Thorp Air Command - T18.net

I just bought a T-18 that is a little tail heavy with 9 pounds of lead strapped to the top of the engine and front baffle. This surprised me since it has a 160 hp O-320 and I know the T-18 was designed with the Lycoming 4-cylinder in mind. I hate dead weight in an airplane. My personal preference is to balance with functional weight than to use ballast.

The current battery is an Odyssey (PC680?) located behind the passenger seat. The first thing I looked at was moving the battery to the firewall but I have my doubts there is room for it, let alone do any maintenance on it when needed (may not be enough to eliminate the lead anyway). I may be able to move it 2 feet forward pretty easy. The original battery box is against the seat back and the Odyssey is mounted further back.

The prop is a Sterba. I was actually thinking about changing it before I knew about the lead. My opinion is that wood props are only suited for "severe clear" flying weather and I expect I will be in the rain some. I also read a thread in this forum about inertia rings being used to help Lycoming/wood prop performance. I don't know what that would weigh but it may be enough to balance the aircraft a little better.

The weight of a metal Sensenich 40+ pounds and would certainly bring the balance point forward. The brochure for their ground adjustable composites did not list a weight for the O-320 prop.

I can't seem to find much weight information on the internet for the composite props. Wood and composite share a downside on repairs when compared to metal. Does anyone have convenient weight numbers for composites? Maybe what theirs was at install?

Are there any recommendations on which way I should go to bring the weight forward or other options I have missed?

Thanks,
Jeff J

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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

Jeff,

Let me be the first to respond to your queries and I'm sure there will be others I hope, along the way. Before I start moving items from one end to the other, I would perform the weight and balance on the airplane to see where it stands as far as the CG is concerned. The thing to remember is that the CG on the Thorp is between F.S. 61 and F.S. 71.

Never heard of dead weight on the engine and if there has been some, someone need to chime in. The ring you refer to is a weight ring secured to the back of the starter ring weighing twelve pounds. I have one and use it when I use the wood prop which I'm about to take off for refinish and install the Sensinich. The prop erosion which occurs with wood or composite props generally occurs when flying fast. In other words, when the airplane is moving fast across the skies and not so much as the propeller rpm. As a precaution, I do back off on throttle if I happen to come across rain; however, if you have a hard finish on the prop, erosion is minimized.

I too had the Odyssey 680 mounted under the passenger seat and while it was easy to get at for whatever needed, I decided to move it to the right side firewall and have a jumper cable accessible for jumper cable if ever needed. By moving the battery to the firewall, I shorten the cable by some six feet. Starter was not getting the required voltage to turn engine fast enough due to poor grounding connection. The cable to starter solenoid is now 12 inches or so long.

Again, do the math before cutting and I'm sure you should be able to find enough space on the firewall for the battery box. You may have to move some components and that shouldn't be too difficult.

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In CowTown USA,
Don D-Day
NX18DD

9 pounds ! OH MY ! Yep..start out with a fresh W & B . Then I would take a closer look behind the seats/baggage compartment all the way to the tail to see if an undecided voter has set up camp back there ! I have heard of some guys finding old radio gear back there (remote compass/ADF gear etc.) that would not tip the scales in your favor ! As Mr. D sez you can put the PC-680 on the firewall and it works quite well . Shorter battery leads are a plus , in addition to moving the battery weight forward .

RB

BaBaLoo

I actually woke up this morning thinking I needed to run that thing across the scales when I get it home.

The tail heavy concern came from the previous owner who had been flying with a heavier engine installed. He was describing how it felt to him. Since it made through phase 1 plus a couple of hours after the upgrade I am assuming it is safe enough to fly but I will improve it if possible. Excess lead is for making fishing sinkers!

I had another thought... Was the T-18 originally designed with the battery on the firewall? It seems like the posts I have read concerning battery installations behind and/or under the seats had to do with installing heavy props. With a wood prop and a Skytech starter, a stock battery on the firewall would make a big difference and possibly explain the excess lead up front. The Odyssey isn't as heavy so moving it may not bring it all the way in but it would help. Additional bonuses for the shorter leads are causing less voltage drop and fewer potential chafing problems.

The more I think about it the more I want at least some of the lead off of it. I think having 4 pounds of lead bolted to the baffling is asking for trouble in the form of cracks and breakage. Baffling doesn't seem to hold up all that well under normal circumstances but then most baffling I see is over 40 years old. I don't like the 5 pounds on the engine but I don't see where it would actually cause a problem.

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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

The original plans call for the battery behind the baggage compartment. Something is wrong in that bird. There is no reason for any weight up front. All good suggestions. First, snoop inside the tail cone. Take anything out that you can. Then W&B. If the plane was built to plans, you should be fine. I have an O320 with an AD wood prop up front. My battery is an Odessey behind the baggage. My W&B is too far forward with no pilot. My fat ass brings the plane into proper balance.
Cubes

I second that ... I have an 0-320 with a CS and an Odyssey on the firewall as well as almost zero avionics (comm/xpnder/MGL EFIS). My W&G is forward but well with in the envelope (let's just say I can haul some serious baggage).

