Wingtips, Washout, and other issues...

[astroboy]

The saga continues:


1)Ollie, in my recent thread on wing structure, you elaborated on how spar components can taper and thin as they approach the wingtip. Do the same principles apply to the wingtip itself? By the time the plan of my current project was scaled up, the wingtip material was 7/16" wide and 1/4" thick. If the loads on the wingtips are minimal, I had in mind something more like 2 bent laminations of 1/16" strip, with maybe a third lamination near the LE.

2)Let's try to save some more weight. The plan has many wing ribs, about half of which I was going to eliminate. Do ribs add much structural strength to the wing? Regarding concerns about covering sag, bear in mind that the wing will be sheeted, top and bottom, back to the spar, and some ways beyond that on top.

3)I intend to install access panels for the aileron servos. I'd like to build these as light as possible so I'm wondering what sort of forces are exerted on them. In other words, does lift consist of positive pressure from beneath the wing, or negative pressure from above?

4)Since the model has elliptical wingtips, I was going to add some washout. Looking at the ribs, below, it looks like it's built into the ribs. Either that, or the airfoil is changing, or--both?


Thanks,
Jeff

[Ollie]

Jeff,
The answers to your questions have to be put in context.
1. What is a rough estimate (target) for the finished flying weight of the model?
2. What is the wing span?
3. What is the wing area?
4. What is the fastest that you think the model can fly.
5. What will you power it with?
6. What rib spacing are you contemplating?
7. What is the purpose of the model in terms of flight characteristics?

[Viper Pilot]

This is gunna be good!!!

I think this will be a great learning thread.

Waiting in anticipation!!!

:cool: :cool: :cool:

Viper

[astroboy]

Originally posted by Ollie
Jeff,
The answers to your questions have to be put in context.
1. What is a rough estimate (target) for the finished flying weight of the model?
2. What is the wing span?
3. What is the wing area?
4. What is the fastest that you think the model can fly.
5. What will you power it with?
6. What rib spacing are you contemplating?
7. What is the purpose of the model in terms of flight characteristics?

Holy kershmoly, Ollie--you're gonna make me work for it! Well, all right:

1) The target weight is 37 ounces, though, based on my latest estimates, that may be a bit optimistic. 38-39 ounces shouldn't be a problem.

2) 43 inches.

3) About 345 square inches.

4) The top speed of the prototype(P-26) translates to about 30 mph. I wouldn't expect, or want, more than maybe 35 mph.

5) Aveox 27/13/4, geared 3.6:1, 8x1700 Sanyo Nimh. The most likely prop will be an APC 12x10E. The power system just arrived the other day; I'll be bench-testing it this weekend.

6) The model's wing will have a flat center section 8.5" wide, with four ribs, or five, if I put one in the center. In the outer panels, the spacing will average three inches, with two exceptions: As the wing thins toward the tips, the spacing will be 1.5" with 1/16" thick ribs, as opposed to 3/32" for the others, to better define the shape. The real question is between the end of the center section and the inboard egde of the ailerons. The plan shows two ribs between those points, and I'm wondering if I can get away with eliminating both of them, which would mean a spacing of 3.875." I thought perhaps a compromise of one 1/16" thick rib.

7) I'd say a little better than scale-like, meaning fairly aerobatic, but I don't care about 3D or unlimited vertical and the like. The benchmark in Motocalc is "at the best lift-to-drag ratio airspeed, the excess-thrust to weight ratio is X." I'd like X to be .4 or thereabouts, where Motocalc indicates performance is quite good.

Hurry back. Viper Pilot is eagerly awaiting your reply. :p

Thanks,
Jeff

[Ollie]

I would go with bent laminations for the wing tip. Because the grain is running in a favorable direction, the crossectional area of the tip can be made smaller than with straight grained construction. Because the crossection at the trailing edge is triangular, the width should be about 2/3 of the original design, four or five laminations of 1/16 balsa. If you use water thinned Titebond and medium balsa, you will save about 1/2 the weight of the original sized tip. Don't forget that the unsupported tip has to resist the covering tension.

The structural contribution of the ribs is to hold the shape and to prevent the top and bottom sheeting from splitting along the grain when stressed. The weight of the ribs is much less than the weight of the top and bottom sheeting. I would use contest balsa for the sheeting and and ribs with a rib spacing closer to 2.5 inches. The weight of a couple of extra ribs can be off set by selecting low density wood for the sheeting.

The covering in the open bays of the wing doesn't deflect much due to pressure associated with lift forces, so I wouldn't worry much about the the strength of any access hatches for the aileron servos. If you really want to save weight, install the servos before covering and if you need access later, just cut away the covering of a rib bay. When the servo repair or replacement is finished, just recover the rib bay.

An elliptical planform without washout tends to produce an elliptical lift distribution. Elliptical lift distributions have minimum induced drag. When washout is employed in such a wing, particularly one with an airfoil of low mean camber, at normal air speeds and above the wing tips can lift down instead of up. This not only reduces lift but increases drag. On the other hand, the washout increases tip stall margin when the plane is slowed in the landing approach. It is your call, which horn of this dilemma you want to sit on.

[Sparky Paul]

Just being curious.. this is the P-26 you're doing?

[astroboy]

Ollie:

Thanks for the reply. The only reason I'm reluctant to just cover over the bay containing the aileron servos is that the model will be painted(including "US ARMY" in big letters across the underside of the wing), so it's not as simple as just patching up the covering if the servo needs attention. I suppose, though, that it is pretty expensive insurance, weightwise.

As to your last point, are you saying that if I wanted to maximize lift at the expense of tip stall safety, that I should rotate the trailing edges of the outer ribs down a bit(somehow, I think the answer to that one is going to be that simple)?

Paul:

That's it. With all those wires and masts, and that radial engine, you can see why I want a big, slow-turning prop. I'm also going to cheat the landing gear out, and maybe back, some. I can just see me doing nothing but cuttin' cookies with this one.

Jeff

[Ollie]

Washin (tip TE down) won't increase lift much because the tips will stall before the rest of the wing reaches maximum lift. Better to leave this wing untwisted (up or down).

Paint can be very heavy. Consider film covering.

[Viper Pilot]

Ollie,

Sheeting question.

Is 1/16" too much for a plane this size??

Or would 1/32" ply (obichi, I think it's called) be OK - cost not a consideration ??

Viper

[Ollie]

Contest balsa (5 to 7 pounds per cubic foot) will be much lighter than 1/32 birch ply or 1/28 obechi veneer. Because obechi veneer does not lie as flat as balsa it is usually reserved for foam core wings where it is glued down all over. A piece of 1/16 X 4 X 36 inch contest balsa will weigh about 1/2 ounce. A piece of 1/28 X 4 X 36 obechi will weigh a little more than an ounce. A piece of 1/32 birch plywood 4 X 36 inches will weigh about 1-3/4 ounces. A piece of 1/64 birch ply 4 X36 inches will weigh about 7/8 of an ounce. The 1/16 contest balsa will make the best sheeting in this case.

[Viper Pilot]

Ollie,

Thanks. That's exactly the info I needed.

VP