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Author Topic: T - tails.  (Read 790 times)

Offline Matt Spencer

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T - tails.
« on: March 15, 2007, 04:23:58 PM »


   Your thoughts .

  Flap wash causing blanketing of stab, -control reversal seems to be 'the worry'

 Im intending to build a F-104 ., Marut-ish fuselage, Berringer wing and stab,
 on top of the fin.
 I cant see any 'bother' , with those little wing flaps ?

                                                                                      M.S.


Offline Mark Scarborough

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Re: T - tails.
« Reply #1 on: March 15, 2007, 05:30:29 PM »
the first thought that comes to my mind is the moment from thrustline that the drag induced by the tail will be. There would, IMHO be some issue with a nose up pitching moment being introduced anytime you deflect the elevators. For up control this wouldnt be much of an issue, but to at least some degree, when you input down elevator, there would  be a counter force introduced as a result of the drag of the elevator. thus, the tail wants to introduce rotation nose down as a result of the tail lifting force, but also a nose up rotation resulting from the drag of the tail deflection so I would expect a trim issue as a result of this, that being difficulty more than normal getting equal inside and outside turn rates. Just my observation as a result of research for my personal project wich places the hori tail above the thrustline substantially more than conventional stunters. perhaps Ted or on of the other gurus can chime in?
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Offline Randy Powell

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Re: T - tails.
« Reply #2 on: March 16, 2007, 12:20:23 PM »
Mark i s right. YOu have to be careful about getting the centerline of the stab too far off the thrustline or you will introduce a substantial pitching moment. I built a "T" tail plane some years ago. It took an awful lot of dialed in down elevator to get the plane to turn the same inside and outside. So much that I didn't have much down control left. And I wasn't all that far off the engine centerline. It's a similar problem with semi-scale stunt planes with low wings. They get far enough off the engine centerline and the only way to resolve the vertical CG issues is to introduce significant dihedral. I suppose you could use a "T" tail configuration with substantial anhedral in the stab to bring it more into line with the thrust line. Hmmm... That's an idea on a jet style plane.
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Offline Jim Pollock

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Re: T - tails.
« Reply #3 on: March 16, 2007, 01:39:26 PM »
Tom McClain's F-104 seemed to fly O.K. with the elevated stab, but it probably had to have some down dialed in also.  I'm sure you can get it trimmed with enough control response.

Jim Pollock   D>K

Offline Matt Spencer

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Re: T - tails.
« Reply #4 on: March 16, 2007, 03:17:33 PM »


  What im thinking is that the flap-wash is the 'big' problem.

  small flaps , small flap wash ? ? .

 and hopefully the tailplane far enough away so its outside the suction, or wingwash.

 Randy, did your T-tail have a rather high vert.stab. Nobler-ish proportions ?

 I think one of the Thunder Chief or R.F. 84s  had an anheadral T tail.

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Online Paul Smith

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Re: T - tails.
« Reply #5 on: March 17, 2007, 04:55:18 AM »
Aside from aerodynamic issues, the structural problems are huge.

Tough enough to make a strong rudder, but add on the tailplane with all its forces?  It's hard enough to secure the flipper in normal position.  Real-plane builders have titanium & such. 
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Offline Tom Niebuhr

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Re: T - tails.
« Reply #6 on: March 17, 2007, 11:05:34 AM »
I had a Carousel (Mackey design) It has a high tail but did not have the problems that are described. (See pictures)

Jack Sheik's "Knight" has a T-tail. I have never seen on fly, but I think there was one at VSC a while back.
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Offline Randy Powell

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Re: T - tails.
« Reply #7 on: March 17, 2007, 01:42:45 PM »
If you look, Jack Sheek's Knight is really pretty conventional. The fuselage cuts down very narrow to allow the "T" tail look, but the stab is, in fact, about a half inch above the thrust line. It just looks like a "T" tail.

The one I build had an 18" tail moment and the stab was easily 2" above the thrust line. As I said, I was finally able to get it to turn relatively equally inside and out, but had something on the order of 20 degrees of down elevator dialed in. May be able to overcome some of that with anhedral in the stab, but it would be a killer to construct.
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Offline Tom Niebuhr

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Re: T - tails.
« Reply #8 on: March 17, 2007, 03:24:36 PM »
Randy,

I don't doubt you a bit. Just that I didn't see the problem with the Carousel.