Lee

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Lee Walton
Houston, TX
N51863,N118LW
KEFD

I have an "old-fashioned" heavy battery behind the baggage compartment - a gell-cell, O-320, a lightweight starter and a lightweight wood prop. Flew it for a few years with no problems before adding the steel ring to my flywheel/ring gear assembly. It was after shedding the original Prestolite geared "boat-anchor" starter that I added the steel ring. The airplane has always had what I consider an ideal weight distribution. Like Lee, I can load a tremendous amount of weight in the baggage compartment and with a passenger, or fly it empty & solo and remain well within the envelope.

Weigh the airplane and do the calculations.

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John Evens
Arvada, Colorado
T-18 N71JE (sold)
Kitfox 7 SS N27JE

I like the battery position in the picture above but I have an oil filter occupying that space already. I found an extra ELT behind the seat so there is ~4 pounds that can disappear. Looking at the aircraft file, this bird appears to have always been heavier in back. The very first tail weight was 57 pounds. Current sheet shows 67 pounds. I will have to figure out the arm for the ELT and see what that does to the balance. It won't be a big change. I am thinking barely noticeable on paper.

_________________
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

BADA BING , BADA BOOM ! Oil filter below PC680 .

RB

Actually, my preference would be to install the filter on the engine and loose the extra hoses. Less weight along with less to inspect and maintain with better access behind the engine. I am already having issues with the hoses being in my way for mag maintenance. I am redoing P-leads today because the terminals were soldered and the mags were grounded through the shielding when the switch was off. With less than 8 hours on the system one of the terminal rings had broken free of the shielding in the cockpit and, while I was looking for that break, I broke one on the mag end with a light tug. I have always agreed with the FAA (AC 43-13-1B chapter 11 sections 14, 15 & 17) on this one, solder really is a poor choice in a high vibration environment.

On the bright side, my battery seems to be fine. Voltage drops to 10 and holds under a load and at rest still shows over 13 volts.

_________________
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

Jeff, what filter options have you looked at that are on the engine? On my Piper 0320 it is on the engine and I like that. Just like you said it has less points for leak and less weight. If you find a decent and affordable one for the 0290 let me know. I think 400$ is a little too much. If you can't find one I might be able to bribe one of my machinist friends to make a set for us (he can cut prices by doing multiple).

One of the best discussions I have encountered on the subject of oil filter options is here:
viewtopic.php?f=35&t=7704&hilit=oil+filter

It hits most of the pros and cons along with some good pictures. There are other good threads as well. I have never changed oil on anything with the filter canted up so don't know if the mess concern is valid there or not. I don't get a filter full from one mounted horizontal. If an aircraft is going to be flown frequently then I think the original screen is plenty good. There are 2 IO-520's here that are past TBO (one is 500 hours past) and they have been just fine without filters. I think we have changed one cylinder out of the 12 but these get flown 2-4 hundred hours a year.

_________________
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'll take a look at that. Sorry for "jacking" your thread off topic. I just realized I did that.

As for the filter verses screen. The filter is easier to change than pulling the screen out so I prefer less mess. Having been an engine builder for team kawasaki I'm a huge proponent of filtration. With the amazing oils we have these days oil break down isn't as much an issue as wear particles. It was designed with a screen so I'm not concerned about a screen being sub-par although anything I can do above and beyond and as simple as adding a filter I think I should do. It's an easy upgrade and if it helps only a slight amount it's worth it in my eyes.

As far as engine wear. I have read that most of all the wear comes from the first few seconds after start up.

IF, (and that is a big IF) that is true, then it would seem like a method to have an electric oil pump to "pre-oil" the innards including the crank and cam in the engine would be the single best thing that could be done to minimize engine part wear. This is probably particularly true for ANY engine that is not used frequently. I believe this pretty much explains why airplane engines that are run daily or really frequently will often easily reach or exceed TBO with little to no problem. While other aircraft engines don't fair well that only fly a few hours a year. This is I believe due to the oil film disappearing from the long times between flights or engine runs. This causes the parts to be starved of oil. Making them susceptible to engine wear on start up and of course the lack of oil makes them extremely vulnerable to corrosion. A "pre-oiler" will not eliminate the corrosion as much but would minimize the wear aspect. Although if a person who has put his plane away for the winter or longer period of time, if they could go out to the hangar and run the pre-oiler on a regular basis, they may at least coat a good portion of the innards with a new coat of oil to help minimize internal engine corrosion.

I wonder if someone has a system to pre-oil the engine. If it was not too heavy or complicated, it might be worth it in the long run.

Filtration is great. And do not forget good AIR filtration. A LOT more air is pumped through the engine than gasoline. And much of the dirt therefore comes from dirty air.

Just a couple thoughts to consider.

I didn't say filtration was bad by any means but you can't really compare a nice, tight kaw racing engine to the sloppy tolerances of an aircraft engine. It is an apples to oranges comparison at best.

If you have the battery for it and are wired so it can be done, the engine can be pre-oiled by cranking without letting it spark. That is how we pre-oil fresh overhauls and engines that have set for a long time except that we normally (but not always) have the plugs out so it will spin faster. Pulling the engine through by hand a few turns before starting would help get the oil moving too. Do it long enough and you will see oil pressure start to come up if your gauge reads low enough. Something like LPS 2 can be also sprayed in the cylinders if an engine has set long enough for the cylinders to dry out.

_________________
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