Radome Cooling System Installation - Structural Substantiation

INTRODUCTION

AIRCRAFTDHC-8-100Exterior radome modification

BOUNDING SPEED242 ktSea-level V_MO,max = 408.45 ft/s

PRIMARY MATERIALSGlass/Epoxy + PTFEFiberglass Style 1581 / EPOCAST 50-1A; PTFE drain

SUBSTANTIATION ROUTEAero → Joints → LaminateRigid-body loads, CLPT, plate/shear and cut-out checks

Structural substantiation of the DHC-8-100 radome cooling-system installation. The assessment converts external aerodynamic demand into component attachment loads, develops conservative composite-joint allowables, and verifies the exhaust laminate, PTFE drain and radome cut-outs using 3D rigid-body equilibrium, CLPT, classical plate/shear methods and stress-concentration factors.

Technical figure from the Radome Cooling System structural substantiation. — Radome Cooling System installation on the DHC-8-100.

DESIGN ASSESSMENT

System Architecture & Load Path

FWD EXHAUSTSLH + RH external outlets

Each exhaust is treated as an external aerodynamic body with lift and drag resolved at its CoG.

AFT INTAKEShield-angle drag body

The exposed shield angle is assessed at the orientation that maximizes drag.

DRAINPTFE local component

Aerodynamic load, fastener demand, annular-plate bending and outlet shear are checked separately.

RADOME INTERFACEFour local cut-outs

Cut-out stress concentrations are assessed against the published radome laminate strengths.

Load-Path Rationale

I treated each cooling-system component as its own aerodynamic body because the exposed geometry and governing coefficient are different for the exhausts, aft-intake shield, and drain. Ultimate lift/drag is applied at each component CoG, the CoG offset is converted into attachment-plane moments, and the combined force/moment set is distributed to the fasteners with 3D rigid-body analysis. The joint check is then separated from the component-material check: the exhaust laminate is substantiated with CLPT, the PTFE drain with annular-plate bending and direct shear, and the radome interface with local cut-out stress-concentration factors. This preserves a traceable load path without forcing unlike failure mechanisms into one model.

Structural components, material systems, thicknesses and laminate orientations.

Component Name	Thickness [in]	Material	No of Plies	Orientation
Meshes	0.08	Chemical-Resistant Polypropylene	-	-
Drain	0.63	PTFE	-	-
Flange Holders	0.06	Fiberglass Cloth - Style 1581 (MIL-C-9084) EPOCAST 50-1A Resin System and Hardener 9816 The Mixing Ratios, Application, and Curing Schedule are per the Manufacturer's Instructions.	6	0°/45°/90°/0°/-45°/-90°
Flange Mesh Restrains	0.125		6	0°/45°/90°/0°/-45°/-90°
Intake Flange	0.125		12	0°/45°/90°/0°/-45°/-90°/0°/45°/ 90°/0°/45°/90°
LH & RH FWD Exhausts	0.125
Radome-Intake Flange	0.125
Shield Angle	0.188		18	0°/45°/90°/0°/-45°/-90°/0°/45°/ 90°/ 0°/45°/90°/0°/ 45°/90°/0°/-45°/-90°

Materials & Joint Allowable Basis

The installation combines a fiberglass/EPOCAST laminate with a PTFE drain and PTFE #10-32 attachment screws. Composite properties are treated as conservative hot-wet lower-bound values and reduced by an additional 25% for fabrication uncertainty before the laminate and joint checks.

LAMINATE BUILD

Symmetric + balanced

Fiberglass Cloth Style 1581 is saturated with EPOCAST 50-1A / Hardener 9816. Wet layup is vacuum-assisted at approximately 20 inHg, cured overnight at room temperature and post-cured for 24 hours.

ALLOWABLE BASIS

Hot-wet lower-bound philosophy

The source does not claim test-derived allowables for the as-built laminate. Instead, conservative hot-wet properties are selected and then reduced by an additional 25% to account for fabrication uncertainty.

LONGITUDINAL TENSION

F_tu1 = 32.11 ksi

Knocked-down laminate tensile strength in the material 1-direction.

TRANSVERSE TENSION

F_tu2 = 25.75 ksi

Knocked-down tensile strength in the material 2-direction.

COMPRESSION

F_cu1 = F_cu2 = 36.14 ksi

Conservative compression strengths retained for the CLPT checks.

IN-PLANE SHEAR

F_su12 = 8.44 ksi

Knocked-down laminate shear strength used by the ply-level failure assessment.

