I have a scratchbuilt Curtiss SBC4 biplane which has retracts which fold up into the fuse. When the LG is down there is a hole or opening which goes thru the fuse; completely thru from side to side. The model flies just fine with the landing gear down. When the gear is retracted it still flys great with positive control; It's quite a gentle plane to handle. Now the funny part, with the gear up it swims like a fish, almost like I am swinging the rudder from side to side when in straight and level flight. It seems to be worse when flying downwind but I'm not sure. It has a very large cowl and the prop, a 3 blade which only extends 2 inches beyond the cowl but has adequate pull on a 90 FS. It does not swim when the gear is down????

Would appreciate any thoughts

What happens while in a turn? Does it tend to hang with the tail low in a steeply banked turn? If you use only ailerons to roll into a turn does the model show signs of adverse yaw? Or does it do the opposite and wants to tighten up all but the shallowest of turns?

I'm betting there's some adverse yaw from aileron only inputs and that it runs around a steep banked turn with the tail hanging low and the engine pointed to the "top" of the circle cone of the turn. That's a sign of too small a vertical tail and the fishtailing you're seeing is a slight bit of dutch roll. It gets worse when you're climbing, right?

Bruce

Thanks for the reply; the vertical tail is quite large. Have not noticed anything wrong with aileron only turns. Have not checked it closely in a climb. Any answer for the fact it does not swim with the the gear down? The wingspan is 64 inches; the vertical tail is 12" high and 11" long at the base. Due to a rotten October the season is over here. It has been snowing off and on for two weeks.

> it runs around a steep banked turn with the tail hanging low and the engine pointed to the "top" of the circle cone of the turn. That's a sign of too small a vertical tail <

I have been troubled by this problem with a small bipe when I only used ailerons. Mixing the rudder to ailerons does help things out. Will adding to the fin area help or must the rudder also be larger? And if this mod is done, will rudder input still be necessary for a decent turn?

If I read your post correctly, your problem with your Helldiver is: yawing And it happens as you've already said in your first post in straight and level flight.

If I read the rest of your description correctly, it doesn't yaw in straight and level flight with the gear down.

Is that because you've only flown at slower speeds with the gear down? Or have you seen it fly without the yawing at high speed with the gear down.

There is an aerodynamic flight problem called Dutch Roll. It is a cyclical yawing that happens at different airspeeds. It is usually accompanied by a coupled rolling. The airplane will yaw right and develop a clockwise roll. At some point, it will reverse the yaw/roll and come back the other way. It yaws left with a counterclockwise roll. It repeats. Sometimes it will increase the amplitude until the aircraft gets into real trouble.

Does the yawing happen only at cruise speeds? not happen at slow speed? And is there rolling, even a slight amount?

Dutch roll is usually caused by not having enough vertical tail stability. At some airspeeds or AOAs, the vertical stab/elevator is inadequate to hold the aircraft steady in yaw. The airplane starts to develop a yaw and the yaw can cause the forward yawing wing to gain lift, while the aft yawing wing loses lift causing a roll. As the tail yaws, it's AOA increases. This generates a force contradicting the yaw and starts to pull the aircraft back straight. As the tail heads back, it picks up speed in the yaw axis and with the tail being insufficient to control the yaw, the tail keeps going. It reaches a sufficient AOA on the other side to stop the yaw etc etc etc.

You won't always see an associated roll. The airplane might want to do it, but there could be a blanking force. If the design has rudder/roll couple for example, that couple could be strong enough to fight the yaw induced roll simply because the rudder induced yaw generated an opposite roll force.

Got any videos? If you do, slow them down and look first for rudder movement. There might be some kind of radio gremlin. Then look for roll axis changes and pitch axis changes. There might be more going on than originally thought.

Curtis had yaw problems with a number of their designs. The P40 for example was modified throughout it's production life in attempts to keep the airplane flying straight.

There is no problem with aileron banks or turns. Yes it has flown WOT with the gear down. There is no coupling of ail and rudder. This occurs in straight and level flight and does not bank, it just wiggles the tail when the gear is up. I appreciate all your replies but you are curing problems which don't seem to exist. If it wasn't weird I would not have written in. "It runs around in a steeply banked turn with the tail hanging down" is Bruces statement, not mine.

The vertical is probably too large.. With the gear down, the lateral area is more balanced.
I had similar wiggles on an Evan 'Simitar", which I cured by cutting a small hunk off the leading edge of the vertical.
With a scale plane though, you don't have that option.

Thank you Paul. Now I have a plane with too small a tail and too large a tail, but kidding aside all of this is informative reading; keep it coming.

The mention of RUDDER coupling was part of the discussion of your airplane and why the yaw might not show a rolling with your airplane. It's quite easy to read about rudder coupling and think the discussion is about aileron problems. Rudder coupling is not aileron coupling. But the mention of Dutch roll and all the following discussion applying it to your situation was pertinent. It was just too much and too complex.

Dutch Roll is a symptom of incorrect lateral area and balance. It most often happens when the vertical tail volume is inadequate. Tail volume considers both the area of the vertical tail and it's distance aft.

BTW, it is quite possible that the area in the vertical tail is too large but too far forward. Or it could simply be too small for where it is. It certainly isn't working right is it.

If you're satisfied with simply having a reason for the hunting, then you've got a couple. If you'd like to experiment with the best test opportunity ever, then use your airplane. Tape some temporary additional rudder on and fly the sucker. If the hunting (or is it fishing?) is reduced you've gotten a cheap and simple solution. If it increases, you've gotten an answer that winds up with a tougher solution.

