Due to the "tuck" inherent in rudder use on a four star, I crashed mine.

I have been told that this characteristic is due to the stabilizer being too high in the airstream.

My question is how far should it be lowered? My thought is 1/4 inch, but I could almost get to 1/2 inch before having to move a pushrod.

Thanks

I am not sure this will cure the pitch down on rudder. A better solution would be to mix it out if you are using a computer radio. Consider most of the current crop of aerobatic airplanes....the CAP 232, Extra 300 series, Giles.....all have the stab on the upper fuselage line similar to your 4*40.

However, it would be interesting to see if it makes a difference. If you change it (1/2" suggested) let us know if it works!

Clair

After giving this more thought, you might check another thing or two: if the plane is slightly on the tail heavy side, it will require a little down trim to fly level. Then when in knife edge, it will exhibit this down trim. It doesn't take much......run a few clicks of up trim while flying, then put it on its side and note the difference.

If the wings and tail surfaces are out of alignment it will affect knife edge flight (and rudder application). For example, if your wings are exactly vertical in knife edge, and the rudder is tilted slightly up from horizontal, it will act like a minor V tail, and push the tail towards the top of the plane, causing the pitch down.

Even on a nicely trimmed plane, too much rudder application will cause the plane to do funny things. Try to use less rudder and see if your situation improves.

All this after years of pattern trimming before we got computer radios......hope this helps!

Clair

You are correct about lowering the stab being one cure for a pitch down with rudder. But check your plane for other things first. Normally, 4*s don't need that modification.

First, you need to make sure the CG is correct. Your previous advice is good in that a slight tail heavy condition will cause you to carry some down trim which causes a pitch down when the weight is off the wings as in knife edge. Conversely, a nose heavy condition will cause a pitch up with rudder.

I would look at CG first and check the wing incidence. I have a modified 4-Star 60 that has been bashed into a "sort of" WW II Kawasaki Tony. It flied knife edge with only a vewry slight correction. Not enough to bother chopping the plane up for. It flies a very slight arc from one end of the field to the other. If I am making knife edge turns, flying up and down the length of the field, I don't notice it since I am using elevator for turns.

As for lowering the stab, I would say half way down. As I recall, the stab is on top the fuselage. I would lower it to the middele of the rear of the fuselage. This is a guess, as are most aerodynamic corrections.

If you look at the Great Planes U-Can-Do, you'll notice the stab is on top. The 2 different ones I have flown both had pronounced pitch down. I notice now, the newer .46 size version has the stab lowered down about half way!

Most Sticks have the stab on the bottom of the fuselage. They tend to pitch toward the top of the plane in knife edge.

One thing you can do that was done to planes many years ago is to give the stab some anhedral. This effectively lowers it.

This isn't knife edge I'm trying to correct. This is normal flight. When I use rudder it pitches down. I've been told that this is a characteristic of four stars that is due to the stab being too high.

A few weeks back (I've been busy with non-RC stuff) I was landing it dead stick. I realized I was too hot and high, and was headed for the ditch. So I decide to slip it a bit.

I forgot about the rudder tuck. Now I'm repairing the fuse and a small bit of the wing. I figure that while I'm doing this, I can make the mods that I want to...

Lower the stab, increase rudder size and cut a bay off of each wing. The problem is that I'm not sure where the optimum placement of the stab would be.

Bruce Tharpe, the original designer of the 4* 40 & 120 made some comments on an RCU thread similar to what you are saying. Now if I can only find the thread :-)
Go to btemodels.com and there is some info comparing the Sig 4*60 to Bruce's Venture 60. Since Bruce designed the 4* 40 &
4* 120 he knows their weekness which he designed out when he designed the Venture 60. Bruce is a great guy and very helpfull, my guess is if you write him at his website listed above he will tell you what you can do to help correct the belly pitching while using rudder.

