WMB

My Feedback: (1)

Hi all, I have a framed up LT-40 that I want to bash. I know if I build it stock, I will get bored with it. To make it more interesting I want to install a Sig 4*60 wing. The span, chord and wingspan are very close to the LT-40. I will need to cut the wing saddle for the correct mating with the new wing. Can I use a rib as a template to cut the fuse saddle? If I draw a line through the rib from LE to TE and make this parallel with the horizontal stab, will it fly ok?
Other mods planned.
Extend rear of fuse to taper to a point.
Convert to tailgragger.
Install two Saito 62? in wing.
Make the elevator a little larger and split it.
Extend the rudder to the bottom of the fuse.
Install floats at a later date.
I'd like it to be fairly fast with great verticle. I don't want soggy performance with the floats on it. I'm really liking my Saito's. I would also like it fairly aerobatic when called upon, no 3-D.
I guess my biggest question is the wing incidence and how to get it right the first time.
Thanks, MikeB

HighPlains

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I experimented years ago with a buddy's Eagle 63 years ago with a wing swap. I put a small (400 sq in) wing on it. Most trainers have a bunch of decalage built in, with positive incidence on the wing and loads of down thrust. Since I didn't reduce the down thrust, I needed to maintain the postive incidence on the wing.

Another experiment on another trainer (also an Eagle), we removed the covering from the bottom of the wing and grafted on ribs to the existing ribs to make a semi-symetrical wing.
This flew pretty well, with the original incidence. Flew much better inverted.

I'd put the wing about 1 degee positive to the tail, and decrease the down thrust to about 1 degree. Fly and adjust the down thrust based on flight test.

WMB

My Feedback: (1)

What is decalage? I had thought about adding material to the bottom of the ribs of the stock wing, but thought the wing would end up too thick.
Thanks for the info

HighPlains

My Feedback: (1)

The relationship of the wing, stab, and thrust lines are the decalage.

Most trainers are set up with some degree of self recovery, which requires a force arrangement that tends to lift the nose when the airplane has a speed increase. A lot like most GA airplanes.

Ed_Moorman

My Feedback: (1)

Use the 4*60 wing, but clip the tips one rib. As big as a 4*60 wing is, you may need to take off 2 ribs. The fuselage length should be around 75%-80% of the wing span. Measure the fuselage length, then divide by .75 and .8. The span should be in this range for a sport lane.

Also build the wing flat, no dihedral.

If you don't have an incidence meter, after you make the cutout for the wing saddle, get a small level and put it on the stab.
Prop the tail up until the level bubble zeros. The stab is now level.
Measure up to the exact center of the LE and also to the exact center of the TE. These two should be exactly the same measurement. Check the level to make sure you haven't moved the plane.

WMB

My Feedback: (1)

Thanks for the info! Planning on getting this ready for next year. Have a couple others to finish up.

WMB

My Feedback: (1)

"Also build the wing flat, no dihedral."

I'm wondering about putting a slight amount of anhedral in the wing. Will this effect how it flies or just look different? I would like it to have a decent knife edge if possible.

Thanks

da Rock

Whatever rudder roll couple is caused by dihedral, you could expect anhedral to do but in the opposite direction for the roll. How much? About as much.........

There was a $40 trainer sold for a month or so last year. Basically an LT-40, EagleII, etc etc but only $40. One of our guys got a dozen of them and we all slapped them together. A couple of us did the wings with anhedral. Easy to do since the wing joiners fit so slopply you could do whatever you wanted with almost no effort except for the rib matchup. Mine fought every turn the pilot wanted. Add rudder and it'd fight harder.

How to predict how much of what is going to happen with your combination of random parts? WAG

Ed_Moorman

My Feedback: (1)

A high wing location is the equivalent of about 3 degrees of dihedral, so you need 3 degrees of anhedral to counter the wing location. With regard to rudder roll coupling, the following are the cases:

High wing, sweep back and dihedral cause roll in the direction of rudder
Low wing location, sweep forward and anhedral cause roll opposite to the rudder.

This being the case, you need dihedral or sweep back in a low wing plane and you need sweep forward or anhedral in high wing planes. I have used anhedral for years on high wing planes. I am attaching a few photos.

If you build a low wing plane with no dihedral or sweep, it will roll backwards to the rudder. I have shown this several times to people who built planes that way.

If the LT-40 had a 60 inch wing, you'll need 1 5/8" anhedral on each side or 3 1/4" total. The anhedral on one side is half span times sine 3 deg. You'll need a scientific calculator.

The Big Stick 40 does not have enough and still rolls with rudder. I got lazy and just flipped the dihedral brace over without checking.

If you ever do try anhedral, you'll get lots of comments from people who don't have a clue. One person who hadn't seen one of mine fly, came up and told me not to fly because it was unstable and was going to flip over in flight. I went ahead and flew. If you ever have any ideas like this, turn a 4*40 or other low wing plane over and look at it. It becomes a high wing plane with anhedral.

buzzard bait

OK, roll coupling can be eliminated on low wing plane with dihedral, so logically you can do the same with a high wing plane using anhedral. But what's with the little black mid-wing in the second pic? Jim

da Rock

Are you sure of this?
Dihedral is a major cause of rudder-roll couple. When the rudder yaws the airplane, the advancing wing's dihedral angle gives it increased lift and it does so. The retreating wing's diherdral causes it to lose lift and it rolls down.

buzzard bait

As someone noted above, a high wing plane will have a dihedral effect even if the wing has no dihedral at all. Picture a skid induced by rudder: pressure builds under the outside wing, which will raise it, just like dihedral does = positive roll coupling. So if you want to cancel it, use some anhedral.

Conversely, consider a low wing plane with no dihedral in the same circumstances: pressure builds on top of the outside wing, which will lower it = negative roll coupling. So if you want to cancel it, use some dihedral.

Lots of people seem to think if reducing dihedral on, say, an Astro Hog, is better, then eliminating altogether is best. But then you wind up with a plane that has negative roll coupling.

Jim

da Rock

You're talking about roll effect and roll stability. That's not roll couple.

A low wing without dihedral has negative roll stability. Add dihedral and you can balance out that roll instability. And the airplane can be designed to have neutral roll stability by giving a low wing the right amount of dihedral. But if it's got dihedral, and you yaw it with the rudder, it will tend to roll in the direction of the yaw, which is rudder roll couple.

buzzard bait

No, a low wing plane without dihedral, besides being unstable, will also have negative roll coupling. See above.

da Rock

roll couple

roll stability

Two very different things.