Engine oil pressure

[Guest andrewluck]

I've been trying to raise the oil pressure at idle for the Hawker as I find the caution lights keep coming on when taxiing.The relevant file appears to be tri4.ngn which contains the following lines: -- Oil Pressure vs rpm -- -- begin 1D Table -- - x poly - 0.0 0.0 0.4 0.0 62.5 1.0 8.33 41.67 - end format - -- end 1D Table --Can anyone tell me how the values in this table work?ThanksAndrew Luck18 miles SW EGSH

[Guest Chris Wallace]

OK, I'll give this a shot but these tables are a bit difficult to explain without illustrated examples etc. Plus I may have some of the details mixed up, but here goes:The overall purpose of the table is to define a function that maps engine RPM (for turbine engines, I think this is the N1 RPM?) to oil pressure (in PSI). For convenience, we'll refer to the independent value of engine RPM as 'x', and the dependent value of oil pressure as 'y'. Each line of the table represents a "breakpoint" which is the x-value at which the specified function applies, then for a table, a set of coeffients for a quadratic polynomial function which is used for x-values up to that breakpoint. Generally:b p0 p1 p2translates to a breakpoint x-value of "b", and a corresponding mapping function of y = p0 + (p1 * x) + (p2 * x^2).For your example, since there are three lines in the table, there are three distinct regions to the mapping function:0.0 0.0translates to breakpoint x=0.0, function y=0.00.4 0.0 6.25translates to breakpoint x=0.4, function y=0.0 + 6.25 x1.0 8.33 41.67translates to breakpoint x=1.0, function y=8.33 + 41.67xSo the overall mapping function is composed of three regions:

 x <= 0.0		 y = 0.0 0.0 < x <= 0.4   y = 62.5 * x 0.4 < x < 1.0	y = 8.33 + (41.67 * x)

In real-life terms, what you see is the oil pressure increasing linearly from 0 to 25 PSI as N1 goes from 0% to 40%, then it would increase linearly but at a slower rate from 25 to 50 PSI as N1goes from 40% to 100%. To decrease the oil pressure for a given RPM, you just have to reduce the polynomial coefficients. Take care to make sure that the mapping functions on either side of each breakpoint converge to the same value, otherwise there will be a noticeable "jump" when the independent value hits the breakpoint. In the original table for example, looking at the breakpoint value 0.4, the value on the "left" side is y=62.5 * 0.4 = 25.0 and the value on the "right" side is y = 8.33 + 41.67 * 0.4 = 25.0 That's a happy coincidence :-)So to decrease oil pressure by 10% across all RPM values, use the following table values:0.0 0.00.4 56.251.0 7.5 37.5This is only one way to solve your original problem however...if all you want to do is prevent the low pressure annunciator light from coming on, you can do it by lowering the threshold at which the alarm is raised, rather than changing the way that the engines model oil pressure. I don't have the Hawker AMP file here in front of me now, but I'll follow up with how to do this if I get a chance later.Chris WallaceOttawa, Canada

[Guest andrewluck]

ChrisThanks for that. A clear and precise explanation of what I was looking for.I've also been looking at the AMP file but the low oil pressure warnings are generated at 5psi which seems rather low. However, I was an avionics engineer, not engines, so I may be completely wrong here :-).I shall take a look at the figures that your equations give and see if I can get a faster ramp up to the orange section of the gauge.As more people start to look at panel design we really need a central repository for all this type of information to prevent everyone from redesigning the wheel over and over.Andrew Luck18 miles SW EGSH

[Guest Chris Wallace]

>I've also been looking at the AMP file but the low oil >pressure warnings are generated at 5psi which seems rather >low. However, I was an avionics engineer, not engines, so I >may be completely wrong here :-). It's odd that you're getting an oil pressure annunciator light...I took the Hawker out for a run last night and the oil psi was always up around 25 as long as the engines were spooled up and running. I never saw the oil pressure light unless I cut one of the engines. If the threshold is 5 psi as you say, it's strange that you'd ever see the light at all.Chris WallaceOttawa, Canada

[Guest andrewluck]

ChrisA close inspection of my AMP file revealed the cause of the problem. While the oil pressure gauge definition has an entry for low pressure of 5, the MWS definition actually defines when the light is going to come on and in my file that was set to 20.An idle rpm of 25% gives an oil pressure of 15psi so that's why the warning light was coming on.I think this AMP file came with the Fly!2 sound enhancement pack and the author had also messed with the pressures. I'd subsequently changed the aircraft name in the NFO file and re-podded which rolled the modified AMP file in as well.Now I just need to check the POH for a Hawker 800 and see what the idle oil pressure should be.Andrew Luck18 miles SW EGSH