Changing the sweep?

[gouch]

I have some foam cores i plan on fitting a couple of 400 pusher motors to.
The cores have been cut to give a certain sweep. If i cut the roots to give more sweep will this be a problem to the airfoil given it is cut at a set sweep?
I'm thinking of doing this to create a more rearward c of g and to give it some more stability? ( i'm under the impression sweep induces stability) I don't think there is any washout in the cores.

What do you think?

Cheers
Paul

[Captain Ed]

Paul, I think you'll achieve the your desired results with a slight sweepback. I've done that myself to correct a cg problem. It has a similar effect as moving a wing rearward on the fuselage. Stability should improve since sweepback has an effect similar to dihedral. The wing chord will increase giving you a greater cg range. Sounds good to me as long as you don't overdo it. Ed

[Ollie]

The down side is that the more you sweep the wing back, the more wash out you need to maintain tip stall margin. Untapered planforms can tolerate more sweepback than tapered planforms because the less the taper the more the tip stall margin.

[gouch]

Thanks Ed, that was my theory to correct the twin motor weight at the rear/ CG issue I would have.

Ollie, the cores don't appear to have any washout at all, they
are a reflexed type airfoil (if that makes any difference). Maybe i can persuade the washout with some carefull strapping tape.

They are made from EPS, is their another way to induce some washout in these wings?
The root chord is 290 mm the tip is 175mm with elevons counted, elevons are tapered ala Zagi style.
The std sweep is 135 mm back from root LE. at a half span of 640 mm.
Cheers
Paul

[Ollie]

See:
http://aero.stanford.edu/WingCalc.html
for an online calculator that will allow you to take the effects of sweep, taper and washout into effect.

How much wash out, if any, that you want depends on how you want to trade off tip stall margin for inverted handling. If you only intend to fly right side up and do positive G maneuvers then you may want 4 to 6 degrees of washout. What ever tip stall margin you get from washout while upright will be subtracted from the no washout case when inverted. Upright washout becomes washin for negative G maneuvers like flying inverted.

I have no idea how you might warp in washout with tape while avoiding unwanted distortions like spanwise curvature.

[Captain Ed]

An option to washout is to add some airfoil to the leading edge at the tips. Sand some foam to a shape that blends in with the existing airfoil and slide it over the existing leading edge. It can be taped or glued. I haven't done it myself but it's pictured in an RC modeling design book. The author added maybe 15% of the chord to about the last 30% of the leading edge.

[Ollie]

You could forget the washout and sweep the wing forward slightly. The lengthend nose would allow the desired CG without excess weight for the nose and the sweep forward would combat tip stall both upright and inverted without resorting to washout. you could increase yaw stability with a bigger vertical tail. The result would be something like the Bowman's Hobby JW. See:
http://www.bowmanshobbies.com/jw.html

[Sparky Paul]

I wouldn't worry about tip-stall and washout.
Both are overemphasized and take up way too much time considering what to do about.
Build it, fly it.
If it's a tip-staller, then considering raising the ailerons. If it isn't, fly it!

[gouch]

LOL, I love the logic, sparky!

I'll start building tonight!

Cheers
Paul