Interlaminar shear allowable: the cited fiberglass data span approximately 5.5–10.4 ksi. The analysis starts from the 6.5 ksi typical ILSS noted in the composite reference and applies the same 25% reduction, giving 4.875 ksi for the pull-through assessment.

Joint allowable philosophy

Evaluate the fastener and laminate/sheet as one load-transfer system. The usable joint allowable is the weakest physically applicable failure path.

P_{in-plane} = \min (P_{shear}, P_{bearing})

P_{out-of-plane} = \min (P_{tension}, P_{pull-through})

Fastener shear alone does not define the joint.Composite bearing is corrected for geometry outside the test envelope.Pull-through is checked separately for out-of-plane demand.

Composite bearing treatment. A 40 ksi glass-laminate ultimate bearing basis is reduced when the installed joint falls outside the cited test envelope. The source applies edge-distance and small-diameter knockdowns, while no strength increase is credited for laminates thicker than the 0.118 in test baseline.

S17114 / 94701A833 · #10-32 PTFE countersunk screws

ThreadUNF-2A · ASME B18.6.3 Nominal shank0.19 in Lengths1.0 in / 1.5 in MaterialPTFE plastic Conservative tensile basisF_tu = F_ty = 3.00 ksi Shear strengthF_su = 0.725 ksi

For the installed PTFE screws, the shank/threads govern well before the surrounding composite bearing or pull-through limits. The source obtains the following isolated fastener capacities:

f_{su} = F_{su} \times A_{s} = 0.725\pi\ \ \frac{{0.192}^{2}}{4} \times 1000

= 20.56\ {lb}_{f}

f_{tu} = F_{tu} \times A_{m} = 3\pi\ \ \frac{{0.14771763}^{2}}{4} \times 1000

= 51.41\ {lb}_{f}\ \

The composite pull-through check uses the Abbott semi-empirical relation with α = 0.75 for glass laminate and the knocked-down interlaminar shear value τ_max = 4.875 ksi.

$F_{pull} = \frac{2\alpha t\pi D_{f}\tau_{\max}}{2.9 - 0.018\frac{t}{D_{s}} - 0.51\left( \frac{t}{D_{s}} \right)^{2}}$

Composite-joint geometry used to determine bearing knockdown factors.

Definition	Symbol	FWD/LH&RH Exhaust Assemblies	In the AFT Intake Assembly – Joint 1	In the AFT Intake Assembly – Joint 2
Accumulated thickness of all components	t_joint [in]	0.06+0.125+0.125=0.31	0.125+0.125=0.25	0.125+0.188=0.313
Shank diameter	D_s [in]	0.19	0.19	0.19
Thickness to diameter ratio	t_joint/D_s	1.63	1.32	1.65
Edge distance	e [in]	0.5	0.5	0.44
Edge distance to diameter ratio	e/D_s	2.63	2.63	2.315

FWD EXHAUST1,638 lb_f

Knocked-down composite bearing capacity; pull-through = 2,400.56 lb_f.

AFT INTAKE J11,321 lb_f

Knocked-down bearing capacity; pull-through = 1,469.41 lb_f.

AFT INTAKE J21,486 lb_f

Knocked-down bearing capacity; pull-through = 2,467.3 lb_f.

DRAIN BEARING221.45 lb_f

PTFE bearing capacity; still above the screw shear limit.

Ultimate in-plane and out-of-plane joint allowables used in the component checks.

Assembly	In Plane Strength [lb_f]	Out of Plane Strength [lb_f]
FWD/LH&RH Exhaust	20.56	51.41
AFT Intake – Joint 1	20.56	51.41
AFT Intake – Joint 2	20.56	51.41
Drain Component	20.56	51.41

Source note: the original summary repeats “AFT Intake – Joint 1” for the third row; it is labeled “Joint 2” here to match the preceding joint definitions and calculations.

LOAD CASES FORMULATION

REGULATORY SOURCE BASISFAR Part 25 criteria used to formulate the structural load envelope

Flight-load framework

FAR 25.321 and 25.331–25.351

General flight loads and symmetric maneuver response
Flight maneuver envelope and design airspeeds
Limit maneuver factors, gust and turbulence loads
Fuel/oil, high-lift, rolling and yaw conditions

Flight loads are treated as limit loads.

Emergency landing

FAR 25.561

Forward: 9 g
Downward: 6 g
Upward: 3 g
Sideward: 3 g airframe / 4 g seats & attachments
Rearward: 1.5 g

Emergency-landing values are ultimate loads.

