Fuel flow - Backside of the curve

I have seen several references to " going down hill and getting a rush of fuel".....

Does anybady have a CLUE as to what front-to-back the G-forces are on the fuel in a stunt model during maneuvers?

Obviously, straight and level with the model flying at a constant speed, there is NO front-to-back forces to deal with. What happens when you go into a maneuver and now are going straight up? Or into a maneuver and goes straight down? What does the fuel "see"?

Inquiring minds want to know!

Regards,

>I have seen several references to " going down hill and
>getting a rush of fuel".....
>
>Does anybady have a CLUE as to what front-to-back the
>G-forces are on the fuel in a stunt model during maneuvers?
>
>Obviously, straight and level with the model flying at a
>constant speed, there is NO front-to-back forces to deal
>with. What happens when you go into a maneuver and now are
>going straight up? Or into a maneuver and goes straight
>down? What does the fuel "see"?
>
>Inquiring minds want to know!

It's certainly not as simple as which way gravity goes! I have a bunch of hand calculations, but it will take me a while to collate them into a decent post. I might also add that you can't just look at it in one axis. The lateral acceleration is also a huge factor. There are also a bunch of unknown factors (like the angle between the body angle and the flight path) that could potentially make a large difference in the fuel delivery pressure, but vary from airplane to airplane and day to day.

One thing that leaps out at you, comparing what the engine does in flight VS. what the fuel pressure does - it's so weakly correlated that you could almost disregard the fuel pressure, at least in the short term. For instance, in every corner, the fuel pressure goes way up for a little while as the airplane deccelerates due to maneuvering loads - yet, most of the time, the engine goes into a 2-stroke!

<>There are also a bunch of unknown factors (like the angle between the body angle and the flight path) that could potentially make a large difference in the fuel delivery pressure, but vary from airplane to airplane and day to day.<>

Brett,

would the use of a clunk tank attenuate the effect of some of these factors?

After all, if the clunck is kinda free to move (read - to be moved) around by the forces that are an effect of the unknown factor you mentioned above would it not be reasonable to believe that the clunck - with its capacity of being in constant variation (wow, is "constant variation" a paradox?) would be a neutralizor of the effect of these forces on the fuel delivery pressure?

Lemme rephrase this, I can see that stupid look on your face when confronted with a stupid way of putting ideas into phrases, I don't wanna get as an answer a

?????????????????????
SO...

Regular tanks are affected by the body angle and the flight path because they don't have any option.

The clunk seems to have the tendency of adjusting it's position when these forces change, thus squaring things up...

Like James Moody's song goes, "Am I insane of do I really see heaven in your eyes?" , I mean what I wrote makes any sense or is this post a complete idiocy?

**********************************

Changing subjects, one thing I know, that tuned pipe made some of my wildest dreams come true.

You should have wised me up on them pipes when I was tamin' that
OS .32 F that you saved from being castrated, remember?

Well, I learned a lot in the process of making it operational.

I am thinking of gettin a side exaust header and piping that little sucker.

If the 25 VF runs a 10 X 3 prop aat 12,000 rpm, the .32 should be able to swing a 10.5 X 3.5 and allowing me to put it on a bigger plane.

The VF seems to be "confy" with 50 " span, 525 sq inches and not too much a thick airfoil.

I understand Bob Hunt is using a piped RO Jett .40 on his 47.5 oz. 630 sq" Genesis.

This .32 would be a little butt kicker on a Shark .45 if it was light, whaddya think?

************************

Do you believe, Brett, that most cats down here in Rio think nitro is the devil's brew?

It'll make the engine run too hot, it'll dstroy it, it's only for cold countries, bla bla bla...

And most guys use a simple 4 by one (20%) homemade fuel, methanol and castor.

On a Fox they'll use the "Poppa and Momma" mix, 3 by one, (25%).

Some guys use Castrol M-50 motorcycle oil and make a mixed oil fuel.

I found that Wild Cat 15% nitro with 16% oil (synth/castor) plus a glassfull that good, thick, degummed castor we have here (it goes to 24%) is just fine.

No synth made my .32 get stuck, I had to open it up and clean off the coal, and only synth gives me the creeps.I had some three lean, mean runs while working on the 32, and the castor saved it. Looks like castor polymerizes at hgh temps and gets "thikerer", becaus te engine at first went like a rocket and after a lap or two it just idled.

**********************

Here in Rio and Sao Paulo on a summer day there is a big change on the temperature depending on what time it is.

