You know the "clunk" thats on the end of the supply line? How does it draw fuel when the plane is in a nose dive for very long when the tank is near empty? The tube will not allow the "clunk" to fall all the way to the front of the tank. What keeps the engine from dying in a nose dive or loop?

Just wanna make sure I'm not missing anything before I take her up.


Thanks,




Wings

You are correct. In a prolonged nose dive with a nearly empty tank it is possible to draw air into the tube. But it is very difficult to actually make this happen. Generally you are in a nose down attitude for a very short time. Some air in the line does not automatically cause the engine to quit. And if the throttle is at idle (very likely), then it takes a very long time to draw in enough air to make a difference.

The tank is pressurized which forces the fuel into the only opening available, the intake tube. There also is a small vacuum which helps this as well. If the fuel supply is extremely low you may get some air into the line. Time your first flight to get a good estimate on flight length then take off a couple of minutes for buffer. Always land before your time runs out. In 30 years, I've only dead sticked because of no fuel 1 time. This WILL work!

P.S. Keep in mind that you'll use more fuel at full throtle than 1/2 throtle. My timing was always made flying an entire flight at near full throtle. (Engine already broken in.)

It's never possible to draw air into the pickup line. In a vertical nose dive with the engine running, the airplane is accelerating downward faster than free fall and the fuel is still at the rear of the tank. If the aircraft had dive brakes like some old dive bombers, it might be a problem when the engine wasn't running, but for all normal configurations, the fuel never moves to the front of the tank in flight.

LouW,


I think your right. But, I am still thinking hard about it. Because if the tank is pressurized, due to the laws of physics, the fluid will be moving the same speed as the plane and I think it may be possible for the fuel to be at the front of the tank. Kinda like the old question, why don't you smack into the back of a commercial plane if you jump in the air. I think its called the law or relativity. Its been a while since physics, I dont remember the names of the laws, but remember the principles.

I think the plane will be accelerating the fuel at the same rate as the plane because it is pressurized. So, you very well may be right, but I'm not conviced that the fuel stays at the back.

I guess all this is just food for thought, but the important thing is that according to you experienced pilots, then engine is unlikely to die.

Thanks guys,


Wings

I just did some emperical testing (with a 1/4 full water bottle) to see what acceleration is required to slosh the water to the upper end of a decending tank. Not much, but it is tough to keep it there. It has to be continually accelerating, not just speeding earthward, or that old devil gravity pulls the fluid forward. Pressurization doesn't change gravity, it just forces fuel already in the lines out against the force of gravity.

I've done some fairly extended "dive bombing" runs of six or seven seconds without faltering, but I can't say how full the tank was at the time. I do tend to do most of my verticals early in the flight, as I do worry about sucking air as the fuel runs low.

Once I get up I tend to loose track of time. I've sucked tanks dry dozens of times every season. Keep the nose slightly down. Regain as much upwind distance as you can if you need to and loose height with shallow spirals early on in the glide.

The pressure doesn't effect where the fuel goes in the tank. A falling body is accelerating downward at 32ft per second per second (1 g). In free fall, the fuel is weightless, and could float around in the tank. But with the engine developing any thrust at all (even at idle) the acceleration in a vertical dive is enough to keep the fuel at the rear of the tank. You are right, it is physics.

Ya, now that I think about it , I guess pressure doesn't matter. If you were on a commercial airliner and it was ACCELERATING fast enough, and you jumped in the air you would hit the back of the plane.

Actually, it is quite possible to suck air in a vertical dive with low fuel, I've done it a few times. However, you usually run out of altitude before you run out of fuel, because it does take a few moments for all the fuel in the line to empty and for the engine to die. At low throttle, it's even longer.

A 6-7 second dive? You've timed it? That's a LOOOOOOOONG time, you must be nearly a dot when you start the dive. I suspect your dives are a lot shorter than that.

