{
 "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": "wig-pitch-height-stability",
 "name": "WIG pitch and height stability (moving centre of pressure)",
 "kind": "standard",
 "category": "Aircraft design",
 "group": "Ground-effect design",
 "year": null,
 "question": "Why are ground-effect craft hard to keep stable in pitch?",
 "tags": [
  "vehicle design",
  "ground-effect design",
  "rc design",
  "aerodynamics"
 ],
 "aliases": [
  "wig stability",
  "ground effect pitch stability",
  "irodov criterion",
  "ekranoplan stability",
  "wig centre of pressure",
  "height stability ground effect",
  "wig model pitch up",
  "ground effect porpoising",
  "wig cg location"
 ],
 "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/wig-pitch-height-stability/",
 "sheet_txt": "https://xemstudio.com/objects/wig-pitch-height-stability/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": [
   "In ground effect the centre of pressure moves with both pitch and height, so a craft can be stable in one and not the other. The Irodov criterion requires the aerodynamic centre in height to lie upstream of the centre in pitch (Rozhdestvensky).",
   "Remedies: a large tail kept out of ground effect (KM up to 50% of wing area), a reverse-delta planform that limits c.p. shift, or for flying wings, profiling and automatic damping, which have a small stable range (Rozhdestvensky).",
   "NASA found ground-effect wings had height stability at positive and instability at negative angles of attack, favouring positive-AoA cruise (NASA TN D-926); over waves, ram pressure raised lift at the cost of pitch stability (Frontiers 2022).",
   "Used on: KM (Caspian Sea Monster), Lippisch X-113.",
   "The file is a reference example built by Xem Studio from the numbers above. Sizes the sources do not fix are illustrative; scale the example to your own vehicle."
  ],
  "table": {
   "columns": [
    "parameter",
    "value or range",
    "source"
   ],
   "rows": [
    [
     "Irodov criterion",
     "centre in height upstream of centre in pitch",
     "Rozhdestvensky 2006"
    ],
    [
     "Height stability sign",
     "stable at +AoA, unstable at -AoA",
     "NASA TN D-926"
    ],
    [
     "Tail size for stability (KM)",
     "up to 50% of wing area",
     "Rozhdestvensky 2006"
    ],
    [
     "Reference build",
     "reverse delta 700 x 400, anhedral 15",
     "computed"
    ]
   ]
  }
 },
 "sheet_text": "WIG pitch and height stability (moving centre of pressure) — dimension sheet\nSource: Xem Studio, https://xemstudio.com/objects/wig-pitch-height-stability/  (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- In ground effect the centre of pressure moves with both pitch and height, so a craft can be stable in one and not the other. The Irodov criterion requires the aerodynamic centre in height to lie upstream of the centre in pitch (Rozhdestvensky).\n- Remedies: a large tail kept out of ground effect (KM up to 50% of wing area), a reverse-delta planform that limits c.p. shift, or for flying wings, profiling and automatic damping, which have a small stable range (Rozhdestvensky).\n- NASA found ground-effect wings had height stability at positive and instability at negative angles of attack, favouring positive-AoA cruise (NASA TN D-926); over waves, ram pressure raised lift at the cost of pitch stability (Frontiers 2022).\n- Used on: KM (Caspian Sea Monster), Lippisch X-113.\n- The file is a reference example built by Xem Studio from the numbers above. Sizes the sources do not fix are illustrative; scale the example to your own vehicle.\n\nparameter  value or range  source\nIrodov criterion  centre in height upstream of centre in pitch  Rozhdestvensky 2006\nHeight stability sign  stable at +AoA, unstable at -AoA  NASA TN D-926\nTail size for stability (KM)  up to 50% of wing area  Rozhdestvensky 2006\nReference build  reverse delta 700 x 400, anhedral 15  computed\n\nFiles elsewhere (not hosted by Xem Studio; license as stated by the publisher):\n- Progress in Aerospace Sciences (K.V. Rozhdestvensky, 2006): Wing-in-ground effect vehicles (review) — web — Publisher's page; dimensions are facts — https://www.hassanhameed.com/wp-content/uploads/2016/01/wingingroundeffectvehicles-130120003005-phpapp01.pdf\n- NASA Langley (Fink & Lastinger): Aerodynamic Characteristics of Low-Aspect-Ratio Wings in Close Proximity to the Ground, NASA TN D-926 (1961) — web — Publisher's page; dimensions are facts — https://ntrs.nasa.gov/api/citations/19980231058/downloads/19980231058.pdf\n- Frontiers in Aerospace Engineering (2022): Review of ground-effect experiments on rectangular, reverse-delta and delta wings — web — Publisher's page; dimensions are facts — https://www.frontiersin.org/journals/aerospace-engineering/articles/10.3389/fpace.2022.975158/pdf\n\nFree files (Xem Studio Design License, Xem Studio; STEP + STL, 1 parts): https://xemstudio.com/files/hardware/wig-pitch-height-stability/index.json\nAPI: https://xemstudio.com/api/geometry?object=wig-pitch-height-stability\n",
 "sources": [
  {
   "publisher": "Progress in Aerospace Sciences (K.V. Rozhdestvensky, 2006)",
   "title": "Wing-in-ground effect vehicles (review)",
   "url": "https://www.hassanhameed.com/wp-content/uploads/2016/01/wingingroundeffectvehicles-130120003005-phpapp01.pdf",
   "formats": [
    "web"
   ],
   "license": "Publisher's page; dimensions are facts",
   "license_verified": true,
   "hosted": false
  },
  {
   "publisher": "NASA Langley (Fink & Lastinger)",
   "title": "Aerodynamic Characteristics of Low-Aspect-Ratio Wings in Close Proximity to the Ground, NASA TN D-926 (1961)",
   "url": "https://ntrs.nasa.gov/api/citations/19980231058/downloads/19980231058.pdf",
   "formats": [
    "web"
   ],
   "license": "Publisher's page; dimensions are facts",
   "license_verified": true,
   "hosted": false
  },
  {
   "publisher": "Frontiers in Aerospace Engineering (2022)",
   "title": "Review of ground-effect experiments on rectangular, reverse-delta and delta wings",
   "url": "https://www.frontiersin.org/journals/aerospace-engineering/articles/10.3389/fpace.2022.975158/pdf",
   "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": [
  "ground-effect-height-span-ratio",
  "ground-effect-height-chord-ratio",
  "lippisch-reverse-delta-wig",
  "ekranoplan-wing-tail-layout",
  "tandem-wig-configuration",
  "power-augmented-ram-par"
 ],
 "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": "Why does my WIG model pitch up near the ground?",
   "a": "Its centre of pressure moves with height; Irodov's criterion and a large out-of-effect tail address this (Rozhdestvensky)."
  }
 ],
 "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": "wig-pitch-height-stability-wing",
   "formats": [
    "step",
    "stl"
   ],
   "price_usd": 0,
   "license": "Xem Studio Design License",
   "step": "https://xemstudio.com/files/hardware/wig-pitch-height-stability/wig-pitch-height-stability-wing.step",
   "stl": "https://xemstudio.com/files/hardware/wig-pitch-height-stability/wig-pitch-height-stability-wing.stl",
   "note": "Lippisch reverse delta: straight leading edge, trailing edge swept forward to the tips, span 700, root 400 mm, anhedral 15°, endplates 40 mm (scale to your craft)"
  }
 ],
 "markdown": "https://xemstudio.com/objects/wig-pitch-height-stability/index.md"
}