TwistMaster Saito 30 revisited

Back from the Wichita contest and a great weekend of flying, friends, food and fun, thought I would post an update on the continuing saga of the TwistMaster. We had a pretty good breeze yesterday, according to the weather channel 15 to 20 with 25 mph gusts and I had no problems what so ever putting the TwistMaster through it's paces.

First the Saito 30 is now running flawlessly on a clunk tank plumed conventionally.. two tubes, the vent to the front inside corner and the clunk. Thank you Bob Z, my 70's mentality refused to believe I couldn't make a hard tank work as well as a silly old clunk tank so the lesson was a tough one. As posted previously I tried every configuration of metal tank ever dreamed up and never was able get rid of the sag overhead. As soon as I switched to the clunk the problem went away. Even though I have lived it, I am still having a hard time believing that a simple plastic clunk tank can work better than a uniflow hard tank but with this engine it really does. To re-cap the Saito 30 set up.. 9.5 - 6 Master Airscrew prop, 20% nitro 24% oil 50% castor, Muffler has the exhaust nipple cut off and the new exit hole was reamed to .280. Clunk tank mounted on center of carb.

The other problem I have been fighting for the last year and a half was a simple matter of some minor trim adjustments. Mike Pratt spent some of his precious and extremely appreciated time helping me get the airplane trimmed. The big problem I was having with it yawing on hard outsides is all but gone and it no longer flops and wallows through the square eight. He watched me fly it, said add 1/4 ounce of tip weight, make the flaps the same length and the only problem on outsides was it had too much elevator throw and was stalling. After analyzing the elevator throws it was pretty obvious I had more down than up which was the reason I was only seeing the problem on outsides.

I added the tip weight and cut the outboard flap down. Wasn't much I could do at the contest about the elevator so during the official flight I was careful not to over control the outside corners. Wow what a difference, Just another example of how valuable a little help from someone like Mike can be. Thank you Mike, Bob Z. and everyone else that has offered help and suggestions.. I would never have reached this point the way I was going about it.

Bob,
Thanks for the updates.Be sure you bring the TwistMaster with you next time you come to Florida. Regards to family.
Roger

Not to be picky, But I think that it was me who suggested the clunk tank. That is the standard "Dallas" setup. We have been spreading the word about the virtues of the clunk tank with the 4 stroke.

I went back and checked the thread because I thought I had responded.

Either way, I am glad you liked it.

Also, everyone read the symptoms Bob described. That is the classic uniflow problem with the 4 cycle.

The suction venting is really what cures most of the problems. Bob Z found the same thing on metal tanks. The clunks just seem to be much better at the end of the tank.

Zilla,

Sorry bout that, so many people have helped so much it is hard to remember who sugested what.

I do apreciate it.

Bob - Bradley (Zilla) is correct on the clunk tank - it was his idea. He has done extensive flying and testing with one. After reading his article and speaking with him, I am setting up "The Plane" with a clunk. In almost 50 years of modelling, I've never tried one so I'm quite anxious. My Staggerwing is almost complete and since it features a modular engine/tank mount, I can use just about any set-up. I will definitely try the clunk. Thanks, Brad.
Bob

Two questions I have here:

1) How much extra outboard flap did you have? Normally a plane, even without assymetry, will benefit with a little extra outboard flap. Matt has never had too much outboard flap, even though he uses equal panel wings. (Of course, he doesn't carry this to extremes.) That little extra outboard flap allows you to carry a little more tip weight (and maybe that was part of your problem. Just simply not enough) and the extra tip weight translates into better tension overhead.

2) How much line tension do you have in the overheads. I know there have been other discussions on the clunk tank as the magic fixer for the 4-stroke, and no one has offered an explaination. In fact, most people who have even used it have been dumbfounded as to why it works. I cannot imagine a clunk tank working better than a properly designed metal tank unless:

a) you are losing overhead tension and the fuel is falling out of the wedge (but the clunk would follow the fuel) This doesn't seem altogether likely. Or
b) you have an excessive amount of yaw on the plane (possible) and the fuel is flowing towards the front of the tank. Matt has encountered a bit of this problem with his large props, first on .53 and now on the pipe set up. The anwwer here was provided by Brett with a tapered metal tank design where the back part of the wedge is a 1/4 to 1/2 inch wider than the front. In uniflow configuration this works beautifully until the end. We are going to be experimenting with a carbon fiber variant of this in the future.