I remember that at the NATS in Sao Paulo, some years ago Roberto Magri was using a ST .60, and at 10 AM and 2 PM the needle setting was the same.

The last flight was at 5 PM, and it was a some 6 or 7 degrees cooler.

He fired up that old loop scavenger, went to the handle and realized he needed a couple of clicks on the needle valve.

He waved at Bené Rodrigues, that was launching his ship and made a gesture of "tighten the NVA".

NOW, this is the evil gossipy part of the tale:

Instead of closing Bené opened a little more tha NVA, and the plane took off too darn rich.

And Bené and Magri were fightn' for the first place.

And Magri missed the clover...

I think it was a fatality, a mistake that can happen to any human being, specially if this human being is tuning up his opponent's engine...

Paulo Gomes knows better than Magri; his launcher is his ever loving wife, and I figure that as long as he behaves like a good boy with her his needle settings aren't bound to get funny...

God bless you!

In Christ,

Ion

> One thing that leaps out at you, comparing what the
>engine does in flight VS. what the fuel pressure does - it's
>so weakly correlated that you could almost disregard the
>fuel pressure, at least in the short term. For instance, in
>every corner, the fuel pressure goes way up for a little
>while as the airplane deccelerates due to maneuvering loads
>- yet, most of the time, the engine goes into a 2-stroke!
>
>
> Brett

Brett, what data do you have on fuel pressure, and how do you know that it goes way up in a corner? I would be interested in this.

I wouldn't think that that fuel pressure would go up in a corner. It definitely gets slammed around, but I don't know that this equates to increased pressure. And if the plane turns abruptly, the engine goes in one direction and the fuel in another (airplane makes "square" corner from level flight to pinted up, engine now goes up while fuel forces would continue to be applied at right angles, parallel to level flight and away from the engine) wouldn't this decrease the fuel pressure at the venturi? Also, if "its so weakly correlated that you could almost disregard fuel pressure", why does adjusting the tank height as little as a business card width make such a difference in the engine run?

Now, in level flight we have one "g" pushing down on the fuel and as many "g's" as you have line tension (perhaps 3) pushing out on the fuel. So the fuel load is always leaning out (as long as you have line tension) and applying pressure to the fuel because of the increased (sideways) "g" load. The outward force, combined with accelleration and a constant turn, force the fuel to the outside, rear. However, when the plane is pointed up, we now have to add gravity to the draw. Point the plane up, and the engine runs leaner, point the plane down--well, don't do this for long while you are holding it or, with no other forces in play all the fuel will flow to the front of the tank and the pick up tube will uncover and the engine will stop--but in flight we still have the outward forces (and maybe acceleration) keeping the pickup tube covered and the fuel to flow, but gravity working in its favor on the way to the engine causing it to go richer. You get the same effect on the test stand when you raise or lower the tank. (Tank high, engine rich. Tank low, engine lean.) And it doesn't take much. Even adding muffler pressure to the tank makes a great difference in the run which must be adjusted for by the needle. And muffler pressure only adds about 1 1/2% to atmospheric pressure.

To me, fuel pressure is significant in both its effects and how your set up your engine. Again, there may be other significant forces, but this surely is one of them.

>
>> One thing that leaps out at you, comparing what the
>>engine does in flight VS. what the fuel pressure does - it's
>>so weakly correlated that you could almost disregard the
>>fuel pressure, at least in the short term. For instance, in
>>every corner, the fuel pressure goes way up for a little
>>while as the airplane deccelerates due to maneuvering loads
>>- yet, most of the time, the engine goes into a 2-stroke!
>>
>>
>> Brett
>
>Brett, what data do you have on fuel pressure, and how do
>you know that it goes way up in a corner? I would be
>interested in this.

Analysis - the delta in the fuel pressure from X-axis acceleration is
sigma*ax*x

where

sigma=fuel density (assume about .233 slugs/cu. ft., or weigh some fuel)
ax = X-axis acceleration
x= distance from top of fuel head to spraybar inlet. Figure at least 8" for a 6 ounce tank (~6" for the x dimension of the tank, + 2" to get from the front of the tank to the spraybar).

For the acceleration, assume that the peak is unaffected by engine feedback. You start an 8 g corner with a 4 lb airplane, with an L/D of about 5. Calculation of drag left to reader, calcultion of acceleration is drag/mass. But the direction is unambigously +X, meaning the pressure goes up from this effect.

