EP250 Air-Launch (Airdrop) Setup

Caution

Status: sim-predicted, not yet flight-validated. Everything on this page comes from PX4 SITL (Gazebo) testing. No air-launch has been flown. Treat this as a development procedure to be flight-validated, not a cleared operational procedure — the figures (pitch angle, altitude loss, recovery reliability) are sim-predicted. Parameter changes are per Firmware & Software Updates — coordinate with RFD.

What it is

The EP250 can be carried aloft under a multirotor, released, and fly away — instead of a multicopter take-off and forward transition. The aircraft is carried horizontal (wings level, nose forward — the easiest way to hang it under a carrier, and the attitude it flies away in), dropped at altitude, arms directly in fixed-wing, and accelerates away under its own power. It remains a full VTOL, so after the mission it back-transitions and lands vertically as normal.

The same firmware still flies normal multicopter-take-off missions — the airdrop behaviour is off by default and switched on by a single parameter (VT_DROP_EN). One aircraft, one firmware, two launch methods.

Parameters to set

Parameter	Airdrop value	Normal value	Purpose
VT_DROP_EN	1	0	Master switch. 1 = airdrop (drop-launch); 0 = normal MC take-off + transition.
VT_DROP_PITCH	35	—	Commanded nose-up pitch (deg) held through the launch — the altitude-loss knob (see testing below).
VT_DROP_DUR	5	—	Maximum launch-hold time (s) before handing to the normal fixed-wing controller (TECS).
VT_DROP_SPD	35	—	Groundspeed (m/s) at which the launch hands to TECS — whichever comes first with VT_DROP_DUR.
CBRK_VTOLARMING	159753	0	Circuit-breaker that allows arming in fixed-wing (a VTOL normally refuses to arm unless in multicopter mode).
COM_PREARM_MODE	2	0	Always prearmed — motors locked out (guaranteed zero thrust) while captive on the carrier, until armed at release.
FW_LAUN_DETCN_ON	0	—	Catapult/hand-throw launch detection off — a drop has no forward-acceleration spike.
COM_DISARM_PRFLT	0	10	Don’t auto-disarm before take-off — there is a brief armed-but-not-yet-flying window at release.

Danger

CBRK_VTOLARMING removes a safety interlock (fixed-wing arming). With it set, the aircraft can be armed in fixed-wing on the ground — motors run with no transition. Keep COM_PREARM_MODE 2 so the props stay locked until the deliberate arm at release, and handle the aircraft accordingly.

How the launch works

1. Captive carriage — carried horizontal under the carrier, prearmed (motors locked, zero thrust). With VT_DROP_EN the aircraft is put straight into fixed-wing while disarmed (it is already at the fixed-wing attitude, so there is no hover/transition).
2. Release + arm — the aircraft is dropped and armed in fixed-wing within ~0.2 s.
3. Drop-launch hold — throttle goes to 100 %; the aircraft holds VT_DROP_PITCH nose-up and wings level for up to VT_DROP_DUR (5 s) or until VT_DROP_SPD (35 m/s) groundspeed, whichever comes first.
4. Hand-off — control passes to the normal fixed-wing controller (TECS); the aircraft climbs to its commanded altitude and flies the mission.
5. Recovery + land — at the end of the mission it back-transitions and lands vertically, exactly like a normal sortie.

What the testing showed (sim-predicted)

A hover drop means zero airspeed at release — the wing makes no lift until roughly 30 m/s, so left to itself the aircraft sinks a long way before it flies. The key finding: the commanded nose-up pitch (VT_DROP_PITCH) controls how much height is lost — a nose-up attitude at full throttle tilts the thrust vector to help support the weight, trading forward acceleration for less height loss.

Sim sweep (drop from 100 m AGL, EP250 at the as-flown mass):

VT_DROP_PITCH	Altitude lost	Hand-off speed
0° (level)	~50 m	~43 m/s
20°	~28 m	~38 m/s
30°	~17 m	~37 m/s
35° (chosen default)	~13 m	~35 m/s
40°	~10 m	~32 m/s

• 35° is the chosen default — about 13 m lost with a comfortable ~35 m/s hand-off (above the 30 m/s minimum airspeed). Lower pitch loses more height but hands off faster; higher pitch loses less but hands off slower (less stall margin).
• Drop height: 100 m AGL keeps the recovery comfortably under a 120 m (400 ft) AGL ceiling, with ~13 m used on the launch. Airspace compliance is the operator’s responsibility.
• Drop in fixed-wing, not multicopter — the multicopter controller cannot hold the horizontal carriage attitude at zero airspeed.
• Marginality (important). The zero-airspeed drop is dynamically marginal in the sim — at 35° it recovered on about 5 of 6 runs; the occasional failure is a stall on the edge of controllability. This is a real limitation of the manoeuvre and the main reason it must be flight-validated and further tuned before any operational use.

Reverting to normal flight

Set VT_DROP_EN 0 — the aircraft does a normal multicopter take-off and forward transition. The drop-launch parameters are ignored when VT_DROP_EN is 0, so one airframe configuration covers both launch methods.