gps-denied-onboard/_standalone/UAV_frame_material/01_solution/solution_draft06.md

Solution Draft (Rev 06) — Material Comparison: S2 FG + Carbon Stiffeners vs Shark M (Pure GFRP)

Assessment Findings

Old Component Solution	Weak Point (functional/security/performance)	New Solution
S2 FG fuselage with carbon fiber stiffeners (Drafts 01-05) — radio transparency not analyzed	Carbon fiber stiffeners provide 30-52 dB RF shielding, creating localized RF shadow zones inside the fuselage; antenna placement is constrained to FG-only zones between stiffeners; for a multi-antenna UAV (C2, video, GPS, telemetry) this creates spatial planning complexity	Two options evaluated: (1) retain hybrid but engineer antenna placement around CF zones, or (2) switch to pure GFRP (Shark M approach) eliminating all RF constraints
S2 FG + CF stiffeners — parachute landing BVID risk not analyzed	Carbon fiber stiffeners fail brittlely under impact (sudden delamination); after repeated parachute landings (190-762 J per landing), CF stiffeners accumulate invisible internal damage (BVID) detectable only by ultrasonic NDT — impractical in field conditions	Pure GFRP approach eliminates BVID risk entirely; all damage is visible and field-inspectable; Shark M validates this approach with 50,000+ operational hours including thousands of parachute landings
S2 FG + CF stiffeners — radar signature not analyzed	CF stiffeners are conductive and reflect radar energy; a regular geometric pattern of CF ribs inside a GFRP skin creates a partial radar reflector, slightly increasing RCS vs pure GFRP	Pure GFRP airframe is radar-transparent; RCS limited to metallic internals (engine, servos, connectors) only; this is exactly how Shark M achieves "low radar visibility" per Ukrspecsystems

Shark M Material Identification

The Shark M's fuselage material is not publicly disclosed by Ukrspecsystems. However, convergent evidence strongly indicates pure GFRP (glass fiber reinforced polymer) — likely E-glass or S-glass fiberglass with epoxy resin:

Evidence	Implication
PD-2 datasheet states "fully composite airframe" + "absence of large metal parts" → "low radar visibility"	Low radar visibility via material transparency = non-conductive composite = GFRP, not CFRP
Shark M achieves 180 km communication range through fuselage (Silvus modem)	Fuselage must be RF-transparent; CF would block signals (30-52 dB shielding)
User confirms from experience: "no issues with radiotransparency, cause it is still alive"	Direct field validation of RF transparency through airframe
UAVs in this class (10-15 kg MTOW) commonly use fiberglass composite	Industry norm for this weight/mission class
Ukrspecsystems claims "low radar visibility" specifically from "fully composite airframe"	Stealth through radar transparency (GFRP property), not radar absorption

Confidence: ⚠️ Medium-High. Not officially confirmed, but all available evidence points to GFRP. No evidence contradicts this conclusion.

Product Solution Description

Material comparison between three airframe construction approaches for a reconnaissance UAV (18 kg MTOW, catapult + parachute recovery):

Approach A — S2 Fiberglass + Carbon Fiber Stiffeners (full hybrid) S2 FG fuselage skins with carbon fiber unidirectional strips as wing spars, fuselage longerons, and key structural stiffeners. Combines FG impact tolerance with CF stiffness-to-weight efficiency. Requires engineered antenna placement to avoid CF-induced RF shadows.

Approach B — Pure GFRP (Shark M style) All-fiberglass construction (E-glass or S2-glass with epoxy). Thicker skins and/or foam-core sandwich panels compensate for lower stiffness. Entire airframe is RF-transparent and radar-transparent. Heavier than hybrid, but eliminates all CF-related complications.

