Stick Jammer

You are correct. It should have read "The velocity of an object in free fall has no limit, it increases at a constant rate as long as the object continues toward the earth. It's final velocity is determined by how long it was free-falling."

Bill Mixon

Guess it depends on how long you extend the downline. I have tested this with glow engines and the engine would run dry on a long downline. I wish I had specific details such as the altitude, amount of fuel, length of the tubing, etc..
On most attempts it took a lot of altitude to completely draw the fuel out of the line with the engine idling.
This was with a relatively low drag airframe so it did take a lot more altitude than any "normal" downline manuever would ever require.
The times that the engine did continue running after pulling out of the dive the engine would hesitate very briefly as the power was brought back up and the small amount of air pulled through the line.

sillyness

My Feedback: (25)

Good to know. I'm not pretending to know everything for fact, but I'm solid with the theory. I'm strongly considering purchasing a cheap camera to mount in a high drag GS plane to get conclusive results. Of course, seeing who I work for, I will have to classify the tapes of my model aircraft's fuel tank. I also really want a new snowboard though... hmmm... this debate may have to wait 'till after ski season.

CAPtain232

My Feedback: (40)

Hey fellas... If you want to use the camera as proof. There have been several DISCOVERY shows on TV where they show how astronauts train in a zero G environment by going up in the cabin of a jet plane, do a hardover and they literally seem to float in zero gravity........ This however is short lived. The jet has to maintain a certain amount of acceleration to continue with the zero G environment. We know that it is not actually a zero G condition, rather a condition of acceleration versus gravity that allows them to float. Until that point where the plane can no longer accelerate faster than what gravity is trying to pull you back to Earth, you can float, but at or after that point, you better hang on because you ARE coming down. The fuel in a glow fuel tank will do the same thing, problem is that this time period is VERY short. In conclusion, when you nose over pointing the nose to the ground, back off the throttle

sillyness

My Feedback: (25)

In the "Vomit Comet" they start their zero G time while flying UPWARD... they get the zero G by flying a parabolic course while the pilot is pushing forward to keep zero G on the airplane. He does this by matching free-fall acceleration around the parabolic arc. Once the nose is pionted down, he can no longer maintain zero G (for several reasons) and must recover from the dive before the plane overspeeds.

Red B.

Funny, the same question was discussed in August, 2004, see this [link=http://www.rcuniverse.com/forum/m_2075848/anchors_2081207/mpage_1/key_/anchor/tm.htm#2081207]thread[/link].

/Red B.

Geistware

My Feedback: (16)

On a down line during a dead stick, I can understand the plane being in free fall with the fuel. If the engine is running, it would be pulling the plane through the air. Not fast, but faster than free fall!

canavanbob

My Feedback: (20)

Are you guys talking about vertical downward flight or 45 degree downward flight? Anything other than vertical downward flight adds a completely new force to the equation. The force of gravity is vertical. A fuel tank at 45 degree angle with a motion component away from vertical will force the fuel up the ramp of the tank floor.

v-snap

My Feedback: (18)

I vote for the fuel moving foward in the tank due to gravity..My Staudacher with a tartan twin on gas is a good example. Any downline with power at idle will cause the engine to quit, sounds nice and quite as it winds down. Following a pull to level flight, if in time it will re-fire. No cough here just slowly dies at idle. If in a spin it will run untill I stop the spin, and let the plane continue on a downline. This is not an extremely low idle either, a comfortable one on the ground. It will not do this at 1/4 throttle, but it doesn't look as good as that nice slow downline at idle..

This problem has a tendency to keep the box on a smaller side, do-able and looks quite nice..

sillyness

My Feedback: (25)

Ever heard of prop braking? That's why we use wide props. When you hear your engine revving up on a downline, it is because your engine is being turned over against compression by the prop... creating a big brake, just like a Mack Truck. The prop can create more DRAG than any other part of the airplane.

v-snap

My Feedback: (18)

Mine defintely doesn't sound like it is revving up, it just goes quite. I guess you can see it as increasing in RPM due to air passing through the prop, running the engine dry due to the increase in RPM without a increase in fuel flow..Forgot I already figured this one out.

canavanbob

My Feedback: (20)

I don't see the connection between down line braking and by statement. As long as the plane has some forward angle, the gravitational force on the bottom of the tank has a vertical force vector and a horizontal force vector. This forces the fuel forward and down relative to the bottom of the tank. This force makes the fuel want to go toward the nose of the airplane. However there is also a force generated due to the forward motion of the airplane that pushes the up the ramp that is the bottom of the tank. This force pushes the fuel toward the tail. Acceleration and deceleration will also affect the equilibrium of the fuel. Any kind of violent aerobatics simply adds orders of magnitude to the complexity of the analysi

sillyness

My Feedback: (25)

Sorry Bob... the downline braking wasn't directed at you.

There is absolutely no force ever generated by motion. It's Newtonian physics. Force=Mass*Acceration. Thus, the speed on a 45 degree down-line has no effect on the fuel. Now... lets say you are doing this downline at idle. Would you agree that your speed is constant? What forces are acting on the fuel? The force of gravity pushing down the 45 degree ramp that is the bottom of the fuel tank? There are no other forces because there is no other acceleration. F=m*0=0.

As for the prop, I can hear mine wind up on a downline indicating braking. As soon as I level off and slow down the RPM decays back to idle. You have to vector add the velocity of the prop's rotation with the forward velocity of the airplane. At zero airspeed, the relative wind is coming at the backside of the prop's airfoil generating lift (thrust). As you get faster and faster, the relative wind starts moving around to be inline with the prop, then as you get faster it moves further forward until it impacts the front of the prop. Now, the prop's airfoil is a wing flying at a negative AOA... it is producing thrust in the forward direction (drag). The prop is trying to turn the motor and more drag is created by the compression of the motor. The way we get rid of the drag with a fixed pitch prop is to increase throttle... the higher rotational speed moves the relative wind to the rear of the blade again. Most complex aircraft just increase the prop pitch.

