{
 "version": "2026-10-20",
 "license": "Dimension sheets: CC BY 4.0. Standard and published-size files: CC BY 4.0. Xem Studio's own designs (files_license = Xem Studio Design License): free to use and sell things from, not to redistribute as files — https://xemstudio.com/license/. Other publishers' files: their own license, linked beside them.",
 "slug": "gurney-flap-wickerbill",
 "name": "Gurney flap / wickerbill",
 "kind": "standard",
 "category": "Car design",
 "group": "Car aero",
 "year": null,
 "question": "What is a Gurney flap (wickerbill) and how tall should it be?",
 "tags": [
  "vehicle design",
  "car aero",
  "rc design",
  "aerodynamics"
 ],
 "aliases": [
  "gurney flap",
  "wickerbill",
  "wicker bill",
  "gurney lip",
  "trailing edge tab",
  "gurney flap height",
  "gurney flap percent chord",
  "Liebeck gurney flap",
  "wing lip spoiler",
  "rc wing wickerbill",
  "gurney tab"
 ],
 "overall_mm": {
  "length": null,
  "width": null,
  "height": null
 },
 "official_cad": false,
 "xem_status": "free-files",
 "les": false,
 "free_files": 1,
 "url": "https://xemstudio.com/objects/gurney-flap-wickerbill/",
 "sheet_txt": "https://xemstudio.com/objects/gurney-flap-wickerbill/sheet.txt",
 "sheet": {
  "basis": "Design rules and figures from the cited sources; example geometry by Xem Studio",
  "overall_mm": {
   "length": null,
   "width": null,
   "height": null
  },
  "notes": [
   "A Gurney flap, also called a wickerbill, is a small tab at the trailing edge set at right angles to the pressure side (Wikipedia, citing Van Dam and Storms & Jang). It raises lift by changing the trailing-edge Kutta condition and creates a pair of counter-rotating vortices behind it.",
   "Liebeck tested a 1.25%-chord flap (0.0125c) and found better lift and, unexpectedly, lower drag; he stated that flaps much taller than about 0.0125c would probably show drag penalties (NASA TM-4071).",
   "Dan Gurney first used it in 1971 on Bobby Unser's car; HPA describes it as a way to add downforce once wing angle is maxed out.",
   "Used on: Eagle Indy cars (Dan Gurney, 1971), Sikorsky S-76B horizontal stabiliser, Bell JetRanger.",
   "The file is a reference example built by Xem Studio from the numbers above (gurney_h = 1.25% of 250 mm chord (Liebeck).). Sizes the sources do not fix are illustrative; scale the example to your own vehicle."
  ],
  "table": {
   "columns": [
    "parameter",
    "value or range",
    "source"
   ],
   "rows": [
    [
     "Liebeck test height",
     "0.0125c (1.25% chord)",
     "NASA TM-4071"
    ],
    [
     "Height above which drag penalty expected (Liebeck)",
     "> ~0.0125c",
     "NASA TM-4071"
    ],
    [
     "Typical height",
     "1-2% of chord",
     "Wikipedia (Storms & Jang 1994)"
    ],
    [
     "Club-car wing gurney",
     "about 10-20 mm tall",
     "HPA rear wing"
    ],
    [
     "Splitter edge gurney/upkick",
     "about 20-30 mm",
     "HPA splitter"
    ],
    [
     "First use",
     "1971 (Dan Gurney, Bobby Unser)",
     "Wikipedia"
    ],
    [
     "Liebeck paper introducing it",
     "AIAA 76-406 (1976)",
     "Wikipedia"
    ]
   ]
  }
 },
 "sheet_text": "Gurney flap / wickerbill — dimension sheet\nSource: Xem Studio, https://xemstudio.com/objects/gurney-flap-wickerbill/  (sheet CC BY 4.0; files Xem Studio Design License, https://xemstudio.com/license/)\nBasis: Design rules and figures from the cited sources; example geometry by Xem Studio\nCatalog version: 2026-10-20\n\nSizes: see table below\n- A Gurney flap, also called a wickerbill, is a small tab at the trailing edge set at right angles to the pressure side (Wikipedia, citing Van Dam and Storms & Jang). It raises lift by changing the trailing-edge Kutta condition and creates a pair of counter-rotating vortices behind it.\n- Liebeck tested a 1.25%-chord flap (0.0125c) and found better lift and, unexpectedly, lower drag; he stated that flaps much taller than about 0.0125c would probably show drag penalties (NASA TM-4071).\n- Dan Gurney first used it in 1971 on Bobby Unser's car; HPA describes it as a way to add downforce once wing angle is maxed out.\n- Used on: Eagle Indy cars (Dan Gurney, 1971), Sikorsky S-76B horizontal stabiliser, Bell JetRanger.\n- The file is a reference example built by Xem Studio from the numbers above (gurney_h = 1.25% of 250 mm chord (Liebeck).). Sizes the sources do not fix are illustrative; scale the example to your own vehicle.