The effect on the engine of mixture/fuel pressure variation, is, in my opinion and based on years of observation, a very long-period effect. Hence my comments about the engine responding to other, faster time-constant effects (like load and exhaust tuning) in directions opposite to the direction that the fuel pressure changes.

OK, some numbers to Brett’s equation, and it will be more clear.

1/ The drag in corner could be close to 25N. Sorry for metric units, but if you take weight 1500g (optimistic weight – means strong acceleration) than acceleration can be about 1.5G (richening)

2/ The centrifugal force in corner is ~15G for 3.5m radius. If the AoA is ~6 deg (means nose toward the center of loop), the tangent is 0.10 and thus acceleration is 1.5G – leaning - exactly opposite to acceleration from drag. That is very important, because lighter model or more flaps means lower AoA and thus richening in corner, while heavier model or less flaps means leaning in corner. It also means that richening / leaning in corner can be “trimmed” little bit by CG position or flap to elevator ratio.

3/ The thrust after the corner is certainly not greater than thrust at 0 speed (on piped low pitch application) so acceleration after the corner is certainly not more than 1G ands its effect is less than nose up effect.

4/ The up hill and down hill effect is very confusing. I note GRAVITY has NO direct effect to fuel pressure. If you point nose up in flight, the fuel pressure does not change. It is because the gravity applies to the mass of fuels as well as to mass of model. It is different if you do it on ground in hands. Hand supports model frame, but not to mass of the fuel. It means hands apply acceleration to model. The only fuel pressure difference is caused only by aerodynamic forces on wing (lift, drag) or prop. So real in-flight leaning nose up can appear only in case that engine pulls more up hill. It means model must first slow down, prop gets more AoA, makes more thrust = force in x axle and that is that acceleration applied to frame and not to fuel. Here comes the fuel pressure difference and now the engine can jump “on” or switches to 2 cycling. That “jump on” makes positive feedback, so the up hill segment can be rally accelerated (means more than 1G acceleration) in some case and down hill segment can be really too slow. But I would say (that is my feeling and preference) that the acceleration uphill and downhill would be in real well adjusted power train little under 1G (means little slowing up hill and little accelerating downhill).

5/ The level flight at or over 45 deg make the same effect like the corner, but without effect of drag – just because it is constant speed flight. So here applies only leaning because of centrifugal force and positive AoA. It means the leaning is up to 1.5G at tightest loop overhead.

> sigma=fuel density (assume about .233 slugs/cu. ft., or
>weigh some fuel)

Isn't the symbol for density usually "rho"?

OK, so now we can calculate dP by calculating a for each vector component (along x, y and z). This is probably the hard part, but not too bad.

If x is the length of the airplane, ignoring y and z:

In turns, you see a deceleration, since drag goes up and airspeed goes down (as Igor mentions). So this is simple. Conclusion: fuel head goes up during turns.

On an upward leg, you're acting against gravity (which acts on both airplane and fuel). If the airplane is held nose-up, the fuel will collect in the back of the tank, and I don't have any reason to believe that, if the airplane is flying upwards at constant velocity, it will behave any differently. If the airplane decelerates upwards faster than g, then the fuel will surge to the forward part of the tank, but I think everyone agrees that if there is a deceleration, it can't be more than g (10 ms^-2) or the plane won't make the top of a square. Conclusion: fuel head at the needle valve decreases.

On the downward leg, if the airplane accelerates faster than g, the fuel gets "left behind", otherwise it collects at the front of the tank. On modern stunters, the engine "brakes" on the downward leg, so acceleration downwards is checked. In any case, the needle is now lower than the fuel, so conclusion: pressure at the needle increases on the downward leg.

Is ignoring y and z warranted? I think it is, simply because the distance between the fuel in the tank projected along those directions are small compared to that in the x direction.

>>>Is ignoring y and z warranted<<<

You cannot ignore y and z acceleration. If you look to my point 2/ in previous message, you see that its effect is similar to acceleration from drag just oriented opposite. May be I did not write it clearly:

Wing in corner makes lift. The angle of attack is say ~6 degrees. It means fraction of lift is converted to x axle. That fraction leans the mixture – it is opposite to drag, which makes it rich. My example shows number where those two effects are balanced – they cancel each other. But you can have lighter model with lot of flaps which gets rich in corner, but also model which gets lean in corner – everything depends which effect is stronger.

