Hi,

I'm building my third plane now (sig somethin' extra) and got to the point where I need to sand some edges.
I was wondering, is it really essential to sand the trailing parts of the ailerons, elevator, rudder etc. round?
I like it it better when it's square, because it looks better, easier to cover and less sanding [img]i/expressions/face-icon-small-smile.gif[/img]

So is it?

greetings,

Richard

There is an ongoing debate about what trailing edge shape is best.

The squared off trailing edge seems to assist in preventing control surface flutter. It doesn't cause significant if any) added drag at the typical R/C model flying speeds. So go ahead and leave them squared off if you desire. (do sand off the edge a bit to remove lazer burn, which some colors of covering won't hide.)

ok and what about the leading edges of the fin and stabilo? Could I let them square too or will I get in trouble
flying my plane when I do that?

thanks,

Richard

You can get away with the square LE on the tailplanes... its not a god idea, as it will mean increased drag. I've seen several planes wth the flat slab tailplanes left with squared LE's that flew OK at low speed... but they had a tendancy to having rudder/elevator flutter at high speed.


You could easilly get away without rounding the tailplane LE's on a trainer... I wouldn't do it on the SE.

Rounding the edges on a tailplane takes very little time in comparison to the the rest of the time put into a model so why risk it, even on a trainer. I use a medium grit 3M sanding sponge or medium grit sandpaper (80-120 or so) on a block and it only takes a few minutes to complete.

I agree with leaving the trailing edges square provided we are only talking 1/16 to 1/8 inch. A 1/4 inch seems a bit blunt to me and, as indicated above, rounding only takes a few minutes.

Having said that, getting things rounded evenly so they look nice takes a little care, perhaps a little practice, good lighting, and a sanding block with enough grit to get the job done fairly quickly (but not too quickly) but it isn't that hardest or most precise thing you are ever going to do when building a model.

I've found that square edges on controls increase their effectivness...or at least make them "wilder" In a not so scentific test I built a Tidewater Pronto a long while back. They tell you to leave all edges square. Not knowing any different at the time, I followed the directions [img]i/expressions/face-icon-small-happy.gif[/img]. A flying buddy was building one too, but "knew better" and rounded everything.
Talk about 2 different planes.... Mine was very snappy, spun very well, etc; all around a great flying plane. My buddies on the other hand was sluggish. We matched control throws, ballance points, etc. His never flew as well as mine.
To this day, I leave the edges square on "snappy" models, and round them on more sedate planes.

Just my $0.02

Is this about having trouble doing a good covering job on the rounded corners? 'Cause doing an excellent job of that is not hard once you've done it.

Sure the plane will fly with square edges. It won't even look or behave very much worse... just a little on both counts. I'm just wondering why somebody'd want to leave them square. If it's a matter of needing info on shaping or covering or some other technique, sharing is what we're here for.

Looks like the square/rounded, fin/rudder, stab/elevator has a lot to do with what the bulder likes. A square edge is easy to cover if it is straight, not so if curved. I like rounded edges, I think it just looks niced.

Jerry

I'm currently bulding my first plane, and I really was nervous at first of not being able to round the edges, but I found out it was really quite easy. Here's my technique, and it takes only a few minutes to great effect:

I have a few sanding blocks, store bought. They are fairly small (about 5 inches by 3 inches, found them very cheap in the painting section of a homedepot-like store).

First, I'll use 220-grit sandpaper on one sanding block, and run the sanding block in a curving motion perpendicular and/or a little diagonally to the leading/trailing edge. Note that I will do only one side of the edge at a time, and that I run the sanding block on it's entire long end (the 5 inches). After a small portion of the leading/trailing edge is rounded off a bit, I move the sanding block a little to the side and do the same thing.

Continue doing this until one entire side of the stabilizer or elevator is done, then turn it around and do the other side.

After this is done, I use the 320 grit sandpaper on a second block to lightly sand any irregularities in my sanding.

And finally, I use some more 320 grit sandpaper, not mounted on a sanding block, wrap it around the edge tightly, and run it down the length of the edge. This smoothes it out completely.

Oh. another point, if I have some really hard balsa (like my current elevator), I'll use 150 grit instead.

Anyway, I hope this makes any sense, it's rather difficult to explain in words without seeing it done. (and I've never been good at explaining things!) I guess I've become rather proficient at sanding the edges, this is my third stabilizer/elevator afte really screwing up the hinge slots. I'm hopeless in that department [img]i/expressions/face-icon-small-smile.gif[/img]

As you are sanding along the lenght of the leading edge, you may need a few extra short strokes on the ends as they get short changed just a bit when sanding and can end up a little higher than the middle if you don't correct for this.

Sanding at a slight angle along and around the nearly finished leading edge can help get an even smooth curve. I'll also use a fairly soft medium grit sanding sponge rather than a block as you can control how much you are taking off by how much pressure you apply while also allowing the sponge to conform a bit to the curve which eliminates any flat spots.

Sighting along the leading edge in good light will reveal any high or low spots that may need to be addressed. The hardness of the balsa often may not be consistent along the entire leading edge and you may need to sand the hard areas just a bit more to keep things even.

When sanding a wing leading edge the exact shape is not critical, but you do want a smooth curve transitioning from the upper and lower surface of the ribs or leading edge sheeting and you want the stagnation point to be in the correct location as shown on the view of the airfoil section. This is what determines the front edge of the chord line and if it is too far above or below the intended point it will effect incidence and angle of attack. Also don't let the leading edge get too pointy, particularly on a trainer, as the airflow stays attached better on a wing with a slightly blunter leading edge at the low reynolds numbers involved on an R/C aircraft. Too pointy a leading edge will increase stall speed and in some cases cause a sharper stall break to occur.

Leading edge should always have a radius. On some models, like trainers, with large fin and stabilizer, you might get away with square L.E.

Trailing edge, however, should be square to reduce flutter on fast models (control surface oscillation).

Ideally, fin and stabilizer should not be flat but have real airfoil section (symmetrical, like NACA 0012 or something).

This does not only reduce drag, it also makes rudder and elevator more effective.

You will need a effective rudder on a 2-engine airplane, one engine may stop and you might stall a poorly designed fin if it is not very large.

Some models requires a lot of elevator to raise nose during landing flare. You will need a effective stabilizer for that.

The picture shows an old Airplane from passed times, however capable of Mach 2+. Look at the trailing edges !

/Bo