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Old 04-18-2008, 01:41 PM
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JollyPopper
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Default More Important?

Is it more important to have the crankshaft on the thrust line of the air frame or to have the fuel tank outlet level with the spray bar in the carb? I have an application that doesn't allow for both.
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Old 04-18-2008, 01:47 PM
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Default RE: More Important?

It is not terribly critical either way for most airplanes. On a trainer type airplane, I'd try to get the spary bar of the carb lined up with the c/L of the tank. This makes for more friendly starting and running the engine. For a pattern plane, I'd try to keep the prop centerline (crankshaft) near the centerline of the fuselage. This makes for a more neutral-flying plane less trimming when inverted, etc.
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Old 04-18-2008, 02:15 PM
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Default RE: More Important?

All airplanes are indeed a group of compromises flying in a tight formation.
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Old 04-18-2008, 03:17 PM
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Default RE: More Important?

Hi!
The thrust line is where You put the engine ...! No where else.
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Old 04-18-2008, 06:55 PM
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Default RE: More Important?

Been there before. Have you tried a different tank type?
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Old 04-18-2008, 07:08 PM
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Default RE: More Important?

Get the fuel tank right and don't worry about the thrust line.
Jim
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Old 04-19-2008, 08:06 AM
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Default RE: More Important?

Put the crankshaft on the thrust line, THEN modify the area the tank goes in so that the center of the tank (where the fuel line to the engine comes out on MOST tanks) is even with the spraybar in the carb. USUALLY, if it is within 1/4" you will have no problems.

Modifying (hogging out the formers) is normally necessary for ANY inverted engine installation to lower the tank. Mounting the engine sideways rarely requires any mods to the tank area and mounting the engine vertical occasionally requires shimming of the tank to raise the tank to the correct height.

As someone else said, A model plane is a series of compromises.
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Old 04-19-2008, 09:28 AM
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Default RE: More Important?

I have never seen the slightest evidence that the position of the thrust line, within reasonable limits, is important. Many full scale designs have used different thrust lines depending on the engine. The Great Lakes Trainer was first sold with an upright engine and a low thrust line. Then they inverted the engine and moved the thrust line up. Same with the Gypsy Moth vs. Moth Minor. The SE-5a was first built with direct drive, and a fairly low thrust line. Then they changed to a geared drive with a prop shaft above the engine, which raised the thrust line. There are countless examples of this.

When I built my 4 Star 60 and wanted to change the nose, I side-mounted the engine, cowled it in, and moved the thrust line up some. I never worried about the thrust line, and it is a beautiful flier.

At the extremes, yes, the thrust line has an effect. Flying boats with engines mounted up on pylons sometimes have some upthrust to counteract the effect of the high thrust line. But in that case the thrust line is well above the entire rest of the airplane.

Jim
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Old 04-19-2008, 10:54 AM
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JollyPopper
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Default RE: More Important?

OK, let me clarify a couple things. First, I have never been much of a believer that the thrust line had much importance. If it did, it would eliminate high wing multi engine aircraft that clearly have the engines way above the thrust line as well as pylon mounted engines. Anyway, that's another story.

The tank in this airplane is where it is and it ain't gonna move. The front of the tank bay is 2 1/8 inches wide flaring to 2 1/4 at the rear of the opening and is only 3 inches deep. It has a six ounce tank in it that fills it up. I could possibly move the tank up 1/4 to 1/2 inch, but the centerline of the tank is almost 2 inches below the spray bar. I don't know that 1/4 to 1/2 inch would do that much. The problem is that the engine is difficult to prime. Once it is running, it is fine. But after priming it and filling the supply line to the carb, then attaching the glow driver, the fuel has run back into the tank. And this turkey will not run unless the supply line is full. It will fire immediately and die before it draws fuel.

The engine is mounted on horizontal hardwood rails that came with the kit and are according to plans that came with the kit. I could eliminate those rails and bolt a mount to the firewall and drop the engine below the thrust line but that would require removing the floor of the fuselage that extends to the front of the engine bay, also according to plans. In other words, I can't drop the engine without removing the floor of the engine bay.

