I just repaired a twist that crashed because the engine died in a down-line at the complete wrong time. Now that I have the plane back together again I want to make sure that it doesn't happen again so I added a small fuel filter thinking that the additional volume of fuel in the line will help, but while testing on the ground, the plane will only run for about 10 seconds with the nose down. Is this normal or should I expect more time than that? Any thoughts on how to keep the engine going during long nose down maneuvers?

Thanks,
schu

Did you happened to check the clunk in the tank to see if the impact moved it to the front of the tank .The engine should run as good in a downline position as any other .Its sounds like a for sure out of fuel condition .check the clunk ,lines and watch the fuel line going to the carb for fuel .I have a twist and had to put the tank right up against the top to get anywhere near the center of tank position with the engine mounted upright .
What engine are you using, prop ,fuel & plug?

Sounds like your clunk is too stiff or for some reason it will not double back towards the front of the tank.

take the tank out and check it.

Engine should run nose down until it runs out of fuel. Was this 10 second test at WOT? If so, that may be normal. If at idle, that seems a bit short, but will depend upon the lenght of the lines. Check to see if the fuel line goes dry, and that is why the engine is shutting off. If the lines are still wet, then you have a mixture or some other issue.

If it is a dry line issue, the only good solutions are 1) don't do 10 second dives, and/or 2) use a header tank setup to extend the time you can be in a dive. and/or 3) use a bladder tank which will totally eliminate the issue.

Cheers

Edit: Probably not a clunk issue. If the clunk is forward, it would either die all the time (pinched line) or only nose up. It is shutting down when nose down.

Is the clunk supposed to come back to the front of the tank? My fuel line is too stiff for that. Is there special fuel line that you put in the tank? I always thought that the fuel line bending over in the tank would pinch it.

The 10 second test was at idle and the fuel line did go dry, so it sounds like I have a major issue that needs to be resolved before I fly.

The plane is really to small for a header tank so I don't think I'll go that route, what is a blader tank? I've never heard of that before.

Thanks for all the replys,

schu

I keep a supply of Prather fuel line, which seems to be thin-walled enough that it will flop over inside the tank and come to the nose on downlines. For plumbing exterior to the tank, the thicker-walled varieties seem a bit more durable. Also, I believe Great Planes tanks include a length of thin-walled tubing for the clunk line. Not sure where I got the Prather, but Tower probably has it.

However, whenever I have the nose pointed down, I also have throttle at idle, and the engine should be sucking fuel slowly enough to come down without sucking dry. Seems to me a 10-second downline is rather long.

G'day Mate,
No the clunk should not come to the front of the tank as it will be kinked if it did & you will not get any fuel at all to the motor, try this test, get a clear drink bottle anything will do, as long as you can see whats happening inside, part fill it with water, hold it in your hand, & take it from behind your back on the right up & over your head to behind your back on the left, & watch what the liquid inside does, this simulates what happens to your fuel in the tank in a dive, the plane is travelling fast enough to stop the fuel from running to the front of the bottle, you will find you can do this at quite a slow speed before the liquid moves forward. so unless you have a tuning problem or you clunk is touching the back of your tank, your motor should keep running fine.

Allen is correct. As long as the plane is traveling forward the fuel should be at the back of the tank. None of my clunks move forward.

ASOLUTELY!

Now let's not totally rewrite the laws of physics just yet . If the model is flying downhill and not accelerating then gravity does its job and the fuel runs towards the front of the tank. With a part empty tank this could quite easily uncover the clunk. In a steady dive the fuel level in the tank will always be parallel to the ground.

It seems to me that the fuel would be at the back of the tank only if the plane was accelerating or headed up.

It your running the line dry its a tank deal,when you say downline are you standing the plane on its head or just at say a 45 degree angle.In flight like allen said the fuel goes to the back of the tank and shouldn't be a prob.Myself I worry more about the nose of the plane being pointed up than down ,can't say that I have even ran out of fuel going down ,the Plane has always made it to the ground *LOL* Good luck and lets us know what you find.

Some prefer that the clunk flop forward to the front of the tank when the airplane is nose down, hence the reason for installing thin walled and very flexible fuel line such as Prather fuel line which allows the clunk to flop back and forth in the tank as the airplane pitches nose up or nose down. The fuel line will not kink but form a bend as the clunk falls forward in the tank, therfore keeping the clunk submerged in fuel. When the attitude of the airplane goes to nose up the clunk will flop to the back of the tank. The danger of this method of installation is the clunk hanging up in the forward end of the tank and the engine dying when the airplane goes nose up.
In any case, the clunk will be submerged in fuel whether it is in the front or rear of the tank during level flight. The fuel line inside the tank should be long enough to place the clunk to within 1/4 inch of the rear of the tank.

And, if the airplane is not accelerating, fuel will indeed move to the front of the tank during nose down attitudes because of the force of gravity. And even if it is accelerating, the acceleration must be enough to overcome gravity to move the liquid fuel uphill to the back of the fuel tank.

