Thorp Air Command - T18.net

I was researching something unrelated and stumbled across a good cooling article in the new letters. http://thorp18.com/forum/newsletters/T- ... r_1-44.pdf PDF page 164 of 376 or page 144 looking at the bottom of the pages.

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Thorp T18
O-320-B3B (160 HP)
68x74 Sterba Propeller

"The joke in aviation is, 'If you want to make a million, you'd better start with £10m.' " -Bruce Dickinson

I didn't know what pics you needed and it appears I have them all turned sideways. I'll get the extra pictures and post again. As for the upper lip, the fiberglass curves in from the top a few inches pointing upwards and aft towards the top of the cowl. I think that's what you were asking. I'll get a picture of it tomorrow when I pull the cowl apart again to check the tappet adjustments.


I do not have any seal from that point. I guess the extra seal I have left I could make one. It appears riveting it to the fiberglass would be the best option then it resting on the top of the baffling shelf although I don't like putting extra holes into fiberglass since it tends to work its way through. It seems like riveting that to the baffle shelf would still allow it to be pushed down by air. I think if I extended the shelf a little with aluminum sheet metal so it's fully under the front lower lip would reduce this issue. What do you think about that?



The only issue with spreading them wider is the fact that the cowl is only a certain width. If I spaced it out further I don't think my screw holes will line up because my top cowl sheet metal wouldn't extend further out. I could probably space them a little for a test but I'm not sure I could get more than a couple washers between the cowl and the bar. On the fuselage there is a sheet metal flap that extends 2-2.5" into the engine bay and is curved inboard. I think that's what you were asking about. So it funnels the air into the side outlets. I can get better pics of this with the cowl off for you.



I'll get measurements on those intake and outlets to compare the flow. I think the side outlets are too small as well. They are noticeably smaller than the chipmonk cheek cowls everyone else around here has. That was the reason for the AP recommending to cut the bottom cowl since my side exits are narrow with not much way to make them bigger without making a whole new cowl. He even suggested building something with a piano hinge so I could open and close it like cowl flaps based on outside temp. I'm waiting to do any major mods like this. The picture between my exhaust stacks was to show I could probably get 1/2-3/4" spacers in my bottom cowl to see if that increases the flow out the bottom and cools temps prior to doing any major modifications to the cowl.



Ya, I thought about that as well. Since my #4 doesn't extend to the 6 oclock position on the cylinder like my #3 does I thought about taking thin sheet metal and sliding it down behind the cylinder and riveting it into place after I have it curved and all the way to the 6 oclock position. If I make that I can put a bend at the end which would allow me to use a spring to hold pressure against the cylinders. It never had the springs before and the baffle I have is made out of quite thick material and not easy to bend. I think they did this for that exact reason. Just by how hard it was to bend without putting a micrometer to it I would say easily 80 thou or 100 thou thick.

1albee: I don't plan on doing any major mods at this time since temps are getting better and weather is about to break from 90's down to 70's this weekend. I'm holding out on major mods until after I get my first oil change on this new top end. The oil consumption I think is really good for a break in compared to my piper. It was eating over a quart an hour when I replaced 2 cylinders last year.

Jeff J: can you get the dimensions of your side exits? Are you using a 320 or 290? My exits seem really narrow compared to the other type cowl I see commonly.

Cubes: I was only doing the touch and gos to get comfortable with the landings after 8 months of not flying tail wheel. I did 5 of them and it was on a 9000 foot runway so I was letting it get down to a crawl before taking off again. I will not be doing them any further. I will be spending long xc time in the plane to give it more break in time.

As for the probes. This is a used unit. I have not tested calibration on the sensors. I assume by the "distance from the heads" you are talking EGT probes which I have down to within 1/16th of an inch of each other as measured from the exhaust flange. The CHT probes are screwed into the bottom port on the cylinder heads. I did not cut any wires. The extra length I zip tied neatly away from everything since I might transfer this over to my piper. I didn't want to cut wires shorter. Some are different lengths from the previous owner but they were properly cut at the 25 pin side and had new pins put on. No splices anywhere in the system. When I get the cowl off wednesday I can see if I have enough length to swap my cht's from front to rear cylinders to check them.

Probes and unit were all purchased as a combo from JPI (I have a jpi 700). I am the second owner and have all the original things from the first owner. No new probes or replacement probes added.

The left side gill on mine has an opening about 3 1/8 inches as measured in the middle of the opening. The right measured 2 3/8. Both sides are about 22 inches tall giving me about 69 square inches on the left and about 52 square inches on the right. I am running an O-320 that had 3 hrs SMOH when I bought it. I don't think there is much difference in physical size of any of the Lycoming 4 cylinder engines and, in theory anyway, more power would generate more heat. I was trying to find something in the drawings last night concerning cowls but came up empty. I don't know what the reason is for having a wider opening on one side versus the other. It may have to do with spinner/cowl alignment since the engine does not sit square on the front or it may have just happened that way. Either way, mine stays below 350* CHT indicated on the rear cylinders with the cooler of the two being on the left. My oil cooler is on the baffle behind number 4 and I don't know what bearing that has on the discussion either. My EGT probes are positioned about a 1/2 inch different so their readings differ quite a bit (60-70 degrees). I do not have probes on the front cylinders.

I am NOT advocating making changes to your cowl at this time. I think your last round of data indicated your temps were getting better. Having said that, if you were to make adjustments to the cowl, I do not think it would be that big of a deal. If you have nutplates holding the cowl to the bow (cowl mount) you may gain enough slack to experiment by removing the nutplates and switching to nuts or you might be able to relocate the holes on the bow or even make new bows and drill them to match the holes in your existing cowl. The real issue would be filling any gap created between top and lower halves but even that would not have to be overly painful. Especially on a trial basis where it would not have to be pretty.