I was using clunk tanks on a Saito 50 in 1998. It was great until the controls locked and SPLAT! The head and everything just broke off. What a bummer. I didnt revisit 4s until Zilla came around a few years later then he started that up again and we have found that the clunk tank is a good tank for the 4s. It is also cheap and abundantly available in many shapes and sizes. Easy to build and it solves your troubles.

There was a post that about 2s having a charge waiting in the crank case right behind the charge that is already in the cylinder. 4s does not have this. I think this is one of the reasons the cluck works. The clunk goes where the fuel goes. There is no two ways about it. On a metal this cant happen with a fixed pick up. In the overheads I bet the fuel does fall away at the top of the arc, some of the fuel not all, and the clunk just goes right with it. Then at the bottom of the arc the fuel slides back out and so does the clucnk. On a 2s with the charge in the case waiting the fuel draw and consumption is much better and stronger. Therefore it can fight through this. The 4s fuel draw is marginal compared so the tank is critical. The pick up has to be in the main portion of fuel remaning in the tank at all times. If the amount of fuel drops around the Pick up the motor will think it is running out and go over lean. A 2s will just break and lean out during the lower fuel pressure time. The 4s cant run like this. Pinch the line while running a 2s and it runs hard and lean. Pinch the line on a 4s and it dies. There is no need for a clunk uniflow and all that either. You can run it if you want but it really doesnt matter since you are not dealing with a 2-4 run and all that. The clunk will stay in the fuel and it will run steady as long as the tank height is correct. Besides these are RC motors developed for RC and in RC they use clunk tanks on pressure.

I would liek to see someone take the insides out of a metal tank and make it s clunk tank on the inside. I bet it would run just fine. Just a hunch.

By the way this is just my thoughts on the subject of why it happens this way.

LAST EDITED ON Aug-13-02 AT 09:22 AM (CDT)

>I would liek to see someone
>take the insides out of
>a metal tank and make
>it s clunk tank on
>the inside. I bet
>it would run just fine.
> Just a hunch.

We'll find out soon enough. I have a half-built metal-shell clunk tank on the workbench even as we speak. If it works, it will be way better than plastic tanks, because you can actually fit it into existing airplanes, and you don't have to go in 2 oz increments. And it's not quite so Mickey-Mouse.

Uncle Jimby cut a plastic tank in half, took out a section, and then melted it back together. Surprisingly, this actually seemed to work, and stayed together for 4 flights the other day.

Hi Doug. As to metal clunk tanks, they have been used, in the late 60's and early 70's. A similar system used a moving pick up tube. The pickup was in the shape of an "L" and swiveled inside the main feed tube. You can see this on some older stunt plane plans. Say that fast three times. The metal clunk was in an article, I belive in MAN, and was nothing more than a fuel tube with a weight on the end. Not unlike the commercial tanks.

I completely understand what you mean about fitting it in current planes. When it comes to 2s that is. Since the 4s uses less fuel there is no reason a 6oz or 4oz for that matter shouldnt fit in any normal sized plane. That is my only beef with Clunks. They dont alwasy fit. When going in a small plane. There is a simple fix for that though. Just build your plane a little wider in the future and bingo all problems solved and both tanks can be used. I little extra width wont hurt anything. I have been using clunk tanks on all my competition planes since 96 and have had very few problems. When I do the fix is very easy and chaep and can be done in minutes. The thing I really like about using plastic clunks is there is no art to making them. You can build them in any congif you want and still get a good seal and test it out. With metal you need to know how to solder really well to make a properly. I have never been able to do this. With Sullivan hard tanks or Hays blow molded there is no need.

Please keep us up on how well it works I have been wanting test this for some time but never got around to it.

What do you mean "So mickey mouse"

>What do you mean "So mickey
>mouse"
>

Ted and I were talking about this this weekend. You sort of bend some tubes, test fit a bunch of times to see if they go more-or-less the right spot, then assemble the tank and sort of "twist and hope" that the tubing winds up where you want. It's just not a reliable or very repeatable process. And then there's nothing really keeping it from changing over time. It's very unsatisfying, and you never know for sure whether this tank is the same as the next one. They usually run OK, but they're never quite the same.

Compare this to a regular tank - everything is solidly assmbled, can't move, and can be built to 1/64" time after time. I can build 10 tanks and they will all be the same.

