I have a new Thunder Tiger Pro 46 which cuts out in a 3D move. Such as a roll or loop. Does the fuel in the tank stay at the back of the tank when performing a loop or a dive.
I was told that the fuel line inside your tank needs to be flexible enough to move to the front of the tank when the nose is down, as in a dive. But I don't think this actually happens.
I think that centrifugal force keeps the fuel at the rear of the tank.

The klunk line should NEVER fold towards the front. The end of the klunk should be at least 3/8" away from the back wall of the tank.

Dr.1

I agree with Dr1. The fuel line inside the tank should be flexible enough to move freely to all 4 back corners of the tank, but should NOTbe able to fold forward. An easy way to prevent the line folding forward, but retaining the flexiblity is to put a piece of brass tubing in the middle of the fuel line. If your fuel line is folding towards the front of the tank, fix that.

Also, just a quick clarification: a simple roll or loop does not really constitute "3D". And a plane with a proper fuel set-up will not cut out in 3D moves, much less a roll or a loop.

Dr1 is correct, the fuel line should never move to the front of the tank. In fact, I usually put a small piece of stiff tubing in the clunk line to keep it from moving to the front of the tank (see attached diagram). When you are in maneuvers like you described inertia will keep the fuel at the back of the tank.

Hope this helps

Ken

You might want to check your tank. I was having a problem with mine, I was getting only about 1/2 the time in the air, and whenever I went to a nose high attitude the engine would sputter and stall (interestingly enough, in a simple, tight loop the engine was fine). I took the tank out of the plane and found that the hose the klunker is on came off the brass tube. When I'd go to a nose high attitude all the fuel would go the back of the tank, and the brass tube would be out of the fuel.....so.....no gas, no go.

Since I fixed the tank it's been working great.

Oh, and centrifugal force is created by objects spinning, what your thinking of is inertia (object at rest stays at rest, object in motion stays in motion). In a dive the fuel would tend to 'float' in the center of the tank.

The problem with the fuel stiffner is that when you have approximately 1/3 of a tank left or less, and you go nose down, the engine can become fuel starved.

The thought is that you don't want to have the clunk stuck at the front of the tank where it can get lodged using standard tubing.

However if your tubing is flexible enough (you WANT thinner tubing for this) this should not be a problem at all, and you CAN permit the clunk to travel forward and rearward freely, provided that it cannot wrap around the fuel inlets themselves.

A longer piece of inlet tubing can prevent the latter.

You should always take the tank out and make sure that there is absolutely NO way that the clunk can get stuck and try to make it get stuck as much as you can, to check.

Many of the commercial tanks, such as the Dubro tanks, come with the thinner tubing that permits forward movement of the clunk, while prohibiting it from wrapping around the inlets or getting stuck.

If your fuel in inside your tank is too flexible and your clunk move forward it will pinch your line. A pinched fuel line will kill your motor. If you are running lean it will kill it almost instantly.
If you are suffering a lack of fuel it may be evidenced by bubbles in your fuel line. Check it after your dead stick landing.

This still doesn't happen. Remember, you are dealing with inertia here. Even with only a small amount of fuel in the tank, if you put the plane in a nose down attitude the fuel will STILL remain in the back of the tank. It's nothing but pure physics. The inertia of the fuel and plane keeps the fuel in the back of the tank. The only time fuel will flow to the front of the tank when the plane is nose down is when there is no forward motion on the plane. I'm sorry to say this, but there is NO circumstances where the clunk should ever be in the front of the tank. As Missleman said, if the clunk were somehow to go forward the fuel line would be pinched anyway and not pickup any fuel.

Ken

No you don't have it quite right.

The fuel is traveling the same speed as the airplane. It will "lay flat" along the bottom, if the plane noses down, the fuel flows downhill starving the engine.

The only time that the fuel stays at the back of the tank is when the plane is accelerating at a relatively high rate (say 6/8 "G" dependant upon inclination). Then inertia kicks in.


This is pure physics...



There is no problem with the clunk traveling forward provided that it cannot get entangled or stuck. If anything this is preferable as it permits the clunk to remain where the fuel is.

The fuel line does not get pinched if you have things set up right... especially it's length.

You mean if it the line is too flexible....

If the fuel tube is of proper length and build this is not problematic.

Usually people are replacing the provided tubes with ones that have walls that are in turn too thick.... (or too thin).

In a case such as this the tubing can pinch off as you've said.

Proper sized tubing will prohibit this as well as a little common sense.

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If you nose down w/o the clunk traveling with the fuel, you'll lean out and kill your engine, something you can also see after your dead sitck landings.

Keeping the clunk in only the rear quadrant starts affecting flight at approximately 1/3 of tank of fuel. At this point the clunk will become exposed to the air inside the tank during nose down flight where-in the plane is not accelerating foward at approximately 6/8 of a gee or more dependant upon inclination.... and that in turn means it happens quite often.

That does bring up a question for p40hawker.
How much fuel is in the tank when this happens?
Does it happen when the tank is near full or toward the end of your normal flight time.
I had something similar happen recently with my airplane. Same motor TT Pro .46,
Turned out my problem was that my motor just did not like 15% fuel. It would run fine until I pushed it with some aerobatics. I went back to 10% fuel and have had no problems since.
Fuel pickup issues in your tank are the most likely cause but not the only possible one.