You can also lower the aerodynamic center of the rudder itself - decrease the chord at the top and increase it at the bottom - the Cap series is an extreme example.

You could also truly lower the vertical fin and rudder, decrease the vertical fin height and add a subfin, then lower the rudder to match.

Bill.

I guessed that you were not into knife-edge, but the rudder effect is the same.....if it misbehaves in knife edge it will do somewhat the same thing flying level, and vice-versa.

Let us know how your experiments go!

Clair

only a suggestion... what if your vertical stab is cocked to the side (IE: Not 90 deg from the hor stab) wouldn't this make for downward pitch when rudder is being used (instead of directing side / side it is acting as a slight elevator)
I dont have much experience with the planes, I have my 1st 4* on the table being built now.
now this would only give down pitch in theory to one direction, and up in the other.
Just a thought to consider...

If the vstab is not vertical, it will act like an elevator, meaning turning one way will make the plane go up, turning the other will make it go down. The problem inherent in four-stars is that the airplane will pitch toward the belly no matter what direction of rudder is used.

I have cut 1/2" off of the fuse, now I am planning on gluing the hstab back on, after I re-cover it.

I have chopped the wing by about 4" on each side (1 bay + 1" which allows the wing to fit in my trunk.)

I have increased the size of the rudder at the bottom, and replaced the top of the vstab with an area connected to the rudder.

I still need to re-cover everything, and tighten the ailerons. Once that is done, I will be flying again.

I received this response from Bruce:

You're right about the stab lowering helping the pitch coupling with
rudder. Your guess is as good as mine on the FS40. My guess is 1/2"
will be an improvement, but 3/4" or 1" would be better. Sorry, don't
have a firm figure for you. Another thing I did on the V60 was to
angle the rudder hinge line about 15 degrees, the idea being that
rudder deflection would automatically give a hint of "up" elevator
action due to the angle. You might want to try that in addition to
moving the stab.

I've flown my modded 4* once now. Due to some of the modifications (control surface/throw enlargements) my 4* is a real handful.

It is a real eye opener when you can whip the plane around one wheel. The lowered hstab has indeed improved the tuck problem, although it does still tuck a bit. Snap rolls don't look quite right anymore, although I think this is too much aileron, which overpowers the snap. I will probably put dual rates on to 50% for the ailerons and see if that helps. Once the field melts and dries, I will be further refining the flight of this bird. I think that this is a good mod, although I would recommend doing the modifications one at a time if possible.


For a recap, her is what I did...

Clipped the wings 4" per wing. - Increase roll rate, landing speed, decrease float. 1 bay is 3", removed another inch to make fit in trunk. Definitely recommend.
Increased area on all three control surfaces. - Increased controls to allow for tighter maneuvers. Jury is still out on how good of a mod this was, although the large surfaces look very cool.
Lowered the horizontal stab. - Tried to remove a trait in the four stars that resulted in my crash. (While using the rudder, the plane pitches downward.) Jury is still out on whether this was a good mod.

It's been a while since I posted to this...

My four star has been lots of fun until two weeks ago. The tucking with rudder has been reduced, although I have not tried a slip lately.

It did have some glitches, including a tendency to severely balloon when pulling out of a loop. I was playing with the CG to try and correct this.

Two weeks ago, I ran into a snag in my redesigned built-up hstab. Turns out, it wasn't strong enough. I was coming out of a split S and half the hstab ripped off. Fortunately, it was the side without the control horn, and I pulled it to the fastest, smoothest landing I've ever done. My plane was flying at least 2x my normal landing speed, and I greased it in!

I repaired this and a week later (last Saturday) I took it out with a stronger hstab. I took off, came around a couple times, then realized I was hearing vibration and flutter. I come in, and it is flying fast. Waiting for the flare, I'm thinking one more second, one more second. I pull back for the flare, and nothing. No flare, nothing. Smack, the plane slams into the ground. I see the mains rip off (common on a four star), then I see the tail separate. It ripped the light ply apart.

