I put together my first fuel tank tonight and the metal tubes had to be cut back to about 1/4". I had to do this because it was running into the back of the engine and kinking the fuel lines otherwise. It's goinig on a Sig Sr Kadet with an OS .91. Could this present any problems? If so, how can I fix them? I don't want any fuel lines falling off.

Here's a pic. Not the greatest, but I think you can see what I'm talking about.

Kind of short. I would have just moved the tank back a bit.

If you cut a short, 1/8" length of fuel line and open it up with forcepts and put it over the fuel tubing, the put tubin on the tank, and roll/push the short piece up over the nipple, is should lock it on OK. Should.

Don

I don't think that's an option without moving the firewall. I'm supposed to use silicone sealer and press the tank up against the firewall. Could I put some silicone around the fuel tubes themselves? Would that help hold the fuel lines?

Thanks!

OK, I'm confused. THe fire wall is 1/4" thick, so you tubes should be long enough to extend beyond the fire wall enough to put the fuel lines on. How are you getting the engine close enough to the firewall to kink the lines??? Could you take a photo and post it?

Don

Here some pics. The lid(for lack of a better term) extends throught the firewall only leaving about 3/4" to the back of the engine. This is all the room I have for the metal tubing and fuel lines.

Thanks!

I guess I'm used to the four strokes setting an inch and a half off the firewall. You will be OK, but use the retainers I mentioned before. Let me know if you need a photo of what I'm talking about.


A couple points, don't tighten the bung (lid) to tight as those ARF tanks tend to split open some times. Also, make sure you don't have sharp edges on the brass tubing that could cut the lines. You will not have much luck trying to replace the lines later without pulling the tank. Use some wet or dry and round the edge just a bit. Then make sure you rinse the tubes out with some fuel to get any grit out that may have collected, then install them into the tank.

One last thing, I usually use a short, 1/4' to 1/2" piece of fuel line on the vent tube inside the tank. Just a bumper to speak on the tube so it doesn't have a chance to abrade the top of the tank.

Don

G'day Mate,
Move your engine forward on the mount & rebolt, very unusual that it has to be that close to the firewall. Then you will have plenty of room for your fuel lines.
Even if it is cowled, the cowl can be slipped forward to match. It won't matter, if you have 3mm clearance from cowl or 8mm.
If you are worried about balancing, you can adjust the balance of the plane by moving the batteries, later.

Thanks Don. I could use some pics if it's not to much of a bother.

I sanded down the sharp edges on the brass tubing to avoid cuts in the fuel line already.

I'm not sure about what you are talking about or how to do it on the vent line. How would that help and don't you run the risk of cutting off the vent line with tubing on the end of it?

Thanks Alan0899, I can't move the engine more forward because of the cowling. I'm already a quarter to an half inch to far forward now. If I go much farther the cowling won't fit.

This isn't a "recommended" engine. It's bigger. I know that it will happen with a little help from you folks. It's just a little mod I've got to figure out how to get done with your kind help.

Thanks!

You're putting an OS 91 two stroke in a Sig Senior Kadet. WOW!!!! I think your problem isn't going to be fuel tank lines, etc, etc. It's going to be slowing the plane down enough for landing. You might want to consider swinging a very low pitch prop with that engine. I've flown two Sig Senior Kadets, one with an MDS .68 I think and one with an OS 70FS. They both floated and floated and floated. The Kadet would probably fly fine with a .46.

Good luck,,,,,let us know how it turns out.

When I built my Four Star, I cut the brass fuel lines too short, so that less than 1/4" of fuel tubing was biting on the brass tubes. I pull tested in the shop and they seemed secure. I even put retainer clips on them. The first time I went to fly it, between the heat and the fuel, the lines slipped off and I couldn't get them to stay on. I ended up pulling out the tank and installing longer brass tubes.

That engine is WAY to large for that plane, and if this is your first plane, which your questions clearly indicate it is, that's just nuts.

Brad

I know a guy who has a OS LA .40 in a kadet.

Putting an OS 91 on a Kadet Senior is like shooting birds with a cannon. Talk about your overkill.
Moving the engine that far back is going to upset the recommended Center of Gravity and you will probably end up with a very poor flying airplane. Get a smaller engine or move the engine forward where it is supposed to be and fly it without a cowling.

According to your profile, this is not your first plane, and you appear to have been flying for a while (longer than me).

So the question is: Why such a big engine on this plane? Are you trying to do something unusual with it?

Measuring is ok, but nothing beats a test fit.

1/4" is getting close to too short. It would be better to have just a bit more tube and it's easy to find room for that. The fuel tank inserts into the fuselage right. So insert it 1/4" less than it'd be if bumped up against the inside of the firewall. Worried that it might not be supported by the round hole in the firewall? Fix that with a spacer. It's easy to make one.