\n\nparameter  value or range  source\nLiebeck test height  0.0125c (1.25% chord)  NASA TM-4071\nHeight above which drag penalty expected (Liebeck)  > ~0.0125c  NASA TM-4071\nTypical height  1-2% of chord  Wikipedia (Storms & Jang 1994)\nClub-car wing gurney  about 10-20 mm tall  HPA rear wing\nSplitter edge gurney/upkick  about 20-30 mm  HPA splitter\nFirst use  1971 (Dan Gurney, Bobby Unser)  Wikipedia\nLiebeck paper introducing it  AIAA 76-406 (1976)  Wikipedia\n\nFiles elsewhere (not hosted by Xem Studio; license as stated by the publisher):\n- NASA (NTRS): A Water Tunnel Study of Gurney Flaps, NASA TM-4071 (Neuhart & Pendergraft, 1988) — web — Publisher's page; dimensions are facts — https://ntrs.nasa.gov/api/citations/19890004024/downloads/19890004024.pdf\n- Wikipedia: Gurney flap — web — Publisher's page; dimensions are facts — https://en.wikipedia.org/wiki/Gurney_flap\n- High Performance Academy: Aerodynamic performance devices for club level cars: rear wing theory — web — Publisher's page; dimensions are facts — https://www.hpacademy.com/courses/aerodynamics-fundamentals/aerodynamic-performance-devices-for-club-level-cars-rear-wing-theory\n- High Performance Academy: Aerodynamic performance devices for club level cars: front splitters — web — Publisher's page; dimensions are facts — https://www.hpacademy.com/courses/aerodynamics-fundamentals/aerodynamic-performance-devices-for-club-level-cars-front-splitters\n\nFree files (Xem Studio Design License, Xem Studio; STEP + STL, 1 parts): https://xemstudio.com/files/hardware/gurney-flap-wickerbill/index.json\nAPI: https://xemstudio.com/api/geometry?object=gurney-flap-wickerbill\n",
 "sources": [
  {
   "publisher": "NASA (NTRS)",
   "title": "A Water Tunnel Study of Gurney Flaps, NASA TM-4071 (Neuhart & Pendergraft, 1988)",
   "url": "https://ntrs.nasa.gov/api/citations/19890004024/downloads/19890004024.pdf",
   "formats": [
    "web"
   ],
   "license": "Publisher's page; dimensions are facts",
   "license_verified": true,
   "hosted": false
  },
  {
   "publisher": "Wikipedia",
   "title": "Gurney flap",
   "url": "https://en.wikipedia.org/wiki/Gurney_flap",
   "formats": [
    "web"
   ],
   "license": "Publisher's page; dimensions are facts",
   "license_verified": true,
   "hosted": false
  },
  {
   "publisher": "High Performance Academy",
   "title": "Aerodynamic performance devices for club level cars: rear wing theory",
   "url": "https://www.hpacademy.com/courses/aerodynamics-fundamentals/aerodynamic-performance-devices-for-club-level-cars-rear-wing-theory",
   "formats": [
    "web"
   ],
   "license": "Publisher's page; dimensions are facts",
   "license_verified": true,
   "hosted": false
  },
  {
   "publisher": "High Performance Academy",
   "title": "Aerodynamic performance devices for club level cars: front splitters",
   "url": "https://www.hpacademy.com/courses/aerodynamics-fundamentals/aerodynamic-performance-devices-for-club-level-cars-front-splitters",
   "formats": [
    "web"
   ],
   "license": "Publisher's page; dimensions are facts",
   "license_verified": true,
   "hosted": false
  }
 ],
 "xem": {
  "status": "free-files",
  "sheet_free": true,
  "mesh_price_usd": null,
  "step_price_usd": null,
  "tolerance_mm": null
 },
 "related": [
  "rear-wing-single-element",
  "rear-wing-dual-element",
  "swan-neck-wing-mount",
  "wing-endplates",
  "front-splitter",
  "splitter-tunnels-front-diffuser"
 ],
 "fits": [],
 "dimensions": "exact (nominal, no printing tolerance)",
 "dimension_rule": "All geometry on Xem Studio is exact (nominal, true size) with no printing tolerance. Where a part meets another part, the object lists its mating features (feature, exact size, what it mates with, fit class: press, slide, run, socket, clamp or designed). Apply your process's allowance to those features only; 'designed' gaps are part of the design and stay as drawn.",
 "mating": [],
 "faq": [
  {
   "q": "How tall should a Gurney be?",
   "a": "Liebeck used 1.25% of chord and expected drag penalties above that; 1-2% chord is the commonly cited range."
  },
  {
   "q": "Why is it called a wickerbill?",
   "a": "Wickerbill is another name for the same trailing-edge tab (Wikipedia)."
  }
 ],
 "files_license": {
  "name": "Xem Studio Design License",
  "holder": "Xem Studio",
  "url": "https://xemstudio.com/license/",
  "note": "Xem Studio's own design: free to use, print, modify and sell things made from it; not to be republished or redistributed as files."
 },
 "offers": [
  {
   "file": "gurney-flap-wickerbill-rear-wing",
   "formats": [
    "step",
    "stl"
   ],
   "price_usd": 0,
   "license": "Xem Studio Design License",
   "step": "https://xemstudio.com/files/hardware/gurney-flap-wickerbill/gurney-flap-wickerbill-rear-wing.step",
   "stl": "https://xemstudio.com/files/hardware/gurney-flap-wickerbill/gurney-flap-wickerbill-rear-wing.stl",
   "note": "gurney rear wing: span 1600 × chord 250 mm at 6°, endplates 300 mm, inverted NACA 4412 section (mounts from below)"
  }
 ],
 "markdown": "https://xemstudio.com/objects/gurney-flap-wickerbill/index.md"
}