Just a matter of preference. I always got my clunk tanks to work OK with some fiddling. One thing that helped was to make a triangular brass thing that captured the tubes at each point of the triangle. Once it's right, solder everything in place and then it can't move as easily. Now you only have to deal with disturbing the stopper as you take the tubes on and off.

Yep I can see that about consistancy. With your setup you can get it exact before you ever take off. With mine I have to figure out all the shims every time. That is a beating for sure.

>...In the overheads I bet
>the fuel does fall away
>at the top of the
>arc, some of the fuel
>not all, and the clunk
>just goes right with it.
> Then at the bottom
>of the arc the fuel
>slides back out and so
>does the clucnk...
>DMoon

This was my question about overhead tension. If there is no tension, then the fuel is falling away (just as the airplane is). But if you have any tension at all, then the fuel is staying up there just as the airplane is. (You won't get some of the fuel falling down and some staying up there. Gravity doesn't work that way, nor does centrifugal force.)

My bet is there is a fair amount of yaw in the nose and that, with the yaw, the front of the tank is farther away than the back and the fuel is flowing to the front of the tank and uncovering the uniflow on a uniflow setup. When Al put his uniflow tube in the front corner on his metal tank, he solved that problem. On a clunk, the clunk just follows the fuel. But it can also do some funny things and I have never liked them.

LAST EDITED ON Aug-13-02 AT 11:55 AM (CDT)

>This was my question about overhead
>tension. If there is
>no tension, then the fuel
>is falling away (just as
>the airplane is). But
>if you have any tension
>at all, then the fuel
>is staying up there just
>as the airplane is.
>(You won't get some of
>the fuel falling down and
>some staying up there.
>Gravity doesn't work that way,
>nor does centrifugal force.)
>
>My bet is there is a
>fair amount of yaw in
>the nose and that, with
>the yaw, the front of
>the tank is farther away
>than the back and the
>fuel is flowing to the
>front of the tank and
>uncovering the uniflow on a
>uniflow setup. When Al
>put his uniflow tube in
>the front corner on his
>metal tank, he solved that
>problem. On a clunk,
>the clunk just follows the
>fuel. But it can
>also do some funny things
>and I have never liked
>them.
>
>
>Leonard Neumann

Tension! Brads Impact makes more tension in the overheads than most people like in level flight. Yet with a metal tank the motor gets no fuel in the overheads and it just sags away. This has to mean that the fuel is moving somewhere. Away from the outside is the only place left. The fuel doesnt completley fall away but if does some this will change the amount of pressure at the pick up and then motor will sag. Fuel draw on the 4s is way more critical(because it is so weak) than on a 2s. The fuel head pressure(searching for the right term) at the pu cannot change one bit or the motor will not run steady. A business card shim can be hurtful or helpful. These are very sensative setups.

Moving the uniflow vent in the tank supports this. When the uniflow is back where the pu is and the plane is level the fuel is all forced to the outside and creating an even amount of pressure at the pu. Well when the fuel gets low at the pu it starts getting air from the Uniflow tube and it tries to run lean. In a 4s this will mean it will sag on you. Moving the uniflow away then you have nothing but fuel at the pu. No chance for airbubbles to get in there. So even if the fuel head drops at the pu a little bit there is no air there to get into the line like before. You will get a leaner run at this point due to the pressure change but it wont try to die on you. With a clunk you dont have to worry about all this stuff. The clunk follows the fuel. Just shim it right and the runs should be good.

I bet Al's runs arent as good as they can be on a clunk tank. We have tried the standard vented metal tank on 4s. It ran better but not as good as on the tank it is designed to use.

Leonard,

In an earlier effort to correct the roll/yaw problem I cut the inboard flap down by 6 inches making the outboard flap 6" longer than the inboard flap. At Wichita I cut the outboard flap so it was only slightly longer than the inboard flap bringing the whole flap thing back into normal proportions. Now I have a 6" trim tab on both wings The TwistMaster is fairly light so still have plenty of flap area.

Overhead tension is not a problem, I have more overhead tension with the Saito 30 then I had with the FP-40. Some of this is due to the airplane being in better trim but I had the 40 so de-tuned in order to keep it from running away it was putting out less usable power than the 30. The plane has no rudder or engine offset and the leadouts have only a degree or two of rake, it isn't flying with the nose yawed out, in fact I cannot see any part of the outboard wheel when in level flight. I think Doug has pretty well covered the clunk/hard tank explanation and it sure makes sense to me. I was a die hard, built/modified 3 metal tanks of different configurations plus two stock metal tanks before finally fixing the problem with a darned old plastic clunk.