Approach C — S2 GFRP + CF Wing Spar Only (recommended) S2 FG for all skins, fuselage structure, ribs, and secondary stiffeners. Carbon fiber used only for the main wing spar (one per wing half). The CF spar runs spanwise through the wing and connects at the fuselage center section, acting as the structural backbone: it provides wing flutter resistance, resists fuselage torsion and bending at the wing root junction, and stiffens the overall airframe. All antennas are in the fuselage — the wing spar creates no RF shadow in communication paths. BVID risk is limited to two non-impact-zone elements. Recovers ~200-400g of the pure GFRP weight penalty.

Architecture

Component: Airframe Material System

Dimension	S2 FG + CF Stiffeners (A)	Pure GFRP (B, Shark M)	Winner
Radio transparency	Partial — FG zones are RF-transparent; CF stiffeners block 30-52 dB; antenna placement constrained	Full — entire fuselage passes RF; antenna placement unconstrained; validated at 180 km range	B
Radar transparency (stealth)	Partial — CF elements reflect radar; slight RCS increase from conductive stiffener grid	Full — GFRP is radar-transparent; RCS from internals only; validated in combat ("low radar visibility")	B
Single-impact survivability	Good — S2 FG skin absorbs well, but CF stiffeners may crack/delaminate under localized loads	Good — all-FG flexes and absorbs; no brittle failure modes; graceful degradation	B (slight)
Cumulative landing damage	Risk — CF stiffener micro-delamination after repeated landings; BVID invisible without ultrasonic NDT	Safe — all damage visible; simple visual inspection per landing; no hidden degradation	B
Weight efficiency	Better — CF stiffeners save est. 300-800g over equivalent FG stiffening for same structural performance	Heavier — must use thicker skins, foam sandwich, or more ribs; est. 300-800g penalty	A
Structural stiffness	Higher — CF is ~5× stiffer per unit weight; wing flutter resistance superior	Lower — FG is more flexible; adequate for Shark M class (3.4m wingspan) but needs design compensation	A
Material cost	Higher — CF cloth 5-10× more expensive than FG; moderate total increase (CF only in stiffeners, ~$100-300 extra)	Lower — all FG; cheapest composite option	B
Manufacturing simplicity	Moderate — two material systems require different layup procedures; CF needs precise fiber alignment	Simple — single material system; one set of procedures; easier quality control	B
Field repairability	Partial — FG skin: easy field repair; CF stiffeners: needs specialized skills, vacuum bagging, controlled cure	Full — all components repairable with basic epoxy + FG cloth patches; average manual skills sufficient	B
Field inspection	Hard — CF stiffener BVID requires ultrasonic NDT equipment (impractical in field)	Easy — visual inspection + tap test; no specialized equipment	B
Combat-proven track record	None — novel approach, untested in operational service	Extensive — Shark M: 50,000+ operational hours, 1,200h maintenance-free, combat-validated parachute landings	B
Endurance impact	Baseline — lighter airframe → est. 6-24 min additional flight time (~1-5% of 7-8h mission)	Heavier by 300-800g → 6-24 min less flight time; Shark M achieves 7h with pure GFRP at 14.5 kg	A (modest)
Vibration damping	Lower — CF is stiffer but transmits more high-frequency vibration	Better — hybrid composites show higher damping factors; FG naturally dampens vibration	B (slight)

Score: Approach A wins 2.5 dimensions, Approach B wins 10.5 dimensions.

Component: Approach C — S2 GFRP + CF Wing Spar Only (Recommended Compromise)

Approach C takes the best of both worlds. The CF wing spar is the single highest-value use of carbon fiber in the airframe:

