Possible fuel starvation?

[Skunkworks85]

13 hours ago, FredG said:

The difference in pressure at the inlet to a hose compared to the outlet of a hose (the pressure equivalent of voltage drop across a resistor) is proportional to the fourth power of the radius of the hose.  So, if the radius is halved, the pressure drop in that length of hose increases sixteen times.  A 20% reduction in hose diameter results in a 2.4 fold reduction in pressure (which is the same as 2.4 fold increase in resistance to flow).  Small changes in hose diameter have very large effects on the resistance to flow in that hose (and, very large decreases in flow).

You are confusing pressure and flow. 

[FredG]

The Hagen-Poiseuille equation from Wikipedia is provided below.  It is useful because it shows the relationships between pressure, flow, hose length, hose diameter, and fluid viscosity.

 

As the pressure at the inlet increases in comparison to the outlet (deltaP), the flow (Q) increases (this is analogous to the Ohm's law relationship whereby an increase in voltage in a circuit results in an increase in current flow at a given resistance).  As the radius of a hose decreases or the length of the hose increases, the pressure at the outlet decreases in comparison to the inlet (this is analogous to a voltage drop with increasing resistance).  The decrease in pressure is proportional to the length of the hose (as noted by Corey) whereas the decrease in pressure is the fourth power of the radius of the hose.

 

Jim, your question, "What diameter hose is necessary to flow lets say 30 liters per hour (roughly 8 gal per hour - much more than I ever see on my fuel flow gauge)?" requires you also specify a hose length and a working pressure (I will assume the fluid is gasoline).  Otherwise, there is no one answer.

 

 

 

 

where:

• Δp is the pressure difference between the two ends,
• L is the length of pipe,
• μ is the dynamic viscosity,
• Q is the volumetric flow rate,
• R is the pipe radius,
• A is the cross sectional area of pipe.

[Jim Meade]

Length is the length from the CTSW tank to the fuel pump.  I'll make a wild guess of 4 meters, though I am fully willing to stand corrected if someone has a known length.  Pressure will be gravity pressure or whatever other pressure is operative for the CTSW fuel system.

[Madhatter]

16 hours ago, FredG said:

The Hagen-Poiseuille equation from Wikipedia is provided below.  It is useful because it shows the relationships between pressure, flow, hose length, hose diameter, and fluid viscosity.

 

As the pressure at the inlet increases in comparison to the outlet (deltaP), the flow (Q) increases (this is analogous to the Ohm's law relationship whereby an increase in voltage in a circuit results in an increase in current flow at a given resistance).  As the radius of a hose decreases or the length of the hose increases, the pressure at the outlet decreases in comparison to the inlet (this is analogous to a voltage drop with increasing resistance).  The decrease in pressure is proportional to the length of the hose (as noted by Corey) whereas the decrease in pressure is the fourth power of the radius of the hose.

 

Jim, your question, "What diameter hose is necessary to flow lets say 30 liters per hour (roughly 8 gal per hour - much more than I ever see on my fuel flow gauge)?" requires you also specify a hose length and a working pressure (I will assume the fluid is gasoline).  Otherwise, there is no one answer.

 

 

 

 

where:

• Δp is the pressure difference between the two ends,
• L is the length of pipe,
• μ is the dynamic viscosity,
• Q is the volumetric flow rate,
• R is the pipe radius,
• A is the cross sectional area of pipe.

I think you've lost most of us with this. My first engineering job was a turbine test engineer for Lycoming and we had another engineer throwing equations at me until my soon to be wife sitting in front of me who also was a mathematician told me to not take him seriously. For me just give me some wrenches, parts and instrumentation and I'll figure it out. No offense to you it's just your thing. My wife won't even let me use the company checkbook because my numbers never add up to her standards ( but I have a credit card ).

 

[FredG]

The equation looks complicated, but it really isn't.  It says that for a given fuel pressure, if the hose length doubles, the flow rate is halved.  If the hose diameter (or radius) is halved, the flow decreases 16-fold.  That really is the message relevant to those of us who build, modify, or maintain airplanes.  