20 is quite a lot of elevator differential! I will be playing with a non-conventional set up in the near future. I am wondering if built-in stab incidence is the key. Bob Whitely advocates this with his conventional airplanes.

In his "Sea Vixen" article Jack Sheeks mentions that they had to lengthen the tail moment to get better response for outside maneuvers. He also eliminated the moving flap between the booms.

I talked to Jack about this a few months ago. I noticed that the stabilier chord looked like it had been reduced, and he said that this was also to get a faster turn. At that time he also said that the major problem was solved with the longer tail moment, and that it probably did not need the center flap to locked out.

Going back to the Carousel. it only has a center section flap between the booms. On that airplane it worked fine. Several Carousels were built, including Jim Vornholt's Nats winner.

This might prove that the longer tail moment is one key factor. We can probably get away with a high tail surface, but the question really is how high is too high, since the negative affect mentioned above would also be exagurated with the longer moment.

A key difference today is our ability to adjust the elevator bias and movement.

When I get home, I will measure the relative height of wing/thrust line/ elevator on both the Sea Vixen and Carousel plans.
« Last Edit: March 17, 2007, 04:14:59 PM by Tom Niebuhr »
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Offline Matt Spencer

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Re: T - tails.
« Reply #9 on: March 17, 2007, 06:58:00 PM »


  Tom,
          Do you ever feel a kick or bump as the tail plane goes through the wing wash ,
          into corners ,or on different sized loops.

          With the Sea vixen I was going to try a flying tailplane pivoting on a carbon fibre tube, about 10 m.m. dia.
         between the tops of the fins. The full size sea vixin ran a flying stab.I think modified from a fixed stab. elevator set up.

        The Caurosel looks preety neat. does it feel different to a conventional job to fly ?

                                                                                                                                            M.S.

Offline Clint Ormosen

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Re: T - tails.
« Reply #10 on: March 18, 2007, 04:40:10 AM »
Tom, you need to kit the Carousel. Pretty plane.
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Offline Tom Niebuhr

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Re: T - tails.
« Reply #11 on: March 18, 2007, 09:28:57 AM »
Matt,
The "Carousel" doesn't have anything wierd going on in flight at all. It feels like a conventional airplane. Mine had low time, because I had other problems, that were unrelated to the design. The airplane is semi-taken apart at this time.

The Sea Vixen is an interesting subect. I don't know about using a flying stab ,howver. They have been tried before, with varying degrees os success. i just have no experience on that subject. There has been some written on them if you search here or on Stuka. I think the "Vixen" is different enough without making it more complicated.

Clint,
I have no intentions of kitting the "Carousel" at this time. Jigging the stab for proper alignment, the very narrow booms, and confined space for the elevator horn, make it tricky to build. As with other airplanes, I could make Laser ribs available if someone wants them.

I over complicated mine by running pushrods through both booms, using a second bellcrank in the wing linked to the main bell crank. This caused a problem at the neutral area that made landing difficult but had no effect in the air. Again, this was not a problem with the airplane design.. just a problem that I caused myself.

Randy,
I took some measurments from the "Carousel" and "Sea Vixen" plans.

"Carousel":  Center of thrust is 1" above the wing centerline; Stab is 3" above wing centerline, or 2" above the thrust line. Hinge to hinge measures 16"

"Sea Vixen": Thrust line is at the centerline of wing, Stab is 3 3/8" above, centerline. Hinge to hinge measures 19", but I have not calculated with reference to the COL or CG. With the sweep back the effective tail moment is about 1.5" shorter.

Again, Jack Sheeks had lengthened the tail moment,  and locked out the center flap between the booms, but in a telephone conversation I had with Jack, he felt that the lockout was probably an overkill, and that the center flap section could be used. I have never seen a "Sea Vixen" fly. The only one I have seen a picture of is on the Victoria Site from Australia. www.vicstunt.com
The builder is listed as John Goodge and it is powered by a OS .46LA. (2006) Maybe someone can find John to learn more about the flying characteristics.
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