Now, something is rotten in Denmark here. I can't believe the manufacturer of this kit meant for the fuel tank to be that much lower than the spray bar. I must have done something wrong. However, I built this kit in 1976? or so and have long since lost the plans. I built it long before RCU was even thought about and I was not aware of thrust lines and spray bars being on the same plane as fuel tank center lines and all that stuff. That being said, it has flown well for all this time. I was just thinking, since I have read about all these details here on RCU, if the fuel tank c/l were even with the spray bar, it would be much easier to start and less prone to losing its prime in the air. This is an original Goldberg Falcon 56 with a Tower .46 in it.

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Old 04-19-2008, 01:26 PM
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Default RE: More Important?

Sr. Falcon, I bet. Why have the fuel tank so low? On mine the hatch was hollowed out and the tank was quite a bit higher. I ran an OS 40 FP on it with no trouble, and without lowering the thrust line.

Alternatively, cut off the nose, side mount the engine, and make a balsa cowl.

Actually, I would try to preserve the thrust line just for the looks.

Jim
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Old 04-19-2008, 02:14 PM
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Default RE: More Important?

Hi!
Senior Falcon! There are two options. either tilt the engine90 degrees to the left which means get rid of the wooden bearers and mount a plastic engine mount (Dave Brown mount).
Second : Place the tank higher.
Both are easy to do! The second means adding balsa sides to the fuselage so the tank can sit higher.

Both these modifications take just an hour to do. I would choose the first one.
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Old 04-19-2008, 05:34 PM
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Default RE: More Important?

If you do not want to mount the engine sideways (preferred choice is to mount it sideways), remove the cross bar over the tank and make a new hatch cover that is hollow (or curved using sheet balsa) and attach the cover by using magnets. Radio Shack has small, but quite powerful, rare earth magnets. Imbed the magnet into the fuselage side and imbed a small STEEL plate into the hatch.

I THINK it is Edmund's Scientific that has square magnets (about 60 of them for around $6 ). Each magnet is 1/4" square and has a 6 lb+ pull. I have one plane that uses 4 magnets on a 9" long 4" wide hatch (one in each corner). Doing maneuvers (Loops, rolls, spins, etc) at speeds over 90 mph at times, the hatch has never even shifted position.
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Old 04-20-2008, 11:08 AM
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Default RE: More Important?

A couple good ideas here. Rotating the engine on its own axis doesn't solve anything. I would have to remove the right side cheek for that to work and if I dropped the engine below the thrust line, I would still have to remove the floor. However, building the sides of the tank bay up sounds like an intriguing idea. I hadn't thought of that. Might look kind of goofy with the wing lower than the tank bay walls and the turtle deck at its present height. However, it would solve the problem. That would be an easy fix. Re-routing the nose wheel and throttle controls might be a little tricky, but it's worth a look. Or, like Campy says, make an arched hatch cover, move the controls, and retain with magnets. I like the magnet idea better than the cross bar anyway. Hadn't thought of that either.
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Old 04-20-2008, 11:14 AM
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Default RE: More Important?

Or fabricate an inverted "U" shaped hatch cover using the legs of the "U" to raise the height of the tank bay rather than building up the sides of the fuse and use magnets to retain it. I REALLY like that idea. Campy, are the magnets that Radio Shack sell small enough to embed in the fuselage sides without being obrtusive?
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Old 04-20-2008, 12:10 PM
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Default RE: More Important?

Yes. If I remember correctly (I haven't bought any in awhile), the rare earth magnets were about 1/8" in diameter.

If you are concerned about the thickness of the fuselage side, just CA some scrap balsa to the inside of the fuselage where you plane to put the magnet.

The way I do it is to make a hole where I plan to put the magnet, then "attach" the magnet to the flat side of a 11 blade. When I am happy with the fit, I put the CA in the hole, insert the magnet and "wipe off" the 11 blade against the side of the fuselage to leave the magnet in the hole.

