Mike Pratt Skyray question?

I think you are using the modern schnuerle Fox 25 BB, not some baffle version? This is probably a lot of power for the Skyray, and I believe it will want to run fast. I'd guess you would be better off running the tank as a uniflow, and using a lower pitch prop on the Fox 25, setting the mixture to run a little on the rich side of a two cycle. It is tough to setup a uniflow on a profile, because you often need a very rich setting on the ground to account for the pressure head you get in level flight. This is made worse by the fact that your tank edge probably sits pretty far outboard of the spraybar.

Muffler pressure (with uniflow or normal) may help out, since it typically dominates the pressure, making your setup less sensitive the changes in the pressure head of the tank.

You didn't say what size venturi you were running. I would recommend a .257 or .261 with that set up using the Fox or an ST needle of .155 diameter. If the venturi is too big, you will get too much variance in speed from beginning to end (too little suction). Also, don't set the engine too rich. If your lap times get too fast, go to longer lines, but the 4 inch pitch should work fine with the right line length.

I hope this helps.

Sorry to disagree but the problem isn't the tank, he either has to large a venturi or an airleak, my guess would be an airleak because he said it worked better with the overflow as the vent. I am using Chicken Hoppers on every profile we fly (6) and am not having one single problem with any of them...

Tim,

Wished I could be more help but have no expierence with the Fox, you already checked the pluming and tank for leaks which leaves the NVA try a piece of fuel line over the needle to make sure no air is getting in through the threads.

I am not an chicken hopper or uniflow expert, just someone fighting the same problems. After fighting air leaks, fuel foaming, tanks have turned into their own sport. I use 35 stunts and older fox 25's on skyrays and other profiles.

I have lots of problems with the 2 chickenhoppers I own. Mine seem to work best when either the uniflow is capped or both lines are open. With the uniflow capped, the engine (35 stunt) speeds up the whole flight like yours does.

With the uniflow open and vent capped, the motor would cut on any tight turn, like it is getting air in the feed line line. I think the uniflow line may be too close to the pickup. It has been suggested that I restrict the uniflow line ( makes bubbles smaller) , but I am just capping it for now. My flying time is limited and messing with tanks eats too much of it.

I get a good enough run out of a Perfect conventionally vented tank for now (at my skill level). A perfect 3 oz tank should fit fine and give pleanty of run time. The skyray has lots of room for a tank. Also I think the standard GRW tank is about the same size as the hopper tank , and should fit anywhere a hopper tank fits.

You can take the back off the hopper tank and see waht is going on. If the uniflow line is within a 1/2 inch of the feel line, move it and run uniflow. Also try restricting the uniflow line (solder or crimp it to about 1/2 size) and extend it from behind the engine out into clean air facing forward.

I hope this helps

Now foam is a possible problem, as noted elsewhere in this thread. If you can shake your jug of fuel and watch it foam up like a pint of Budweiser, you have a foam problem. You should never have to add more than one or two drops (as in little bitty drops!) of Armor-All (original forumula) in the fuel to kill the foaming. If you have a foam problem after shaking the jug, add one drop and shake it again. The bubbles should disappear very quickly. If not, add the second drop. Do it again and you will see no more is needed. I hate using Armor-All because it contains other components besides the anti-foaming agent and I don't want to contaminate my fuel. So keep it to one or two drops. This is another case of "if some works, more is not better."

Adding a foam pad under the tank and more rubber bands seemed to correct the foaming problem, but the uniflow problem escapes me. The chickenhoppers have me scratching my head. I doubt I will use another.

This is on a fox 35 powered shoestring.

I would be curious as to what your venturi size is. According to my chart it should have about a .257 or .261 for stunt with the installed (.155) Fox or ST needle.

or the stock fox 25 bushing setup for both the needle and choke area

fox super fuel or sig 10% plus castor.

if it was fuel or venturi related, why would it run well withthe uniflow capped?

If someone has one of these (stock) and a calipers, maybe they could take a reading just to compare the stock venturi with my numbers. Just remember, a lot of CL engines are not "stunt" engines as set up by the factory, even though they may have a venturi and CL needle. Take the same engine and run it under pressure, and you will have no problem. It may be as simple a thing as changing out to a smaller venturi to solve your problem with uniflow (and make a better stunt engine out of it to boot).

Or, to put it another way, if one engine works on uniflow and another does not, it is not uniflow as such that is the problem. I would look first at the engine that does not work with the system rather than blaming the tank. Uniflow plumbing does work.

Taffinder's fuel won't foam if it's in fact the same exact formula that Doug used before he retired from the business end of this hobby.

Leonard Neumann

Yes, around the Chicago area I think. There is a 1/4 or 1/2 page add in one of the latest Stunt News's. Look in the last three issues, the add should be in there. I don't have access to my copies of SN where I am at right now.

In these quantities we ship motor freight which costs us about $1.75 per gallon from Texas. UPS charges more than that shipping in case lots, plus adds the $4.00 a gallon HAZMAT.

Leonard Neumann

I ordered a case a couple of seasons ago and was quite happy (except for the UPS Hazard fee!), but recently have been using Sig since I pick it up at the yearly Sig Contest.

Now if I can just get Fred K. to pick up some fuel for me on his way back and forth to Purdue, maybe I'll switch back!

As long as the needle mixture appears to remain constant throughout the flight, I would say the Uniflow is doing its job.

FWIW

Curt

Beginner in Michigan

Agree. There has been too much whining about Perfect tanks by some of the regulars here.

When you're done chasing problems with your Uni tanks, return to the old standby.

I agree with many of statements that were posted on this subject by modelers. Profile models are a little different because they are easily affected by engine vibration (Fox .35 & OS .35S engines). The reason for this is the piston and connecting rod are traveling in a horizontal axis or plane to the profile fuselage. Think about jumping on the end of a diving board. The force is transmitted to the diving board and the board deflects, once the force is removed, the board springs back. This same thing is happening to the nose of your profile model. So why do build profile models at all? Because, they are easy to build and most of the vibration problems can be eliminated by making sure that the front end of the model is very stiff.

Here are a few suggestions to keep in mind when you build your next profile stunter.

(1) Make the plywood doublers extend past the high point of the wing. This distributes the vibration and load over a larger area of the fuselage.

(2) Longer maple engine mounts. This may not be necessary on some of the modern kits but it is a real good idea on the older ones. How many of you have broken the nose off your Ringmaster of Flight Streak.

(3) Never cut out the engine opening in the outside plywood doubler otherwise the engine mounts will act like a tuning fork and vibrate back and forth. Many of the kits manufactured today still cut out the engine opening in the outside doubler. The reason for this is so the builder can’t install the doubler on the wrong side of the fuselage. Make a new doubler without the cutout and make sure it is installed on the inboard side of the fuselage.

(4) Always make a large radius fillets on the fuselage and wing joint from a mixture of epoxy and micro balloons or Epoxolite putty. Again, this will help distribute the vibration over a larger area of the wing and fuselage joint.

(5) Laminate .5 carbon fiber matt between the doublers and the profile fuselage. This really helps strengthen the nose and the weight penalty is almost none existent.

(6) The addition of a 3/8” or ½” balsa cheek cowl to the outside of the inboard plywood doubler is a good idea only it must tie into the leading edge of the wing to be really effective.

(7) Mount the fuel tank with “J” hooks and rubberbands. Isolate the tank form the fuselage with a thin sheet of foam rubber. The rubberbands and the foam rubber will dissipate and /or soak up a lot of the vibrations and help keep the fuel from foaming.

All of the above suggestions would add very little weight to the model but greatly improve the consistency of your engine runs.

Mike Pratt

As they say everyone needs to find what works for them..

At the time my main re-entry ship was a Flight Streak built from the Top Flight Kit with an OS 35 S on the nose. During this time I also helped the wife build a Ringmaster and we threw together a couple of Ukey-35's.. Could not get a decent engine run on anything.. I was really getting frustrated, couldn't even think of trying to fly the pattern until I had an engine that would run for more than 2 minuets and quit in the wrong place at the wrong time.. Finally after a crash caused by a flame-out I took the OS off the Streak plugged and re-drilled the mounts for an OS 25 FP. It just so happened to have a Chicken Hopper tank on it at the time and I left it. I think this was the 3rd tank and many re-plum's of the second I tried. I followed the advice I saw in a post by Brett Buck, Air filter, 9-4 prop, stock muffler and the results were fantastic.. I could finally fly the full pattern with the Streak and had great engine runs... But wait.. I wasn't through learning...

Now I needed a better stunt ship so I built a Twister in Ringmaster dress and the TwistMaster was born. Beefed up nose and all... So hey I shouldn't have any trouble with the 35 S on this, after all I built several stock Twisters in the 70's with 35 S's and never had a problem. Wrong... here I was with the same old engine run problems all over again. Tried different tanks, foam etc. I was sure the basic problem was vibration but was not able to find the right combination to a good engine run. Finally out of desperation just before my first contest in 25 years I pulled the 35S and stuck on a 40 FP with an OS 25 FP venturi and two head gaskets.. First test flight it did the full pattern without nary a burp.. Made the contest and it is still flying great to this day...

In the meantime I was given an overweight full fuselage Oriental.. in went the OS 35 S and the results were as expected the engine ran great, no weird runs or unexpected burps great combination till I missed up and pulled too much fuel out.. Quit exiting the clover but it wasn't the engines fault....

Anyway the only conclusion I can come to out of all this is that profiles and vibrating engines do not mix. There are those that can find the right combination to get them to work but I am not one of them. The ones I have seen work usually have a 1/2" balsa slab on the inside of the nose. Why fight it when you can pick up a 25 FP for $50.00 or even get one of the new LA's for not much more.... put the fox and Max S in a full fuselage ship and it will run great.

So the problem is vibration and sticking a 25 FP or LA on it will fix it no matter what tank you are using..

Tanks really don't cause these sorts of problems. This is some internal issue with the engine, or a vibration problem.

>It would be nice to be
>able to concentrate on just
>flying without so much diddling
>with the engine.

That's *precisely* the reason I like the 20/25FP - it's almost a turn-key system for a profile model, and plenty strong enough for a Twister.

To cure most of your engine run problems, build a full-fuse airplane.

Basically, if the engine doesn't needle properly--first off, don't fly it. And if it goes lean in the air, bring it out level and fly it out.

To check this, pull the needle out, and if you can look through the spray bar, do. Otherwise, attach a tube to it and blow through it, or attach your fuel pump to it, and squirt some fuel through it to clean it out (backwards, if you can.) You don't always see the "junk" so this is a good proceedure if you just suspect a problem.

The "junk in the needle" situation can happen at any time--with or without filters. It is easily correctable (just clean it out) but very frustrating if you aren't aware that this is the problem.

Leonard Neumann