FILLETS-DID YOU KNOW?

The combination of low pressure from the side of a rounded fuselage and the low pressure on the trailing edge part of the wing on a low wing airplane results in a very low pressure area at the fuselage-wing junction. air in this area tends to detach causing tubulence and a slight yaw instability or tail waggle.
When Douglas aircraft started building transport planes, they had this yaw instability problem with the DC-1. Dr. Theodor Von Karmann was retained to find the solution. His work resulted in the large fillets seen on this and subsequent planes....(I read it)

I Actually did know that. I also have finally figured out how to put in an epoxy/filler fillet that continually changes radius and gets larger from LE to TE.

Its also why some jet fighter types have pinched fuse sections in the wing root area--as to do with controlling the pressure gradient in that area.

so how do you shape the fillets on a flapped stunter to control the pressure gradient flow?? With a spoon?

"Inquirying minds want to know" How do you do it?

A spoon might work but I finally used a kidney-shaped plastic trowel. Start at the TE with the largest R part of the trowel and then, as it is drawn toward the LE to shape the fillet, rotate it continuously toward the small R end. You can cut the trowel from one of your old credit cards. I find once they're maxed out, I have no more use for them!

It definitely takes some practice and I would suggest that it is only for appearance sake.

It also helps weight-wise to use an under fillet of balsa if you plan to make them large.

FWIW

Fillets are a whole topic by themselves.

First, poorly designed fillets contribute to drag. This typically isn't a problem with a stunt ship as the drag contribution is a small percentage of the total. Even the total drag of a stunt ship isn't practically a problem. It equals thrust so draggy airplanes like semi-scale Bearcats can fly just fine by using more power. We aren't concerned with efficiency.

Second, the aerodynamicly minimum drag intersection is a straight 90 degree joint. Prop driven airplanes sometimes had airflow problems, but in general fillets began getting smaller in the early forties and are virtually nonexistent today. The P-40 and Spitfire had big fillets, the later Corsair, Hellcat, and Bearcat, to name just a few have none. The P-80 had small fillets, offhand, I can't think of any Mach 2 fighters or modern jet transports with any. With jet transports, typically the fillet would consist of a flat surface extending down from the wide part of the rounded fuselage to meet the wing top surface at a near 90 degree angle. So, large fillets on stunt ships typically aren't necessary. If you like the way they look, fine, but generally they are constructed heavy and occupy some of the inboard end of the wing wasting that lift producing area.

If you want or need fillets, and I do when I model some semi-scale airplanes, at least make them light. Balsa fillets are the lightest practical material, particularly when they are formed from thin sheet or carved and hollowed (real thin) like on my Mustangs. I don't understand an obsession with building light and then applying a large, heavy, fillet of some solid putty material. Fillets can weigh several ounces and probably offer no compensating benefit in performance.

Fillets are desirable on profiles where they add to the fuselage footprint for a more stable wing/fuselage joint.

>Fillets are a whole topic by themselves.
>
>First, poorly designed fillets contribute to drag. This
>typically isn't a problem with a stunt ship as the drag
>contribution is a small percentage of the total. Even the
>total drag of a stunt ship isn't practically a problem. It
>equals thrust so draggy airplanes like semi-scale Bearcats
>can fly just fine by using more power. We aren't concerned
>with efficiency.
>
>Second, the aerodynamicly minimum drag intersection is a
>straight 90 degree joint. Prop driven airplanes sometimes
>had airflow problems, but in general fillets began getting
>smaller in the early forties and are virtually nonexistent
>today. The P-40 and Spitfire had big fillets, the later
>Corsair, Hellcat, and Bearcat, to name just a few have none.
> The P-80 had small fillets, offhand, I can't think of any
>Mach 2 fighters or modern jet transports with any. With jet
>transports, typically the fillet would consist of a flat
>surface extending down from the wide part of the rounded
>fuselage to meet the wing top surface at a near 90 degree
>angle. So, large fillets on stunt ships typically aren't
>necessary. If you like the way they look, fine, but
>generally they are constructed heavy and occupy some of the
>inboard end of the wing wasting that lift producing area.
>
>If you want or need fillets, and I do when I model some
>semi-scale airplanes, at least make them light. Balsa
>fillets are the lightest practical material, particularly
>when they are formed from thin sheet or carved and hollowed
>(real thin) like on my Mustangs. I don't understand an
>obsession with building light and then applying a large,
>heavy, fillet of some solid putty material. Fillets can
>weigh several ounces and probably offer no compensating
>benefit in performance.
>
>Fillets are desirable on profiles where they add to the
>fuselage footprint for a more stable wing/fuselage joint.
>
>Al