The same goes on with side acceleration – the nose is out of circle – typically 1 or 2 degrees. Side acceleration from centrifugal force makes rich mixture. We fly constant speed, so it should not be variable, but side wind can point nose in (thanx rudder with left wind) make mixture lean on downwind side where you fly figures and also if you fly overhead the acceleration is less by gravity – so it also leans mixture overhead.

one clue is taking a look at the kind of tanks that work.
Most of the PA tanks have the fuel pickup towards the rear of the tank, usually in the rear, outboard corner. Making a tank with the fuel pickup forward usually requires some major changes in the tank shape to force the fuel to pool in the front of the tank- wedge shapes with the front 3/4 in. wider than the rear, canting the whole tank, etc. That indicates that the fuel must be mostly sloshing around the outboard rear corner.

Bill,

Let's remember that the thrust required from the prop reflects the model's total drag. Acceleration from 'rest' to 'cruise airspeed' is an example of a (relatively) gradually decreasing thrust load. That's why a lot of Fox35 style takeoffs ran from warbling in/out 4-2, and settled into the classic purring 4 cycle in about 1 lap. A stunt model has a large wing, and needs little AoA, consequently little C(L) for level flight. The turn into the first climb in the Rev WO, per Wild Bill, may be as high as 35 g. The WING has to make (most, at least of) that lift.

Induced Drag coefficient changes by the square of the change of Lift coefficient. In the same air and conditions, C(D(i))could increase to 1,225 times as much as in the 'cruise' run-up to the corner (that's 35 squared.) Even if Induced Drag in cruise is small, over a thousand times as much isn't.

I.Newton had a few things to say about mass, force and acceleration, so we can SWAG the 'cruise' Lift Coefficient from weight and wing area, then derive the 'cruise' C(D(i)) -- and its amount in an ideal corner from the g load needed. Play with these for a crude idea of the decelerations involved.

The long and excellent thread on "peak" would have pleased me more if the RPM limiting effect of mixture setting were addressed more thoroughly... Also, to get a dyno map of an engine's torque and power capabilities, as Dick Fowler suggested, takes several runs with different loads, so that the maximum value of each factor can be tuned for and recorder. That means many props andmuch twiddling. We can't map the ENGINE's torque and HP output with one prop; we CAN profile its range (within the ultimate, max load envelope) on that prop. Similarly for fuel blend, etc. As I recall, most of the P.G.F. Chinn and Billinton curves for crossflow engines showed torque peaking around 10,000RPM, even when HP peaked nearer to 20,000 RPM. That's quite a spread.

"THE CURVE" (Torque & HJP vs RPM) connects the dots at best performance , or maximum load, the engine shows along the RPM range.

Go back to the original Hunt/Pappas Flying Models Tuned-Pipe article! The intent is to tune the "SYSTEM" to allow the engine to go betond the effective torque peak (as modified by pipe resonance effects) and go OFF tune at the desired RPM. As load (to me mostly prop and accelerations) varies the pipe is either able to reach the OFF-tune upper RPM, where resonance failure prevents winding-out (reduced load), or the RPM is dragged back down to where the boost increases (increased loads). A moderately narrow boost range helps keep airspeed more steady: boost increases to prevent slowing, and off-boost "strangling' prevents runaway.

And the prop load differences between cruise and maneuvering vary the prop RPM to regulate pipe response.

As Brett and other mentioned, there is a considerable "centrifugal" load on all elements of the model -- several g, horizontal and out from the center. This is much greater than the 1 g static condition, where we can prove that static fuel head affects RPM. Dynamic fuel head is more nearly the "height" from the uniflow air vent in the tank to the NV jet. Line losses in the metal and plastic length are significant, and apparently subdue the fore/aft accelerations on the "column" of fuel in the supply line. If there is a significant fore/aft acceleration effct, the prop load changes seem to be much greater. The engine tends to lean rather than richen from any forward surge of fuel during the sharp decelerations in maneuvers. And the sharper maneuvers do not last long enough to establish steady conditions; they are brief and violently transitional.

Why not order a video from the Gluedobbers to actually see what fuel does in a clunk tank during the pattern? It's worth the $20.00.

