Need help with wing Thickness questions, and other tips

[morphideous]

I'm at a point where I think it would be wise to get some help from fellow modelers...

I've been scratch building a Cessna 310Q, but tweaking the design some for my style of flying (And having fun designing and trying new things) Instead of the normal split flaps used on a 310, I've designed my wing to use slot flaps. I've been unable to find information that states what should be done with the max thickness of the wing though when using this type of flap. The chord is 7 inches with flaps up, 5 inches with flaps down. The wing is about 1 inch thick, give or take a 1/16th of an inch. This puts the max thickness of the wing at about 18%-23% of the chord, if we're going off of the 5 inch chord when flaps are down. The max thickness is about 14%-16% if the flaps are up and we go off the 7 inch chord.

In my research, it's rare to see a max thickness of a commonly used wing over 16%. The model aircraft aerodynamic books I have, and a few full size aerodynamic books have plenty of NACA wing designs with large thickness, such as 21%.

What I'm wondering is if I need to lower the thickness of my wing. I was planning on doing 16% thickness, which it is about if we go by flaps up. When I lower the flaps, the wing just looks too thick. :)

I'd like to know what other models feel is a good choice in my situation.

My flying style for this plane will be slow and easy. The wing is 42 inches wide, the flying weight is about 20oz (Don't have a scale, just another plane that's about 19oz and this on a see-saw balances close to that) The wing tip chord is 5 inches(Including ailerons). I don't care how fast the plane can go (30mph is more than enough), as much as I care about being able to fly it slow on landings. The motors put out about 14oz of thrust each (8x4 prop, bp21 motor on a 2S 1800Mah lipo).

My other concerns are stall tendencies with a thick wing, or other bad flying characteristics.

I'll attach some photo's to show how the wing currently is designed, and hopefully give you an idea of how it looks a little thick.

Another thing, the bottom of the wing is flat, and I'm wondering if I should keep it flap, or taper it up in the back, or taper down. If you look at the picture taken from the side, you can see the flaps are rectangular, and look a little odd because the wing curves down to the flaps, then go straight out for 2 inches.. How big of a problem is this, and how should I fix it?

I'm also completely open to _any_ other ideas or tips on the building of this plane.

Thanks for the help. It's been a fun project and I'm excited to getting it to the flying field to see how it goes..

:)

[BMatthews]

Due to the scale effects of how air flows at our speeds and sizes wing thicknesses of much over 12 to 14% start resulting in more drag and less efficiency.

[morphideous]

Do I go off of the 5 inch chord when the flaps are down, or 7 inches when flaps are off to get the 12%-14% max thickness?

[Sparky Paul]

The cambering due to flap deflection doesn't affect the thickness.. that's determined by the thickness of the around the spar area, usually.
The precentage is your 1 inch thick, 7 inch chord value.

[green66]

I choose wing thickness based on: A. the required bending strength, and B. the desired stall resistance....... considering both against the add'l drag at higher speeds caused by more thickness.

For a given structure, sheeting thickness, etc, bending stress varies inversely with the cube of thickness. To illustrate, if thickness is decreased from, say 15% to 13%, that's 13/15 = 13.3% reduction, which will mean .867^3 = roughly 35% reduction of bending load capability, assuming the sheeting doesn't fail in buckling first. The point here is not to get reckless thinning a wing if you're not sure of the loads it'll encounter.

More thickness usually broadens the drag bucket, up to a point as BMatthews points out, so decambering the inboard portion of span can also be considered to avoid tip-stalling tendencies.

[morphideous]

Thanks for the info.

I just want to make sure once more, since no one has said this out right regarding slot flaps vs plain flaps. By using the slot flaps (Which create a half inch gap between the back of the wing and front of the flap) I don't go by just the chord up to that gap, but I calculate it the % thickness from the chord that goes beyond the gap and includes the flaps. The downside is when the flaps are out, the thickness would be about 22%, and I can't find any documentation stating how the thickness, when slot flaps are out and the gap is there, should be calculated. It wouldn't be a good thing if the thickness is too much and the wing is so thick it loses a fair bit of it's lift right when flaps are put out. ;)

[Sparky Paul]

Section thickness doesn't change with flap.
An excerpt from NACA 824.. Summary of Airfoil Data, pg 46
All that is accounted for is the flap deflection angle on lift.

[green66]

Forget about the flap deflection. Non-deflected is the condition to base chord on for thickness calcs, i.e. use your 7" chord. When you see polar data for airfoils under flapped conditions, the thickness is based on the chord of the non-deflected airfoil.

[BMatthews]

Just a thought but if you're using a flap design that produces a gap as it extends then be sure to use some form of soft gasket so it seals when raised to the flat position. Air flowing up from below to the upper surface will result in a loss of lift performance.