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Cowl flaps and scat tubes


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I have all the cowls off for the first time while I’m replacing the cowl flap control cable and noticed there there’s a scat tube that’s not connected anywhere. It comes from the round opening just above the air filter but the other end was not connected. Any idea where it goes? 

Also, any tips on cowl flap control cable replacement appreciated. 

Thanks

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9 hours ago, Skates97 said:

I hate to disagree with Clarence, but I don't think he has a cooling shroud on the fuel pump. It should go to the generator.

You may be correct, I looked up the OP’s stated N# and it doesn’t come up with the FAA, so I can’t figure out the year and serial #.  I’ve looked through all 3 C model IPC’s, none are really clear in what hoses go where.

Clarence

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8 hours ago, HRM said:

As long as we're on this topic, does anyone know where this SCAT tube is supposed to go? In the photo it is just lying there, not attached to anything. Comes out of the doghouse.

IMG_2256.thumb.JPG.175e3ea74c04746dfcebc08f5c833efa.JPG

I believe that it should go to the fuel pump cooling shroud, which your pump does not appear to have.  The Mooney IPC’s are not really to clear on these details.  The hole in the front left baffle should be cabin heater inlet, and the generator cooling isn’t shown on the front right baffle.

Clarence

4E657317-AF4D-4049-BAAB-B817CBFA79A3.jpeg

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1 hour ago, M20Doc said:

I believe that it should go to the fuel pump cooling shroud, which your pump does not appear to have.  The Mooney IPC’s are not really to clear on these details.  The hole in the front left baffle should be cabin heater inlet, and the generator cooling isn’t shown on the front right baffle.

Clarence

Thanks, this makes sense. I have an alternator and it may have been a conversion from a generator and no longer needs that tube.

The hole in the left front has a rubber plug in it.

I thought maybe the SCAT went to a cooling port on the mag or something, which it doesn't have, but then there isn't one on the other mag--wouldn't it be symmetrical? I cannot find anything within range of that tube that has a fitting to receive it. Also, the string that wraps around the outside is disintegrating, meaning that it may have been flapping free for some time.

I went to the drawing you posted where you can clearly see the port at the back of the doghouse (they even provided a cutaway), but nowhere, anywhere does it discuss what attaches to it; i.e., the SCAT hose, and then where it goes. 

I have always had issues with that side (C2 & C4) being hot and now I am wondering if I should just plug that port, which would force more air down over the cylinders. Apparently that is what is happening on the other side and whatever once received the air from that port no longer exists.

image.png.f99b46768b230cde46cb66b23884e5b7.png

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3 hours ago, HRM said:

Thanks, this makes sense. I have an alternator and it may have been a conversion from a generator and no longer needs that tube.

The hole in the left front has a rubber plug in it.

I thought maybe the SCAT went to a cooling port on the mag or something, which it doesn't have, but then there isn't one on the other mag--wouldn't it be symmetrical? I cannot find anything within range of that tube that has a fitting to receive it. Also, the string that wraps around the outside is disintegrating, meaning that it may have been flapping free for some time.

I went to the drawing you posted where you can clearly see the port at the back of the doghouse (they even provided a cutaway), but nowhere, anywhere does it discuss what attaches to it; i.e., the SCAT hose, and then where it goes. 

I have always had issues with that side (C2 & C4) being hot and now I am wondering if I should just plug that port, which would force more air down over the cylinders. Apparently that is what is happening on the other side and whatever once received the air from that port no longer exists.

image.png.f99b46768b230cde46cb66b23884e5b7.png

I think that makes a lot of sense.  I had issues with cyl #2 running hot and it was poor baffling on that side; anything that causes pressure loss in the plenum above the cylinders is going to cause loss of cooling...IMHO.

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6 hours ago, HRM said:

Thanks, this makes sense. I have an alternator and it may have been a conversion from a generator and no longer needs that tube.

The hole in the left front has a rubber plug in it.

I thought maybe the SCAT went to a cooling port on the mag or something, which it doesn't have, but then there isn't one on the other mag--wouldn't it be symmetrical? I cannot find anything within range of that tube that has a fitting to receive it. Also, the string that wraps around the outside is disintegrating, meaning that it may have been flapping free for some time.

