How to use ExternalForce in Unreal Engine

[jijnt12345]

How to use ExternalForce in Unreal Engine to achieve aircraft catapult takeoff or arrested landing

[seanmcleod70]

I'd suggest taking a look at some of the JSBSim based aircraft models that are used in FlightGear with an aircraft carrier, for example here are some snippets from @Zaretto's F-14 model.

https://github.com/Zaretto/f-14b/blob/d592041d50d74916a3e45c172333d8882151c0fa/Systems/catapult.xml#L35-L49

       <pure_gain name="systems/catapult/cat-force">
          <input>inertia/weight-lbs</input>
          <gain>3</gain>
       </pure_gain>

       <switch name="systems/catapult/cat-final">
           <default value="0"/>
           <test  logic="AND" value="systems/catapult/cat-force">
               systems/catapult/cat-launch-cmd == 1
               systems/catapult/cat-pos-norm lt 0.999
               systems/catapult/cat-pos-norm gt 0.0
               gear/unit[0]/WOW ne 0
           </test>
           <output>external_reactions/catapult/magnitude</output>
       </switch>

So you can see he's generating a catapult force that is equal to 3 times the aircraft's weight, and assuming the catapult is engaged etc. it outputs this force to the property external_reactions/catapult/magnitude while the nosewheel has a Weight On Wheel != 0.

Then in the FDM for the F-14 he defines the location and direction of the catapult external force.

https://github.com/Zaretto/f-14b/blob/d592041d50d74916a3e45c172333d8882151c0fa/f-14b.xml#L3084-L3095

        <force name="catapult" frame="BODY">
            <location unit="IN">
                <x> 667 </x>
                <y>   0 </y>
                <z>   0 </z>
            </location>
            <direction>
                <x>   1 </x>
                <y>   0 </y>
                <z>   0 </z>
            </direction>
        </force>

@Zaretto I was expecting the catapult force to be applied to the nose landing gear, but looking at the locations it doesn't look like it's applied to the nose landing gear. Any particular reason for the choice of the location for the catapult external force?

        <contact type="BOGEY" name="NOSE_LG">
            <location unit="IN">
                <x> 197 </x>
                <y> 0 </y>
                <z> -92 </z>

So in your Unreal based application all you need to do is set the magnitude for the external reaction property.

[Zaretto]

@seanmcleod70 according to commit 8b3eafbd "Revised hook geometry to prevent the aircraft being pulled under deck" - but this message doesn't tell the full story of the time spent (probably about a month on and off) where I was struggling with FG and JSBSim to get the carrier operations to work correctly.

It is probably something that I will revisit.

[seanmcleod70]

No problem, was just wondering if there was a specific reason.

In theory I guess with the force applied at the nose landing gear during the catapult launch there would then also be a pitching up moment so you would need to model an additional restraining force on the nose landing gear to prevent it lifting off, and I guess the main landing gear being behind the cg would react with the deck and there would be some compression of the main landing gear? Plus in theory with the main landing gear compression there would be a small increase in AoA as well during the catapult launch?

So I guess not completely trivial 😉

[Zaretto]

What I didn't know at the time was that the F-14 nosegear can 'kneel'[1] and this is how all cat launches take place. The aircraft attitude will pretty much be fixed when the because the nosegear is attached to the shuttle by the launch bar. The current F-14 has gear kneel modelled.

Once the holdback bar is released the forces involved are huge and there is an initial adjustment and settling [2] it's fairly well planted onto the deck until it starts flying so I don't think there would be any significant change to alpha or compression.

---

[1] https://www.youtube.com/watch?v=zbuldkYstD4
[2] https://youtube.com/clip/Ugkxx47jxAJm3W7XlVjRpM62fjo-b2Pr5o_U?si=LitZayV2l6Domzpp

[seanmcleod70]

Eyeballing the DCS HUD - Real F-14 Pilot Flies the DCS Tomcat it looks like without kneeling the pitch attitude is about +0.5 deg and when kneeling about -1.5 deg, so roughly a 2 deg pitch down?

Also saw mention of the nose gear strut compressing by 14 inches when kneeling. Looking at your gear locations that would equate to ~3.15 deg pitch down?

So assuming roughly -1.5 AoA during launch that would also reduce the drag a bit compared to no kneeling, and I'm assuming part of the reason for the kneeling is to add a pitch-up impulse as the nose gear is released just before the end of the deck? To boost the nose-up trim set via the horizontal stabilizers during launch.

The F-18 makes use of toed-in rudders during launch to add an additional pitch up moment.