PROBLEM...Where can I find a "Brett Buck" at the hobby store? I work with the practical, not the mathematical. Sure, I can do the SIMPLE math when doing modeling, because the hard stuff has been done already. Numbers for stunt have been signed, sealed, and delivered.
As for “FUEL DRAW” when you point the nose DOWN what happens. Gravity forces the fuel into the ports for a split second causing a RICH run, that’s your breaking going downhill. What happens then fuel head takes over and the fuel pick up tube can’t draw any more fuel your engine goes lean then stops because of fuel starvation. I’m I wrong? Go out and run your plane, YOU HOLD ITLEVEL, at 45 seconds or so point the nose down engine riches for about .8th of a second, after that .8th of a second the engine speeds up EVERYTIME! This is true for every engine out there. During the pattern WHAT is the time between nose down at 45 degree’s and level 5’ off the deck? About five 10ths of a second, this is about max, so you have 3000th. Of a second to work with. CAN you imagine 100’ foot lines, you point the nose down, and it breaks, and then speeds up at about 15’ off the deck? “WALKER PULL OUT”

Scott (needs to take a pill my head hurts) Riese

>PROBLEM...Where can I find a "Brett Buck" at the hobby
>store? I work with the practical, not the mathematical.
>Sure, I can do the SIMPLE math when doing modeling, because
>the hard stuff has been done already. Numbers for stunt
>have been signed, sealed, and delivered.
> As for “FUEL DRAW” when you point the nose DOWN what
>happens. Gravity forces the fuel into the ports for a split
>second causing a RICH run, that’s your breaking going
>downhill. What happens then fuel head takes over and the
>fuel pick up tube can’t draw any more fuel your engine goes
>lean then stops because of fuel starvation. I’m I wrong? Go
>out and run your plane, YOU HOLD ITLEVEL, at 45 seconds or
>so point the nose down engine riches for about .8th of a
>second, after that .8th of a second the engine speeds up
>EVERYTIME! This is true for every engine out there. (snip)
>
>Scott (needs to take a pill my head hurts) Riese

While holding the plane in hand and pointing the nose down you can quickly shut off the engine (our preferred method if something goes wrong after start up and we need to shut it off--just pick it up and point the nose down). This is because gravity simply takes over and all the fuel flows away from the pick up. With no more fuel coming in the tube, it runs out what is there and quits. In flight, however, the fuel is being thrown against the side of the tank at, perhaps, three times the force of gravity. that, plus the acceleration effect keeps the fuel from uncovering the fuel pick up until it is very nearly empty. Then you can do a loop and shut it off, but not before.

However, you are still correct that gravity is pushing (pulling?) the fuel down the pick up line when the plane is pointed down which increases fuel draw and helps to make the engine go rich.

Newman wrote...While holding the plane in hand and pointing the nose down you can quickly shut off the engine (our preferred method if something goes wrong after start up and we need to shut it off--just pick it up and point the nose down).

True to the preferred method. HOWEVER, if the tank is full, before, lets say 45 seconds you'll have a hard time making your engine quit. Especially if running muffler pressure.
My statement is the .8th of the second of reaction time when your engine has to go rich and then lean. Does this make sense to you guy's?

scott(I wonder what a Brett Buck goes for these day's)Riese

Disclaimer: I put my fuel pickups at the back of the tank. The Palmer tank has the fuel pickup halfway back. Take a look at a profile chicken hopper tank. Isn't the fuel feed into the little tank halfway back on the big tank?

Yes we do need a video that is focused on what goes on in the tank.

Jim, We did just that on the Gluedobbers video. You can see exactly what goes on with the fuel in the tank during the entire stunt pattern . The camera is positioned so you can see the side and rear of the tank at the same time.

Scott, two things...

The STATIC fuel head and the DYNAMIC fuel head are very different. What I meant to make clearer is that the "resultant" direction of all forces on the fuel, in flight, at a given instant, is how we should view the 'height' to the needle valve jet (or whichever other reference point it actually is.) With, say 3.5 g centrifugal load, 10 g "vertical" (pitch, along the MODEL'S axes in a round figure) and 1 g from gravity at whatever angle the wing makes from vertical, we have a different picture. The fuel effectively "weighs" its static weight TIMES the momentary g load. The length of plumbing from pickup to NV jet is pretty much at right angles to the much larger g loads, so has VERY little effect. The weight of fuel in the tubing is not only small, but pretty much trapped against surging: the pickup end is (most of the time) submerged, and the jet end empties through a small diameter restriction.

Second, to convince me that the static fuel head check is the definitive factor, I'd like someone to try it while moving on the same radius of motion at the model's airspeed ... and if possible with the same g loads in the model's pitch direction(s).

Not sniping at anyone, just trying to share some thoughts. I do enjoy playing with numbers, and try to reduce the values which work out as strongest, most significant, to simple concepts most of us can grasp.