Load-Case Selection Rationale

This is an exterior modification, so the governing structural demand is established from flight aerodynamic loading rather than cabin emergency-landing inertia. I bounded the external shapes at sea-level maximum operating speed: the exhausts are treated as maximum-lift 2D external surfaces while retaining maximum drag, the aft-intake shield is aligned with the freestream to maximize drag, and the drain follows the same conservative lift/drag approach. The resulting aerodynamic forces are then multiplied by the source 1.5 ultimate factor before the attachment and material checks.

SEA-LEVEL DENSITY

ρ = 0.00238 slug/ft³

Used to maximize dynamic pressure for the stated operating-speed assessment.

MAX OPERATING SPEED

V_MO,max = 242 kt

Equivalent to 408.45 ft/s. Each external shape is evaluated separately using the aerodynamic coefficient that best represents its geometry.

Bounding Aerodynamic Loads

Each exposed component is idealized separately at the source sea-level maximum operating condition. Hoerner lift/drag data are used to define the governing coefficient for each geometry, and the resulting force is multiplied by the source 1.5 ultimate factor before structural assessment.

Bounding external aerodynamic loads retained for structural substantiation.

Component	Idealization / coefficient	Ultimate lift [lb_f]	Ultimate drag [lb_f]
LH/RH FWD Exhaust	2D lifting surface; C_L=1.5, C_D=0.1	26.96	1.83
AFT Intake Shield	Rear-open semicircular body; C_D=1.20	0	5.13
Drain	Small lifting/drag body; AR=0.556, C_L,max=1.078, C_D=0.1	1.003	0.083

D_{u} = 1.5 \times 1.22\ = \boxed{1.83}\ {lb}_{f}

L_{u} = 1.5 \times 17.97\ = \boxed{26.96}\ \ {lb}_{f}

CLASSICAL ANALYSIS

Ultimate aerodynamic loads are applied at each component CoG and transferred to the local fastener pattern. The offset between the force line of action and attachment plane generates the moments used in the component and laminate checks below.

1Aerodynamic demand

Resolve lift/drag at the component CoG.

2Moment build-up

Apply CoG offsets to obtain M_x, M_y and M_z.

3Fastener reactions

Distribute force and moment with 3D rigid-body analysis.

4Strength checks

Compare joints, laminate/plate stresses and radome cut-out demand to allowables.

Load Transfer & Fastener Reactions

Ultimate forces are applied at each component CoG. The CoG offset generates attachment-plane moments, and the complete force/moment set is distributed to the local fastener group using 3D rigid-body equilibrium. The exhaust provides the largest attachment demand.

REPRESENTATIVE EXHAUST LOAD TRANSFERCoG (0.086, 0.001, 0.981) inL = 26.96 lb_f · D = 1.83 lb_f · H_max = 2.965 in

M_{x,Base}=26.96\times0.981=\pm26.45\ {lb}_{f}\cdot in

M_{x,Top}=26.96(2.965-0.981)=\pm53.49\ {lb}_{f}\cdot in

M_{y,Base}=1.83\times0.981=1.80\ {lb}_{f}\cdot in

M_{y,Top}=1.83(2.965-0.981)=-3.63\ {lb}_{f}\cdot in

M_z=\pm26.96(0.086)-1.83(0.001)=\pm2.32\ {lb}_{f}\cdot in

Fastener Reaction Forces

Maximum 3D rigid-body fastener reactions from the source solutions.

Assembly	Max shear [lb_f]	Max tension [lb_f]	Max compression [lb_f]
Exhaust	3.55	2.09	1.96
AFT Intake	0.73	0.10	0.11
Drain	0.25	0.01	0.01

GOVERNING FASTENER DEMANDExhaust attachment

Tension 2.09 lb_f

Compression 1.96 lb_f

Shear 3.55 lb_f

PASS

Minimum joint capacities are 20.56 lb_f in-plane and 51.41 lb_f out-of-plane. Compressive reaction is distributed through the attachment surface.

Component Strength Assessment

METHOD

Classical Laminate Plate Theory

The laminate is characterized by its ABD stiffness matrix, then ply direct, bending and total stresses are recovered through the thickness.

FAILURE CRITERIA

Tsai-Wu · Hill · Hoffman

The source spreadsheet evaluates ply-level margins using multiple composite failure criteria rather than relying on a single scalar stress allowable.

Exhaust Component

LAMINATE0.125 in · 12 plies

STACK0/45/90/0/−45/−90/0/45/90/0/45/90°

LOAD INPUTAerodynamic forces + CoG moments

EFFECTIVE AREA

Non-flanged plate only

Only the 4.01 × 3.69 in non-flanged region is credited with carrying the aerodynamic load, increasing the running loads relative to using the full component area.