[Zaretto]

Eyeballing the DCS HUD

Often I've found that collecting data from other simulations especially cockpit instrumentation can be unreliable source of data.

From NATOPS[1]

At shuttle release, the energy stored in the nose strut is released, rotating the aircraft to the initial flyaway attitude of approximately 12−15 degrees nose−up on the VDI and HUD.

However we can't currently model the force and pitch change in JSBSim because there is no way to compress the strut so instead I move the contact point below deck. ISTR that when I tried a few ways to simulate the push up force (at the end of the catapult) none of them worked well enough; mainly because JSBSim doesn't model the catapult either.

Whilst researching this I remembered that I started working on carrier launch inside JSBSim and when I checked I found this message (from 2017) in jsbsim-devel [2]

On 06/10/2017 09:25, Richard Harrison wrote:
I have previously tried to use an external force attached at the right place, together with a matching hold back and the result is that the aircraft tends to move as the forces aren't in the right place and it only takes small disturbances to cause a twitch that feeds back through the gear and results in a small movement. The only solution I can see for this is to allow any contact point to receive an external force and also to be locked to a specific point in space.
...
The only way I can keep the aircraft locked in position is to have the external force acting through the same point as the mean engine location.

So that answers the question about why I don't have the contact point in the correct place; and possibly opens the subject of carrier handling (or a generic version) being incorporated into JSBSim directly; which is probably out of scope for this discussion.

---

[1] Section III, 8.4.2 "Catapult Launch" https://server.3rd-wing.net/public/Bureau_VF31/Docs%20F-14%20r%C3%A9el/NAVAIR%2001-F14AAP-1%20-%20NATOPS%20Flight%20Manual%20-%20F-14B.pdf

[seanmcleod70]

The catapult take-off is a lot easier than the arrested landing.

For the arrested landing you'll need to write some code to work out whether the tailhook has engaged one of the arrestor wires.

You can take a look at the FlightGear source code to see how they do it in FGJSBsim::update_external_forces().

https://gitlab.com/flightgear/flightgear/-/blob/release/2024.1/src/FDM/JSBSim/JSBSim.cxx#L1429

Once FlightGear detects that the tailhook has engaged an arrestor wire it sets a property and then relies on the aircraft model to calculate and return the force applied by the arrestor wire via the property - systems/hook/force

https://github.com/Zaretto/f-14b/blob/d592041d50d74916a3e45c172333d8882151c0fa/Systems/hook.xml#L137-L160

        <fcs_function>
            <description>
                Hook deceleration force.
            </description>
            <function>
                <product>
                    <p>systems/hook/arrestor-wire-available</p>
                    <p>systems/hook/funcs/hook-operational-efficiency</p>
                    <sum>
                        <!-- Simulate the operation of arrestor wire;
                         1. overspeed will reduce the effiency, up until
                         2. up until a 15kts overspeed at which point the arrestor will will no longer work
                         3. the arrestor wire will take 60 seconds to reset
                         4. arrestor-wire-available; this is 1 or 0 as a function of the maximum contact speed. -->
                        <p>forces/fbx-prop-lbs</p>
                        <product>
                            <p>systems/hook/hook-decel-multiplier</p>
                            <p>inertia/weight-lbs</p>
                        </product>
                    </sum>
                </product>
            </function>
            <output>systems/hook/force</output>
        </fcs_function>

FlightGear then goes ahead and uses systems/hook/force to set an external force, external_reactions/hook/magnitude, with it's value.

fgSetDouble("/fdm/jsbsim/external_reactions/hook/magnitude", fgGetDouble("/fdm/jsbsim/systems/hook/force"))

With the FDM developer specifying a location and direction for this external hook force in their FDM.

https://github.com/Zaretto/f-14b/blob/d592041d50d74916a3e45c172333d8882151c0fa/f-14b.xml#L3110-L3121

        <force name="hook" frame="BODY">
            <location unit="IN">
                <x>  300 </x>
                <y>    0 </y>
                <z>    0 </z>
            </location>
            <direction>
                <x>  -0.9995 </x>
                <y>   0.0    </y>
                <z>   0.01 </z>
            </direction>
        </force>

[jijnt12345]

Which function should be called in Unreal Engine to activate external force?

[seanmcleod70]

Which function should be called in Unreal Engine to activate external force?

As I mentioned above, all you need to do is update the JSBSim property based on the name of the external force that you have defined in your aircraft's FDM, for example based on the F-14 example it would be external_reactions/catapult/magnitude.

If you're specifically asking how to update JSBSim properties from Unreal have you looked at the documentation?

https://github.com/JSBSim-Team/jsbsim/tree/master/UnrealEngine#extended-commands-and-properties