Fuel will go to the back of the tank as the plane accelerates downwards, but you can reach terminal velocity. At that point, gravity + engine thrust = drag, and veloticy becomes constant, and all your fuel goes to the front of the tank. Getting to terminal velocity is a bit of a trick though, as, like I said above, you usually run out of altitude first. Though I think it's easier in a spin, as you reach terminal velocity faster. However the spinning motion also pushes the fuel and clunk around a bit, and might or might keep it from happening.

So, unless you are trying to make it happen, it almost never does.

Also consider that when you are landing, the plane is constantly DEcellerating, so the fuel will slosh to the front.

The bottom line is... whatever the reason, we all seem to be able to land with our engines running.

You have a valid point regarding terminal velocity dives. During the early thirties, test pilots (at least according to the movies) would climb the experimental aircraft as high as it would go and put it in a vertical dive (presumably with the engine at idle). When it wouldn't accelerate any more the drag was equal to the weight which seemed to be important to the early engineers. The drama was all in the resulting pull out which some made and some didn't. I don't seriously think that they still do this with airplanes like the FA-18.

When the aircraft approaches terminal velocity, fuel could indeed fall to the front of the tank. With a lightly loaded biplane, or even something like a SIG Senior, you might approach terminal velocity but the initial altitude would likely higher than most folk ever fly. For a typical sport, aerobatic, or 3D type I doubt it would ever happen.

When an aircraft is slowing down for a landing, the deceleration is not enough to do more than slightly slope the fuel level. If the tank is almost empty, it could cause a shutdown

The point is that wings isn't missing anything. It is not a problem.

Lou, oh, yeah, I think we all agree it's not a problem .

You need to realize that during a down line, the fuel is traveling slower than the plane and is usually still in the back of the tank. I know I may not be saying this correctly, but just remember that the fuel and plane are moving and the plane us usually moving faster than the fuel can so the fuel tends to stay near the back of the tank unless you are doing some serious braking.

<<A 6-7 second dive? You've timed it? That's a LOOOOOOOONG time, you must be nearly a dot when you start the dive. I suspect your dives are a lot shorter than that. >>

Enough time for a fellow standing behind me to say to another "Watch this." And for the other fellow to say "Yeah, Charlie is always worth watching, snicker, snicker."

40 mile/hr * 1hr/60min * 1min/60sec * 5,280ft/mile = 58 ft/sec

58ft/sec * 7 sec = 406 ft , so if it was a fairly steep angle of -60º (sin60º=0.866) and 406 ft is the hypotenuse, then that allows for 400 ft of altitude with a 355 ft vertical decent and still 45 ft to pull out. 400 ft isn't all that high. I plugged the 40 mph, but I start slow and build up as it desends, so it probably is close. I fly thick winged 4-strokes that don't tear up the sky. Seven seconds in a Quickie 500 would put you four seconds underground.


Who says I wasn't paying attention in trig class?

Glad to see I'm not the only nerd here.

The engine is supposed to be running when you land?[X(]

Charlie,
Great job there yeah, 400ft is not that high up at all, so yeah, looks reasonable. 40mph in a 60degree dive is slow even for a trainer, I think, but hey, climb to 800ft, and it's easily possible. And I know many guys take .40 size trainers up over 800ft all the time. I was thinking vertical dives, but close enough. I'll have to time some dives next time I'm at the field .

The tube is long enough inside my tank for the clunk to reach the front of the tank. Also, wouldn't the clunk move at the same rate as the fuel in a dive (while the plane is accelerating)? At terminal velocity (plane not accelerating anymore) both the fuel and the clunk would move to the front of the tank. Rignt?

Not if the movement of the clunk is restrictied by the tubing, which is the case on mine at least.


Wings,

ass you dive, the fule slaushes to the back of tank (inertia)... clunk can then be in fule and engine still runs, but you can run out of fule in dive if there is too litle to fill teh back of the tank and have the clunk under.

sorry if this was already posted