AERODYNAMIC AMPLIFICATION

10× ultimate load/moment

The already-ultimate aerodynamic force and derived moments are increased tenfold to cover interference, rough-surface effects and additional drag forms.

M_z LOAD PATH

Reacted at the exhaust base

The source excludes M_z from the plate CLPT input and assumes it is carried by the exhaust base interface.

ALLOWABLE CONSERVATISM

25% laminate knockdown retained

Ply allowables already include the separate material knockdown established above.

CLPT GOVERNING RESULTHill criterion

Minimum margin 69%

Laminate knockdown 25%

Applied load amplification 10×

PASS

The exhaust remains positive-margin after both the material-property knockdown and the separate tenfold amplification of ultimate aerodynamic forces and moments.

Drain Component

The PTFE drain is split into two mechanics problems: annular-plate bending at the base and direct shear through the outlet. This keeps the hand calculation aligned with the actual load path instead of forcing the full geometry into one idealization.

BASE IDEALIZATION

Annular plate · outer edge simply supported · inner edge free

A uniform line moment is applied at the inner radius. The Roark annular-plate coefficient is used at the inner edge where the tangential unit moment governs [Roark’s Formulas for Stress and Strain, 9th Ed., annular-plate case].

The source uses the more conservative coefficient for b/a = 0.3 and ν = 0.3 even though the PTFE Poisson ratio is 0.46.

M_{t, max} = K_{M, t b} M_{o}

$F_{bt} = \frac{6M_{t}}{t^{2}} = \frac{6 \times - 0.269}{{0.125}^{2}} = 0.1033\ ksi$

Drain base - PASSCalculated bending stress is 0.1033 ksi versus PTFE F_cy = 1.45 ksi; the source therefore reports a high positive margin.

The outlet portion of the drain component will be checked against shear loads. The load along the x- and y-axes equal 0.1003 lb_f and 0.083 lb_f, respectively. Hence, the shear stress at the XY surface is:

$F_{s - xy} = \frac{\sqrt{{1.003}^{2} + {0.083}^{2}}}{0.554} = 1.82\ psi$

The ultimate shear strength for the PTFE Plastic is 725 psi. Therefore, it passes by observation.

Radome’s Cut-Outs

Four local openings are assessed as geometric stress raisers in the 0.1875 in radome laminate. Gross- or net-section stress-concentration factors are selected to match the cut-out geometry, then applied to the source far-field stresses.

GROSS-AREA FACTOR

K_tg

Peak edge stress referenced to gross far-field stress.

NET-AREA FACTOR

K_tn

Peak edge stress referenced to net-section far-field stress after the opening is removed.

K_{tg} = \frac{\sigma_{\max}}{\sigma}

K_{tn} = \frac{\sigma_{\max}}{\sigma_{n}}

CUT-OUT IDEALIZATION

The Exhaust Hole

Due to the sharp corner of the exhaust’s hole, it will be approximated as a 3.7”x4.0” rectangular hole with 0.125” corners. Considering a/b value of 0.925 and the minimum r/2b value of 0.05, the resulted K_t value is 4.62 [Peterson’s Stress Concentration Factors-3rd edition by Pilkey- Chart 4.62a]. The utilized r/2b is 0.033. Hence, the K_t value is scaled up by 25%. Therefore, K_t=4.62x1.25=5.78

CUT-OUT IDEALIZATION

The Drain Hole

The drain’s hole will be approximated as a 0.866” diameter circular hole due to its rounded corners. The resulted K_tg and K_tn values are 3.23 and 2.45, respectively [Peterson’s Stress Concentration Factors-3rd edition by Pilkey- Chart 4.1].

CUT-OUT IDEALIZATION

Aft Intake Hole

The aft intake hole has a slot-like shape. Considering a=1.5 and H=4.5, the resulted a/H value is 0.333. For conservatism, a/H value of 0.3 was used. The resulted K_tn value is 2.8 [Peterson’s Stress Concentration Factors-3rd edition by Pilkey- Chart 4.59].

Gross/net stress-concentration factors selected for the three cut-out geometries.

