I'm working on my Hunter build, and have a question regarding fully enclosed engine installations and jet pipes.

The kit comes with a simple single walled jet pipe, and I'm thinking of replacing it with a dual walled pipe, primarily to help keep the rear fuselage cool. The engine is fully enclosed within a sealed bypass, and I can't see how a dual walled pipe would work in this case. There is no airflow path to get bypass air into the rear fuselage for the ejector part of the dual wall pipe to extract. I'm assuming that the front face of the pipe fits inside the end of the bypass ducting to make a continuous flow path from the intake to the rear of the plane. Does both the inner and outer bypass path of the pipe fit inside the engine bypass?

The kit was also supplied with a spun aluminum bell-mouth, which I assume fits inside the rear of the bypass to protect the glass bypass from the heat.[]

I have seen a picture where some holes were cut into the contraction portion of the bypass, to spill some air into the rear fuselage. Is this standard?

Any advice on installing dual walled pipes on a bypass, including pictures would be appreciated.

Thanks

Paul

Paul

you might check with Joey over at JT hobbies near benbrook. he has been making pipes for a while, including keith sullivan's t-33... he might be able to take your existing single wall and add a second "wall" to it...

This is a great question, again the kind of stuff I come onto RCU to read about.

Most installation I have seen (read that "all of the installations I have seen" ) have the front of the outer pipe OUTSIDE of the bypass...more on the alternative later in this post....

The normal mechanism for getting cool air to go through the outer pipe is a venturi effect, formed at the rear of the plane near the nozzle. This generates a very strong negative pressure in the fuse. Of course if the fuse is completely sealed, then no air will go down the outer pipe. I have seen this handled in a number of ways. The original bandit had a NACA duct in the top of the hatch to let the air in. The DL cyclone I built had a cutout in the bypass (I think Avonds did this as well) to let high pressure air into the fuse....slick. The BVM F-4 I built has a couple of slots cut under the inlets to let in air.

In a lot of planes the gear openings are enough, or given the gear door fits I have seen on some models I don't think those guys need to worry about getting air into the fuselage ;-)

Now a VERY slick system is to have the bypass air come from the bypass. I remember Tad from Golden West talking about this years ago, not sure if anyone else has done it but I know Tad had the idea like 6 years ago. I still think this idea is very good, having the outer pipe mate completely to the bypass. I know Tad has done a couple of custom installs in Super Bandits here in LA, those planes are awesome, it really made a difference.

Now of course, the insulation properties of the pipe exist even without airflow, but they are very greatly diminished!

I have seen the naca inlets quite often. to my best non-engineering knowledge, think this would be the better solution. there is probably a very fine line between a non or negative pressure in the fuse, vs. a positive pressure in the fuse, which i would imagine to be bad, very bad.

edit> i agree with matt in that there are only a handful of builders who could get the fit and finish of all the potential openings sealed tight enough to cause problems.

Matt,

Thanks. Great info.

On a related note, the diameter of the rear opening of the bypass, and hence the diameter of the supplied single walled pipe is about 3". The rear exit hole in the rear fuselage is also about 3" and the pipe is at least 30" long, maybe longer. I don't have the opening in the fuselage to install anything much larger in diameter. If I go for a dual walled pipe, this means that the inner (exhaust) pipe diameter will have to be smaller by maybe at least 1/2" diameter, maybe more, in order to keep the outer pipe diameter under 3"

Will a smaller diameter pipe cause too much blockage and hence thrust loss or a hot running motor? I'm using a P-80 for power.

Thanks

Paul

That pipe sounds too small to me (2.5 inch inner), ask Tam (although I think he is still in china)

The gap between the inner and outer does not have to be 1/2 inch smaller, could be less and you will still get the effect.

On my little F-106, I used a dualpipe setup similar to what Mr Matt first described. The outer pipe is fed by the interior of the model, not the bypass. The air that feeds the interior of the model comes from a forward facing slot formed by the inlet's boundary layer splitter plate and the fuselage. It's ram air, and should provide cooling around the bypass itself, before exiting the outer tailpipe passage. [8D]

Mr Matt's second concept he described, I think is used on the BVM F-86 with the RAM500 installation, I believe

Here is what I did, simple yet very efective. I took 6 of the dubro Pushrod Exits and placed them equal distance apart from each other and right at the portion of the bypass where it starts to turn up towards the pipe. I placed them facing the rear intake. This way it is picking up the left over air so to speak that spills over from the intake inside the bypass and routes it towards the outer portion of the duall wall pipe.

I wish my camera was working I would take a picture for you. My crude graphic here hopefully will explain it a little better. The red circles is where I placed the Exits. Hope this helps.

Avonds cut 1 NACA shaped openings (as they are not true NACA ducts) on each side of the intake ducting to let air spill into the fuse for pipe cooling. The single wall pipe is short of the tail cone by about 1/2" to create the venturi effect that Matt was talking about. Using a single wall pipe, heat proofing using BVM heat shield or ceramic blanket is a must for ground ops as air is not flowing well through the fuse on the ground.