While at Wichita, I happened to see Bob Bookins(sp) flying a small war bird of some vintage with a Saito 30. At the top of everything overhead you could hear a noticeable sag. I looked over at the gentleman I was sharing the shade with and said "I will bet $100.00 right now that plane has a hard tank on it". After Bob landed I walked over to the circle and sure enough it was a conventional wedge. I told Bob about the clunk thing and hope he tries one.

Brett,
I made a metal clunk tank a few years back that I had some real success with. The only real problem I had with it was it rattled (a lot). I eliminated the rattle by cutting a groove around the outside of the brass clunk and installing a small o-ring in the groove. The o-ring would contact the sides of the tank.

Len,

I don't know it's the clunk, per se. I'm using a chambered metal tank (sort of a highly modified chickenhopper layout) that is vented much as Brad described previously. The vent is in the center, front, inside corner of the tank and the pickup in the opposite ouside back corner with standard overflow. There are 3 chambers in the tank with a 1/2oz or so chamber around the pickup. I've had zero problems with this arrangement. Great in the overheads without the starvation I've seen in some setups. Only real problem is that it gives you absolutely no warning when it quits. It just thuds along and stops.

I'm quite happy with the setup. And with the chamber around the pickup tube, the head pressure remains pretty steady throughout the flight and allows me to use a ridiculously large venturi and still get very steady runs. I suspect a similar thing is seen with the clunks. My first real "no brainer" setup in a very long time. Just flip and go.

Nice to concentrate on my flying and not worry about the engine run.

Some history from an older person: Such metal tanks were sold under the name Clank Tank. The first plastic ones were labeled Clunk Tank, a reference to the former. Those were humorous times. Folks these days don't know from onomatopoeia, except maybe Ted, who occasionally waxes highfalutin'.

>b) you have an excessive amount
>of yaw on the plane
>(possible) and the fuel is
>flowing towards the front of
>the tank. Matt has
>encountered a bit of this
>problem with his large props,
>first on .53 and now
>on the pipe set up

To some extent, this must be part of the correct explanation, at least for the poor performance of the standard wedge tank at the end of the pattern. In our testing, we had to cock the metal tank severely inside the fuselage to even finish the pattern! I do think the 4-cycle can yaw the plane more than the 2-cycle. I do not know why, maybe just brute force. All of the test planes we have used can maintain line tension with the leadouts extremely far forward.

I really think that the bulk of most people’s problems, at least that I have observed, have really been associated with the use of uniflow. To this end, I think Doug explained my feelings perfectly. The uniflow venting, by its nature, has a very constricted flow rate compared to a suction setup. This almost always makes the engine go OVER lean when pointed up. That was Uncle Jimby’s problem. Also, the needle is much harder to set since the engine must be set RICH on the ground and will go leaner in the air. It is very tough to needle a 4-cycle, and predicts results, when the engine is very rich. The suction setup allows you to set the engine at peak (or close to it) and then the engine will naturally richen slightly in flight. This shows you that the suction system has a higher head pressure in flight than the uniflow system.

As far as repeatable results making several clunk tanks that perform the same, I once again bring up Uncle Jimby at the Nats. I built that tank (like 10 minutes), and set the shim, and the performance was nearly DEAD ON! I really think the location of the suction vent is not an exact requirement, as long as it sufficiently to the front inboard so as to be immediately uncovered as the airplane takes off. I have moved the suction vent up and down severely and it showed no effect at all! But, by any elementary fluid dynamics, it shouldn’t care where it is located as long as it is not in the fuel. The only other tube to locate is the clunk line, which should leave the tank at the exact same location on each tank, at least ensuring that the “hill” the fuel climbs is the same every time. Even though, again, we have tried moving the exit of the clunk, and at it had little, if any effect (this includes the PA 65).