Ron B.
F4Fguy

As usual Al bangs the nail squarely on the head.Square is best,especially on a slab sided conventional stunter with mid or low-mid wing.If you feel you must have them for esthetic reasons or have to for scale reasons,make them from sheet.The ones on the Zero are 1/20"-4# sheet.I have no idea what they weigh,I don't have a scale that sensitive.In any case,and in this case in particular,they're probably blanked by the cowl flaps.I know this is bad,but I've got to cool the engine somehow.

Who the heck are you that comes up with stuff that only Al should be able on doin'??

Ain't you Al using a nickname?

Talking serious, your work is OUSTANDING!!!!!!!

Do you have a site?

God bless you!

In Christ,

Al-

>Second, the aerodynamicly minimum drag intersection is a straight 90 degree joint. Prop
>driven airplanes sometimes had airflow problems, but in general fillets began getting smaller
>in the early forties and are virtually nonexistent today.

I agree - to a point. However, the NACA reports on this seem still to advocate a minimal fillet, even on such 90-degree intersections, where a straight slab side intersects the airfoil contour. This is probably because of boundary layer interference between accelerated flow approaching the wing's "high point" and the slower flow over the fuselage. While this is probably a lot like a tip plate phenomenon, perhaps the overall fuselage shape affects the flow to the point where some small buffer zone (mild fillet) is needed for the flows to accomodate eachother. As you said, prop wash complicates things. This would be further aggravated in designs where the fuselage is already narrowing along the root; while the slab side and wing can still be intersecting in the lateral plane at 90 degrees, the air flows are attempting to diverge...area rule and all that. I have heard of general aviation designers of light planes deliberately sacrificed performance in order to use the buffeting around the root as a stall warning. I remember this sound that should have concerned my flight instructor more(!) on the Beech "Musketeer".

Of course in more complex situations, fillets are often necessary. John Roncz's program suggested that no fillets be designed into the Genesis sailplane. Jim Marske disagreed and was proved right by an otherwise good design that nonetheless still suffers a couple L/D point handicap due to flow separation requiring turbulators. Area rule (really area gradient) application here would have helped.

I do agree that fillets can actually cause more drag in aerobatic maneuvers - like at the flap roots - and overall the cost or savings in drag is miniscule compared to line and induced drags anyway. But they look nice, and as you pointed out, they are significant to the root juncture strength of profiles. Balsa fillets are best where strength is not a factor - lighter and easier to form; I like mine. FWIW.

Al,

Who is this F4 guy???

This guy is so good, are you sure it ain't you?

Tell me about this cat, he gets me droolin' when i see his pics...

God bless you!

In Christ,

When the AT6/SNJ aircraft were being modified to look kinda like the A6M2 Zero Sen fighters, they added the wing fillets and the plane gained about 15 mph cruise speed.!! (this was for the movie Tora, Tora, Tora)

And this proves that modern airplanes with Little or no fillets are poorly designed?

Ron B.
F4Fguy

No,Al,only that very few modern aircraft have true "low" wings.As you point out in your earlier post,with square intersections at the fuselage,no fillet is required.

Modern? That bucket of bolts came from a 1939 design!! The wing is at the bottom of the fuselage, the fillets weren't that big, just for show and the speed increase came as a surprise. The smaller canopy may have contributed to the increase in speed as well. The entire jist of my comment was that it is just a comment about fillets, not a critcism of your designs nor philosophy of design, which, by the way,is about the best uot there.
As to stunt models, I still use the flexible, light and easy leather fillets from Walter Umland. They work.

Ron,

I disagree about there being few true low wings. All business jets and commercial airliners sit the fuselage onto the wing center section or wing box, as do most home builts. Modern military aircraft typically use shoulder wings but that is probqbly clearance for ordinance.

As for fillet construction, Ron, we didn't convince anybody of anything. These posts seldom do. Even leather fillets are heavier than balsa and I would think would require some putty to fair them into the wing and fuselage where balsa can just be sanded flush.

Oh well, we do our thing and they can do theirs.