# 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](https://ukrspecsystems.com/drones/shark-m-uas)
96. Ukrspecsystems PD-2 Datasheet: [https://www.unmannedsystemstechnology.com/wp-content/uploads/2016/06/PD_2.pdf](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/](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](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](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/](https://ieeexplore.ieee.org/document/10329805/)
101. EM Shielding of Continuous CF Composites — 52 dB: [https://www.mdpi.com/2073-4360/15/24/4649](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](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](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](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/](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](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/](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/](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](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](https://www.scientific.net/AMM.842.178)
111. Hybrid Composite Wing Spar Analysis: [https://yanthrika.com/eja/index.php/ijvss/article/view/1476](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](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/`