[Madhatter]

I'm not questioning the complexity, just that most don't think that way. We had design engineers for the aerospace companies I worked for and it was my job to evaluate the end result. Their calculations did not always achieve the desired result and sometimes were counter-productive. Sometimes it just took someone with a simple guage, some tools and common sense to solve the problem. I did the testing and my wife did the data analysis, that's the way I like it. I see myself more as a glorified mechanic as apposed to an engineer, doing equation calculations is no fun for me. Except for some material strength calculations I have never had to solve  equations, and have never met an A&P in 50 years who did, we use charts and graphs. To each his own however.

[FlyingMonkey]

To add additional complexity to this topic...Madhatter didn't you replace the masts on the fuel caps with curved tubes like you see on a lot of older airplanes?  If so that has to provide better/higher pressurization than the masts with their sharp 90° bends and restrictions.  Probably lower drag too...

[Madhatter]

1 hour ago, FlyingMonkey said:

To add additional complexity to this topic...Madhatter didn't you replace the masts on the fuel caps with curved tubes like you see on a lot of older airplanes?  If so that has to provide better/higher pressurization than the masts with their sharp 90° bends and restrictions.  Probably lower drag too...

 I didn't change the caps. My concern is that in rain I will get some water ingestion. I have a modified cub with the caps but any significant rain is generally avoided in this category aircraft.

[FlyingMonkey]

1 hour ago, Madhatter said:

 I didn't change the caps. My concern is that in rain I will get some water ingestion. I have a modified cub with the caps but any significant rain is generally avoided in this category aircraft.

Hmm...I know I saw somebody put the tube-style on their caps, not sure who it was. 

[Tom Baker]

3 hours ago, FlyingMonkey said:

Hmm...I know I saw somebody put the tube-style on their caps, not sure who it was. 

It was Roger. His old airplane still has them in place.

[FlyingMonkey]

18 minutes ago, Tom Baker said:

It was Roger. His old airplane still has them in place.

Thanks.

[Skunkworks85]

On 6/17/2023 at 3:41 PM, FredG said:

The equation looks complicated, but it really isn't.  It says that for a given fuel pressure, if the hose length doubles, the flow rate is halved.  If the hose diameter (or radius) is halved, the flow decreases 16-fold.  That really is the message relevant to those of us who build, modify, or maintain airplanes.  

"if the hose length doubles, the flow rate is halved"

Without dusting off my fluid dynamics books, there is some other factor at play here. If I double my garden hose length, I will see 1/2 the volumetric flow?  I can tell you right now, I can fill a 5 gallon bucket right out of the spicket at an very similar rate as a 100ft hose. 

 

 

[Anticept]

For the 100 ft garden hose, the hose itself isn't much of a restriction. The valve at the spigot is.

An open hose has next to no pressure difference from the inlet and outlet.

But that said yes perhaps saying doubling a pipe length halves the volumetric flow rate might be an oversimplification.

[Skunkworks85]

5 hours ago, Anticept said:

For the 100 ft garden hose, the hose itself isn't much of a restriction. The valve at the spigot is.

An open hose has next to no pressure difference from the inlet and outlet.

But that said yes perhaps saying doubling a pipe length halves the volumetric flow rate might be an oversimplification.

I understand this, But fundamentally, what is different from my example to the fuel hose system from wing to the shut off valve in a CT? From a physics stand point, nothing. 

 

FregG is stating that his post is fact based on a misapplied equation. (to be 100% honest I have not looked at it enough to understand what is wrong)

 

[Anticept]

It's above ky pay grade.

All I have to do is install lines, and make sure they flow a minimum 150% of max expected engine demand even when filled barely above useable and call it good.

Madhatter's 3/8 AN probably does 35 gallons an hour!

[FredG]

Skunkworks:  I will make this easy for you.  Hypothetically, you have 50 feet of garden hose with an inlet pressure of 75psi and outlet pressure of  73psi (the delta P is 2psi over the 50 feet).  If you double the length of hose, the delta P becomes 4psi (the PSI at 100 feet would be 71psi).  My error was to apply the formula to absolute pressure and not to the change in pressure from hose inlet to outlet (ie, delta P or pressure drop).  The proportional decrease that resulted from doubling the hose length (IF the original hose met the pressure drop value provided above) is the value (73/75) or 0.97 (ie, not very much, as you intuited).

