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longitudinal dihedral

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Old 12-20-2010, 02:58 AM
  #1  
pl365322
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Default longitudinal dihedral

Hi guys.
As I am setting up my Jamara (Fly-Fly) Discus CS 2.6m, I measured the longitudinal dihedral (Vé) and found it at 0°. I beleive this is not right. I thought setting it up to 1 - 1.5° by shimming the elevator.
Any thought, advise, experience is welcome.
Thanks.
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Old 12-20-2010, 06:10 AM
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Ranfred Radius
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Default RE: longitudinal dihedral

I think you are referring to "angle of incidence"? If thats the case, then yes a degree or two should be fine..

Randy
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Old 12-20-2010, 06:19 AM
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Default RE: longitudinal dihedral

Thanks Ranfred but no.Angle of incidence is refering to the fuselage axle. The call "V" angle is the angle between the wing cord axle and the elevator axle. That give you the way the plane fly, tail up or down and of course move the wing flying incidence.
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Old 12-20-2010, 07:53 AM
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Allie33
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Default RE: longitudinal dihedral

++++++++++++++++

This looks like a debatable subject that has an answer best explained in terms that the layman can understand. Somewhat akin to "water seeking its own level" in simplicity. If anyone wants to know about this water principle theory pm me.

In my limited experience any moderate deviation from an incidence of 0-0 involving a symmetrical wing and stabilizer had to be compensated by elevator offset. That is what it took to make my CMP Leo fly level ,hands off, at cruising speed.

I could go on and on but I won't.

AW
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Old 12-20-2010, 08:38 AM
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Default RE: longitudinal dihedral

well, it doesn't really reply to my question. Do not mix wing incidence with V longitudinal dihedral. The V angle is related to wing incidence by construction, CG and flying caracteristics.
Modifying the V angle modify the flying wing incidence and flying caracteristic. At 0 stabilizer angle, the wing incidence will be 0 (in my Discus CS case), wich will be not right. As I cannot move the wing incidence, moving the stabilizer incidence will move the wing incidence but will also move the way the plane will fly, tail up or down, not having the max of airfoil performance.
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Old 12-21-2010, 02:25 AM
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Default RE: longitudinal dihedral

in reply to Paul in the aerodynamic thread,I agree with you about WING incidence but the LD is not an angle compare to fuse ref. line but the angle between wing and elevator. Of course if the stabilizer is level with the fuse ref line, it gives you only the wing incidence. Now, imagine that the stab is NOT level with the ref line and I have a neg angle of 1° on the stab (which is what they recomand for the DG 1000 Fly-Fly -Jamara during test in French RC mag magazine) by shimming the back of the stab. I will induce an angle between wing and stab. That's the angle I need.
Taking the case of my Discus, I have all angles set at 0 (stabilizator, wing vs fuse ref line). If I shim the back of the stab, I will fly tail down, which I don't like but also increase the wing flying incidence which I might need.

In reply to Matt on the same thread, my Discus CS is the Fly-Fly / Jamara kit.No flaps, fixed stab but still possibility of shimming.
Looking to all books and website, the LD is the angle between wing and stab, not the fuse ref line.
Shimming the back of the stab change that angle. That's the angle I would like to know, to see what is best so that I could know the amount of shim I have to ad....if need!
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Old 12-21-2010, 09:13 AM
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Tall Paul
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Default RE: longitudinal dihedral

You argue against every suggestion!
Why?
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Old 12-21-2010, 09:34 AM
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Default RE: longitudinal dihedral

Paul,
I am not argueing, just trying to have information, understand and know how to set the stab incidence.
Sorry if you missunderstand me.
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Old 12-21-2010, 10:22 AM
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Default RE: longitudinal dihedral

Let me see if I can help at all. pl365322 is correct when he is talking about Longitudunal dihedral. There is actually such a thing, however most of us know it as Decalage. Basically, all that it is is the diference between the wing incidence and the horizontal stab incidence. Basically, the way I learned it is that decalage (or longitudinal dihedral) is:

