Covering
09-05-2002, 06:08 PM
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Covering
You will get some sagging between ribs no matter what....
The first defense is applying the covering with the "grain" running along the span. Monokote shrinks just a bit more along the length of the roll than it does across the roll. Apply the covering with the length of the roll running along the span of the wing.
Second, tecnique in appcation can reduce sagging. (and make covering MUCH easier...)
Don't cut the covering.... yet.
Start at the bottom of the wing, at the trailing edge (TE). (just hot enough to make it stick.)
Tack down the wingtip corner of the covering with enough hanging the rear to wrap 1/2 inch over to the top of the trailing edge along the span, and 1/4 inch over the top at the high point of the wingtip. (assumes a normal 45 deg bevel wingip of a trainer)
Pull tight along the trailng edge and tack down at the dihedral break TE.
Pull forward and inboard to remove maximum amount of wrinkles (but not too tight!) and tack down at the dihedral break leading edge (LE).
Pull forward and outboard, again to minimize wrinkles, and tack down at wingtip LE.
NOW you can cut the covering to size for the wing panel. Allow overlap of 1 inch at the center, and clear to the top of the leading edge.
Tack down starting along the TE at center, gently pullng out wrinkles, but avoiing pulling hard enoough to cause wrihkles along the LE. Then shift to LE center, then 1/4 and 3/4 along TE.... Going around the panel almost as if cross-torqueng a straight 6 cyl engine's cylinder head.
You may find that you need to pull the tacked corners free to remove some wrinkles...
gently iron down all overhanging coverng. (wingtip... just over the beveled lower surface, let the edges hang.) (You WILL have covering ironed onto the upper surface of the trailing edge! Its on purpose.)
Increase iron temp to the shrink temp for the covering, and iron down all the overhanging covering. Trim the wingtip covering to 1/4 inch out from the wood, slit many times between the LE and spar(or high point) to allow the covering to lay down and iron it onto the upper surface.
Don't shrink the main panel areas yet.
repeat for second wing panel.
Top... similar, except: No overhang over the TE or wingtip. LE wraps to bottom of the wing for at least 1/2 inch of double-coverage. And at some point along TE of each panel... don't seal down 1/2 inch width until you've finished the shrinking process. This is where the heated air gets out. (I normally leave a bit near a wingtip unsealed)
NOW, with all panels covered and sealed down, you can start shrinking. Work from bottom along TE toward LE, then top , along TE toward main spar. Seal down at main spar as you get there. (forward 1/3 of top will be saggy... at this point; looks bad.)
Heat gun prevfered for shrinkng the forward upper surface... but an iron held from touching the covering can do it. Yo are trying to avoid adhesion on the wing ribs as long as possible durring shrink. (allows maximum spanwize pull) But, don't shrink between ribs without shrinking the rest... that causes maximum sag.
*******
Then there's the "turbulator spar" trick. Yo can inset a 1/8 inch square stick along the wing appx 1/2 way between LE and main spar to aid in maintaining covering from sagging. Easiest to do f you cut the slots in ribs before building the wing.... Stack ris, cut all at once by sanding. (piece of sandpaper glued on 1/8 inch square stock and trimmed carefully to width. Stop sanding just before the 1/8 stock is flush... gotta allow for paper/grit thickness.)
****
Slight sagging in aft 60% of wing... ignore it. it won't ever be enough to matter.
The first defense is applying the covering with the "grain" running along the span. Monokote shrinks just a bit more along the length of the roll than it does across the roll. Apply the covering with the length of the roll running along the span of the wing.
Second, tecnique in appcation can reduce sagging. (and make covering MUCH easier...)
Don't cut the covering.... yet.
Start at the bottom of the wing, at the trailing edge (TE). (just hot enough to make it stick.)
Tack down the wingtip corner of the covering with enough hanging the rear to wrap 1/2 inch over to the top of the trailing edge along the span, and 1/4 inch over the top at the high point of the wingtip. (assumes a normal 45 deg bevel wingip of a trainer)
Pull tight along the trailng edge and tack down at the dihedral break TE.
