DESIGN NOTES

1. Purpose

This document defines a normative airframe geometry and a set of mechanical interfaces for 10 inch class quadcopter platforms intended for attritable use.

The objective is interchangeability. An attritable platform is procured in volume, from more than one supplier, on short lead time, and is expected to be consumed. Under those conditions the airframe's most valuable property is not peak performance. It is that any two units, from any two sources, accept the same components and behave the same way in flight.

Existing commercial frames in this class do not support that. Product lines are revised without notice, dimensions vary between listings of nominally the same part, and no supplier in the consumer supply chain publishes a layup schedule or holds batch traceability. A standard cannot be anchored to a product. It has to be anchored to a specification, with a controlled model as the reference.

2. Reference model

The normative geometry is defined by the master CAD model, not by this document. Where this document and the model disagree, the model governs. Dimensions below are extracted from the model and are stated so that conformance can be checked without CAD access.

The MK4 V2 10 inch family, a clone lineage derived from the GEPRC MK4 10 inch, approximately conforms and may be used as an informative example. It is not the normative reference. No manufacturer of record exists for that family and its published dimensions vary by up to 16mm of wheelbase between sources.

3. Why 10 inch

Larger platforms carry more. 13 and 15 inch airframes lift heavier payloads with better endurance, and where the mission permits the logistics, they are the better aircraft. That is not in dispute.

The 10 inch class earns its place on a different axis. It is the smallest airframe that clears the capability floor for a useful mission: enough payload margin for a real sensor package, enough endurance to reach an area and work it, enough disc area to hold position in wind. Below 10 inch those margins collapse.

At the same time it sits inside the commercial supply chain. Motors, ESCs, propellers, and frames in this class are produced in volume for the consumer and light commercial market. When the airframe is consumable, that matters more than any single performance figure. A platform available in quantity, at cost, from multiple suppliers, on short lead time is worth more in an attritable role than a superior platform available fifty at a time.

10 inch is not the optimum. It is the floor, and it is where capability and availability intersect most favorably.

4. Normative geometry

4.1 Layout rationale

The geometry is a moderate wide-X: lateral motor spacing exceeds fore-aft by a factor of 1.2518, with arms at 51.38° from the fore-aft centerline.

Widening laterally opens the side-to-side gap between adjacent propeller discs, which is where tip vortex interference costs the most hover efficiency. On a platform whose value is endurance, efficiency converts directly to time on station.

Two second-order effects follow. Compressing the fore-aft dimension lowers pitch inertia, which quickens the axis that is normally sluggish on a heavy quadcopter, since the battery lies along the centerline and contributes nearly all its mass to pitch. Conversely roll inertia scales with the square of the lateral arm while roll torque scales linearly, so roll becomes slower and more damped. For a loaded platform that damping is desirable behavior, not a defect.

Propellers sitting further outboard also permit lower camera tilt without propeller tips entering frame, which matters at the 15 to 20 degree cruise attitudes this class flies.

4.2 Propeller clearance

Disc edge-to-edge clearance against a 254mm propeller:

No overlap on any axis. Fore-aft is the binding constraint at 9.06% of propeller diameter.

Consequence to be documented, not corrected. At that clearance the rear discs operate partially within front-motor downwash in hover and low-speed forward flight. Expect rear motor temperatures above front, and a pitch-axis gyro trace noisier than roll. This is inherent to the geometry. Integrators shall not attempt to tune it out as a control problem.

Propeller limit. Maximum non-overlapping propeller is 277mm (10.91 in), fore-aft limited. Left-right would permit 346mm (13.65 in). No propeller larger than 254mm shall be fitted without a documented overlap analysis.

5. Structure

5.1 Arm plate

Why woven and not UD. Unidirectional laminate is stiffer along the fiber axis and splits on side impact. Woven laminate flexes slightly more and survives side impact. For an airframe expected to take propeller strikes and hard landings, side-impact survival outranks peak stiffness.

Why standard or intermediate modulus. High modulus fiber is stiffer and more brittle. Stiffness is not the governing property here; impact tolerance is. Do not over-specify upward.

Why symmetric and balanced. Symmetric means the ply stack mirrors about the midplane, which prevents warp on cure and under thermal load. Balanced means every +45° ply is matched by a -45°, which prevents bend-twist coupling. An arm that twists as it bends feeds that motion straight into the gyro.