Regards,
doubledee

Wow, what a mixed bag of results and recommendations.

I'm not sure I want to go with flexible tubing as that seems like it will pinch the line or the clunk will get stuck. I think I'm going to add a little more fuel line and try to get the idle downline into the 12-15 second range. I figure that if I'm above idle the fuel is going to go to the back, and I shouldn't need more than 12-15 seconds since that is a pretty long time to fall.

I'll report back if I have any more issues with this.

Thanks,
schu

Ok so you can go with the guys that know or thoses who are just hangin in the wind.
Clunk lines shouldn't bend.
Fuel tends to go to the back of the tank during a vertical free fall.
If one did a 10, never mind 15 second vertical decent during an aerobatic compitition you'll be needin' you second plane to finish.
Enjoy.

Well, you know what they say about opinions. Yeah, fuel falls at a slower rate than airplanes, that's why it moves to the back of the tank, while on the other hand, clunks fall at a faster rate than airplanes and that's why they fall to the front of the tank.
Some prefer flexible fuel line in the tank and some don't, but that doesn't mean one way is better than the other.
I thought balls dropped off the leaning tower of Pisa had settled that arguement hundreds of years ago, guess not.

Have a nice day.
doubledee

You possibly have a point there.
Relax about the bendy clunk lines.. they're nothing but trouble.
A 10-15 second vertical decent is a very, very long time. Even if it was a stalled free fall (see the rule above) or during a spin (not the rule above)... it's a life time!
For gods sake, make sure you have a spade and all the necessary "excavation" warning signs when retrieving the aeroplane... that hole could a danger to passers by.
Relax and enjoy the flying... you'll do just fine with 10 seconds.

If you take a bucket of water and swing it the water will stay in the bottom of the bucket, more or less. As long as the plane is moving as fast as you are swinging the bucket the fuel will stay at the back of the tank. You don't need to accellerate just moving forward at one G.

Ten seconds on a vertical down line would put all my planes well below terra firma. Five would be about max.

I'm not flaming or nothing ,but how can a plane go downhill and not accelerate,what your saying here is the fuel will out run the plane .Sorry don't think so .Now if the plane is stopped for some reason,and pointed down then I could see your point ,but where both the fuel and plane are going at the same rate of decent if the fuel is at the back of the tank I''m sure it will stay there. Inertia will take over gravity any day of the week.

If the low speed needle is a bit on the rich side, especially if the idle is set on the slow side, you can get a dead stick situation very easily when the nose is pointed down for a few seconds without worrying about the fuel or clunk position in the tank.

Try simply checking low speed tuning and set the idle a bit higher and see if all does not go well.

Ernie

Here's an experiment for you to try. Jump out of a plane carrying a jar full of fuel (you could use water but let's stay with fuel ) and watch what happens to that fuel when you reach terminal velocity as a model does when it's in a dive. Don't forget to pull the rip cord if you want to report the results to us

Do what the heli guys do, install a small stunt tank with a standpipe instead of a clunk, about a one ounce will do fine and it can be mounted next to the engine or just behind it on the twist.

No thanks ,my better judgment tells me not to jump out of a perfectly good plane for any reason .But you can if you like .

In a downline the plane should be accelerating at least at 1G - probably more if the engine is running and turning the prop (OK you 3d guys - I know about the wide props and propeller breaking). If you are reaching terminal velocity with your planes in a downline (from 400 or so feet in most cases?)- your planes are either built a lot stronger than mine or have a lot more drag than any of mine and you are probably holding your downlines a lot longer than I am brave enough to do.

Hmmm....Unless you're operating under a fairly-high power level, I sincerely doubt that the model is accelerating at 9.8 M/s/s, which is standard Earth gravitational acceleration. Drag will make it less than that amount. Even so, if the model only fell at that speed, the fuel would be in "free fall" and would just vibrate around the tank. In order to make the fuel move to the back of the tank, and stay there, the acceleration would have to be MORE than 9.8 M/s/s. This can be done momentarily, and is well-exceeded in turning maneuvers for the duration they are held (loops can also be considered "turning" maneuvers for the purpose of this discussion). However, in a dive, with a constant line, most models will stabilize rather quickly...and if it's coming down at less than 9.8 M/s/s, then the fuel will run to the front of the tank.

9.8 M/s/s come out to 32 F/s/s. This means that you'll fall about 480 feet in 5 seconds, and 1,760 feet in 10 seconds. This is assuming there is no drag of any kind. As soon as the model encounters drag, that is, ALL the time, then it is accelerating at LESS than gravitational acceleration. The fuel in the tank, though, is not so restrained, and tries to accelerate at the 32 F/s/s. It moves to the front of the tank, as a result.

So, sorry, regular physics shows that the fuel won't stay in the back of the tank unless the model is acclerating faster than 32 F/s/s. If you're diving from only a few hundred feet, and are taking 10 seconds, the model is NOT accelerating faster than gravitational acceleration, and the fuel in the tank will slosh to the front of the tank.