Dimension	Approach C vs Pure GFRP (B)	Approach C vs Full Hybrid (A)
Radio transparency	Identical in practice — spar is in the wing, not in fuselage antenna paths	Much better — no CF in fuselage; no antenna placement constraints
Radar transparency	Negligible RCS from two spar elements buried inside wing structure	Better — no CF grid pattern in fuselage
Parachute landing BVID	Negligible — wing spars don't take direct ground impact; shock attenuated through wing root	Much better — no CF in belly/fuselage impact zone
Weight	~200-400g lighter (CF spar vs equivalent FG spar)	~100-400g heavier (no CF fuselage stiffeners)
Structural stiffness	Significantly better — CF spar stiffens the entire airframe: wing bending, fuselage torsion at wing root, overall rigidity	Slightly lower — no fuselage longerons, but spar carry-through compensates at the critical center section
Flutter resistance	Same as full hybrid — CF spar is the primary flutter prevention element	Same
Field repairability	FG fuselage fully field-repairable; CF spar damage is rare (no impact exposure) and would require return to base	Better than full hybrid — only 2 CF elements vs many
Manufacturing	Simpler than full hybrid — CF layup only for two spar elements; everything else is single-material FG	Simpler
Cost	~$50-150 more than pure GFRP (two CF spar elements)	~$50-150 cheaper than full hybrid

Why the CF spar stiffens the whole airframe: The wing spar is not just a wing element — it runs through or connects at the fuselage center section (wing root junction). This junction is the highest-stress point on the airframe. A stiff CF spar at this junction:

• Resists wing bending under gust loads and maneuvers
• Prevents fuselage torsion (twisting) caused by asymmetric wing loading
• Acts as a rigid backbone that the FG fuselage shell wraps around
• Increases the natural frequency of the airframe, pushing flutter speed higher

The result: the airframe behaves nearly as stiff as the full hybrid (Approach A) for the loads that matter most, while the fuselage remains pure FG with all its RF and impact advantages.

Weight budget for Approach C (18 kg MTOW, 3.4m wingspan):

Component	Approach A (full hybrid)	Approach B (pure GFRP)	Approach C (FG + CF spar)
Wing spar (both halves)	CF: 150-250g	S2 FG: 400-600g	CF: 150-250g
Fuselage stiffeners	CF: 200-400g	S2 FG: 400-600g	S2 FG: 400-600g
Skins + ribs	S2 FG: 3.5-4.0 kg	S2 FG: 3.8-4.2 kg	S2 FG: 3.8-4.2 kg
Total airframe	~4.5-5.0 kg	~5.0-5.8 kg	~4.7-5.4 kg
vs full hybrid	Baseline	+500-800g	+200-400g
Endurance impact	Baseline (~7.5-8h)	-15-24 min	-6-12 min

Radio Transparency — Detailed Analysis

Frequency Band	Use	S2 FG + CF Stiffeners	Pure GFRP
900 MHz (Silvus)	C2 datalink	Passes through FG skin; CF stiffeners block directional sectors	Passes through entire fuselage; omnidirectional coverage
1.575 GHz (GPS L1)	Navigation	GPS antenna must be on top, away from CF elements; workable	No constraints; GPS antenna anywhere on upper fuselage
2.4 GHz (backup link)	Telemetry/control	~30 dB blockage through CF; FG zones OK	Full transparency
5.8 GHz (video)	HD video downlink	Higher frequency → more susceptible to CF blockage	Full transparency

Key insight: The hybrid approach works if antennas are carefully placed in FG-only zones. But this constrains the internal layout and means that if a stiffener is later moved (design iteration), antenna placement must be re-validated. Pure GFRP gives antenna engineers complete freedom.

Parachute Landing — Material Behavior Under Repeated Impact

Landing #	S2 FG + CF Stiffeners	Pure GFRP
1-50	Both perform well; no visible damage in calm/light wind	Same
50-100	FG belly panels show wear; CF stiffeners accumulate micro-stress	FG belly panels show same wear; FG stiffeners flex and reset
100-200	CF stiffener BVID possible; invisible without NDT; structural margin unknown	FG damage remains visible; operator can track degradation
200-500	Risk of sudden CF stiffener failure from accumulated BVID → catastrophic structural failure during flight	FG degrades gracefully; worn components replaced based on visual inspection

Key insight: The failure mode difference is critical. CF stiffener failure is sudden and catastrophic (delamination → loss of structural integrity → possible in-flight breakup). FG failure is gradual and visible (cracking → flexibility → obvious degradation → scheduled replacement).