Why this is written as a performance requirement. No supplier in this supply chain publishes a layup schedule or holds batch traceability. A dictated ply sequence would be unverifiable and therefore unenforceable. Conformance is established by test on receipt, not by supplier declaration.

5.2 Arm bracing

Arms shall be braced at an intermediate station along their length. Bracing at the motor tip is non-conforming.

Tip bracing ties the four arms into a closed ring, maximizing torsional stiffness. Intermediate bracing makes each arm a cantilever from the brace station to the motor. That trade is made deliberately for three reasons:

• Higher first bending mode. The free length governing arm bending under thrust is shortened, raising that mode's frequency. A 10 inch propeller at this class's operating RPM produces a passing frequency low enough to excite big-frame arm modes. Moving the structural mode upward reduces gyro noise, which reduces required software filtering, which reduces phase delay in the control loop.
• Better crash behavior. A tip-braced arm loads its brace in shear during a propeller strike and cracks near the motor mount, which is the costly region. An unconstrained tip deflects instead of transferring load into a joint.
• Field repairability. Arms are replaceable without disassembling the brace structure.

The accepted cost is higher tip deflection under load, since motor and propeller mass sit on an unconstrained cantilever. This is the reason for the load margin requirement in 5.3.

5.3 Structural qualification

Conformance of the arm laminate shall be established by three-point bend test per ASTM D790 on coupons cut from each production lot.

Thresholds shall be established by testing the reference airframe, not taken from published fiber data. Published fiber properties describe the fiber. Delivered parts are governed by fiber volume fraction, void content, and cure quality, none of which the fiber datasheet constrains.

6. Mechanical interfaces

The 35mm stack height is the load-bearing number in this table. It admits a triple stack, or a stack plus a full-size digital video module with airflow around it. On a platform carrying navigation, video, and a separate receiver, vertical volume is what prevents the build becoming a wiring compromise.

Dual motor and FC patterns widen the supplier envelope without cost. Required patterns are mandatory; secondary patterns are permitted and encouraged.

Drone Components

1. Carbon frame for 10-inch propellers
2. Brushless motors 3115 900KV
3. Electronic Speed Controllers (ESC): 55A
4. FPV camera — depending on configuration (analog daytime, analog thermal Caddx IRC-640CA or Caddx IRC-384CA)
5. Video transmitter (VTX): 2.5 W
6. Propellers: 1050
   
   

Part 2: Geometry and Design Improvements

Revision: 0.1 draft Status: Proposed changes to the Part 1 baseline. Nothing here is normative until adopted. Scope: Airframe geometry, structure, and mechanical interfaces. Propulsion, avionics, and payload integration remain out of scope.

10. Fore-aft disc clearance

Grade B.

Problem. Fore-aft motor spacing is 277.011mm against a 254mm propeller. Clearance is 23.011mm, or 9.06% of disc diameter. That is the binding constraint on the entire airframe and it was inherited, not chosen.

At that clearance rear discs operate inside front-motor downwash in hover and low-speed forward flight. The consequences are asymmetric rear motor loading, elevated rear motor temperature, and a pitch-axis gyro trace consistently noisier than roll. Part 1 documents this as a characteristic to be accepted. It does not have to be.

Options.

Recommendation. Target 15% fore-aft clearance at 292.1mm, holding lateral spacing at 346.763mm. That is +15.1mm fore-aft for a 9.6mm growth in diagonal and it moves the platform from marginal to comfortable on the axis that governs.

The 20% and 25% options push the geometry toward true-X, which surrenders the pitch inertia advantage the wide-X layout was providing. 15% is where the curve turns.

What it breaks. Frame plates, arms, and any 3D printed part referencing arm positions. Nothing in the electronics or propulsion chain. Motors, ESCs, propellers, stacks, and cameras are unaffected.

What it does not break. Commonality with the clone-frame supply base is already nominal, since those frames vary by up to 16mm of wheelbase between suppliers. Moving 15mm off baseline forfeits less than it appears to.

12. Brace station placement

Grade A.

Problem. Part 1 requires intermediate bracing and explains why. It does not specify where. Brace position is currently inherited.

Brace station is the single most influential geometric variable on arm dynamics. It sets the cantilever free length, which sets the first bending mode, which is the entire reason for choosing intermediate bracing over tip bracing.

Proposed change. Specify brace station as a fraction of arm length from the root, with tolerance. Determine the value by resonance survey, not by inheritance.