Very often scale modelers have problems with their first attempt at a subject. They then test for a solution. You hear often of a scale modeler building a certain airplane with a larger tail. Very often it's the vertical tail. But what you're seeing so often happens that some builders do their first attempts with larger tails to begin with.

Here's a wild thought - though I liked the comment about you're flying slower with the gear down - with the gear out you've widened your moment arm for lateral forces; like a tight rope walker carrying a pole or extending his arms. You're also adding a lot of drag. In free flight if we had a plane that "rooted" or "hunted" (nose worked side-to-side) we checked the tail surfaces for warpage and alignment. But they had a lot of dihedral. If you're swimming in straight flight I would suspect vertical stab out of alignment or twist.

An airplane that yaws but doesn't roll, might not roll because there is actually a rudder couple. Even if the rudder/fin hasn't got enough tail volume to keep the airplane straight doesn't mean the rudder/fin won't have enough force to generate a couple. The wing might want to generate a roll when yawed, but the rudder/fin couple could cancel it out.

BTW, about "Dutch roll"........ It wasn't a roll after all.

Back when the term came about, the story goes........
The guys trying to sort the behavior were first told that the airplane had this funny rolling behavior. So they started off on the crusade to find the answer to rolling behavior. Who'da thunk to look first at the tail. Rolling is a wing thing ain't it. Well, you'd look first at the wing for rolling right?

So after they'd worn out all their pencils, one of them thought to see if maybe the dumb airplane had enough vertical tail and taaa daaaa......

So they found out that it wasn't really a roll problem. And they decided to make light of all the overtime they'd worked on the roll problem by calling the problem, "Dutch roll", as in "Dutch uncle". A Dutch uncle ain't really an uncle, right. And Dutch roll really isn't about roll.

Keep in mind also that when the gear is extended, not only does the airplane probably fly slower than it can with the gear up, but........

All that stuff hanging down there is going to leave a really messy wake. It's going to blanket the underside of the airplane and probably play hell with how the lower wing is working. Maybe even mess up the flow over the lower wing.

And the big holes that are open are up in the fuselage........... half the fuselage is those holes......... hmmmmmmmmmmmmmmm

Dave, either way if fine. It doesn't matter if it's all on one or you just split it up. Adding the area is good either way. This is why you see sub fins on many float equpped full sized planes. It's to balance off the large area of the floats ahead of teh CG.

Adding the area will definetly reduce the rudder input or eliminate it. But don't go over the top or it'll tend to want to wind into the turn.

The problem may have nothing to do with the rudder
Not having seen YOUR plane - it is impossible to ascertain what is happening
I have had - and it is very possible - the canopy is causing the problem--seen it before on other models and duplicated the problem in tests
Depending on the flying attitude of the model - turbulace-created by the canopy , sets up a side to side rythmic wiggle
old timers called it the "Quick Fly" dance. ( I ain't old enough to remember ------)
I first saw it when I attached a cup on top of a model for a "egg drop" . the model danced hard. moving th cup back- stopped the wiggle.
tape some carboard on top to streamline the front of the canopy - or similar tests.

Scratch,
By any chance is there an opening from the cowl area through to the wheel well portion of the fuse or is it sealed off by a firewall?

Flugg: it's closed by a wall.

Dick: It does have a large, 2 place slab sided canopy with quite a blunt windscreen which runs from the mid point of the wing to about 4 inches from the tail. Never considered the canopy. Will check this come spring. Would the extended gear counteract the effect of the canopy? If you go to google Curtiss helldiver page 1 and look at the title "1-1" you can see what my plane looks like. Should convince that the tail is large enough? There was a monoplane and a bipe called helldiver; mine is the bipe.

I know those planes - - the canopy thing can be tested with some tped on bits of thin hard foam /balsa cardboard etc. this is one of those things that happen when "everything is just right" you can't calculate it - it is just a watch and see thing-- and quite a project - I have some local friends who do vintage scale stuff - Orville Lloyd and Noal Hess- Noal has a room full of trophies from many years of scale stuff. Orville did the Liberty Sport - full scale and loves vintage scale military

Large isn't the only measurement that matters. Look up the definition of tail volume.

BTW, the P40 Warhawks tail was large enough. The airplane would yaw as yours does. It also happened to have too little lateral stability on takeoffs. How did Curtis solve the problem?

Curtiss moved the vertical tail farther aft. They didn't change anything about the size. They simply moved the sucker 25" back. The versions that had that modification were some of the F, K, Ls, and all of the M and Ns and later. Then it really was large enough.

If your canopy is removable, try flying without it.

The position of the front (windscreen) of the canopy is where we found the problem to be--as if--the airflow around /over it created a lowered pressure/ turbulance/ whatever you want to call it -- at a point somwhere around the fin/rudder.the fin/ rudder and the plane went into an occilation due to lowered effectiveness of the fin--- that lowered pressure area. Anyway we could create this problem-- by simply positioning the cup along the square flat topped fuselage. It was quite interesting at the time.
Even on automobiles - this intersection and angle of the windscreen has shown to create problems
As I drive a ancient 4 banger now - I really have lost interest in th car "streamlining.'

It will be greatly interesting to see what scratchonly discovers with his testing.

It's also interesting that the airplane that's giving the problems is a biplane. One would think that the canopy or for that matter anything aft of the wings' LEs would be in such stirred up air to begin with that they'd have a problem disrupting it enough more to cause any kind of problem.

And truth is, there could be a couple of things causing or contributing to the wiggle, all at the same time.

Hey, was it an old Chuck Berry song about, "...a wiggle in her walk" Wonder if she has a "giggle in her talk"???