After bringing the plane back in, I was talking to others at the field, and they said it looked tail heavy.

So, I finished the repairs, rebalanced the plane, which required the battery to move up 6 inches.

I flew yesterday. The plane now tends to buck over as the speed increases on the ground. Okay, I can deal with that. The plane seems faster in the air. Okay, I can deal with that. Wait. What's that? I hear vibration. Ugh. Time to bring it in.

I start thinking, OK I'll float it in like I used to. The thing starts falling out of the air. That's interesting, it didn't do that before. So I bring it around, keep some power on it, and fly it to the ground. Ugly landing, but everything is still intact. I taxi it back to me and notice the vibration still occurs on the ground, so I don't shut it down. I then go through some tests, and note that the vibration occurs at 1/3 to 1/2 throttle, but not at idle, nor at full throttle.

I hold all of the control surfaces to see if that is the issue. Nope. No flutter. Something else is causing it. (BTW, I have a different thread discussing this issue.) After playing around a bit more I decide to fly it again, as it appears that the vibration is engine related, and I want to fly. Takeoff was the same, tends to nose over. Liftoff was fine, bring it around to land, and I now need about 1/8 throttle to keep it in the air, which I like. Not time to do touch and goes, as I need to catch my breath after every landing. Land, taxi, then decide to go home.

You seem to have some rather uncommon opinions of the Four Star 60. I would never suggest that your opinions or observations are wrong or unfounded, merely that I am surprised to hear them. I have seen many Four Star 60s fly at my club, and have never seen nor heard any of things that you describe. I own two of them, also. Neither one of them exhibits any of the idiosyncrasies that you have mentioned, except for a slight amount of "fall off" in true knife-edge flight. As far as the vibrations and strange sounds are concerned, perhaps it is your covering vibrating. Sometimes slightly loose or very taught covering will amplify engine vibes and sound like flutter. There is a good amount of open space on the back of the fuselage that could be doing this. The gear coming off is not necessarily any more common to a Four Star in a bad landing than it is in any other model airplane during a bad landing.

Lachlan

I am working with the four star 40, with 4 inches taken off each wing, enlarged control surfaces, and now a couple of tail repairs that ended up adding weight. This makes it not float, which in my opinion is a good thing.

As for the engine vibration, I have another thread that talks about it here http://www.rcuniverse.com/forum/Vibr...2119719/tm.htm . I doubt it is covering vibrating as I can see the muffler vibrating when it is at the throttle setting that vibrates it.

As for my observations, remember we are talking about the 4*40, and as you can see in this thread, Bruce Tharpe, the designer of the 4*40 confirmed the issues I spoke about before I made any changes.

Yeah, the Four Star 40. I'm sorry to have made that mistake. I remember reading every single post in your thread, and somehow whiffed on the 40 part of it. Anyway, I have seen Bruce's changes to the kit in his version, and I think they are both aesthetically pleasing and most likely aerodynamically sound. I sure do know what you mean regarding floating, though. This plane can practically soar with power off and still have a 15 minute flight! Again, sorry about my mistake. Best of luck with your 40, and with the oscillating .46! Sounds like some good suggestions to help track down the vibes in the other thread.

Lachlan

I find it interesting that others haven't noted the pitch down tendency with rudder application, because I sure have, and it's a commonly known and discussed issue. (At least with the 40.) Mine will tuck tightly towards the belly during attempted knife edge unless I apply a lot of up.

It also shows up dramatically in things like stall turns.

You said that the tendency was reduced with your mods to the horizontal stab. Can you tell me how much you ended up lowering it?
Dennis-

I essentially set the hstab on the pushrods.

I want to be able to properly land my plane, using throttle to regulate height, and elevator to regulate speed. If I am forced to practically kill the engine to get it down, I consider the plane to be a floater. It's a personal preference, and one that makes me more adaptable to different types of planes.