So, you are correct, the effect of doubling hose length is a doubling of the pressure drop through the hose. If the original hose had a small pressure drop (as in my example in the paragraph above), then twice the small pressure drop is still a relatively small pressure drop.  If the original hose had a larger pressure drop, then twice the pressure drop may be a problem in some settings.

Similarly, the pressure drop increases as the fourth power of the radius.  In this case, small or modest pressure drops have the potential to exponentiate to values that matter.  If you halved the diameter of the hose above, the pressure drop would be 16psi and the outlet pressure would be 59psi, or a 21% reduction.  So, in this case, doubling the length results in a 2psi pressure drop increasing to a 4psi.  Halving the hose diameter results in a 2psi pressure drop increasing to a 16psi pressure drop.

Bottom line, my error above was to confuse delta pressure across a run of hose with actual pressure.  In this post, I have corrected the error and provide results I believe are correct regarding the effect of changes in hose length and hose diameter.

[Madhatter]

[FlyingMonkey]

When did our airplane forum become a physics and fluid dynamics forum??  

[Anticept]

Why can't it?

[FlyingMonkey]

55 minutes ago, Anticept said:

Why can't it?

Maybe it should be renamed to "CTFluids"...  

[Tom Baker]

3 hours ago, FlyingMonkey said:

Maybe it should be renamed to "CTFluids"...  

There is a sub forum for that. https://www.ctflier.com/forum/72-fuels-fluids/

[Jim Meade]

I moderated a national agricultural forum for 20 years.  At the start 1n 1995, we had a lot of land grant university academics posting and they brought a lot of differing perspectives to the forum.  Within a few years, the naysayers and Luddities had driven them out with denial, badmouthing, and personal disrespect.  No science for the farmers.  No formulas.  Nothing that involved absolutes.  It was all about "well, I think" and "this always worked for me" and "these guys can't farm so they teach".  The forum epitomized H. L. Mencken's quote, "Before a man speaks it is always safe to assume he is a fool. After he speaks, it is seldom necessary to assume it."  The forum lost any value for the enterprising farmer and ended up being the common herd with nothing really to say, unless it was how to hook up a plow differently.

The concept of the FAQ was supposed to be a way to avoid repeating the same question over and over.  It failed.  No one reads the FAQ and all are fascinated by their own questions.  The idea of moving concepts to a new forum has never broken the tendency to thread creep and I predict never will.  Move it if you want.  Many will be more comfortable if it is not easily seen.

Do you have a copy of "Aerodynamics for Naval Aviators"?  Throw it out.  It has formulas in it.

 

[FlyingMonkey]

8 hours ago, Jim Meade said:

The idea of moving concepts to a new forum has never broken the tendency to thread creep and I predict never will.  Move it if you want.  Many will be more comfortable if it is not easily seen.

I see this forum as "conversational".  When people have conversations in person, the topics sometimes shift and wander, and that happens here.  No biggie.

I don't think anybody is getting upset about any of this.  Some like equations and jab at the "seat of the pants" guys, some don't like equations and poke a little fun at the eggheads.  It's all good.

[Towner]

I read every post on here from beginning to end. I find it all interesting, and educational, for an owner of a CT. Usually the bleed is fine with me. I admit, I do feel for the guys trying to buy or sell a plane as the original post, just to have it bleed into something way off topic, but otherwise, let it bleed. I’ll still read it and chances are I’ll enjoy it and learn something. Even the formulas!

[FredG]

FWIW, I am not trying to jab at anybody.  See my post above in which I acknowledge (in response to Skunkworks85) my initial incorrect application of an equation relating the pressure loss in a hose to hose length and diameter and the correction application of the equation (to a problem relevant to us as aircraft owners).  

My goal is to understand how various systems of my experimental aircraft work so that I can maintain and improve them with a high likelihood of success.

1 hour ago, Towner said:

"let it bleed"

Hmmmm, speaking of bleeding, that might be a good name for a record album...