Longitudinal dihedral can also mean the angle between the zero lift axis of the two surfaces instead of between the root chords of the two surfaces. This is the more meaningful usage because the directions of zero-lift are pertinent to longitudinal trim and stability while the directions of the root chords are not
HOWEVER:, mister pl365322, you are mistaken on a couple of other things. First:

I thought setting it up to 1 - 1.5° by shimming the elevator
Shimming the elevator is incorrect, I'm sure this is a typo and you meant to say "horizontal stabilizer" as you did mention that later. Shimming the "elevator" does nothing as it is a moving surface.

moving the stabilizer incidence will move the wing incidence but will also move the way the plane will fly, tail up or down
Moving the stabilizor incidence will NOT, repeat, wil NOT, move the wing incidence. As has already been mentioned, the wing incindence is the angle between the chord line of the airfoil and the longitudunal axis of the fuselage. You mentioned that this is fixed on your discus, so it will not change no matter what you do to the stab. Here is what is really happening when you raise the trailing edge of the stab. You are changing the decalage (or as you like, LD) and by doing this you will create more downforce on the tail of the aircraft, basically the same as pulling on the stick and givingit some up elevator. What this will do is push the tail down, increasing the angle of attack of the wing. DONOTCONFUSEANGLEOFATTACKANDANGLEOFINCIDENCE, THEYAREDIFFERENT!!

but will also move the way the plane will fly, tail up or down, not having the max of airfoil performance
No it will not affect the airfoil performance. The tail flying high or low has zero effect on airfoil performance. It is strictly a visual preference thing. The airfoil performance is all aboutangle of attack, in other words, the guy wiggling the sticks is the determining factor as to whether the airfoil is flying at its max efficiency. Some airfoils are most effecient at 2² and some at 4², and any other angle. Every one is different. The way to get the max out of your airfoil is strictly a go fly, and learn the airplane thing. You may find that your glider may be happiest flying tail high, even with the incidence in the stab, it may be happier tail low, who knows until you fly it. The only thing that tail low vs. tail high will affect as far as performance goes, is fuselage drag. It may be a bit draggier with the tail hanging down a bit, but that can't be helped.

Now, as to your original question, if it were at all possible to change the wing, that would be the best bet, about 1 ² would be lots. However, since you mentioned that it is fixed, you have to do the stab. I would guess, just from my experience, that the 1-1.5² you suggested will be plenty.

I hope this helps out a bit in understanding what you need. I apologize if I seem a bit abrupt, but as Tall Paul mentioned, your tone is a bit argumentative so I was slightly heated when I started typing.

PS
Thanks Ranfred but no.Angle of incidence is refering to the fuselage axle. The call "V" angle is the angle between the wing cord axle and the elevator axle
The word you want is axis

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Old 12-21-2010, 10:45 AM
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Default RE: longitudinal dihedral

thanks rhall999 for the great explanation.
I am so sorry to everyone about how you understand and pick my writing tone up and I apologise. It was not my intention, beleive me.
I just did want to clarify what I was looking for. I always found that in this site.
I will set the LD of the Discus to 1° as a start, then I think I have to wait some time to test the flying perf as we have 50cm of snow now. Amazing for this period and even, in 12 years, I have never seen that in Belgium !

Thanks again to all for posting and I apologise again to everyone.
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Old 12-21-2010, 06:08 PM
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Default RE: longitudinal dihedral



On a more educational level if you were doing the design of the model yourself then what you would do is decide on what speed range you want the model to be most efficient over.  And by "range" I mean a narrow range chosen for how you'll fly the model.  Then you would figure out what the wing's Coefficient of lift and thus its angle of attack would be for the middle of that speed range.  Then you'd set the wing's incidence angle to the fuselage's median axis to that angle.  This would ensure that the fuselage is pointed into the oncoming air at the most streamlined angle possible.  Typically you would set the wing's incidence angle to something slightly higher than the angle of attack at the best L/D speed so that it's still close enough at the best L/D speed and still axis on for slightly slower or faster flight depending on which aspect you want to optimize.

