# inverted tangens

## Recommended Posts

Hi,I try to calculate the flight path angle by using vertical speed dividied by ground speed and inverted tangens of that. I have looked in the sdk and there are atg and atg2 which one to use and what is the difference?BrgdsEugen

##### Share on other sites
 Help AVSIM continue to serve you!Please donate today!

atg is the arc tangens you might no, it needs one argument between -1

##### Share on other sites

Ah I think I got something wrong here atg is not limited to -1 1 like asin or acos, but if you have vertical movement y/x gets infinite and that will cause trouble, so atg2 might be better then. I'm so used to atg2 that I seldom use atg any more.Arne Bartels

##### Share on other sites

Hi Arne,So it could look something like this in an autopilot ?%((A:Autopilot vertical hold var, ft/min) (A:Autopilot airspeed hold var, ft/min) atg2)%!3.2f!Cant get it to work...regardsEugen

##### Share on other sites

Why? It works fine. The result is of course pretty small especially since the result is in radians, so at 100kt IAS and 100ft/min VSPD you end somewhat at 0.01 radians. If you convert to degrees, it looks better of course:%((A:Autopilot vertical hold var, ft/min) (A:Autopilot airspeed hold var, ft/min) atg2 rddg)%!3.2f! This gives 0.56

##### Share on other sites

(A:Autopilot airspeed hold var, ft/ min)Huh? Never seen that one before. I've made something similar, just to display the descent angle, not hooked up with the autopilot in any ways. Keep in mind that indicated airspeed used in groundspeed "operations" tend to go a little wacko when variable winds or gusty windspeeds are introduced to the system :DMost for fun though:GRADIENT PROFILE: %(60 (A:GROUND VELOCITY,knots) / (A:VERTICAL SPEED,feet per minute) 0.01 * * abs)%!3.1f!°Replace the

##### Share on other sites

Oops, I didn't actually pay attention if the code I placed does make sense or not, it was only to demonstrate the math behind it.Hey what are you doing with this code, (A:GROUND VELOCITY,knots) and (A:VERTICAL SPEED,feet per minute) definetely OK, but the rest does look, as if you made a tanx=x approximation. Defenetely OK for the usual small angles, but I haven't checked the factors (Why should I, if you say it works).Arne Bartels

##### Share on other sites

Hi Guys,(A:GROUND VELOCITY,knots) and (A:VERTICAL SPEED,feet per minute)That are two different units of measuring speed? knots (nautical miles/hour) and feet/ minute right? wouldn't you need common units to calculate the fpa?RegardsEugen

##### Share on other sites

There are different units used, but if you convert them properly by hand, it's OK.I had deeper look at the formula, it looks like on of those famous "rule of thumbs" so popular in aviation (I'm only a "desktop" pilot and have never learned them properly, so I have to recalculate every time). Mainly it takes into accont that for small angles (approx. <5° ; or so) tan(x) is approximately x (x is here in radians, NOT degree). (vertical speed in kt)/(groundspeed in kt)=tan(flightpathangle in radians) is approximately(vertical speed in kt)/(groundspeed in kt)=flightpathangle in radiansfrom radians to degrees you need to multiplicate by 180/pi which is very roughly 60to convert vertical speed in ft/min to kts you have to divide it roughly by 100.So you end up with a formula60 / (A:GROUND VELOCITY,knots)*(A:VERTICAL SPEED,feet per minute)/ 100which is Karls formula. I assume the rough estimate for 180/pi and feet/min to kts balance the errors a bit.A bit exacter would be 180/pi=57.29578 and 1feet/min=0.009876kts=(1.0/101.260)kts, but that doesn't look to be remembered easily.So Karls formula is an approximation of the mathematically exact solution, but for small angles it is pretty good.Arne Bartels

##### Share on other sites

Heh, what I used was actually the description in my E6-B (ASA's flight computer) manual; Feet Per Mile vs Feet Per Minute paragraph. A climb of 300 feet per mile will result in a 3

##### Share on other sites

Just to see the differences I calculated an example:assume 1000ft/min and 100kts gives with atan 2.8268° Karls Method 3° +0.17° or 6% error the method with enhanced factors,but without atan 2.8291° +0.002° or .08% error.Ah by the way, *180/pi doesn't need to be calculated in the XML gauges because rddg does the same, also the ft/min to kt can be done with the units in the (A: statements.Arne Bartels

## Create an account

Register a new account

• Tom Allensworth,
Founder of AVSIM Online

• ### Hot Spots

• Flight Simulation's Premier Resource!

AVSIM is a free service to the flight simulation community. AVSIM is staffed completely by volunteers and all funds donated to AVSIM go directly back to supporting the community. Your donation here helps to pay our bandwidth costs, emergency funding, and other general costs that crop up from time to time. Thank you for your support!