Don't forget to inlet the steel plates so they sit flush. FWIW - I used some scrap sheet metal (24 gauge ?) I had gotten at Home Depot and cut strips 1/4" wide x 1/2" long using the cutoff wheel on my Dremel.
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Old 04-20-2008, 04:08 PM
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Default RE: More Important?

One more question: Does putting two magets togetner, north to south, double the attraction or is the attraction between the same magnet and a piece of steel just as strong?
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Old 04-20-2008, 09:37 PM
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Default RE: More Important?

Jolly, what matters is the relationship between the carb and the fuel tank, so rotating the engine on its own axis DOES solve the problem, by lowering the carb - it's a common way of getting carb and tank into alignment. But it's true that the nose has to be cut.
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Old 04-21-2008, 09:29 AM
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Default RE: More Important?

BuzzardBait, you're right in that rotating the engine on its own axis would lower the carb. My earlier post was wrong. But it would only lower it by, what? A half inch, 3/4 of an inch. I need about two inches to bring them into alignment. Rotating this engine would no doubt help the problem but not cure it. I think.[)]
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Old 04-21-2008, 03:21 PM
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Default RE: More Important?

Functionality or esthetics?
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Old 04-21-2008, 03:35 PM
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Default RE: More Important?


ORIGINAL: JollyPopper

One more question: Does putting two magets togetner, north to south, double the attraction or is the attraction between the same magnet and a piece of steel just as strong?
Magnet to magnet is a little stronger, however the magnet to steel plate is easier to align (and cheaper - less magnets to buy)
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Old 04-22-2008, 02:37 AM
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Default RE: More Important?

how much higher would the tank be in that former over the top of it were completely removed and the tank risen to flush mount to the hatch? would that put it in line with the carb? if so remove the former, pad the under tank area with foam to raise the tank up flush in the front and then move that former back just behind the tank, then you wouldn't have to move any controls or anything, just the former! forget about thrust line unless you need it to fly level inverted with as little control movement as possible. this plane is meant to fly, it will get ground sick if you are too particular! just move that tank up and make it easier on yourself! or you dont have to do anything besides this: i had an evolution .46 that the tank had to be mounted about 2 inches below, so what i did was, every time i refueled, when i disconnected the fill line i squirted about a thimble full of fuel into the mouth of the carb and that would run it long enough to draw fuel. i never did have any trouble out of it (i was hand flipping the prop, btw, and it always started like a champ unless i forgot to shoot the carb full of fuel. that also never flooded it.)
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Old 04-22-2008, 09:12 AM
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Default RE: More Important?

Actually, the "former" is not really a former. It was not part of the original plan. The hatch cover is secured in the back with two 3/16 dowels, much like a wing, and has a screw in the front that screws into a blind nut in that cross piece. No matter what I decide to do, the cross piece is going to go away. I have ordered magnets from Radio Shack to secure what ever hatch I decide to go with. Campy was right. Radio Shack sells rare earth super magnets 3/16" in diameter, and I am going to use those to secure the hatch. The control rod for the nose wheel actually touches the top of the tank, so if I fabricate a domed cowl, that would have to be moved, probably to below the tank and then move the arm on the strut below the bearing. The throttle control can be changed to a cable configuration rather than control rod so it would be flexible enough to go over, around, or under the tank. I just wonder how goofy that thing would look with a domed tank cover that would be substantially higher than the wing?

A lot of good ideas here, guys. We'll get it right yet.

Oh, and to answer your original question, fozjared, moving the tank right up against the existing hatch cover would give me 1/2 to 3/4 of an inch. I need 1 3/4 to 2 inches to bring the center of the tank level with the spray bar. I think part of the problem is that the Tower .46 is a tall engine, and from center of crankshaft to spray bar is a greater distance than the OS .40H that was originally on the airplane. I don't recall the spray bar in that engine being as far above the tank as this one is, but maybe that's just because I didn't know they had to be level and just never paid attention. Unfortunatley, I don't have the old OS to check against the Tower.
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