With that out of the way THEN you set the decalage or LD angle to achieve the required lift at the tail for the optimum CG location.  And that angle is typically best set by trial and error unless you've got some good software to crunch the numbers for you.

So you see, there's more to this than meats the eye.  In your case the wing's angle is fixed to whatever it is.  The final shim you need to set the stabilizer to will depend on where the CG is located.  That's why I suggested to set it to -1 to -1.5 degrees to the wing and then see where the model trims out.  If you have up or down trim then shim the stabilizer to where it flattens the angle and fly again.  When you have it where the elevator is really close to being in line with the stabilizer then you're close enough.

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Old 12-22-2010, 01:15 AM
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Default RE: longitudinal dihedral

Thanks Matthews for the info. I did design a few planes from very slow sailplane (wich I used to learn flying) to intermediate and F3A. That was a lot of fun and dust ...Now with reduce evening time, I only modify kits.
Awaiting better weather condition here in Belgium and Italy (where I fly most) to see the results of this post, take care.
I hope to read you again sometime.
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Old 12-22-2010, 12:10 PM
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Default RE: longitudinal dihedral

That T tail floater bare bones shot looks VERY nice.  Is that one of your own designs?
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Old 12-22-2010, 01:46 PM
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Default RE: longitudinal dihedral

yes it is. I design and built it from scratch being fed up of kit to learn flying. That was around 1990. She is now still flying with latest technology as Lipo and outrunner motor after a few mods. I am glad you appreciate that picture. That's the blue and yellow glider from the 1st pic you can see. It is the latest version of it. I have lost her twice from the time she flys, out of view under good thermal lift but always found her back without damage. An amazing gentle floater. she flys like a butterfly
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Old 12-23-2010, 10:47 AM
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Default RE: longitudinal dihedral

TWICE you almost lost it! ! ! 

I lost one plane the same way.  Got it back but not before weather had done some damage.  Now I make it a point to fly no higher or farther than where I can still see the stabilizer.  Once I can't see the stabilizer along with the wing I know that losing vision on the wing is just a bad blink away and that it's time to come back in or lower.

Anyhow, it looks like it would be a superb model for calmer days or lazy sunny summer evenings where the sun can shine on the bottom of the wings while you're getting in one more flight before darkness falls.
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Old 12-24-2010, 06:25 AM
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Default RE: longitudinal dihedral

First time , I just lost it, out of view.It was a nice Italian summer afternoon.Flying over a corn field, she picked up a strong lift and in few minutes, I couldn't see her. Tryed to push elevator down but nothing...The owner of the fields phone me few days later (we are good friend!). The glider land exactly at the vertical of his land in between of 2 corn field, no damage at all.
2nd time was also a warm partly cloudy summer afternoon.I went under a cloud and up she went so quickly...but I was aware this time and give motor plus down elevator till I saw her again and landed safely...
Fantastic floater. 2.20m WS, 1.2kg, Gottingen airfoil.
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Old 12-24-2010, 10:53 AM
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Tall Paul
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Default RE: longitudinal dihedral

Diving out of a thermal....
Now I prefer spinning.
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Old 12-24-2010, 11:38 AM
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Default RE: longitudinal dihedral

You are surely right but this glider doesn't want to spin...As it have no ailerons, it just do a slow flat spin...when things goes as thought. Not easy to pull her down when she doesn't want...
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Old 12-29-2010, 02:11 AM
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Default RE: longitudinal dihedral

I thought I'd add a small bit of info that has helped me thru the years..most of you guys will catch this pretty quickly... whenever I design or use three vues, I usually set the fuse at zero and also the stab, I tend to put the incidence on the wing at whatever I chose 0,1.0,1.5 degrees I put it on the wing so that I can adjust it in the field or if it needs it at all. sometimes I even adjust the fuse and wing for the plane to fly level or slightly nose down..anyways the point is, how do want the plane to look when flying...it takes some research to understand all thats involved to get a plane to fly right... as stated previously in this forum there certainly is alot going on when your plane is flying.....
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