Design > Stunt design
Flap trailing edge shape?
Mike Scholtes:
I did a search but can't find a discussion of current thinking on flap TE shape. The fact that Chris Cox is apparently using a square TE makes me think there is more to this than I used to. The options are (1) tapered thickness from flap LE to the TE, the "classic" shape; (2) rounded TE but keeping same thickness from LE to TE; (3) square TE with same thickness from LE to TE, a rectangle in cross section. I am at the point of finishing the flaps on a 660 inch modern stunter so need to make a decision. One advantage of squared TE is that it can later be rounded if performance is not as hoped.
We are basically locked indoors here on the West Coast so I may have more time to work on model stuff. I am one of the fortunate few who actually can work from home, which leaves me more time than I used to have.
Air Ministry .:
Good stiff quater grain sheet , flat . is stiffer than good stiff quatergrain tapered 'V'eed .
You see all out here . Looks kinda unfinished Square Edge . Specially with a few paint drips .
The Caudron / Hurricane , Flat 1/4 sheet ( think about 5/16th - 8 mm on a 60 ship , onless the 1/4s rock hard )
V front at hinge , round - Half Round actually LL~ Rear Edge .
The theorys they interact sooner by the angle of a tapered flaps angle , so fly smoother - more effective .
And any extra drags to ones advantage . Someones on about Sq rear edge , on the main forum maybe .
Using 40 / 60 / & 80 weight , sandpaper on flat block , to rough out tapered flaps , these days . Good'n dusty .
H^^
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Doing a tapered one ball pen on bench & run flap edge past it , both ways up . Get twoo lines , maybe 1 mm or 1/16th in. apart .
If youre lucky . You can even use black & red pens , get two black with a red on Center spliting them . or Suchlike .
Then I draw a 1/2 ( or 2/3 ) chord line . Block off / sand to outer rear & half chord lines . Both Sides .
Go & have a coffe and rearage the brain cells & eye sight , in case somethings cheating .
Return , eyeball along , hold up to ligh . etc Etc .
THEN . . . . . . Mark at say 1/2 inch back from the front . Not Less ! Nuther line spanwise half or 2/3 fwd on new flank .
MASK the front 1/2 inch . so as not to intrude on its exactude .Could even do it on the rear 1/2 flank too .
Block Sand Down , cross wise mainly , Double Cross Hatch with the 60 on a longish block . Lengthwise with 80 on a long block .
:P
So you get a thicker stiffer flap at half chord , with a uniform taper . 2 or three swipes on the ' creases ' and theyre gone for ever .
Often sit on a styro fruit box on the bench to raise to eye level . Or on a cold day you stick bent over head at bench edge hight . LL~
Mark the eqaul thingos for L E cut , Coarse pare on a long block . Carpenters joints free so as excess pressures not applied
to sanding block or razor plane . :(
I leave the flank Fwd around 1.5 deep , till final sanding - radius then .
BEFORE you do ANY whittling , A piece of say 1/8 ply 1/2 wide , to rest a blade on , to thin slot the hinge recesses . then the steak kinfe.
Then the 6 in steel rule end . In 15 mm . :P
Thisisso theyre all SQUARE and CENTRAL and suchlike . Or its between the knees sitting down , to do em , or clear the dope out ,
with the flaps shaped . Carefull to leave / reistae the centerline & other marks whilst theyre still required .
Right. that deserves a COFFE . But Im at the Library . If anyone thinks theyre getting five dollars outta ME for a COFFE , theyre sorely
deluded . >:(
Matt Colan:
I leave my flaps square at the end. Frank Williams told me once that with a hard, squared off edge, the air will continue off the end of the flap before it drops off and gives the effect of a larger flap. My flaps are also not tapered, 1/4” boards beveled are the leading edge. I hate tapering flaps and once I heard from Paul Walker there is a benefit to straight non-tapered flaps being stiffer than tapered, I never looked back. My last two airplanes have been like this and my new one as well
Ken Culbertson:
--- Quote from: Matt Colan on March 24, 2020, 07:59:08 AM ---I leave my flaps square at the end. Frank Williams told me once that with a hard, squared off edge, the air will continue off the end of the flap before it drops off and gives the effect of a larger flap. My flaps are also not tapered, 1/4” boards beveled are the leading edge. I hate tapering flaps and once I heard from Paul Walker there is a benefit to straight non-tapered flaps being stiffer than tapered, I never looked back. My last two airplanes have been like this and my new one as well
--- End quote ---
To some degree I think it depends on the plane BUT recent experience with my Twister (RIP) which I elected to keep slab may be why it cornered like a big ship. My latest PA ship (RIP) which cornered even better had extremely stiff rather small 3/8" flaps with a 1/8" flat back. I am 100% on the flat back for aerodynamics but think that the "slab" simply adds stiffness. It also adds just a bit of additional under camber and slightly less on the top than tapered flaps. The rocket scientists among us can let us know if it is the shape of the upper or lower surface that matters.
Ken
Ted Fancher:
Hmmm? Tapered versus not tapered flap cross section???
I'm curious what aerodynamic factor one expects to be affected positively or negatively in terms of improving the performance of the stunt pattern of the airplane thus equipped? ...and why?
By far the most important question that occurs to me.
Ted
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