Pull forward and inboard to remove maximum amount of wrinkles (but not too tight!) and tack down at the dihedral break leading edge (LE).
Pull forward and outboard, again to minimize wrinkles, and tack down at wingtip LE.
NOW you can cut the covering to size for the wing panel. Allow overlap of 1 inch at the center, and clear to the top of the leading edge.
Tack down starting along the TE at center, gently pullng out wrinkles, but avoiing pulling hard enoough to cause wrihkles along the LE. Then shift to LE center, then 1/4 and 3/4 along TE.... Going around the panel almost as if cross-torqueng a straight 6 cyl engine's cylinder head.
You may find that you need to pull the tacked corners free to remove some wrinkles...gently iron down all overhanging coverng. (wingtip... just over the beveled lower surface, let the edges hang.) (You WILL have covering ironed onto the upper surface of the trailing edge! Its on purpose.)
Increase iron temp to the shrink temp for the covering, and iron down all the overhanging covering. Trim the wingtip covering to 1/4 inch out from the wood, slit many times between the LE and spar(or high point) to allow the covering to lay down and iron it onto the upper surface.
Don't shrink the main panel areas yet.
repeat for second wing panel.
Top... similar, except: No overhang over the TE or wingtip. LE wraps to bottom of the wing for at least 1/2 inch of double-coverage. And at some point along TE of each panel... don't seal down 1/2 inch width until you've finished the shrinking process. This is where the heated air gets out. (I normally leave a bit near a wingtip unsealed)
NOW, with all panels covered and sealed down, you can start shrinking. Work from bottom along TE toward LE, then top , along TE toward main spar. Seal down at main spar as you get there. (forward 1/3 of top will be saggy... at this point; looks bad.)
Heat gun prevfered for shrinkng the forward upper surface... but an iron held from touching the covering can do it. Yo are trying to avoid adhesion on the wing ribs as long as possible durring shrink. (allows maximum spanwize pull) But, don't shrink between ribs without shrinking the rest... that causes maximum sag.
*******
Then there's the "turbulator spar" trick. Yo can inset a 1/8 inch square stick along the wing appx 1/2 way between LE and main spar to aid in maintaining covering from sagging. Easiest to do f you cut the slots in ribs before building the wing.... Stack ris, cut all at once by sanding. (piece of sandpaper glued on 1/8 inch square stock and trimmed carefully to width. Stop sanding just before the 1/8 stock is flush... gotta allow for paper/grit thickness.)
****
Slight sagging in aft 60% of wing... ignore it. it won't ever be enough to matter.
09-07-2002, 07:05 PM
#2
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Covering
I have found that if I avoid sticking the covering to the ribs until the shrinking is done, I get less sag.
When you have finished shrinking, go ahead and go over the ribs to ensure the covering has adhered if you want... but I've also found that the shrink temp makes the covering stick down just fine.
BTW... a cheap travel iron works just fine. I haven't bought a "covering " iron in 20 years of using Monokote. The shrink temp of Monokote is usually just barely above the minimum "Steam" line. (start a bit below... the thermostats aren't calibrated too great, but they are consistent once you know the setting to use.) A hair blow-dryer on hottest setting will barely do any good with Monokote... but it shrinks Towerkote/Econokote just fine.
When you have finished shrinking, go ahead and go over the ribs to ensure the covering has adhered if you want... but I've also found that the shrink temp makes the covering stick down just fine.
BTW... a cheap travel iron works just fine. I haven't bought a "covering " iron in 20 years of using Monokote. The shrink temp of Monokote is usually just barely above the minimum "Steam" line. (start a bit below... the thermostats aren't calibrated too great, but they are consistent once you know the setting to use.) A hair blow-dryer on hottest setting will barely do any good with Monokote... but it shrinks Towerkote/Econokote just fine.