	$\mathbf{K}_{\mathbf{tg}}$	$\mathbf{K}_{\mathbf{tn}}$	$\mathbf{K}_{\mathbf{t}}$	Reference
Exhaust’s hole	-	-	5.78	[Peterson’s Stress Concentration Factors-3rd edition by Pilkey- Chart 4.62a]
Drain’s hole	3.23	2.45	-	[Peterson’s Stress Concentration Factors-3rd edition by Pilkey- Chart 4.1]
Aft intake’ hole	-	2.8	-	[Peterson’s Stress Concentration Factors-3rd edition by Pilkey- Chart 4.59]

RADOME CUT-OUT ASSESSMENTLocal stress-concentration check

Highest tabulated stress 9.54 ksi

Minimum tensile strength 22 ksi

Minimum compressive strength 25 ksi

PASS

The accompanying stress table lists the AFT intake cut-out at 9.5368 ksi, while the source narrative identifies 5.77 ksi at the exhaust cut-out. Both values remain below the published radome laminate strength basis used by the source assessment.

Calculated concentrated stresses at the radome cut-out edges.

	Considering
	$\mathbf{K}_{\mathbf{tg}}$	$\mathbf{K}_{\mathbf{tn}}$	$\mathbf{K}_{\mathbf{t}}$
Exhaust’s hole			5768.44
Drain’s hole	637	642
Aft intake’ hole		9536.8

SUBSTANTIATION OUTCOME

JOINT DEMANDPASS

Governing 3.55 lb_f shear / 2.09 lb_f tension is below the 20.56 / 51.41 lb_f source joint allowables.

EXHAUST LAMINATE+69%

Minimum source CLPT margin by Hill criterion after 25% material knockdown and 10× aerodynamic amplification.

PTFE DRAINPASS

Base bending stress = 0.1033 ksi versus F_cy=1.45 ksi; outlet shear = 1.82 psi versus 725 psi.

RADOME CUT-OUTSPASS

Highest tabulated concentrated stress = 9.5368 ksi versus 22 ksi tensile / 25 ksi compressive strength basis.

✓

Structural substantiation complete

The source assessment demonstrates positive reserve for the evaluated aerodynamic load path, fastener groups, composite exhaust laminate, PTFE drain and radome cut-outs.

REFERENCES

Composite Materials & Analysis Methods

Composite Materials Handbook - MIL-HDBK-17 / CMH-17
Analysis & Design of Composite & Metallic Flight Vehicle Structures - Richard Abbott
Lamina Stress Analysis - Document AA-SM-101-108
The Strength of Bolted Joints in Multidirectional CFRP Laminates
Roark’s Formulas for Stress and Strain - 9th Edition
Peterson’s Stress Concentration Factors - 3rd Edition

Regulatory & Aerodynamic Load Basis

Federal Aviation Regulations - 14 CFR Part 25
Fluid-Dynamic Lift - S. F. Hoerner
Fluid-Dynamic Drag - S. F. Hoerner

Composite Fabrication & Material Data

MIL-C-9084 - Fiberglass Cloth specification
MIL-P-22241B - Polytetrafluoroethylene plastic sheet and film
EPOCAST 50-1A Resin System / Hardener 9816 manufacturer instructions
PTFE material and sheet technical data cited in the source report

Fastener, Washer & Aircraft Structure Data

ASME B18.6.3 - Machine screw dimensional standard
S17114 / 94701A833 screw technical data
93785A500 washer technical data
SRM-100 - General X-Band Nose Radome Repair Procedures

INTRODUCTION

DESIGN ASSESSMENT

System Architecture & Load Path

Materials & Joint Allowable Basis

Symmetric + balanced

Hot-wet lower-bound philosophy

Ftu1 = 32.11 ksi

Ftu2 = 25.75 ksi

Fcu1 = Fcu2 = 36.14 ksi

Fsu12 = 8.44 ksi

LOAD CASES FORMULATION

Flight-load framework

Emergency landing

ρ = 0.00238 slug/ft³

VMO,max = 242 kt

Bounding Aerodynamic Loads

CLASSICAL ANALYSIS

Load Transfer & Fastener Reactions

Fastener Reaction Forces

Component Strength Assessment

Classical Laminate Plate Theory

Tsai-Wu · Hill · Hoffman

Exhaust Component

Non-flanged plate only

10× ultimate load/moment

Reacted at the exhaust base

25% laminate knockdown retained

Drain Component

Annular plate · outer edge simply supported · inner edge free

Radome’s Cut-Outs

Ktg

Ktn

The Exhaust Hole

The Drain Hole

Aft Intake Hole

SUBSTANTIATION OUTCOME

REFERENCES

Composite Materials & Analysis Methods

Regulatory & Aerodynamic Load Basis

Composite Fabrication & Material Data

Fastener, Washer & Aircraft Structure Data

F_tu1 = 32.11 ksi

F_tu2 = 25.75 ksi

F_cu1 = F_cu2 = 36.14 ksi

F_su12 = 8.44 ksi

V_MO,max = 242 kt

K_tg

K_tn