Be careful with your baffle to the cowl clearances. If engine movement on the mount causes anything solid to hit the cowl you will get vibration and possible damage to the baffle and/or the cowl. It may not happen over night but eventually it will wear a hole or break a baffle or both.

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Thorp T18
O-320-B3B (160 HP)
68x74 Sterba Propeller

"The joke in aviation is, 'If you want to make a million, you'd better start with £10m.' " -Bruce Dickinson

Oh wow, I thought those measurements you put out were of the intakes. Mine are considerably thinner. I can barely get my hand in that back area with my hand flat. I would be safe to say 2" or less on those exhaust vents for mine. I'll measure them tomorrow and calculate the input/output ratio. Even though I'm only 135hp the flow might not be enough out the cowls to get good air exchange. I might even borrow an old airspeed indicator and check my pressure gradient in the cowl as indicated in that newsletter you linked. Thanks for that good piece of info!

I keep looking up info on chrome cylinders and if they take longer to break in than steel cylinders. #4 is chrome plated with cast rings. The others were fresh honed with new chrome rings. That could possibly account for some of it if they take longer. The 440 on climb with only a few hours I'm comfortable with. I'm going to put a good amount of hours on it this weekend and the outside temps are calling for low 70's instead of 90's like we have experienced. I'm going to keep checking things and making small changes to the baffles just to tweak it but ultimately no big changes until that first oil change and after I get cam guard back in the oil as that is what I ran prior to the top overhaul. If i'm still high temps at that point then I might look into purchasing the different cowl design because I don't really like this cowl anyways.

I found this this morning and decided to use it to illustrate the point I was trying to make about clearances.



The inlet ramp is worn and cracked where it is hitting the rivet to the right. The ramp is cutting the baffle tab on the cylinder even though there is a full quarter of an inch clearance between the 2 parts with the engine off. This installation can't have more than 40 hours on it...likely less.

_________________
Thorp T18
O-320-B3B (160 HP)
68x74 Sterba Propeller

"The joke in aviation is, 'If you want to make a million, you'd better start with £10m.' " -Bruce Dickinson

So Ship, I guess I don't have a ramp on the upper cowl. Mine is fiberglass so the only way I can think to add one would be fiberglass one since rivets into fiberglass will most likely oval the holes and wear out. Here are some pics of it. Also I wasn't sure how well new fiberglass would bond to 40 year old fiberglass.

https://drive.google.com/file/d/0BwwDkR ... sp=sharing
https://drive.google.com/file/d/0BwwDkR ... sp=sharing
https://drive.google.com/file/d/0BwwDkR ... sp=sharing

So, here are the updates so far along with my flight today.

A&p checked valve clearances. #4 was loose so he put them back in spec. Checked timing and it's solid at 25*. He didn't recommend reducing it.

The #4 exhaust is 1/2" from the intake runner with no heat muff like the right bank has on it. So I insulated the #4 runner.
Built extensions on the baffles to wrap around the #4 cylinder and the head. There is a picture showing the new shiny ones under the old ones. The old ones barely wrapped around.
I used aluminum tape into the intake portion of the cowl to seal the gap around my intake pipe. There was a gap that I think was causing pressurization of the lower cowl which would slow cooling.
I put the CHT probes all in boiling water and found them to all be within 3* so the probes are all similar.
Plugs all looked took. Only some slight oil on a couple lowers. Color was consistent on all plugs with a light brown. AP said they looked good to maybe a touch rich. Checked resistance and they were all under 2k ohms except 1 of them at 3k ohms.

Today it was nice and cool out. Upper 50's when we took off.
On climb out the rear cylinders were still 50-70* higher on cht than fronts. At a 500fpm climb I maintained 420's on #4 and slightly less on #3. 3&4 were within 10-15* throughout the flight.
In cruise my rear cylinders were 380-390* and fronts 350-370. Egt's consistently warmer on the rears with an 80-100* spread from all egt based on my JPI gauge. Cruise at 2500 rpms

So things have stabilized between rear cylinders now. I did some testing with leaning to rich of peak with the gauge. With the cooler temps today I didn't have to lean much to get to peak. Maybe 1/4" out with my mixture. #2 peaked first. I still get reduced performance and drop in rpms if I push full throttle at any aspect of flight. That makes me think rich although with leaning not taking much to hit peak I think I might be lean at the end of my throttle play which could contribute to uneven mixture and higher cht. I have an MA3 SPA carb which is discontinued. I was quoted over 1000$ for an overhaul "if" they could find the parts. I've also thought about switching to a throttle body injection system which would be just over 1000$ instead of trying to throw money at an old carb overhaul that might not even help.

Thoughts on the mixture settings and routes to go to look into what seems to be a carb issue as well?

Well, the flights yesterday in upper 70* weather were still 420-430 on climb out then they drop down fairly fast to the 380-390 on the rear cylinders. 3/4 seem to stay about 5* apart now.

I think the higher temps on climbout as well as the bog when pushing full throttle on take off or in cruise flight is a lean condition on the carb. I have a ma3 spa with part number 10-3364. The other one recommended for 290 is a 10-3565. Spruce lists one that one but not the 3364 I have.

I'm nervous to just put a larger fuel nozzle in it since they are around $100. If that doesn't work it could just be worn out parts although everything appeared in order when I removed it to check this problem last year (it has always existed since purchase). I also don't want to spend 900$ on an overhaul and it still be too lean of a carb. What to do, what to do.

Anyone have an o290 or continental 300 carb that I could use to check?