The target. Blade passing frequency for a 3-blade 10 inch propeller:

The operating band spans roughly 150 to 350 Hz. The first arm bending mode must sit clear of it, and clear means above 350 Hz with margin, since below 150 Hz is not achievable on a structure this stiff and inside the band is disqualifying.

Requirement to add. First arm bending mode ≥ 450 Hz [ASSUMED margin — set from resonance survey], measured with motor and propeller mass fitted.

What it breaks. Frame plates if the station moves. Nothing else.

13. Motor mount reinforcement

Grade A.

Problem. The motor mount is four M3 holes through 7mm plate at the end of a cantilever. It is the highest-stress region on the airframe and the documented failure location for frames in this class, where arms crack near the motor mount under repeated impact.

Through-holes in laminate concentrate stress at the hole edge and expose cut fiber ends to bearing load. Bolt preload crushes the laminate through-thickness. Repeated prop strikes work the hole oversize, and once the hole is oval the joint loses preload and the arm begins to fret.

Proposed change. Three options, in increasing cost:

1. Chamfered holes and increased edge distance. Minimum edge distance 2× hole diameter. Chamfer both faces to remove the stress riser at the cut edge and prevent motor wire chafe. Effectively free.
2. Load-spreading washers. Large-OD washers under bolt heads to distribute preload over more laminate area. Adds two grams and a line item.
3. Bonded metallic inserts. Aluminum or steel bushings bonded into oversize holes, carrying bearing load in metal rather than laminate. Best solution, highest cost, requires bonding process control the supply chain does not have.

Recommendation. Adopt 1 and 2 as normative. Note 3 as an available upgrade path.

What it breaks. Nothing for options 1 and 2.

14. Damage tolerance and arm replaceability

Grade A.

Problem. Part 1 cites field repairability as a justification for intermediate bracing but does not specify it as a requirement. If arm replacement is a design goal it needs to be stated and verified, not assumed to fall out of the bracing choice.

Proposed requirements.

• Any single arm shall be replaceable without removing the flight controller stack, video system, or battery mounting hardware.
• Arm replacement shall require no more than one tool type.
• Replacement time by a trained operator shall not exceed 10 minutes [ASSUMED].
• Arm bolt torque shall be specified. Overtorque crushes laminate; undertorque permits fretting. Neither is left to judgment.

Proposed inspection criteria. Add to Part 1 section 7 a post-flight arm inspection standard defining what damage is acceptable, what is repairable, and what condemns the part. Delamination visible at any free edge, any crack propagating from a fastener hole, and any permanent set in the arm should all condemn.

What it breaks. Nothing. This is documentation of intent that already exists.

Plate-and-standoff versus printed monocoque

Grade C. Architectural.

Problem. Part 1 assumes plate-and-standoff carbon construction because that is what the inherited frame is. That assumption was never tested against the alternative.

The counterexample is established at scale: an interceptor built on a 3D printed monocoque airframe, produced at <cite index="46-1">10,000 units per month at $2,100 unit cost, reaching 343 km/h and 3,000m cruise altitude.</cite> That is not a prototype architecture.

What printing resolves. Every problem Part 1 section 5 exists to solve disappears:

• No layup schedule to specify, because there is no laminate.
• No lot-to-lot carbon variance, because material comes from a controlled powder or filament lot.
• No coupon testing regime, because process control lives on a machine under your control rather than in a supplier's declaration.
• No dimensional variance between suppliers, because geometry comes from the master model directly.
• No tolerance stack across plates, standoffs, and fasteners, because the part is one piece.

It also collapses the receiving inspection burden that Part 1 section 7 imposes, which is the most operationally expensive requirement in the document.

What printing costs.

• Lower specific stiffness than carbon laminate. A printed monocoque must be thicker and shaped to compensate, which is achievable but constrains internal volume.
• Anisotropy in the build direction. Layer adhesion is the weak axis and part orientation becomes a structural decision.
• Loss of arm replaceability. A monocoque damaged at one arm is a condemned airframe, not a repair. This is only acceptable if the platform is genuinely attritable rather than merely cheap.
• Production capacity becomes a capital problem rather than a purchasing problem.

The decision this forces. Plate-and-standoff is correct for a repairable, reusable platform where arms are consumables and airframes are not. Printed monocoque is correct where the airframe itself is the consumable.

Part 1 is currently written for the former while the document title claims the latter. That inconsistency should be resolved explicitly rather than left implicit.