By the way, Al Rabe does not use a clunk. He uses a metal tank with a very severe wedge, and the last I saw him flying the BBQB, it worked as well as mine. No problem… At that time, he was venting to the inboard front of the tank, just like mine. Al has since changed his venting, but I really feel that his troubles were in no way associated with the venting of the tank. It is really not important though. By the way, the venting that Al described, a uniflow to the front OUTBOARD of the tank, is what I call “Russian uniflow”. My friend Jonuz Abcnynzki (Polish National Champion) showed me that venting idea (it is also in the plans of the “classic” 70’s era Russian .35 stunter in a PAMPA from a few years ago). The idea being that engine is uniflow vented in level fight but suction vented when the nose is pointed up. I think that venting might work better with the 4-cycle than uniflow, but again, setting the needle would be more “complicated”. To that end, why bother?

$3.00 and very light. Why question what the Stunt Gods have provided?

Bob Z,

What size motor is in the staggerwing? I know you like the small 4s motors alot. They are cool arent they. My 26-C I got from the Don Dixon estate runs good on 30% nitro and spins 12000 on the ground. I am going to under prop it so I can get to the real HP range whish is 17500 to 21000. Cant wait! I thought I would let you onto a tank that I tried out this weekend and it worked great. It is a Hays blow molded tank. The 4oz one is what I have on P-force for my 26. It only really needs a 2oz tank. The cool thing about the Hays is it there is no stopper or anything to have to put together. There is only one way to plumb it. The pressure line will always be uncovered in flight. It takes 3 minutes to build if you take your time. And they are about 3$. Check em out they come in cool shapes to. They are very very strong. They are the standard stuff in the YS setups. They dont blow apart from the crank pressure. I will use these whenever I can. Every one builds exactly the same so the setup will be very consistant from tank to tank. There is also no tubing involved. So it cant go bad. These are neat. I am going to try them out on my PA 65 next.

Doug, I am not doubting your findings, just questioning your theory. If you have good "pull" overhead from centrifugal force, then the fuel is going to be in the outboard side of the tank as well. It has to be. If the engine, alone, is holding the plane up, and not centrifugal force. then, perhaps, the fuel will fall away from the pick up tube when it is in a fixed location as in a metal tank. However, Brad's comments about having to angle the metal tanks, and suggesting that there might be more yaw there (as I suspect -- and I might add that yaw is not always obvious) still leads me to surmise that the fuel is moving forward in the tank, not away from the outside. This would also lead me to think that Brett's metal tank design would work equally well, if not better than the clunk tank. Again, I am guessing, but it would be an interesting experiment to try.

>...By the way, Al Rabe does not use a clunk.
>He uses a metal tank with a very severe
>wedge, and the last I saw him flying the
>BBQB, it worked as well as mine. No problem…
>At that time, he was venting to the inboard
>front of the tank, just like mine. Al has
>since changed his venting, but I really
>feel that his troubles were in no way
>associated with the venting of the tank.
> It is really not important though. By the
>way, the venting that Al described,
>a uniflow to the front OUTBOARD of the tank,
>is what I call “Russian uniflow”. My friend
>Jonuz Abcnynzki (Polish National Champion)
>showed me that venting idea (it is also in
>the plans of the “classic” 70’s era Russian
>.35 stunter in a PAMPA from a few years ago).
> The idea being that engine is uniflow vented
>in level fight but suction vented when the
>nose is pointed up. I think that venting
>might work better with the 4-cycle than
>uniflow, but again, setting the needle would
>be more “complicated”. To that end, why bother?
>...

I think this is (or was) a common misconception that fuel flows to the back of the tank. Since a well trimmed stunter can well have 3g's to the outside compared to 1 g of force down (from gravity) The fuel remains basically to the outside of the wedge at all times. If the nose yaws out (and it does naturally since it is ahead of the center of gravity and the plane is constantly turning in) then the fuel will actually flow more to the front than to the back of the tank. The only exception might be when the plane abruptly changes directon which can cause some "sloshing" in the tank. But just pointing the nose up does not necessarily mean that the fuel is now going to the back of the tank (any more than pointing the nose down means it all falls to the front.)

Maybe we need to start another thread on tank theory.

LAST EDITED ON Aug-14-02 AT 08:41 AM (CDT)

One thing you and Brad are forgetting. The clunk doesnt move forward either just like the fixed pu. Take a clunk tank and turn it upside down long ways exits towards the bottom. The clunk just lays against the tank but it doesnt move forward. It just siwngs around in the back of the tank. It does not move forward. I know this becuase I had one get stuck forward once. The clunk does not move forward. If a severly yawed condition existed the fuel moving forward would effect both tanks.

You are probably correct about fuel not falling away from the outside. I just cant come up with anything else.