#### Archived

This topic is now archived and is closed to further replies.

# "Groundspeed" on a moving surface in FSX ?

## Recommended Posts

Hi All,In FSX, is there a variable that gives the speed of an aircraft relative to the surface it moves on ??Note: Not "ground velocity", because that gives the speed relative to the earth, NOT the moving surface :-)Regards, Rob Barendregt

##### Share on other sites

No hints ??Or simply not existing ?Regards, Rob

##### Share on other sites

Hi Rob Are you trying to measure AC Carrier speed?

##### Share on other sites

>> Hi Rob>> Are you trying to measure AC Carrier speed?>>Hi,It's indeed related to aircraft carrier operation :-)But I'm not trying to measure the carrier speed, because I already know that (since that's the value of the "Ground Velocity" variable).What I'm looking for is a variable that gives the "groundspeed" of the aircraft related to the carrier deck it drives on ....I already found a workaround by using the Nosewheel rotation variable, and, given the nosewheel radius, derive aircraft speed from that, but that is rather a cumbersome solution.So I'm looking for a more direct one.Cheers, Rob

##### Share on other sites

Hi Rob,I guess what you mean is kinda measuring the speed of an aircraft *rollig* on a surface like a car? I don't think it has a velocimeter cable attached to the wheels :-) but you might try VELOCITY BODY Z, which is by definition its True longitudinal speed, relative to aircraft axis (not Earth).Tom

##### Share on other sites

>Hi Rob,>>I guess what you mean is kinda measuring the speed of an>aircraft *rollig* on a surface like a car? I don't think it>has a velocimeter cable attached to the wheels :-) but you>might try VELOCITY BODY Z, which is by definition its True>longitudinal speed, relative to aircraft axis (not Earth).>>Tom>>Hi Tom,Thanks for the tip; I'll give it a try...Cheers, Rob

##### Share on other sites

Rob,If you have FSX w/Accelator or SP2, this is the variable you're looking for:SURFACE RELATIVE GROUND SPEED (knots, feet per second, etc)Tom

##### Share on other sites

Hi Tom,That's probably it !!But since I don't have SP2 installed yet (yes, I know, it's one way or the other ...lol), do you know if this value can also be negative (unlike the GroundVelocity variable) and if it is the groundspeed in the exact aircraft-longitudinal direction or the groundspeed in all directions relative to the surface ??I.o.w: is it "VELOCITY BODY Z Relative to the surface" ?? Because I'm really reluctant to install SP2 (or require users of my new gauge to have SP2 installed) if I'm not sure this variable is exactly what I need...Thanks sofar ....Cheers, Rob

##### Share on other sites

Rob If the carrier has an ILS/DME maybe you could use the DME ie, time/distance.

##### Share on other sites

Yes, VBZ can be negative, and is only the longitudinal direction; there is a VB X var that measures lateral displacement.But I guess you might want to make your own tests :-)Tom

##### Share on other sites

>Rob,>>If you have FSX w/Accelator or SP2, this is the variable>you're looking for:>>SURFACE RELATIVE GROUND SPEED (knots, feet per second, etc)>>Tom>>>>>>>>>Actually, it's an Acceleration-only variable.It's probably exactly what he's looking for.SURFACE RELATIVE GROUND SPEED The speed of the aircraft relative to the speed of the first surface directly underneath it. Use this to retrieve, for example, an aircraft's taxiing speed while it is moving on a moving carrier. It also applies to airborne aircraft, for example when a helicopter is successfully hovering above a moving ship, this value should be zero. The returned value will be the same as GROUND VELOCITY if the first surface beneath it is not moving.

##### Share on other sites

Hi Guys,Thanks for all the info provided !!Actually, I needed this for an enhancement of my VTOL gauges, where I want to provide the ability to control a vehicle "driving" accurately on a moving surface like an Aircraft Carrier.As some of you may know, this "VTOL" solution is based on the principle of overwriting the FDE-calculated values by gauge-calculated ones.Since I don't want to force people to buy Accelleration to be able to use it (thanks Ed, for clearing this up !) I'll revert back to my old trick, where the RPM of the nosewheel is a good measure for taxiing speeds.Works well, unless the aircraft "hops" a bit on deck because of the carrier movements.And Tom, I was unaware of VELOCITY BODY Z (thanks for that tip); I have tested that, and started to use it for some other functions instead of using GROUND VELOCITY. Works actually a lot better for the purpose I need it for :-)Cheers, Rob

##### Share on other sites

• 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!