made a mistake . . . tell me it wasn't bad

[emersunn]

Well, looking over this thread: http://www.rcgroups.com/forums/showthread.php?s=&threadid=80547

about JK thermic construction and they talked about: "did you build the wings with 100mm dihedral or 140mm?"

Uh oh! I looked at the plans and sure enough it was 70mm for each wing =140mm. I only did 70mm on my JKT.

To save a little pride I must say that in the instructions the drawing shows 100mm from the workbench to one wingtip. In the plans it shows half the plane with 70mm from horizontal to one wingtip.

So if you do the math, I am only 30mm short from the directions, but I am 70mm short on the plans. Sadly enough, the directions say to go by the plans because they are more current info.

So since I now have a wing that is 50% flatter than it should be should I:

1) fix it before I even launch it
2) fly it and see if I like the way it handles
3) leave it be because there are benefits to a flatter wing
4) stop kicking myself and banging my head against the wall, I am disturbing my family

I assume this plane will need more aileron attention by the pilot (me). That is fine, I have flown planes that need a lot of aileron input.

So is it a bad thing? What are the benefits, if any? What are the drawbacks?

Measure thrice and glue once, :confused: Paul

[Ollie]

The result of less dihedral is that the plane will be less spirally stable and there will be less yaw to roll coupling. As the angle of bank steepens, the need to hold opposite aileron to prevent the circle from developing into a spiral dive will occur at a shallower angle of bank. At the same time, up elevator will be needed to keep the turn level and not steepen too much. This means that it will take more of the pilot's concentration to turn with precision. This means that it can't be circled safely near the limits of vision when the wing may disappear monentarily. Be prepared to steer with rudder during a highstart or winch launch.

[markdrela]

35mm under each wing works out to only 1.6 degrees EDA. As a result, this glider is going to be a real bear to thermal well, particularly at large distances. If this is your first aileron thermal glider, it will probably be unacceptable. I say cut it apart and try again.

With the Thermic's short tail arm, I'd go for at least 5 degrees EDA (109mm under each tip) to quash most of the spiral instability. Even 8 degrees (174mm) wouldn't produce any unfavorable side effects for thermal flying, and there are lots of advantages. Slope flying is another matter.

[emersunn]

Thats what I thought, Mark. I will have to do the wing over again. The good news is everything else is ready. I am not comfortable enough at distance keeping it in a thermal circle. I do not plan to slope with this ship. So I will go with the recommended angle.

PS thanks for doing the math - I was wondering what the angle would be. What's the equation? Wing Length / mm from horizontal / ?????? = degree angle???

[markdrela]

Originally posted by emersunn
What's the equation?

For simple V-dihedral...

elevation_of_each_tip = semispan * sin(EDA)

so the inverse formula is...

EDA = arcsin(elevation/semispan)


FWIW, the standard aero definition of "dihedral angle" and EDA is from the horizontal. Always. So a V-dihedral wing with 5 degrees of dihedral has a 10 degree break angle in the center. People who refer to the center break angle as "the dihedral angle" should get out of the habit. This is non-standard usage and can cause unfortunate building errors like in your case.

[emersunn]

Yes, that is exactly the confusion I had. The diheral angle was measured in the plans and the break angle was measured in the instructions. They should just give the recommended dihedral angle and let me figure out the rest.

The good news is it was a lot easier to re-do than I thought. The epoxy is now drying on a nice 5.5 degree dihedral angle. Plus I get to tell the guys that Mark helped me build it. :)

[Bernie Wolfard]

I wanted to build my Thermic for our clubs February Floater contests, basically RES only contests. I asked Andre if he had any info on making the Thermic RES instead of aileron and I also asked for the X tail version. I received a nicely hand drawn line drawing of a two break polyhedral wing with the breaks outside of structural elements. To create the poly breaks I cut a narrow vee in the top of the wing down to the bottom skin, filled the vee with filled epoxy, bent the tip to the correct dihedral angle and weighted it in position until the epoxy cured. I then covered the breaks with 2-oz glass. Some doubted whether the bottom skin would bend far enough without breaking but there were no problems. Doing it like this I didn’t change the built in tip washout.

The results are a very light RES plane with a modern airfoil. It flies rings around the Oly IIs and other gassbags. It’s will out sink rate them in a thermal then speed and up and way out L/D them to the next thermal. With the X tail it turns well and has no instabilities. It is simple and easy to fly and just plane fun, a relaxing break from a full house ship.

My only problem is the wing is too thin to get anything more than a sub micro servo into it for spoilers and I have burned out 2 naro servos holding the spoilers closed. My solution has been to tape the spoilers shut and plan my landings carefully. Actually my second problem is someone else in the club has converted a Hi Aspect to RES, so winning our club RES meets is now pretty hard. Oh well, I can live with second.

If I can remember I will post pictures of my RES (actually now an RE no S) Thermic when I get home this evening.

Bernie

[emersunn]

Sounds neat. Plase post pictures. I have "repaired" my mistake and I am close to maiden flights. I need to do the little details and program the TX.

[Ric Duley]

Plus I get to tell the guys that Mark helped me build it. :D ;) :p :cool: