combatpigg

My Feedback: (3)

Howdy all...got a couple of questions. I've got a Nelson FAI engine and have had all kinds of trouble keeping pipe seals from blowing out. The replacement seals from PS only last 3 minutes if I'm lucky.

Has anyone here come up with a solution?

if not.........

Does anyone know what the temperature is at the exhaust port?

I'm about ready to contact Hylomar to see if they have any advice, but I thought I might get lucky here first.

[email protected]

We use VITON o-rings and they last, or seem to last forever. I'm surprised that Daves Orings are failing- have you got plenty of cooling air around the exhaust stack? regards trevor hendersonnz

combatpigg

My Feedback: (3)

Trevor, thanks for the reply...and the ray of sunshine! To answer the question about cooling, I'd say NO, I haven't made any special effort to duct air back there. It's just a simple cowl with a big hole for the cylinder head to poke through and a smaller one for the carb. The rear of the cowl is wide open, the idea was to direct air past the bulge in the pipe.

I'm using a single wrap of music wire around the cylinder and through the provided holes on the pipe to hold the pipe to the engine. I've got a pedestal in back to support the stinger.

The engine came with what looks like a hunk of silicone tubing for a seal and the PS seals are red O rings, I guess these are VITON?

garys

The Nelson FAI engines went through pipe o-rings, but not in three minutes. If I remember right, we usually changed them after three flights or so (although I guess that would probably be closer to five minutes of run time..30-40 seconds on the ground and just over a minute in the air per flight). As Trevor said, make sure you're getting a bit of cooling in that area, in addition, make sure there aren't any burrs or sharp edges on the front of the pipe that may be damaging the o-rings as you put the pipe on.

combatpigg

My Feedback: (3)

Thanks Gary, your experience gives me some idea what is normal to expect.

The need for a soft connection between the engine and pipe I understand, I just think it could be beefed up somehow.

Is a single loop of music wire good enough to hold the pipe to the engine? In the past one of the provided holes on the pipe broke out. I wasn't sure if it was because of me making it too tight or not.

What kind and size of fuel line are you using?

I'm using a length of what looks like thin wall latex to go through my line pincher shutoff.
The line goes from the tank, through the shutoff, then to a Perry inflight mixture control valve. From there the line is medium silicone up to the NVA. The nipple on the needle is kind of a loose fit for medium tubing, not as snug as I would like. Maybe there is a type or brand of tubing that is just right?

Ed Smith

See the pictures. This is a cowling from an F3D racer. The shroud is a tight fit on the upper case. This forces the ducted air through the fins, around the back and then over the pipe. There is about 1/8" clearance around the pipe.

I never had overheating issues with this configuration.

Ed S

combatpigg

My Feedback: (3)

Ed, thanks for the look at your cowl.

What are you using for pipe seals? The inside diameter of my pipe where it slips over the O-rings is about .851". The outside diameter of the O-rings as they sit on the exhaust stack is about .850". It takes no effort to slip the pipe onto these O-rings. I find it hard to believe that this is a working set up?
I had plenty of sucess with the original seal with the rectangular cross section. I have not had 1 good run with these replacement O-rings.

wrstew

My Feedback: (30)

How about a wrap or two of teflon tape before the o-ring goes on to tighten the fit? You could slice a thin ribbon to "shim" the fit. Might work? Silicone O-rings might help also.......Randy

combatpigg

My Feedback: (3)

Hi Randy, I didn't think about teflon tape, I'll give that a try. I attacked the problem from the opposite end this morning. I made a press in bushing to tighten up the fit of the O.D. of the O-rings. It helped, but I didn't put enough of a squeeze on the O-rings.
On second thought, if I shim the I.D. of the O-rings with teflon tape....that will lift them slightly out of the machined groove that serves to anchor them. Still worth a try, thanks

luv to race

My Feedback: (1)

Your very lose slip fit when putting the pipe on is much of the probelm. sounds like you are sayint the ID of the pipe is larger than the OD of the header on the motor? The pipe shouldn't just slip on. It should take a little extra effort and a little oil on the o-ring to get the pipe to slip on.

So you have to look at what pipe you are running. We always used a special made Macs pipe, Macs made those pipes for PS and the FAI engine. The pipe I see in the photo is NOT what we always ran back in those days. If you keep running that same pipe. You already know that you need a much larger OD o-ring. I'd be more than happy to measure the OD of a MB motor, but all my stuff is in Germany as we are headed to the worlds today. MB's o-rings last 25-50 flights if you take care of them. When we get back I can mail you some of those o-rings, if the OD is larger than you say the Nelson is.

Also.. The first few inches of that pipe as if comes off the motor header... is way too hot. So you for sure need to get some cooling back there somehow. The piston won't take that kind of heat, and it will cause the rear of the piston to swell. So work on that too...

Randy Bridge

Ed Smith

There is a technology to "O" rings and their fit. For a given O ring the diameter and width of the groove in which it sits is very important. The diameter of the pipe going over the O ring is also very important. There is a very close relationship between this diameter and the outside diameter of the header. Tolerances on those dimensions must be held to within +-0.002.in.

If an O ring is continually failing and is of the correct materia,l the groove/pipe dimensions are probably incorrect. Stuffing the groove with teflon tape will not work. The ring will not seal. Another thing, ONE "O" Ring only in a groove.

Ed S

combatpigg

My Feedback: (3)

Thanks very much Randy! I'm going to make a couple tiny NACA ducts somehow and graft them onto the existing cowl. After speaking with Dave S. this morning the original O-rings for this header were cut out of standard silicone tubing. I'll bet routing cool air will do the trick along with locating some of the original tubing to make seals out of. PS doesn't have much in the way of spare parts left for this engine, so I'll be happy to keep it going for another couple years.

What max size prop would you say this engine is happy with for the long haul?

Don't tell me 6x3, that wouldn't be nice.

Ed, they cut the groove in the exhaust stack too wide for a single O-ring, so I'm going to track down high temp silicone tubing and slice my own rings.

GSJames

Be careful. NACA inlet ducts only work if they are in an area of higher than ambient pressure. The inlet must be in a high pressure area, and the parallel wall exit duct must be in an area of low pressure, otherwise there won't be a decreasing pressure gradient along the duct and it won't work. If the air has been accelerated to a speed significantly higher than the free stream, the pressure will be low. Good ol' Bernoulli.

combatpigg

My Feedback: (3)

Gary, thanks for your response and good evening to you.

I've got to wonder, wouldn't any spot I choose on the front of this 200 mph plane qualify as higher than ambient?

What is ambient at 200?

At some point I've got to rely on instinct or give up....how would I ever know what area of my cowl is above ambient?[]

freeair

the best you can do is buy the best quality 0-RINGS which are VITON , they work out cheaper than the fuel costs per runs of say 5.

combatpigg

My Feedback: (3)

As it turns out, I'm getting good life out of slices of 5/8" silicone tubing, thanks for all your advice. Especially about proper cooling.

Now, next question.....

Where should the liner be set? I'm asking about the deck height. I'm running it now at .225" and feel that I'm working in the right direction, but just wondered if any of you guys have notes to save me some time.
I plan on running this in a hand launched flying wing with 475 sq inches and about 52inches of span. I figure a 7x7 prop could get the plane into the 200mph zone without straining the engine too much or making the needle settings too critical.
The problem is future repair parts, so I've got to make this work with the advice of others who've already made all the mistakes, that's only fair, right?
[..please don't send me to the extreme speed forum to research how to run this FAI engine, they'll just tell me to buy a Jett].