Recommendation. Resolve the intended attrition rate first. If mean airframe life is measured in tens of sorties, keep plate construction and treat this section as future work. If mean life is a single sortie, plate construction is the wrong architecture and this becomes the priority item in the program.

16. Environmental and finish requirements

Grade A.

Absent from Part 1 entirely.

• Edge sealing. All cut carbon edges shall be sealed. Unsealed cut edges wick moisture into the laminate and initiate delamination. Cheap to specify, expensive to omit.
• Chamfer. All edges contacting wiring shall be chamfered. Cut carbon severs motor wire insulation.
• Fastener corrosion. Specify plating or stainless. Carbon in contact with aluminum or plain steel forms a galvanic couple, and carbon is strongly cathodic. This is a known and frequently ignored failure mode.
• Isolation. Where aluminum standoffs contact carbon plate, specify an isolating washer or coating.

17. Summary of proposed changes

Grade A items are adoptable immediately and cost nothing but drawing revisions. Section 10 is the highest-value change and should be evaluated against supply chain reality. Section 15 is a program decision, not a revision, and should not be made without first fixing the intended attrition rate.

18. Open items carried forward

• Fore-aft clearance target requires a decision between 10%, 15%, and 20%.
• Brace station fraction and first-mode floor require resonance survey.
• Arm bolt torque undetermined.
• Arm replacement time target assumed.
• Intended mean airframe life undefined, which blocks section 15.
• Flexural thresholds from Part 1 section 5.3 still outstanding.

Target Specification for CAD

1. Master geometry

Origin at frame center. +X right, +Y forward, +Z up. Model is the authority; this sheet exists so the model can be checked without CAD.

1.1 Motor centers

1.2 Propeller clearance, 254mm disc

Fore-aft remains the binding axis. Maximum non-overlapping propeller is 292mm (11.50 in). No propeller above 254mm without documented overlap analysis.

1.3 Geometry tolerances

2. Arm

Thickness is constant because plan-form taper is achievable on the same flat plate in the same CNC operation. Thickness taper is not, and buys nothing that plan-form taper does not.

2.1 Brace station

Bracing is at an intermediate station. Tip bracing is non-conforming.

Target: 0.55 [ASSUMED — confirm by survey]. Shorter free cantilever raises the first bending mode, which is the objective. Pushing beyond 0.60 begins to approach tip-brace behavior and forfeits the crash and repairability advantages the intermediate brace exists to provide.

Model the brace station as a driven parameter so it can be swept after the resonance survey without rebuilding the arm.

2.2 Motor mount

Edge distance drives motor pad width. Size the pad from the bolt pattern plus 2D edge distance, not the other way around.

2.3 Structural target

Blade passing frequency spans roughly 150–350 Hz across the operating band for a 3-blade 254mm propeller. The first arm mode must clear 350 Hz with margin. 450 Hz is the working target until survey data replaces it.

3. Mechanical interfaces

35mm stack height is a hard floor, not a target. It admits a triple stack, or a stack plus a full-size digital video module with airflow. Do not let plate spacing erode during detail design.

4. Finish and environment

5. Maintainability

Condemning damage: delamination visible at any free edge, any crack propagating from a fastener hole, any permanent set in the arm.

6. Modeling notes

Build the model so the following are driven parameters, not baked geometry:

1. Fore-aft spacing. It moved once and may move again. Everything downstream should rebuild from it.
2. Brace station fraction. Will be swept against survey data.
3. Arm thickness. Locked at 7.0mm now, but the taper and pad geometry should not assume it.
4. Stack height. Driven by plate spacing; keep the 35mm floor as a constraint the model enforces.

Derive the diagonal, arm angle, and motor coordinates from fore-aft and left-right spacing rather than entering them. They are outputs, not inputs, and hand-entered values will drift out of sync the first time spacing changes.

7. Carried-forward open items

• Fore-aft target of 292.1mm assumes the 15% clearance decision from Part 2 §10. Confirm.
• Brace station 0.55 is judgment. Resonance survey governs.
• First-mode floor of 450 Hz is a working assumption.
• Flexural thresholds outstanding pending reference coupon test.
• Arm bolt torque undetermined.
• All geometry tolerances assumed; set from production capability data.
• Architecture question from Part 2 §15 remains open. This spec assumes plate-and-standoff construction. If intended mean airframe life is a single sortie, this spec is the wrong answer and printed monocoque should be evaluated before CAD work begins.