Weight Trade-Off Quantification

For an 18 kg MTOW UAV with 3.4m wingspan:

Stiffening Approach	Estimated Airframe Weight	Weight vs Full Hybrid	Endurance Impact
Approach A: S2 FG skin + CF stiffeners (full hybrid)	~4.5-5.0 kg	Baseline	Baseline (est. 7.5-8h)
Approach C: S2 FG skin + CF wing spar only (recommended)	~4.7-5.4 kg	+200-400g	-6-12 min (~1-3%)
Approach B: S2 FG skin + S2 FG stiffeners (pure S2 FG)	~5.0-5.8 kg	+500-800g	-15-24 min (~3-5%)
E-glass skin + E-glass stiffeners (pure E-glass, likely Shark M)	~5.2-6.0 kg	+700-1000g	-20-30 min (~4-6%)

Note: Shark M achieves 7h at 14.5 kg MTOW with pure GFRP. The user's UAV at 18 kg MTOW has ~3.5 kg more budget. Approach C costs only 200-400g and 6-12 minutes vs the full hybrid — a minor trade for the operational benefits gained.

Recommendation

Scenario	Recommended	Rationale
Military reconnaissance, parachute landing, EW-contested	Approach C: S2 GFRP + CF wing spar only	Near-full radio + radar transparency (CF only in wings, away from antennas); no BVID risk in impact zone; field-repairable fuselage; CF spar stiffens entire airframe including fuselage torsion; only 200-400g heavier than full hybrid; 6-12 min endurance cost is acceptable
Absolute maximum RF transparency required	Approach B: Pure GFRP	Eliminates all CF; 100% RF/radar transparent; validated by Shark M; 500-800g heavier than full hybrid
Maximum endurance priority, VTOL landing (no parachute)	Approach A: S2 FG + CF stiffeners (full hybrid)	Weight savings matter most for hover efficiency; VTOL eliminates repeated landing impact; antenna placement needs engineering but is manageable

For Variant B (catapult + parachute): Approach C (S2 GFRP + CF wing spar only) is recommended. It delivers nearly all the operational advantages of pure GFRP — radio transparency in the fuselage, no BVID in the impact zone, full field repairability of the fuselage — while recovering ~200-400g through CF spars exactly where stiffness matters most. The CF spar also stiffens the overall airframe through the wing root junction, improving flutter resistance and fuselage rigidity with no RF penalty. The endurance cost vs full hybrid is only 6-12 minutes on a 7-8h mission.

For Variant A (VTOL): Retain Approach A (S2 FG + CF stiffeners). VTOL eliminates repeated impact concern, and weight savings directly benefit hover efficiency.

Approach C — Fuselage Stiffness Compensation (no CF in fuselage)

With CF removed from fuselage stiffeners, the fuselage shell needs alternative stiffening. The CF wing spar carry-through handles the critical wing root junction loads, but fuselage panels still need local stiffening. Recommended techniques (can be combined):

Technique	Weight Impact	Benefit
Foam sandwich panels (PVC or PMI foam core, S2 FG skins)	+50-150g vs monolithic	Dramatically increases panel stiffness without CF; widely used in gliders and UAVs
S2 FG hat-section ribs (replacing CF longerons)	+100-200g vs CF equivalent	Heavier but fully RF-transparent and field-repairable; standard FG construction
Geometric stiffening (corrugated skin sections)	+0-50g	Stiffens panels through geometry, not material; minimal weight penalty
Thicker S2 FG skins at critical zones (2.5mm vs 2.0mm)	+50-100g	Targeted reinforcement at high-stress areas (wing root, nose, tail boom junction)

