View Full Version : Is there an easier way :)

Wozza

November 6th, 2009, 21:38

Hi

Im trying to adjust the DEFLECTION v DYNAMIC PRESSURE but the maths leaves me wallowing and wanting :D

As the table is liner there has to be a simple maths to work out the IN/X param as a speed at sea level ? why they didnt just use speed as the

X param is beyond me given how simple the table is

Thanks

Wozza

sparks

November 7th, 2009, 06:38

Here's the math:

V_fps = SQRT(2*q/rho)

q = (rho/2) * V_fps^2

V_mph = V_fps * 3600/5280

Alt (ft) rho

00000 0.002378

05000 0.002049

10000 0.001756

15000 0.001496

20000 0.001267

25000 0.001065

30000 0.000889

35000 0.000736

40000 0.000582

45000 0.000459

50000 0.000361

55000 0.00026

60000 0.00019

A good approximation for rho:

rho = 0.002378 * (1.0 - 0.00000688 * altitude)^4.256

This is pretty easy to code into a spreadsheet.

Wozza

November 7th, 2009, 09:49

Here's the math:

V_fps = SQRT(2*q/rho)

q = (rho/2) * V_fps^2

V_mph = V_fps * 3600/5280

Alt (ft) rho

00000 0.002378

05000 0.002049

10000 0.001756

15000 0.001496

20000 0.001267

25000 0.001065

30000 0.000889

35000 0.000736

40000 0.000582

45000 0.000459

50000 0.000361

55000 0.00026

60000 0.00019

A good approximation for rho:

rho = 0.002378 * (1.0 - 0.00000688 * altitude)^4.256

This is pretty easy to code into a spreadsheet.

WOOOOOOSH!! thats the sound of what you typed passing way over my head :D

spreadsheet I thought you used those to stop paint from getting on the floor ;)

Thanks for taking the time to reply but Im at a total utter loss when it comes to these formulas...I can understand the physics/theory behind it but not the maths (yeah Im lazy but have killed of way to many brain cells)

maybe i didnt explain it well enough,My way of thinking in sim not real world is that at say 500ft at a certain speed the Y param will take effect rather than spend mths working out the maths there has to be a simple rule of the thumb to convert the X var to a "knots mph kmh" times,divided added or minus numberX = dynamic pressure

Thanks

Wozza

Wozza

November 7th, 2009, 11:11

Hi

Was sitting here thinking and thought why not do some in sim testing :)

what I did was set the elevator X1=0 Y1=1, X2=50 Y2=1 X3=51 Y3=0

then bump up to 2000ft and dived with engine off and waited till elevator stopped working :) not the most mathematical solution but an interesting way to while away the hrs.

ball park figures so far

X of 25 = 95mph

X of 50 = 145mph

X of 100 = 195mph

X of 150 =245mph

X of 200 = 295 mph

whilst not perfect there is a trend forming,Ive only tested with one aircraft so It will be interesting to see if its the same trend for different aircraft\airfiles

if not at least I have sufficient info to fudge my project :D

Wozza

fliger747

November 7th, 2009, 12:22

Over Iraq yesterday someone was talking to bagdad from FL570... pretty low Rho up there!!!!

Sometimes physically plotting the "curves" will cause the light bulb to illuminate if perusal of the formulas and tables do not initially.

Cheers: t

sparks

November 8th, 2009, 10:05

mph to dynamic pressure at sea-level...

Wozza

November 8th, 2009, 18:53

mph to dynamic pressure at sea-level...

Ahhh now thats easier :)

now if i can work out something along these lines for the prop table

Id be able to fix those as well ;) yeah I know a whole different kettle of fish

thanks agsin

Wozza

fliger747

November 9th, 2009, 00:37

Herr Carter;

In Jerry's old workbook series the generated tables were visible in graphical form. Another application is Aircraft Airfile Manager which also generates the tables in graphical form and are directly editable by airthemetic entries to the ordinates.

Cheers from Dubai.... T

Wozza

November 9th, 2009, 21:02

Herr Carter;

In Jerry's old workbook series the generated tables were visible in graphical form. Another application is Aircraft Airfile Manager which also generates the tables in graphical form and are directly editable by airthemetic entries to the ordinates.

Cheers from Dubai.... T

Ich bin dummkopf :D

Using the graph above Ive now set the controls to freeze just over tha max permissible speed this stops the stupid crash reset if you push the aircraft slightly outside the envelope. Ive set the max speed to 300 mph in the cfg but if you manage to get to that speed your going to a big smoking hole in the ground ;)

I use AAM but the graphs make no sense to me ;) as for the workbook I dont have excel so no luck there ...then I remembered that someone smarter than me did a 1% airfile for my cfs2 texan using the workbook.So I found that and had a look at the prop tables. A quick cut and paste and it looks like Im getting pretty close to real world fuel burn/speed at a quoted rpm/mp settings,though there are some weird prop pitch numbering in there it appears to work ok

Now to fix the tail coming up late on take off and the roll issues and I think I can put this sucker to bed :D

Thanks

Wozza

PS when using airwrench to set the balance up I almost always end up with an aircraft that feels(to me) to nose heavy is this right ? or have I missed something ?

Wozza

November 13th, 2009, 21:04

Hi Tom or anybody for that matter :D

You ever sat in a G model texan? (yeah slightly off topic :) )

I only have info on the C model and cant work out where they stuck the

engine primer and the starter on the G model.Imagine my surprise when I found out they completely changed the cockpit layout in different models

Thanks

Wozza

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