I went to the drawing you posted where you can clearly see the port at the back of the doghouse (they even provided a cutaway), but nowhere, anywhere does it discuss what attaches to it; i.e., the SCAT hose, and then where it goes. 

I have always had issues with that side (C2 & C4) being hot and now I am wondering if I should just plug that port, which would force more air down over the cylinders. Apparently that is what is happening on the other side and whatever once received the air from that port no longer exists.

image.png.f99b46768b230cde46cb66b23884e5b7.png

Some of the temperature issues on the left bank may be the gaps in the baffle joints caused by missing screws.  I doubt that the small Scat duct has much effect, consider on the J model that the oil cooler is mounted on the back left baffle.

Once the baffles are absolutely tight, you can alter the width of the Lycoming inner cylinder baffle to increase air flow to the cylinders.

Clarence

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4 hours ago, M20Doc said:

Some of the temperature issues on the left bank may be the gaps in the baffle joints caused by missing screws.  I doubt that the small Scat duct has much effect, consider on the J model that the oil cooler is mounted on the back left baffle.

Once the baffles are absolutely tight, you can alter the width of the Lycoming inner cylinder baffle to increase air flow to the cylinders.

Clarence

Trying to learn here; not being argumentative.  I had small gaps (maybe 1/2") in my baffle seals and closing those helped a lot (15-20 degrees cooler).  A small scat tube has a pretty good cross-sectional area... on a par with the seal gaps.  As you say, the baffles need to be 'absolutely tight.'  So, I'm having a hard time understanding why a scat duct wouldn't be significant?  Also, an oil cooler presents a pretty significant restriction to airflow; plus it's engineered to be there.  That is, the 'leakage' through the cooler would seem to be much less than what would flow through an open scat duct.

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@Nick X


my 65C... the front small hose went direct to the generator... The generator didn’t have a fan like most alternators do...

Getting a hose clamp to hold the hose on that shaped aluminum part was interesting...

 

other air cooling devices...

Two blast tubes were welded to the top of the dog house and aimed directly at each mag...

I’m pretty sure something was cooling the mechanical fuel pump... but I don’t recall the answer... or the hose at the top of the dog house...

So... if your alternator has a fan... and your generator has been removed... it’s cooling hose probably got repurposed or removed...

While you are in there... check on the seals around the starter and alternator... some of these have been lost or changed over time...

CHTs are dependent on nicely controlled air flow... anything that changes randomly can really harm the air flow balance...

so... if that 1” hole up front isn’t going to the right place... it might be a smart thing to get it plugged properly...

You don’t want to add any pressure below the cylinders... this would disrupt some flow from the top of the cowl to the bottom..l

PP thoughts only, not a mechanic...

Best regards,

-a-

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On 12/7/2019 at 7:36 PM, Nick X said:

 

Skitch Image.jpg

That little hole with the mesh was a mystery to me back when I got my plane.  There was nothing on the other side. The consensus here seemed to indicate it was ducted to cool my now long-defunct generator, so I sealed it up to help my cowl efficiency.  The C model doghouse cools rather poorly in my opinion, so every little bit should help.  I've never had anything off the back of the doghouse like you do to cool my engine driven fuel pump. I'd take the lack of a cooling shroud on this pump more seriously if someone could convince me it was important, but no one has said anything about it.

 

 

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Maintaining tight baffle metal pieces, sealing every conceivable gap, maintaining the forward seal and installing it in the groove in the cowl should result in a high pressure differential from above to below the cylinders which should help with cooling.

I’ve been unable to find the fuel pump cooling shroud in the IPC, but a number of Mooneys I’ve worked on have them and they’re not the standard metal version similar to a Piper Cherokee.

Clarence

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34 minutes ago, Raptor05121 said:

Mine goes to the mechanical fuel pump- no shroud. Just zip-tied in place to blast the pump

That is exactly like mine...and I don’t have a tube going to my generator, even though the generator has a provision to attach a tube. Now I am wondering whether my arrangement is correct....  there is no outlet at the rear of the doghouse for an additional tube. Should I use the scat tube to cool the fuel pump or the generator?

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