I've read many trainer ads from various kit manufacturers whose trainers include flat-bottom airfoils, plenty of dihedral and "built-in washout".

I've never built a model that had built-in washout, and I was simply wondering how it is incorporated into the design of the wing. How is washout built into the wing? Do the ribs vary in shape from one to the next to give the wing somekind of twist to it? This is what I was wondering because I don't see how you can build a wing (especially if flat-bottomed) over the plans, flat on a building board.

Any light shed on how "built-in washout" is built in will be greatly appreciated.

Thanks in advance,

Dan

some kits will number each rb progressively from root to tip, and have a little "tab" on the ribs at the trailing edge. The tabs get larger as you progress toward the wingtip.

The GP PT-60, you start building it straight, and incorporate the twist as you apply the wing sheeting there are plywood guide pieces to brace the wingtips on as the sheeting dries, thus locking in the washout. (similar to the 1/2 inch wedge idea... but its closer to 1 inch of washout on this kit's wing)

(that's the 2 most common ways)

You can put the washout in durring covering if the wing is un-sheeted... just twist as you apply the covering and shrink it. (harder to get it even this way)

Washout effect can also be done aerodynamically with a changing rib shape. Just changing the LE from sharp at root to very round at tip will give a bit of the stall characteristics desired in a trainer.

The previous comments are correct. "Jig tabs" and other means are used to build a precise warp into the wing.

Washout of course refers to a wing that is twisted in such a way as to place the LE of the outer portion of the wing at a lower AOA than the root. The idea being to have a wing that stalls at the root, migrating outward toward the tip. Lateral stability is the goal...a model is less likely (in theory) to flip over on it's back when stalled.

This stuff works great with full size aircraft, but seems to me that in models..."black magic" prevails.

I've seen guys show up with SQUARE leading edges, bowed wing panels and you-name-it, and the model flew fairly well. ????? Go figure. Further, the build quality of a modelers first attempt or two is often so poor....aerodynamics go right out the window. And we're worried about wash-out?

rant complete....


Fly safe!

Ollie...

You have a point in the latter portion of your post. It does matter, however...

I think we're dealing with a trainer here...

I'm trying to find someone who can SPELL "aerodynamic" !!!


"I hate ARF's"

depending how much washout you want ,get a strip of bulsa or what ever and taper it from zero to what washout you need the zero is from the root of the wing and the washout is at the tip , just remeber when you placeyour washout template under your wing it is done from the trailing edge of the wing and not the leading edge or the centre of the wing eg from the root zero at the tip 1/4 inch hope this helps claraifys this the tip at the back is tilted up

Back in the 1970s our oldest club member always had a Sig 71" Wing Span J3-Cub that he built from a kit. It was a gorgeous plane to watch in the air. Unfortunately, he crashed it very frequently. This usually happened during the landing. All of a sudden the plane would snap roll into the ground. A few years later I purchased the same J3-Cub kit plane that another member had built after I totaled my only plane. I was amazed at how easily the plane would snap roll during the landing. I was determined to solve the problem, but my only focus was to fly it better. After 4-6 bad crashes due to snap rolls into the ground, I lost my desire to keep rebuilding it. The plane gave absolute ZERO warning of a coming snap roll during the landing. Well, I could not get that problem out of my mind, so I built the same Sig 71" WS J3-Cub from a kit. That is when I realized where the problem was. This model has the weakest wing imaginable. The wing twists very easily. The plane must not be flown without the struts. I have seen one fly without struts and one wing folded during a loop. The struts were very strong. The top of the cabin(or the bottom of the wing), upon which the wing sat, was NOT a solid, rigid structure. After a number of hard landings the top of the cabin(or the bottom of the wing) could "crush" little. When that happened, the struts could "push" up and twist the wing into a washin configuration resulting in a snap roll demon. I therefore built the struts to have adjustable ends where it attached to the wing bottom. After I assembled the wing to the plane at the field I would check the washout at the wing tips and adjust the strut ends if needed. Not one time did I ever have an uncommanded snap roll with this plane, not matter how slow I flew it. Eventually I totaled this plane when the nylon clevis to the elevator opened during a dive and the plane went straight in. I then switched to steel clevises with locking clips made by Sullivan. I now have a SPAD 71" J3-Cub that I designed and built, and it uses the same wing struts I used on my last Sig-kit J3-Cub. I check for washout every time I assemble the wing to the fuselage. I must confess that I did have two uncommanded snap rolls with this plane. I had not flown a 71" J3-Cub in about 4 years and had completely forgotten how to take one off safely. I was giving it full throttle and jerking the elevator for a takeoff. A J3-Cub will not accept such abuse and will snap roll in rebellion, like a spirited stallion. After"it" taught me again how it needs to be taken off, I have not had one uncommanded snap roll. I'll have it in the air in about 3 hours. The sky is blue, about 50 degrees, and wind under 5 MPH. The perfect flying day. I think everyone needs to have a large J3-Cub. It can teach you what a lousy pilot you have become.

Full scale Cubs had about 3 degrees of washout. Probably a good idea on the models too.