Airfoils With Vs. Without Stationary Flaps

[Serge_Krauss]

Another curiosity, FWIW post...

I've been theorizing about the sheet wings over on the current Design Board thread, particularly concerning the apparent need for large amounts of out-thrust on the Australian flapped stunters that seem to work so well. I haven't come to terms with that yet, other than thinking that they might be operating at a greater AoA higher up. Looking through old NACA reports though, I found no evidence for sufficiently violating theory in comparisons of wing sections varying from 6% to 18% in thickness. However...

In looking through that stuff, I ran across another bit of historical research that tends to support our findings that a wing with stationary flaps seems to have a better coefficient-of-lift curve than one of the same wing section ('airfoil'), plan-view shape, and area without them. I posted in SSW Forum and here what X-Foil showed with my favorite section (higher maximum lift with gentler stall), and I'd also seen in another earlier NACA report how they found similar advantages for undeflected thin flaps. Now I see that in TN 763, they compare the old NACA 23015 with a plain flap to a 23018 section with a Gwinn flap (very thin) extension, giving overall thicknesses of 15.0% and 14.4% respectively. These are not symmetrical ('stunt') sections, but I might expect them to show pessimistic values because of their flatter bottoms. So when they show an advantage, it might be smaller than what we would see with symmetrical sections. Even though NACA liked the plain flap best, I like the thin Gwinn best at zero deflection, which is equivalent to what we use on our flapless models.

In the graphs, the zero-deflection curve is the one in the middle of all curves (the 4th of 7, counting from top or bottom. I don't know whether the graphs will appear large enough below to read the data, so I'll summarize what I found.

1) CL Curve Slopes (zero Deflection):  Plain Flap (plain airfoil shape) - less than .07,   Guinn Flap (thin extension) - greater than .075

2) Maximum Lift Coefficient:    Plain -  1.09,      Gwinn -  1.19

3) The Gwinn configuration had a bit more drag.

The NACA "five-digit" sections, most usually the 230xx series wing sections, were popular during the late 1930's and into the 1940's. I believe they were used on some WW II aircraft, including the Corsair, B-25, Lancaster, Helldiver, FW-190, P-38, Wildcat, Avenger, Hellcat, Bearcat, Guardian, Halifax, Me-410, .. They remained used in private aircraft like the King- and Queen-Airs, Bonanza, Staggerwing, Aero Commander, Baron, Cessna Citation (!), Helio Currier, and many homebuilts, including all the RV's through #8. They had little c.p. motion with their far forward "high point" and therefore were popular with flying wing designers. 'pretty popular sections. That's the FWIW in the FWIW.

SK

[Tim Wescott]

Interesting.  Thanks for posting that.

[Scientifiction .]

Intresting to see the 60 Deg. deflection . Presumably at 0 Deg. A.o.A.  ?  

What the actual angle of attack IS in Loops and our Five Foot Radius ' square Corners , would take at least a fixed posn. slowmo camera , or telemetry . ?  

ditto etc . tempted to throw Pic of my Yak here , with ' ice cream cone ' airfoil . Was Light 43 odd Oz. ' before shipping ' . unfortunately trashed .
Tests at a rearish C. G.  ( fairly touchy as in required a steady hand ) had it poping a tight clean corner if you wung over /   ,
and hit Full Up at the 15 Ft. Mark saw it well clear and flat ( no bounce ) at around ten foot ish .

repairs saw it wrentaWreckish , and touching 50 + Oz . ( its 53 odd span , ish . 53 or 4 ) . Scaled at 40 / 60th area for a 40 from My original 60 drg. of 192000.
Id left the U/C (  undercarrage / chassis ) off , as retacts on a Yak fighter are perpendicular to the pintal ( pivot ) .
 the Sym. section 'd crank the leg  fwd , just . down

Airfoils With Vs. Without Stationary Flaps

Both Actually !   flap ends out on fifth open bay , there . ( Ailerons are FIXED )

Better Picture ; pretty much think I coudve stuck with this rather than the general w.o.f.t.a.m.
co-ordinated rudder deflection seems helpfull to counter yaw , or o.m.g. into top of hourglass & into clover , in wind .
 etc .& inertia .( of tip Wt. )



Seems to me tapered ( less % chord outboard ) flaps are better suited to a Stiff Breeze / turbulance .
though experiments with the little Phantom ( Noblerish ) with the last bay ( flap ) fixed & 1/8 Sht run tru aft from highpoint to & 1/2 span ( o.a. )
reduced vortice wakes working hard so their destabilisation was quatered .



The Commie 60s 2/3rd span suckers ' we ' presumed were to allow the outer wing to fly ' clean ' or unstalled . at Least ' outside ' buffet @ stall approach .
Presumably .

Other trick is to use flaps as ' trimmers ' presumably as per Beringer , setting the wing to + bouyancy , as it were .
The 72s about Airworthy again , Will tryn get a camera on it , airborne . works as per This theory . v Smooth in still air .

