail is kind of funny if you break it a bit down. X-tail was tha favoured weapon in the 70-80ts and all moving elevator led to the possibillity of a further back CG (atleast in Norway back then with the Sagitta 900 etc. ).
But moving to all moulded fuses it was difficult to make them light compared to the V-tails. Remember aome used polyester also. You would save a lot on a V-tail build and in most cases the duration task was flown hanging in front.
Then more and more producers find their way making light X-tails and also that the overall weight was now so low that a little weight in the tail could be sacrifised (planes were also designed for a higher weight).

What I think is funny is that many have forgotten the difference in all moving elevator versus flipper tail in CG matters. Hey we even shifted the CG in the air! (although with other airfoils than today)

Tuomo

Jan 14, 2007, 05:00 AM

What I think is funny is that many have forgotten the difference in all moving elevator versus flipper tail in CG matters. Hey we even shifted the CG in the air! (although with other airfoils than today)

With modern radios and accurate servos it is bossible the equalize (at least to an extent) the feel of v and x-tail with expo and differential up-down movement. This is certaily quite a work. Making x-tail model fly well can be a more straightforward process. At least it has been to me :o

Another issue is how much a more forward CG sacrifices the preformance. Maybe not so much -- and it is (more or less) compensated by reduced drag of a v-tail.

BTW Surely you remember that it is possible to make an all-moving v-tail. (if that is seen as the answer to get the best out of both worlds.)

http://rc-sailplane.com/elita/elita_v-tail.jpg

Anyway, the discussion goes on and on... For an average contest flyer the choise is very puzzling. Not only because of most manufacturers producing both types of tail, but also because of the differences between F3B and J. In J there is a strong movement towards x-tail. However, despite some succesfull planes sporting x-tail, namely Crossfise and Europhia, B seems to be a dicipline where v-tail rules.

Andy W

Jan 14, 2007, 06:13 AM

What I think is funny is that many have forgotten the difference in all moving elevator versus flipper tail in CG matters. Hey we even shifted the CG in the air! (although with other airfoils than today)

Could you please expand on that comment? If I interpret it correctly, are you saying CG moves as AOA changes with a "flipper tail"?

first flight had high spectator entertainment value

Hope you don't mind if I save that one for later use - I'll need it! :D
..a

OVSS Boss

Jan 14, 2007, 07:58 AM

Tuomo, interesting looking ship, details please.

jojoen

Jan 14, 2007, 02:21 PM

Tuomo, interesting looking ship, details please.
That is the Elita V-tail from Jaro Muller.
This design was first seen on the Compact by Milan Janek. The unofficial record in Dynamic Soaring were for some time done with the Compact from Milan.
The Elita came a bit later than the Compact.
It was Jaro Muller or maybe his engineer Jano Tulak (now working at Aerodesign.sk with Aris) that made this type of tail on the Elita. They shifted the horns from inside to the upside instead. It was a difficult task to get the rods moving freely on the top side but they made it.

jojoen

Jan 14, 2007, 02:23 PM

Could you please expand on that comment? If I interpret it correctly, are you saying CG moves as AOA changes with a "flipper tail"?

I am talking back in time when we had a "screw" shifting a lead bar inside the fuse. This made it possible to have different CG's in the air. When you use camber the lifting point moves and so should the CG. But on newer airfoils this is not so critical.

aeajr

Feb 05, 2007, 05:42 PM

What an interesting discussion! And, having read it all, I know so much more yet I still would be at a loss to determine which would be a better design for an american style TD competition plane.

My Thermal Dancer has an X tail. That is what I have, so I think I will fly that! :)

Robglover

Feb 05, 2007, 06:28 PM

I just wandered in to this nice thread. It is exposing several good things.

One more thing that I think is often missed in the discussion of V tails is that there seems to always be some cross coupling from yaw to pitch. In my somewhat limited experience with V tails I found that I could set one up so that a pure yaw input induced no pitch at only one speed. If I set it up to act well at low speed then it would pitch with high speed yaw inputs, or vice versa. Has anyone else had this problem?

Wing-span

Feb 05, 2007, 06:58 PM

Yes

Wing-span

Feb 05, 2007, 07:19 PM

Let me explain. I was able to fly my Xperience Pro-V on Sunday in dead calm conditions. I was flying just before sunset. There was no thermal activity so the glider was able to cruised around for maximum 'hang' time. These still conditions really revealed the pitch yaw coupling with the flying surfaces (ruddervators - flappers!) moving identical amounts. My TX has the facility to mix this out. But I will leave that till next time, maybe. I was having too much fun to bother altering things and I also found it really easy to make the required adjustments directly with the elevator stick. Up or down a touch.