12.1.2 Intake tube modification

This entry is part 11 of 50 in the series 12 - Engine / Propeller

After the engine was installed we discovered that the upper cowling was hitting the engine in a couple of places.

This is the number 5 cylinder intake tube. It’s hard to see, but it is in contact with the inside of the upper cowling.

There is also contact on the number 6 cylinder intake tube and the main intake port.

Now there are three ways to fix this.
1) Lower the engine. I don’t like this approach. When installing the engine, the airframe is leveled, the engine is leveled and engine location is determined based on the location of the prop in the opening of the cowling. I could shim the top of the engine mount, but that would change the thrust vector. It would also require modifying the cowling to accommodate the new location of the prop.

2) Create “bumps” in the cowling. This is done fairly often. Especially when using non-standard engines or engines with turbo-chargers and intercoolers. The cowling mod can be done over a large enough area so that it’s not too unsightly, so I’m going to call this “plan B” for the intake tubes. It will, however be” plan A” for the main air intake.

3) Modify the intake tubes. This is a tricky approach. This engine has what is called a “balanced induction”. Which means (among other things) that all the intake tubes are the same length. One of the ways these tubes are modified is by cutting off the part that’s too high and welding a flat plate over it. I don’t like this method because you’re drastically reducing the size of the tube which means that you’ll be changing the volume of air that can get to the cylinder.

What I’m thinking of is to change the routing of the tube. By my calculations, the length of the tube will be about 3/4″ shorter after the modification. That makes it “plan A”.

The first step is to find someone who can do it. I searched and found a company that has made a whole set of tubes for a low-profile installation. But they haven’t made any for quite a while and they’re expensive.  After exchanging emails with them I got the impression that they weren’t very interested.

I trolled around the Lancair forums (they run the same engine) and had no luck there. Then I found out the company that made the low-profile tubes were done for the Venture Questair. This basically a propeller driven rocketship. Very small (2 people) and very fast (300 knots). So I started trolling the Questair forums. No luck there either.

Then something interesting happened. I decided to sell the small 2-1/4″ backup instruments I had got from Albert. I put them on Ebay and everything but the airspeed indicator sold (not many experimental aircraft need a 400kt airspeed indicator). But one of the bidders asked if I was interested in selling even though he didn’t meet the reserve. I decided to left him have it. Turns out he has a… Questair! I told him what I was trying to do and he put me in touch with a guy that’s done a few intake modifications.

So now I have to come up with a plan of attack. The first thing I did was to build a jig. I figure the guy doing the mod won’t have an engine handy and he’ll need to know if it’s going to fit.

This way he’ll be able to see if the tube ends will be in the correct location. Then I created a template that shows the shape (or outline) of the tube from two dimensions. You can see where I’ve already done that on the picture above.

Next, using the side view I identified where the cowling was and determined where to make the cuts.

This will lower the profile so it doesn’t hit the cowling. A straight piece will then be added to compensate for the loss of run length.

To test this, I need to build a mock up of the intake tube. So I got some 1-1/2″ PVC and built a duplicate tube.

Then marked the locations of the cuts.

Made the cuts.

Cleaned up the ends and attached.

Then I made a cut on the straight portion and added a piece about 1-3/16″ long.

The final product.

On the jig.

And this is the number 6 cylinder intake tube and the PVC mock-up.

The ends align and based on measurements, it should clear the cowling, cylinders and other hardware on the engine. But the only way to know for sure is to send it down to Malcolm and see how looks in the engine.

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