If you're getting flameouts during rolls, what happens during a loop or other downhill maneuvers doesn't really matter.
Add me to the growing list of people who do not think the fuel tubing should ever move forward. I've used the setup Ken drew. On smaller tanks, I've even gone so far as to solder a small section of brass tubing directly to the klunk so I only needed one piece of flexible line in the tank.

Pull your tank and confirm that the klunk can easily move to all 4 rear corners and that it can't move forward, and potentially pinch closed, when the nose is down. Also confirm that the klunk doesn't hang up on the rear of the tank as it's moving around.
I think the whole discussion about the fuel being in the front or back of the tank is a red herring. If you're having trouble with rolls, there's something else going on.

For most maneuvers, there will be enough fuel at the rear of the tank to keep the engine running. There are a few things to keep in mind:
1) It's not a good idea to do long, steep descents as the fuel is getting lower. Aerobatics should be done while the tank has a good amount of fuel in the tank. If you're going to do descending maneuvers, get used to the idea that you're not gong to be running the tank nearly dry during your flights.
2) Most (all?) descending maneuvers should be done with reduced throttle. This drastically reduces the fuel flow. Even if a small bubble gets in the line, the duration of a typical downhill maneuver won't be long enough, due to the low fuel flow because of being at reduced throttle, to reach the engine. By the time the bubble, if there even is one, gets to the carb, the engine will be powered back up and would barely notice it, unless...

How do you set the high speed needle on your engine? Do you just peak it and back off a bit? Do you then perhaps give it the famous "pinch test" and figure it's ok? Following is my suggestion for tuning. I learned this somewhere around 25 years ago from my instructor, who flew masters pattern. I figured if it was good enough for him, it was good enough for me:

Tuning – High speed needle
The following method is designed for sport flying where one wants reliable engine performance (minimum deadsticks) and long engine life (part of which comes from avoiding lean runs).

I’ll start off with a reminder that all needle adjustments should be done from behind the engine and that the plane should be firmly anchored or held. Never attempt to adjust the needle while holding the plane in the air. A small engine blip or wind gust could result in fingers in the prop.

Begin by gradually leaning the engine until you find the peak RPM. Once you find this, turn it several clicks rich. Once you’ve found this slightly rich setting, with a firm two handed grip on the plane, point the nose straight up. While you are doing this, remember to keep the propeller arc out of anyone’s face. When you raise the plane’s nose, one of three things will happen to the engine RPMs.

If the engine speeds up slightly and holds that speed for 10 to 15 seconds, pat yourself on the back (after you put the plane down), you’ve got the needle set pretty well.

If the engine slows down, or does not speed up a bit, it’s too lean. Lower it immediately, turn it a few clicks richer and try again.

If the engine speeds up significantly, it could probably be a bit leaner. Put it back down, set it a bit richer and try again.

End of tuning guide.

PS. My instructor was also the one who taught me that the klunk should NEVER go to the front of the tank and that a piece of rigid tubing would help prevent such a thing.

There is just not enough space in the typical tank for a .45 size plane to allow ANY fuel line to bend to the front of the tank without kinking. While it might look ok on the bench, when you factor in all the things happening in the air, including turbulence, and the fact that the line is now subject to a vacuum, setting up a klunk that can move forward is a recipe for flameouts. As I said in my previous post, the fuel moving to the front of the tank is a red herring. If he's having problems with rolls, something else is going on.

The clunk on any type of line will not function reliably if it goes to the front of the tank. The flameout situation is a mixture or mixture /glow plug situation.

I have tried with no success more than once to induce fuel starvation to kill motors that had suffered throttle problems and would be at anything from 1/4 to 1/2 throttle. They have ranged from a SIG Seniorita to a H-9 Sukhoi with a ZDZ 80.
The seniorita sould be flown until it was barely visible and put into a vertical dive that lasted for close to 15 seconds the engine never missed a beat. The Sukhoi displayed the same situation so I flew it around until it went dry and dead sticked it. It had my prop on it so that was my best choice. The seniorita flew on for about 35 minutes until I drove it onto the runway and broke the prop. It was not my prop.
Any way the point is that empirical findings compiled from 36 years of RC flying suggest that the clunk staying to the rear of the tank supplies fuel until the tank runs dry regardless of flight position, including waterfalls transitioning to a spin to a crash where the engine never missed[8D] a beat.

I am using 15% fuel. I also have several fuel tanks laying around with the factory equiped fuel line and clunk, and none of them will allow the clunk to move to the front of the tank.

This problem occurs soon after take off. As long as I am flying in a circle it seems ok, but if I do a loop or roll it cuts out.

I can demonstrate otherwise with any airplane in which you can see it's fuel tank.

Like I said in an earlier post. I have the same motor and it did the same thing when I ran 15% fuel. Not saying this is your problem but if you can borrow a tank of 10% from a friend at the field try it out and see if it helps.

You can see the tank while a plane is doing "waterfalls transitioning to a spin..."? Wow, good eyesight!