This is why full size aircraft have to be able to feather their props (90 degree blade angle... pointed into the wind) should an engine fail. There is SO MUCH DRAG on a rotating prop that it can make an airplane uncontrollable. A C-130 at 150 kias with #1 engine flamed out at a flat blade angle (23 degrees positive pitch) sees as much drag on windmilling prop (around 10,000 pounds) as the other motors are capable of producing thrust. This creates a huge yawing moment and directional control problems. There are several other safety systems installed to help prevent this from happening, but it would take pages and pages.

RTK

My Feedback: (1)

I agree with all the theory and math. But if desertpig has put a camera in a plane and there is proof as to what happens, then we must not know exactly what are planes are doing acceleration/decceleration wise on down lines.

canavanbob

My Feedback: (20)

An airplane flying around the earth does not fly in a straight line when flying at a given altitude. It is actually flying in a circle around the center of the earth. This generates centrifugal force which would make the plane go off into space. This force is counteracted by gravitational force. This is angular acceleration and it also produces force. Just as a ball on a string twirled around your head generates a force.

Stick Jammer

Not the case in either instance. A dead stick plane cannot travel toward the earth at free-fall acceleration due to air resistance. If the engine is running at idle on a GS plane during a vertical down-line the plane is actually accelerating slower than if it were dead stick due to the braking effect of the prop. In order for the plane to exceed free-fall acceleration on a vertical down-line the throttle has to be substantially opened.

Stick Jammer

We don't have enough details of this account to prove or disprove the issue. We don't know for sure if this plane was in a true vertical dive and what the acceleration rate was. On the other hand, we do know the laws of physics that apply to this issue and they can't be changed. I stand behind the statement that if the plane is in a true vertical dive it would have to be accelerating faster than the acceleration rate of free-fall to hold the fuel at the rear of the tank. This simply isn't possible with the engine at idle on our models.

quist

My Feedback: (198)

Then why aren't we sucking air on long downlines?

If I am flying to long and the fuel is getting low in the tank my engine lets me know when it cuts out for a moment. On long downlines my engine accelerates smooth when I pull out.

sillyness

My Feedback: (25)

I think we do get small airbubbles in the lines but not enough to make a difference, seeing as how the motors are barely sipping gas at idle. Also... the down-lines are short duration, reducing the air ingestion further. Just my thoughts. Like I said... I hope to install a camera in my own plane next season to get a conclusive answer for myself. I'm not afraid to be wrong... I just want to a reason that makes physical sense.

mAvRiCk-inactive

Well I've gone 500-600 ft up with my 40% Edge 540T and I go strait down and at the 200 foot mark I pull out gently. My engine doesn't even plane on quiting! ALso, I will try this weekend with my smoke on and I'll try to get it on tape!! Later

Dan

I believe DesertPig is right!! The fuel stays in the back of the tank for sure..

Bill Mixon

You should go up higher to say 3000+ (I know, I know) and try it. 400' isn't enough downline for it to happen.

JoeAirPort

My Feedback: (41)

I always back off on the throttle on down lines so fuel draw isn't an issue. Whatever is in the fuel line is enough. Like someone else said, how long can you keep going down? 5 seconds? We just don't fly that high.

Now hovering, that is a fuel draw issue.

LarryC

Somebody once suggested that any sufficiently advanced technology is indistinguishable from magic. I think it's just magic that makes it work.

I have a Creek Hobbies Quest, YS-110, Tetra tank. The Tettra comes with a very flexible clunk line that I felt would always stay with the fuel, whereever it went into the tank. After flight # 10, I started getting dead sticks. I would find the clunk hung up in the upper portion of the tank at the front, where no fuel stays for long.

This morning, I replaced the line to the clunk with a much stiffer line. I took the plane much higher than I would ever fly normally and tried power off and partial power downlines. Never a hiccup, even though the fuel in the front of the tank idea was in my mind. I'm convinced that the fuel does go to the front, even though there are some legitimate arguments presented here stating otherwise. My reasoning is simple. The airplane is slowly gaining speed in the downline, with the prop creating drag along with the airframe, while the heavy fuel, also accelerating but no extra wind drad, would seem to have no reason to do anything but go to the front of the tank.

So, I'm staying with the idea that there is some magic involved, and I sure don't the absolute answer. A simple test a full scale aerobatic pilot could do is simple. Strap a jar half full of water in the cockpit, point the plane down, and see where the water goes.

Fun for discussion though.

LarryC

JoeAirPort

My Feedback: (41)

This is exactly what my common sense tells me. In engineering school (and high school) they taught us that two objects having two different weights fall at the same acceleration neglecting air drag. Since the plane has air drag and the fuel does not, obviously the fuel falls faster than the plane. So if we're coming out of a hammer head, and assuming the engine is at idle and having no effect on the plane's acceleration, the fuel will be at the fron of the tank.

Swat Cop

My Feedback: (11)

Um No, unless your airplane is hovering with the nose down (not possible) the fuel is at the rear of the tank. The fuel floats weightless at the rear of the tank or held there by G forces. NASA trains astronauts for weightlessness in space by putting a large transport in a long downline. I have personally experienced this in a skydive plane (pilot goofing off) and its pretty cool. This thread has turned into a great amature physics discussion but if you think about its common sense. If the fuel ever went to the front of the tank you would have a deadstick because the clunk don't go there.