A ancient ( 60s ) American Aircraft modellor ? ( the old blottish paper B&W type ) Had an Article proscibing
flaps ( For C/L stunt ) were first used to stop the ' pitch ' into the corner - whereby the Tail first Dropped ,
before a Inside was entered . Looking oafish .
promulgated tail end was trown out to counter length of nose ( engine & Tank ) to posn. of acceptable balance unbalasted !
If I recall Wright .
Now DRAG as you mentioned .

Proposed that power to Weight WASNT the correct primary equation , But Power to Drag . And make it as light as practical .
By balancing to output to the drag , the airspeed was controlled at at the Light weight ( Lb Drag exceeds Lb Wt . maybe 3/2  ? )
 
havnt found trace of this ( a copy - lately ) but presume was common knowledge as in your National press ! .

Anyone find that & we're in for a read . Mayve been Larry scarinzi ( scuse the spelink ) or that era .  

[Brett Buck]

  This is mostly consistent with Al's car hood results.

     Brett

[Serge_Krauss]

Yes, I'd forgotten that for the moment. What I liked here was that the guys actually put the wing through the spectrum of AoA's with the flap stationary in neutral, a useful verification for flapless types. It gets too complicated for my waning attention span when the flaps are deflected while changing AoA.

[Scientifiction .]

When one finks about it . Overly ? Simplifying Facts ,  

If ' we ' take the ' airfoil ' Centerline as being from the L.E. centerline ( center of front radius ) to the rear point of the Flap ( deflected )
it turns the section into LIFTING ( or the inverse ) when in manouvres .

If the Flap Deflection Matches the rear ( aft of highpoint ) curve ( flat flaps ) it looks pretty obvious ,

Flaps chopped from the wing section would appear to interupt the contiuity of curve under All deflections .

So what we need is a flexible flap that curves across the chord .  

[Chuck_Smith]

Serge,

I wonder if the CFM used really accounts for the differing stall characteristics between thin and thick airfoils?

I'd expect a thin airfoil to begin to stall at the LE and have a fairly pronounced transition, whereas a thick airfoil will generally stall from the TE with a slower progression forward. Which is another way of saying a thin airfoil will have a quicker, harder stall. Years ago I remember showing this in the UB wind tunnel. We were doing drag determination by measuring the flow in the wake, and the thin sections would stall so fast we never really got good data of drag vs AoA at the top of the curve.  

And lest we forget, the lift and moment curves are for 2D flow, so they don't take into account aspect ratio which will also affect the stall characteristics as well as modifying the Cl curve.

Tru dat about the 5 digits, and they were easy to produce as well so they were indeed common. They would allow a large enough spar to take the loading and big enough fuel tanks without being overly draggy unless... you got into compressibility and then they were not-so-nice.

It was also common to use the 4 digits for the tail surfaces. 0010, 0012 etc.

[Chuck_Smith]

When one finks about it . Overly ? Simplifying Facts ,  

If ' we ' take the ' airfoil ' Centerline as being from the L.E. centerline ( center of front radius ) to the rear point of the Flap ( deflected )
it turns the section into LIFTING ( or the inverse ) when in manouvres .

If the Flap Deflection Matches the rear ( aft of highpoint ) curve ( flat flaps ) it looks pretty obvious ,

Flaps chopped from the wing section would appear to interupt the contiuity of curve under All deflections .

So what we need is a flexible flap that curves across the chord .  

Maybe my next design will use feathers...

[Serge_Krauss]

The NACA tests were done using 10" chord wings of aspect ratio 6 (span = 60"). The speed in the wind tunnel was 80 mph, which combined with dynamic pressure slightly above atmospheric (16.3 psi), gave average Reynolds numbers of about 609,000. I don't know whether they corrected to infinite aspect ratio or just compared at AR = 6, but this is not far from where airfoil characteristics change for our models. I'd have to look that up. These particular tests were done in 1940, after they had learned to account for serious effects of tunnel-wall and apparatus interference. So we don't have computer interpretations, and the values should be realistic. An earlier report showed vary interesting differences between behavior of small thin vs. thicker and larger sections, because of RN effects. That is mirrored in our thin horizontal tail's appearing to need sharper leading edges.

[Howard Rush]

Thanks for finding this cool report, Serge.  Now we know the official name for stunt flaps.  The fine print in the report said that the data were "corrected for tunnel effects to aspect ratio 6 in free air."  I also looked to see if they sealed the flaps.  They did. 

What I liked here was that the guys actually put the wing through the spectrum of AoA's with the flap stationary in neutral, a useful verification for flapless types.

I used XFoil to compare an Impact airfoil with flaps fixed at zero to some good combat-plane airfoils.  The Impact airfoil had higher Clmax.  Albert von Doenhoff's cousin knew what he was doing when he designed the Flite Streak.