You can either use a Forstner bit and drill an appropriately sized hole, or simply glue together the right thickness wood scraps with the right size hole inside them. Then glue the spacer to the inside of the firewall. Use epoxy. And after it's set, mix some more and coat the entire inside of the firewall, spacer and all, and the inside of the fuselage as well.

When you use a greatly larger engine than recommended for any model, you're bolting on a lot more than the structure was designed for. So you need to consider that the front of that airplane is going to need a bit of help. Do what you think would be necessary. Later on, you will have a chance to autopsy and see how your reinforcment worked. If it doesn't happen soon, then you'll get feedback from that. That suggests you did enough. If it does happen soon, that's also good, because you'll see early on if your improvements worked or not. And have a chance to refine your hot rodding skills.

Also, think about the prop that motor is going to prefer. Is the stock landing gear going to give enough ground clearance? It's especially critical on landing. Stock gear is often marginal for the recommended engine's weight. Or is usually marginal when a heavier engine is used. And that's not even considering the prop clearance, just the added weight.

Lot's of things happen when you hotrod an existing model design. And a little forethought can make the hotrod run. Without it, you wind up with a one-shot hotrod. You've already run into a very insignificant result of a shoehorn job. There are lots more to come.

Good thing about our hobby is that we can try almost anything we can come up with. Best thing about our hobby is that we can usually make almost anything work. And no research animals are killed in the testing. Well, other than a few earthworms on occassion.

Thanks for the ideas da Rock.

Is there anyway I can just put some kind of glue around the fuel tubes to help hold them?

This is my second trainer plane. My first one bit the dust after I got in a little trouble with it and it didn't have enough power to pull me out of it. This is also the setup my instructor had on his plane and it worked great. Unfortunately, he lives 500 miles away and is difficult to get a hold of.

Thanks!

What is the need for a .91 in a Kadet? What structural reinforcements have you made to take this much power – to the firewall, fuselage, wings, empennage???

You should have at least 1” of brass tubing sticking out of the fuel tank for the fuel lines to slide on. You need to extent the tubing and bend it (use a tubing bender so it doesn’t kink) so it runs up behind the engine, and inside the firewall. If there is not enough tubing to do this, then get some more brass tubing and make new lines.

Guys who fly Kadet Sr.’s on floats or glider tow use .61's, and that is all the power it should need. If you want to do 3D, then get a Twist 60, Showtime or Funtana to put your .91 on. Kadet's fly well on a .46 AX, and will even fly with an OS .40 FP or LA.

If this was my plane, I’d pull the .91 and install a .61 with new motor mounts. This way the engine should fit the cowl. If you are intent on using a .91, then you may want to consider building up a wood cowl like the Kadet Sr. kit utilizes.

Hogflyer

Your best and safest bet is to re-plumb the tank. Even with the small clearance, you can make the brass tubes about 1 1/2" long with a bend upward behind the motor. You can then spread them to either side once you get them through the firewall. Your fuel lines were having to do this anyways, however the lines were kinking as you stated. The brass tubes will not kink and can route them around the motor as you please. This will take away the worry of lines coming off, and not having to remove the tank to get your lines back on if for some reason they have to be removed.

Get a Hayes tank.

Man no plane i have has an 1" of tubing sticking out. Maybe 3/4" closer to 1/2" but a 1/4 is cutting it close. I like hot rods. I hot rodded a super cub. Put a 480 brushless and wings off of a 540 mini edge. Is pretty cool. Hollowed out the front to fit a 2100 lipo. Well it flys anyway kinda keeps you on your toes. Good luck with this.

come on........you guys dont know how to use your left hands????... throttle management...just so you guys know a 91 size engine is the same size case as a 61 basically the same weight as well just that extra power. Last time i checked he is not a complete newB so stop bashing him on his engine choice and help him out with his problem. Back on topic. IMO that brass tube length is OK. i have had a few planes at that length and never had a problem with them. What i did do though was put mini tie wraps on them and seemed to work great.

LOOK at the pictures on the post just before yours. Help Carbon Fiber ........
It a Sig Kadet Senior and the cowl and engine were moved forward .65 inches
See the 1/4 x 1/4 balsa blocks that were glued and screwed to the firewall.
This was overdone as only one block was needed on each side. You can
probably move the engine and cowl forward about .75 inches max.
Good Luck with your Sig Senior.

Don't use glue or caulk! Eventuall you'll have to replace the lines and then you'll have a problem ever getting a good seal again.

Take a nail set or centerpunch and flare the ends of the metal tubing slightly. This really helps prevent the fuel hose from slipping off. You can slip on small cable ties to further hold it, but it really isn't necessary.

This also get's rid of the problem with the brass tubing rubing through the fuel tubing. Because when they come out fo the brass they are straight so there is nothing rubing against anything.

Austin