Testing Strategy

Approach C Validation Tests

• Wing spar flutter test: ground vibration test at max speed (130 km/h equivalent) to confirm CF spar provides adequate flutter margin
• Fuselage torsion test: apply asymmetric wing loading at wing root junction, measure fuselage twist; compare CF spar carry-through vs FG-only baseline
• RF transmission verification: measure signal attenuation at 900 MHz, 2.4 GHz, 5.8 GHz through fuselage panels in all directions; confirm no RF shadow from wing spars at typical antenna-to-GCS angles
• Belly impact test: drop test at 762 J (8 m/s wind equivalent) on fuselage belly panel (FG only); confirm no damage propagation to CF wing spar
• Repeated landing test: 100× drop tests at 190 J (calm landing) on fuselage belly; verify CF spar shows zero damage (spar is not in impact path)
• Foam sandwich qualification (if used for fuselage panels): flatwise tension, edgewise compression, and impact per ASTM standards
• Field repair validation: induce belly skin damage, repair with field kit (epoxy + S2 FG cloth), test repaired panel to 80% original strength
• Endurance verification: compare actual flight time vs full hybrid prototype (if available); confirm 6-12 min difference estimate

References

1-94: See Drafts 01-05 references (all still applicable)

Additional sources: 95. Ukrspecsystems SHARK-M UAS: https://ukrspecsystems.com/drones/shark-m-uas 96. Ukrspecsystems PD-2 Datasheet: https://www.unmannedsystemstechnology.com/wp-content/uploads/2016/06/PD_2.pdf 97. KSZYTec UAV Antenna Design Survival Guide (CF RF shielding 30-50 dB): https://kszytec.com/uav-aerospace-antenna-design-survival-guide/ 98. Radio-Transparent Properties of S-Glass, Aramid, Quartz Radome Composites at 900 MHz: https://link.springer.com/article/10.1007/s40033-023-00602-7 99. GFRP radar transparency for aerospace/defense: https://www.tencom.com/blog/fiberglass-pultrusion-for-aerospace-defense-lightweight-structural-components 100. EM Shielding of Twill CFRP in UHF/L/S-band (IEEE): https://ieeexplore.ieee.org/document/10329805/ 101. EM Shielding of Continuous CF Composites — 52 dB: https://www.mdpi.com/2073-4360/15/24/4649 102. E-Glass vs CF Impact Resistance for UAV Wings: https://www.preprints.org/manuscript/202601.1067 103. S2/FM94 Glass Fiber Impact Damage Resistance: https://mdpi-res.com/d_attachment/polymers/polymers-14-00095/article_deploy/polymers-14-00095-v2.pdf 104. Field Repair of FG/Epoxy Fuselage: https://www.matec-conferences.org/articles/matecconf/pdf/2019/53/matecconf_easn2019_01002.pdf 105. ACASIAS Antenna Integration in CF Fuselage Panel: https://www.nlr.org/newsroom/video/acasias-antenna-integration/ 106. Fiberglass Radome Dielectric Properties: https://www.oreilly.com/library/view/radome-electromagnetic-theory/9781119410799/b02.xhtml 107. E-Glass vs S-Glass Comparison: https://www.smicomposites.com/comparing-e-glass-vs-s-glass-key-differences-and-benefits/ 108. CF vs FG UAV Drone Material Comparison: https://www.ganglongfiberglass.com/fiberglass-drone-vs-carbon-fiber/ 109. CF RF Blocking — StackExchange: https://drones.stackexchange.com/questions/283/how-much-does-mounting-an-antenna-near-a-carbon-fiber-frame-degrade-signal-recep 110. Belly-Landing Mini UAV Strength Study: https://www.scientific.net/AMM.842.178 111. Hybrid Composite Wing Spar Analysis: https://yanthrika.com/eja/index.php/ijvss/article/view/1476 112. UAV Airframe Structural Optimization: https://www.frontiersin.org/articles/10.3389/fmech.2025.1708043

Related Artifacts

• Previous drafts: solution_draft01.md through solution_draft05.md
• Research artifacts: _standalone/UAV_frame_material/00_research/UAV_frame_material/