Induction negative pressure calculation

[ToughBirds]

Can someone direct me to a method of calculating the negative pressure experienced by an induction air hose that is connected between the air filter and intake manifold?

In most small aircraft, the hose used to connect the air filter housing to the engine air intake is SCAT, SCEET, CEET, or CAT. However, none of those is recommended for negative pressure applications. So, I'm looking for a hose that is rated for that application, but I can't find a reference to how much negative pressure is found inside that hose.

I will cross-post this to the automotive thread as well.

 

[rb1957]

won't the drop in static pressure inside the hose be due to the dynamic pressure of the airflow in the hose ?

so this'd be q = 1/2*rho*v^2

my guess for airflow velocity (since i guess that'll be the next question) could be determined by the mass flow through the engine

 

[ToughBirds]

rb1957,

I believe I can calculate the velocity from the volume and intake hose diameter. That is, CFM (based on RPM and displacement) and I.D. of the intake hose.

Since I'm a computer guy, not an aerospace engineer, can you tell me the value for rho in the equation you posted?

Thanks for the help.

 

[rb1957]

air mass density = 1.225kg/m^3 = 0.0765 lbm/in^3
(if i can do my conversions right !)

 

[rb1957]

oops lbm/ft^3 (sigh)

 

[thruthefence]

Calculations aside, if you are experiencing enough pressure drop across your inlet filter, you are having other problems beside collapsing a scat duct.

 

[ToughBirds]

thruthefence,

I have seen references to the pressure drop across the filter of anywhere from 1.5 to 3 psi. I don't know what it takes to fully collapse a scat duct (since they're not recommended for negative pressure applications I suppose the manufacturers don't want to put a number on paper).

I have seen CEET duct rated at not more than 4 PSI negative, so at up to 3 psi due to the filter alone it's getting pretty close for comfort.

However, even partially collapsing the duct will have an impact on air flow, no?

There have also been reported cases of power loss on takeoff that were blamed on collapsing scat duct. The air filters were not found to be a problem, so one can guess that under full-power conditions scat duct isn't quite up to the task.

 

[thruthefence]

Professionally interested, can you provide a link from the SDR or MDR reporting system to elaborate on this? A 3psi delta seems huge.